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Molecular and Cellular Biology, August 2005, p. 6694-6706, Vol. 25, No. 15
0270-7306/05/$08.00+0     doi:10.1128/MCB.25.15.6694-6706.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Mammalian Polyhomeotic Homologues Phc2 and Phc1 Act in Synergy To Mediate Polycomb Repression of Hox Genes

Kyo-ichi Isono,1,2 Yu-ichi Fujimura,1,2 Jun Shinga,1,2 Makoto Yamaki,2 Jiyang O-Wang,1 Yoshihiro Takihara,3 Yasuaki Murahashi,1,2 Yuki Takada,1,2 Yoko Mizutani-Koseki,1,2 and Haruhiko Koseki1,2*

RIKEN Research Center for Allergy and Immunology, 1-7-22 Suehiro, Tsurumi-ku, Yokohama 230-0045, Japan,1 Department of Molecular Embryology, Graduate School of Medicine, Chiba University, 1-8-1 Inohana, Chuo-ku, China 260-8670, Japan,2 Department of Stem Cell Biology, Research Institute for Radiation Biology and Medicine, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8553, Japan3

Received 2 December 2004/ Returned for modification 27 January 2005/ Accepted 4 May 2005

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.

* Corresponding author. Mailing address: RIKEN Research Center for Allergy and Immunology, 1-7-22 Suehiro, Tsurumi-ku, Yokohama 230-0045, Japan. Phone: 81-45-503-9689. Fax: 81-45-503-9688. E-mail: koseki{at}rcai.riken.jp.

Molecular and Cellular Biology, August 2005, p. 6694-6706, Vol. 25, No. 15
0022-538X/05/$08.00+0     doi:10.1128/MCB.25.15.6694-6706.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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