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D.M. Pillers, J. Pang; Discovery of a Novel Internal Exon in Dp260 that Results in Protein with Structural Changes Supports Concept of Unique Function of Retinal Dystrophin . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2262.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Most Duchenne and Becker muscular dystrophy patients have been shown to have defects in retinal electrophysiology characterized by a reduction in the ERG b-wave amplitude under scotopic conditions. Genotype-phenotype correlations in both mouse and man have demonstrated that the presence or absence of the ERG changes correlates with whether or not the patient has a mutation that affects Dp260, an isoform of dystrophin that is predominantly expressed in retina. We sought to determine whether there were any unique structural aspects of Dp260 that could contribute to its role in retinal electrophysiology, as opposed to the structural role that full-length dystrophin Dp427 plays in muscle. Methods: Dp260 was amplified by long RT-PCR from mouse eye mRNA. The product was sequenced in two directions and the sequence was compared to that of Dp427 in GenBank. 3D-JIGSAW was used to perform comparative structural modeling. Results: The amplimer contained exon R1 followed by exon 30 followed 3' through the remainder of the Dp427 exons, in predicted order. A novel 36 bp exon with the open reading frame preserved was identified. A polyclonal antibody raised against this exon demonstrated onWestern blot analysis that Dp260 does contain the exon. Comparison of the Dp260 structure versus the Dp427 in the vicinity of the novel exon by 3-D modeling showed disruption of the spectrin-like repeat and exposure of a group of amino acids on the peptide surface. RT-PCR of a panel of mouse tissues showed that the exon is uniquely expressed in retina. Conclusions: The DMD gene is remarkable in that it is large, complex, and encodes multiple overlapping proteins known as isoforms. Our data suggest that these isoforms may have unique functional roles, with Dp260 being predominantly expressed in retina and contributing to normal retinal electrophysiology as defined by the electroretinogram. The isoforms differ structurally from each other in that their transcripts are derived from different 5' promoters and first exons, and their 3' ends are alternatively spliced but otherwise overlap with Dp427 in their gene structure. Dp260 differs from the other isoforms by containing a novel internal exon in the center of the rod domain that results in structural changes of the protein as identified by 3-D modeling and which may lead to its unique functional aspects demonstrated by retinal electrophysiology.
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