DNA methylation imprinting marks and DNA methyltransferase expression in human spermatogenic cell stages

Paternal imprinting marks were shown to be erased in the mouse primordial germ cells and progressively re-established throughout the male germ line development, starting in fetal prospermatogonia and continuing post-natally through the onset of meiosis. We here evaluated imprinting marks in human adult spermatogenic cells and analyzed mRNA and protein expression of DNA Methyltransferases (DNMTs). Spermatogonia A, primary and secondary spermatocytes, round spermatids and elongated spermatids/spermatozoa were isolated by micromanipulation from testicular biopsies of men with normal spermatogenesis. DNA methylation at two imprinted genes, H19 and MEST/PEG1, was analyzed using bisulphite genomic sequencing and DNMTs expression was determined by qRT-PCR and immunofluorescence. H19 was completely methylated at the spermatogonia stage in the analyzed individuals and MEST/PEG1 was completely demethylated, with the exception of few CpGs. The analysis of DNMT1, DNMT3A and 3B expression showed peaks of mRNA transcripts in primary spermatocytes and in mature ejaculated spermatozoa, with DNMT1 transcript level being the most abundant in all cell stages. Immunolocalization showed that DNMT proteins are present throughout the spermatogenic cycle, with stage-specific shuttling between the nucleus and cytoplasm. We conclude that, in humans, methylation imprints are established in spermatogonia A and are maintained in subsequent stages up to elongated spermatid/spermatozoa. Additionally, DNA methyltransferases are expressed throughout human spermatogenesis, possibly maintaining the methylation patterns in order to avoid the transmission of imprinting errors by the male gamete.