From structure and dynamics to novel therapeutic development for muscular dystrophy.

Abstract

Dystrophin is defective in Duchenne (DMD) and Becker (BMD) muscular dystrophies, which are debilitating X-linked diseases that currently have no cure. Dystrophin links the actin cytoskeleton at its N-terminus and a glycoprotein complex (DGC) embedded in the sarcolemma at its C-terminus, apparently providing mechanical stability to the muscle during contraction. Due to the large size (427 kD) and filamentous nature of dystrophin, studies of its function and attempts to develop effective therapeutics have developed slowly, despite intensive efforts. Utrophin (395 kD) is a homolog of dystrophin that has shown therapeutic promise in mdx mice, which lack dystrophin. Utrophin is endogenously expressed in the cytoskeleton of fetal and developing muscle but is replaced by dystrophin as the muscle matures (8-10). Both dystrophin and utrophin belong to the spectrin superfamily of actin-binding proteins, which carry out diverse functions in the cytoskeleton of most cells. Of the many proteins included in this superfamily, dystrophin and utrophin are among the least studied in terms of structural dynamics, limiting the understanding of their function at the sarcolemma. In order to target the root of dystrophin malfunction in muscular dystrophy, we need to better understand the native functions of dystrophin and utrophin. Lack of structural information about dystrophin and its interactions adds to the complexity of tying clinical presentations to the diverse disease-causing mutations, and hinders therapeutic advancement in gene or drug therapy. There are numerous mouse-model studies, but there are varied results across several parameters tested, and no construct or drug has been found that restores normal muscle force in the mdx mouse. Exon-skipping morpholinos are expensive to produce with variable delivery and efficacy to muscle groups and require a customized oligo design for each mutation, making it difficult to test them individually in mouse models. In order to (a) understand disease mechanisms and (b) design better therapies rationally, we need more fundamental information about the structures and interactions of specific regions of dystrophin and utrophin. That is the goal of this project.