Discovery of a Novel Dystrophin Isoform in the Heart
Supervisor / Principle Investigator:
Kenji Rowel Q. Lim
MD Class of 2022
Duchenne muscular dystrophy (DMD) is an X-linked recessive neuromuscular disease affecting 1 in 3500-5000 males. DMD is characterized by progressive muscle degeneration due to loss-of-function mutations in the dystrophin (DMD) gene coding for dystrophin, a cytoskeletal protein that stabilizes muscle cell membranes. Since dystrophin functions similarly in skeletal and cardiac muscles, DMD patients experience various degrees of cardiomyopathy besides skeletal muscle degeneration. To develop a new therapy for DMD, our group uses the canine X-linked muscular dystrophy Japan model (CXMDJ). These dogs harbor an acceptor site point mutation on DMD intron 6, causing exon 7 exclusion from the final mRNA which produces a premature stop codon. Interestingly when we performed Western blot on cardiac muscle from non-treated CXMDJ dogs, dystrophin bands of near full-length size were present at detectable levels. A previous study reported a patient with an out-of-frame exon 3-7 deletion, however, dystrophin-positive fibres were detected using antibody against exon 8. We hypothesized that the dystrophin isoform detected in non-treated dogs is the product of a new transcription start site (TSS) on exon 8 of the DMD gene. qPCR showed elevated expression of dystrophin past the putative TSS in left ventricle and transverse septum samples from dystrophic dogs. Immunohistochemistry confirmed that dystrophin-positive signals were specific to the endothelial layer of the blood vessels in the heart. Furthermore, these signals were observed when antibody against exon 8 after the putative TSS was used. Western blot using antibody against exon 8 after the putative TSS also showed bands of near full-length size dystrophin in cardiac muscle from dystrophic dogs. These data support our hypothesis of a novel heart-specific dystrophin isoform. Characterization of this new isoform will have important implications both for the treatment of DMD and our understanding of dystrophin’s regulation and biological roles in the heart.