Carcass composition, meat quality, leg muscle status, and its mRNA expression profile in broilers affected by valgus-varus deformity

Valgus-varus deformity (VVD) is a common leg disease in commercial broilers, which seri-ously affects animal welfare and causes economic losses. Up to now, most of the studies on VVD have been on skeleton, whereas there are fewer studies on VVD mus-cle. In this study, carcass composition and meat quality of 35-day-old normal and VVD Cobb broilers assess the effect of VVD on broiler growth. Molecular biology, morphology, and RNA sequencing (RNA-seq) were used to study the difference between normal and VVD gastrocnemius muscle. In comparison with the normal broilers, the breast muscle and leg muscle of the VVD broilers had lower shear force, notably lower crude pro-tein, lower water content, cooking loss, and deeper meat color (P < 0.05). The morphological results showed that the weight of skeletal muscle was significantly higher in the normal broilers than that in the VVD broilers (P < 0.01), the diameter and area of myofibrils in the affected VVD were smaller than in the normal broilers (P < 0.01). Quantitative real-time PCR (qPCR) of gastrocnemius muscle revealed that the expression of myasthenic marker genes, fast myofiber marker genes, and apoptosis-related factors were significantly higher in the VVD broilers than in the normal broilers (P < 0.01). In total, 736 differentially expressed genes (DEGs) were identified firstly in the normal and VVD leg muscle by RNA-seq. Gene ontology (GO) enrichment indicated that these DEGs were mainly involved in the multicellu-lar organismal process and anatomical structure devel-opment. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that DEGs are significantly enriched in proteasome. Protein interaction analysis obtained that DEGs with high interaction were protea-some-related coding genes and ubiquitin-related genes, these DEGs were closely associated with muscle atro-phy. These show that VVD has an adverse effect on growth characteristics, slaughter characteristics, and meat quality in broilers, which may cause leg muscle atrophy. This study provides some reference values and basis for studying the pathogenesis of VVD in broilers.