Dynamic expression of DNMT3a and DNMT3b isoforms during male germ cell development in the mouse

In the male germ line, sequence-specific methylation patterns are initially acquired prenatally in diploid gonocytes and are further consolidated after birth during spermatogenesis. It is still unclear how DNA methyltransferases are involved in establishing and/or maintaining these patterns in germ cells, or how their activity is regulated. We compared the temporal expression patterns of the postulated de novo DNA methyltransferases DNMT3a and DNMT3b in murine male germ cells. Mitotic, meiotic and post-meiotic male germ cells were isolated, and expression of various transcript variants and isoforms of Dnmt3a and Dnmt3b was examined using Quantitative RT-PCR and Western blotting. We found that proliferating and differentiating male germ cells were marked by distinctive expression profiles. Dnmt3a2 and Dnmt3b transcripts were at their highest levels in type A spermatogonia, decreased dramatically in type B spermatogonia and preleptotene spermatocytes and rose again in leptotene/zygotene spermatocytes, while Dnmt3a expression was mostly constant, except in type B spermatogonia where it increased. In all cases, expression declined as pachynema progressed. At the protein level, DNMT3a was the predominant isoform in type B spermatogonia, while DNMT3a2, DNMT3b2, and DNMT3b3 were expressed throughout most of spermatogenesis, except in pachytene spermatocytes. We also detected DNMT3a2 and DNMT3b2 in round spermatids. Taken together, these data highlight the tightly regulated expression of these genes during spermatogenesis and provide evidence that DNMTs may be contributing differentially to the establishment and/or maintenance of methylation patterns in male germ cells.