BACKGROUND: Noninvasive prenatal testing (NIPT) for monogenic diseases by use of PCR-based strategies requires precise quantification of mutant fetal alleles circulating in the maternal plasma. The study describes the development and validation of a novel assay termed circulating single-molecule amplification and resequencing technology (cSMART) for counting single allelic molecules in plasma. Here we demonstrate the suitability of cSMART for NIPT, with Wilson Disease (WD) as proof of concept. METHODS: We used Sanger and whole-exome sequencing to identify familial ATP7B (ATPase, Cu++ transporting, β polypeptide) gene mutations. For cSMART, single molecules were tagged with unique barcodes and circularized, and alleles were targeted and replicated by inverse PCR. The unique single allelic molecules were identified by sequencing and counted, and the percentage of mutant alleles in the original maternal plasma sample was used to determine fetal genotypes. RESULTS: Four families with WD pedigrees consented to the study. Using Sanger and whole-exome sequencing, we mapped the pathogenic ATP7B mutations in each pedigree and confirmed the proband's original diagnosis of WD. After validation of cSMART with defined plasma models mimicking fetal inheritance of paternal, maternal, or both parental mutant alleles, we retrospectively showed in second pregnancies that the fetal genotypes assigned by invasive testing and NIPT were concordant. CONCLUSIONS: We developed a reliable and accurate NIPT assay that correctly diagnosed the fetal genotypes in 4 pregnancies at risk for WD. This novel technology has potential as a universal strategy for NIPT of other monogenic disorders, since it requires only knowledge of the parental pathogenic mutations.

Congenital adrenal hyperplasia (CAH) is an autosomal recessive disorder that has been included in newborn screening programs. Current approaches to gene testing for CAH are facing challenges because of the complexity of the CYP21A2 locus and genetic heterogeneity of the disease. A comprehensive analysis of CAH (CACAH) combining long-range locus-specific PCR and long-read sequencing (LRS) was developed to perform full sequence analysis of 5 common CAH candidate genes, including CYP21A2, CYP11B1, CYP17A1, HSD3B2, and StAR. In a blind retrospective study, the clinical utility of CACAH was evaluated in 37 samples by comparing to standard CAH testing using multiplex ligation-dependent probe amplification (MLPA) plus Sanger sequencing. Of the 37 clinical samples, a total of 69 pathogenic variants were identified, comprising 65 CYP21A2 variants, 2 HSD3B2 variants, and 2 CYP17A1 variants. For CYP21A2, the most frequent variant was c.518T > A (29.2%), followed by c.293−13C/A > G (21.5%). Compared with the current CAH testing using MLPA plus Sanger sequencing, the CACAH assay showed 100% specificity and 100% sensitivity, and precisely determined the junction sites of deletions/insertions and cis–trans configuration of multiple variants without analyzing family samples. Moreover, CACAH identified a case carrying 2 copies of CYP21A1 with the c.1451_1452delinsC variant on the same chromosome, which was not confirmed by MLPA plus Sanger sequencing. LRS-based CACAH can determine all genotypes of CAH accurately and reliably in one assay, presenting a comprehensive approach for CAH genetic diagnosis and carrier screening.