Abstract

Limb-girdle muscular dystrophy type 2D (LGMD2D) is caused by autosomal recessive mutations in the α-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 α-sarcoglycan. The floxed neomycin (Neo)-cassette retained at the targeted H77C α-sarcoglycan locus caused a loss of α-sarcoglycan expression, resulting in muscular dystrophy in homozygotes, whereas Cre-mediated deletion of the floxed Neo-cassette led to recovered H77C α-sarcoglycan expression. Contrary to expectations, mice homozygous for the H77C-encoding allele expressed both this mutant α-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 α-sarcoglycan-deficient mice prevented the disease. Adding to the case that the behavior of mutant α-sarcoglycan is different between humans and mice, mutant human R77C α-sarcoglycan restored the expression of the sarcoglycan–sarcospan complex when introduced by adenoviral vector into the skeletal muscle of previously created α-sarcoglycan null mice. These findings indicate that the α-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.

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