CHARACTERIZATION OF REVERTANT MUSCLE-FIBERS IN DUCHENNE MUSCULAR-DYSTROPHY, USING EXON-SPECIFIC MONOCLONAL-ANTIBODIES AGAINST DYSTROPHIN

Most Duchenne muscular dystrophy (DMD) patients have genetic deletions or point mutations in the dystrophin gene that alter the reading fram e of dystrophin mRNA. This causes early termination of translation, an d no dystrophin (Or, less commonly, a truncated N-terminal dystrophin fragment) is produced. In many DMD patients, however, a small proporti on of muscle fibers show strong dystrophin staining, and these ''rever tant fibers'' are thought to arise by a mechanism that restores the re ading frame. Exon-specific monoclonal antibodies (mAbs) have now been used to determine, for the first time, which exons are removed, in ord er to correct the reading frame in individual muscle fibers. Thus, 15 revertant fibers in a DMD patient with a frameshift deletion of exon 4 5 were shown to correct the frameshift by the additional deletion of e xon 44 (or perhaps exon 46 in some fibers) from the dystrophin mRNA, b ut not by larger deletions. This result was consistent with reverse tr ansciption (RT)-PCR and sequencing of a minor dystrophin mRNA with an exon 43/46 junction in this biopsy. In a DMD patient with a frameshift deletion of exons 42 and 43, however, larger deletions than the minim um necessary were used to correct the frameshift. In this patient, who produces a half-size N-terminal dystrophin fragment in all fibers, we were able to show that the revertant dystrophin replaces the truncate d dystrophin in revertant-fiber sarcolemma. The results are consistent with somatic mutations in revertant-fiber nuclei, which result in rem oval of additional exons from dystrophin mRNA. They do not clearly dis tinguish between additional somatic deletions and somatic effects on d ystrophin mRNA splicing, however, and both mechanisms may be operating .