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185
I. PRINCIPLES OF GENETIC SCREENING
A.
Genetic screening should screen for a disease that: is serious, is relatively common, and has
an effective treatment.
B.
Genetic screening should be relatively inexpensive, easy to perform, valid, and reliable.
C.
Genetic screening should have the necessary resources for diagnosis and treatment readily
available.
D.
Genetic screening provides the following advantages: informed choice, improved under-
standing in the family affected by the genetic disorder, early treatment when available, and
reduction in births of affected homozygotes.
E.
A genetic screening test must have the appropriate sensitivity and specificity (Table 17-1).
1. Sensitivity
is the ability to identify affected individuals and is measured as the
proportion of
true positives.
Thus, if a test detects only 65 out of 100 affected individuals as positive (true
positives; A), the test has a sensitivity of 65%. Note that 35 affected individuals are missed
(false negatives; C) by the test. For example, if a patient who actually has strep throat gets
a rapid strep test that comes back positive, then this is a true positive. However, if a patient
who actually has strep throat gets a rapid strep test that comes back negative, then this is
a false negative.
2. Specificity
is the ability to identify unaffected individuals and is measured as the
propor-
tion of true negatives
. Thus, if a test detects 90 out of 100 unaffected individuals as negative
(true negative; D), the test has a specificity of 90%. Note that 10 unaffected individuals are
falsely diagnosed as positive (false positives; B) by the test.
3. Positive predictive value
of a screening test is the proportion of positive tests (A
B) that
are true positives (A).
II. LIMITATIONS OF GENETIC SCREENING
A.
Genetic screening is never 100% accurate. For example, mosaicism can confound cytoge-
netic results even though the accuracy of the genetic test approaches 100%. In addition,
human error is always a possibility.
B.
Genetic screening cannot detect the presence of disease. For example, a genetic test for
hereditary breast cancer cannot predict who will get the disease.
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Board Review Series Genetics
C.
Genetic screening may not detect all the mutations causing the disease. For example,
autosomal dominant breast cancer, cystic fibrosis, and Marfan syndrome all have multiple
mutations, which may cause the disease. In general, it is not practical to test for all
mutations.
D.
Genetic screening can lead to complex ethical and social considerations, which include but
are not limited to:
■
Discrimination by employers of insurance companies
■
Lack of effective treatment for the condition (e.g., Huntington disease, familial Alzheimer
disease)
■
Results may affect family members who do not wish to know about their risk for a
genetic disorder
■
Family members who are at risk for a genetic disorder may not wish to share this infor-
mation with other family members
■
“Survivor guilt,” where those who do not test positive for a mutated gene may feel guilty
when others in the family do test positive
■
Inappropriate anxiety in carriers
■
Inappropriate reassurance if test is not 100% sensitive.
III. PREIMPLANTATION GENETIC SCREENING (PGS)
The indications for PGS include:
■
Couples who are at high risk of having a child with a genetic defect
■
Subfertile couples
■
Infertile couples
In PGS, the couple uses in vitro fertilization technology along with intracytoplasmic sperm
injection to avoid the presence of extraneous sperm. Intracytoplasmic sperm injection is not
done at every center that does PGS screening. The blastulas are biopsied at the 8-cell stage
where one or two blastomeres are removed for chromosomal or DNA analysis. The 1 to 2 blas-
tulas that are unaffected by the genetic disorder that the baby is at risk for are introduced into
the uterus of the mother. Implantation must then occur for a successful pregnancy. The
success rate (i.e., successful implantation/pregnancy) for this procedure is 25% per cycle of
treatment.
t a b l e
17-1
Sensitivity, Specificity, and Positive Predictive Value of a Genetic Screening Test
Disease Status
Screening Test Result
Affected Individual
Unaffected Individual
POSITIVE
TRUE POSITIVE (A
65)
FALSE POSITIVE (B
10)
NEGATIVE
FALSE NEGATIVE (C
35)
TRUE NEGATIVE (D
90)
A 65
Sensitivity
—— ——— 65% Proportion of True Positives
A
C
65
35
D 90
Specificity
—— ——— 90% Proportion of True Negatives
D
B 90 10
A
Positive Predictive Value
—— 86%
A
B
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Chapter 17
Genetic Screening
187
IV. PRENATAL GENETIC SCREENING
The indications for prenatal screening include:
■
Advanced maternal age
■
Previous child with chromosome abnormality
■
Family history of a chromosome abnormality
■
Family history of a single gene disorder
■
Family history of a NTD
■
Family history of a congenital structural abnormality
■
Consanguinity
■
A poor obstetric history
■
Certain maternal illnesses
Prenatal genetic screening for genetic disorders includes the following techniques:
A. Amniocentesis.
Amniocentesis is a transabdominal sampling of amniotic fluid and fetal
cells.
Amniocentesis is performed at weeks 14 to 18 of gestation and is indicated in the fol-
lowing situations: the woman is over 35 years of age, a previous child has a chromosomal
anomaly, one parent is a known carrier of a translocation or inversion, one or both par-
ents are known carriers of an autosomal recessive or X-linked recessive trait, or there is a
history of neural tube defects. The sample obtained may be used in various ways which
include:
1.
-Fetoprotein (AFP) assay on the amniotic fluid is used to diagnose neural tube defects
(NTDs); AFP is increased. Amniotic fluid levels of AFP are much more sensitive in diag-
nosing NTDs than maternal serum levels of AFP.
2.
Spectrophotometric assay of bilirubin
on the amniotic fluid is used to diagnose hemolytic dis-
ease of the newborn (i.e., erythroblastosis fetalis) due to Rh-incompatibility.
3.
Lecithin-sphingomyelin (L/S) ratio and phosphatidylglycerol assay
on the amniotic fluid is used
to determine lung maturity of the fetus.
4.
DNA analysis on the fetal cells.
A wide variety of DNA methodologies are available to diag-
nose chromosomal abnormalities and single gene defects, which include: karyotype
analysis, Southern blotting, or RFLP analysis (restriction fragment length polymorphism).
B.
Chorionic Villus Sampling (CVS).
CVS is a transabdominal or transcervical sampling of the
chorionic villi to obtain a large amount of fetal cells for DNA analysis. CVS is performed at
weeks 11 to 12 of gestation (i.e., much earlier than amniocentesis). The major advantage of
CVS is that it offers first trimester prenatal diagnosis although it has a 1% to 2% risk of miscar-
riage.
The sample obtained may be used in various ways which include:
1.
Karyotype analysis
2.
FISH analysis
3.
Multiplex l igation-dependent probe amplification (MLPA)
4.
Biochemical assays
5.
DNA analysis
C.
Ultrasonography (USG).
USG is a valuable, noninvasive technique that conveys no risk to the
mother or fetus. USG is performed at weeks 16 to 18 of gestation and is routinely offered to
all pregnant women in the United States as a “dating” scan at week 12 of gestation. USG is
commonly used in obstetrics to: date a pregnancy, diagnose a multiple pregnancy, assess
fetal growth, determine placenta location, determine position and lie of fetus, and monitor
needle or catheter insertion during amniocentesis and CVS. In addition, USG can detect cer-
tain congenital structural anomalies which include:
■
Extra digits
associated with short-limb polydactyly syndromes
■
Hypoplasia of the mandible
associated with cleft palate
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Board Review Series Genetics
■
Nuchal pad thickness
associated with Down syndrome, trisomy 13, trisomy 18, Turner
syndrome,
-thalassemia, and congenital heart defects
■
Dramatic deficiency in the cranium
associated with anencephaly
■
Herniation of cerebellar tonsils
(called the “banana sign”) and a distortion of the forehead
(called the “lemon sign”) associated with an open myelomeningocele
■
A sac in the occipital region
associated with a posterior encephalocele
■
Rocker bottom feet
associated with Trisomy 18
■
Air trapped in the stomach and duodenal bulb
associated with duodenal atresia and Down
syndrome
■
Echogenic bowel
associated with meconium ileus and cystic fibrosis
D. Cordocentesis (CD).
CD is the transabdominal percutaneous sampling of fetal blood from the
umbilical cord vessels. The sample obtained may be used in various ways which include:
1.
Rh isoimmunization management
2.
Chromosome analysis to resolve problems associated with mosaicism found in amnio-
centesis or CVS.
E.
Maternal Serum Screening (MSS).
MSS is a sampling of the maternal blood. MSS is performed
at weeks 15 to 20 of gestation. The sample obtained may be used in various ways which
include:
1.
-Fetoprotein (AFP) assay on the maternal serum is used to diagnose neural tube defects
(NTDs); AFP is increased. However, maternal serum levels of AFP are not as sensitive in
diagnosing NTDs as amniotic fluid levels of AFP. Pregnant women with maternal serum
AFP level above a certain arbitrary cut-off level are offered detailed ultrasonography.
Maternal AFP screening and ultrasonography have led to a striking decline in NTDs in the
United States. Other causes of increased maternal serum AFP levels include: incorrect
gestational age, intrauterine fetal bleed, threatened miscarriage, multiple pregnancy, con-
genital nephrotic syndrome, and an abdominal wall defect.
2.
Down syndrome (Trisomy 21).
The triple test for trisomy 21 includes: low
-fetoprotein levels
(T
T
AFP)
in maternal serum; low unconjugated estriol (TTestriol) in maternal serum; and high
human chorionic gonadotropin (c
c
hCG)
in maternal serum. Another possible marker is inhibin-
A
which is high in maternal serum. A maternal age of
35 years of age and a positive triple
test do not give an absolute diagnosis of trisomy 21, but indicate an increased probability.
Pregnant women with an increased probability of carrying a trisomy 21 baby are offered
amniocentesis or CVS for chromosomal analysis.
3.
Fetal cells in the maternal circulation.
It has been demonstrated that fetal cells are present in
the maternal circulation in the first trimester of pregnancy. With advances in enriching
the population of fetal cells and excluding maternal cell contamination, this may provide
and entirely noninvasive method for prenatal diagnosis of chromosome and DNA abnor-
malities in the future.
The rationale for neonatal genetic screening is the prevention of subsequent morbidity in the
child. In the United States, all 50 states provide neonatal genetic screening to all newborns for
galactosemia, phenylketonuria, congenital hypothyroidism, and sickle cell anemia. Neonatal
genetic screening is expanded to include up to 30 genetic disorders in North Carolina and
Oregon.
A. Galactosemia (see Chapter 12–II-A).
1.
Neonatal genetic screening utilizes a small amount of blood obtained from a heel-prick to
assay red blood cell GALT (galactose-1-phosphate uridylyltransferase) enzyme activity and iden-
tify GALT isoforms by isoelectric focusing. In addition, total red blood cell galactose-1-
phosphate
and galactose concentrations are measured.
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Chapter 17
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189
2.
In classic galactosemia, affected neonates have GALT enzyme activity
5% of normal val-
ues. In Duarte variant galactosemia, affected neonates have GALT enzyme activity 5% to
20% of normal values.
3.
Molecular genetic testing is used clinically for: confirmatory diagnostic testing, prog-
nostication, carrier testing, and prenatal diagnosis. Molecular genetic testing methods
include:
a.
Targeted mutation analysis.
A targeted mutation analysis panel is available for 6 common
GALT gene mutations: Q188R, S135L, K285N, L195P, Y209C, F171S, and the Duarte vari-
ant (N314D).
b.
Gene sequence analysis.
GALT gene sequence analysis is used to identify private mutations.
B.
Phenylketonuria (see Chapter 12–III-A).
1.
Neonatal genetic screening utilizes a small amount of blood obtained from a heel prick to
assay plasma phenylalanine concentration using the Guthrie card bacterial inhibition assay, flu-
orometric analysis,
or tandem mass spectrometry (this technique can be used to identify
numerous other metabolic disorders on the same sample).
2.
In classic phenylketonuria, affected neonates have plasma phenylalanine concentrations
16.5 mg/dL in the untreated state. In non-PKU hyperphenylalaninemia, affected neonates
have plasma phenylalanine concentrations
2 mg/dL but 16.5 mg/dL when on a nor-
mal diet.
3.
Molecular genetic testing is used clinically for: confirmatory diagnostic testing, prognosti-
cation, carrier testing, and prenatal diagnosis. Molecular genetic testing methods include:
a.
Targeted mutation analysis.
A targeted mutation analysis panel is available for 15 com-
mon PAH gene mutations and very small deletions.
b.
Mutation scanning.
Mutation scanning identifies virtually all point mutations in the PAH
gene. Mutation scanning is performed by denaturing HPLC (high performance liquid
chromatography) which is a fast and efficient method to detect locus-specific point
mutations.
c.
Gene sequence analysis.
PAH gene sequence analysis is used to identify private mutations.
d.
Duplication/deletion analysis.
Multiplex ligation-dependent probe amplification (MLPA)
identifies large duplications and deletions when no PAH gene mutations have been
identified by mutation scanning or gene sequence analysis.
C. Congenital Hypothyroidism.
1.
Neonatal genetic screening utilizes a small amount of blood obtained from a heel prick 2
to 5 days after birth to assay thyroxine (T
4
)
concentration initially with a follow-up thyroid-
stimulating hormone (TSH) assay if the T
4
value is low.
2.
In congenital hypothyroidism, affected neonates have low serum total T
4
and low serum free
T
4
concentrations along with high serum TSH concentrations. Neonates with TSH concen-
trations
60 mU/L (birth to 24 hours) or 20 mU/L (after 24 hours of birth) are recalled
for evaluation.
3.
In neonates 1 to 4 days of age
, normal serum total T
4
concentration is 10 to 22
g/dL and
normal serum free T
4
is 2 to 5 ng/dL. In neonates 1 to 4 weeks of age, normal serum total T
4
concentration is 7 to 16
g/dL and normal serum free T
4
is 0.8 to 2 ng/dL. Serum T
4
con-
centrations are higher in normal neonates
1 to 4 weeks of age versus adults due to a TSH
surge soon after birth.
4.
Congenital hypothyroidism is most commonly caused (85% of cases) by thyroid dysgenesis
(e.g., agenesis, hypoplasia) during embryological development. The other 15% of cases
are caused by inborn hereditary errors of thyroid hormone synthesis.
5.
Congenital hypothyroidism is the most common treatable cause of mental retardation.
6.
Prevalence.
The prevalence of congenital hypothyroidism is L 1/4,000 births.
7.
Clinical features include:
few if any clinical features are present because maternal T
4
crosses
the placenta so that T
4
concentrations are
25% to 50% of normal; birth length and weight
are normal; lethargy, slow movement, hoarse cry, feeding problems, constipation,
macroglossia, umbilical hernia, large fontanels, hypotonia, dry skin, hypothermia, and
jaundice may be observed.
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