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Published: Monday, March 07, 2011 - 22:55

In a report published online, the journal Nature reports of a potentially important breakthrough in the search for a non-invasive test for Down’s syndrome to replace amniocentesis and CVS currently used to confirm or rule out a diagnosis of this and other chromosomal disorders. The study was carried out by researchers at the Cyprus Institute of Neurology and Genetics and was supported by the University of Cyprus.

One of the most difficult issues that some pregnant women have to contend with is a decision on whether to undergo an invasive diagnostic test when either their age or a screening test identifies them as having a high risk for having a baby with Down’s syndrome. Down’s syndrome is seen in 1 in every 700 live-births. The increased risk with advancing maternal age is very real. A mother aged 20 has a 1 in 1500 chance of having a Down’s syndrome baby. This risk will have risen almost 15-fold by the time she is 40 with a risk at 1 in 110.

The extreme concern about the invasive diagnostic tests i.e. amniocentesis or CVS is that they carry an inherent risk of causing a miscarriage. That risk is procedure related and could happen regardless of whether the baby is affected or not.

The Holy Grail in this area has therefore been to identify a reliable, simple, ideally inexpensive test which does not carry a risk to the pregnancy, the so-called Non-Invasive Prenatal Diagnosis (NIPD).

Differentially methylated regions (DMRs)

Extensive investigation of methylation differences between the mother and the fetus has led to the identification of differentially methylated regions (DMRs). In this study, the researchers described a strategy using the methylated DNA immune-precipitation (MeDiP) methodology in combination with real-time quantitative PCR (qPCR) to achieve fetal chromosome dosage assessment. Crucially, this can be performed non-invasively through the analysis of fetal-specific DMRs. They report to have achieved non-invasive prenatal detection of Trisomy 21(Down’s syndrome) by determining the methylation ratio of normal and Trisomy 21 cases for each tested fetal-specific DMR present in maternal peripheral blood, followed by further statistical analysis. The application of this fetal-specific methylation ratio approach is reported to have provided correct diagnosis of 14 trisomy 21 and 26 normal cases. That is a sensitivity of 100% and specificity of 100%.

Affordable and universally available test

While it is early days, the researchers are very confident that their technique can be replicated with similar success in larger studies. What’s more, they claim that their approach involves the application of MeDiP and real-time qPCR, which are accessible in all basic diagnostic laboratories as they require no major and expensive infrastructure, are technically easier and of lower cost, conferring an additional advantage of this methodology in comparison to next-generation sequencing technologies, which are of high cost and not easily accessible to diagnostic labora­tories.

This approach is also said to be an improvement in compari­son to the currently applied methodologies such as the use of sodium bisulfite conversion, of the DNA, which can cause up to 96% DNA degradation and can complicate even further the quantification of limited amounts of ffDNA.

Fetal gender

Additionally, they say this diagnostic approach is not affected by fetal gender or the presence of informative polymorphic sites in contrast to previous studies using methods related to SNP analysis, fetal-specific RNA marker identification or epigenetic-genetic ratio analysis. This apparent advantage is of high importance as the technology will be available to any pregnant woman. The researchers go on to speculate that this methodology has the potential of being used for prenatal diagnosis of other aneuploidies (chromosomal disorders) such as Edward’s syndrome (Trisomy 18), Patau’s syndrome (Trisomy 13) and Klinefelter’s syndrome (47XXY)

Non-invasive (risk-free) blood test for Down’s Syndrome developed; appears highly reliable

Amniocentesis  and CVS  are invasive and carry a risk of causing miscarriage

By Dr Joe Kabyemela, MD