Gene expression, X-inactivation, and methylation during spermatogenesis: the case of Zfa, Zfx, and Zfy in mice.

While it has become clear that X-inactivation in the female soma is complete in mouse (in contrast to being "patchy" in man), the degree of X-inactivation in the testes has not been ascertained. We have compared autosomal and X-linked zinc finger homolog expression and X-linked and Y-linked zinc finger homolog methylation in an attempt to elucidate this question. Using RTPCR, we have extended earlier studies of Zfx and Zfa expression in developing testes and find that Zfa expression starts at the time of X-inactivation while Zfx expression is continuous. Cell separation studies did not preclude continued expression of Zfx in adult germ cells. The methylation status of four CCGG residues in the Zfx promoter was studied using PCR bridging this region before and after DNA digestion with the isoschizomers Msp I and Hpa II, the latter being methylation sensitive. Hpa II resistant Zfx promoter DNA was found in all female tissues, but not in male tissues, including the testes. Previous studies have shown that Zfy is expressed at meiosis (like Zfa and unlike Zfx). Despite its expression, the Zfy gene is adjacent to, or contains, highly methylated CCGG sites since hybridization after Msp I digestion detected multiple small fragments that were not released after DNA digestion with Hpa II. Thus, Zfx is not methylated in sperm, while Zfy is, in contrast to their apparent patterns of expression.