The Polyproline Site in Hinge 2 Influences the Functional Capacity of Truncated Dystrophins

Mutations in dystrophin can lead to Duchenne muscular dystrophy or the more mild form of the disease, Becker muscular dystrophy. The hinge 3 region in the rod domain of dystrophin is particularly prone to deletion mutations. In-frame deletions of hinge 3 are predicted to lead to BMD, however the severity of disease can vary considerably. Here we performed extensive structure-function analyses of truncated dystrophins with modified hinges and spectrin-like repeats in mdx mice. We found that the polyproline site in hinge 2 profoundly influences the functional capacity of a microdystrophin(Delta R4-R23/Delta CT) with a large deletion in the hinge 3 region. Inclusion of polyproline in microdystrophin(Delta R4-R23/Delta CT) led to small myofibers (12% smaller than wild-type), Achilles myotendinous disruption, ringed fibers, and aberrant neuromuscular junctions in the mdx gastrocnemius muscles. Replacing hinge 2 of microdystrophin(Delta R4-R23/Delta CT) with hinge 3 significantly improved the functional capacity to prevent muscle degeneration, increase muscle fiber area, and maintain the junctions. We conclude that the rigid a-helical structure of the polyproline site significantly impairs the functional capacity of truncated dystrophins to maintain appropriate connections between the cytoskeleton and extracellular matrix.