Hybrid Spectrin Type Repeats Produced by Exon-skipping in Dystrophin

Dystrophin is the protein whose defect underlies Duchenne Muscular Dystrophy, DMD, a common (1:3500 male births) and fatal condition in which muscle tissue deteriorates leading to death in the second or third decade of life. Dystrophin is coded for by the largest human gene, and one of the most complex. It is translated from at least 7 distinct promoters, with the largest transcripts (which are the ones involved in DMD) containing 79 exons over >2.5 Mbp [K.F. O'Brien, L.M. Kunkel, Dystrophin and muscular dystrophy: past, present, and future, Mol. Genet. Metab. 74 (2001) 75-88, H.M. Sadoulet-Puccio, L.M. Kunkel, Dystrophin and its isoforms, Brain Pathol. 6 (1996) 25-35]. Exacerbating this complexity, it has recently been shown that dystrophin is subject to extensive alternative RNA processing, potentially producing a wide variety dystrophin variants [M. Sironi, R. Cagliani, U. Pozzoli, A. Bardoni, G.P. Comi, R. Giorda, N. Bresolin, The dystrophin gene is alternatively spliced throughout its coding sequence FEBS Lett 517 (2002) 163-166]. The structure of the dystrophin protein is highly modular, with the most common module being a motif termed the spectrin type repeat, or STR, of which there are 24. Each STR is roughly coded for by two exons, and the most common type of multiple exon-skipping events start and end at introns in the middle of STRs [R.G. Roberts, A.J. Coffey, M. Bobrow, D.R. Bentley, Exon structure of the human dystrophin gene Genomics 16 (1993) 536-538, M. Koenig, L.M. Kunkel, Detailed analysis of the repeat domain of dystrophin reveals four potential hinge segments that may confer flexibility, J. Biol. Chem. 265 (1990) 4560-4566]. This would produce fractional STR modules, however, the concept of STRs as proteins domains makes the viability of such fractional motifs questionable. However, certain of these events produce pairs of potentially complementary fractional domain that might reassemble into a hybrid STR motif. We have constructed model fragment corresponding to one such exon-skipping event, and show that the hybrid STR so produced is viable, and furthermore that some of the properties of the protein containing it differ substantially of the native, un-skipped parent.