DNA MUTATION

MADE BY - GIREESHA SHARMA

Defining the term “Mutation”, what it is?

• Mutation is a natural process that changes a DNA sequence. And it is more common than you may think. As a cell copies its DNA before dividing, a "typo" occurs every 100,000 or so nucleotides. That's about 120,000 typos each time one of our cells divides.

• Most commonly, a single base is substituted for another. Sometimes a base is deleted or an extra base is added. Fortunately, the cell is able to repair most of these changes. When a DNA change remains unrepaired in a cell that will become an egg or a sperm, it is passed down to offspring. Thanks to mutation, we all have some new variations that were not present in our parents.

• People commonly use the terms "mutant" and mutation" to describe something undesirable or broken. But mutation is not always bad. Most DNA changes fall in the large areas of the genome that sit between genes, and usually they have no effect. When variations occur within genes, there is more often a consequence, but even then mutation only rarely causes death or disease. Mutation also generates new variations that can give an individual a survival advantage. And most often, mutation gives rise to variations that are neither good nor bad, just different.

• Mutation is a permanent change in the DNA. Mutation in germ cells are transmissible, in somatic cells its non - transmissible but may contribute to change (malignant transformations). Changes in the nucleotide sequence of the DNA, may occur in somatic cells aren’t passed to offspring, may occur in gametes and passed to offspring. It happens regularly, almost all mutations are natural. Chemicals and UV radiations cause them. Many mutations are repaired by enzymes.

CHAPTER – 1 TYPES OF MUTATION

• Where do mutations occur?• Mutations can be grouped into two main categories based on where they occur:

somatic mutations and germ-line mutations. • Somatic mutations take place in non-reproductive cells. Many kinds of somatic

mutations have no obvious effect on an organism, because genetically normal body cells are able to compensate for the mutated cells. Nonetheless, certain other mutations can greatly impact the life and function of an organism. For example, somatic mutations that affect cell division (particularly those that allow cells to divide uncontrollably) are the basis for many forms of cancer.

• Germ-line mutations occur in gametes or in cells that eventually produce gametes. In contrast with somatic mutations, germ-line mutations are passed on to an organism's progeny. As a result, future generations of organisms will carry the mutation in all of their cells (both somatic and germ-line).

• What kinds of mutations exist?• Mutations aren't just grouped according to where they occur —

frequently, they are also categorized by the length of the nucleotide sequences they affect.

• • Changes to short stretches of nucleotides are called gene-level

mutations, because these mutations affect the specific genes that provide instructions for various functional molecules, including proteins. Changes in these molecules can have an impact on any number of an organism's physical characteristics. As opposed to gene-level mutations, mutations that alter longer stretches of DNA (ranging from multiple genes up to entire chromosomes) are called chromosomal mutations. These mutations often have serious consequences for affected organisms. Because gene-level mutations are more common than chromosomal mutations, the following sections focus on these smaller alterations to the normal genetic sequence.

• Base substitution

• Base substitutions are the simplest type of gene-level mutation, and they involve the swapping of one nucleotide for another during DNA replication. For example, during replication, a thymine nucleotide might be inserted in place of a guanine nucleotide. With base substitution mutations, only a single nucleotide within a gene sequence is changed, so only one codon is affected. Although a base substitution alters only a single codon in a gene, it can still have a significant impact on protein production. In fact, depending on the nature of the codon change, base substitutions can lead to three different subcategories of mutations. The first of these subcategories consists of missense mutations, in which the altered codon leads to insertion of an incorrect amino acid into a protein molecule during translation; the second consists of nonsense mutations, in which the altered codon prematurely terminates synthesis of a protein molecule; and the third consists of silent mutations, in which the altered codon codes for the same amino acid as the unaltered codon.

TYPES OF MUTATION • Small-scale mutations, such as those affecting a small gene in one or a few

nucleotides, including:• Point mutations, often caused by chemicals or malfunction of DNA replication,

exchange a single nucleotide for another. These changes are classified as transitions or transversions. Most common is the transition that exchanges a purine for a purine (A ↔ G) or a pyrimidine for a pyrimidine, (C ↔ T). A transition can be caused by nitrous acid, base mis-pairing, or mutagenic base analogs such as 5-bromo-2-deoxyuridine (BrdU). Less common is a transversion, which exchanges a purine for a pyrimidine or a pyrimidine for a purine (C/T ↔ A/G). An example of a transversion is the conversion of adenine (A) into a cytosine (C). A point mutation can be reversed by another point mutation, in which the nucleotide is changed back to its original state (true reversion) or by second-site reversion (a complementary mutation elsewhere that results in regained gene functionality). Point mutations that occur within the protein coding region of a gene may be classified into three kinds, depending upon what the erroneous codon codes for:– Silent mutations, which code for the same (or a sufficiently similar) amino acid.– Missense mutations, which code for a different amino acid.– Nonsense mutations, which code for a stop and can truncate the protein.

•

• Insertions and deletions• Insertions and deletions are two other types of

mutations that can affect cells at the gene level. An insertion mutation occurs when an extra nucleotide is added to the DNA strand during replication. This can happen when the replicating strand "slips," or wrinkles, which allows the extra nucleotide to be incorporated .

• Strand slippage can also lead to deletion mutations. A deletion mutation occurs when a wrinkle forms on the DNA template strand and subsequently causes a nucleotide to be omitted from the replicated strand.

INSERTION AND DELETION METHOD-

• Large-scale mutations in chromosomal structure, including:– Amplifications (or gene duplications) leading to

multiple copies of all chromosomal regions, increasing the dosage of the genes located within them.

– Deletions of large chromosomal regions, leading to loss of the genes within those regions.

– Mutations whose effect is to juxtapose previously separate pieces of DNA, potentially bringing together separate genes to form functionally distinct fusion genes (e.g., bcr-abl). These include:

• Chromosomal translocations: interchange of genetic parts from nonhomologous chromosomes.

• Interstitial deletions: an intra-chromosomal deletion that removes a segment of DNA from a single chromosome, thereby apposing previously distant genes. For example, cells isolated from a human astrocytoma, a type of brain tumor, were found to have a chromosomal deletion removing sequences between the "fused in glioblastoma" (fig) gene and the receptor tyrosine kinase "ros", producing a fusion protein (FIG-ROS). The abnormal FIG-ROS fusion protein has constitutively active kinase activity that causes oncogenic transformation (a transformation from normal cells to cancer cells).

• Chromosomal inversions: reversing the orientation of a chromosomal segment.

– Loss of heterozygosity: loss of one allele, either by a deletion or a recombination event, in an organism that previously had two different alleles.

• Frameshift mutations• Insertion or deletion of one or more nucleotides during

replication can also lead to another type of mutation known as a frameshift mutation. The outcome of a frameshift mutation is complete alteration of the amino acid sequence of a protein. This alteration occurs during translation because ribosome’s read the mRNA strand in terms of codons, or groups of three nucleotides. These groups are called the reading frame. Thus, if the number of bases removed from or inserted into a segment of DNA is not a multiple of three, the reading frame transcribed to the mRNA will be completely changed. Consequently, once it encounters the mutation, the ribosome will read the mRNA sequence differently, which can result in the production of an entirely different sequence of amino acids in the growing polypeptide chain.

• To better understand frameshift mutations, let's consider the analogy of

words as codons, and letters within those words as nucleotides. Each word itself has a separate meaning, as each codon represents one amino acid. The following sentence is composed entirely of three-letter words, each representing a three-letter codon:

• THE BIG BAD FLY HAD ONE RED EYE AND ONE BLU EYE.

• Now, suppose that a mutation eliminates the sixth nucleotide, in this case the letter "G". This deletion means that the letters shift, and the rest of the sentence contains entirely new "words":

• THE BIB ADF LYH ADO NER EDE YEA NDO NEB LUE YE.• This error changes the relationship of all nucleotides to each codon, and

effectively changes every single codon in the sequence. Consequently, there is a widespread change in the amino acid sequence of the protein. Lets consider an example with an RNA sequence that codes for a sequence of amino acids:

• AUG AAA CUU CGC AGG AUG AUG AUG

• With the triplet code, the sequence shown in figure 5 corresponds to a protein made of the following amino acids:Methionine-Lysine-Leucine-Arginine-Arginine-Methionine-Methionine-Methionin

• Now, suppose that a mutation occurs during replication, and it results in deletion of the fourth nucleotide in the sequence. When separated into triplet codons, the nucleotide sequence would now read as follows.AUG AAC UUC GCA GGA UGA UGA UGThis series of codons would encode the following sequence of amino acids:Methionine-Asparagine-Phenylalanine-Alanine-Glycine-STOP-STOP

• Each of the stop codons tells the ribosome to terminate protein synthesis at that point. Consequently, the mutant protein is entirely different due to the deletion of the fourth nucleotide, and it is also shorter due to the appearance of a premature stop codon. This mutant protein will be unable to perform its necessary function in the cell.

• What causes mutations?• Mutations can arise in cells of all types as a result of a variety of factors,

including chance. In fact, some of the mutations discussed above are the result of spontaneous events during replication, and they are thus known as spontaneous mutations. Slippage of the DNA template strand and subsequent insertion of an extra nucleotide is one example of a spontaneous mutation; excess flexibility of the DNA strand and the subsequent mispairing of bases is another.

• Environmental exposure to certain chemicals, ultraviolet radiation, or other external factors can also cause DNA to change. These external agents of genetic change are called mutagens. Exposure to mutagens often causes alterations in the molecular structure of nucleotides, ultimately causing substitutions, insertions, and deletions in the DNA sequence.

• What are the consequences of mutations?• Mutations are a source of genetic diversity in

populations, and, as mentioned previously, they can have widely varying individual effects. In some cases, mutations prove beneficial to an organism by making it better able to adapt to environmental factors. In other situations, mutations are harmful to an organism — for instance; they might lead to increased susceptibility to illness or disease. In still other circumstances, mutations are neutral, proving neither beneficial nor detrimental outcomes to an organism. Thus, it is safe to say that the ultimate effects of mutations are as widely varied as the types of mutations themselves.

• Mutation generates new alleles -• The whole human family is one species with the same genes. Mutation

creates slightly different versions of the same genes, called alleles. These small differences in DNA sequence make every individual unique. They account for the variation we see in human hair color, skin color, height, shape, behavior, and susceptibility to disease. Individuals in other species vary too, in both physical appearance and behavior.

• Genetic variation is useful because it helps populations change over time. Variations that help an organism survive and reproduce are passed on to the next generation. Variations that hinder survival and reproduction are eliminated from the population. This process of natural selection can lead to significant changes in the appearance, behavior, or physiology of individuals in a population, in just a few generations.

• Once new alleles arise, meiosis and sexual reproduction combine different alleles in new ways to increase genetic variation.

• Mutation VS Variation – • It's useful to think of mutation as a process that creates genetic variation.

We often refer to a mutation as a thing—the genetic variation itself. This approach can be useful when it comes to a gene associated with a disease: the disease allele carries a mutation, a DNA change that compromises the protein's function. However, this approach gives mutation a bad name.

• It’s important to remember that losing the function of a gene doesn’t always affect health. For example, most mammals have hundreds of genes that code for olfactory receptors, proteins that help us smell. Losing one of these genes probably doesn’t make all that much difference.

• In contrast to variations that cause disease, there are many more examples of variations that are neither good nor bad, but just different—like blood types and eye color. Just like with disease alleles, the process of mutation creates these more neutral variations. But with neutral variations, it can be impossible to tell which allele is the "normal" one that existed first and which is the "mutant"—and the distinction is often meaningless.

• Proteins and Switches – Mutation creates variations in protein-coding portions of genes that can affect the protein itself. But even more often, it creates variations in the "switches" that control when and where a protein is active and how much protein is made.

• Lactase is an enzyme that helps infants break down lactose, a sugar in milk. Normally the gene that codes for lactase is active in babies and then turned off at about age four. When people who don't make lactase consume milk, they experience gas, nausea, and discomfort. But some people have a variation in a genetic switch that keeps the lactase gene active. This variation is called "lactase persistence," and people who have it can keep milk in their diets even as adults.

• Other drivers of mutation – Environment factors:• Radiation, chemicals, byproducts of cellular metabolism, free radicals,

ultraviolet rays from the sun—these agents damage thousands of nucleotides in each of our cells every day. They affect the nucleotides themselves: converting one base to another, knocking a base off its backbone, or even causing a break in the DNA strand.

• DNA Repair - Most of the time, mutation is reversed. DNA repair machines are constantly at work in our cells, fixing mismatched nucleotides and splicing broken DNA strands back together. Yet some DNA changes remain. If a cell accumulates too many changes—if its DNA is so damaged that repair machinery cannot fix it—it either stops dividing or it self-destructs. If any of these processes go wrong, the cell could become cancerous.

• When we put on sun screen, we are protecting ourselves against mutation in somatic cells—the cells that make up the body and are not involved in reproduction. Only when DNA changes are carried in egg and sperm cells are they passed to the next generation. Believe it or not, a certain amount of sloppiness is built into the system. Without mutation there would be no variation, and without variation there would be no evolution.

• Nomenclature• In order to categorize a mutation as such, the "normal" sequence must be

obtained from the DNA of a "normal" or "healthy" organism (as opposed to a "mutant" or "sick" one), it should be identified and reported; ideally, it should be made publicly available for a straightforward nucleotide-by-nucleotide comparison, and agreed upon by the scientific community or by a group of expert geneticists and biologists, who have the responsibility of establishing the standard or so-called "consensus" sequence. This step requires a tremendous scientific effort. (See DNA sequencing.) Once the consensus sequence is known, the mutations in a genome can be pinpointed, described, and classified. The committee of the Human Genome Variation Society (HGVS) has developed the standard human sequence variant nomenclature, which should be used by researchers and DNA diagnostic centers to generate unambiguous mutation descriptions. In principle, this nomenclature can also be used to describe mutations in other organisms. The nomenclature specifies the type of mutation and base or amino acid changes.

• Nucleotide substitution (e.g., 76A>T) — The number is the position of the nucleotide from the 5' end; the first letter represents the wild type nucleotide, and the second letter represents the nucleotide that replaced the wild type. In the given example, the adenine at the 76th position was replaced by a thymine.– If it becomes necessary to differentiate between mutations

in genomic DNA, mitochondrial DNA, and RNA, a simple convention is used. For example, if the 100th base of a nucleotide sequence mutated from G to C, then it would be written as g.100G>C if the mutation occurred in genomic DNA, m.100G>C if the mutation occurred in mitochondrial DNA, or r.100g>c if the mutation occurred in RNA. Note that, for mutations in RNA, the nucleotide code is written in lower case.

• Amino acid substitution (e.g., D111E) — The first letter is the one letter code of the wild type amino acid, the number is the position of the amino acid from the N-terminus, and the second letter is the one letter code of the amino acid present in the mutation. Nonsense mutations are represented with an X for the second amino acid (e.g. D111X).

• Amino acid deletion (e.g., ΔF508) — The Greek letter Δ (delta) indicates a deletion. The letter refers to the amino acid present in the wild type and the number is the position from the N terminus of the amino acid were it to be present as in the wild type.

• Contribution of mutations• The contribution of mutations is different in the tissues. This may be due to

different mutation rates by cell division and the different number of cell divisions in each tissue.

• Furthermore, knowing the mutational processes, mutation rates and the process of tissue development, can show the history of individual cells. For that, used cellular genome sequencing.

• Mutation rates• Mutation rates vary across species. Evolutionary biologist have theorized

that higher mutation rates are beneficial in some situations, because they allow organisms to evolve and therefore adapt more quickly to their environments. For example, repeated exposure of bacteria to antibiotics, and selection of resistant mutants, can result in the selection of bacteria that have a much higher mutation rate than the original population (mutator strains).

• According to one study, two children of different parents had 35 and 49 new mutations. Of them, in one case 92% were from the paternal germline, in another case, 64% were from the maternal germline.

• Harmful mutations• Changes in DNA caused by mutation can cause errors in protein

sequence, creating partially or completely non-functional proteins. Each cell, in order to function correctly, depends on thousands of proteins to function in the right places at the right times. When a mutation alters a protein that plays a critical role in the body, a medical condition can result. A condition caused by mutations in one or more genes is called a genetic disorder. Some mutations alter a gene's DNA base sequence but do not change the function of the protein made by the gene. One study on the comparison of genes between different species of Drosophila suggests that if a mutation does change a protein, this will probably be harmful, with an estimated 70 percent of amino acid polymorphisms having damaging effects, and the remainder being either neutral or weakly beneficial.Studies have shown that only 7% of point mutations in non-coding DNA of yeast are deleterious and 12% in coding DNA are deleterious. The rest of the mutations are either neutral or slightly beneficial.

• If a mutation is present in a germ cell, it can give rise to offspring that carries the mutation in all of its cells. This is the case in hereditary diseases. In particular, if there is a mutation in a DNA repair gene within a germ cell, humans carrying such germ-line mutations may have an increased risk of cancer. A list of 34 such germ-line mutations is given in the article DNA repair-deficiency disorder.

• An example of one is albinism. A mutation that occurs in the OCA1 or OCA2 gene. Individuals with this disorder are more prone to many types of cancers, other disorders and have impaired vision. On the other hand, a mutation may occur in a somatic cell of an organism. Such mutations will be present in all descendants of this cell within the same organism, and certain mutations can cause the cell to become malignant, and, thus, cause cancer.

• A DNA damage can cause an error when the DNA is replicated, and this error of replication can cause a gene mutation that, in turn, could cause a genetic disorder. DNA damages are repaired by the DNA repair system of the cell. Each cell has a number of pathways through which enzymes recognize and repair damages in DNA. Because DNA can be damaged in many ways, the process of DNA repair is an important way in which the body protects itself from disease. Once a DNA damage has given rise to a mutation, the mutation cannot be repaired. DNA repair pathways can only recognize and act on "abnormal" structures in the DNA. Once a mutation occurs in a gene sequence it then has normal DNA structure and cannot be repaired.

• Beneficial mutations• Although mutations that cause changes in protein sequences can be

harmful to an organism, on occasions the effect may be positive in a given environment. In this case, the mutation may enable the mutant organism to withstand particular environmental stresses better than wild-type organisms, or reproduce more quickly. In these cases a mutation will tend to become more common in a population through natural selection.

• For example, a specific 32 base pair deletion in human CCR5 (CCR5-Δ32) confers HIV resistance to homozygotes and delays AIDS onset in heterozygotes. One possible explanation of the etiology of the relatively high frequency of CCR5-Δ32 in the European population is that it conferred resistance to the bubonic plague in mid-14th century Europe. People with this mutation were more likely to survive infection; thus its frequency in the population increased. This theory could explain why this mutation is not found in southern Africa, which remained untouched by bubonic plague. A newer theory suggests that the selective pressure on the CCR5 Delta 32 mutation was caused by smallpoxinstead of the bubonic plague.

• Another example is Sickle-cell disease, a blood disorder in which the body produces an abnormal type of the oxygen-carrying substance hemoglobin in the red blood cells. One-third of all indigenous inhabitants of Sub-Saharan Africa carry the gene, because, in areas where malaria is common, there is a survival value in carrying only a single sickle-cell gene (sickle-cell trait). Those with only one of the two alleles of the sickle-cell disease are more resistant to malaria, since the infestation of the malaria plasmodium is halted by the sickling of the cells that it infests.

• Prion mutations• Prions are proteins and do not contain genetic material.

However, prion replication has been shown to be subject to mutation and natural selection just like other forms of replication.

• Somatic mutations• Main article: Loss of heterozygosity• See also: Carcinogenesis• A change in the genetic structure that is not inherited from a parent, and also

not passed to offspring, is called a somatic cell genetic mutation or acquired mutation.

• When analyzing somatic mutations present in the cells of multicellular organisms, can know its origin and its past.

• Cells with heterozygous mutations (one good copy of gene and one mutated copy) may function normally with the unmutated copy until the good copy has been spontaneously somatically mutated. This kind of mutation happens all the time in living organisms, but it is difficult to measure the rate. Measuring this rate is important in predicting the rate at which people may develop cancer.

• Point mutations may arise from spontaneous mutations that occur during DNA replication. The rate of mutation may be increased by mutagens. Mutagens can be physical, such as radiation from UV rays, X-rays or extreme heat, or chemical (molecules that misplace base pairs or disrupt the helical shape of DNA). Mutagens associated with cancers are often studied to learn about cancer and its prevention.

• Gain-of-function research• The aim of gain-of-function (GOF) research is to

genetically engineer increased transmissibility, virulence, or host range of pathogens. As such, it has been extremely controversial. As a Nature editorial put it in October 2014, "revelations over the past few months of serious violations and accidents at some of the leading bio safety containment labs in the United States has burst the hubris that some scientists, and their institutions, have in their perceived ability to work safely with dangerous pathogens." There is a current moratorium on such work in the United States.

CHAPTER – 2 Mutation Related Diseases

• DNA is constantly subject to mutations, accidental changes in its code. Mutations can lead to missing or malformed proteins, and that can lead to disease.

• We all start out our lives with some mutations. These mutations inherited from your parents are called germ-line mutations. However, you can also acquire mutations during your lifetime. Some mutations happen during cell division, when DNA gets duplicated. Still other mutations are caused when DNA gets damaged by environmental factors, including UV radiation, chemicals, and viruses.

• Few mutations are bad for you. In fact, some mutations can be beneficial. Over time, genetic mutations create genetic diversity, which keeps populations healthy. Many mutations have no effect at all. These are called silent mutations.

• But the mutations we hear about most often are the ones that cause disease. Some well-known inherited genetic disorders include cystic fibrosis, sickle cell anemia, Tay-Sachs disease, phenylketonuria and color-blindness, among many others. All of these disorders are caused by the mutation of a single gene.

• Most inherited genetic diseases are recessive, which means that a person must inherit two copies of the mutated gene to inherit a disorder. This is one reason that marriage between close relatives is discouraged; two genetically similar adults are more likely to give a child two copies of a defective gene.

• Diseases caused by just one copy of a defective gene, such as Huntington's disease, are rare. Thanks to natural selection, these dominant genetic diseases tend to get weeded out of populations over time, because afflicted carriers are more likely to die before reproducing.

• Scientists estimate that every one of us has between 5 and 10 potentially deadly mutations in our genes-the good news is that because there's usually only one copy of the bad gene, these diseases don't manifest.

• Cancer usually results from a series of mutations within a single cell. Often, a faulty, damaged, or missing p53 gene is to blame. The p53 gene makes a protein that stops mutated cells from dividing. Without this protein, cells divide unchecked and become tumors.

• Sickle Cell• These are the sickle-shaped blood cells of someone with sickle cell

anemia, a genetic disease common among those of African descent.• Sickle cell anemia is the result of a point mutation, a change in just

one nucleotide in the gene for hemoglobin. This mutation causes the hemoglobin in red blood cells to distort to a sickle shape when deoxygenated. The sickle-shaped blood cells clog in the capillaries, cutting off circulation.

• Having two copies of the mutated genes cause sickle cell anemia, but having just one copy does not, and can actually protect against malaria - an example of how mutations are sometimes beneficial.

•

• Heart and Muscle Disease• One basic mutation in mitochondrial DNA has been suggested as a

cause for heart and muscle disease. All human cells have mitochondria and they also hold DNA. Mitochondria produce energy through the metabolism of calories that you consume from food along with the oxygen you inhale.

• In this process, mitochondria also create oxygen radicals, which can harm the mitochondrial DNA, resulting in a greater number of mutations in body tissues. Unlike a cell's DNA, mitochondrial DNA is found around the cell's nucleus. It is also inherited only from the mother. It is when mitochondrial DNA damage builds up - typically associated with ageing - the body no longer produces energy as effectively and efficiently. Tissue and organ deterioration follows and disease may occur. Unfortunately, the energy metabolism of mitochondria make them particularly susceptible to mutations. With improved understanding of mitochondrial DNA mutations, however, we can pave the way for new treatments for heart and muscle disease.

• Breast and Ovarian Cancer• Our knowledge that breast and ovarian cancer is caused by genetic factors

in a small percentage of the population is not new information but our discoveries of specific genetic mutations have occurred more recently - over the past two decades. In the mid 1990s, it was found that the gene BRCA1, when mutated, significantly raised a person's odds of developing breast cancer. Shortly thereafter, another gene known as BRCA2 was identified, which also has been found to raise a person's risk of breast cancer.

• Researchers now are aware that any mutations in these two genes can also raise a person's risk of ovarian cancer. In fact, this risk is higher for individuals of Ashkenazi Jewish background. Research has also shown that BRCA1 mutations may increase the risk of other cancers such as colon or prostate. Mutations in the BRCA2 gene are suggested to increase the risk of other cancers such as those of the pancreas or prostate. More studies are being performed now to investigate any possible links between these two genes and other forms of cancer such as skin or lung cancer.

• Alzheimer's Disease• Alzheimer's disease tends to run in families, which means that if a

person in your family suffers from the disease, your risk of developing the disease is higher than the general population. One reason for this elevated risk is due to mutations within genes in your DNA. Researchers have now identified a handful of genes that play a role in the development of early-onset Alzheimer's disease but they have yet to conclusively identify a gene that triggers late-onset Alzheimer's disease. The inherited genes that have been identified include APP, presenilin-1 and presenilin-2. Due to the dominant inheritance of these three genes, you only need to inherit a single copy of the gene mutation for development of the disease. The relationship to early-onset Alzheimer's disease means that those who do inherit a mutated form of the gene are likely to develop Alzheimer's disease prior to age sixty-five.

• Angelman syndrome is a neuro -genetic disorder characterized by severe intellectual and developmental disability, sleep disturbance, seizures, jerky movements (especially hand-flapping), frequent laughter or smiling, and usually a happy demeanor.

• AS is a classic example of genomic imprinting in that it is caused by deletion or inactivation of genes on the maternally inherited chromosome 15 while the paternal copy, which may be of normal sequence, is imprinted and therefore silenced. The sister syndrome, Prader-Willi syndrome, is caused by a similar loss of paternally inherited genes and maternal imprinting.

• AS is named after a British pediatrician, Harry Angelman, who first described the syndrome in 1965.[1] An older, alternative term for AS, "happy puppet syndrome", is generally considered pejorative and stigmatizing so it is no longer the accepted term. People with AS are sometimes referred to as "angels", both because of the syndrome's name and because of their youthful, happy appearance.

• There is currently no cure available. The epilepsy can be controlled by the use of one or more types of anticonvulsant medications. However, there are difficulties in ascertaining the levels and types of anticonvulsant medications needed to establish control, because AS is usually associated with having multiple varieties of seizures, rather than just the one as in normal cases of epilepsy. Many families use melatonin to promote sleep in a condition which often affects sleep patterns. Many individuals with Angelman syndrome sleep for a maximum of 5 hours at any one time. Mild laxatives are also used frequently to encourage regular bowel movements, and early intervention with physiotherapy is important to encourage joint mobility and prevent stiffening of the joints. Speech and Language Therapy is commonly employed to assist individuals with Angelman syndrome and their communication issues.

Those with the syndrome are generally happy and contented people who like human contact and play. People with AS exhibit a profound desire for personal interaction with others. Communication can be difficult at first, but as a child with AS develops, there is a definite character and ability to make themselves understood.

People with AS tend to develop strong non-verbal skills to compensate for their limited use of speech. It is widely accepted that their understanding of communication directed to them is much larger than their ability to return conversation. Most afflicted people will not develop more than 5–10 words, if any at all. Seizures are a consequence, but so is excessive laughter, which is a major hindrance to early diagnosis.

• Color blindness, is the inability or decreased ability to see color, or perceive color

differences, under normal lighting conditions. Color blindness affects a significant percentage of the population.There is no actual blindness but there is a deficiency of color vision. The most usual cause is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to the optic nerve. This type of color blindness is usually a sex-linked condition. The genes that produce photopigments are carried on the X chromosome; if some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome (in females, a functional gene on only one of the two X chromosomes is sufficient to yield the needed photopigments).

• Color blindness can also be produced by physical or chemical damage to the eye, the optic nerve, or parts of the brain. For example, people with achromatopsia suffer from a completely different disorder, but are nevertheless unable to see colors.

• The first scientific paper on this subject, Extraordinary facts relating to the vision of colours, was published by the English chemist John Dalton in 1798 after the realization of his own color blindness. Because of Dalton's work, the general condition has been called daltonism, although in English this term is now used only for deuteranopia.

Color blindness is usually classified as a mild disability, however there are occasional circumstances where it can give an advantage. Some studies conclude that color blind people are better at penetrating certain color camouflages. Such findings may give an

evolutionary reason for the high prevalence of red–green color blindness. There is also a study suggesting that people with some types of color blindness can distinguish colors

that people with normal color vision are not able to distinguish.

• There is generally no treatment to cure color deficiencies. ″The American Optometric Association reports a contact lens on one eye can increase the ability to differentiate between colors, though nothing can make you truly see the deficient color.″ Optometrists can supply colored spectacle lenses or a single red-tint contact lens to wear on the non-dominant eye, but although this may improve discrimination of some colors, it can make other colors more difficult to distinguish. A 1981 review of various studies to evaluate the effect of the X-chromosome contact lens concluded that, while the lens may allow the wearer to achieve a better score on certain color vision tests, it did not correct color vision in the natural environment.

• Cystic fibrosis (CF), also known as mucoviscidosis, is a genetic disorder that affects mostly the lungs but also the pancreas, liver,kidneys and intestine. Long-term issues include difficulty breathing and coughing up sputum as a result of frequent lung infections. Other symptoms include sinus infections, poor growth, fatty stool, clubbing of the finger and toes, and infertility in males among others. Different people may have different degrees of symptoms.

• CF is an autosomal recessive disorder. It is caused by the presence of mutations in both copies of the gene for the protein cystic fibrosis transmembrane conductance regulator (CFTR).Those with a single working copy are carriers and otherwise mostly normal.CFTR is involved in production of sweat, digestive fluids, and mucus.[4] When not functional usually thin secretions become thick. The condition is diagnosed by a sweat test and genetic testing. Screening of infants at birth takes place in some areas of the world.

There is no cure for cystic fibrosis. Lung infections are treated with antibiotics which may be given intravenously, inhaled, or by mouth. Sometimes the antibiotic azithromycin is used long term. Inhaled hypertonic saline and salbutamol may also be useful. Lung transplantation may be an option if lung function continues to worsen. Pancreatic enzyme replacement and fat soluble vitamin supplementation are important, especially in the young. While not well supported by evidence, many people use airway clearance techniques such as chest physiotherapy.[1] The average life expectancy is between 37 and 50 years in the developed world.[6] Lung problems are responsible for death in 80% of people.

• CF is most common among people of Northern European ancestry and affects about one out of every three thousand newborns.About one in twenty five are carriers. It is least common in Africans and Asians. It was first recognized as a specific disease by Dorothy Andersen in 1938, with descriptions that fit the condition occurring at least as far back as 1595.The name cystic fibrosis refers to the characteristic fibrosis and cysts that form within the pancreas.

• While there are no cures for cystic fibrosis, there are several treatment methods. The management of cystic fibrosis has improved significantly over the past 70 years. While infants born with cystic fibrosis 70 years ago would have been unlikely to live beyond their first year, infants today are likely to live well into adulthood. Recent advances in the treatment of cystic fibrosis have meant that an individual with cystic fibrosis can live a fuller life less encumbered by their condition. The cornerstones of management are proactive treatment of airway infection, and encouragement of good nutrition and an active lifestyle. Pulmonary rehabilitation as a management of cystic fibrosis continues throughout a person's life, and is aimed at maximizing organ function, and therefore quality of life. At best, current treatments delay the decline in organ function. Because of the wide variation in disease symptoms, treatment typically occurs at specialist multidisciplinary centers, and is tailored to the individual. Targets for therapy are the lungs, gastrointestinal tract (including pancreatic enzyme supplements), the reproductive organs (including assisted reproductive technology (ART)) and psychological support.

• The most consistent aspect of therapy in cystic fibrosis is limiting and treating the lung damage caused by thick mucus and infection, with the goal of maintaining quality of life. Intravenous, inhaled, and oral antibiotics are used to treat chronic and acute infections. Mechanical devices and inhalation medications are used to alter and clear the thickened mucus. These therapies, while effective , can be extremely time consuming too.

• Down syndrome also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is typically associated with physical growth delays, characteristic facial features, and mild to moderate intellectual disability. The average IQ of a young adult with Down syndrome is 50, equivalent to the mental age of an 8- or 9-year-old child, but this varies widely.

• Down syndrome can be identified during pregnancy by prenatal screening followed by diagnostic testing, or after birth by direct observation and genetic testing. Since the introduction of screening, pregnancies with the diagnosis are often terminated. Regular screening for health problems common in Down syndrome is recommended throughout the person's life.

• Education and proper care have been shown to improve quality of life. Some children with Down syndrome are educated in typical school classes, while others require more specialized education. Some individuals with Down syndrome graduate from high school and a few attend post-secondary education. In adulthood, about 20% in the United States do paid work in some capacity with many requiring a sheltered work environment. Support in financial and legal matters is often needed. Life expectancy is around 50 to 60 years in the developed world with proper health care.

• Down syndrome is the most common chromosome abnormality in humans,

occurring in about one per 1000 babies born each year. In 2013 it resulted in 36,000 deaths down from 43,000 deaths in 1990. It is named after John Langdon Down, the British doctor who fully described the syndrome in 1866. Some aspects of the condition were described earlier by Jean-Étienne Dominique Esquirol in 1838 and Édouard Séguin in 1844. The genetic cause of Down syndrome—an extra copy of chromosome 21—was identified by French researchers in 1959.

• Hemophilia - is a group of hereditary genetic disorders that impair the body's ability to control blood clotting, which is used to stop bleeding when a blood vessel is broken. Haemophilia A (clotting factor VIII deficiency) is the most common form of the disorder, present in about 1 in 5,000–10,000 male births. Haemophilia B (factor IX deficiency) occurs in around 1 in about 20,000–34,000 male births.

• Like most recessive sex-linked, X chromosome disorders, haemophilia is more likely to occur in males than females. This is because females have two X chromosomes while males have only one, so the defective gene is guaranteed to manifest in any male who carries it. Because females have two X chromosomes and haemophilia is rare, the chance of a female having two defective copies of the gene is very remote, so females are almost exclusively asymptomatic carriers of the disorder.

• Female carriers can inherit the defective gene from either their mother or father, or it may be a new mutation. Although it is not impossible for a female to have haemophilia, it is unusual: daughters which are the product of both a male with hemophilia A or B and a female carrier will have haemophilia, while the non-sex-linked haemophilia C due to coagulant factor XI deficiency, which can affect either sex, is more common in Jews of Ashkenazi (east European) descent but rare in other population groups.

• Haemophilia patients have lower clotting factor level of blood plasma or impaired activity of the coagulation factors needed for a normal clotting process. Thus when a blood vessel is injured, a temporary scab does form, but the missing coagulation factors prevent fibrin formation, which is necessary to maintain the blood clot. A haemophiliac does not bleed more intensely than a person without it, but can bleed for a much longer time. In severe haemophiliacs even a minor injury can result in blood loss lasting days or weeks, or even never healing completely. In areas such as the brain or inside joints, this can be fatal or permanently debilitating.

• Though there is no cure for haemophilia, it can be controlled with regular infusions of the deficient clotting factor, i.e. factor VIII in haemophilia A or factor IX in haemophilia B. Factor replacement can be either isolated from human blood serum, recombinant, or a combination of the two. Some haemophiliacs develop antibodies (inhibitors) against the replacement factors given to them, so the amount of the factor has to be increased or non-human replacement products must be given, such as porcine factor VIII.

• If a patient becomes refractory to replacement coagulation factor as a result of circulating inhibitors, this may be partially overcome with recombinant human factor VII (NovoSeven), which is registered for this indication in many countries.

• Klinefelter syndrome is the set of symptoms that result from two or more X chromosome in males.The primary feature is sterility. Often symptoms may be subtle and many people do not realize they are affected. Sometimes symptoms are more prominent and may include weaker muscles, greater height, poor coordination, less body hair, smaller genitals, breast growth, and less interest in sex. Often it is only at puberty that these symptoms are noticed. Intelligence is usually normal; however, reading difficulties and problems with speech are more common. Symptoms are typically more severe if three or more X chromosomes are present.

• Klinefelter syndrome usually occurs randomly. An older mother might increase the risk slightly. The condition is not inherited from one's parents.The underlying mechanisms involves at least one extra X chromosome in addition to a Y chromosome such that there is a total of 47 or more chromosomes rather than usual 46. KS is diagnosed by the genetic test known as a karyotype.

• While there is no cure a number of treatments may help. Physical therapy, speech and language therapy, counselling, and adjustments of teaching methods may be useful. Testosterone replacement may be used in those who have significantly low levels. Enlarged breasts may be removed by surgery. About half of males affected with the help of assisted reproductive technology have a chance of having children; however, this is expensive and carries risks. The condition has a nearly normal life expectancy.

• Klinefelter syndrome is the most common chromosomal disorder, and it occurs in 1:500 to 1:1000 live male births. It is named after Harry Klinefelter who identified the condition in the 1940s. In 1956 it was determined to be due to an extra X chromesome. Mice also can have the XXY syndrome, making them a useful research model.

• The genetic variation is irreversible. Often individuals that have noticeable breast tissue or hypogonadism experience depression and/or social anxiety because they are outside of social norms. This is academically referred to as psychosocial morbidity. At least one study indicates that planned and timed support should be provided for young men with Klinefelter syndrome to ameliorate current poor psychosocial outcomes.

• Phenylketonuria is an inborn error of metabolism involving impaired metabolism of phenylalanine, one of the amino acids. Phenylketonuria is caused by absent or virtually absent phenylalanine hydroxylase (PAH) enzyme activity.

• Protein-rich foods or the sweetener aspartame can act as poisons for people with phenylketonuria. The role of PAH is to break down excess phenylalanine from food. Phenylalanine is a necessary part of the human diet and is naturally present in all kinds of dietary protein. It is also used to make aspartame, known by the trade name Nutrasweet, which is used to sweeten low-calorie and sugar free soft drinks, yogurts, and desserts. In people without PKU, the PAH enzyme breaks down any excess phenylalanine from these sources beyond what is needed by the body. However, if there is not enough of the PAH enzyme or its cofactor, then phenylalanine can build up in the blood and brain to toxic levels, affecting brain development and function. PKU is rare, but important to identify, because if caught early it is very treatable. It is not contagious, and it is lifelong, but with early diagnosis and consistent treatment, the damaging effects can be minimal or non-existent.

• Untreated PKU can lead to intellectual disability, seizures, and other serious medical problems. The best proven treatment for classical PKU patients is a strict phenylalanine-restricted diet supplemented by a medical formula containing amino acids and other nutrients. In the United States, the current recommendation is that the PKU diet should be maintained for life. Patients who are diagnosed early and maintain a strict diet can have a normal life span with normal mental development.

• PKU is an inherited disease. When an infant is diagnosed with PKU, it is never the result of any action of the parents or any environmental factor. Rather, for a child to inherit PKU, both of his or her parents must have at least one mutated allele of the PAH gene. Most parents who are carriers of PKU genes are not aware that they have this mutation because being a carrier causes no medical problems. To be affected by PKU, a child must inherit two mutated alleles, one from each parent. PKU is not curable. However, if PKU is diagnosed early enough, an affected newborn can grow up with normal brain development and live a normal life in terms of educational achievement, career success, etc., by managing and controlling phenylalanine ("Phe") levels through diet, or a combination of diet and medication.

• When Phe cannot be metabolized by the body, a typical diet that would be healthy for people without PKU causes abnormally high levels of Phe to accumulate in the blood, which is toxic to the brain. If left untreated, complications of PKU include severe intellectual disability, brain function abnormalities, microcephaly, mood disorders, irregular motor functioning, and behavioral problems such as attention deficit hyperactivity disorder, as well as physical symptoms such as a "musty" odor, eczema, and unusually light skin and hair coloration. In contrast, PKU patients who follow the prescribed dietary treatment from birth, may have no symptoms at all. Their PKU would be detectable only by a blood test.

• To achieve these good outcomes, all PKU patients must adhere to a special diet low in Phe for optimal brain development. Since Phe is necessary for the synthesis of many proteins, it is required for appropriate growth, but levels must be strictly controlled in PKU patients.

• PKU is not a food allergy or a digestive problem. Eating "forbidden" foods does not cause an immediate reaction. The phenylalanine from that food remains in the person's system, however, and as Phe accumulates over time they may experience concentration and mood problems, as well as eczema and other symptoms. For children, developmental problems may occur if levels are elevated frequently or remain elevated for a significant amount of time.

• Optimal health ranges (or "target ranges") are between 120 and 360 µmol/L or equivalently 2 to 6 mg/dL, and aimed to be achieved during at least the first 10 years, to allow the brain to develop normally.

• Sickle-cell disease (SCD), is a hereditary blood disorder, characterized by an abnormality in the oxygen-carrying haemoglobin molecule in red blood cells. This leads to a propensity for the cells to assume an abnormal, rigid, sickle-like shape under certain circumstances. Sickle-cell disease is associated with a number of acute and chronic health problems, such as severe infections, attacks of severe pain ("sickle-cell crisis"), and stroke, and there is an increased risk of death.

• Sickle-cell disease occurs when a person inherits two abnormal copies of the haemoglobin gene, one from each parent. Several subtypes exist, depending on the exact mutation in each haemoglobin gene. A person with a single abnormal copy does not experience symptoms and is said to have sickle-cell trait. Such people are also referred to as carriers.

• The complications of sickle-cell disease can be prevented to a large extent with vaccination, preventive antibiotics, blood transfusion, and the drug hydroxyurea/hydroxycarbamide. A small proportion requires a transplant of bone marrow cells.

• Almost 300,000 children are born with a form of sickle-cell disease every year, mostly in sub-Saharan Africa, but also in other parts of the world such as the West Indies and in people of African origin elsewhere in the world. In 2013 it resulted in 176,000 deaths up from 113,000 deaths in 1990.The condition was first described in the medical literature by the American physician James B. Herrick in 1910, and in the 1940s and 1950s contributions by Nobel prize-winner Linus Pauling made it the first disease where the exact genetic and molecular defect was elucidated.

• Folic acid and penicillin•

Children born with sickle-cell disease undergo close observation by the pediatrician and require management by a haematologist to assure they remain healthy. These patients take a 1 mg dose of folic acid daily for life. From birth to five years of age, they also

have to take penicillin daily due to the immature immune system that makes them more prone to early childhood illnesses.

• Malaria chemoprophylaxis• The protective effect of sickle-cell trait does not apply to people with

sickle cell disease; in fact, they are more vulnerable to malaria, since the most common cause of painful crises in malarial countries is infection with malaria. It has therefore been recommended that people with sickle-cell disease living in malarial countries should receive anti-malarial chemoprophylaxis for life.

Vaso-occlusive crisis

• Most people with sickle-cell disease have intensely painful episodes called vaso-occlusive crises. However, the frequency, severity, and duration of these crises vary tremendously. Painful crises are treated symptomatically with pain medications; pain management requires opioid administration at regular intervals until the crisis has settled. For milder crises, a subgroup of patients manage on NSAIDs (such as diclofenac or naproxen). For more severe crises, most patients require inpatient management for intravenous opioids; patient-controlled analgesia (PCA) devices are commonly used in this setting. Diphenhydramine is also an effective agent that doctors frequently prescribe to help control itching associated with the use of opioids.

• Acute chest crisis• Management is similar to vaso-occlusive crisis, with the addition

of antibiotics (usually a quinolone or macrolide, since cell wall-deficient ["atypical"] bacteria are thought to contribute to the syndrome), oxygen supplementation for hypoxia, and close observation. Should the pulmonary infiltrate worsen or the oxygen requirements increase, simpleblood transfusion or exchange transfusion is indicated. The latter involves the exchange of a significant portion of the patients red cell mass for normal red cells, which decreases the percent of haemoglobin S in the patient's blood. The patient with suspected acute chest syndrome should be admitted to the hospital with worsening A-a gradient an indication for ICU admission.

• Hydroxyurea• The first approved drug for the causative treatment of

sickle-cell anaemia, hydroxyurea, was shown to decrease the number and severity of attacks in a study in 1995 (Characheet al.) and shown to possibly increase survival time in a study in 2003 (Steinberg et al.). This is achieved, in part, by reactivating fetal haemoglobin production in place of the haemoglobin S that causes sickle-cell anaemia. Hydroxyurea had previously been used as a chemotherapy agent, and there is some concern that long-term use may be harmful, but this risk has been shown to be either absent or very small and it is likely that the benefits outweigh the risks.

• Transfusion therapy• Blood transfusions are often used in the management of sickle-cell disease in

acute cases and to prevent complications by decreasing the number of red blood cells (RBC) that can sickle by adding normal red blood cells.In children prophylactic chronic red blood cell (RBC) transfusion therapy has been shown to be efficacious to a certain extent in reducing the risk of first stroke or silent stroke when transcranial Doppler (TCD) ultrasonography shows abnormal increased cerebral blood flow velocities. In those who have sustained a prior stroke event it also reduces the risk of recurrent stroke and additional silent strokes.

• Bone marrow transplants• Bone marrow transplants have proven effective in children. Bone marrow

transplants are the only known cure for SCD. However, bone marrow transplants are difficult to obtain because of the specific HLA typing necessary. Ideally, a twin family member (syngeneic) or close relative (allogeneic) would donate the bone marrow necessary for transplantation.

• Tay–Sachs disease is a rare autosomal recessive genetic disorder. In its most common variant (known as infantile Tay–Sachs disease), it causes a progressive deterioration of nerve cells and of mental and physical abilities that begins around six months of age and usually results in death by the age of four. The disease occurs when harmful quantities of cell membrane components known as gangliosides accumulate in the brain's nerve cells, eventually leading to the premature death of the cells. A ganglioside is a form of sphingolipid, which makes Tay–Sachs disease a member of the sphingolipidoses. There is no known cure or treatment.

• The disease is named after the British ophthalmologist Waren Tay, who in 1881 first described a symptomatic red spot on the retina of the eye; and after the American neurologist Bernard Sachs of Mount Sinai Hospital, New York, who described in 1887 the cellular changes of Tay–Sachs disease and noted an increased disease prevalence in Ashkenazi Jewish people. Research in the late 20th century demonstrated that Tay–Sachs disease is caused by a genetic mutation in the HEXA gene on (human) chromosome 15.

• A large number of HEXA mutations have been discovered, and new ones are still being reported. These mutations reach significant frequencies in specific populations. French Canadians of southeastern Quebec have a carrier frequency similar to that seen in Ashkenazi Jews, but carry a different mutation. Cajuns of southern Louisiana carry the same mutation that is seen most commonly in Ashkenazi Jews.

• HEXA mutations are rare and are most seen in genetically isolated populations. Tay–Sachs can occur from the inheritance of either two similar, or two unrelated, causative mutations in the HEXA gene.

• As an autosomal recessive disorder, two Tay–Sachs alleles are required for an individual to exhibit symptoms of the disease. Carriers of a single Tay–Sachs allele do not exhibit symptoms of the disease but appear to be protected to some extent against tuberculosis. This accounts for the persistence of the allele in certain populations in that it confers a selective advantage—in other words, being a heterozygote is advantageous.

• There is currently no cure or treatment for Tay–Sachs disease. Even with the best care, children with infantile Tay–Sachs disease die by the age of 4. Although experimental work is underway, no current medical treatment of the root cause yet exists. Patients receive supportive care to ease the symptoms or extend life. Infants are given feeding tubes when they can no longer swallow. Improvements in life-extending care have somewhat lengthened the survival of children with Tay–Sachs disease, but no current therapy is able to reverse or delay the disease's progress.

• In late-onset Tay-Sachs, medication (e.g., lithium for depression) can sometimes control psychiatric symptoms and seizures, although some medications (e.g., tricyclic antidepressants, phenothiazines, haloperidol, and risperidone) are associated with significant adverse effects. In 2011, researchers discovered that Pyrimethamine can increase ß-hexosaminidase activity, thus slowing down the progression of Late-Onset Tay–Sachs disease.

• Turner syndrome (TS) is a condition in which a femaleis partly or completely missing an X chromosome. Signs and symptoms vary among those affected. Often there is a short and webbed neck, low-set ears, low hairline at the back of the neck, short stature, and swollen hands and feet at birth. Typically they arewithout menstrual periods, do not develop breasts, and are unable to have children. Heart defects, diabetes, and low thyroid hormone occur more frequently. Most people with TS have normal intelligence. Many, however, have troubles with spatial visualization such as that needed for mathematics. Vision and hearing problems occur more often.

• Turner syndrome is not usually inherited from a person's parents. There are no known environmental risks and the mother's age does not play a role. Turner syndrome is due to a chromosomal abnormality in which all or part of one of the X chromosomes is missing or altered. While most people have 46 chromosomes, people with TS usually only have 45. The chromosomal abnormality may be present in just some cells in which case it is known as TS with mosaicism. In these cases the symptoms are usually fewer and possibly there are none at all. Diagnosis is based on physical signs and genetic testing

• There is no cure for Turner syndrome. Treatment, however, may help with symptoms. Human growth hormone injections during childhood may increase adult height. Estrogen replacement therapy can promote development of the breasts and hips. Medical care is often required to manage other health problems with which TS is associated.

• Turner syndrome occurs in between 1 in 2000 to 1 in 5000 females at birth.

All regions of the world and cultures are affected about equally. People with TS have a shorter life expectancy, mostly due to heart problems and diabetes. Henry Turner first described the condition in 1938. In 1964 it was determined to be due to a chromosomal abnormality.

• As a chromosomal condition, there is no cure for Turner syndrome. However, much can be done to minimize the symptoms. For example:

• Doctors might use a shot of a growth hormone known as Genotropin (Pfizer)• Growth hormone, either alone or with a low dose of androgen, will increase

growth and probably final adult height. Growth hormone is approved by the U.S. Food and Drug Administration for treatment of Turner syndrome and is covered by many insurance plans. There is evidence that this is effective, even in toddlers.

• Estrogen replacement therapy such as the birth control pill, has been used since the condition was described in 1938 to promote development of secondary sexual characteristics. Estrogens are crucial for maintaining good bone integrity, cardiovascular health and tissue health. Women with Turner Syndrome who do not have spontaneous puberty and who are not treated with estrogen are at high risk for osteoporosis and heart conditions.

• Modern reproductive technologies have also been used to help women with Turner syndrome become pregnant if they desire. For example, a donor egg can be used to create an embryo, which is carried by the Turner syndrome woman.

• Uterine maturity is positively associated with years of estrogen use, history of spontaneous menarche, and negatively associated with the lack of current hormone replacement therapy.

CHAPTER -3DETECTION OF DNA MUTATION

• There are few mutation detection method present in our labs which I am going to discuses here.

Genetic disorders are traditionally categorized into three main groups: single-gene, chromosomal, and multifactorial disorders.

• Single gene or Mendelian disorders result from errors in DNA sequence of a gene and include autosomal dominant (AD), autosomal recessive (AR), X-linked recessive (XR), X-linked dominant and Y-linked (holandric) disorders.

• Chromosomal disorders are due to chromosomal aberrations including numerical and structural damages. Molecular and cytogenetic techniques have been applied to identify genetic mutations leading to diseases. Accurate diagnosis of diseases is essential for appropriate treatment of patients, genetic counseling and prevention strategies. Characteristic features of patterns of inheritance are briefly reviewed and a short description of chromosomal disorders is also presented. A combination of genes and environmental factors is involved in multifactorial disorders such as congenital heart disease, most types of cleft lip/palate, club foot, and neural tube defects If the DNA sequence is mutated and the alteration is not repaired by the cell, subsequent replications reproduce the mutation.

• A variety of mechanisms can cause mutations ranging from a single nucleotide alteration to the loss, duplication or rearrangement of chromosomes.

• Mendelian Disorders• Gregor Mendel discovered a set of principles of

heredity in the mid-19th century; characteristic patterns of inheritance are determined on the basis of these principles. Single gene disorders arising from errors in DNA sequence of a gene are categorized into autosomal dominant (AD), autosomal recessive (AR), X-linked recessive (XR), X-linked dominant and Y-linked (holandric) disorders.

• In autosomal dominant disorders , damage in one allele of a pair of the gene leads to the deficiency , e.g. a mutation in FGFR3 gene can cause achondroplasia . A parent with an autosomal dominant disorder has a 50% chance of transmitting the disease to her/his child. The range of signs and symptoms of some diseases in different people vary widely (variable expressivity), e.g.

• some people with Marfan syndrome (due to mutation in FBN1) have only mild symptoms (such as being tall and thin with long, slender fingers), while others have life-threatening complications involving the heart and blood vessels as well. Furthermore, some individuals exhibit signs and symptoms of a given disorder while others do not, even though they have the disease-causing mutation (i.e. a proportion of people with a particular mutation show the condition in this type of disorders), e.g. many people having mutation of the BRCA1 gene will develop breast cancer during their lifetime, while some people will not. In other words, in a pedigree a healthy individual has at least one affected parent and one affected child (skipped generation) .

• Mutations in both alleles (loss of function) of a gene are required to cause the defect to appear in an autosomal recessive disorder, i.e. an affected person has got one abnormal allele from one heterozygous parent. In this type of disorders, there is a 25% chance of having an affected offspring for heterozygous parents. In case of common autosomal recessive disorders or traits (sickle cell anemia due to a specific mutation in HBB gene encoding beta globin or nonsyndromic autosomal recessive hearing loss due to mutations in GJB2 gene encoding connexin 26), sometimes a homozygous affected person marries a heterozygous carrier; such an example, in which apparently dominant transmission of this disorder occurs, is called pseudodominant inheritance.

• In an X-linked disorder , the mutated gene is located on the X chromosome. A recessive mutation can lead to the disease. The gene in chromosome X should be mutated to cause the condition; hence, an X-linked recessive disorder is carried by females, while usally affects males.

• Some of genetic conditions follow none of the mentioned patterns of inheritance; mitochondrial diseases, trinucleotide expansion disorders and genomic imprinting defects have non-Mendelian or nontraditional pattern of inheritance

• Chromosomal Disorders• Typically, somatic cells proliferate via division called mitosis while germ cells

are produced through meiosis division. Meiosis involves a reductional division followed by an equational division, Meiosis I and II, respectively.

• Oogenesis begins in the female fetus at 12 weeks, but it is stopped in a stage of meiosis I (when the homologous chromosomes have replicated and paired as bivalents or tetrads) at about 20 weeks. At puberty usually only one oocyte is released per month; a primary oocyte completes meiosis I and produces one secondary oocyte and one polar body. Chromosomal aberrations including numerical (due to errors at chromosome pairing and crossing-over) and structural damages lead to chromosomal disorders . Aneuploidy is usually due to failure of segregation of chromosomes in meiosis I or meiosis II (non-disjunction, premature disjunction or anaphase lag) ; examples of numerical aberrations include Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), Klinefelter syndrome (XXY syndrome), Turner syndrome (monosomy X) and trisomy X. Chromosomal errors in oocytes are increased dramatically with maternal age. Non-disjunction or chromosome lag during mitosis can lead to mosaicism

• With the development of new technologies for more accurate understanding of the genome and potential gene therapies, the detection of mutations has an increasingly central role in various areas of genetic diagnosis including preimplantation genetic diagnosis (PGD), prenatal diagnosis (PND), presymptomatic testing, confirmational diagnosis and forensic/identity testing. Two groups of tests, molecular and cytogenetic, are used in genetic syndromes. In general, single base pair mutations are identified by direct sequencing, DNA hybridization and/or restriction enzyme digestion methods. However, there are two approaches for genetic diagnosis; indirect approach depends on the results from a genetic linkage analysis using DNA markers such as STR(short tandem repeat) or VNTR (variable number tandem repeat) markers flanking or within the gene. The direct approach for diagnosis essentially depends on the detection of the genetic variations responsible for the disease.

• Cytogenetics and Molecular Cytogenetics• Conventional Karyotyping: Chromosome

studies are advised in the following situations: suspected chromosome abnormality, sexual disorders, multiple congenital anomalies and/or developmental retardation, undiagnosed learning disabilities, infertility or multiple miscarriage, stillbirth and malignancies. Traditionally, the microscopic study of chromosomes is performed on compacted chromosomes at a magnification of about 1000 at metaphase.

• Preparation of a visual karyotype is done by arresting dividing cells at metaphase stage with a microtubule polymerization inhibitor such as colchicine; the cells, then, are spread on a glass slide and stained with Giemsa stain (G-banding). Chromosomes are studied by making a photograph or digital imaging and subsequent assembling of chromosomes. Human chromos-omes are categorized based on position of centromere; in metacentric chromosomes, centromere is located in the middle (chromosomes 1, 3, 16, 19 and 20), chromosomes 13, 14, 15, 21, 22 and Y are acrocentric (the centromere near one end),

• and other chromosomes are sub-metacentric. Chromosome arms are defined by region number (from centromere), band, sub-band and sub-sub-band numbers, e.g. 12q13.12 refers to chromosome 12, long arm, region 1, band 3, sub-band 1, sub-sub-band 2 (read chromosome 12, q, 1, 3, point, 1, 2). High resolution banding needs fixation before the chromosomes are fully compacted. The convenient methods of chromosome banding are G-(Giemsa), R-(reverse), C-(centromere) and Q-(quinacrine) banding.

• Fluorescence in situ hybridization (FISH): FISH is applied to provide specific localization of genes on chromosomes. Rapid diagnosis of trisomies and microdeletions is acquired using specific probes. Usually a denatured probe is added to a metaphase chromosome spread and incubated overnight to allow sequence-specific hybridization. After washing off the unbound probe, the bound probe is visualized by its fluorescence under UV light; thus, the site of the gene of interest is observed as in situ . This technique is used to check the cause of trisomies, microdeletion syndromes, etc.

• Comparative genomic hybridization (CGH): CGH, a special FISH technique (dual probes), is applied for detecting all genomic imbalances. The basics of technique is comparison of total genomic DNA of the given sample DNA (e.g. tumor DNA) with total genomic DNA of normal cells.

• Typically, an identical amount of both tumor and normal DNAs is labeled with two different fluorescent dyes; the mixture is added and hybridized to a normal lymphocyte metaphase slide. A fluorescent microscope equipped with a CCD camera and an image analysis system are used for evaluation. Technical details have been described in numerous CGH publications. Copy number of genetic material (gains and losses) is calculated by evaluation software. CGH is used to determine copy number alterations of genome in cancer and those cells whose karyotype is hard or impossible to prepare or analyze. In array-CGH, metaphase slide is replaced by specific DNA sequences, spotted in arrays on glass slides; so its resolution is increased.

• Molecular Diagnostics• In addition to genetic causes of disorders, predisposition to a

disease or treatment options could be revealed by determining DNA variations. Molecular diagnostics provide a way for assessment of the genetic makeup of human; it combines laboratory medicine with molecular genetics to develop DNA/RNA-based analytical methods for monitoring human pathologies.

• A wide range of methods has been used for mutation detection. Molecular methods for identification of the disease-causing mutations could be classified as methods for known and methods for unknown mutations.

• Several criteria, however, have to be met for choosing a suitable method; for example the following points should be considered: type of nucleic acid (DNA or RNA), kind of specimen (e.g., blood, tissues, etc.), the number of mutations, and reliability of the method.

• A) Known Mutations• Many different approaches have been used for

identifying known mutations. Usually starting with the polymerase chain reaction (PCR).

• DNA microarray: DNA “chips” or microarrays have been used as a possible testing for multiple mutations. In this technology, single DNA strands including sequences of different targets are fixed to a solid support in an array format. On the other hand, the sample DNA or cDNA labeled with fluorescent dyes is hybridized to the chip. Then using a laser system, the presence of fluorescence is checked; the sequences and their quantities in the sample are determined.

• DNA Sequencing: As a powerful technique in molecular genetics, DNA sequencing provides analysis of genes at the nucleotide level. The main aim of DNA sequencing is to determine the sequence of small regions of interest ( 1 kilobase) using a PCR product as a template. ∼Dideoxynucleotide sequencing or Sanger sequencing represents the most widely used technique for sequencing DNA. In this method,

• 1.double stranded DNA is denatured into single stranded DNA with NaOH. A Sanger reaction consists of a single strand DNA, primer, a mixture of a particular ddNTP with normal dNTPs (e.g. ddATP with dATP, dCTP, dGTP, and dTTP). A fluorescent dye molecule is covalently attached to the dideoxynucleotide. ddNTPs cannot form a phosphodiester bond with the next deoxynucleotide so that they terminate DNA chain elongation.

• 2.This step is done in four separate reactions using a different ddNTP for each reaction . DNA sequencing could be used to check all small known and unknown DNA variations.

• Multiplex ligation-dependent probe amplification (MLPA): MLPA is commonly applied to screen deletions and duplications of up to 50 different genomic DNA or RNA sequences. Altogether gene deletions and duplications account up to 10%, and in many disorders up to 30% of disease-causing mutations. In this technique, briefly,

• 1.the probe set is hybridized to genomic DNA in solution. Each probe consists of two halves; one half is composed of a target specific sequence and a universal primer sequence, and other half has other more sequences, a variable length random fragment to provide the size differences for electrophoretic resolution.

• 2.A pair of probes is hybridized on the target region adjacently so that they can then be joined by use of a ligase; the contiguous probe can be amplified by PCR. After PCR amplification, the copy number of target sequence i.e. deletion or duplication of target sequence can be determined and quantified using the relative peak heights.

• B) Unknown Mutations• Single Strand Conformational Polymorphism (SSCP): SSCP is one of

the simplest screening techniques for detecting unknown mutations (microlesions) such as unknown single-base substitutions, small deletions, small insertions, or microinversions.

• A DNA variation causes alterations in the conformation of denatured DNA fragments during migration within gel electrophoresis.

• The logic is comparison of the altered migration of denatured wild-type and mutant fragments during gel electrophoresis.

• In this technique, briefly, 1.DNA fragments are denatured, and renatured under special conditions preventing the formation of double-stranded DNA and allowing conformational structures to form in single-stranded fragments .

• 2. The conformation is unique and resulted from the primary nucleotide sequence. Mobility of these fragments is differed through nondenaturing polyacrylamide gels; detection of variations is based on these conformational structures.

• Denaturing Gradient Gel Electrophoresis (DGGE): DGGE has been used for screening of unknown point mutations. It is based on differences in the melting behavior of small DNA fragments (200-700 bp); even a single base substitution can cause such a difference. In this technique, 1. DNA is extracted

• 2. dna subjected to denaturing gradient gel electrophoresis.• 3. As the denaturing condition increases, the fragment

completely melts to single strands. • 4. Detection of mutated fragments would be possible by

comparing the melting behavior of DNA fragments on denaturing gradient gels.

• 5. Approximately less than 100% of point mutations can be detected using DGGE. Maximum of a nearly 1000 bp fragment can be investigated by this technique.

• Heteroduplex analysis: A mixture of wild-type and mutant DNA molecules is denatured and renatured to produce heteroduplices. Homoduplices and heteroduplices show different electrophoretic mobilities through nondenaturing polyacrylamide gels. In this technique, fragment size ranges between 200 and 600 bp. Nearly 80% of point mutations have been estimated to be detected by heteroduplex analysis.

• Restriction fragment length polymorphism (RFLP): Point mutations can change restriction sites in DNA causing alteration in cleavage by restriction endonucleases which produce fragments with various sizes. RFLP is used to detect mutations occurring in restriction sites .