EPIMEDIOLOGY OF GENETIC DISEASES

Framework Introduction

Types of genetic diseases

Epidemiology of genetic diseases

Preventive and social measures

Promotional measures

Specific protection

Early diagnosis and treatment

Advances in genetics

DNA technology

The human genome project

Human genome diversity project

Gene therapy

Genetic engineering

INTRODUCTION

Genetic epidemiology is a relatively new discipline that seeks to elucidate the role of genetic factors and their interaction with environmental factors in the occurrence of disease in populations (Khoury et al., 1993).

Basic principles of genetics -Mendel and Galton by end of the 19th century

Genetic makeup is one of the determinants of health and is responsible for a large proportion of infant mortality and childhood disability in developed countries

Types of genetic diseases-

Single gene inheritanceChromosome abnormalitiesPoint mutationsMultifactorial inheritance

Single gene inheritance

also called Mendelian or monogenetic inheritance.caused by changes or mutations that occur in the

DNA sequence of a single gene.  There are more than 6,000 known single-gene

disorders, which occur in about 1 out of every 200 births.

 inherited in recognizable patterns:

1. autosomal dominant,

2. autosomal recessive,

3. X-linked.

autosomal dominant

Early onset- eg: brittle bone disease, osteogenesis imperfectLate onset- eg: . Huntington disease, adult polycystic disease of the kidney, familial cancer syndrome, tuberculous sclerosis, neurofibromatosis

Autosomal recessive

Carriers are healthy themselves but have reproductive riskEg- hemoglobin disorders, cystic fibrosis, phenylketonuria and Werdnig - hoffman disease

X-linked inheritance

X-linked inheritance

Unaffected carrier females(with two X chromosomes) and affect mainly, but not exclusively male.

E.g. duchenes muscular dystrophy, fragile X mental retardation and G6PD deficiency

About 60% of carriers of X linked disorders might be detected by family studies.

Family carriers are high genetic risk; in each pregnancy there is 25% risk of affected son and 25% risk of carrier daughter

Chromosome abnormalities  abnormalities in chromosome number or structure

can result in disease. Abnormalities in chromosomes typically occur due to

a problem with cell division.eg:  Down syndrome or trisomy 21, turner syndrome

(45,x), klinefelters syndrome (47,XXY)

Point mutations

Sickle-cell hemoglobin is the result of a specific, single-base change in the β-globin gene.

β –thalassemia can be due to any one of more than 100 different mutation in and around the β-globin gene.

Cystic fibrosis is caused by any of more than 400 different changes in and around the cystic fibrosis trans-membrane conductance regulator gene.

Multifactorial inheritance

When in combination of small variation of genes, in combination with environmental factors, predispose to or produce the condition.

Tend to recur in families but DONOT show mendelian pattern of inheritance.

cancer is an eg of very common multifactorial disorder

Multifactorial inheritance-

Contribution of genetic and congenital disorders of infant and child mortality in atypical developed country

Main causes of the death at <1 year

% Main causes of the death at 1 to 4 years

%

Perinatal factors 38 Accidents 31

Congenital & genetic disorders 25 Congenital & genetic disorders

23

Sudden infant death syndrome 22 neoplasms 16

Infections 9 Infections 11

other 6 other 9

Role of genetic predisposition in some common disorder

Disease Remarks

Coronart heart disease

Familial hypercholesterolaemia Serum cholesterol Blood pressure Familial hyperlipidemias High levels of fibrinogen, homocystine, Lp(a) lipoprotein &

apolipoprotein E4

Cancer Retinoblastoma Familial polyposis coli Breast: chromosome 17, Colon cancer Neurofibromatosis

Asthma & allergy Specific genes that affects plasma IgE level

Diabetes IDDM- chromosome 6 NIDDM:

Mental disorder schizophrenia

Alzheimer disease strong familial tendency with increase prevalence with advancing age

Category Estimated birth /1000

Commonest diagnosis

Single-gene disorders

Dominant 7.0 Familial hypercholesterolemia, Adult polycystic disease of the kidney,

Huntington disease, Neurofibromatosis, Chondrodystropy

X-linked 1.33 Muscular dystrophy, Haemophilia and Christmas disease, Colour vision disorders, X-linked mental retardation., Glutathione deficiency

Recessive 1.66 Cystic fibrosis, Phenylketonuria, Amino-acid disorder, Werdnig-Hoffman disease,

ThalassaaemiasChromosomal 3.49

Autosomes 1.69 > 70% Down SyndromeSex chromosome 1.8 Mostly Klinefelter and turner syndromesCongenital abnormalities

52.8

26.6 Congenital heart disease, Club foot, CDH, pyloric stenosis, cleft palate/lip

No genetic component 26.2 -

Other multifactorial 10.06 Strabismus, Inguinal hernia, Epilepsy, Diabetes, Mild mental retardation

Genetic, unknown type 1.2 -

Total genetic 51.34

Total genetic + non-genetic congenital anomalies

77.54

DISEASE PREVALENCE IN COUNTRY

Cleft Lip +CP Cleft lip ± Cleft palate 0.93 /1000 live births. Cleft palate alone 0.17 / 1000

Talipes (club foot) 1-2 in every 1000 live birth

Developmental dysplasia of the hip One in 1,000 children is born with a dislocated hip, and 10 in 1,000 may have hip subluxation. (No Indian data)

Congenital heart diseases Incidence is 8-10 per 1000 live births

Congenital Deafness 5.6 to 10 per 1000 live birth

Congenital cataract 1-15/10,000 children

Retinopathy of maturity 20 TO 22 % IN neonatal ICU

Sickle cell anemia 5 TO 34 %

Beta Thalassemia 3 TO 4 %

Promotional Measures1) Eugenics

Science which aim to improve genetic endowment of human population

Negative eugenics:

To reduce the frequency of hereditary disease and disability in the community to be as low as possible.

debarring those with hereditary disease from producing children.

Eg; Hitler

Can eliminate genetic defects? (fresh mutation, marital alliance)

Positive eugenics:

encouraging carriers of desirable genotypes to assume burden of parenthood.

Limitations- Valuable traits have multifactorial determination

No control over transmission

2) Euthenics:

Euthenics deals with human improvement through improving mutual interaction btw hereditary and environment.

Eg mental retarded –exposure to +ve environment stimulation --improved IQ

3) Genetic Counseling-

at risks of an inherited disorder, are advised of-

the consequences and nature of the disorder,

the probability of developing or transmitting it and

the options open to them in management and FP to prevent/avoid or ameliorate

importance of genetic Counseling is because of-

The predictive nature genetic information

Impact of knowledge of genetic risk for the individual and family

Correct information on risk,

Availability of management and prenatal diagnosis.

The main components are

A correct diagnosis

Estimation of genetic risk (pedigree, investigations involving other family members)

The provision of information on existence of risk and on any option for avoiding it;

Accessibility for long-term contact

Types of genetic counselingProspective Retrospective

True preventionIdentify the heterozygous and explain risk on marrying another heterozygousEg sickle cell anaemia, thalassemia

More commonDisorder already occurredMethods suggested- contraception, MTP, sterilization

Consanguineous marriageDegree of consanguinity Genetic make-up

1st degree- between brother and sister (rare in India)

50% genetic material commonMax P of expression of autosomal recessive traits

2nd degree- with fathers own sister OR mothers own brother

25% genetic material common

3rd degree- with fathers sisters son/daughter OR mothers brothers son/daughter(common in India)

12.5% genetic material common

4th degree- btw distant relatives Minimal risk of auto recessive

Early diagnosis and treatment-

1. Detection of genetic carriers

2. Perinatal diagnosis

3. Screening of new born infant

4. Recognising pre clinical cases

1) Detection of genetic carriers-

Genetic population screening-

A “screening test” is applied to the whole population

A screening programme is a public health policy. The classical requirements are

A common and potentially serious condition

A clear diagnosis in each case

Sound knowledge of the natural history of the condition

An effective and acceptable method of treatment or prevention

Affordable test

Flow chart of genetic screening and Perinatal diagnosis for carriers of a recessive gene

Antenatal screening and Perinatal diagnosis

Test Reason

Scan Fetal viabliltyNumberGestational age

Blood test HaemoglobinABO and rhesus blood groupsHepatitis B virusHIV

Carrier screening Haemoglobin disorderTay-Sach diseaseCystic fibrosis

Maternal serum AFP or triple screen

Neural tube defectsDown syndrome

Routine fetal anomaly scan

RBSK- 6 to 7 per 100 born with birth defect ( 17 lac annually , 9.6 % of all newborn

deaths)

Aim - early identification and early intervention for children from birth to 18 years to cover 4 ‘D’s viz. Defects at birth, Deficiencies, Diseases, Development delays including disability.

0-6 years age group -(DEIC) level while for 6-18 yrs-through existing public health facilities First level of screening -delivery points (mo, ANM)

48 HRS to 6 wks-ASHA

6wks to 6yrs- outreach screening …mobile health team at anganwadi

6yrs to 18 yrs- school

Treatment and intervention at zero cost to family

Mobile health team- MO (AYUSH) 2, ANM, Pharmacist

Defects at Birth 1. Neural tube defect

2. Down's Syndrome

3. Cleft Lip & Palate / Cleft palate

4. Tallipes (club foot)

5. Developmental dysplasia of the hip

6. Congenital cataract

7. Congenital deafness

8. Congenital heart diseases

9. Retinopathy of Prematurity

2) Perinatal diagnosis

1. Fetal Anomaly Scan

2. Amniocentesis

3. Chorionic villus sampling

4. Foetal blood sampling(cordocentesis)

5. Foetal tissue biopsy

6. FISH(fluorescent in situ hybridization)

7. Polymerase chain reaction

1) Fetal Anomaly Scan

To confirm intrauterine gestation, gestational age, and fetal viability and number

Congenital abnormalities scan at >19 weeks.

Gross morphological abnormalities can be detected

Scanning is generally offered to women belonging to recognized risk groups

e.g. those with DM, raised serum AFP level, twins or H/O fetal abnormality or possible exposure of teratogen.

2) Amniocentesis

fetal DNA from amniotic fluid is examined for genetic abnormalities.

14th-16th week of pregnancy

The fetal cells are separated, grown in a culture medium, then fixed and stained. Under a microscope the chromosomes are examined for abnormalities.

Used in prenatal diagnosis of chromosomal abnormalities and fetal infection

The most common abnormalities detected are Down syndrome, Edward syndrome[Trisomy 18] and Turner syndrome[Monosomy X].

Usually genetic counseling is offered prior to amniocentesis.

3) Chorionic villus sampling Prenatal diagnosis

Done by catheter passed through uterine cervix or by inserting needle in abdominal cavity

It entails getting a sample of the chorionic villus (placental tissue) and testing it.

Carried out 10-13 weeks after the last period

4) Fetal blood sampling ( cordocentesis )

Fetal blood is obtained after 18 weeks safely by USG-guided trans-abdominal needle puncture of fetal cord insertion.

Fetal loss is 1-2%.

Used for Perinatal diagnosis of blood disorder, but now commonly used for the rapid karyotyping of fetal lymphocytes when a major malformation has been detected by USG.

5) Fetal tissue biopsy

At 19-20 weeks.

Sample like fetal skin, muscle liver are taken to diagnose the disease.

6) FISH (fluorescent in situ hybridization)

New method for detecting numerical chromosome abnormalities in non dividing cells, it uses fluorescent DNA probes for specific sequences

7) Polymerase chain reaction Technique to amplify a single or few copies of a piece of DNA across several

orders of magnitude, generating millions or more copies of a particular DNA sequence.

Application of PCR

Isolation of genomic DNA,

Amplification and quantitation of DNA

early diagnosis of malignant diseases such as leukemia and lymphomas

3) Screening of new born infants

Disorder Neonatal assayPhenylketonuria PhenylalanineCongenital hypothyroidism Thyroid stimulating harmoneSickle cell disease Haemoglobin electrophoresisCystic fibrosis Immunoreactive trypsinDuchenne muscular dystrophy

Creatine phosphokinase

Congenital adrenal hyperplasia

17-hydroxy-progesterone

Congenital dislocation of hip Ortolani and barlow manoeuvres

4) Recognising pre clinical cases - Objectives of different types of genetic population-screening programmesType of service Condition Preventive or screening action

Primary prevention

Rhesus haemolytic disease Postpartum use of anti-D globulin

Congenital rubella Immunization of girls

Congenital malformation Addition of folic acid to maternal diet Control of maternal diabetes Avoidance of mutagens and teratogens

such as alcohol, certain drugs and possibly tobacco

Antenatal screening

Congenital malformation Ultrasound fetal anomaly scan, maternal serum alfa protein estimation

Chromosomal abnormalites Noting maternal age and maternal serum factors

Inherited disease Checking family history Carrier screening for haemoglobinopathies,

tay-Sachs disease

Neonatal screening

Congenital malformation Examination of the newborn for early treatment e.g. congenital dislocation of hip

Phenylketonuria, congenital hypothyroidism, sickle cell disease

Biochemical tests for early treatment

Advances in genetics

1. The human genome project

2. Human genome diversity project

3. Gene therapy

4. Genetic engineering

The human genome project-

to understand the genetic makeup of the human species.

the project also has focused on several other nonhuman organisms such as E. coli, the fruit fly, and the laboratory mouse.

The project began in 1990 headed by James D. Watson at the U.S. National Institutes of Health

Objectives-

identify all the approximately 20,000-25,000 genes in human DNA,

determine the sequences of the 3 billion chemical base pairs that make up human DNA,

store this information in databases,

improve tools for data analysis,

transfer related technologies to the private sector, and

address the ethical, legal, and social issues (ELSI) that may arise from the project.

The human genome project-

Agencies involved-

UNESCO , Genome data base , HUGO , National institute of health / dept of energy (USA) , Medical research council (UK) , European union , genetion france

Some of the findings-Causation of diseaseAll human races are 99.99 % alike, so racial differences are

genetically insignificant.Most mutation are in the male and are agents of change.

They are also more likely to be responsible for genetic disorders.

understanding of how we evolved as humans and diverged from apes 25 million years ago.

Human genome diversity project Started by Stanford University's Morrison Institute and a collaboration of scientists around the

world.

Aim- increase understanding of human evolution

HGDP has attempted to map the DNA that varies between humans, which is less than 1% different.

Benefit

Yield new data on various fields of study ranging from disease surveillance to anthropology. The Morrison Institute has maintained that diversity research could create definitive proof of the origin of individual racial groups.

Potential gain lies in research on human traits.

Disease research.

Diversity research could help explain why certain racial groups are vulnerable to certain diseases and how populations have adapted to these vulnerabilities

Gene therapy Gene therapy is the insertion of genes into an individual's cells

and tissues to treat a disease, such as a hereditary disease in which a deleterious mutant allele is replaced with a functional one.

Gene therapy may be classified into the following types:

Germ line gene therapy

germ cells, i.e., sperm or eggs, are modified by the introduction of functional genes, which are ordinarily integrated into their genomes. Therefore, the change due to therapy would be heritable and would be passed on to later generations.

Somatic gene therapy

Therapeutic genes are transferred into the somatic cells of a patient. Any modifications and effects will be restricted to the individual patient only, and will not be inherited by the patient's offspring.

Advantages/developments in gene therapy

Potential to treat the blood disorder thalassaemia, cystic fibrosis, and some cancers.

Sickle cell disease is successfully treated in mice.

Treatment for Parkinson's disease, Huntington’s disease

gene therapy can be effective in treating cancer. Eg successfully treated metastatic melanoma, disease affecting myeloid cells.

developed a way to prevent the immune system from rejecting a newly delivered gene.

the world's first gene therapy trial for inherited retinal disease.

Genetic engineering Genetic engineering, recombinant DNA technology, genetic

modification/manipulation (GM) and gene splicing are terms that apply to the direct manipulation of an organism's genes. Genetic engineering uses the techniques of molecular cloning and transformation to alter the structure and characteristics of genes directly.

Genetic engineering techniques have found some successes in numerous applications. Improving crop technology,

The manufacture of synthetic human insulin

the production of new types of experimental mice such as the oncomouse

Manufacture of human growth hormone, vaccine for humans, for hepatitis B.

References-

Muin j. khoury, terri h. beaty, berrnice h, cohen /fundamentals of genetic epidemiology, 1993

The human genome project, national human genome research institute, http://www.genome.gov/10001772

K.park, textbook of preventive and social medicine, 22 edition 2013

http://nrhm.gov.in/images/pdf/programmes/RBSK/For_more_information.pdf

http://www.who.int/genomics/public/geneticdiseases/en/index2.html

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