Person:

Xie, Huafeng

Polycomb repressive complexes (PRCs) play key roles in developmental epigenetic regulation. Yet the mechanisms that target PRCs to specific loci in mammalian cells remain incompletely understood. In this study we show that Bmi1, a core component of Polycomb Repressive Complex 1 (PRC1), binds directly to the Runx1/CBFβ transcription factor complex. Genome-wide studies in megakaryocytic cells demonstrate significant chromatin occupancy overlap between the PRC1 core component Ring1b and Runx1/CBFβ and functional regulation of a considerable fraction of commonly bound genes. Bmi1/Ring1b and Runx1/CBFβ deficiencies generate partial phenocopies of one another in vivo. We also show that Ring1b occupies key Runx1 binding sites in primary murine thymocytes and that this occurs via PRC2-independent mechanisms. Genetic depletion of Runx1 results in reduced Ring1b binding at these sites in vivo. These findings provide evidence for site-specific PRC1 chromatin recruitment by core binding transcription factors in mammalian cells.

SUMMARY Early T cell precursor acute lymphoblastic leukemia (ETP-ALL) is an aggressive subtype of ALL distinguished by stem-cell-associated and myeloid transcriptional programs. Inactivating alterations of Polycomb repressive complex 2 components are frequent in human ETP-ALL, but their functional role is largely undefined. We have studied the involvement of Ezh2 in a murine model of NRASQ61K-driven leukemia that recapitulates phenotypic and transcriptional features of ETP-ALL. Homozygous inactivation of Ezh2 cooperated with oncogenic NRASQ61K to accelerate leukemia onset. Inactivation of Ezh2 accentuated expression of genes highly expressed in human ETP-ALL and in normal murine early thymic progenitors. Moreover, we found that Ezh2 contributes to the silencing of stem-cell- and early-progenitor-cell-associated genes. Loss of Ezh2 also resulted in increased activation of STAT3 by tyrosine 705 phosphorylation. Our data mechanistically link Ezh2 inactivation to stem-cell-associated transcriptional programs and increased growth/survival signaling, features that convey an adverse prognosis in patients.

Recurrent chromosomal translocations involving the mixed lineage leukemia gene (MLL) give rise to a highly aggressive acute leukemia associated with poor clinical outcome1. The preferential involvement of chromatin-associated factors in MLL rearrangement belies a dependency on transcription control2. Despite recent progress made in targeting chromatin regulators in cancer3, available therapies for this well-characterized disease remain inadequate, prompting the present effort to qualify new targets for therapeutic intervention. Using unbiased, emerging CRISPR-Cas9 technology to perform a genome-scale loss-of-function screen in MLL-AF4-positive acute leukemia, we identified ENL (eleven-nineteen leukemia) as an unrecognized dependency particularly indispensable for proliferation in vitro and in vivo. To explain the mechanistic role for ENL in leukemia pathogenesis and dynamic transcription control, we pursued a chemical genetic strategy utilizing targeted protein degradation. Acute ENL loss suppresses transcription initiation and elongation genome-wide, with pronounced effects at genes featuring disproportionate ENL load. Importantly, ENL-dependent leukemic growth was contingent upon an intact YEATS chromatin reader domain. These findings reveal a novel dependency in acute leukemia and a first mechanistic rational for disrupting the YEATS domain in disease.