Ubiquitin-specific proteases 7 and 11 modulate Polycomb regulation of the INK4a tumour suppressor.

An important facet of transcriptional repression by Polycomb repressive complex 1 (PRC1) is the mono-ubiquitination of histone H2A by the combined action of the Posterior sex combs (Psc) and Sex combs extra (Sce) proteins. Here, we report that two ubiquitin-specific proteases, USP7 and USP11, co-purify with human PRC1-type complexes through direct interactions with the Psc orthologues MEL18 and BMI1, and with other PRC1 components. Ablation of either USP7 or USP11 in primary human fibroblasts results in de-repression of the INK4a tumour suppressor accompanied by loss of PRC1 binding at the locus and a senescence-like proliferative arrest. Mechanistically, USP7 and USP11 regulate the ubiquitination status of the Psc and Sce proteins themselves, thereby affecting their turnover and abundance. Our results point to a novel function for USPs in the regulation and function of Polycomb complexes.

Role for the MOV10 RNA helicase in polycomb-mediated repression of the INK4a tumor suppressor.

Several lines of evidence point to a role for noncoding RNA in transcriptional repression by Polycomb group (PcG) proteins, but the precise mechanism remains unclear. Here we show that human MOV10, a putative RNA helicase previously implicated in post-transcriptional gene silencing, co-purifies and interacts with components of Polycomb-repressive complex 1 (PRC1) from human cells. Endogenous human MOV10 is mostly nuclear, and a proportion associates with chromatin in an RNA-dependent manner. Small hairpin RNA (shRNA)-mediated knockdown of MOV10 in human fibroblasts leads to the upregulation of the INK4a tumor suppressor, a known target of PcG-mediated repression, accompanied by the dissociation of PRC1 proteins from the locus and a reduction in trimethylation of histone H3 on Lys27 (H3K27me3). As well as prompting reassessment of MOV10's role in other settings, our findings suggest that it is directly involved in transcriptional silencing by PcG complexes.

Several distinct polycomb complexes regulate and co-localize on the INK4a tumor suppressor locus.

Misexpression of Polycomb repressive complex 1 (PRC1) components in human cells profoundly influences the onset of cellular senescence by modulating transcription of the INK4a tumor suppressor gene. Using tandem affinity purification, we find that CBX7 and CBX8, two Polycomb (Pc) homologs that repress INK4a, both participate in PRC1-like complexes with at least two Posterior sex combs (Psc) proteins, MEL18 and BMI1. Each complex contains a single representative of the Pc and Psc families. In primary human fibroblasts, CBX7, CBX8, MEL18 and BMI1 are present at the INK4a locus and shRNA-mediated knockdown of any one of these components results in de-repression of INK4a and proliferative arrest. Sequential chromatin immunoprecipitation (ChIP) reveals that CBX7 and CBX8 bind simultaneously to the same region of chromatin and knockdown of one of the Pc or Psc proteins results in release of the other, suggesting that the binding of PRC1 complexes is interdependent. Our findings provide the first evidence that a single gene can be regulated by several distinct PRC1 complexes and raise important questions about their configuration and relative functions.