Sequencing cell-free fetal DNA in pregnant women with GCK-MODY: a proof-of-concept study.

Context Persons with monogenic diabetes due to inactivating glucokinase (GCK) variants typically do not require treatment, except potentially during pregnancy. In pregnancy, fetal GCK genotype determines whether treatment is indicated, but non-invasive methods are not clinically available. Objective To develop a method to determine fetal GCK genotype non-invasively using maternal cell-free fetal DNA. Design, and main outcome measure This was a proof-of-concept study which used information from 1) massive parallel sequencing of maternal plasma cell-free DNA, 2) direct haplotype sequences of maternal genomic DNA, and 3) paternal genotype to estimate relative haplotype dosage of the pathogenic variant-linked haplotype. Statistical testing of variant inheritance was performed using a sequential probability ratio test (SPRT). Patients and setting Three pregnant women with causal GCK variant. Results In each of the three cases, plasma cell-free DNA was extracted once between gestational weeks 24 and 36. The fetal fraction of cell-free DNA ranged between 21.8 - 23.0%. Paternal homozygous alleles that were identical to the maternal GCK variant-linked allele were not overrepresented in the cell-free DNA. Paternal homozygous alleles that were identical to maternal wild-type-linked allele were significantly overrepresented. Based on the SPRT, we predicted that all three cases did not inherit the GCK variant. Postnatal infant genotyping confirmed our prediction in each case. Conclusions We have successfully implemented a non-invasive method to predict fetal GCK genotype using cell-free DNA in three pregnant women carrying an inactivating GCK variant. This method could guide tailoring of hyperglycemia treatment in pregnancies of women with GCK monogenic diabetes.