Non-invasive prenatal diagnosis: progress so far

Abstract

The prenatal diagnosis or exclusion of aneuploidy and single gene disorders has necessitated invasive testing by chorionic villus sampling or amniocentesis. The current use of ultrasound and biochemical markers provides screening but not diagnostic testing for these conditions. The risk of procedure related miscarriage is well recognized, along with the physical discomfort and the mental anguish many couples experience when deciding whether to accept the offer of invasive testing. For these reasons, the ‘holy grail’ of prenatal diagnosis has been to find non-invasive techniques which avoid these risks and discomfort. During the 1990s considerable effort was expended on the isolation of the small number of fetal cells reaching the maternal systemic circulation during pregnancy. This proved to be quite a challenge; fetal erythroblasts and trophoblasts could indeed be found after very significant enrichment and complex sorting protocols, but isolating a pure source was very challenging and anyway concerns existed that some of these fetal cells might have persisted from a previous pregnancy. One of the constituents of maternal plasma is cell free DNA (cfDNA), derived mostly from maternal white blood cells. In 1997, Lo proved that in pregnant women, some of the cfDNA in maternal blood was derived from the feto-placental unit and named this cell free fetal DNA (cffDNA). Y-chromosomal DNA could be reliably identified in the blood of pregnant women carrying a male fetus from as early as the first trimester, using targeted PCR reactions. Women carrying a female fetus should have no Y-chromosomal DNA circulating in their blood, and these Y-specific PCR reactions would in this case fail to produce a product. His work also showed that this cffDNA is cleared from the circulation within a couple of hours of the birth of the baby. Trophoblasts are the mostly likely source of this DNA, and it is also clear now that fetally derived cell free mRNA is also present in maternal plasma. cffDNA accounts for between 3 and 6% of all cfDNA in maternal blood and with very sensitive PCR reactions can be detected as early as 4 weeks’ gestation. Since 1997 significant advances in molecular biology have brought non-invasive prenatal diagnosis (NIPD) to a point where it is now in common use in a small number of defined prenatal situations. However, the next 5e10 years will see it being utilized ever more widely, and probably even in the prenatal testing for common aneuploidies. The first non-invasive techniques using cffDNA relied on the identification of genes or alleles that are not present in the mother but are present in the baby, either because they have been