Pig H3K4me3, H3K27ac and gene expression profiles reveal reproductive tissue-specific activity of transposable elements

Regulatory sequences and transposable elements (TEs) account for a large proportion of the genome sequences of species, while their roles in gene transcription, especially tissue-specific expression, remain largely unknown. Pigs are an excellent animal models for studying the biology of the genome sequences owing to their great diversity of wild and domesticated populations. Here, we integrated H3K27ac ChIP-seq, H3K4me3 ChIP-seq and RNA-seq data from 10 tissues of the same 7 fetuses and their consanguineously related 8 adult pigs to annotate the regulatory elements and TEs for their links with histone modifications and mRNA expression across varying tissues and development stages. The association analyses of mRNA expression with H3K27ac and H3K4me3 peak activity revealed that H3K27ac peaks showed stronger associations with gene expression than H3K4me3. We revealed that 1.45% of the TEs overlapped with H3K27ac or H3K4me3 peaks, of which the majority displayed tissue-specific activity. We identified a TE subfamily (LTR4C_SS) with binding motifs for SIX1 and SIX4 that showed specific enrichment in adult and fetal ovary H3K27ac peaks. We also revealed widespread expression of TEs as part of exons or promoters of genes from the RNA-seq data, including 4688 TE-containing transcripts that displayed development stage and tissue-specific expression. Notably, 1967 TE-containing transcripts were enriched in the testes. We highlighted that an LTR acting as a testis-specific alternative promoter in SRPK2 (a cell cycle-related protein kinase) in our pig data, MLT1F1, was also conserved in humans and mice, suggesting an ancient embedding of the TEs in testis-specific expressed genes or parallel evolution. Collectively, our work demonstrates that TEs are deeply embedded in the genome and exhibit important tissue-specific biological functions, particularly in the reproductive organs.