In vivo CRISPR/Cas9 screening identifies Pbrm1 as a regulator of myeloid leukemia development in mice.

CRISPR/Cas9 screening approaches are powerful tool for identifying in vivo cancer dependencies. Hematopoietic malignancies are genetically complex disorders in which the sequential acquisition of somatic mutations generates clonal diversity. Over time, additional cooperating mutations may drive disease progression. Using an in vivo pooled gene editing screen of epigenetic factors in primary murine hematopoietic stem and progenitor cells (HSPCs), we sought to uncover unrecognized genes that contribute to leukemia progression. We, first, modeled myeloid leukemia in mice by functionally abrogating both Tet2 and Tet3 in HSPCs, followed by transplantation. We, then, performed pooled CRISPR/Cas9 editing of genes encoding epigenetic factors and identified Pbrm1/Baf180, a subunit of the polybromo BRG1/BRM-associated factor SWItch/Sucrose Non-Fermenting chromatin-remodeling complex, as a negative driver of disease progression. We found that Pbrm1 loss promoted leukemogenesis with a significantly shortened latency. Pbrm1-deficient leukemia cells were less immunogenic and were characterized by attenuated interferon signaling and reduced major histocompatibility complex class II (MHC II) expression. We explored the potential relevance to human leukemia by assessing the involvement of PBRM1 in the control of interferon pathway components and found that PBRM1 binds to the promoters of a subset of these genes, most notably IRF1, which in turn regulates MHC II expression. Our findings revealed a novel role for Pbrm1 in leukemia progression. More generally, CRISPR/Cas9 screening coupled with phenotypic readouts in vivo has helped identify a pathway by which transcriptional control of interferon signaling influences leukemia cell interactions with the immune system.