Covalent modification of histones is fundamental in orchestrating chromatin dynamics and transcription. One example of such an epigenetic mark is the mono-ubiquitination of histones, which mainly occurs at histone H2A and H2B. Ubiquitination of histone H2A has been implicated in polycomb-mediated transcriptional silencing. However, the precise role of the ubiquitin mark during silencing is still elusive. Here we show in human cell lines that ZRF1 (zuotin-related factor 1) is specifically recruited to histone H2A when it is ubiquitinated at Lys 119 by means of a novel ubiquitin-interacting domain that is located in the evolutionarily conserved zuotin domain. At the onset of differentiation, ZRF1 specifically displaces polycomb-repressive complex 1 (PRC1) from chromatin and facilitates transcriptional activation. A genome-wide mapping of ZRF1, RING1B and H2A-ubiquitin targets revealed its involvement in the regulation of a large set of polycomb target genes, emphasizing the key role ZRF1 has in cell fate decisions. We provide here a model of the molecular mechanism of switching polycomb-repressed genes to an active state.

The reactivation of the INK4-ARF locus, which is epigenetically repressed by Polycomb proteins in healthy cells, is a hallmark of senescence. One mechanism of reactivating Polycomb-silenced genes is mediated by the epigenetic factor ZRF1, which associates with ubiquitinated histone H2A. We show that cells undergoing senescence following oncogenic Ras expression have increased ZRF1 levels, and that this binds to the p15INK4b, ARF and p16INK4a promoters. Furthermore, ZRF1 depletion in oncogenic Rasexpressing cells restores proliferation by preventing Arf and p16Ink4a expression, consequently bypassing senescence. Thus, ZRF1 regulates the INK4-ARF locus during cellular proliferation and senescence, and alterations in ZRF1 may contribute to tumorigenesis.

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