An intragenic deletion/inversion event in the DMD gene determines a novel exon creation and results in a BMD phenotype
Duchenne and Becker Muscular Dystrophy (DMD and BMD) are caused, in the majority of cases, by deletions in the dystrophin gene (DMD). Here we describe the unprecedented case of a BMD patient carrying a large out-of-frame intragenic deletion, together with an inversion in the DMD gene, resulting in the inclusion of a novel exon in the transcript. Multiplex PCR amplification revealed the presence of a 48–52 exon deletion, but transcript analysis identified two unexpected products, neither of them including exon 53. The shorter mRNA derived from the juxtaposition of exons 47–54 (in-frame), while the longer one resulted from the inclusion of a novel 73-bp exon between exons 47 and 54. Sequence analysis revealed that the inserted sequence derived from an inverted portion of intron 53; its inclusion is predicted to determine protein truncation. The presence of a genomic inversion involving exon 53 and flanking regions was confirmed, and inversion/deletion breakpoints were sequenced. The inverted 73-bp sequence displays splicing signals at both ends and thus it is probably recognized as a novel exon when the partially inverted hnRNA is processed. These findings highlight the importance of mRNA analysis on patients that, based on routine DNA screenings, do not follow the reading-frame rule. This is the first reported patient carrying both an intragenic deletion and inversion in the DMD locus. This case might provide further insight into both the mechanisms that determine genomic rearrangements in the DMD locus and the molecular signals that drive exon inclusion.
- Chelly J, Gilgenkrantz H, Hugnot JP, Hamard G, Lambert M, Recan D, Akli S, Cometto M, Kahn A, Kaplan JC (1991) Illegitimate transcription. Application to the analysis of truncated transcripts of the dystrophin gene in nonmuscle cultured cells from Duchenne and Becker patients. J Clin Invest 88:1161–1166PubMedGoogle Scholar
- Dubowitz V (1985) Muscle biopsy, a practical approach, 2nd edn. Baillière Tindall, PhiladelphiaGoogle Scholar
- Krawczak M, Reiss J, Cooper DN (1992) The mutational spectrum of single base-pair substitutions in mRNA splice junctions of human genes: causes and consequences. Hum Genet 90:41–54Google Scholar
- Nicholson LV, Davison K, Johnson MA, Slater CR, Young C, Bhattacharya S, Gardner-Medwin D, et al (1989b) Dystrophin in skeletal muscle. II. Immunoreactivity in patients with Xp21 muscular dystrophy. J Neurol Sci 94:137–146Google Scholar
- Nobile C, Toffolatti L, Rizzi F, Simionati B, Nigro V, Cardazzo B, Patarnello T, et al (2002) Analysis of 22 deletion breakpoints in dystrophin intron 49. Hum Genet 110:418–421Google Scholar