Several Distinct Polycomb Complexes Regulate and Co-Localize on the INK4a Tumor Suppressor Locus

Abstract

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.

Variable Requirements for DNA-Binding Proteins at Polycomb-Dependent Repressive Regions in Human HOX Clusters

“…RYBP-PRC1 complexes are able to associate with chromatin independently (39) and contain unique compositions that are associated with different PcG targets (9). It has also been demonstrated that different PRC1 complexes can be observed at the same locus, such as at the INK4A/ARF locus, although the functional significance of these different complexes coexisting at promoters has not been fully characterized (40).…”

Transcriptional regulation of cellular senescence

“…Conversely, knockdown of the polycomb components CBX7, CBX8, BMI1 or MEL18 results in INK4A derepression and proliferative arrest. (Jacobs et al, 1999;Itahana et al, 2003;Gil et al, 2004;Dietrich et al, 2007;Maertens et al, 2009). Interestingly, polycomb group proteins are also essential for stem cell self-renewal in both mice and humans (Lessard and Sauvageau, 2003;Molofsky et al, 2003;Park et al, 2003;Cales et al, 2008), and this is in part due to their ability to bind and repress the INK4A-ARF locus.…”

Genome-scale transcriptional analysis reveals key genes associated with the development of type II diabetes in mice

“…CBX8, part of the polycomb group repressive complex 1, belongs to the CBX protein family, which is homologous to the Drosophila polycomb protein (17) and regulates the proliferation of diploid human and mouse fibroblasts through direct binding to the INK4A-ARF locus (18). Furthermore, a previous study demonstrated that the ectopic expression of CBX8 leads to cellular immortalization (19).…”