ADDITIONAL DYSTROPHIN FRAGMENT IN BECKER MUSCULAR-DYSTROPHY MAY RESULT FROM PROTEOLYTIC CLEAVAGE AT DELETION JUNCTIONS

Becker muscular dystrophy is usually caused by intragenic dystrophin gene deletions that result in production of an internally deleted protein. Previous studies have detected what appears to be a unique dystrophin degradation product that appears only in muscle biopsies from patients with Becker muscular dystrophy. This dystrophin fragment is always seen in addition to the "full-size" dystrophin of the expected size for a given gene deletion. It is only found in biopsies from patients with mutations in the deletion-prone region encompassing exons 45-53, but it does not appear to correlate with any observable phenotype at the clinical level. By correlating the size and locations of dystrophin gene deletions with the size of this degradation product, together with use of region-specific dystrophin antisera, we find that proteolytic cleavage may occur at the deletion break-points, perhaps due to alterations of the secondary and/or tertiary structures of the protein. This cleavage results in loss of the carboxy-terminal domains that are thought to be important for interactions between dystrophin and other membrane-bound proteins.