14 February 2007
Some laboratories already use the presence of free fetal DNA present in the maternal bloodstream to determine fetal sex, and there is a great deal of interest in exploiting the existence of circulating fetal DNA for other forms of non-invasive prenatal genetic analysis. A recent paper in The Lancet reports a new technique of determining the chromosomal copy number of fetuses from maternal blood samples.
Single nucleotide polymorphisms (SNPs) – sites of common genetic variation between individuals – were used to distinguish fetal from maternal DNA in each sample. The researchers then calculated the ratio of the unique fetal signal based on these SNPs to the combined maternal and fetal signal, and found that three of the samples analysed showed a discrepancy between the fetal and maternal levels of SNPs located on chromosomes 13 and 21, suggestive of the presence of aneuploidy (abnormal chromosomal copy number) in the fetus. Subsequently, it was shown that the chromosomal number had been correctly determined in 58 of the 60 samples tested; one case of trisomy 21 was not detected, and trisomy 21 was indicated in one normal fetus. The authors propose that, with further refinement, this method could be a useful complement to current prenatal screening and diagnostic tests [Dhallan et al. (2007) Lancet 369(9560):474-81].
Comment: The utility of the technique described in this paper depends on the sensitivity and specificity, and to determine this, a trial of a much larger number of pregnancies would be required to calculate the true false negative and false positive rate. However, this and other reports together suggest that the prospects for non-invasive prenatal diagnosis of genetic abnormalities are good.