Testis-specific histone H1t gene is hypermethylated in nongerminal cells in the mouse

The testis-specific histone Hit gene is expressed only in pachytene primary spermatocytes during spermatogenesis. There is a correlation between the s pecific binding of testis nuclear proteins to a rat histone Hit promoter se quence, designated the H1t/TE element, and the onset of transcription in pr imary spermatocytes, Our laboratory has shown that mice bearing the rat gen e with a deletion of the TE promoter element and replacement with a heterol ogous stuffer DNA fragment fail to express the rat Hit transgene in any tis sue. In this study we report that five CpGs located within the Hit proximal promoter, including two CpGs located within the essential TE promoter elem ent, contain unmethylated cytosines in vivo in genomic DNA derived from pri mary spermatocytes where the tilt gene is expressed. All seven CpGs are hyp ermethylated in vivo in genomic DNA derived from liver cells where gene exp ression is repressed. Further, in vitro methylation of an Hit promoter-driv en reporter plasmid markedly reduced expression in a transient transfection assay system. These results suggest that cytosine methylation may contribu te to the transcriptional silencing of the testis-specific histone H1t gene in nonexpressing tissues such as liver.