Trimethylation of histone H3 lysine 4 is an epigenetic mark at regions escaping mammalian X inactivation.

It is now estimated that 150-200 genes clustered in several discrete regions escape X inactivation in somatic cells of human females by unknown mechanisms. Here, we show that although the human female inactive X chromosome is largely devoid of histone 3 lysine 4 trimethylation (H3K4me3), regions that are known to escape X inactivation, including the pseudoautosomal regions, are enriched with this modification. Also, H3K4me3, unlike H3K4me2 and H4 and H3 acetylation, is restricted to discrete regions on metaphase chromosomes. In contrast to humans, there are only a few genes that are known to escape X inactivation in the mouse. Therefore, we examined mouse female somatic cells with H3K4me3 to identify candidate regions with genes that escape X inactivation. We found the mouse female inactive X in somatic cells and the male inactive X in meiosis to have seven discrete regions that are enriched with H3K4me3. Furthermore, RNA polymerase II is largely excluded from the XY body at male pachytene except for several discrete regions on the X and Y suggesting the presence of regions that also escape sex chromosome inactivation during male meiosis.