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BCOR and BCORL1 Mutations Drive Epigenetic Reprogramming and Oncogenic Signaling by Unlinking PRC1.1 from Target Genes

ABSTRACT: Polycomb repressive epigenetic complexes are recurrently dysregulated in cancer. Unlike polycomb repressive complex 2 (PRC2), the role of PRC1 in oncogenesis and therapy resistance is not well defined. Here, we demonstrate that highly recurrent mutations of the PRC1 subunits BCOR and BCORL1 in leukemia disrupt assembly of a non-canonical PRC1.1 complex, thereby selectively unlinking the RING-PCGF enzymatic core from the chromatin targeting auxiliary subcomplex. As a result, BCOR-mutated PRC1.1 is localized to chromatin but lacks repressive activity, leading to epigenetic reprogramming and transcriptional activation at target loci. We define a set of functional targets that drive aberrant oncogenic signaling programs in PRC1.1 mutated cells and primary patient samples. Activation of these PRC1.1 targets in BCOR-mutated cells confers acquired resistance to treatment while sensitizing to targeted kinase inhibition. Our study thus reveals a novel epigenetic mechanism that explains PRC1.1 tumor suppressive activity and identifies a therapeutic strategy in PRC1.1 mutated cancer.

ORGANISM(S): Homo sapiens

PROVIDER: GSE167869 | GEO | 2022/02/28

REPOSITORIES: GEO

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Project description:BCOR is a component of a variant Polycomb group repressive complex 1 (PRC1) complex. Recently, we and others reported recurrent somatic BCOR loss-of-function mutations in myelodysplastic syndrome and acute myelogenous leukaemia (AML). However the role of BCOR in normal hematopoiesis is largely unknown. Here, we explored the function of BCOR in myeloid cells using myeloid murine cell models with Bcor conditional loss-of-function or overexpression alleles. Bcor mutant bone marrow cells showed significantly higher proliferation and differentiation rates with reduced protein levels of RING1B, a ubiquitin ligase subunit of PRC1 family complexes. Global RNA expression profiling in murine cells and AML patient samples with BCOR loss-of-function mutation suggested that loss of BCOR expression is associated with proliferation and myeloid differentiation and decreased stem cell quiescence. Further, we used a MLL-AF9 murine model of AML and found that loss of Bcor increased serial replating efficiency, enhanced MLL-AF9 in blocking cell differentiation, and increased expression of Evi1 which is associated with leukemic transformation. Our results strongly suggest that BCOR plays an indispensable role in maintaining hematopoietic stem cell (HSC) quiescence by inhibiting myeloid stem cell proliferation and differentiation and offer a mechanistic explanation for how BCOR regulates gene expression such as Hox genes. We took advantage of a mouse conditional Bcor allele which has exons 9 and 10 been flanked by LoxP sites to allow their removal via expression of Cre recombinase. Excision of these exons results in a frame shift and a premature stop codon causing nonsense mediated decay and/or carboxy-terminal deletion of the BCOR protein. CD34+ cells were sorted from control or Bcor mutant cells cultured under myeloid stem/progenitor conditions (IL3, 6 and SCF). We used microarrays to detail the global programme of gene expression in Control and Bcor mutant CD34+ cells.

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BCOR and BCORL1 Mutations Drive Epigenetic Reprogramming and Oncogenic Signaling by Unlinking PRC1.1 from Target Genes

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