fetal dna in maternal serum
DESCRIPTION
Fetal DNA in maternal serumTRANSCRIPT
FETAL DNA IN MATERNAL SERUM
DR KAPIL DEV
Trisomies
• Trisomy 21 is due to an extra chromosome 21 also called Down Syndrome and is the most common trisomy at the time of birth.
• Trisomy 18 is due to an extra chromosome 18. also called Edwards Syndrome, is associated with a high rate of miscarriage.
• Trisomy 13 is due to an extra chromosome 13. also called Patau Syndrome, is associated with a high rate of miscarriage
How are trisomies diagnosed?
• Current methods of fetal genetic testing typically involve obtaining samples of amniotic fluid, placenta, fetal blood, or rarely, other fetal tissues or fluids.
• The invasive techniques required for obtaining fetal samples (eg, amniocentesis, chorionic villus biopsy, fetal venipuncture, fetoscopy-guided biopsy) place the fetus at risk of injury or death.
Chorionic villi sampling (CVS)
• A small amount of tissue from the developing placenta.
• tested chromosomes.• CVS is typically performed between 11 and 14 weeks
of pregnancy.• CVS is associated with a small risk of miscarriage - 1-
2%
Amniocentesis
• withdraws a small amount of fluid that surrounds the fetus.
• tested for chromosomes.• An amniocentesis is usually performed
around or after the 15 weeks of pregnancy. • Amniocentesis is associated with a small risk
of miscarriage- 1%
• Complications associated with invasive prenatal diagnostic tests:
• Pregnancy loss (0.15%-1.0%)• PROM• Vaginal bleeding• Worsening sensitization of alloimmunization• Infection• Placental mosaicism
• Fetal DNA in maternal plasma or serum was first detected by PCR amplification of a Y-specific fetal sequence, followed by the analysis with agarose-gel electrophoresis and ethidium-bromide staining.
• In a subsequent study Lo et al• Developed a real-time quantitative PCR assay to measure the concentration of
fetal DNA in maternal plasma and serum.• This system is sensitive enough to detect the DNA equivalent of a single target
cell.• Other advantages include a large dynamic range of over five orders of
magnitude, a high through put, and the use of a homogeneous amplification and detection system that minimizes the risk of carry-over contamination.
• The main limitation of PCR analysis of Y-chromosomal sequences in maternal plasma is that this approach can be used only in pregnancies involving male fetuses.
• First report of free fetal DNA in maternal circulation. (Lo YMD et al. Lancet 1997;350:485-7)
• Fetal DNA clears rapidly from maternal circulation after the baby is delivered. (Lo YMD et al. Am J Hum Genet 1999;64:218-24)
• First report of free fetal RNA in maternal circulation. (Poon LLM et al. Clin Chem 2000;46:1832-4)
• Prenatal diagnosis of fetal RHD status by molecular analysis of maternal plasma. (Lo YMD et al. N Engl J Med 1998;339:1734-8)
Background:Fetal Nucleic Acids in Maternal Plasma
Library preparation
Cluster Generation
Massively Parallel Sequencing
Data Analysis
• Alignment (chromosome matching) using human genome database
• One matching error per 36 bases allowed• Interpretation of results:
% of matches on chromosome 21
Z score for each sample
Trisomy Detection – Fetal fraction matters
• Trisomy 21• Disomic Chr
• Total: 100
• (Maternal: 90)• (Fetal: 10)
• 10% fetal DNA in circulation
• Total: 105
• (Maternal: 90)• (Fetal: 15)
20% fetal DNA in circulation
Total: 100(Maternal: 80)(Fetal: 20)
Trisomy 21
Total: 110(Maternal: 80)(Fetal: 30)
Disomic Chr
Trisomy detection via cell-free DNA analysis depends on the fetal fraction (proportion of fetal to maternal cell-free DNA within the sample)
The higher the fetal fraction the easier it becomes to detect aneuploidy
Some Limitations
• Diagnostic testing is recommended following all positive results .Chance pregnancy affected ≥ 50%
• Negative result does not ensure unaffected pregnancy .
• Cost efficacy
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