Non Invasive Prenatal Testing Flashcards
NIPT is based on the presence of what?
- Cell-free fetal DNA: accounts for approx 5-10% of total DNA in maternal plasma
- Detectable from 4 wks and increases during gestation (cleared from circulation within 2 hrs of delivery)
- Small fragments (most under 200bp)
What were the initial uses for NIPD?
- Fetal sexing (for X-linked disorders such as DMD)
- RhD status of foetus
Why is NIPD used for RhD testing?
- RhD neg mother with RhD pos child
- Mother becomes sensitised and produces immune response against fetus
NIPD is only currently used for RhD status in high risk (sensitised) women - what is the main reason why ppl think it should be expanded out to all?
- All pregnant women currently offered injection of anti-D antibodies following amnio/CVS at third trimester and following birth
- However, 40% of these are unnecessary as they have a RhD negative child
What are the benefits of using NIPD for fetal sexing?
- for DMD: avoid a CVS for female foetus
- for CAH: dexamethasone for female foetus to prevent virilisation - can avoid side effects if male
What single gene disorders are reported via NIPT?
- Acondroplasia
- Apert syndrome
- Beta thalassaemia
- CF
- DM
- HD
When is relative mutation dosage used?
In pregnancies where both parents carry the same mutation - the mother will be heterozygous for the location of interest
What is relative mutation dosage based on?
- Allelic ratio between mutant and wild type alleles determined by counting maternal and fetal contributions
How are relative mutation dosage results interpreted?
- if fetal DNA is homozygous mutant then overall mutant fraction will be higher than the wildtype
- if fetal DNA is heterozygous then the overall mutant and wildtype fractions will be equal
- if fetal DNA is homozygous wildtype then the overall mutant fraction will be lower than the wildtype
What are the two main benefits of using NGS over digital PCR for NIPT?
- More cost effective as samples can be multiplexed
- More mutations covered in single NGS assay
The current method for trisomy screening in the U.K. is the combination test (maternal serum biomarkers followed by invasive testing) - what are the issues with this method?
- The invasive test has a miscarriage rate of 1%
- Screening focuses on phenotypic features and not genetic pathology
- Sensitivity and specificity are sub optimal
- Combined use results in strict gestational window
What are the three different possible approaches for trisomy detection via NIPT?
- Heterozygous SNPs on chr 13, 18 or 21 (trisomy ratio would be 1:2 instead of 1:1)
- Measure copy number of chr 13, 18, 21 by accurate measurement of fetal-specific sequences (detection by methylation difference)
- Measure copy number of chr 13, 18, 21 by very accurate measurement of fetal and maternal sequences
What is the difference in methylation status of some genes in placenta vs blood?
- HYPERmethylated in placenta
- HYPOmethylated in blood
What is the benefit of using NIPT over an invasive test for women at high risk from their biochemical screen?
- Because the risk level is set at 1 in 150, most women offered invasive testing will find their baby does not have a trisomy
- The invasive test has a 1% risk of miscarriage
- NIPT is a much safer test meaning many women will avoid an invasive test
Is a cfDNA test diagnostic?
No - an invasive diagnostic test is needed to receive a definitive diagnosis
