Mammalian polyhomeotic homologues Phc2 and Phc1 act in synergy to mediate polycomb repression of Hox genes.

The Polycomb group (PcG) gene products form multimeric protein complexes and contribute to anterior-posterior (A-P) specification via the transcriptional regulation of Hox cluster genes. The Drosophila polyhomeotic genes and their mammalian orthologues, Phc1, Phc2, and Phc3, encode nuclear proteins that are constituents of evolutionarily conserved protein complexes designated class II PcG complexes. In this study, we describe the generation and phenotypes of Phc2-deficient mice. We show posterior transformations of the axial skeleton and premature senescence of mouse embryonic fibroblasts associated with derepression of Hox cluster genes and Cdkn2a genes, respectively. Synergistic actions of a Phc2 mutation with Phc1 and Rnf110 mutations during A-P specification, coimmunoprecipitation of their products from embryonic extracts, and chromatin immunoprecipitation by anti-Phc2 monoclonal antibodies suggest that Hox repression by Phc2 is mediated through the class II PcG complexes, probably via direct binding to the Hox locus. The genetic interactions further reveal the functional overlap between Phc2 and Phc1 and a strict dose-dependent requirement during A-P specification and embryonic survival. Functional redundancy between Phc2 and Phc1 leads us to hypothesize that the overall level of polyhomeotic orthologues in nuclei is a parameter that is critical in enabling the class II PcG complexes to exert their molecular functions.

The mouse Edr2 (Mph2) gene has two forms of mRNA encoding 90- and 36-kDa polypeptides.

The vertebrate Polycomb Group (PcG) genes encode proteins that form large multimeric and chromatin-associated complexes implicated in the stable repression of developmentally essential genes. Here we have isolated a 2.5-kb cDNA for Edr2, a mouse homolog of the Drosophila PcG gene Ph, although it was originally identified as a 3.8-kb cDNA. However, little is known about molecular basis of the 3.8-kb cDNA. Genomic and RNA analyses have shown that Edr2 locates on Chromosome 4 as a single copy gene and is transcribed into at least two transcript isoforms about 3.0 and 4.4 kb in length, most likely corresponding to the 2.5- and 3.8-kb cDNAs, respectively. The largest open reading frames in the 2.5- and 3.8-kb cDNAs encode 36- and 90-kDa polypeptides, respectively. The 36-kDa protein is a truncated form lacking of the N-terminal region of the 90-kDa protein. Interestingly, it has been demonstrated that the 3.0-kb mRNA accumulates at a much higher level than the 3.8-kb mRNA in mouse embryos and mature tissues. Immunostaining assay of mammalian cells has shown that the 36-kDa form tagged with HA colocalizes with the other PcG protein Mel18 in nuclei, suggesting that the smaller protein is capable of forming maltimeric complex with other PcG proteins. Therefore, the 36-kDa protein might function generally as a PcG protein.