| *601411 | |||||||||||||||||||||||||||||||||||||||||||||||||||
| SARCOGLYCAN, DELTA; SGCD | |||||||||||||||||||||||||||||||||||||||||||||||||||
| HGNC Approved Gene Symbol: SGCD | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Cytogenetic location: 5q33.3 Genomic coordinates (GRCh37): 5:155,753,766 - 156,194,798 (from NCBI) | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Gene Phenotype Relationships | |||||||||||||||||||||||||||||||||||||||||||||||||||
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| Description | |||||||||||||||||||||||||||||||||||||||||||||||||||
| The dystrophin-glycoprotein complex (DGC) is a multisubunit protein complex that spans the sarcolemma and provides structural linkage between the subsarcolemmal cytoskeleton and the extracellular matrix of muscle cells. There are 3 main subcomplexes of the DGC: the cytoplasmic proteins dystrophin (DMD; 300377) and syntrophin (SNTA1; 601017), the alpha- and beta-dystroglycans (see 128239), and the sarcoglycans (see, e.g., SGCA; 600119). | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Cloning | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Nigro et al. (1996) identified a fourth member of the sarcoglycan family through expressed sequence tag (EST) database searching and cDNA library screening. The protein, designated delta-sarcoglycan, shows 70% identity at the amino acid level to both the human and rabbit gamma-sarcoglycan (SGCG; 608896) sequences. Nigro et al. (1996) reported that delta-sarcoglycan is a putative transmembrane glycoprotein with a small intracellular domain, a single transmembrane hydrophobic domain (amino acids 35 to 59), and a large extracellular C terminus of 231 amino acids. They noted that the extracellular domain of delta-sarcoglycan contains 4 cysteine residues that are a common feature of all sarcoglycans. Northern blot analysis revealed greatest expression in skeletal and heart muscle and weaker expression in smooth muscle. Immunochemical staining revealed that delta-sarcoglycan is localized at the sarcolemma. Jung et al. (1996) used peptide sequence of purified rabbit skeletal muscle delta-sarcoglycan to search an EST database for homologous human sequences. They identified an EST from a human placenta cDNA library that encodes a 256-amino acid polypeptide representing the full-length coding region of delta-sarcoglycan. In a note added in proof, they stated that the sequence reported by Nigro et al. (1996) differs in the C-terminal region of their own sequence, suggesting the existence of 2 isoforms of delta-sarcoglycan in skeletal muscle. | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Mapping | |||||||||||||||||||||||||||||||||||||||||||||||||||
| By human-rodent hybrid analysis and FISH analysis, Nigro et al. (1996) localized the SGCD gene to chromosome 5q33. Okazaki et al. (1996) mapped the Syrian hamster cardiomyopathy gene to hamster chromosome 9qa2.1-b1 (see 'Animal Model' below). | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Molecular Genetics | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Nigro et al. (1996) identified a homozygous frameshift mutation (601411.0001) in the SGCD gene of 8 Brazilian families with limb-girdle muscular dystrophy type 2F (LGMD2F; 601287). Following up on the observation of delta-sarcoglycan mutations in Brazilian muscular dystrophy patients, Duggan et al. (1997) studied Duchenne-like and limb-girdle muscular dystrophy patients who were known not to exhibit gene mutations of dystrophin, alpha-, beta-, or gamma-sarcoglycan. They identified 2 American female patients with novel nonsense mutations of delta-sarcoglycan (601411.0002 and 601411.0003). Microsatellite mapping showed likely consanguinity in the first patient through homozygosity for 13 microsatellite loci covering a 38-cM region of chromosome 5. The second patient was heterozygous. Both girls showed clinical symptoms consistent with Duchenne-like muscular dystrophy. Duggan et al. (1997) concluded that delta-sarcoglycan deficiency occurs in multiple ethnic groups. Furthermore, most or all patients showed a deficiency of the entire sarcoglycan complex, adding support to the hypothesis that these proteins function as a tetrameric unit. Cardiomyopathy in the hamster is a model of human hereditary cardiomyopathy and is divided into hypertrophic cardiomyopathy (HCM; BIO 14.6 strain) and dilated cardiomyopathy (DCM; TO-2 strain) inbred sublines, both of which descended from the same ancestor and are due to mutations in the gene encoding delta-sarcoglycan (Sakamoto et al., 1997; Nigro et al., 1997). Hypothesizing that DCM is a disease of the cytoskeleton and sarcolemma, Tsubata et al. (2000) focused on candidate genes whose products are found in these structures. They screened the human SGCD gene in patients with DCM (CMD1L; 606685) by SSCP analysis and DNA sequencing. Mutations affecting the secondary structure of SGCD were identified in 1 family (S151A; 601411.0006) and in 2 sporadic cases (K238del; 601411.0005). Immunofluorescence analysis of myocardium from 1 of these patients demonstrated significant reduction in SGCD staining. No skeletal muscle disease occurred in any of these patients. In a consanguineous family of Arab origin, in which homozygosity for an A131P mutation in the SGCD gene had been identified in 3 sibs with LGMD2F (601411.0007), Bauer et al. (2009) also identified heterozygosity for the S151A mutation in 7 unaffected family members, 4 of whom were compound heterozygous for the S151A and A131P mutations. Comprehensive clinical and cardiac investigation in all of the compound heterozygous family members revealed no signs of cardiomyopathy or limb-girdle muscular dystrophy. Bauer et al. (2009) questioned the pathologic relevance of the S151A variant, and of the SGCD gene itself, in dilated cardiomyopathy. | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Animal Model | |||||||||||||||||||||||||||||||||||||||||||||||||||
| The BIO14.6 hamster is a widely used model for autosomal recessive cardiomyopathy. These animals die prematurely from progressive myocardial necrosis and heart failure. Nigro et al. (1997) demonstrated that the cardiomyopathy results from a mutation in the delta-sarcoglycan gene, which maps to the same region as the disorder. Thus, the Syrian hamster cardiomyopathy represents the first animal model identified for human sarcoglycan disorders. Holt et al. (1998) investigated the feasibility of sarcoglycan gene transfer for LGMD using a recombinant SGCD adenovirus in the BIO14.6 hamster. They demonstrated extensive long-term expression of delta-sarcoglycan and rescue of the entire sarcoglycan complex, as well as restored stable association of alpha-dystroglycan with the sarcolemma. Importantly, muscle fibers expressing delta-sarcoglycan lacked morphological markers of muscular dystrophy and exhibited restored plasma membrane integrity. Holt et al. (1998) concluded that the sarcoglycan complex is requisite for the maintenance of sarcolemmal integrity, and primary mutations in individual sarcoglycan components can be corrected in vivo. To investigate mechanisms in the pathogenesis of cardiomyopathy associated with mutations of the dystrophin-glycoprotein complex, Coral-Vazquez et al. (1999) analyzed genetically engineered mice deficient for either the Sgca (600119) or Sgcd gene. They found that only Sgcd-null mice developed cardiomyopathy with focal areas of necrosis as the histologic hallmark in cardiac and skeletal muscle. The authors observed absence of the sarcoglycan-sarcospan (SG-SSPN) complex in skeletal and cardiac membranes in both animal models. Loss of vascular smooth muscle SG-SSPN complex was only detected in Sgcd-null mice and was associated with irregularities of the coronary vasculature. Administration of a vascular smooth muscle relaxant prevented onset of myocardial necrosis. These data indicated that disruption of the SG-SSPN complex in vascular smooth muscle perturbs vascular function, which initiates cardiomyopathy and exacerbates muscular dystrophy. To examine the long-term in vivo supplement of the normal SGCD gene driven by cytomegalovirus promoter, Kawada et al. (2002) analyzed the pathophysiologic effects of the transgene expression in TO-2 hamster hearts by using recombinant adeno-associated virus vector (rAAV). The transgene preserved sarcolemmal permeability detected in situ by mutual exclusivity between cardiomyocytes taking up intravenously administered Evans blue dye and expressing the SGCD transgene throughout life. The persistent amelioration of sarcolemmal integrity improved wall thickness and the calcification score postmortem. Furthermore, in vivo myocardial contractibility and hemodynamics, measured by echocardiography and cardiac catheterization, respectively, were normalized, especially in the diastolic performance. The survival period of the TO-2 hamsters was prolonged after the transduction of the SGCD gene, and the animals remained active, exceeding life expectancy of animals without transduction of the responsible gene. These results provided the first evidence that somatic gene therapy is promising for human DCM treatment, if the rAAV vector can be justified for clinical use. Heydemann et al. (2007) developed several lines of transgenic mice that expressed human SGCD with the S151A mutation (601411.0006) specifically in heart. All transgenic mouse lines demonstrated elevated prenatal or perinatal lethality, and surviving animals suffered cardiomyopathy and sudden death at a young age. Nontransgenic control mice expressed wildtype Sgcd only at the cardiomyocyte plasma membrane. However, S151A SGCD transgenic mice expressed mutant SGCD in cardiomyocyte nuclei and showed partial nuclear mislocalization of other sarcoglycan subunits. In addition, lamin A/C (LMNA; 150330) and emerin (EMD; 300384) mislocalized from the nuclear periphery and nuclear membrane, respectively, to a more generalized nuclear distribution in close proximity to mutant SGCD. Heydemann et al. (2007) concluded that the S151A SGCD mutation acts in a dominant-negative manner to produce protein trafficking defects that disrupt nuclear localization of lamin A/C and emerin and plasma membrane sarcoglycan. In mice, Millay et al. (2008) showed the deletion of the gene encoding cyclophilin D, Ppif (604486), rendered mitochondria largely insensitive to the calcium overload-induced swelling associated with a defective sarcolemma, thus reducing myofiber necrosis in 2 distinct models of muscular dystrophy. Mice lacking delta-sarcoglycan (Scgd-null mice) showed markedly less dystrophic disease in both skeletal muscle and heart in the absence of Ppif. Moreover, the premature lethality associated with deletion of Lama2 (156225), encoding the alpha-2 chain of laminin-2, was rescued, as were other indices of dystrophic disease. Treatment with the cyclophilin inhibitor Debio-025 similarly reduced mitochondrial swelling and necrotic disease manifestations in mdx mice, a model of Duchenne muscular dystrophy (see 310200), and in Scgd-null mice. Thus, mitochondrial-dependent necrosis represents a prominent disease mechanism in muscular dystrophy, suggesting that inhibition of cyclophilin D could provide a new pharmacologic treatment strategy for these diseases. Li et al. (2009) generated delta-sarcoglycan/dystrophin (DMD; 300377) double-knockout mice (delta-Dko) in which residual sarcoglycans were completely eliminated from the sarcolemma. Utrophin (UTRN; 128240) levels were increased in these mice but did not mitigate disease. The clinical manifestation of delta-Dko mice was worse than that of mdx mice. They showed characteristic dystrophic signs, body emaciation, macrophage infiltration, decreased life span, less absolute muscle force, and greater susceptibility to contraction-induced injury. Li et al. (2009) suggested that subphysiologic sarcoglycan expression may play a role in ameliorating muscle disease in mdx mice. | |||||||||||||||||||||||||||||||||||||||||||||||||||
| ALLELIC VARIANTS (Selected Examples): | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Table View | |||||||||||||||||||||||||||||||||||||||||||||||||||
| .0001 MUSCULAR DYSTROPHY, LIMB-GIRDLE, TYPE 2F | |||||||||||||||||||||||||||||||||||||||||||||||||||
| SGCD, 1-BP DEL, 656C | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Nigro et al. (1996) identified a homozygous deletion of nucleotide 656 in exon 7 of the delta-sarcoglycan gene (601411.0001) in 8 patients from 4 Brazilian families with limb-girdle muscular dystrophy type 2F (LGMD2F; 601287). They reported that this deletion produces a frameshift resulting in premature truncation of the translatable protein after 5 codons. Nigro et al. (1996) reported that this mutation prevents the translation of the entire 'Cys cluster,' resulting in a truncated protein. Results of immunofluorescence studies on frozen muscle samples indicated that the primary delta-sarcoglycan deficiency leads to disruption of the entire sarcoglycan complex as a primary or secondary effect. | |||||||||||||||||||||||||||||||||||||||||||||||||||
| .0002 MUSCULAR DYSTROPHY, LIMB-GIRDLE, TYPE 2F | |||||||||||||||||||||||||||||||||||||||||||||||||||
| SGCD, ARG165TER [dbSNP:rs121909295] | |||||||||||||||||||||||||||||||||||||||||||||||||||
| In a 17-year-old girl with LGMD2F (601287), Duggan et al. (1997) identified homozygosity for 493C-T transition in the SGCD gene, resulting in an arg165-to-stop (R165X) substitution. This patient was adopted, and the parents were not available for study; however, homozygosity for the R165X mutation was confirmed by direct sequence analysis and genotyping of microsatellite markers flanking the delta-sarcoglycan gene. The mutation was not identified in 100 normal chromosomes. The patient had clinical symptoms consistent with a Duchenne-like muscular dystrophy phenotype. | |||||||||||||||||||||||||||||||||||||||||||||||||||
| .0003 MUSCULAR DYSTROPHY, LIMB-GIRDLE, TYPE 2F | |||||||||||||||||||||||||||||||||||||||||||||||||||
| SGCD, TRP30TER [dbSNP:rs121909296] | |||||||||||||||||||||||||||||||||||||||||||||||||||
| In a 5-year-old girl with LGMD2F (601287), Duggan et al. (1997) found heterozygosity for an 89G-A transition in the SGCD gene, resulting in a trp30-to-ter (W30X) substitution. The father was shown to be a carrier of the W30X mutation. The mutation was not identified in 100 normal chromosomes. A second mutant allele was not identified despite sequencing of the entire coding sequence. The patient had clinical symptoms consistent with a Duchenne-like muscular dystrophy phenotype. | |||||||||||||||||||||||||||||||||||||||||||||||||||
| .0004 MUSCULAR DYSTROPHY, LIMB-GIRDLE, TYPE 2F | |||||||||||||||||||||||||||||||||||||||||||||||||||
| SGCD, GLU262LYS [dbSNP:rs121909297] | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Moreira et al. (1998) reported a C-to-A transversion at position 784 of the SGCD gene, causing a glu262-to-lys substitution of the translated protein in a girl with limb-girdle muscular dystrophy type 2F (LGMD2F; 601287). The phenotype was as severe as that presented by other LGMD2F patients with truncating mutations. | |||||||||||||||||||||||||||||||||||||||||||||||||||
| .0005 CARDIOMYOPATHY, DILATED, 1L | |||||||||||||||||||||||||||||||||||||||||||||||||||
| SGCD, 3-BP DEL, 710AGA/711GAA | |||||||||||||||||||||||||||||||||||||||||||||||||||
| In 2 presumably unrelated patients with dilated cardiomyopathy (CMD1L; 606685), Tsubata et al. (2000) identified a 3-bp deletion, either a deletion of AGA at nucleotide 710 or a deletion of GAA at nucleotide 711, in exon 9p of the SGCD gene. The deletion resulted in the loss of a codon encoding lysine at position 238. A screening of 200 control individuals as well as the phenotypically normal parents of these children failed to identify the same abnormality. Deletion of lys238 was predicted to result in a change in the secondary structure of SGCD in which the folding of the protein is disrupted, consistent with a disease-causing mutation. | |||||||||||||||||||||||||||||||||||||||||||||||||||
| .0006 CARDIOMYOPATHY, DILATED, 1L | |||||||||||||||||||||||||||||||||||||||||||||||||||
| MUSCULAR DYSTROPHY, LIMB-GIRDLE, TYPE 2F, DIGENIC, INCLUDED | |||||||||||||||||||||||||||||||||||||||||||||||||||
| SGCD, SER151ALA [dbSNP:rs121909298] | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Tsubata et al. (2000) identified a heterozygous 451T-G transversion in exon 6 of the SGCD gene, resulting in a ser151-to-ala (S151A) substitution, in a family with dilated cardiomyopathy (CMD1L; 606685) in 3 generations. The grandfather died suddenly with congestive heart failure at the age of 38 years. Two daughters died suddenly with congestive heart failure at ages 14 years and 36 years. The second of these daughters had 2 sons, 1 of whom died suddenly at age 17 with congestive heart failure, while the other underwent cardiac transplantation at the age of 21 years. The S151A mutation was not detected in 200 controls. Trabelsi et al. (2008) identified a heterozygous S151A mutation in a patient with autosomal recessive limb-girdle muscular dystrophy (LGMD2F; 601287). He was also found to carry a heterozygous partial duplication of exon 1 of the SGCB gene (600900), which is responsible for LGMD2E (604286), although the consequence of the variant on SGCB production was unknown. However, Trabelsi et al. (2008) suggested that this patient had 'double heterozygosity,' or digenic inheritance. The patient showed muscle weakness at age 3 years, complicated by cardiomyopathy at age 13 years. In a consanguineous family of Arab origin, in which homozygosity for an A131P mutation (601411.0007) in the SGCD gene had been identified in 3 sibs with LGMD2F, Bauer et al. (2009) also identified heterozygosity for the S151A mutation in 7 unaffected family members, 4 of whom were compound heterozygous for the S151A and A131P mutations. Comprehensive clinical and cardiac investigation in all of the compound heterozygous family members revealed no signs of cardiomyopathy or limb-girdle muscular dystrophy. Bauer et al. (2009) questioned the pathologic relevance of the S151A variant, and of the SGCD gene itself, in dilated cardiomyopathy. | |||||||||||||||||||||||||||||||||||||||||||||||||||
| .0007 MUSCULAR DYSTROPHY, LIMB-GIRDLE, TYPE 2F | |||||||||||||||||||||||||||||||||||||||||||||||||||
| SGCD, ALA131PRO | |||||||||||||||||||||||||||||||||||||||||||||||||||
| In 3 affected sibs with LGMD2F (601287) from a consanguineous family of Arab origin, Bauer et al. (2009) identified homozygosity for a 391C-G transversion in exon 6 of the SGCD gene, resulting in an ala131-to-pro (A131P) substitution. The unaffected parents and 4 unaffected sibs, as well as 4 other unaffected family members, were heterozygous for the A131P mutation. | |||||||||||||||||||||||||||||||||||||||||||||||||||
| REFERENCES | |||||||||||||||||||||||||||||||||||||||||||||||||||
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