DMD/BMD - GENETICS: EP.113 Lack of correlation between DMD exon 2 duplication splicing choices and phenotype severity

Abstract

Duchenne muscular dystrophy (DMD) is an X-linked disorder caused by mutations in the DMD gene. About 75% of DMD mutations are deletions or duplications while the remaining are small mutations. Exon 2 duplication is the most frequent duplication and is associated with variable phenotypes, ranging from mild to severe. We profiled the muscle biopsy splicing pattern of DMD exon 2 duplication in seven DMD patients (identified by MLPA) using the Agilent High Sensitivity assay. Four patients showed classical DMD phenotype, with loss of ambulation within 13-year-old while in the other three patients ambulation was lost >age 15. We found four different transcripts due to different splicing choices: the exon2-exon2 duplicated transcript (79%), the intron1-exon2-exon2 transcript, which incorporates an upstream region of intron 1, generating a premature stop codon (13%), a transcript with both exon 2 skipped (3%) and a transcript skipping one exon 2, which generates a normal messenger (6%), this last represented in all seven DMD muscles. We did not identify a phenotype-specific DMD splicing pattern in the seven exon 2 duplicated DMD patients. All DMD patients show a similar percentage of wild type transcript (6%), and the very low amount of the transcript skipping both exon 2 suggests that dystrophin rescue due to the alternative ATG translation start site in exon 5 may not play a major role in disease severity. Our results therefore indicate lack of correlation between DMD exon 2 duplication splicing choices and DMD phenotype.