Abstract B010: Determining the role of BRD9 in human hematopoietic stem cell aging
Authors: Adelman Emmalee R. , Figueroa Maria E. 
Published: December 2022

Abstract
Aging is the number one non-modifiable risk factor for cancer, including myeloid malignancies, such as Acute Myeloid leukemia (AML). Intrinsic changes within hematopoietic stem cells (HSC) with aging contribute to HSC dysfunction and may predispose to leukemia. We have previously shown that aged human HSC present a reduction of H3K27ac at active promoters and enhancers of genes associated with immune and cancer pathways. Concurrent with this epigenetic deregulation there is deregulation of immune-associated genes and downregulation of the bromodomain containing protein-9 (BRD9). This protein is a component of the non-canonical BAF (nc-BAF) chromatin remodeling complex, which binds to both active enhancers and promoters. Notably alteration of BRD9 has been shown to perturb immune and cancer associated pathways, with BRD9 regulating interferon-stimulated genes. However, little is known about the role of this protein in human hematopoiesis. Therefore, we hypothesize that chromatin remodeling due to loss of BRD9 with HSC aging leads to reduction of H3K27ac at active enhancers and promoters, contributing to HSC dysfunction. To investigate this, we treated human CD34+ hematopoietic stem and progenitor cells (HSPC) with dBRD9-A (100 nM) a chemical degrader of BRD9. Loss of BRD9 resulted in enhanced myeloid differentiation in vitro with a concurrent block in erythroid differentiation. Interestingly, amongst the myeloid populations upregulated with loss of BRD9 were monocytic myeloid derived suppressor cells (M-MDSC), immunosuppressive cells that are involved both in cancer pathogenesis and aging. Treatment with the BRD9 bromodomain specific inhibitor, I-BRD9 (1 µM), suggests that while the role of BRD9 in myeloid differentiation is largely bromodomain independent, the bromodomain may regulate erythroid differentiation. Similarly, loss of BRD9 leads to more myeloid biased colonies, suggesting that loss of BRD9 may recapitulate certain features of aged HSC. At the transcriptional level, RNA-seq revealed that loss of BRD9 resulted in few, but important differentially expressed genes, including downregulation of MPO, LIF, EPX, and GZMB (FDR &lt; 0.05). However, gene set enrichment analysis showed that both bromodomain inhibition and loss of BRD9 resulted in downregulation of interferon and STAT3 associated genes, which are known BRD9 targets. Chromatin profiling of H3K27ac with CUT&RUN in young HSPC showed that with loss of BRD9, there is a reduction of H3K27ac at active enhancers that we had previously observed have loss of H3K27ac with human HSC aging (n=2). These active enhancers are associated with immune signaling and regulate hematopoietic transcription factors such as KLF6, RUNX1/2/3, and HIF1A. In conclusion, we have found that the chromatin remodeling protein BRD9 contributes to the reprogramming of age-associated enhancers and regulates human myeloid and erythroid differentiation in vitro, and that its loss may result in an expansion of M-MDSC, which has implications for age-related disorders such as immune dysfunction and cancer.
Citation Format: Emmalee R. Adelman, Maria E. Figueroa. Determining the role of BRD9 in human hematopoietic stem cell aging. [abstract]. In: Proceedings of the AACR Special Conference: Cancer Epigenomics; 2022 Oct 6-8; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2022;82(23 Suppl_2):Abstract nr B010.