ONLINE MUTATION REPORT Multiple exon skipping and RNA circularisation contribute to the severe phenotypic expression of exon 5 dystrophin deletion

Deletion and duplication of one or more exons in the dystrophin gene account for 70% of patients with Duchenne and Becker muscular dystrophies (DMD and BMD) and other allelic clinical entities such as raised serum creatine kinase and X linked dilated cardiomyopathy (XLDC). The severity of the resulting phenotype can be generally predicted by whether these mutations lead to translation frame disruption and premature termination of protein synthesis. Nevertheless, the occurrence of severely affected patients with in frame deletions as well as mild phenotypes associated with frameshift, indicate that factors other than the frame disruption should contribute to the clinical severity. Exceptions to the “frame rule” are found in about 8% of patients with mutations occurring both at the 5′ and 3′ end of the dystrophin gene, 3 4 although they seem to predominate in the 5′ region. Despite extensive clinical, immunocytochemical, and transcriptional studies, the basis of genotypephenotype correlation in these “atypical” cases remains controversial and its clarification will surely provide relevant information about normal and abnormal dystrophin function. Several reports suggest a role for alternative splicing in altering the clinical phenotype by modulating the editing of the translation reading frame. 5 6 Patients with BMD carrying the frameshift deletions of exons 3–7 and 45 show alternative splicing phenomena theoretically restoring the reading frame. 7 However, the relevance of these events in contributing to a milder phenotype is still unclear. 8 9 Supporting the bridging role of the dystrophin splicing machinery as active modulator between genotype (deletion mutation) and protein production, cell specific somatic exon skipping has been documented in skeletal muscle revertant fibres in DMD. Production of circular RNA molecules, resulting from the adjoining of the donor splice site of a 3′ exon to the acceptor site of a 5′ exon, represents a peculiar splicing behaviour described to date in a limited number of human genes, dystrophin included. These circular RNAs represent scrambled transcripts supposed to have originated as a corollary of alternative splicing events. Nevertheless, the “exon skipping circular RNA” hypothesis cannot account for all circular molecules, and their origin as well as their function remains unclear. 16 Some circular dystrophin RNAs are transcribed at low levels in normal human skeletal muscle; however, their production has been found to be abolished in tissues from patients with DMD carrying deletions in the 5′ region of the gene.