Emerging role of noncanonical polycomb repressive complexes in normal and malignant hematopoiesis.

Polycomb group (PcG) proteins are the key epigenetic regulators of normal hematopoiesis and the dysregulation of their functions is closely involved in the pathogenesis of hematological malignancies. These proteins function in the multimeric complexes called polycomb repressive complex (PRC) 1 and 2. In addition to canonical PRC1, four noncanonical PRC1 complexes have been identified. In contrast to canonical PRC1, which is recruited to its target sites in a manner dependent on H3K27me3, noncanonical PRC1 complexes are recruited to their target sites independently of H3K27me3. Among them, PRC1.1, consisting of PCGF1, RING1A/B, KDM2B, and BCL6 corepressor (BCOR) or BCLRL1, regulates diverse biological processes, including pluripotency, reprogramming, and hematopoiesis. PRC1.1 has been implicated in myelopoiesis and lymphopoiesis and is targeted by somatic gene mutations in various hematological malignancies. These findings revealed the more complex regulation of epigenetic cellular memory by PcG proteins than we expected and propose PRC1.1 as a novel therapeutic target in hematological malignancies.