A new pseudoexon activation due to ultrarare branch point formation in Duchenne muscular dystrophy.

Deep-intronic variants that create or enhance a splice site are increasingly reported as a significant cause of monogenic diseases. However, deep-intronic variants that activate pseudoexons by affecting a branch point are extremely rare in monogenic diseases. Here, we describe a novel deep-intronic DMD variant that created a branch point in a Duchenne muscular dystrophy (DMD) patient. A 7.0-year-old boy was enrolled because he was suspected of DMD based on his clinical, muscle imaging, and pathological features. Routine genetic testing did not discover a pathogenic DMD variant. We then performed muscle-derived dystrophin mRNA analysis and detected an aberrant pseudoexon-containing transcript. Further genomic Sanger sequencing and bioinformatic analyses revealed a novel deep-intronic splicing variant in DMD (NM_004006.2:c.5325+1759G>T), which created a new branch point sequence and thus activated a new dystrophin pseudoexon (NM_004006.2:r.5325_5326ins5325+1779_5325+1855). Our study highlights the significant role of branch point alterations in the pathogenesis of monogenic diseases.