Maintenance of leukemic cell identity by the activity of the Polycomb complex PRC1 in mice

Leukemia is a complex heterogeneous disease often driven by the expression of oncogenic fusion proteins with different molecular and biochemical properties. Whereas several fusion proteins induce leukemogenesis by activating Hox gene expression (Hox-activating fusions), others impinge on different pathways that do not involve the activation of Hox genes (non-Hox-activating fusions). It has been postulated that one of the main oncogenic properties of the HOXA9 transcription factor is its ability to control the expression of the p16/p19 tumor suppressor locus (Cdkn2a), thereby compensating Polycomb-mediated repression, which is dispensable for leukemias induced by Hox-activating fusions. We show, by genetically depleting the H2A ubiquitin ligase subunits of the Polycomb repressive complex 1 (PRC1), Ring1a and Ring1b, that Hoxa9 activation cannot repress Cdkn2a expression in the absence of PRC1 and its dependent deposition of H2AK119 monoubiquitination (H2AK119Ub). This demonstrates the essential role of PRC1 activity in supporting the oncogenic potential of Hox-activating fusion proteins. By combining genetic tools with genome-wide location and transcription analyses, we further show that PRC1 activity is required for the leukemogenic potential of both Hox-activating and non-Hox-activating fusions, thus preventing the differentiation of leukemic cells independently of the expression of the Cdkn2a locus. Overall, our results genetically demonstrate that PRC1 activity and the deposition of H2AK119Ub are critical factors that maintain the undifferentiated identity of cancer cells, positively sustaining the progression of different types of leukemia.