Abstract Mutations in the well-known Myostatin ( MSTN ) produce a “double-muscle” phenotype, which makes it commercially invaluable for improving livestock meat production and providing high-quality protein for humans. However, mutations at different loci of the MSTN often produce a variety of different phenotypes. In the current study, we increased the delivery ratio of Cas9 mRNA to sgRNA from the traditional 1:2 to 1:10, which improves the efficiency of the homozygous mutation of biallelic gene. Here, a MSTN Del273C mutation with FGF5 knockout sheep, in which the MSTN and FGF5 dual-gene biallelic homozygous mutations were produced via the deletion of 3-base pairs of AGC in the third exon of MSTN , resulting in cysteine-depleted at amino acid position 273, and the FGF5 double allele mutation led to inactivation of FGF5 gene. The MSTN Del273C mutation with FGF5 knockout sheep highlights a dominant “double-muscle” phenotype, which can be stably inherited. Both F0 and F1 generation mutants highlight the excellent trait of high-yield meat with a smaller cross-sectional area and higher number of muscle fibers per unit area. Mechanistically, the MSTN Del273C mutation with FGF5 knockout mediated the activation of FOSL1 via the MEK-ERK-FOSL1 axis. The activated FOSL1 promotes skeletal muscle satellite cell proliferation and inhibits myogenic differentiation by inhibiting the transcription of MyoD1, and resulting in smaller myotubes.

AMSTNDel273Cmutation withFGF5knockout sheep by CRISPR/Cas9 promotes skeletal muscle myofiber hyperplasia via MEK-ERK-FOSL1 axis