Valérie Allamand

Missense mutations in the adhalin gene linked to autosomal recessive muscular dystrophy

Adhalin, the 50 kDa dystrophin-associated glycoprotein, is deficient in skeletal muscle of patients having severe childhood autosomal recessive muscular dystrophy (SCARMD). In several North African families, SCARMD has been linked to chromosome 13q, but SCARMD has been excluded from linkage to this locus in other families. We have now cloned human adhalin cDNA and mapped the adhalin gene to chromosome 17q12-q21.33, excluding it from involvement in 13q-linked SCARMD. However, one allelic variant of a polymorphic microsatellite located within intron 6 of the adhalin gene cosegregated perfectly with the disease phenotype in a large family. Furthermore, missense mutations were identified within the adhalin gene that might cause SCARMD in this family. Thus, the adhalin gene is involved in at least one form of autosomal recessive muscular dystrophy.

β–sarcoglycan: characterization and role in limb–girdle muscular dystrophy linked to 4q12

β–sarcoglycan, a 43 kDa dystrophin–associated glycoprotein, is an integral component of the dystrophin–glycoprotein complex. We have cloned human β–sarcoglycan cDNA and mapped the β–sarcoglycan gene to chromosome 4q12. Pericentromeric markers and an intragenic polymorphic CA repeat cosegregated perfectly with autosomal recessive limb–girdle muscular dystrophy in several Amish families. A Thr–to–Arg missense mutation was identified within the β–sarcoglycan gene that leads to a dramatically reduced expression of β–sarcoglycan in the sarcolemma and a concomitant loss of adhalin and 35 DAG, which may represent a disruption of a functional subcomplex within the dystrophin–glycoprotein complex. Thus, the β–sarcoglycan gene is the fifth locus identified (LGMD2E) that is involved in autosomal recessive limb–girdle muscular dystrophy.

Disruption of the β-Sarcoglycan Gene Reveals Pathogenetic Complexity of Limb-Girdle Muscular Dystrophy Type 2E

Limb-girdle muscular dystrophy type 2E (LGMD 2E) is caused by mutations in the beta-sarcoglycan gene, which is expressed in skeletal, cardiac, and smooth muscle. beta-sarcoglycan-deficient (Sgcb-null) mice developed severe muscular dystrophy and cardiomyopathy with focal areas of necrosis. The sarcoglycan-sarcospan and dystroglycan complexes were disrupted in skeletal, cardiac, and smooth muscle membranes. epsilon-sarcoglycan was also reduced in membrane preparations of striated and smooth muscle. Loss of the sarcoglycan-sarcospan complex in vascular smooth muscle resulted in vascular irregularities in heart, diaphragm, and kidneys. Further biochemical characterization suggested the presence of a distinct epsilon-sarcoglycan complex in skeletal muscle that was disrupted in Sgcb-null mice. Thus, perturbation of vascular function together with disruption of the epsilon-sarcoglycan-containing complex represents a novel mechanism in the pathogenesis of LGMD 2E.

Mutations in COL6A3 Cause Severe and Mild Phenotypes of Ullrich Congenital Muscular Dystrophy

Ullrich congenital muscular dystrophy (UCMD) is an autosomal recessive disorder characterized by generalized muscular weakness, contractures of multiple joints, and distal hyperextensibility. Homozygous and compound heterozygous mutations of COL6A2 on chromosome 21q22 have recently been shown to cause UCMD. We performed a genomewide screening with microsatellite markers in a consanguineous family with three sibs affected with UCMD. Linkage of the disease to chromosome 2q37 was found in this family and in two others. We analyzed COL6A3, which encodes the alpha3 chain of collagen VI, and identified one homozygous mutation per family. In family I, the three sibs carried an A-->G transition in the splice-donor site of intron 29 (6930+5A-->G), leading to the skipping of exon 29, a partial reduction of collagen VI in muscle biopsy, and an intermediate phenotype. In family II, the patient had an unusual mild phenotype, despite a nonsense mutation, R465X, in exon 5. Analysis of the patients COL6A3 transcripts showed the presence of various mRNA species-one of which lacked several exons, including the exon containing the nonsense mutation. The deleted splice variant encodes collagen molecules that have a shorter N-terminal domain but that may assemble with other chains and retain a functional role. This could explain the mild phenotype of the patient who was still ambulant at age 18 years and who showed an unusual combination of hyperlaxity and finger contractures. In family III, the patient had a nonsense mutation, R2342X, causing absence of collagen VI in muscle and fibroblasts, and a severe phenotype, as has been described in patients with UCMD. Mutations in COL6A3 are described in UCMD for the first time and illustrate the wide spectrum of phenotypes which can be caused by collagen VI deficiency.

Merosin-deficient congenital muscular dystrophy, autosomal recessive (MDC1A, MIM#156225, LAMA2 gene coding for alpha2 chain of laminin).

Congenital muscular dystrophies (CMDs) are a highly heterogeneous group of neuromuscular disorders. A subgroup displays a specific deficiency in a protein of the extracellular matrix, the α2 chain of laminin-2 (merosin). A number of mutations in the gene encoding this protein have been identified in patients who present with a severe phenotype and white matter changes.

Sense from nonsense: therapies for premature stop codon diseases

Ten percent of inherited diseases are caused by premature termination codon (PTC) mutations that lead to degradation of the mRNA template and to the production of a non-functional, truncated polypeptide. In addition, many acquired mutations in cancer introduce similar PTCs. In 1999, proof-of-concept for treating these disorders was obtained in a mouse model of muscular dystrophy, when administration of aminoglycosides restored protein translation by inducing the ribosome to bypass a PTC. Since, many studies have validated this approach, but despite the promise of PTC readthrough therapies, the mechanisms of translation termination remain to be precisely elucidated before even more progress can be made. Here, we review the molecular basis for PTC readthrough in eukaryotes and describe currently available compounds with significant therapeutic potential for treating genetic disorders and cancer.

Characterization of δ-sarcoglycan, a novel component of the oligomeric sarcoglycan complex involved in limb-girdle muscular dystrophy

The sarcoglycan complex is known to be involved in limb-girdle muscular dystrophy (LGMD) and is composed of at least three proteins: α-, β-, and γ-sarcoglycan. δ-Sarcoglycan has now been identified as a second 35-kDa sarcolemmal transmembrane glycoprotein that shares high homology with γ-sarcoglycan and is expressed mainly in skeletal and cardiac muscle. Biochemical analysis has demonstrated that γ- and δ-sarcoglycan are separate entities within the sarcoglycan complex and that all four sarcoglycans exist in the complex on a stoichiometrically equal basis. Immunohistochemical analysis of skeletal muscle biopsies from patients with LGMD2C, LGMD2D, and LGMD2E demonstrated a reduction of the entire sarcoglycan complex in these muscular dystrophies. Furthermore, we have mapped the human δ-sarcoglycan gene to chromosome 5q33-q34 in a region overlapping the recently linked autosomal recessive LGMD2F locus.

Selenoprotein function and muscle disease.

The crucial role of the trace element selenium in livestock and human health, in particular in striated muscle function, has been well established but the underlying molecular mechanisms remain poorly understood. Over the last decade, identification of the full repertoire of selenium-containing proteins has opened the way towards a better characterization of these processes. Two selenoproteins have mainly been investigated in muscle, namely SelW and SelN. Here we address their involvement in muscle development and maintenance, through the characterization of various cellular or animal models. In particular, mutations in the SEPN1 gene encoding selenoprotein N (SelN) cause a group of neuromuscular disorders now referred to as SEPN1-related myopathy. Recent findings on the functional consequences of these mutations suggest an important contribution of SelN to the regulation of oxidative stress and calcium homeostasis. Importantly, the conclusions of these experiments have opened new avenues of investigations that provide grounds for the development of therapeutic approaches.

Autophagy is increased in laminin α2 chain-deficient muscle and its inhibition improves muscle morphology in a mouse model of MDC1A

Congenital muscular dystrophy caused by laminin α2 chain deficiency (also known as MDC1A) is a severe and incapacitating disease, characterized by massive muscle wasting. The ubiquitin-proteasome system plays a major role in muscle wasting and we recently demonstrated that increased proteasomal activity is a feature of MDC1A. The autophagy-lysosome pathway is the other major system involved in degradation of proteins and organelles within the muscle cell. However, it remains to be determined if the autophagy-lysosome pathway is dysregulated in muscular dystrophies, including MDC1A. Using the dy(3K)/dy(3K) mouse model of laminin α2 chain deficiency and MDC1A patient muscle, we show here that expression of autophagy-related genes is upregulated in laminin α2 chain-deficient muscle. Moreover, we found that autophagy inhibition significantly improves the dystrophic dy(3K)/dy(3K) phenotype. In particular, we show that systemic injection of 3-methyladenine (3-MA) reduces muscle fibrosis, atrophy, apoptosis and increases muscle regeneration and muscle mass. Importantly, lifespan and locomotive behavior were also greatly improved. These findings indicate that enhanced autophagic activity is pathogenic and that autophagy inhibition holds a promising therapeutic potential in the treatment of MDC1A.

A single homozygous point mutation in a 3′untranslated region motif of selenoprotein N mRNA causes SEPN1-related myopathy

Mutations in the SEPN1 gene encoding the selenoprotein N (SelN) have been described in different congenital myopathies. Here, we report the first mutation in the selenocysteine insertion sequence (SECIS) of SelN messenger RNA, a hairpin structure located in the 3′ untranslated region, in a patient presenting a classical although mild form of rigid spine muscular dystrophy. We detected a significant reduction in both mRNA and protein levels in the patients skin fibroblasts. The SECIS element is crucial for the insertion of selenocysteine at the reprogrammed UGA codon by recruiting the SECIS‐binding protein 2 (SBP2), and we demonstrated that this mutation abolishes SBP2 binding to SECIS in vitro, thereby preventing co‐translational incorporation of selenocysteine and SelN synthesis. The identification of this mutation affecting a conserved base in the SECIS functional motif thereby reveals the structural basis for a novel pathological mechanism leading to SEPN1‐related myopathy.