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Mutation in myostatin (<i>MSTN</i>), a negative regulator of muscle growth in skeletal muscle, resulted in increased muscle mass in mammals and fishes. However, <i>MSTN</i> mutation in avian species has not been reported. The objective of this study was to generate <i>MSTN</i> mutation in quail and investigate the effect of <i>MSTN</i> mutation in avian muscle growth. Recently, a new targeted gene knockout approach for the avian species has been developed using an adenoviral CRISPR/Cas9 system. By injecting the recombinant adenovirus containing CRISPR/Cas9 into the quail blastoderm, potential germline chimeras were generated and offspring with three base-pair deletion in the targeted region of the <i>MSTN</i> gene was identified. This non-frameshift mutation in <i>MSTN</i> resulted in deletion of cysteine 42 in the MSTN propeptide region and homozygous mutant quail showed significantly increased body weight and muscle mass with muscle hyperplasia compared to heterozygous mutant and wild-type quail. In addition, decreased fat pad weight and increased heart weight were observed in <i>MSTN</i> mutant quail in an age- and sex-dependent manner, respectively. Taken together, these data indicate anti-myogenic function of <i>MSTN</i> in the avian species and the importance of cysteine 42 in regulating <i>MSTN</i> function.