genetics and fetal development.pdf

8/10/2019 Genetics and Fetal Development.pdf

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Genetics and Fetal Development

Michael Emerson, M.D.


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Genetic Basis of Pregnancy

A. Chromosome abnormalities during pregnancy

1. Origin: gametogenesis, fertilization, post-zygotic cleavage

2. Consequences: preimplantation death, implantation failure,

spontaneous abortion

B. Classes of chromosome aberrations (CA)

1. Aneuploidy

2. Polyploidy

3. Structural rearrangementsC. Aneuploidy during pregnancy

1. Aneuploid oocytes: 15-30%

2. Aneuploid spermatozoa: 3%

3. Fertilization-related CAs: 8%

4. Aneuploid preimplantation embryos: 24%

5. Aneuploid first trimester embryos: 15-25%

6. Aneuploid 2nd and 3rd trimester fetuses: 7%

7. Aneuploid newborns: 0.3-0.5%

8. Structural congenital malformations: 4%9. Mental retardation: 12%

10. Congenital heart defects: 13%

D. Types of CAs during pregnancy

1. Aneuploidy

a. 45, X: 28%

b. Trisomy 16: 31%

c. Trisomy 18: 5%

d. Trisomy 21: 8%

e. Trisomy 22: 10%2. Polyploidy

a. Triploidy: 20%

b. Tetraploidy: 8.6%

3. Structural rearrangements: 5.1%

E. Risk of trisomy conception following abortion of unknown

karyotype

1. Risk of subsequent trisomy abortion: 4.5%

2. Risk of subsequent trisomic liveborn: 0.45%

F. Risk of trisomic offspring subsequent to trisomic abortion: 0.5%Chromosome Aberrations as a Cause of Congenital Malformations

A. Autosomal chromosome aberrations

1. Trisomy 21 (Down)

a. Incidence: 1 in 600-800 live births

b. Risk increases with advancing maternal age

c. Mechanism of origin: nondisjun ction and

nontranslocation

d. Clinical findings: hypotonia, characteristic facies,

cardiac malformations, duodenal atresia

There are three sources of chromosome abnormalities that occur during thecourse of pregnancy. Primarily during the formation of oocytes andspermatozoa during gametogenesis, during the course of fertilization withthe fusion of the oocyte and the sperm and in the postzygotic period. Theconsequences of any kind of chromosome abnormality usually is on a

preimplantation death, implantation failure, spontaneous abortion, stillbirthor an infant born with birth defects.

15% of the population identified in a clinically identified pregnancyspontaneously aborts of which 60-70% are due to chromosome abnormali-ties. We have now begun to define the incidence of chromosome abnormali-ties prior to the third week of gestation and have found surprisingly that asignificant number of pregnancies are lost following implantation. It isestimated that as many as 70% of all conceptions that are identified throughin vitro fertilization programs, for example, may be carrying a chromosomeabnormality. The significance of that is such that one of my colleaguesremarked that "it is a miracle that any one of us is in this room." Followingthe 12th week of pregnancy, in the second and third periods of gestation,the incidence of stillbirths may be 5-10% and again chromosome abnormali-ties play a significant role in each of these stages.

There are three classes of chromosome abnormalities that we will brieflydiscuss with you. Aneuploidy which is defined as a gain or loss of singlewhole chromosomes. Polyploidy in which you have an additional set or setsof chromosomes. In the human, the chromosome complement consists of 23 different kinds of chromosomes so we have speak of triploidy with 69

chromosomes and tetraploidy with 92 chromosomes. Then there areinstances where chromosomal rearrangements occur. The chromosomes physically break and heal or restitute in new forms or arrangements.

This is a normal karyotype in which to emphasize to you that indeed thereare 46 chromosomes, 22 pairs of non-sex chromosomes or autosomes andan X and a Y constituting a male. It is possible for us using various kinds of stains to identify not only individual chromosomes from one another, butgains and losses of specific segments of chromosomes as well. Then youare looking at what constitutes G-banding in which they use trypsin and aGiemsa staining.

In the case of aneuploidy, and the next set of slides will briefly illustrateeach of the classes of chromosome aberrations, there are three chromo-

somes and the total count is now 47 instead of the normal number of 46.This is the characteristic karyotype associated with trisomy 21 in whichthere is in all of the cells presumably an extra chromosome 21.

Triploidy. Each chromosome is represented 3 times throughout the entirecomplement and so the total chromosome number is 69 and this is anexample of polyploidy. Another example of polyploidy is shown in thislower segment in which the total chromosome count is 92, an example of tetraploidy. Each chromosome has its own particular pair. Tetraploidycharacteristically arises after the first postzygotic cleavage in which thechromosomes divide but the cytoplasm does not, so the chromosomenumber immediately goes from 46 to 92. The cell fails to divide, to formtwo daughter cells. You still have only one parental cell remaining but nowthe chromosome number has been immediately doubled. Invariably, this will

lead to a missed abortion early in the first trimester although again, for allof the statements we make, there are exceptions. There have been a few, asmall number of examples of diploid, tetraploid mosaic infants born with

birth defects presumably associated with the fact that a portion of their cellsnow have a doubling of their chromosome number.

Breakage of the chromosome at two particular sites on either of the twoarms and the broken ends of the long part healing and forming this ring. Italso means, of course, that pieces or segments of genetic material have beenlost in the formation of this ring chromosome thereby leading to chromo-some and genetic imbalance and will be associated with either spontaneousabortions, stillbirth or an infant born with a genetic abnormality.

Depending upon the nature and the appearance of the oocytes, it has been


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e. Recurrence risks

1) In trisomic Down syndrome

2) In translocation Down syndrome

2. Trisomy 13

a. Incidence: 1/2,000-4,000 live births

b. Clinical f indings: cleft lip and/or cleft palate,

microphthalmia, polydactyly, parietal scalp lesions,

cardiac and renal anomalies

3. Trisomy 18a. Incidence: 1/2,000-4,000 live births

b. Clinical findings: SGA, hypertonia, contractures,

characteristic facies, short rib cage, overlapping 1st

and 5th fingers, rocker-bottom feet, dorsiflexion of

hallux

B. Sex chromosome aberrations

1. Gonadal dysgenesis (Turner)

a. Cytogenet ic f indings: 45,X, mosaicism,

isochromosome, and structural abnormalitiesb. Clinical findings: short stature, webbed neck, low

hairline, shield-like chest, increased carrying angle

2. Syndromes which appear normal during gestation

a. Triplo-X, 47,XXX

b. Kleinfelter, 47,XXY

c. YY males

C. Structural rearrangements

1. Translocations: balanced and unbalanced

2. Deletions and duplications: cri-du-chat and 5p-3. Inversions

D. Uniparental disomy (UD) and imprinting (I)

1. UD: both chromosomes of a pair derived from one parent

2. I: maternal and paternal genes differentially altered during

meiosis

3. Clinical implications: Beckwith Weidemann syndrome

Gene Mutations as a Cause of Congenital Malformations

A. Pedigree analysis

1. Autosomal dominant: vertical transmission2. Autosomal recessive: horizontal pattern of familial trans-

mission

3. X-linked: oblique pattern of familial transmission

B. Characteristics of autosomal dominant traits

1. Variable in penetrance and expression due to:

a. Genomic imprinting

b. Anticipation due to unstable DNA: myotonic dystrophy

c. Mosaicism: osteogenesis imperfecta

d. Somatic mutation: familial cancer

reported that at least 15-30% of oocytes carry a gain or a loss of a singlewhole chromosome or aneuploidy. In certain studies, this number couldliterally be doubled if one selects, and there is ascertainment bias in thesestudies, oocytes that morphologically appear to be more abnormal than thegeneral population of oocytes. Spermatozoa, you will note, carry quite asignificantly fewer percentage of chromosomally abnormal sperm. We haveto recognize that the large contribution of chromosome abnormalities in theaneuploid class arise through the maternal line.

During the course of fertilization and preimplantation of embryos againyou note the significant contribution that single gain or single loss of chromosomes constitute to embryonic development and these numbers dropoff such that we know in the newborn period that 1 out of 200, at least,newborns is carrying a chromosomal abnormality that is going to signifi-cantly impair the quality and the quantity in many cases of the life of thatnewborn.

When we begin to break down aneuploidy into the various kinds of aneuploidy, you will note certain features. For example, the most commonchromosome abnormality is the loss of one of the sex chromosomes, either the X or the Y. Indeed, the major source of this loss, interestingly enough,is through the spermatozoa. The failure, about 75% of the cases is of a45,X gonadal dysgenesis in the liveborn or Turner's syndrome. About 75%of those cases, interestingly enough, are through the spermatozoa. Thefailure of the X and the Y to be included in a spermatozoa.

A chromosome abnormality that rarely, if ever, is seen in the liveborn population is trisomy 16. This is probably the highest particular form of achromosome abnormality and not related to age as opposed to trisomy 21and many of the other chromosome abnormalities. 45,X also is not relatedto the age of the mother in this particular case. On the other hand trisomy18 and trisomy 21 and 22 are for the most part related to maternal age andalso contribute to a significant portion of the population of chromosomallyabnormal embryos. Triploidy is the second largest or third largest class. Soif one were to try to classify which are the three most common chromo-some abnormalities, one would have to say trisomy 16 an extra 16 with 47chromosomes, 45,X in which one of the sex chromosomes are missing andtriploidy. Later in this presentation, I will describe to you the origin and the

breakdown of the origin of triploidy. Tetraploidy will constitute almost 9%of the different kinds of chromosome aberrations that exist in the chromo-

somally abnormal abortion population. So these are percentages of the 60or 70% that occur with chromosome abnormalities in the first, second andthird trimesters of pregnancy and about 5% will be these structuralrearrangements.

From a practical point of view I wanted to emphasize the following.Suppose one is counseling a woman who has had a spontaneous loss butchromosomal abnormalities were not performed. You do not know thechromosome constitution of that particular previous pregnancy and sheasked the following question. What is the risk of a trisomy if I get pregnanta second time? The answer is two-fold. First, if it occurs, the next preg-nancy is a spontaneous abortion, there is about a 4.5% chance that thatsecond pregnancy which aborted, having had one previous one, is carryinga chromosome abnormality, specifically an extra chromosome. However,

if she asks what is the chance having had a miscarriage before but notknowing the karyotype that she will have a liveborn with trisomy? I wantto emphasize to you that that risk is 0.5% which in reality is not differentfrom the risk to the general population.

Suppose you know what the chromosome constitution was in that first pregnancy. She had a loss, a spontaneous abortion, karyotypes were performed and she is now asking you what is the risk of a trisomy in thesubsequent pregnancy in the offspring and the answer again is 0.5%.

When we talk to women who have had a liveborn with a chromosomeaberration because now the data is different. If the woman had a livebornwith a chromosome abnormality and she was over 35, her risk is related toher age and not to the fact that she has had a trisomy offspring. However,


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C. Characteristics of autosomal recessive traits

1. Risk of being a carrier based on ethnicity:

a. Tay Sachs disease: 1 in 30 Ashkenazi Jews

b. Sickle cell disease: 1 in 10 African Americans

c. Cystic fibrosis: 1 in 20 in Caucasians

d. Thalassemia: Greek and Italians

2. Carrier testing

a. Tay Sachs disease: hexosaminidase activity levels

b. Sickle cell disease: hemoglobin electrophoresisc. Cystic fibrosis: DNA testing (up to 64 of >500 muta-

tions)

d. Thalassemia: CBC and MCV profile

3. Prenatal diagnosis available for TSD, SS, CF and

thalassemia (alpha, beta)

D. X-linked inheritance

1. Fragile X mental retardation (FHR) syndrome

a. Most common genetic form of MR in males

b. Atypical pattern of inheritance1) 20% of male carriers unaffected

2) 50% of female carriers affected

c. Gene defect identified on Xq27

1) Increase in trinucleotide repeats, CGG

2) All males and 50% of females with full mutation

(>200 repeats) are mentally retarded

d. Permutation is first step before full mutation

2. Prenatal diagnosis of fragile X

a. Diagnosis now by DNA analysis (not cytogenetics)b. Requires considerable genetic counseling

E. Multifactorial inheritance

1. Characteristics:

a. Pedigrees do not follow Mendelian expectations

b. Sex effect often observed: pyloric stenosis

c. Recurrence risk depends on sex of affected patient

and number of affected relatives

d. Relationship to affected relatives

e. Severity of defect in relatives2. Neural tube defects (anencephaly and spina bifida)

a. Factors in expression: genetic predisposition (role of

ethnicity), environmental insult, and time

b. Environmental insult appears to involve either abnor-

mal folate metabolism or dietary deficiency

c. Time: neural tube formation between 21 and 28 days

post-conception

3. Implications of multifactorial inheritance

a. Most cases sporadic

if she is a woman who had a trisomy offspring and is now coming to youfor consultation with respect to a second pregnancy and she is less than 35years of age, now her risk will be on the order of 1-2%. We are talking ingeneral terms, not about specific chromosome abnormalities. So it doesdepend on the age of the woman when you are counseling with respect tothis particular factor. Over 35, it remains what her original age-related risk of a chromosome abnormality. Less than 35, the risk is different.

Chromosome abnormalities. Chromosome aberrations as a cause of congenital malformations. This we should be able to go through in quick order. Your notes certainly are relatively complete and I will make certainspecific comments to add to those notes. Basically, we are talking initiallyof autosomal chromosome aberrations. These do not involve the sexchromosomes and so we are speaking of trisomy 21, 13 and trisomy 18.These are the classic trisomies that occur in the abortion and stillbirth andliveborn population.

Facies you see in Down syndrome have oblique palpebral fissures,triangulation of the mouth, the protruding tongue which is a reflection of underdevelopment of the cheeks. Hypoplasia of the processes that give riseto the oral cavity so that the tongue which is normal in size - it is not a largetongue - does not have enough space in the oral cavity to support itself.

You are all familiar with the relationship between advancing maternal ageand Down syndrome. It is interesting to point out that despite our intenseeffort of maternal serum screening and prenatal diagnosis, the incidence of

Down syndrome is actually increasing in the liveborn population and thereare several reports in the last few years documenting this. So the dottedcurve is the distribution, if you will, for the general population and the shiftto your right indicates the risk. So if the overall risk of Down syndrome is1 in 800, then at age 30, where this is potentially achieved, by age 35 therisk has gone up almost 3-fold and certainly has gone up 10-fold by age 40.This again is distribution at the infant or neonatal period. The actualincidence, at the time of amniocentesis, is higher and the time at CVS it iseven higher. So a woman's risk of Down syndrome is actually changingduring the course of her pregnancy depending upon the time that you aredoing your particular evaluation.

These are risk figures that have been developed empirically by simplyobserving groups of women of varying maternal ages and determining what

their risk of a fetus is. So that at age 35-37, the risk of a chromosomeabnormality, not just Down syndrome, approaches 1%. It is this figure thatone introduces when one discusses cost benefit ratio or risk benefit ratiosto women. Because the risk of amniocentesis is usually quoted, for example, at 1 in 200 and therefore the risk to a 35-year-old, at least the risk of a chromosome abnormality is at least 2 times the risk of losing the

pregnancy as a direct consequence of the procedure itself.

For age 40, the risk now is 2% and so if the risk of the procedure is 0.5%, there is indeed a 4 times likelihood of finding a chromosome abnormalityas opposed to causing the loss of a pregnancy. It is these kinds of compari-sons that one uses in counseling patients for prenatal diagnosis.

The origin of these chromosome abnormalities is primarily nondisjunction.

That is to say the normal chromosome number in a somatic cell is 46 andeach chromosome is paired. During gametogenesis, be it oogenesis or spermatogenesis, this chromosome number is reduced in half from 46 to 23.The paired condition becomes unpaired and fertilization of a gamete by asecond gamete containing the same number restores the diploid number, 46in a human cell, and the fact that each chromosome again is representedtwice.

In approximately 95% of the cases of trisomy 21, this is what happens.During the course of gametogenesis, during myosis, the paired condition isretained and it actually happens primarily in myosis-1. If you recall, myosisis a two-step event and so the chromosomes fail to separate from oneanother and both members of the pair of chromosomes are incorporatedinto this gamete so that the chromosome number in the gamete is 24, not


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b. When counseling, never say "never" or 1 in a million

chance of recurrence

Screening for Genetic Diseases

A. Carrier screening in pregnancy according to ethnicity

B. Maternal serum alpha fetoprotein (MSAFP)

1. Neural tube defects affect 1-2/1,000 pregnancies

2. Prenatal diagnosis is possible in 95% of cases

a. Requires ultrasonography and amniotic fluid AFP

analysisb. Applied directly to high risk pregnancies

3. MSAFP screening available to all pregnant women

a. 2-3% of MSAFP tests positive and 10% of these

actually affected with open neural tube defect

b. False positive associated with incorrect gestation,

twinning, omphalocele, cystic hygroma, fetal demise,

congenital nephrosis

c. Protocol for MSAFP screening

1) Best time: 16-18 weeks gestation2) Values >2.5 MoM requires ultrasound and amnio-

centesis

C. Multiple serum marker screening

1. Down syndrome

a. AFP (low), unconjugated estriol (low), and human

chorionic gonadotrophin (hCG)(high)

b. Combine with maternal age, weight, race and diabetes

status

c. From 5-7% of multiple marker screening are positived. Amniocentesis recommended when risk of Down

syndrome >1 in 250 (which equals risk to 35 year old)

e. Over 60% of cases of trisomy 21 detected

2. False positive associated with adverse pregnancy outcome

3. Trisomy 18 using multiple serum markers

a. 0.75 MoM for AFP, 0.6 MoM for unconjugated

estradiol, and 0.5 MoM

for hCG

b. Odds of being affected given positive result is 14 to 1,ie, in 14 such cases, one trisomy 18 detected

4. Triploidy (69 chromosomes) and multiple marker screening

a. Origins of extra set of chromosomes

b. Low hCG (2.0 MoM) indicative of paternal origin

5. Low (


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a. Nuchal thickening in first trimester

b. Pelviectasis in second trimester

c. Echogenic bowel

2. Trisomy 18

a. Omphalocele

b. Choroid plexus cysts

c. Echogenic bowel

3. Trisomy 13

a. Holoprosencephalyb. Cleft lip/palate

c. Kidney anomalies

4. 45, X

a. Nuchal thickening

b. Cardiac malformation

Teratology and Fetal Maldevelopment

A. Definition of a teratogen

B. Teratogenic effects include SAB, congenital malformation and

behavioral dysfunctionC. Periods of greatest sensitivity to teratogen

1. "All or none" period: up to 18 days post conception

2. Period of greatest sensitivity: 21 to 56 days

3. CNS sensitivity exists throughout pregnancy

D. Recognized human teratogens

1. Maternal infections: CMV, rubella, varicella, toxoplasmosis

2. Drugs: retinoic acid, valproic acid, thalidomide

3. Radiation exposure: >5 rads

4. Maternal disease: alcohol, diabetes, maternal PKUD. Nonteratogens: Agent orange, caffeine, LSD, video display

terminals, anesthetic gases

whereas this second daughter cell has the extra chromosome. So youimmediately set up three cell lines. One in which the chromosome number is normal as shown here to the right with 46 chromosomes in a human andeach pair and two daughter cells with chromosome abnormality gain andloss of a chromosome.

In many cases, this will be lethal and so you will only see two cell lines butif this happens to involve the X-chromosome, then it is not unusual to findcells with 45,X, cells with 46,XX and cells with 47 Triplo-X if you will.Depending on the time in which this error occurs that will determine thedistribution of the cells with the chromosome abnormality and potentiallythe impact. Whatever impact they do have will be modified by the presenceof the normal cells in terms of 46 chromosomes and each chromosome isrepresented twice.

The lower corner of a structural change leading to gonadal dysgenesis inwhich there has been loss of the genetic material in the short arm of the X-chromosome. It appears as if there are genetic elements along the entirelength of the X-chromosome, both in the short and the long arm, that affectnormal gonadal formation. So it appears not to matter with respect to thataspect of development whether it involves the short arm or the long arm.However, there are differences in the phenotype of individuals who haveshort arm deletions versus those who have long arm deletions.

These are individuals with gonadal dysgenesis. This is an individual withTriplo-X. Such individuals usually appear normal. They may be slightly

taller. They have a higher incidence of learning disabilities and psychosocial problems in adolescence and adulthood but otherwise, the majority of themfunction very well in society. This particular individual also is

phenotypically normal. Has four X chromosomes but unfortunately, suchindividuals are institutionalized, the few that have been described, becausethis is associated with developmental delays and retardation.

47,XXY, and the major feature is seminiferous tubule dysgenesis. I want toemphasize however that these are not found usually in the pregnancy

population that you as obstetricians would be dealing with unless this wouldoccur through a prenatal diagnosis.

The third category is that of structural rearrangements. I have classifiedthem into three groups. Translocations, in which genetic material has been

moved from one place to another. Usually these are characterized as being balanced. So although there has been a shift in the genetic material, the totalgene content is unchanged and hence this individual would be clinicallynormal. But if the shift has occurred and as a consequence of that, there isloss or gain of genetic material through this translocation, then it reverts

back to the effects seen with aneuploidy in gains of whole chromosomes,loss of whole chromosomes. In this particular case, you have again anunbalanced karyotype but we are talking about segments of genetic material.There are instances of deletions and duplications which are self defining aswell as inversions in which the order of the genetic material has beenchanged.

t me present to you an example of Down syndrome in which the chromosome number is normal with 46romosomes. When one looks at the karyotype, one only sees two 21 chromosomes. However, on more carefulspection, one sees that chromosome 14 does not match the members of the pair and that there appears to be extranetic material on the short arm of chromosome 14. This is a classic example of a translocation involving chromosomesand 14. So this particular karyotype represents an individual with Down syndrome because they have three dosesthe genetic material on chromosome 21, two as separate entities and the third in association with chromosome 14.

he problem is that this particular chromosome rearrangement of 14 and 21 can be carried by one or the other parent.ere is a karyotype of a perfectly normal individual, normal in terms of their physical and mental development, but atk for Down syndrome. Because as you can appreciate, there is only one 21 and the second 21 is attached toromosome 14. This individual is balanced, has all the genetic material a normal cell has but the genetic material hasen rearranged. This individual is normal but at reproductive risk for passing on this structurally altered 14 in


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sociation with this chromosome 21 and that is the nature of these structural rearrangements. They place theospective parent at reproductive risk in terms of the next generation.

here has been breakage of genetic material in chromosome 14 and 21 and if you look carefully, they actually have twontromeres, one from 14 and one from 21. Apparently, genetically one of these centromeres is silenced and isnctioning in terms of chromosome movement but basically it is a fusion of genetic material of the long arms of 14 and. The short arm, if any material is lost, apparently does not contain genetic information that is of any clinical

gnificance. This contributes and attempts to explain why an individual carrying such a chromosome rearrangement ismed balanced and does not have any anomalies, be it physical or functional, associated with that chromosome

arrangement.

here are a number of possibilities that can occur and I am simply going to outline them for you very quickly in thisrtoon here. It is possible for this normal 14 to go with the normal 21 as is shown here. The reciprocal of that will bee structurally altered 14,21 that is shown here. It is possible for the structurally altered 14 carrying the 21 to go withe 21. Now you have a gamete in which you have a double dose as opposed to the normal circumstances, a single dose.double dose is shown here of chromosome material from 21 and that sets up the unbalanced translocation Downndrome pregnancy. The other possibility is that the two 14s go with one another as shown here and the other ssibility is that the 21 would be by itself. So there are six possibilities. There are actually others but these are the sixajor possibilities.

his last sequence, where the two 14s go together and the 21 is by itself, is extremely rare and I would like you to focusand I emphasize to you that from a clinical perspective, these are the four major gametes that we are concerned

out. Immediately, this particular gamete that lacks a 21, only has a 14 and is going to result in a loss and actuallynically a pregnancy that is not going to be clinically identified. In essence, we are really dealing with three clinicalssibilities. One-third, in theory, should be a normal pregnancy. Normal in terms of one 14 and one 21 fertilized by aerm, if this were carried by a female, for example, also carrying one 14 and a 21. So one-third of the live born,eoretically, should be normal physically and chromosomally, mentally and chromosomally. A third of the pregnanciesould carry the balanced rearrangement like the parent. They should have only 45 chromosomes but be normal becauseere will be only two doses of 21 and two doses of 14 when fertilization takes place. So this particular gamete has all

e genetic material present in this particular gamete except that they are together.

third of the pregnancies, theoretically, should have Down syndrome. Why? Because in this particular gamete, theree two doses of 21 genetic material. One- third, one-third, one-third. In reality, that does not occur. In reality, it doespend if the mother or the father is carrying this rearrangement. If it is the mother, then instead of 33% of Downndrome, we only see about 10 or 11%. There is seemingly a selection pressure towards the formation of carriers andeduction in the expectation from one-third to about 10-11% if it is a female. If it is a male, it turns out it is about 45% so these are certainly much higher than the general population but the theoretical expectation is not met. Thereindeed a selection pressure, either at the gametic level or the zygotic level or the postimplantation period, we are notally sure where, against these chromosomally abnormal pregnancies and an enhancement, when we look at the

stribution, of the carrier frequency.

one does chromosome analysis on women and their partners who have experienced three or more spontaneousortions with or without normal liveborn, the possibility of detecting a structural rearrangement such as a 14,21 thatust described to you is about 4.7%. If the couple comes to you and have experienced two abortions, with or withoutrmal liveborn, the incidence is about 2.5%. So it does change depending upon the history of the couple coming to you.

ut one should counsel them that there is approximately a 1 in 20 chance that one will find a chromosomalarrangement that places them at increased reproductive risk for the formation of unbalanced gametes and the formationunbalanced embryos and the possibility of liveborn with birth defects.

st to again illustrate the fact that chromosome abnormalities can involve segments of genetic material with significant


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nsequences, this is a dysmorphic child. You can appreciate certain features in terms of the overall appearance, for ample. The filtrum, the space between the nose and the lips is increased as well as epicanthus is certainly emphasized.

his is an individual that has a deletion in the short arm of chromosome 5 and these present in the newborn period withcharacteristic cry. The cry of the cat - the cri-du-chat. But these can be picked up because there are changes in thermation of the larynx in association with this chromosome abnormality that appears to change the nature of their cryingttern. This is simply to emphasize to you that it takes two chromosome breaks, not a single break, to cause aciprocal translocation of the filled and unfilled chromosomes to exchange genetic material. Broken chromosomes arecky and they heal and many times they heal back in their original configuration. But if these two chromosomes areng in juxtaposition to one another, they can exchange genetic material.

r an inversion to occur, again, there must be two breaks and now the segment of genetic material must undergo a 180tation. You can appreciate that what they are demonstrating in this illustration is a change in the position of thentromere from being off center to being what we term metacentric. One way to identify the possibility that such anversion is taking place. Most of the time, the inversions do not have any clinical consequences and when these areported from the cytogenetics lab, it is important that one provides a clinical interpretation, not just a chromosomeerpretation.

nally, in this sequence of events, I want to emphasize to you the role of a relatively new non-Mendelian pattern of heritance. Again, characteristically, it does not matter if genetic material comes through the maternal or the paternal

e. It matters that there is a mutation present and it does not usually matter if it comes from the mother or the father.hat I am going to illustrate to you is two particular biological phenomena that violate that principle, if you will.

ne is called uniparental disomy and if we look at those words it simply means that two of the chromosomes have comeom one parent. You have a pair of chromosomes, indeed, but somehow the pair came from only the mother or theher. So both chromosomes that appear come from one parent. How can that be? Well, presumably, these particular egnancies began as trisomy pregnancies and if the extra chromosome was from the maternal line, then you had tworomosomes from the mother, one from the father and hence the trisomy. If there is a loss of one of theseromosomes, it is possible that the chromosome that is lost came from the parent that only contributed one of thoseree chromosomes. In this particular example that I am citing, it is from the father. This is called a fetal rescue. The term

beginning to be introduced into the literature and you end up with 46 chromosomes but through various geneticalyses, you can demonstrate that the two members of a pair of chromosomes came from one parent.

matters which chromosome we are talking about. If the extra set involves chromosome 15, you can have various kindssyndromes, Prader-Willi and Angelman's syndrome depending upon whether it is maternal or paternal. If it is

romosome 21 and you have two maternal chromosomes 21, for the most part you may get some intrauterine growthardation and small for gestational age, but in essence, you may get a perfectly normal individual.

mprinting, a second phenomenon, means it does matter where these genes come from. It does matter if they came frome mother or the father because as chromosomes pass through myosis and gametogenesis, the genetic information is

ing processed and certain maternal genes are being turned on and others being turned off and in a similar fashion,rtain paternal genes. These same genes are being turned on and turned off. So at the time of conception and duringmbryonic development, the maternal and paternal genes are not equal. They may be actually complementary to one

other.

his is a child born with Beckwith Weidemann syndrome. This is a syndrome associated with gigantism - overgrowth -they have larger livers and spleens. Omphalocele - large tongues and large for birthweight. When we look at theserticular children, it has become apparent that a significant number of them have two sets of genetic information fromromosome 11P15 meaning the 11th chromosome on the short arm P for the short arm, petite if you will and theynot have genetic information from the mother. The way in which they arise, I have mentioned before. Through

somy or fetal rescue if you will. They may well have started out with three doses of genetic material from the parent.


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wo from the father, one from the mother. The mother's chromosomes are not the genetic segments in this area aret working and hence the filled in portion. The functional genetic information is from the father. Hence, you have toouch information, if you will, in those simplistic terms and you end up with Beckwith Weidemann syndrome.

hat happens if you have two doses from the maternal line? It appears just to be a complement of that. You havewarfism and there are mouse models that seem to mimic this particular situation and the attempts now are to identifye specific gene or genes that are involved. The point I am simply making to you is that genetic information throughe mother and the father are not necessarily equal. This is not true of all of the genetic material, segments of geneticaterial. To have normal development requires that one of the genes, in this particular case, from the father is

nctioning and from the mother it is not. When you disturb that balance, you disturb the normal course of developmentd this is a form of imprinting and there are a series of genetic conditions associated with imprinting.

ene mutation as a cause of genital malformation and I want to emphasize to you pedigree analysis and certainaracteristics of different patterns of inheritance. There is always a possibility that you may be presented with a historyd one of the features of that is to attempt to determine what pattern of inheritance might be involved. This is anempt to summarize that in simplistic terms. Autosomal dominant, the term simply means this. That both males andmales are affected. Males by the filled-in square, females by the filled-in circle. Autosomal, meaning again both malesd females are affected. Dominant means that you see the particular trait generation after generation. There is indeedvertical transmission of the trait under consideration.

autosomal recessive conditions, you see the following. That the previous generation and subsequent generations arermal and so the affected individuals, you see a horizontal pattern of inheritance. You see two normal individuals whoe carriers producing unaffected offspring, in this particular case a male, but autosomal means it could be a female.

X-linked recessive, the pattern of the pedigree is oblique so that one can see carrier females and affected males. Soain, depending upon how the pedigree looked, one has not a vertical or a horizontal but an oblique pattern of nsmission. These are major characteristics which are quickly useful, I believe, in making a determination of thessible patterns of inheritance.

he term penetrance means that the gene is present and will or will not express itself. Penetrance means the degree or rcent to which a gene which is present actually presents itself. So there are conditions like Marfan's in which if youd 100 known carriers of the gene, possibly only 90% of them would actually show the clinical features of the Marfan'sndrome. Different genes have different degrees of penetrance.

xpressivity refers to the degree to which this particular gene expresses itself. Take a condition like polydactyly.lydactyly is an autosomal dominant involving both males and females. The penetrance of that may also be about 75-% but there are individuals who have six fingers, there are individuals who have seven toes and that is an expressionthe expressivity of this gene. That it varies in its expression and so people have different numbers of fingers and toesyou will.

teogenesis imperfecta. Almost all of the cases of osteogenesis imperfecta are autosomal dominant. The expectationen is that we are going to see an affected with an affected offspring and it doesn't always work that way. As you canpreciate, the accordion-like appearance of the bones from numerous fractures and the beaded appearance of the ribsd the beaded appearance of the skull. So our expectation is that generation after generation will be affected withteogenesis imperfecta and there is a 50% risk that an affected parent will have an affected offspring be it a male or emale.

erminal mosaicism. This particular parent in the germ line is carrying a mutation that produces osteogenesis imperfectad this individual has up to a 50% risk of having an affected offspring even though this individual is normal. Wetimate that about in 6% of the cases of osteogenesis imperfecta we are going to see normal parents. Our expectation


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ain is that generation after generation we will this. But this is an exception, if you will, but a defined exception. Sois possible that you cannot counsel on the basis of autosomal recessive. This is an autosomal recessive pattern or digree. Unaffected generation and a horizontal pattern to the affected. But this again, there are alternative explanationsd one of them of course is the germ line mosaicism.

the autosomal recessive conditions, we are concerned about the carrier risk of gene mutations based upon ethnicityd you are familiar with sickle-cell disease and we will elaborate on that in a moment. Concern about carrier testing.ho should we be testing and how do we counsel them and who do we offer prenatal diagnosis, if you will?

he carrier risk for various ethnic groups should be known to you. In the case of Tay Sachs disease among Ashkenaziws it is 1 in 30. Among non-Ashkenazi Jews it is 1 in 300. Sickle cell disease among African-Americans isproximately 1:10 and one can, sitting with a paper and pencil, figure out immediately what the incidence of thissorder is. Two persons of African-American ethnicity, each have a 1 in 10 chance so couples are at risk 1 in 100, 110 times 1 in 10. Since it is a recessive disorder, if they are both carriers, they have a 1 in 4 chance so we can quicklytimate that 1 in 400 individuals of African-American ancestry or ethnicity will be affected with sickle cell disease. Youn apply these same approaches or calculations to Tay Sachs and cystic fibrosis.

ne in 20 among Caucasians, is a carrier for cystic fibrosis. One in 20 times 1 in 20 is 1 in 400. One in every 100 couplesCaucasian background is at reproductive risk for cystic fibrosis. Since they are both carriers, they have a 1 in 4 risk

d therefore we would estimate in the newborn period that there will be 1 in 1,600. The number comes closer to 1 in000 because they use a figure of 1 in 25. I deliberately use the 1 in 20. I think it is a more correct figure. Amongdividuals of Greek and Italian background from the Mediterranean area, we also must be concerned about thalassemia.

y Sachs. As an example of a normal appearing child interacting with its environment at three to four months, graduallyer the next few months, for reasons that are not known, it loses its ability to have contact with the environment andentually at age 4 and 5, it is hospitalized because of recurrent infections, seizures and eventually dies. There is noatment for this and we know the pattern of inheritance. Both parents are carriers and there is a 1 in 4 chance. Half their offspring, of course, will be carriers like themselves.

e can do carrier testing such that we can measure the value or level of hexosaminidase A, one of the forms of xosaminidase, and we can demonstrate different values for noncarriers in heterozygotes and Tay Sachs disease. Sofective has this screening pattern been among Ashkenazi Jews that the majority of newborns with Tay Sachs are of n-Jewish background. In the United States, about 25-30 children are born each year with Tay Sachs and virtually nonethem are of Ashkenazi background even though their risk is 10 times higher than the non-Ashkenazi background. Thisn 300 is actually obtained. One in 300 times 1 in 300 times 1 in 4 is about 1 in 360,000 and there are about 4.4 millionrths. So nonscreening among the non-Ashkenazi is the result of that. So one then is obligated to test the couple inhich only one member is of Ashkenazi background and I think you do have that kind of responsibility.

he problem with Tay Sachs is that the disease manifests itself three months, four months, after conception. In utero,

e damage is already occurring in the central nervous system. This is an accumulation of the ganglioside associated withy Sachs disease so therapy is going to be virtually impossible with the technologies that we have available to us today.nce there is no treatment and they invariably die, prenatal diagnosis is characteristically applied. So this is a definedpulation. The testing is diagnostic. It is accurate. It is inexpensive. There is no treatment. Prenatal diagnosis isailable and cost savings are an issue here in terms of hospitalization. These children spend a significant portion of their duced life in the hospital.

ou are familiar with sickle cell disease. It is a defined population - the African-American population primarily but notclusively. It's diagnostic. It is accurate. It is inexpensive but this is not a fatal disease. Prenatal diagnosis is availablet not very frequently used. So one asks whether or not this would be cost effective in terms of a prenatal diagnosis.

ertainly, I think most of you, if not all of you, are aware of the great harm that has been done because of the


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gmatization and the misinterpretation of the clinical significance of the carrier versus the affected status.

imarily, we have DNA technologies available to us. We know everything that is to be known about sickle cell diseaset how to cure it. We know the molecular change, the single nucleotide base pair, we know the change in the aminoid, we know the change in the protein, we know how it brings about its affect. We can diagnose it as shown here byrtue of DNA technology, but we don't know how to cure it.

ystic fibrosis presents a similar problem. It is the most common, lethal disorder among Caucasians. The incidence, asmentioned, is about 1 in 1,600 to 1 in 2,500. This number varies depending upon the ethnicity of the people being

sted. It is estimated that 1 in 25 may be a carrier. Think about it. In the United States, there are more than 8 millionrriers of cystic fibrosis. There are now nearly 600 different mutations in the CFG. How do you test for each of these0 mutations? So most of the facilities providing the service will tell you that they will test 12 or 30, and although In't want to do any commercials, there is a company that now does 70 of these. But what is the problem? You test for and you get a negative result. This does not mean that that person tested is not a carrier. They may be for a CF

utation that you simply did not test for.

oes that present a problem? It really does present a problem and that problem I am going to try to illustrate to you ins fashion. Suppose for example you do not do any testing for CF and in a sense neither parent is positive. It is a 1 in

500 risk that I have given to you before. Suppose you do testing and here is the dilemma. One of the parents turns out

be a carrier and the other one does not. Here you went ahead to try to "do the right thing". One turns out to be arrier. What you have done is increased the risk to these parents and you have no way to resolve the nature of that risk.r example, if you were able to detect 75% of the mutations and one parent is positive, they have a risk now of aboutn 400 of an offspring of CF. You have increased their risk from 1 in 2,500 to 1 in 400 by doing that carrier screeningd you don't have a way out. That is the problem with carrier screening for CF.

agile X chromosome. Is becoming a very important topic in obstetrics and gynecology. It turns out that the fragileis the most common form of mental retardation in males and there is an atypical pattern of inheritance. It is not the

assic X-linked recessive inheritance pattern that you are familiar with. Twenty percent of the male carriers areaffected. These are nonexpressing males. They carry the gene mutation, if you will, and they are clinically normal.

hat happens here is that they can pass their genetically altered but clinically nonpenetrant chromosome to their ughters who now become obligate carriers and these nonexpressing males will have grandchildren with a fragile-Xromosome. That is part of the problem. Fifty percent of female carriers are affected. You are familiar with X-linkedd carrier females with hemophilia and color blindness. They are not supposed to be affected. Duchenne muscular strophy for the most part. No clinical affect. Not true here and females will be affected.

asically what we have found in the majority but not all of the cases of fragile X is trinucleotide repeats. The elementsaking up the DNA code, you have CGG repeats in these individuals as I will show in a moment what this means, andere are degrees of repeats. These individuals present with a characteristic facies. Long facies, long nose, prominentrs. These are three brothers from the same family and they were originally described from a cytogenetic perspective.

e don't use cytogenetics anymore but if you can appreciate where the circles are in this and this part you will see itas if a piece of the long arm of the X chromosome is separated. As if it were broken off from the X chromosome andnce the name fragile. There are other fragile sites. This was the first one.

ere is the normal event in which you have only 30 repeats. In the person who is affected with the full mutation, thisquence of CGG is repeated an enormous number of times, probably in excess of 200 and maybe several thousand

mes. Individuals who are carrying the mutation may not be affected, some are, but they are at reproductive risk for ssing on this particular chromosome in which the number of repeats has been increased. So these individuals for theost part are unaffected but in the formation of egg or sperm, primarily egg - oocyte formation - a tremendouspansion of the CGG repeats occurs as is illustrated here. For some reason, this expansion is destabilizing in terms of rmal development.


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gain, I am not sure that you will appreciate this but the first column is a male, the second column is a female and whatshown to your right is the expansion as shown here. The numbers representing the increase in the number of cleotide units. This even shows it more significantly where there are three classes. This is small in numbers of the unitsaking up that particular gene. This is the full mutation indicating expansion and this is the premutation in whichpansion has occurred but not fully. Just to demonstrate the numbers that are involved, here is an individual female whoa carrier. One of her X's has 74, one has 30 of these units and the affected male has greater than 700. Here is a maleth 79 who is unaffected and has a female who seemingly also is unaffected but has 81 and therefore is at risk. Another rrier with less than 100 units having a number of children with less than 100 but here is an individual with 90 andpansion takes place.

e don't know or understand this process. We can count the units by virtue of the DNA technology. When it occurs,here it occurs, how it brings about its effect is currently being studied. But we know that if you have 60 or less of theseits in the maternal line that the chances of expansion is less than 1%. But if you have more than 90 of these units, thesepeat CGGs, then the expansion is also almost 100%. That means almost 100% of the time they will have an affectedale if they have a male and it means almost 100% of the time those females carrying the expansion will also be similarlyfected. There is now a whole series of diseases in which disease expression is a consequence of expansion of nucleotide repeats. Huntington's disease, for example, is a repeat of the CAG unit repeated many times to cause thesease and expansion occurs in these particular cases. Myotonic dystrophy is another disorder that you may be familiar th.

ultifactorial inheritance. Again, we have to recognize that dominant inheritance, recessive inheritance, frequently asoccurs, there are numerous exceptions and there are pedigrees that do not follow Mendelian expectations. There arertain conditions in which one sex is affected more so than another. First born males characteristically present in pyloricnosis so I would quickly ask you, suppose you see a female with pyloric stenosis. What does that mean in terms of

e risk to that family? It turns out that the risk to that specific family is much higher than the risk to a family with a maleth pyloric stenosis. Why? Because it is the wrong sex. If it is the wrong sex, it means that this couple is carrying morean the average amount of genetic information that is going to contribute to pyloric stenosis. If you will, it is harder r a female to have pyloric stenosis so if you do observe it, it really means is that this family is closer to the threshold.

wanted to emphasize not only the sex but recurrence risks. The counseling that you provide is determined by how manydividuals are affected. The more individuals that are affected, the higher the risk of recurrence. The relationship. Isto sibling, first degree, second degree relative meaning cousins, aunts and uncles and also the severity of the defect.r example, in cleft lip, is it unilateral? Bilateral? Is it the sex? Cleft lip is more common among males. Two-thirds of

e time it is on the left side. So if it is bilateral in a female, it is the wrong sex, it is very severe, the risk to that familymuch higher and it does change. These characteristics have to be taken into account when one is counseling.

gain, I don't know how well this illustrated but this is the affected population with males and females to illustrate thats is non-Mendelian inheritance. This is the distribution in first degree relatives, siblings for the most part, if it is a malefspring. The distribution will be different depending upon the condition if it is a female offspring. So the risk of

currence is usually in the order of 1-5% but certainly taking into consideration the sex, the degree of severity, thember of individuals, that risk can go as high as 25%.

ne striking example of multifactorial inheritance is that of the neural tube defects and this is to dramatize this. This isyeloschisis, if you will, in which both the posterior and anterior neuropores fail to close and the entire spinal canal hasen exposed. We know that there are three elements responsible for multifactorial conditions such as an open neuralbe defect. There is a genetic predisposition. There are certain groups of people that seem to carry more than theerage number of genes. For example, we will focus in on neural tube defects, those from Wales and Ireland, northern

uropeans. There is an environmental insult and you will hopefully have been reading about the role of folic acid andlate metabolism in the etiology of neural tube defects as well as the possibility that it has a role in congenitalalformations and in cardiovascular disease as well.


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he third element is time because the neural tube closes by day 28 following conception. It originates at approximatelyy 18 and is completed in its development by day 28. So if there is a genetic predisposition, then that insult acting onat genetic predisposition has to act at a certain point in time. If the insult occurs before or after 18 to 28 days, it shouldt have any impact. This is very, very important in terms of giving you a model by which counseling around congenitalalformations have to operate.

aternal serum alpha fetoprotein and multiple marker screening as well as ultrasonography. Neural tube defects in thenited States affect somewhere on the order of 1 to 2 per 1,000 pregnancies and it does vary among ethnic groups. Itcharacteristically much lower among African-Americans, much higher among people from Wales and Ireland and

enatal diagnosis, as you know, is possible.

terms of neural tube defects, when one performs an analysis for alpha fetoprotein, one finds that about 2-3% of thoselues are elevated and of those that are positive, approximately 10% are actually affected so one has to deal with falsesitives and false negatives. In terms of false positives, what contributes to an elevation of AFP that is not associatedth a neural tube defect? The wrong time. Why the wrong time? Because alpha fetoprotein in the maternal serum ising throughout the pregnancy until about 28 weeks gestation and then begins to drop. If you think the pregnancy is16 weeks when it is at 20 weeks, you have a certain expectation of that value at 16 weeks. But if indeed the pregnancy20 weeks, it is going to give you a false elevation because alpha fetoprotein again is rising during the course of theegnancy.

his is a quantitative test so twins and multiple gestation will increase the value of alpha fetoprotein in the maternalrum. It is estimated now that half of all twin pregnancies are detected through maternal serum alpha fetoproteinograms. Fetal demise and cystic hydroma. Alpha fetoprotein is produced in the liver, circulates in the fetal bloodstreamd gets out through the skin and through urination. But if there is a fetal demise, the compartments now, these variousal compartments, the amniotic fluid compartment have altered relationships and this stuff, AFP, simply leaks out inormous quantities.

he best time for screening is between 16 and 18 weeks. Why? I will show you in a moment and the question is, if thelue is greater than 2.5 MoMs it requires at least offering ultrasound and amniocentesis and we can talk about that.

ow, MSAFP is a screening test. A diagnostic test is a yes or a no but this is a screening test because there is overlaptween the affected populations and the unaffected populations.

e arbitrarily draw a line, and in this particular case and deliberately for points of examination, we drew the line at 2oM. We say that any value of AFP in the maternal serum greater than 2 will be considered at risk for a neural tubefect. Any value less than 2 will be considered as not at risk. What is the fallout? Well, if you draw this line arbitrarily,d it is arbitrarily at 2, you have a false positive rate of 4%. That means that 1 out of every 25 women that walk intour office will be considered at risk for a neural tube defect. The detection rate using 2 as a cutoff is 80%. This alsoeans that you are going to miss 20% of the cases of spina bifida or of anencephaly just using this test alone.

in good conscious, you say to me, "Let's change the rules of this game. I don't like this 2 so let's move to 2. 5 wheree are today." What is the fallout, if you will, of shifting the cutoff at 2.5. The false positive rate goes down to 2%. Thatrt I think you like. That means only 1 out of every 50 women will be identified at increased risk. But what is thetection rate? It goes from 80% down to 70%. You are not going to miss two now, you are going to miss three. Thatint has to be gotten across not only to the practitioner but to the patient that you are counseling. I have to tell youat that dynamic has had a positive change. Two or three years ago this was not the case and prospective parents wouldme in hysterical because they were told or understood that they had a neural tube defect and offering ultrasound and

mniocentesis was simply going to confirm that. That has changed and very much to the positive and I think that iscause of the counseling that the obstetrician has been able to provide.

hy do this test at 16 to 18 weeks? Why not at 12 weeks? Why not at 20 weeks? You can, but here it shows you the


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mpiric data why. The solid lines are the unaffected and neural tube defects and this is the overlap. That is 16-18 weeks,t if you do this test at 18-20 weeks, look at the dotted line. You can appreciate the difference between that part of e population, that overlap, and you are going to get, performing this screening test at 18-20 weeks, a greater degreefalse positives. So it is to your advantage to do this screening at 16-18 weeks. I know that you don't control thetients admissions and entry to you but this is the data as to why you want to do this between 16 and 18 weeks. Thefferentiation between the affected and the unaffected population is better but not complete. That is why it is still areening test, but it is better at this particular point in time.

his shows you why ultrasound can be helpful because the dotted line is with ultrasound. The solid line is data based

the last menstrual period. Notice that if you use the first day of the last menstrual period, you again have a greater gree of overlap between the affected and unaffected populations and you have greater separation when you userasound. So that is the first step. I will tell you in our experience that about half of the time when patients come incause they are told they are at increased risk for a neural tube defect, they had the wrong data and this is whererasound can be helpful. We use ultrasound, not the dates, as the final determiner.

multiple marker screening, if I can conclude this portion of the presentation with you, has been applied to Downndrome, trisomy 18, triploidy, X-linked ichthyosis and adverse pregnancy outcome. Now I am going to talk abouttending beyond alpha fetoprotein to the use of unconjugated estriol and human chorionic gonadotropin. This is anolving story. We started with neural tube defects, then it was Down syndrome, now it is trisomy 18 and triploidy, X-

ked ichthyosis and adverse pregnancy outcome which hasn't been fully defined. I don't think that we have seen thed of the value of the multiple marker screening.

ou are familiar with this, I hope. Namely that when you look at alpha fetoprotein for Down syndrome, there is a shiftthe left and that the median of this, if you draw a line and divide the affected population, is about 0.7 versus 1 for thermal population. This is the spina bifida. So a shift, a reduction in alpha fetoprotein has been associated with Downndrome. It has also been found that unconjugated estriol has a similar distribution. The reason that we can useconjugated estriol is because alpha fetoprotein is produced in the liver. Unconjugated estriol is originally synthesizedthe adrenal glands and processed in the placenta but it is an independent organ system that has been involved and it

o has this shift to the left as shown here. Let me say in terms of words that the hCG is increased in Down syndrome

that the median value is about 2.2 for that population. hCG turns out to be the most accurate predictor of Downndrome if you had to use any one of the three.

basically there are seven elements that go into characterizing the risk of Down syndrome to women and this isually, as you know, applied to women less than 35 years of age. It is alpha fetoprotein, unconjugated estriol and hCGhich is a placentally derived biochemical parameter and you combine that with maternal age, because the risk of Downndrome increases with advancing maternal age with the weight. Thin women concentrate, heavy women dilute,rticularly, alpha fetoprotein. For reasons that we are not aware of or can explain, African-American women have about10% natural increase in alpha fetoprotein. So, you either have to have two curves or you have to have a correctionctor. Women who are insulin dependent also have alterations in alpha fetoprotein and you introduce a correction

ctor.

bout 5-7% of the tests are positive and we recommend that when you combine all of these seven parameters, if thek is greater than 1 in 250, you offer them amniocentesis. We can with this approach detect somewhere on the order 60% of the cases of trisomy 21. Again emphasizing that 40% of the time, you are not going to detect this and it ist unusual, unfortunately, for a woman to have this screening test and possibly deliver a child with Down syndrome.

here have been lawsuits with respect to this. Fortunately, the judges have understood the nature of the screening tests.

second application of multiple marker screening is to take certain multiples of the median, because they are predictivetrisomy 18, and these are fixed. Alpha fetoprotein of 0.75, 0.6 for unconjugated estriol and 0.5 for hCG. Then the

APR, the odds of being affected given a positive result, is 14 to 1. This means you would do 14 amniocenteses and


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t one case of trisomy 18. That is a better OAPR than when you apply amniocentesis to 35 year old women for Downndrome. The OAPR there is 100 to 1. You do 100 amnios and you detect one case of trisomy 21 prenatally. Here,e OAPR is 14 to 1 and if, as is being developed, variable MoMs are applied for AFP, unconjugated estriol and hCG,e OAPR will be 10 to 1. You will only do ten amnios and get 1 case.

iploidy. Triploidy arises in three major ways. You can have two sperm entering an oocyte simultaneously - dispermy -d so you have 69 chromosomes. You can have an unreduced sperm in which the chromosome number in that sperm46, not 23, and it appears as larger morphologically. That occurs about 25% of the time and about 10% of the timeu have an unreduced oocyte. Myosis or oogenesis fails and the oocyte has 46 chromosomes. Now it matters where

e extra set of chromosomes come from in terms of these values. Here is a triploidy that resulted in a spontaneousortion. A very small reduced embryo, a very large hydropic placenta and this has 69 chromosomes.

iploidy is a very large embryo with a very small placenta. The level of hCG indicates the origin of the extra set of romosomes. If the hCG is low, then the extra set is maternal in origin. If the hCG is high, then the extra set is paternalorigin. This is the classic example of imprinting. It is the same genetic material from the mother and the father but its been processed and the information and the expression of the genetic information from the father is contributing toacenta formation. The genetic information coming from the maternal line is contributing to the embryonicvelopment. If you have too much, you have an extra set of genetic information from the paternal that is contributingthe placental formation, you get too much, too large a placenta. If you have extra genetic material coming from the

aternal line, the maternal line emphasizing embryonic development, you get too much of the embryo and too little of e placenta. So the levels are telling you not only the possibility of triploidy, but they are also telling you where the extrat may well have come from.

he relationship of genetic disorders in ultrasonography. It plays a role much broader than genetics. But trisomy 21,hen one observes nuchal thickening in the first trimester, pelviectasis in the second trimester - controversial, anhogenic bowel, one has to raise with the couple the possibility of trisomy 21. Omphalocele is an easy observation toake but controversial is the role of choroid plexus cysts and there are controversies in the field as to the significancechoroid plexus cysts in association with trisomy 18. Holoprosencephaly, cleft lip and palate. These are relatively easydistinguish with ultrasonography and raise the issue of trisomy 13. Nuchal thickening and any cardiac malformation

ses the issue of 45,X.

hese are increased in the nuchal thickening, pelviectasis and our data, in terms of data for pelviectasis, is almost 2%w as opposed to other studies showing only a risk of 1 in 385. Echogenic bowel is very important. You have to raiseree elements with echogenic bowel. Chromosome aberration up to 25%, cystic fibrosis up to 25% rounding off andrauterine fetal infection, CMV and toxoplasmosis and finally an adverse outcome in about one-third of the pregnanciessociated with echogenic bowel. The major cause of echogenic bowel is the fetus swallowing blood. We have observedout 4 pregnancies out of 100 in which there were no other ultrasound findings except choroid plexus cysts in trisomyand trisomy 18. But we also counseled the patient that this is what has not been observed and I don't know how yout around that particular conflict at this particular point in time.

pes of teratogens. Drugs taken by mothers, infection, radiation is a big problem and you have to monitor what thevel of exposure is. Most of the time, despite the fact that there may be a series of x-rays, they never exceed 5 rads anderefore the pregnancy is not at increased risk. Hypoxia and hypothermia. Temperature of 102 for more than three dayss been associated with neural tube defects so it matters when the exposure takes place. What was the length of thever? What was the degree of the fever? The timing of where defects occur is very important and to explainationships between limb and heart defects that occur simultaneously in an affected fetus or embryo is because thevelopmental sequence for these two independent organ systems is occurring at the same time.

any agents that we are concerned about are not teratogens. Not teratogens. That doesn't mean that they don't haveeffect on the individual who is exposed but coffee, aspirin, diagnostic radiation and particular video displays, but that


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ey are not associated with birth defects, despite reports to the contrary.

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