Introduction
Duchenne [DMD, MIM: 310200] and Becker [BMD, MIM: 300376] muscular dystrophies are X-linked disorders which result in progressive proximal muscle weakness and degeneration, in conjunction with characteristic elevation of creatine phosphokinase (CK) in blood. DMD is rapidly progressive and typically presents in early childhood with motor delays and gait instability, while BMD is characterized by a later-onset phenotype of skeletal muscle weakness [1]. Both DMD and BMD are caused by mutations in DMD , a ~2.2Mbp sized gene containing 79 exons encoding the dystrophin protein. Dystrophin maintains the structural integrity of striated muscle cells through the formation of a dystrophin-glycoprotein complex, linking the cytoskeleton to the extracellular matrix and thus providing tensile strength to muscle fibers [2].
Given its size (roughly 0.1% of the human genome), a high degree ofDMD allelic heterogeneity exists for DMD and BMD. Deletions are the most common pathogenic variants in DMD (~64% of cases), followed by nucleotide substitutions (22%), duplications (12%), and others (inversions, insertions, etc., ~2%) [3]. These alterations disrupt dystrophin’s reading frame in diverse ways, which can lead to mutated transcripts susceptible to nonsense mediated decay, truncated unstable proteins products targeted for degradation, or reduced activity protein variants [4].
DMD duplications are up to five times under-represented compared to deletions in public databases [3]. DMD duplications have historically posed a technical detection challenge as exonic duplications could not be captured by PCR-based techniques, fell outside interrogated probes by multiplex ligation-dependent probe amplification (MLPA), or were under the limit of detection of chromosome microarray (CMA). Such technical difficulties are coupled with the fact that duplications can adopt different structural configurations in the genome. These include duplications in tandem direct or inverted orientations, or insertions into an entirely different chromosome, which can significantly alter the functional impact of duplications. These limitations can challenge the clinical assessment of newly identified duplications in young asymptomatic individuals based solely on genomic data; for these individuals, further molecular and protein truncation characterizations need to be performed in order to provide an accurate diagnosis.
In this report, we illustrate the molecular characterization of a 1.18Mbp DMD duplication spanning exons 1-7 in a 2-month-old male with mildly elevated CK levels. The duplication was found to expand up to 698Kb beyond DMD ’s promoter regions and had a tandem configuration in direct orientation, which led to our reclassification of the variant as likely benign. Follow-up studies at 13 months of age indicated decreasing CK levels and normal development in this individual, further supporting the likely benign classification of the duplication. We hope our study prompts further characterization ofDMD duplications not only in young asymptomatic individuals but also in symptomatic patients, to further elucidate the impact of genomic duplications on DMD transcription and dystrophin structure and improve our technical and interpretative capabilities for DMD and BMD diagnosis.