Digital droplet PCR (ddPCR) was an accurate and noninvasive method to detect mutations leading to hemophilia A and B in maternal plasma DNA in 15 at-risk pregnancies of 8 to 42 weeks’ gestation, researchers reported.
Additionally, the researchers showed for the first time that targeted massively parallel sequencing (MPS) accurately detected the clinically important int22h-related inversion mutations in maternal plasma DNA from pregnant hemophilia carriers from three families with the disorder.
As costs of sequencing continue to fall, larger studies of pregnant carriers of F8 int22h-related inversions can help make MPS “an essential part in noninvasive prenatal testing of hemophilia carriers,” Irena Hudecova, PhD, of Li Ka Shing Institute of Health Sciences, Hong Kong, and her associates wrote ( Blood. 2017 Jul 20;130:340-7 ).
Diagnosing hemophilia during pregnancy helps optimize care and allows mothers to make informed decisions about whether to terminate pregnancies. But for male fetuses, invasive testing has been the only option. In a prior small study, researchers used noninvasive microfluidics PCR to detect sequence variants of F8, which encodes factor VIII, and F9, which encodes Factor IX. The assay uses a chip that can accommodate about 9,000 reaction wells, making noninvasive screening much more feasible and affordable. But technical difficulties had precluded detection of int22h-related inversions, the inversion mutations of intron 22 in F8 on chromosome X that affect about half of individuals with severe hemophilia ( Blood. 2011 Mar 31;117:3684-91 ).
For the current study, the researchers first designed family-specific ddPCR assays to test for relevant maternal sequence variants scattered across the F8 and F9 genes. Tests of 15 male singleton fetuses produced three unclassified samples, but no misclassifications.
“Because of the scalability of ddPCR, the protocol performed reliably even in cases with fetal DNA fraction lower than 10%,” the researchers wrote. “When an unclassified result is encountered, one either performs more digital analyses on the sample to accumulate more data points, or when the sample is consumed, one may resort to an additional blood draw, possibly at a later gestational age with higher fetal DNA fraction.”
Next, the investigators used MPS to create detailed fetal haplotype maps of the 7.6-Md region of F8 where int22h-related inversions occur. This approach yielded an “accurate and robust measurement of maternally inherited fetal haplotypes,” they wrote. “Our data suggest it is feasible to apply targeted MPS to interrogate maternally inherited F8 int22h-related inversions, whereas ddPCR [is] an affordable approach for the identification of F8 and F9 sequence variants in maternal plasma.”
The study was funded by the Research Grants Council of the Hong Kong SAR Government and the Vice Chancellor’s One-Off Discretionary Fund of The Chinese University of Hong Kong. Dr. Hudecova reported having no financial disclosures. Several coauthors disclosed patents for plasma nucleic acid analysis and ties to Sequenom, Illumina, Xcelom, and Cirina.