SGCG

Abstract: γ-Sarcoglycan is a transmembrane, dystrophin-associated protein expressed in skeletal and cardiac muscle. The murine γ-sarcoglycan gene was disrupted using homologous recombination. Mice lacking γ-sarcoglycan showed pronounced dystrophic muscle changes in early life. By 20 wk of age, these mice developed cardiomyopathy and died prematurely. The loss of γ-sarcoglycan produced secondary reduction of β- and δ-sarcoglycan with partial retention of α- and e-sarcoglycan, suggesting that β-, γ-, and δ-sarcoglycan function as a unit. Importantly, mice lacking γ-sarco- glycan showed normal dystrophin content and local- ization, demonstrating that myofiber degeneration occurred independently of dystrophin alteration. Furthermore, β-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 γ-sarcoglycan, suggesting that programmed cell death contributes to myofiber degeneration. Vital staining with Evans blue dye revealed that muscle lacking γ-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.

Abstract: A spectrum of dystrophin mutations causes Duchenne muscular dystrophy (DMD). The extreme genomic size of the DMD gene (2.2 Mb) leads to kilobase size deletions or duplications in approximately 70% of DMD patients, and the remaining 30% arise from point mutations that introduce premature stop codons or disrupt splicing1. DMD mutations typically completely ablate dystrophin protein expression, while the milder Becker muscular dystrophy (BMD) mutations are most often in-frame deletions that produce truncated, but partially functional dystrophin. DMD patients are clinically defined by loss of ambulation before the age of 12, whereas BMD patients often remain ambulatory beyond age 15, a distinction that generally correlates with the degree of protein expression. Those who lose ambulation between 12 and 15 years are described as intermediate muscular dystrophy (IMD). Many DMD boys now ambulate beyond the age of 12 reflecting their treatment with prednisone or deflazacort, which remain the only medications that alter the natural history of DMD, prolonging ambulation for up to 1 to more than 3 years2-5. Despite the complete absence of dystrophin, the DMD phenotype is associated with a range in rates of progression, supporting that genetic or environmental modifiers help determine clinical course. Animal models, particularly mice, can be used to map genetic modifiers since inbred strains and standardized environmental conditions reduce variation. Like DMD patients, muscular dystrophy patients with a founder mutation in the dystrophin associated protein, γ-sarcoglycan, (SGCG) lose ambulation over a wide age range, consistent with the presence of genetic modifiers6, 7. The Sgcg null mouse was used in a genomewide screen for modifiers of muscular dystrophy and identified the Ltbp4 gene encoding the latent TGFβ binding protein (LTBP) 4 as a modifier of muscular dystrophy8. Given the shared pathological mechanisms between loss of dystrophin and its associated proteins, the sarcoglycans, we reasoned that LTBP4 may modify DMD. LTBP4, a member of the fibrillin superfamily, binds to transforming growth factor (TGF)-β in the extracellular matrix where it regulates TGFβ activity9. We now examined human LTBP4 polymorphisms in 254 nonambulatory patients from the United Dystrophinopathy Project cohort. This United Dystrophinopathy cohort includes 900 DMD patients, most who still ambulate, identified from seven sites representing by far the largest uniformly characterized DMD population. We assessed the effect of LTBP4 genotype on the age at ambulatory loss, since it reflects disease progression, is not influenced by ascertainment bias, and is a milestone for which the month or age is recalled by most patients and/or families. A protective haplotype in LTBP4 was identified.

Abstract: Autosomal recessive muscular dystrophy is genetically heterogeneous. One form of this disorder, limb-girdle muscular dystrophy type 2C (LGMD 2C), is prevalent in northern Africa and has been shown to be associated with a single mutation in the gene encoding the dystrophin-associated protein gamma-sarcoglycan. The previous mutation analysis of gamma-sarcoglycan required the availability of muscle biopsies. To establish a mutation assay for genomic DNA, the intron-exon structure of the gamma-sarcoglycan gene was determined, and primers were designed to amplify each of the exons encoding gamma-sarcoglycan. We studied a group of Brazilian muscular dystrophy patients for mutations in the gamma-sarcoglycan gene. These patients were selected on the basis of autosomal inheritance and/or the presence of normal dystrophin and/or deficiency of alpha-sarcoglycan immunostaining. Four of 19 patients surveyed had a single, homozygous mutation in the gamma-sarcoglycan gene. The mutation identified in these patients, all of African-Brazilian descent, is identical to that seen in the North African population, suggesting that even patients of remote African descent may carry this mutation. The phenotype in these patients varied considerably. Of four families with an identical mutation, three have a severe Duchenne-like muscular dystrophy. However, one family has much milder symptoms, suggesting that other loci may be present that modify the severity of the clinical course resulting from gamma-sarcoglycan gene mutations.

Abstract: Recently, mutations in the genes encoding several of the dystrophin-associated proteins have been identified that produce phenotypes ranging from severe Duchenne-like autosomal recessive muscular dystrophy to the milder limb-girdle muscular dystrophies (LGMDs). LGMD type 2C is generally associated with a more severe clinical course and is prevalent in northern Africa. A previous study identified a single base pair deletion in the gene encoding the dystrophin-associated protein gamma-sarcoglycan in a number of Tunisian muscular dystrophy patients. To investigate whether gamma-sarcoglycan gene mutations cause autosomal recessive muscular dystrophy in other populations, we studied 50 muscular dystrophy patients from the United States and Italy. The muscle biopsies from these 50 patients showed no abnormality of dystrophin but did show diminished immunostaining for the dystrophin-associated protein alpha-sarcoglycan. Four patients with a severe muscular dystrophy phenotype were identified with homozygous, frameshifting mutations in gamma-sarcoglycan. Two of the four have microdeletions that disrupt the distal carboxyl-terminus of gamma-sarcoglycan yet result in a complete absence of gamma-and beta-sarcoglycan suggesting the importance of this region for stability of the sarcoglycan complex. This region of gamma-sarcoglycan, like beta-sarcoglycan, has a number of cysteine residues similar to those in epidermal growth factor cysteine-rich regions.