A perfect Down syndrome screening test?

In a January post, I wrote about a possible new Down syndrome screening test that detected 100% of affected fetuses and had a very low false-positive rate.  Recently a different group of scientists in Cyprus reported they had developed a perfect test to detect Down syndrome: one that correctly detects all Down sydrome fetuses but had zero false positive results.

While the earlier report utilized a time-consuming and expensive technique called massively parallel sequencing, this new study focused on a process called DNA methylation.  Methyl groups are chemical structures that are naturally attached to regions of DNA.  They function to turn genes on or off.  Because DNA from fetuses have different methylation patterns compared to their mothers', fetal DNA can be distinguished from mom DNA in a blood sample taken from the mother.

After enrichment of the methylated fetal DNA from the mother's blood, quantitative PCR was used to amplify specific methylated regions of chromosome 21.  Fetuses with Down syndrome were identified because they have three, rather than two, copies of chromosome 21, and the increased copies were readily detected by the test.  Out of 40 women who were 11 to 14 weeks pregnant when tested, the method was 100% accurate: all of the 14 Down syndrome and all of the 26 unaffected fetuses were correctly identified.

In contrast to a sequencing approach, the techniques used in this study are rather easy and fast to perform and don't require expensive equipment or software.  It is also a lot cheaper.

This is a promising test but there's still a lot more to be done before it makes the leap into clinical use.  The Cyprus researchers are planning to do larger-scale clinical trials with many more women.  Let's hope that the results from those trials are as exciting as this early report.

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