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Emerging role of noncanonical polycomb repressive complexes in normal and malignant hematopoiesis
Experimental Hematology 2018;68:10−14
PERSPECTIVE
Emerging role of noncanonical polycomb repressive complexes in normal and malignant hematopoiesis Yusuke Isshikia,b, and Atsushi Iwamaa,c a Division of Stem Cell and Molecular Medicine, Center for Stem Cell Biology and Regenerative Medicine, Institute of Medical Science, University of Tokyo, Tokyo, Japan; bDepartment of Hematology, Chiba University Hospital, Chiba, Japan; cDepartment of Cellular and Molecular Medicine, Graduate School of Medicine, Chiba University, Chiba, Japan
(Received 19 September 2018; revised 17 October 2018; accepted 19 October 2018)
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. © 2018 ISEH – Society for Hematology and Stem Cells. Published by Elsevier Inc. All rights reserved.
Polycomb group (PcG) proteins comprise the multiprotein complexes called polycomb repressive complexes (PRCs), which play an important role in the transcriptional repression of target genes through histone modifications. Two major complexes have been extensively characterized: PRC1 and PRC2, which add monoubiquitination at lysine 119 of histone H2A (H2AK119ub1) and mono-, di-, and trimethylation at lysine 27 of histone H3 (H3K27me1/ me2/me3), respectively [1] (Figure 1). PRC2 is recruited to nonmethylated CpG islands (CGIs) through a not yet fully understood mechanism. Canonical PRC1 is then recruited through the recognition of H3K27me3 by CBX subunits [2]. Conversely, recent studies revealed an
Offprint requests to: Atsushi Iwama, M.D., Ph.D., Division of Stem Cell and Molecular Medicine, Center for Stem Cell Biology and Regenerative Medicine, Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo, 108-8639 Japan; E-mail: [email protected]
alternative pathway by which de novo PRC1 binding to target sites and H2AK119ub1 results in the subsequent recruitment of PRC2 and deposition of H3K27me3 [3] (Figure 2). The PRC1 working in this alternative pathway is called “noncanonical or variant” PRC1 and its functions remain largely unknown. In this perspective, we summarize emerging knowledge on the functions of noncanonical PRC1 in normal and malignant hematopoiesis. Role of polycomb group ring finger proteins in noncanonical PRC1 in embryonic stem cells PRC1 complexes are divided into subgroups (PRC1.1 to PRC1.6) according to the subtype of the polycomb group ring finger (PCGF) subunits (PCGF1-6) (Figure 1). PCGF proteins heterodimerize with the H2AK119 ubiquitin ligases RING1A or RING1B, which are responsible for the core enzymatic activity of PRC1 [4]. PCGF2/MEL18 and PCGF4/BMI1 act as components of canonical PRC1 and the others (Pcgf1/3/5/6) as components of
0301-472X/© 2018 ISEH – Society for Hematology and Stem Cells. Published by Elsevier Inc. All rights reserved. https://doi.org/10.1016/j.exphem.2018.10.008
Y. Isshiki and A. Iwama / Experimental Hematology 2018;68:10−14
Canonical PRC1 PRC2 EED SUZ12 EZH2/1 1 RBBP4/7
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PRC1.6 PCGF6 F6 6 L3MBTL2 RING1A/B 1A 1A/B
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Figure 1. Canonical and noncanonical PRC1. PRC1 complexes are divided into subgroups (PRC1.1 to PRC1.6) according to the subtype of the PCGF subunits (PCGF1−6).
noncanonical PRC1. PCGF proteins in noncanonical PRC1 have been characterized for its role in the maintenance of stemness (Pcgf1 and Pcgf6) [5−8] and differentiation (Pcgf5 and Pcgf6) [9,10] of embryonic stem cells (ESCs). Among noncanonical PRC1, PRC1.1 consists of PCGF1, RING1A or RING1B, KDM2B, BCL6 corepressor (BCOR) or BCLRL1, RYBP/YAF2, SKP1, and USP7. KDM2B binds to nonmethylated CGIs through its zinc finger-CxxC (ZF-CxxC) DNA-binding domain, thereby recruiting other components of PRC1.1 [11−13] (Figure 2). The depletion of Kdm2b in mouse ESCs induces the derepression of lineage-specific genes and early differentiation [13]. BCOR is also essential for maintaining primed pluripotency in human ESCs [14]. In addition, KDM2B promotes Oct4-induced somatic reprogramming through recruitment of PRC1.1 to CGIs, indicating that PRC1.1 participates in the establishment of pluripotency [15]. Although KDM2B has the Jumonji C domain, its histone demethylase function is controversial. Function of noncanonical PRC1 in hematopoiesis PCGF4/BMI1-containing canonical PRC1 has been characterized for its role in the maintenance of selfrenewal capacity and multipotency of hematopoietic stem cells (HSCs). It transcriptionally represses the cyclin-dependent kinase inhibitor 2A (CDKN2A) locus to maintain the self-renewal capacity of HSCs and developmental regulator genes to maintain the immature state and multipotency of hematopoietic stem and progenitor cells (HSPCs) [16,17]. In contrast, the roles of noncanonical PRC1 have remained largely unknown. In knockdown experiments using mouse hematopoietic progenitor cells, PCGF1 was initially found to restrict the proliferative capacity of myeloid progenitor cells by downregulating Hoxa family genes (Hoxa7, Hoxa9, and Hoxa10) [18]. Mice lacking Bcor exons 9 and 10 (BcorDE9-10/y mice), which express a carboxyl-
terminal truncated BCOR that fails to interact with PCGF1, showed higher proliferation and differentiation rates of myeloid cells in cultures [19]. We also confirmed myeloid-biased hematopoiesis in BcorDE9-10/y mice [20]. Transcriptional profiling revealed the derepression of myeloid regulator genes such as Cebp family and Hoxa cluster genes in the BcorDE9-10/y progenitor cells [20]. Collectively, these findings indicate that PRC1.1 restricts myeloid proliferation and differentiation by transcriptionally repressing myeloid regulator genes such as the Cebp family and Hoxa cluster genes (Figure 3). Kdm2b-deficient mice have significantly decreased numbers of HSPCs [21]. Kdm2b-deficient HSCs have impaired repopulation of hematopoiesis, particularly the repopulation of lymphoid cells; however, it currently remains unclear whether the deletion of Kdm2b impairs the self-renewal capacity of HSCs similar to the deletion of Pcgf4/Bmi1 [21]. In contrast, the forced expression of Kdm2b prevents the exhaustion of the long-term repopulating potential of HSCs following serial transplantation by negatively regulating the expression of CDKN genes such as Ink4a [22]. However, Pcgf5 appeared to be dispensable for hematopoiesis [23]. Function of noncanonical PRC1 in hematological malignancies Among the noncanonical PRC1 genes, BCOR and its homolog BCORL1 are frequently mutated in patients with various hematological malignancies, whereas mutations in other genes are very rare. Somatic mutations in the BCOR gene have been reported in acute myeloid leukemia (AML) with the normal karyotype (3.8%) [24], secondary AML (8%) [25], myelodysplastic syndrome (MDS) (4.2%) [26], chronic myelomonocytic leukemia (7.4%) [25], extranodal natural killer/T-
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Canonical PRC1 PCGF PHCs 2/4 RING1A/B RIING1A/B RIN 1A
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BcorDE9-10/y MDS cells reproduced MDS or evolved into lethal MDS/myeloproliferative neoplasms in secondary recipients. These findings indicate that BCOR mutations promote the initiation and progression of MDS in concert with concurrent driver mutations [20]. Transcriptional profiling revealed the derepression of myeloid regulator genes such as Cebp family and Hoxa cluster genes in Tet2D/DBcorDE9-10 progenitor cells [20]. BCOR loss-of-function mutations are associated with the gene signature of enhanced myeloid cell differentiation and derepression of PRC1 target genes, including HOXA9 in bone marrow cells from MDS and AML patients [19]. The weaker expression of KDM2B in leukemic cells was related to the poor prognosis of patients with AML, whereas the ablation of Kdm2b accelerated KrasG12D-driven myeloid transformation in mice [21]. These findings support the tumor suppressor function of PRC1.1 in myeloid malignancies. BCOR also plays an important role in restricting the transformation of T lymphocytes. Two different Bcor mutant mice, BcorDExon4/y and BcorDExon9-10/y, which lack the BCL6-binding domain and PCGF1-binding domain, respectively, showed a strong propensity to develop T-ALL, mostly in a Notch-dependent manner [20,36]. Chromatin immunoprecipitation sequencing analysis revealed that BCOR targeted many of the NOTCH1 targets, including Myc, and potentially antagonized their transcriptional activation [36]. The loss of BCOR was also reported to promote B-cell lymphomagenesis in Em-Myc mice [37]. In contrast, previous studies revealed the oncogenic activity of noncanonical PRC1. BCOR is known as a corepressor of BCL6 and contributes to the formation of the noncanonical PRC1/BCOR complex containing CBX8 in germinal center B cells, thereby mediating the cooperation between BCL6 and EZH2-containing PRC2. The BCL6noncanonical PRC1/BCOR-PRC2 interaction led to the silencing of B-cell differentiation and cell cycle checkpoint genes to permit immunoglobulin affinity maturation and excessive gene silencing caused malignant transformation [38]. PRC1.1 component genes are significantly overexpressed in primary AML CD34+ cells and the downmodulation of these components strongly reduced cell proliferation in vitro and delayed or abrogated leukemogenesis in humanized xenograft mouse models. PRC1.1 targeted actively transcribed genes involved in metabolism independently of PRC2, which was required for leukemic cell viability [39]. The lentivirus-mediated overexpression of Kdm2b was sufficient to transform hematopoietic progenitors. In contrast, the depletion of Kdm2b in hematopoietic progenitors significantly impaired Hoxa9/Meis1induced leukemic transformation [40]. Transgenic mice overexpressing Kdm2b developed myeloid or B-lymphoid leukemia through the metabolic activation and upregulation of Nsg2 in mice [41]. KDM2B also promotes D
A
PRC2 EZH2/1 SUZ12 Z12 2 EED
PRC1.1
? RING1A/B PCGF1 CGF1 GF
BCOR B CO
KDM2B KDM2 M2 2
non-methylated CGI Figure 2. Two pathways that target PcG complexes to target sites. (A) Classical hierarchical pathway. PRC2, which catalyzes H3K27me3 modifications, is recruited to nonmethylated CGIs through a not yet fully understood mechanism. Canonical PRC1 is then recruited via the CBX subunits that bind to H3K27me3. (B) Alternative pathway. Noncanonical PRC1 complexes bind to target sites and deposit H2AK119ub1, which results in the subsequent recruitment of PRC2 and deposition of H3K27me3. In the case of PRC1.1, KDM2B binds to nonmethylated CGIs through its DNAbinding domain, thereby recruiting other components of PRC1.1.
cell lymphoma (21−32%) [27,28], chronic lymphocytic leukemia (2.2%) [29], T-cell prolymphocytic leukemia (5−8%) [30,31], and acute T-lymphoblastic leukemia (T-ALL) (2−3%) [32,33]. Most of these mutations results in stop codon gains, frame-shift insertions or deletions, splicing errors, and gene loss, leading to the loss of BCOR function [26]. BCORL1 has also been implicated in AML and MDS [26,34]. These findings indicate that BCOR and BCORL1 function as tumor suppressors in various human hematological malignancies. Somatic mutations in BCOR are frequently associated with TET2 mutations in MDS [35]. Based on these findings, we recently generated mice with the combined deletion of Bcor and Tet2 (Tet2D/DBcorDE9-10/y) and demonstrated that they developed lethal MDS. Tet2D/
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Figure 3. Role of PRC1.1 in hematopoiesis. PRC1.1 restricts myeloid proliferation and differentiation by transcriptionally repressing myeloid regulator genes such as the Cebp family and Hoxa cluster genes. It also targets many of the NOTCH1 targets, including Myc in thymocytes, and potentially antagonizes their transcriptional activation, thereby restricting the transformation of T lymphocytes.
nonhematopoietic tumor development such as pancreatic cancer via polycomb-dependent and -independent transcriptional programs [42]. Future perspectives The functions of noncanonical PRC1 are now being clarified. As is the case with PRC2, noncanonical PRC1 appears to have complex functions that are celltype dependent and act in both an oncogenic and tumor-suppressive manner depending on the tumor type. In addition, PRC1.1 appeared to target a variety of active genes involved in metabolism independently of H3K27me3 [39,42], although the underlying molecular mechanisms remain unclear. Moreover, a number of issues have yet to be clarified: 1. redundancy between canonical and noncanonical PRC1 complexes and among noncanonical PRC1 complexes, 2. their role in the self-renewal of HSCs, 3. the reason why only BCOR and BCORL1 are targeted by somatic gene mutations, and 4. the druggability of noncanonical PRC1 components for the manipulation of hematopoiesis or cancer treatments. Because BCOR and BCORL1 are the main targets of somatic mutations, screening for synthetically lethal partners of BCOR and BCORL1 mutations will also be of great interest as pharmacological targets. Further investigations will provide a new
stage for the polycomb-based epigenetics of normal and malignant hematopoiesis. Acknowledgments This work was supported in part by Grants-in-Aid for Scientific Research (15H02544) and Scientific Research on Innovative Areas “Stem Cell Aging and Disease” (26115002) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Conflict of interest disclosure The authors declare no competing financial interests. References 1. Blackledge NP, Rose NR, Klose RJ. Targeting Polycomb systems to regulate gene expression: modifications to a complex story. Nat Rev Mol Cell Biol. 2015;16:643–649. 2. Wang L, Brown JL, Cao R, Zhang Y, Kassis JA, Jones RS. Hierarchical recruitment of polycomb group silencing complexes. Mol Cell. 2004;14:637–646. 3. Blackledge NP, Farcas AM, Kondo T, et al. Variant PRC1 complex-dependent H2A ubiquitylation drives PRC2 recruitment and polycomb domain formation. Cell. 2014;157:1445–1459. 4. Gao Z, Zhang J, Bonasio R, et al. PCGF homologs, CBX proteins, and RYBP define functionally distinct PRC1 family complexes. Mol Cell. 2012;45:344–356. 5. Zhao W, Tong H, Huang Y, et al. Essential role for polycomb group protein Pcgf6 in embryonic stem cell maintenance and a
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PERSPECTIVE
Emerging role of noncanonical polycomb repressive complexes in normal and malignant hematopoiesis Yusuke Isshikia,b, and Atsushi Iwamaa,c a Division of Stem Cell and Molecular Medicine, Center for Stem Cell Biology and Regenerative Medicine, Institute of Medical Science, University of Tokyo, Tokyo, Japan; bDepartment of Hematology, Chiba University Hospital, Chiba, Japan; cDepartment of Cellular and Molecular Medicine, Graduate School of Medicine, Chiba University, Chiba, Japan
(Received 19 September 2018; revised 17 October 2018; accepted 19 October 2018)
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. © 2018 ISEH – Society for Hematology and Stem Cells. Published by Elsevier Inc. All rights reserved.
Polycomb group (PcG) proteins comprise the multiprotein complexes called polycomb repressive complexes (PRCs), which play an important role in the transcriptional repression of target genes through histone modifications. Two major complexes have been extensively characterized: PRC1 and PRC2, which add monoubiquitination at lysine 119 of histone H2A (H2AK119ub1) and mono-, di-, and trimethylation at lysine 27 of histone H3 (H3K27me1/ me2/me3), respectively [1] (Figure 1). PRC2 is recruited to nonmethylated CpG islands (CGIs) through a not yet fully understood mechanism. Canonical PRC1 is then recruited through the recognition of H3K27me3 by CBX subunits [2]. Conversely, recent studies revealed an
Offprint requests to: Atsushi Iwama, M.D., Ph.D., Division of Stem Cell and Molecular Medicine, Center for Stem Cell Biology and Regenerative Medicine, Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo, 108-8639 Japan; E-mail: [email protected]
alternative pathway by which de novo PRC1 binding to target sites and H2AK119ub1 results in the subsequent recruitment of PRC2 and deposition of H3K27me3 [3] (Figure 2). The PRC1 working in this alternative pathway is called “noncanonical or variant” PRC1 and its functions remain largely unknown. In this perspective, we summarize emerging knowledge on the functions of noncanonical PRC1 in normal and malignant hematopoiesis. Role of polycomb group ring finger proteins in noncanonical PRC1 in embryonic stem cells PRC1 complexes are divided into subgroups (PRC1.1 to PRC1.6) according to the subtype of the polycomb group ring finger (PCGF) subunits (PCGF1-6) (Figure 1). PCGF proteins heterodimerize with the H2AK119 ubiquitin ligases RING1A or RING1B, which are responsible for the core enzymatic activity of PRC1 [4]. PCGF2/MEL18 and PCGF4/BMI1 act as components of canonical PRC1 and the others (Pcgf1/3/5/6) as components of
0301-472X/© 2018 ISEH – Society for Hematology and Stem Cells. Published by Elsevier Inc. All rights reserved. https://doi.org/10.1016/j.exphem.2018.10.008
Y. Isshiki and A. Iwama / Experimental Hematology 2018;68:10−14
Canonical PRC1 PRC2 EED SUZ12 EZH2/1 1 RBBP4/7
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BCOR PCGF1 1 Kdm2b RING1A/B
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PRC1.6 PCGF6 F6 6 L3MBTL2 RING1A/B 1A 1A/B
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Figure 1. Canonical and noncanonical PRC1. PRC1 complexes are divided into subgroups (PRC1.1 to PRC1.6) according to the subtype of the PCGF subunits (PCGF1−6).
noncanonical PRC1. PCGF proteins in noncanonical PRC1 have been characterized for its role in the maintenance of stemness (Pcgf1 and Pcgf6) [5−8] and differentiation (Pcgf5 and Pcgf6) [9,10] of embryonic stem cells (ESCs). Among noncanonical PRC1, PRC1.1 consists of PCGF1, RING1A or RING1B, KDM2B, BCL6 corepressor (BCOR) or BCLRL1, RYBP/YAF2, SKP1, and USP7. KDM2B binds to nonmethylated CGIs through its zinc finger-CxxC (ZF-CxxC) DNA-binding domain, thereby recruiting other components of PRC1.1 [11−13] (Figure 2). The depletion of Kdm2b in mouse ESCs induces the derepression of lineage-specific genes and early differentiation [13]. BCOR is also essential for maintaining primed pluripotency in human ESCs [14]. In addition, KDM2B promotes Oct4-induced somatic reprogramming through recruitment of PRC1.1 to CGIs, indicating that PRC1.1 participates in the establishment of pluripotency [15]. Although KDM2B has the Jumonji C domain, its histone demethylase function is controversial. Function of noncanonical PRC1 in hematopoiesis PCGF4/BMI1-containing canonical PRC1 has been characterized for its role in the maintenance of selfrenewal capacity and multipotency of hematopoietic stem cells (HSCs). It transcriptionally represses the cyclin-dependent kinase inhibitor 2A (CDKN2A) locus to maintain the self-renewal capacity of HSCs and developmental regulator genes to maintain the immature state and multipotency of hematopoietic stem and progenitor cells (HSPCs) [16,17]. In contrast, the roles of noncanonical PRC1 have remained largely unknown. In knockdown experiments using mouse hematopoietic progenitor cells, PCGF1 was initially found to restrict the proliferative capacity of myeloid progenitor cells by downregulating Hoxa family genes (Hoxa7, Hoxa9, and Hoxa10) [18]. Mice lacking Bcor exons 9 and 10 (BcorDE9-10/y mice), which express a carboxyl-
terminal truncated BCOR that fails to interact with PCGF1, showed higher proliferation and differentiation rates of myeloid cells in cultures [19]. We also confirmed myeloid-biased hematopoiesis in BcorDE9-10/y mice [20]. Transcriptional profiling revealed the derepression of myeloid regulator genes such as Cebp family and Hoxa cluster genes in the BcorDE9-10/y progenitor cells [20]. Collectively, these findings indicate that PRC1.1 restricts myeloid proliferation and differentiation by transcriptionally repressing myeloid regulator genes such as the Cebp family and Hoxa cluster genes (Figure 3). Kdm2b-deficient mice have significantly decreased numbers of HSPCs [21]. Kdm2b-deficient HSCs have impaired repopulation of hematopoiesis, particularly the repopulation of lymphoid cells; however, it currently remains unclear whether the deletion of Kdm2b impairs the self-renewal capacity of HSCs similar to the deletion of Pcgf4/Bmi1 [21]. In contrast, the forced expression of Kdm2b prevents the exhaustion of the long-term repopulating potential of HSCs following serial transplantation by negatively regulating the expression of CDKN genes such as Ink4a [22]. However, Pcgf5 appeared to be dispensable for hematopoiesis [23]. Function of noncanonical PRC1 in hematological malignancies Among the noncanonical PRC1 genes, BCOR and its homolog BCORL1 are frequently mutated in patients with various hematological malignancies, whereas mutations in other genes are very rare. Somatic mutations in the BCOR gene have been reported in acute myeloid leukemia (AML) with the normal karyotype (3.8%) [24], secondary AML (8%) [25], myelodysplastic syndrome (MDS) (4.2%) [26], chronic myelomonocytic leukemia (7.4%) [25], extranodal natural killer/T-
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Canonical PRC1 PCGF PHCs 2/4 RING1A/B RIING1A/B RIN 1A
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EED EZ EZH2/1 SUZ12 S UZ Z12 Z Z1 12 2 RBBP4/7
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B
BcorDE9-10/y MDS cells reproduced MDS or evolved into lethal MDS/myeloproliferative neoplasms in secondary recipients. These findings indicate that BCOR mutations promote the initiation and progression of MDS in concert with concurrent driver mutations [20]. Transcriptional profiling revealed the derepression of myeloid regulator genes such as Cebp family and Hoxa cluster genes in Tet2D/DBcorDE9-10 progenitor cells [20]. BCOR loss-of-function mutations are associated with the gene signature of enhanced myeloid cell differentiation and derepression of PRC1 target genes, including HOXA9 in bone marrow cells from MDS and AML patients [19]. The weaker expression of KDM2B in leukemic cells was related to the poor prognosis of patients with AML, whereas the ablation of Kdm2b accelerated KrasG12D-driven myeloid transformation in mice [21]. These findings support the tumor suppressor function of PRC1.1 in myeloid malignancies. BCOR also plays an important role in restricting the transformation of T lymphocytes. Two different Bcor mutant mice, BcorDExon4/y and BcorDExon9-10/y, which lack the BCL6-binding domain and PCGF1-binding domain, respectively, showed a strong propensity to develop T-ALL, mostly in a Notch-dependent manner [20,36]. Chromatin immunoprecipitation sequencing analysis revealed that BCOR targeted many of the NOTCH1 targets, including Myc, and potentially antagonized their transcriptional activation [36]. The loss of BCOR was also reported to promote B-cell lymphomagenesis in Em-Myc mice [37]. In contrast, previous studies revealed the oncogenic activity of noncanonical PRC1. BCOR is known as a corepressor of BCL6 and contributes to the formation of the noncanonical PRC1/BCOR complex containing CBX8 in germinal center B cells, thereby mediating the cooperation between BCL6 and EZH2-containing PRC2. The BCL6noncanonical PRC1/BCOR-PRC2 interaction led to the silencing of B-cell differentiation and cell cycle checkpoint genes to permit immunoglobulin affinity maturation and excessive gene silencing caused malignant transformation [38]. PRC1.1 component genes are significantly overexpressed in primary AML CD34+ cells and the downmodulation of these components strongly reduced cell proliferation in vitro and delayed or abrogated leukemogenesis in humanized xenograft mouse models. PRC1.1 targeted actively transcribed genes involved in metabolism independently of PRC2, which was required for leukemic cell viability [39]. The lentivirus-mediated overexpression of Kdm2b was sufficient to transform hematopoietic progenitors. In contrast, the depletion of Kdm2b in hematopoietic progenitors significantly impaired Hoxa9/Meis1induced leukemic transformation [40]. Transgenic mice overexpressing Kdm2b developed myeloid or B-lymphoid leukemia through the metabolic activation and upregulation of Nsg2 in mice [41]. KDM2B also promotes D
A
PRC2 EZH2/1 SUZ12 Z12 2 EED
PRC1.1
? RING1A/B PCGF1 CGF1 GF
BCOR B CO
KDM2B KDM2 M2 2
non-methylated CGI Figure 2. Two pathways that target PcG complexes to target sites. (A) Classical hierarchical pathway. PRC2, which catalyzes H3K27me3 modifications, is recruited to nonmethylated CGIs through a not yet fully understood mechanism. Canonical PRC1 is then recruited via the CBX subunits that bind to H3K27me3. (B) Alternative pathway. Noncanonical PRC1 complexes bind to target sites and deposit H2AK119ub1, which results in the subsequent recruitment of PRC2 and deposition of H3K27me3. In the case of PRC1.1, KDM2B binds to nonmethylated CGIs through its DNAbinding domain, thereby recruiting other components of PRC1.1.
cell lymphoma (21−32%) [27,28], chronic lymphocytic leukemia (2.2%) [29], T-cell prolymphocytic leukemia (5−8%) [30,31], and acute T-lymphoblastic leukemia (T-ALL) (2−3%) [32,33]. Most of these mutations results in stop codon gains, frame-shift insertions or deletions, splicing errors, and gene loss, leading to the loss of BCOR function [26]. BCORL1 has also been implicated in AML and MDS [26,34]. These findings indicate that BCOR and BCORL1 function as tumor suppressors in various human hematological malignancies. Somatic mutations in BCOR are frequently associated with TET2 mutations in MDS [35]. Based on these findings, we recently generated mice with the combined deletion of Bcor and Tet2 (Tet2D/DBcorDE9-10/y) and demonstrated that they developed lethal MDS. Tet2D/
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HSCs
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Myeloid progeitors
Hoxa genes
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T cell
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Granulocyte Macrophage
Figure 3. Role of PRC1.1 in hematopoiesis. PRC1.1 restricts myeloid proliferation and differentiation by transcriptionally repressing myeloid regulator genes such as the Cebp family and Hoxa cluster genes. It also targets many of the NOTCH1 targets, including Myc in thymocytes, and potentially antagonizes their transcriptional activation, thereby restricting the transformation of T lymphocytes.
nonhematopoietic tumor development such as pancreatic cancer via polycomb-dependent and -independent transcriptional programs [42]. Future perspectives The functions of noncanonical PRC1 are now being clarified. As is the case with PRC2, noncanonical PRC1 appears to have complex functions that are celltype dependent and act in both an oncogenic and tumor-suppressive manner depending on the tumor type. In addition, PRC1.1 appeared to target a variety of active genes involved in metabolism independently of H3K27me3 [39,42], although the underlying molecular mechanisms remain unclear. Moreover, a number of issues have yet to be clarified: 1. redundancy between canonical and noncanonical PRC1 complexes and among noncanonical PRC1 complexes, 2. their role in the self-renewal of HSCs, 3. the reason why only BCOR and BCORL1 are targeted by somatic gene mutations, and 4. the druggability of noncanonical PRC1 components for the manipulation of hematopoiesis or cancer treatments. Because BCOR and BCORL1 are the main targets of somatic mutations, screening for synthetically lethal partners of BCOR and BCORL1 mutations will also be of great interest as pharmacological targets. Further investigations will provide a new
stage for the polycomb-based epigenetics of normal and malignant hematopoiesis. Acknowledgments This work was supported in part by Grants-in-Aid for Scientific Research (15H02544) and Scientific Research on Innovative Areas “Stem Cell Aging and Disease” (26115002) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Conflict of interest disclosure The authors declare no competing financial interests. References 1. Blackledge NP, Rose NR, Klose RJ. Targeting Polycomb systems to regulate gene expression: modifications to a complex story. Nat Rev Mol Cell Biol. 2015;16:643–649. 2. Wang L, Brown JL, Cao R, Zhang Y, Kassis JA, Jones RS. Hierarchical recruitment of polycomb group silencing complexes. Mol Cell. 2004;14:637–646. 3. Blackledge NP, Farcas AM, Kondo T, et al. Variant PRC1 complex-dependent H2A ubiquitylation drives PRC2 recruitment and polycomb domain formation. Cell. 2014;157:1445–1459. 4. Gao Z, Zhang J, Bonasio R, et al. PCGF homologs, CBX proteins, and RYBP define functionally distinct PRC1 family complexes. Mol Cell. 2012;45:344–356. 5. Zhao W, Tong H, Huang Y, et al. Essential role for polycomb group protein Pcgf6 in embryonic stem cell maintenance and a
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