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This week content Prenatal diagnosis of genetic diseases -Indications for prenatal diagnosis -e.g. Tay-Sachs disease -Early Prenatal Screening Methods of prenatal diagnosis: Invasive: Amniocentesis & CVS Non-invasive:

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Indications for prenatal diagnosis Advanced maternal age Multiple pregnancy losses (≥3) Known or suspected family history of genetic disease or multifactorial disorder (e.g. heart disease, diabetes, and obesity) Ethnicity at increased risk for genetic diseases Teratogen= any agent that can disturb the development of an embryo or fetus. Abnormal ultrasound finding Abnormal maternal serum screen results

Ethnicity Certain ethnic groups have an increased incidence of specific genetic conditions Causes: 1.Lack of migration 2.Genetic drift 3.Heterozygote advantage within the given population or geographic area.

e.g. Tay-Sachs disease A rare inherited disorder that progressively destroy neurons in the brain and spinal cord. More common in the Ashkenazi Jewish Population=> Jews living along the Rhine River in northern France and western Germany. 3 mutations account for 94% of disease DNA mutation analysis is less accurate for non- Ashkenazi people for whom enzyme analysis is more accurate and appropriate.

Early Prenatal Screening What are we screening for? – Most associate prenatal screening with aneuploidy, commonly Trisomy 21, 18, 13, monosomy X – Trisomy 16=> the most frequent. But there is a lot more than aneuploidy: – Congenital defects – Post dates screening – Complex congenital cardiac defects

Methods of prenatal diagnosis 1.Invasive 2.(i.e. with risk to the foetus or mother): Amniocentesis Chorionic villus sampling 2. Non-invasive (i.e. with no risk to the foetus or mother):

1. Invasive methods of prenatal diagnosis a. Amniocentesis Aspiration of ml of amniotic fluid through the abdominal wall under ultrasound guidance around the 16 weeks of gestation. Result in ~ 14 days to more Risk of abortion 0.5-1%

Prenatal diagnosis by amniocentesis

b. Chorionic villus sampling Enables diagnosis in first trimester (10-11 week of gest.) under ultrasound guidance by transcervical or transabdominal aspiration of chorionic villi (foetal cells). Results can be obtained in 1-3 days. Higher risk of abortion (2-3%) and limb abnormalities if carried before the 9 weeks of gestation.

(a) Transabdominal CVS (b) transcervical CVS Chorionic villus sampling (CVS)

2. Non-invasive methods of prenatal diagnosis

a. Maternal serum Alpha-Fetoprotein Test (AFP) Mostly done around the15-20 weeks of gestation. AFP is produced in the liver of fetus. A standard practice to offer screening for Down‘s (trisomy 21) and Edward syndromes (trisomy 18) using a blood sample obtained from the mother. It can diagnose between 60-70% of Down‘s sy.

b. Ultrasound to detect Dawn syndrome NT= nuchal translucency at the back of the fetal neck

c. FISH Mainly used to detect the presence or absence of: -microdeletions, -microduplications -aneuploidy It is a 3 steps technique: 1. A specific, single-stranded DNA probe is hybridized to its complementary, target DNA sequence (cell is in prophase, metaphase or interphase) 2. Fluorescent antibodies are then hybridized 3. Fluorescent signals are examined under the microscope.

FISH

Pre-implantation genetic diagnosis LSI 21 detect chromosomal rearrangements at the BCL2 locus on chromosome 18q21

Step 1. with restriction enzymes: MstII cuts the specific DNA sequence shown in the figure. MstII will splice exon I of the HbA; however, MstII will not splice HbS because it does not recognize the restriction site due to the A!T mutation. Step 2. Southern blot analysis: - lane 1; sickle cell anaemia - lane 2; sickle cell trait/ carrier status: heterozygous for normal and sickle cell - lane 3; unaffected, not a carrier: homozygous for the normal alleles. d. Direct DNA analysis : RFLP e.g. sickle cell HbS

d. Direct DNA analysis : Sequencing of restriction enzyme products

e. Dot blot Allele-specific oligonucleotides (ASOs) => direct mutation analysis

Linkage Analysis Indirectly detect a patient’s mutation status, when several family members are known to be affected with the same genetic disorder, and the exact mutation is not known. DNA from affected and unaffected family members is analysed for polymorphisms: e.g. microsatellite repeats, RFLPs and VNTRs.

Prenatal treatment In most situations the diagnosis of prenatal abnormalities has a subsequent option of termination of the pregnancy. With the advent of gene therapy prenatal diagnosis will, in time, lead to effective treatment in utero.

Treatment of the autosomal recessive disorder  Congenital adrenal hyperplasia (CAH): Affected female are born with virilisation of the external genitalia (i.e. the abnormal development of male sexual characteristics in a female). There is an evidence that this can be prevented by powerful steroid therapy at early gestational age.