438 DNA methylation patterns and transcriptional regulation during pig fetal skeletal muscle development

Fetal development is controlled by a complex cascade of highly regulated and coordinated gene expression patterns. Epigenetic mechanisms have important roles in regulating development and differentiation. Among such mechanisms, DNA methylation exhibits context-specific associations with gene expression and has been shown to be highly dynamic during developmental processes. We performed whole-genome bisulfite sequencing (WGBS) to assess DNA methylation in pig longissimus dorsi muscle at 41- and 70-d gestation (dg), as well as RNA- and small RNA-sequencing to identify coordinated changes in methylation and expression between myogenic stages. We identified 45,739 differentially methylated regions (DMRs) between stages, and the majority (n = 34,232) were hypomethylated at 70 vs. 41 dg. Developmental DMRs exhibited feature-specific enrichment in gene regulatory regions, as well as in regions proximal to micro-RNAs (miRNAs) that have known roles in myogenesis. Integration of methylation and transcriptomic data revealed strong associations between differential gene methylation and transcript abundance. We surveyed myogenic regulatory factor (MRF) genes to determine if differential methylation was present in expected genomic regions. Within the MYF5 and MYF6 locus, MYF5 was significantly promoter-hypermethylated at 70 dg, whereas MYF6 was significantly hypomethylated upstream of its transcription start site. MYF5 is the earliest MRF to be expressed and primarily functions in myoblast proliferation and determination, while MYF6 functions in muscle cell differentiation. Thus, these patterns were consistent with expected downregulation of MYF5 and upregulation of MYF6 as muscle development progresses and demonstrate that differential methylation is evident at myogenic transcription factors. Differential miRNA methylation was significantly negatively correlated with abundance, and dynamic expression of assayed miRNAs persisted postnatally. Motif analysis revealed significant enrichment of myogenic regulatory factor motifs among hypomethylated regions, suggesting that DNA hypomethylation may function to increase accessibility of muscle-specific transcription factors. We also show that developmental DMRs are enriched for GWAS SNPs associated with muscle physiology and meat quality traits, demonstrating the potential for epigenetic processes to influence phenotypic diversity. Our results enhance understanding of DNA methylation dynamics in pig fetal skeletal muscle and reveal putative cis-regulatory elements governed by epigenetic processes during porcine myogenesis.