Abstract. The Gsg2 (Haspin) gene encodes a serine/threonine protein kinase and is predominantly expressed in haploid germ cells. In proliferating somatic cells, Gsg2 is shown to be expressed weakly but plays an essential role in mitosis. Although the Gsg2 minimal promoter recognized by the spermatogenic cell-specific nuclear factor(s) has been found, to date, the molecular mechanism that differentially controls Gsg2 expression levels in germ and somatic cells remains to be sufficiently clarified. In this study, we analyzed the DNA methylation status of the upstream region containing the Gsg2 promoter. We found a tissue-dependent and differentially methylated region (T-DMR) upstream (-641 to -517) of the authentic promoter that is hypomethylated in germ cells but hypermethylated in other somatic tissues. Profiling of Gsg2 expression and DNA methylation status at the T-DMR in spermatogenic cells indicated that the hypomethylation of the T-DMR is maintained during spermatogenesis. Using the reporter assay, we also demonstrated that DNA methylation at the T-DMR of Gsg2 reduced the promoter activity by 60-80%, but did not fully suppress it. Therefore, the T-DMR functions as a modulator in a DNA methylation-dependent manner. In conclusion, Gsg2 is under epigenetic control.

The oocyte-specific subtype of the linker histone H1 is H1FOO, which constitutes a major part of oocyte chromatin. H1foo is expressed in growing oocytes, through fertilization, up until the two-cell embryo stage, when it is subsequently replaced by somatic H1 subtypes. To elucidate whether an epigenetic mechanism is involved in the limited expression of H1foo, we analyzed the dynamics of the DNA methylation status of the H1foo locus in germ and somatic cells. We identified a tissue-dependent and differentially methylated region (T-DMR) upstream of the H1foo gene, which was hypermethylated in sperm, somatic cells, and stem cell lines. This region was specifically unmethylated in the ovulated oocyte, where H1foo is expressed. 5-Aza-2'-deoxycytidine treatments and luciferase assays provided in vitro evidence that DNA methylation plays a role in repressing H1foo in nonexpressing cells. DNA methylation analyses of fetal germ cells revealed the T-DMR to be hypomethylated in female and male germ cells at Embryonic Day 9.5 (E9.5), whereas it was highly methylated in somatic cells at this stage. Intriguingly, the unmethylated status was continuously observed throughout oogenesis at E9.5, E12.5, E15.5, E18.5, in mature oocytes, and after fertilization, in E3.5 blastocysts. In comparison, male germ cells acquired methylation beyond E18.5. These data demonstrate a continuously unmethylated circuit at the H1foo locus in the female germline.