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The Switch/Sugar non-fermenting (SWI/SNF) nucleosome remodeling complexes are essential for normal hematopoiesis. The Brg1/Brm associated factor (BAF) is a form of mammalian SWI/SNF that is distinguished by the presence of either ARID1A or ARID1B protein. In this study, we used hematopoietic specific Cre mouse models to assess the function of Arid1b in blood development. We found Arid1b loss did not affect steady state hematopoiesis or hematopoietic regeneration. Nonetheless, Arid1b null hematopoietic stem and progenitor cells have reduced ability to reconstitute hematopoietic system compared to wild type cells. Overall, our data indicate Arid1b is largely dispensable for normal hematopoiesis but impairs the regenerative ability of HSPCs., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Arnold et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Acute myelogenous leukemia (AML) is a high-risk hematopoietic malignancy caused by a variety of mutations, including genes encoding the cohesin complex. Recent studies have demonstrated that reduction in cohesin complex levels leads to enhanced self-renewal in hematopoietic stem and progenitors (HSPCs). We sought to delineate the molecular mechanisms by which cohesin mutations promote enhanced HSPC self-renewal as this represents a critical initial step during leukemic transformation. We verified that RNAi against the cohesin subunit Rad21 causes enhanced self-renewal of HSPCs in vitro through derepression of polycomb repressive complex 2 (PRC2) target genes, including Hoxa7 and Hoxa9. Importantly, knockdown of either Hoxa7 or Hoxa9 suppressed self-renewal, implying that both are critical downstream effectors of reduced cohesin levels. We further demonstrate that the cohesin and PRC2 complexes interact and are bound in close proximity to Hoxa7 and Hoxa9. Rad21 depletion resulted in decreased levels of H3K27me3 at the Hoxa7 and Hoxa9 promoters, consistent with Rad21 being critical to proper gene silencing by recruiting the PRC2 complex. Our data demonstrates that the cohesin complex regulates PRC2 targeting to silence Hoxa7 and Hoxa9 and negatively regulate self-renewal. Our studies identify a novel epigenetic mechanism underlying leukemogenesis in AML patients with cohesin mutations.

Sal-like protein 4 (SALL4) is a transcription factor that exists in two splice isoforms, SALL4a and SALL4b, and regulates transcription in embryonic stem cells, hematopoiesis, and acute myeloid leukemia. Constitutive overexpression of SALL4 in mice induces acute myeloid leukemia. Interestingly, a potential benefit of using SALL4 to facilitate ex vivo hematopoietic stem cell expansion has been proposed. However, distinct roles for how SALL4 contributes to normal versus malignant processes remain undefined. Here we show that SALL4b is the predominant isoform in murine hematopoietic stem cells and progenitors. Overexpression of either SALL4 isoform in hematopoietic stem cells or progenitors impairs hematopoietic colony formation and expansion in vitro. Lineage-negative bone marrow overexpressing SALL4b fails to engraft and reconstitute hematopoiesis when transplanted. We found that both SALL4a and SALL4b overexpression impair hematopoiesis, in part through dose-dependent repression of BMI1. Additionally, we have identified the following potential novel SALL4 target genes in hematopoiesis: ARID5B (SALL4a and SALL4b), EZH2, and KLF2 (SALL4a). Lastly, we found that SALL4 expression is variable in acute myeloid leukemia, ranging from no expression to levels comparable to embryonic stem cells. These results show that SALL4 isoforms contribute to only a subset of acute myeloid leukemia and that overexpression of SALL4 isoforms impairs hematopoiesis through repression of BMI1. Together these data demonstrate the sensitivity of hematopoiesis to appropriately balanced SALL4 expression, highlighting the importance of regulating this dynamic in potential therapeutic applications such as ex vivo stem cell expansion., (Copyright © 2015 ISEH - International Society for Experimental Hematology. Published by Elsevier Inc. All rights reserved.)