Identification of seven novel cryptic exons embedded in the dystrophin gene and characterization of 14 cryptic dystrophin exons.

The dystrophin gene, which is mutated in Duchenne and Becker muscular dystrophy, is characterized by its extremely large introns. Seven cryptic exons from the intronic sequences of the dystrophin gene have been shown to be inserted into the processed mRNA. In this study, we have cloned seven novel cryptic exons embedded in dystrophin introns that were amplified from dystrophin mRNA isolated from lymphocytes. All of these sequences, which ranged in size from 27 to 151 bp, were found to be cryptic exons because they were completely homologous to intronic sequences (introns 1, 18, 29, 63, 67, and 77), and possessed consensus sequences for branch points, splice acceptor sites, and splice donor sites. Compared with the 77 authentic dystrophin exons, the 14 cryptic exons were characterized by (1) lower Shapiro's splicing probability scores for the splice donor and acceptor sites; (2) smaller and larger densities of splicing enhancer and silencer motifs, respectively; (3) a longer distance between the putative branch site and the splice acceptor site; and (4) with one exception, the introduction of premature stop codons into their respective transcripts. These characteristics indicated that the cryptic exons were weaker than the authentic exons. Our results suggested that a mutation deep within an intron that changed these parameters could cause dystrophinopathy. The cryptic exons identified provide areas that should be examined for the detection of mutations in the dystrophin gene, and they may help us to understand the roles of large dystrophin introns.