MacroH2A histones are variants of canonical histone H2A that are conserved among vertebrates. Previous studies have implicated macroH2As in epigenetic gene-silencing events including X chromosome inactivation. Here we show that macroH2A is present in developing and mature mouse oocytes. MacroH2A is localized to chromatin of germinal vesicles (GV) in both late growth stage (lg-GV) and fully grown (fg-GV) stage oocytes. In addition, macroH2A is associated with the chromosomes of mature oocytes, and abundant macroH2A is present in the first polar body. However, maternal macroH2A is lost from zygotes generated by normal fertilization by the late 2 pronuclei (2PN) stage. Normal embryos at 2-, 4-, and 8-cell stages lack macroH2A except in residual polar bodies. MacroH2A protein expression reappears in embryos after the 8-cell stage and persists in morulae and blastocysts, where nuclear macroH2A is present in both the trophectodermal and inner cell mass cells. We followed the loss of macroH2A from pronuclei in parthenogenetic embryos generated by oocyte activation. Abundant macroH2A is present upon the metaphase II plate and persists through parthenogenetic anaphase, but macroH2A is progressively lost during pronuclear decondensation prior to synkaryogamy. Examination of embryos generated by intracytoplasmic sperm injection (ICSI) revealed that macroH2A is associated exclusively with female pronuclei prior to loss in late pronucleus stage embryos. These results outline a surprising finding that a maternal store of macroH2A is removed from the maternal genome prior to synkaryogamy, resulting in embryos that execute three to four mitotic divisions in the absence of macroH2A prior to the onset of embryonic macroH2A expression.

In mammalian heterochromatin, cytosine bases of CpG dinucleotides are symmetrically modified by methylation. Patterns of CpG methylation are maintained by the action of Dnmt1, the mammalian maintenance cytosine methyltransferase enzyme. We genetically manipulated the levels of CpG methylation and found that extensive chromatin alterations occur in pericentric heterochromatin. Homozygous mutations in Dnmt1 cause severe hypomethylation of pericentric heterochromatin and concomitant chromatin reorganization involving the histone variant macroH2A. Demethylation-induced alterations in macroH2A localization occur in both interphase and mitotic embryonic stem (ES) cells. Heterochromatin protein 1 (HP1) marks interphase pericentric heterochromatin (chromocenters). MacroH2A immunostaining in Dnmt1–/– cells becomes coincident with chromocenters detected by HP1 content. MacroH2A, but not HP1, is enriched in nuclease-resistant chromatin fractions extracted from Dnmt1–/– cells. Normal localization of macroH2A was restored upon reintroduction of a Dnmt1 transgene into Dnmt1–/– cells. MacroH2A localization was also affected in T-antigen-transformed fibroblasts subjected to the conditional mutation of Dnmt1. Together, these results suggest that pericentric heterochromatin can be maintained in the absence of CpG methylation, but in a significantly altered configuration.