Uncovering the role for MSI2 network in hematopoietic stem cells

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PROJECT SUMMARY/ABSTRACT Hematopoietic stem cells (HSCs) must navigate important cellular fate choices that include a symmetric self-renewal, symmetric commitment or undergo an asymmetric cell division where one of the cells is fated to differentiate. Alterations in this homeostatic program can lead to hematopoietic disorders and malignancies. Myelodysplastic syndromes (MDS) are a heterogeneous set of clonal disorders characterized by ineffective blood cell development. A common pathophysiologic mechanism in MDS is the presence of dysregulated hematopoietic stem and progenitor cells that fail to normally develop into the diverse set of blood cells necessary for normal function. Our laboratory and others identified MUSASHI2 (MSI2) as a central regulator of HSC and hematopoietic progenitor cell self-renewal (Kharas et al. Nature Medicine 2010). Additionally, we identified that Msi2 loss results in a defect in controlling symmetric and asymmetric division, failure to engraft and results in defective maintenance of myeloid lineage biased HSCs in part through control of the TGFβ pathway (Park et al. 2014 Journal of Experimental Medicine). We found that elevated levels of MSI2 expression predicts poor outcome and using a genetic MDS mouse model that overexpresses MSI2 can drive a more aggressive MDS (Taggart et al. 2016 Nature Communications). To determine if MSI2 is part of a regulatory network, we performed proteomics and in vivo shRNA screen for functional regulators of leukemia self-renewal. Based on this screen, we identified SYNCRIP, an RNA binding protein that shares MSI2 targets and is required in leukemia stem cells (Vu et al., 2017 Nature Genetics). Our preliminary data with a conditional knockout for Syncrip indicates that it is also critical for HSC self-renewal. Our proposal will expand our focus from MSI2 to its associated regulatory network and characterize and identify new molecular determinants for HSC and HSPC symmetric self-renewal and asymmetric fate choice. We have adapted new technologies that include a barcoding, single cell RNA-seq and paired daughter HSC assays (FATE-seq). We have also develop a new way to map direct mRNA targets in HSCs called (HYPERTRIBE), (Nguyen et al. Nature Communications 2020).This proposal will identify new regulators of HSPC fate choice which will lead to novel therapeutic strategies to improve outcomes in MDS patients.