B-223 DMD Duplications Identified by Carrier Screening Should NOT Automatically Be Classified as [Likely] Pathogenic: Data from Long-Read Sequencing of 15 Cases

Abstract Background Pathogenic variants in the DMD gene are associated with Duchenne and Becker muscular dystrophy (DMD/BMD). Professional medical societies currently recommend DMD carrier screening for all patients who are pregnant or considering pregnancy. Such testing may identify intragenic duplications of DMD (i.e., involving one or more exons but fully contained with the gene) in patients without a personal or family history of DMD/BMD. These incidentally detected DMD intragenic duplications, however, pose significant challenges for variant interpretation. Pathogenicity of an intragenic duplication depends on its structural configuration. Tandem duplications may cause reading frame disruption and gene loss-of-function; conversely, non-tandem duplications may not disrupt gene function. Current carrier screening assays cannot determine the structural configuration of an intragenic duplication. The current guidelines (e.g., the ClinGen PVS1 decision tree) presume intragenic duplications are in tandem unless proven otherwise. If this presumption of tandem configuration is inaccurate, it may lead to overestimation of variant pathogenicity, potentially resulting in unnecessary invasive procedures (e.g., amniocentesis for prenatal diagnosis) and/or having other negative impacts with regards to prenatal decision-making. This study aims to evaluate the validity of this presumption for incidentally detected DMD intragenic duplications. Methods We performed high-coverage whole-genome long-read sequencing on the PromethION instrument (Oxford Nanopore Technologies) for 15 cases with previously identified DMD intragenic duplications. Among these cases, four had personal or family history of DMD/BMD, while 11 had no known history (i.e., incidentally detected). The structural configurations of the duplications were determined by an in-house bioinformatics tool and manual inspection of the alignment files. Results In all 4 cases with a known personal or family history of DMD/BMD, the DMD intragenic duplications were confirmed to be in tandem, aligning with expectations of their pathogenicity. Interestingly, among the 11 cases with incidentally detected DMD intragenic duplications, seven (64%) were found to involve interspersed duplications. In five of these seven interspersed duplication cases, the involved DMD exons were co-duplicated with distinct segments of the Xp arm located 0.5–30 Mb away, forming complex structural variants. In another case, the duplicated DMD exons have inserted into the Xq arm, about 90 Mb downstream of DMD. The final case involved the duplicated DMD exons inserted into a repetitive region most consistent with 9q12 (i.e., the 9qh region). Conclusions Our study reveals that while DMD intragenic duplications in patients with a personal or family history of DMD/BMD were consistently in tandem, a substantial 64% of incidentally detected DMD intragenic duplications were not in tandem. These findings have important implications for both clinicians and laboratorians. They challenge the presumption of tandem configuration for DMD intragenic duplications in individuals where the variants were identified incidentally, e.g., through carrier screening in those without a family history of DMD/BMD. In these cases, we recommend classifying the DMD intragenic duplications as variants of uncertain significance (VUS), unless there is evidence of tandem configuration.