Beta-sarcoglycan: characterization and role in limb-girdle muscular dystrophy linked to 4q12

Nondystrophinopathic muscular dystrophies including myotonic dystrophy

The spectacular progress concerning dystrophin and its pathology, the dystrophinopathies, has led to a somewhat arbitrarily separated heterogeneous group of nondystrophinopathic muscular dystrophies that currently comprise the Emery-Dreifuss type, the nosologically heterogeneous autosomal-recessive limb-girdle muscular dystrophy, the severe childhood autosomal-recessive muscular dystrophy, the merosin-positive and -negative congenital muscular dystrophies, the autosomal-recessive distal muscular dystrophy of Miyoshi, the facio-scapulo-humeral muscular dystrophy, and myotonic dystrophy, both the adult and neonatal variants. Deficiencies of adhalin in a particular form of limb-girdle muscular dystrophy, and of merosin in a particular form of congenital muscular dystrophy as well as the newly discovered principle of abnormal tri-nucleotide repeats in myotonic dystrophy are evidence of progress that has also amplified the notion of the dystrophinopathies that the protein-deficient muscular dystrophies can now be considered examples of contributions of the dystrophin-glycoprotein complex across the muscle fiber plasma membrane.

Muscular dystrophy, mental retardation and cardiomyopathy not associated with dystrophin deficiency

We report on a male patient aged 38, affected by a syndrome whose characteristic features include onset in early childhood, slow progression, diffuse muscle weakness, mental retardation and cardiomyopathy. Muscle biopsy showed myopathic changes compatible with muscular dystrophy. However, immunostaining for dystrophin as well as 50 and 43 kDa dystrophin-associated glycoproteins (DAGs) was normal. Genetic analysis suggested that direct involvement of the dystrophin gene was highly unlikely. No other family members were affected. Although the clinical picture is reminiscent of Duchenne/Becker muscular dystrophy, the immunologically and genetically documented lack of dystrophin involvement suggests that this particular syndrome is as yet undescribed.

Recent advances. Medical genetics

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Investigation of muscle disease

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Absence of gamma-sarcoglycan (35 DAG) in autosomal recessive muscular dystrophy linked to chromosome 13q12

We have partially sequenced rabbit skeletal muscle gamma-sarcoglycan, an integral component of the dystrophin-glycoprotein complex. Specific antibodies were produced against a gamma-sarcoglycan peptide and used to examine the expression of gamma-sarcoglycan in skeletal muscle of patients with severe childhood autosomal muscular dystrophy linked to chromosome 13q12 (SCARMD). We show by immunofluorescence and Western blotting that in skeletal muscle from these patients gamma-sarcoglycan is completely absent and alpha- and beta-sarcoglycan are greatly reduced in abundance, whereas other components of the DGC are preserved. In addition, we show that in normal muscle alpha-, beta-, and gamma-sarcoglycan constitute a tightly associated sarcolemma complex which cannot be disrupted by SDS treatment.

Utrophin: a structural and functional comparison to dystrophin

Utrophin is an autosomally-encoded homologue of dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene. Although, utrophin is very similar in sequence to dystrophin and possesses many of the protein-binding properties ascribed to dystrophin, both proteins are expressed in an apparently reciprocal manner and may be coordinately regulated. In normal skeletal muscle, utrophin is found at the neuromuscular junction (NMJ) whereas dystrophin predominates at the sarcolemma. However, during development, and in some myopathies including DMD, utrophin is also found at the sarcolemma. This re-distribution is often associated with a significant increase in the levels of utrophin. At the NMJ utrophin co-localizes with the acetylcholine receptors (AChR) and may play a role in the stabilization of the synaptic cytoskeleton. Because utrophin and dystrophin are so similar, utrophin may be able to replace dystrophin in dystrophin deficient muscle. This review compares the structure and function of utrophin to dystrophin and discusses the rationale behind the use of utrophin as a potential therapeutic agent.

Clinical and histopathological features of abnormalities of the dystrophin-based membrane cytoskeleton

The majority (approximately 70%) of cases of childhood and adult onset muscular dystrophies in males, and approximately 10% of dystrophy in girls and women, show underlying primary abnormalities of dystrophin. Approximately 2% of childhood/adult onset muscular dystrophy patients have a primary defect of one of the three sarcoglycan proteins identified to date (alpha, beta, gamma). The finding of a sarcoglycan deficiency in muscle generally does not reflect the primary underlying defect, and thus testing of biopsies for sarcoglycans should be used only after normal dystrophin findings, and in conjunction with gene mutation testing. Approximately 30% of neonatal onset congenital muscular dystrophy has been shown to be due alpha 2-laminin (merosin) deficiency. alpha 2-laminin is a component of the myofiber basal lamina, and this protein interacts with the dystrophin-based membrane cytoskeleton. Due to the similar clinical and histopathological features of the different etiologies of muscular dystrophies, molecular testing of peripheral blood DNA and muscle biopsy protein are a critical part of the clinical work-up of dystrophy patients. Many patients carrying a Becker dystrophy or limb-girdle dystrophy diagnosis should be re-evaluated with molecular tests to provide accurate genetic counseling to their families.

Beta-sarcoglycan (A3b) mutations cause autosomal recessive muscular dystrophy with loss of the sarcoglycan complex

The dystrophin associated proteins (DAPs) are good candidates for harboring primary mutations in the genetically heterogeneous autosomal recessive muscular dystrophies (ARMD). The transmembrane components of the DAPs can be separated into the dystroglycan and the sarcoglycan complexes. Here we report the isolation of cDNAs encoding the 43 kD sarcoglycan protein beta-sarcoglycan (A3b) and the localization of the human gene to chromosome 4q12. We describe a young girl with ARMD with truncating mutations on both alleles. Immunostaining of her muscle biopsy shows specific loss of the components of the sarcoglycan complex (beta-sarcoglycan, alpha-sarcoglycan (adhalin), and 35 kD sarcoglycan). Thus secondary destabilization of the sarcoglycan complex may be an important pathophysiological event in ARMD.