Silencing of meiotic genes in somatic cells by E2F6

Abstract

E2F6, a member of the E2F-family of transcription factors, is a retinoblastoma protein-independent transcriptional repressor. E2F6 associates with polycomb group (Pc-G) multiprotein complexes that contain histone H3 methyltransferases, suggesting that E2F6 represses genes by covalent histone modification. However, genes that are repressed by E2F6 via a mechanism that involves histone H3 methylation have not been identified. Using cDNA microarray experiments comparing wild-type and E2f6-/- mouse embryonic fibroblasts, we now found that E2F6 is required to silence the meiosis-specific genes SMC1β and STAG3 in somatic cells. Re-expression of E2F6 in E2f6-/- cells was sufficient to restore their repression. E2F6 binds in vivo to the promoters of these genes through a conserved binding site. Transcriptional repression of SMC1β and STAG3 by E2F6 involves multiple mechanisms, including methylation of histone H3 on lysine 9 and lysine 27. Our findings suggest a molecular mechanism for the stable transcriptional silencing of meiotic genes in somatic cells by E2F6. Previous SectionNext Section

E2F proteins are transcription factors that regulate genes with key roles in cell cycle progression, synthesis of nucleotides, DNA replication, DNA repair, and apoptosis (for a review, see Ref. 1). E2F is a family of eight proteins (E2F1 through E2F8) that can be divided into three major subgroups. E2F1, E2F2, and E2F3a are potent transcriptional activators. Their major role is to activate genes that are involved in proliferation and apoptosis. In contrast, E2F3b, E2F4, and E2F5 function primarily as active transcriptional repressors through binding to the retinoblastoma tumor suppressor protein and related pocket proteins. E2F6, E2F7, and E2F8 lack transactivation domains and are pocket protein-independent repressors (2–10).

A mechanism for transcriptional repression by E2F6 was suggested by the observation that E2F6 associates with polycomb proteins and histone methyltransferases (11–13). A contribution of E2F6 to the biological function of polycomb proteins in vivo is supported by the phenotype of mice with a targeted deletion of E2f6, which present with homeotic transformations of the axial skeleton that are very similar to those identified in polycomb-deficient mice (14). However, evidence that E2F6 represses genes in vivo through covalent histone modification is still missing. Indeed, in a recent study it was found that E2F6 represses a set of genes, including the BRCA1 tumor suppressor gene, by competing for activating E2Fs (15). No evidence for a role of histone H3 methylation in repression of these genes was found on these promoters. Here we report a novel function of E2F6 to stably repress certain meiotic genes in somatic cells through multiple mechanisms including methylation of histone H3 on lysine 9 and lysine 27