A group of scientists at the Chinese University of Hong Kong explored the limit of noninvasive prenatal testing by performing genome-wide sequencing of maternal plasma DNA at 195× and 270× haploid genome coverages. Combined with the use of a series of bioinformatics filters, fetal de novo mutations could be detected with a positive predictive value that was two orders of magnitude higher than previously reported.

A de novo BRAF mutation was noninvasively detected in a case with cardiofaciocutaneous syndrome. The maternal inheritance of the fetus could be ascertained on a genome-wide level without the use of maternal haplotypes, hence greatly increasing the resolution of such analysis. Finally, we showed that certain genomic locations were overrepresented at the ends of plasma DNA fragments with fetal or maternal selectivity.

Plasma DNA obtained from a pregnant woman was sequenced to a depth of 270× haploid genome coverage. Comparing the maternal plasma DNA sequencing data with the parental genomic DNA data and using a series of bioinformatics filters, fetal de novo mutations were detected at a sensitivity of 85% and a positive predictive value of 74%. These results represent a 169-fold improvement in the positive predictive value over previous attempts. Improvements in the interpretation of the sequence information of every base position in the genome allowed us to interrogate the maternal inheritance of the fetus for 618,271 of 656,676 (94.2%) heterozygous SNPs within the maternal genome. The fetal genotype at each of these sites was deduced individually, unlike previously, where the inheritance was determined for a collection of sites within a haplotype.

These results represent a 90-fold enhancement in the resolution in determining the fetus’s maternal inheritance. Selected genomic locations were more likely to be found at the ends of plasma DNA molecules. We found that a subset of such preferred ends exhibited selectivity for fetal- or maternal-derived DNA in maternal plasma. The ratio of the number of maternal plasma DNA molecules with fetal preferred ends to those with maternal preferred ends showed a correlation with the fetal DNA fraction. Finally, this second generation approach for noninvasive fetal whole-genome analysis was validated in a pregnancy diagnosed with cardiofaciocutaneous syndrome with maternal plasma DNA sequenced to 195× coverage.

The causative de novo BRAF mutation was successfully detected through the maternal plasma DNA analysis.