Dystrophin-related muscular dystrophies

The golden retriever model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked disease caused by mutations in the DMD gene and loss of the protein dystrophin. The absence of dystrophin leads to myofiber membrane fragility and necrosis, with eventual muscle atrophy and contractures. Affected boys typically die in their second or third decade due to either respiratory failure or cardiomyopathy. Despite extensive attempts to develop definitive therapies for DMD, the standard of care remains prednisone, which has only palliative benefits. Animal models, mainly the mdx mouse and golden retriever muscular dystrophy (GRMD) dog, have played a key role in studies of DMD pathogenesis and treatment development. Because the GRMD clinical syndrome is more severe than in mice, better aligning with the progressive course of DMD, canine studies may translate better to humans. The original founder dog for all GRMD colonies worldwide was identified in the early 1980s before the discovery of the DMD gene and dystrophin. Accordingly, analogies to DMD were initially drawn based on similar clinical features, ranging from the X-linked pattern of inheritance to overlapping histopathologic lesions. Confirmation of genetic homology between DMD and GRMD came with identification of the underlying GRMD mutation, a single nucleotide change that leads to exon skipping and an out-of-frame DMD transcript. GRMD colonies have subsequently been established to conduct pathogenetic and preclinical treatment studies. Simultaneous with the onset of GRMD treatment trials, phenotypic biomarkers were developed, allowing definitive characterization of treatment effect. Importantly, GRMD studies have not always substantiated findings from mdx mice and have sometimes identified serious treatment side effects. While the GRMD model may be more clinically relevant than the mdx mouse, usage has been limited by practical considerations related to expense and the number of dogs available. This further complicates ongoing broader concerns about the poor rate of translation of animal model preclinical studies to humans with analogous diseases. Accordingly, in performing GRMD trials, special attention must be paid to experimental design to align with the approach used in DMD clinical trials. This review provides context for the GRMD model, beginning with its original description and extending to its use in preclinical trials.

The Human Genome Project: implications for the treatment of musculoskeletal disease

The ultimate goal of the Human Genome Project is the determination of the molecular sequence of the entire human chromosomal complement. Realization of this goal will include characterization of all the genes that cause or predispose to disease, which will most certainly lead to the development of powerful new tools for diagnosis, prevention, and treatment in all medical fields, including orthopaedics. The authors review the fundamentals of human genetics and gene mapping, summarize the progress of the Human Genome Project thus far, and discuss the implications of this research as it relates to the treatment of musculoskeletal diseases.

Preimplantation single cell analyses of dystrophin gene deletions using whole genome amplification

Primer extension preamplification (PEP) increases the scope and capacity of single cell genetic diagnosis by generating sufficient template to perform multiple subsequent DNA analyses using the polymerase chain reaction. We report the simultaneous analysis of single cells at five commonly deleted dystrophin exons and at the ZFX/ZFY loci. Ninety three percent of PEP reactions with single amniocytes, chorionic villus cells and blastomeres were successful, and a blinded analysis of single lymphoblasts from affected males resulted in 93% diagnostic accuracy, demonstrating its applicability in preimplantation prevention of Duchenne muscular dystrophy. Transfer of unaffected male embryos and improved diagnostic reliability are achieved with the ability to perform replicate multilocus analyses from the same blastomere.

Localization of dystrophin relative to acetylcholine receptor domains in electric tissue and adult and cultured skeletal muscle

Two high-affinity mAbs were prepared against Torpedo dystrophin, an electric organ protein that is closely similar to human dystrophin, the gene product of the Duchenne muscular dystrophy locus. The antibodies were used to localize dystrophin relative to acetylcholine receptors (AChR) in electric organ and in skeletal muscle, and to show identity between Torpedo dystrophin and the previously described 270/300-kD Torpedo postsynaptic protein. Dystrophin was found in both AChR-rich and AChR-poor regions of the innervated face of the electroplaque. Immunogold experiments showed that AChR and dystrophin were closely intermingled in the AChR domains. In contrast, dystrophin appeared to be absent from many or all AChR-rich domains of the rat neuromuscular junction and of AChR clusters in cultured muscle (Xenopus laevis). It was present, however, in the immediately surrounding membrane (deep regions of the junctional folds, membrane domains interdigitating with and surrounding AChR domains within clusters). These results suggest that dystrophin may have a role in organization of AChR in electric tissue. Dystrophin is not, however, an obligatory component of AChR domains in muscle and, at the neuromuscular junction, its roles may be more related to organization of the junctional folds.

Dystrophin as a focal adhesion protein. Collocalization with talin and the Mr 48,000 sarcolemmal protein in cultured Xenopus muscle

Monoclonal antibodies against dystrophin and the postsynaptic 58 kDa protein from Torpedo electric organ were used to localize homologs of these proteins in cultured skeletal muscle (Xenopus laevis). The Xenopus homolog is an Mr 48,000 protein and, like dystrophin, is a sarcolemmal protein. Both proteins localized precisely to talin-positive sites, hence with each other, on the substrate-apposed sarcolemma. Therefore, the first sites of appearance of dystrophin on cultured muscle cells are focal adhesions, i.e. specific sites of cytoskeleton/extracellular matrix interaction. These data also add to evidence that dystrophin and the 58 kDa act together.