Chantal T. Ly

gamma-Sarcoglycan is a transmembrane, dystrophin-associated protein expressed in skeletal and cardiac muscle. The murine gamma-sarcoglycan gene was disrupted using homologous recombination. Mice lacking gamma-sarcoglycan showed pronounced dystrophic muscle changes in early life. By 20 wk of age, these mice developed cardiomyopathy and died prematurely. The loss of gamma-sarcoglycan produced secondary reduction of beta- and delta-sarcoglycan with partial retention of alpha- and epsilon-sarcoglycan, suggesting that beta-, gamma-, and delta-sarcoglycan function as a unit. Importantly, mice lacking gamma-sarco- glycan showed normal dystrophin content and local- ization, demonstrating that myofiber degeneration occurred independently of dystrophin alteration. Furthermore, beta-dystroglycan and laminin were left intact, implying that the dystrophin-dystroglycan-laminin mechanical link was unaffected by sarcoglycan deficiency. Apoptotic myonuclei were abundant in skeletal muscle lacking gamma-sarcoglycan, suggesting that programmed cell death contributes to myofiber degeneration. Vital staining with Evans blue dye revealed that muscle lacking gamma-sarcoglycan developed membrane disruptions like those seen in dystrophin-deficient muscle. Our data demonstrate that sarcoglycan loss was sufficient, and that dystrophin loss was not necessary to cause membrane defects and apoptosis. As a common molecular feature in a variety of muscular dystrophies, sarcoglycan loss is a likely mediator of pathology.

Sarcoglycan is a multimeric, integral membrane glycoprotein complex that associates with dystrophin. Mutations in individual sarcoglycan subunits have been identified in inherited forms of muscular dystrophy. To evaluate the contributions of sarcoglycan and dystrophin to muscle membrane stability and muscular dystrophy, we compared muscle lacking specific sarcoglycans or dystrophin. Here we report that mice lacking (delta)-sarcoglycan developed muscular dystrophy and cardiomyopathy similar to mice lacking (gamma)-sarcoglycan. However, unlike muscle lacking (gamma)-sarcoglycan, (delta)-sarcoglycan-deficient muscle was sensitive to eccentric contraction-induced disruption of the plasma membrane. In the absence of (delta)-sarcoglycan, (alpha)-, (beta)- and (gamma)-sarcoglycan were undetectable, while dystrophin was expressed at normal levels. In contrast, without (gamma)-sarcoglycan, reduced levels of (alpha)-, (beta)- and (delta)-sarcoglycan were expressed, glycosylated and formed a complex with each other. Thus, the elimination of (gamma)- and (delta)-sarcoglycan had different molecular consequences for the assembly and function of the dystrophin-glycoprotein complex. Furthermore, these molecular differences were associated with different mechanical consequences for the muscle plasma membrane. Through this in vivo analysis, a model for sarcoglycan assembly is proposed.

The dystrophin-glycoprotein complex (DGC) is critical for muscle membrane stability. The sarcoglycans are transmembrane proteins within the DGC, and the function of the sarcoglycans is unknown. Mutations in sarcoglycan genes cause autosomal recessive muscular dystrophy. We have identified a new sarcoglycan gene with high homology to alpha-sarcoglycan highlighting the redundancy of the DGC. This gene, named epsilon-sarcoglycan, has an identical intron-exon structure to alpha-sarcoglycan, and is more broadly expressed. The characterization of epsilon-sarcoglycan should make it possible to determine if it, like the other sarcoglycan genes, is mutated in muscular dystrophy.

Mutations in dysferlin were recently described in patients with Miyoshi myopathy, a disorder that preferentially affects the distal musculature, and in patients with Limb-Girdle Muscular Dystrophy 2B, a disorder that affects the proximal musculature. Despite the phenotypic differences, the types of mutations associated with Miyoshi myopathy and Limb-Girdle Muscular Dystrophy 2B do not differ significantly. Thus, the etiology of the phenotypic variability associated with dysferlin mutations remains unknown. Using genetic linkage and mutation analysis, we identified a large inbred pedigree of Yemenite Jewish descent with limb-girdle muscular dystrophy. The phenotype in these patients included slowly progressive, proximal, and distal muscular weakness in the lower limbs with markedly elevated serum creatine kinase (CK) levels. These patients had normal development and muscle strength and function in early life. Muscle biopsies from 4 affected patients showed a typical dystrophic pattern but interestingly, in 2, an inflammatory process was seen. The inflammatory infiltrates included primarily CD3 positive lymphocytes. Associated with this phenotype, we identified a previously undescribed frameshift mutation at nucleotide 5711 of dysferlin. This mutation produced an absence of normal dysferlin mRNA synthesis by affecting an acceptor site and cryptic splicing. Thus, splice site mutations that disrupt dysferlin may produce a phenotype associated with inflammation.