Bull Spermatozoa DNA Methylation Landscape Displays Unique Features Revealed by Multi-Scale Analysis

Spermatogenesis involves structural modifications including epigenetic remodeling of the genome. It allows the sperm genome to be compacted and protected, a prerequisite to the transmission of paternal genetic heritage to the next generation. Therefore, any environmental changes that impact epigenetic remodeling during spermatogenesis may alter sperm function and male fertility. DNA methylation is instrumental in this epigenetic remodeling, and alterations of methylation profiles have been associated with male infertility. While bull semen is widely used in artificial insemination, the literature describing DNA methylation in bovine sperm is scarce. The aim was to characterize the sperm methylome in cattle.

The methods used were contrasted in terms of resolution (global rate vs base-level resolution) and genome coverage (genome-wide vs candidate regions). Global quantification by LUminometric Methylation Assay (LUMA) in six species revealed a species-specific behavior of DNA methylation in sperm. Methylation level was lower in bovine semen (~45%) than in other species (~70 to 80%), and this level was not affected by cryopreservation. Somatic cells showed similar level in all species. Breed-dependent variations were detected, suggesting genetic factors in the determination of methylation level. To map DNA methylation at a genome-wide scale, we used reduced representation bisulfite sequencing (RRBS) and Methylated-DNA Immunoprecipitation (MeDIP-chip). Differentially methylated regions between tissues (tDMRs) were identified when sperm, liver, fibroblasts and monocytes were compared. Some of these tDMRs were in the vicinity of genes enriched in important pathways for sperm physiology, and most of them were hypomethylated in sperm. Pyrosequencing validation of these tDMR is currently ongoing.