A common disease-associated missense mutation in alpha-sarcoglycan fails to cause muscular dystrophy in mice

Abstract

Limb-girdle muscular dystrophy type 2D (LGMD2D) is caused by autosomal recessive mutations in the alpha-sarcoglycan gene. An R77C substitution is the most prevalent cause of the disease, leading to disruption of the sarcoglycan-sarcospan complex. To model this common mutation, we generated knock-in mice with an H77C substitution in alpha-sarcoglycan. The floxed neomycin (Neo)-cassette retained at the targeted H77C alpha-sarcoglycan locus caused a loss of alpha-sarcoglycan expression, resulting in muscular dystrophy in homozygotes, whereas Cre-mediated deletion of the floxed Neo-cassette led to recovered H77C alpha-sarcoglycan expression. Contrary to expectations, mice homozygous for the H77C-encoding allele expressed both this mutant alpha-sarcoglycan and the other components of the sarcoglycan-sarcospan complex in striated muscle, and did not develop muscular dystrophy. Accordingly, conditional rescued expression of the H77C protein in striated muscle of the alpha-sarcoglycan-deficient mice prevented the disease. Adding to the case that the behavior of mutant alpha-sarcoglycan is different between humans and mice, mutant human R77C alpha-sarcoglycan restored the expression of the sarcoglycan-sarcospan complex when introduced by adenoviral vector into the skeletal muscle of previously created alpha-sarcoglycan null mice. These findings indicate that the alpha-sarcoglycan with the most frequent missense mutation in LGMD2D is correctly processed, is transported to the sarcolemma, and is fully functional in mouse muscle. Our study presents an unexpected difference in the behavior of a missense-mutated protein in mice versus human patients, and emphasizes the need to understand species-specific protein quality control systems.

Alpha sarcoglycan is required for FGF-dependent myogenic progenitor cell proliferation in vitro and in vivo

“…This could be explained by the fact that human cells show a reduced proliferative potency compared with those of mice, and that other factors, such as the residual expression of dystrophin (Klinge et al, 2008;Vainzof et al, 2000), might act at the level of fiber stability rather than in the regeneration process. The phenotypic difference observed between LGMD2D and Sgca-null mice, together with the large variability in the severity of the human disease, might also reflect the mutation type of the SGCA gene together with species-specific compensatory and regulatory mechanisms (Kobuke et al, 2008).…”

Therapeutic Possibilities in the Autosomal Recessive Limb-Girdle Muscular Dystrophies

“…There are both sarcoglycan null mouse models, as well as a mouse model with a disease-specific missense mutation. [52][53][54][55][56][57] The sarcoglycan null mice and the delta sarcoglycan-deficient cardiomyopathic hamsters (BIO14.6, BIO53.58, CHF147, TO-2) are also wellestablished animal models for sarcoglycan-deficient cardiomyopathy. The spectrum of therapeutic approaches that has been used to treat the dilated cardiomyopathy in the hamster models is very broad and reaches from classical pharmacotherapy approaches 58,59 to gene replacement therapy, 60 -62 cellular therapies, [63][64][65] and recombinant growth factor therapies.…”

“…Mice homozygous for the H77C-encoding allele, the most prevalent cause of LGMD2D, did not develop muscular dystrophy. 56,57 The studies suggest that the mutant protein that causes LGMD2D in humans is fully functional in the mouse muscle. Despite the lack of a histological phenotype, the mouse models have helped to gain insight into the molecular mechanisms leading to LGMD2D, caused by the most common mutation in the ␣-sarcoglycan gene, and to test new therapeutic approaches.…”

Lost after translation

“…Un allèle faux-sens se comportant comme un allèle nul Le numéro de mars 2008 du journal Human Molecular Genetics contient deux articles [1,2] portant sur la pathogénicité de la mutation fauxsens R77C dans le gène de l'alphasarcoglycane (SGCA). La pathologie de ce gène entraîne chez l'homme une dystrophie musculaire progressive de sévérité variable [5][6][7][8].…”

“…Les deux équipes mentionnées ci-dessus [1,2] ont effectué avec succès le remplacement du résidu His par le résidu Cys dans le gène murin. À leur grande surprise les souris obtenues (Sgca H77C/H77C ) ne manifestent aucun symptôme clinique ou histologique de dystrophie musculaire, et la protéine mutée est normalement exprimée et positionnée au niveau du sarcolemme.…”