Prenatal Genetics

Question 1

Screening definition

Answer 1

1. examination of asymptomatic individuals to detect those with a high probability of having a given disease; typically through inexpensive and non-invasive tests
2. High sensitivity-detection rate; apply to general population to identify a narrow group at risk
   ex: ultrasound, mammogram, blood in stool

Question 2

Diagnosis definition

Answer 2

1. determination of the presence of a disease
2. Typically more invasive, expensive, and with high specificity
   ex: biopsy

Question 3

Advanced maternal age (indication for prenatal diagnosis)

Answer 3

age when risk of fetal abnormality is greater than risk of miscarriage or complications from procedure itself
Ex: down syndrome risk increases exponentially; age 35 1/365

Question 4

Structural chromosome abnormality (indication for prenatal diagnosis)

Answer 4

Risk depends on type of anomaly and parent of origin

Question 5

Previous child with de novo chromosomal aneuploidy (indication for prenatal diagnosis)

Answer 5

Risk of recurrence for ANY chromosomal abnormality is 1/100

Question 6

Genetic disorder amenable to prenatal diagnosis

Answer 6

Risk is based on family hx and on results of a screening test
Ex: cystic fibrosis; Jewish panel

Question 7

Sex determination (indication for prenatal diagnosis)

Answer 7

Ex: family hx of a boy with an X-linked disorder

Question 8

Neural tube defect (indication for prenatal diagnosis)

Answer 8

1. used when there is a family hx of NTD
2. Or when maternal serum is positive for alpha-fetoprotein
   * Ultrasonography has a higher detection rate and specificity for prenatal diagnosis of NTD

Question 9

Amniocentesis purpose

Prenatal diagnosis

Answer 9

1. Performed when there is a concern for serious abnormal

2. Performed at 15-16 wks

Question 10

Amniocentesis procedure

Answer 10

1. Obtain a sample of amniotic fluid via spinal needle thru ultrasound guidance
2. Centrifugation –> fetal cells & supernatant
3. Fetal cells –> cytogenetic studies, FISH, CGH
4. Supernatant –> tested for alpha-fetoprotein

Question 11

High levels of alpha fetoprotein can be seen in (4)

Answer 11

1. Neural tube defects (99% sensitivity)
2. Fetal blood contaminaiton
3. Fetal abnormalities such as omphalocele
4. Placental disorders

Question 12

Complications of amniocentesis (5)

Answer 12

1. fetal loss risk: 1/300-500
2. hemorrhage
3. amniotic fluid leackage
4. infection
5. respiratory distress at birth
6. Hep B carriers and HIV+ –> incr risk of vertical transmission

Question 13

Chorionic villous sampling (CVS)
purpose & procedure
Prenatal diagnosis

Answer 13

1. Biopsy of the chorion frondosum
2. Taken btwn 10-12 wks (early stage)
3. Transcervically or transabdominally

Question 14

Disadvantages of CVS (2)

Answer 14

1. NO alpha fetoprotein determination

2. 2% rate of discordance (ambiguous results) btwn placenta and fetal make-up due to chromosomal mosaicism

Question 15

Complications of CVS (6)

Answer 15

1. fetal loss risk: 1/100 or 1-1.5%
2. Higher risk of infection via cervix/vagina
3. Bleeding and sub-chorionic hematoma
4. premature rupture of membranes
5. Rh factor sensitization
6. limb abnormalities and oro-mandibular hypogenesis (mouth and jaw)

Question 16

Ultrasonography purpose & procedure

Prenatal diagnosis

Answer 16

1. Detection of structural abnormalities
2. At 16-20 wks, can detect 15-50%
3. After 20-24 wks, can detect cardiac anomalies 50-80%
4. Most important tool for major fetal anomalies

Question 17

Anomalies detected by Ultrasonography (4)

Answer 17

1. isolated such as cardiac defect (one issue)
2. Aneuploidy: multiple anomalies or markers
   Ex: trisomy 13-NTD, cleft palate, cardiac abn
3. single-gene disorders ex: skeletal dysplasia, IPKD kidney disease
4. multi-factorial disorders
   Ex: cardiac anomalies, NTD,

Question 18

Fetal Blood sampling (FBS) Indications

Prenatal diagnosis

Answer 18

• no longer commonly used for prenatal diagnosis
  1. Cytogenetic diagnosis: quick results, impending fetal death, mosaicism
  2. congenital infections
  3. congenital immunodeficiencies
  4. coagulopathies
  5. Typically done after 20 wks

Question 19

FBS technique

Answer 19

A needle is inserted into umbilical cord, intrahepatic vein, or heart (last two higher risk)
to get a sample of fetal blood

Question 20

FBS complications

Answer 20

1. Fetal loss risk: 2.8%
2. Hematoma/hemorrhage
3. bradycardia-HR ahrrythmia

Question 21

Preimplantation genetic diagnosis (PGD) purpose & indications
prenatal diagnosis

Answer 21

1. IVF; done on polar body or single cell biopsy from blastomere
2. used when couples are at a risk for a specific genetic disorder

Question 22

PGD limitations

Answer 22

1. Increased number of probes increases rates of false positive/negative diagnoses
2. limited sample material may not represent all cells of embryo (mosaicism)
3. narrow window of testing
4. high cost

Question 23

Detection rate

Answer 23

rate of affected fetuses detected by screen divided by total of affected fetuses

• sensitivity*
• False positives are wrongly diagnosed

Question 24

History & Ethnicity

Prenatal screening

Answer 24

1. Karyotypic risk (anomaly, translocation, hx or mental retardation)
2. Ethnicity: nebulous due to multi-racial
   * Jewish at high risk for many
   * African Am-sickle cell & beta thalassemia
   * Jewish and caucasians - cystic fibrosis

Question 25

Universal screening

Answer 25

1. Preconception or early pregnancy carrier screen for +100 single gene disorders
2. Less expensive; $350
3. Uses saliva or blood sample from both parents
   * drawback: ethnicity not taken into account

Question 26

Fragile X screening

Answer 26

1. History: female infertility, premature ovarian failure, unexplained MR, developmental disabilities

Question 27

Down syndrome screening

Answer 27

• Most common cause of mental retardation
  1. serum markers: see different levels of fetal/placental proteins in 1st & 2nd trimester
• alpha fetoprotein levels are lower
  2. Sonographic markers: thickness of skin behind neck - lymphatic fluid build up
  3. Maternal age > 35

Question 28

Cell-free fetal DNA (non-invasive)

Answer 28

1. For most common aneuploidies
2. Detection uses real time PCR
3. Detection as early as 32 days
4. Source of fetal cells- placental apoptosis; small amount in maternal circulation
5. This is cleared from maternal blood after birth, so unlikely misdiagnosis from previous pregnancy
   100% accuracy in trisomy 21

Question 29

Cell-free fetal DNA limitations

Answer 29

1. Only for singletons, limited data on twins
2. doesn’t detect abnormalities picked up by other screening tests
3. 1-3% samples do not have enough free fetal DNA
4. only high risk women

Question 30

Neural tube defect higher risk indications (5)

Answer 30

1. high maternal serum AFP (5-spina bifida; 7-10 anencephaly-skull)
2. teratogen exposure
3. history of previous child with NTD
4. patient has NTD
5. uncontrolled insulin dependent diabetes mellitus –> high glc is a teratogen in heart and neural tube
   * 90% families w/ no previous hx or risk factors get NTD

Question 31

NTD screening

Answer 31

1. family hx
2. blood sample AFP
3. ultrasound

Question 32

Comparative Genomic Hybridization (CGH)

5 advantages; 2 disadvantages

Answer 32

Compares pt and control DNA in expression

1. Higher resolution thank karyotype
2. Avoid culturing amniocytes or chorionic villi
3. doesn’t require dividing cells
4. better quality control; automation
5. faster turnaround time
6. not able to detect balanced translocations, inversions, deletions, triploidy
7. Can detect copy number variants of uncertain clinical significance

Question 33

Biochemical assays: DNA analysis or enzyme assays on chorionic villi or amniotic fluid cells

Answer 33

Only when both parents are carriers