Intronic breakpoint definition and transcription analysis in DMD/BMD patients with deletion/duplication at the 5′ mutation hot spot of the dystrophin gene

Dystrophin mutations occurring at the 5′ end of the gene frequently behave as exceptions to the “frame rule,” their clinical severity being variable and often not related to the perturbation of the translation reading frame. The molecular mechanisms underlying the phenotypic variability of 5′ dystrophin mutations have not been fully clarified. We have characterized the genomic breakpoints within introns 2, 6 and 7 and identified the splicing profiles in a cohort of DMD/BMD patients with deletion of dystrophin exons 3–7, 3–6 and duplication of exons 2–4. Our findings indicate that the occurrence of intronic cryptic promoter as well as corrective splicing events are unlikely to play a role in exons 3–7 deleted patients phenotypic variability. Our data suggest that re-initiation of translation could represent a major mechanism responsible for the production of a residual dystrophin in some patients with exons 3–7 deletion. Furthermore, we observed that the out-of-frame exon 2a is almost constantly spliced into a proportion of the dystrophin transcripts in the analysed patients. In the exons 2–4 duplicated DMD patient, producing both in-frame and out-of-frame transcripts, this splicing behaviour might represent a critical factor contributing to the severe phenotype. In conclusion, we suggest that multiple mechanisms may have a role in modulating the outcome of 5′ dystrophin mutations, including recoding mechanisms and unusual splicing choices.