Single Cell RNA Sequencing Identifies a Crucial Role for ASXL1 in Neutrophil Development
Authors: Braun Theodore , Lusardi Theresa , Enright Trevor , Schonrock Zachary , Coblentz Cody , Mohamed Hisham , Druker Brian J. , Maxson Julia E. 
Published: February 2020

Single Cell RNA Sequencing Identifies a Crucial Role for ASXL1 in Neutrophil Development
Additional sex combs-like 1 (ASXL1) is a polycomb-associated protein that is essential for normal hematopoiesis. ASXL1 is recurrently mutated across the spectrum of myeloid malignancies including myelodysplastic syndromes, myeloproliferative neoplasms and Acute Myeloid Leukemia. ASXL1 mutations are also found in the premalignant disorders clonal hematopoiesis of indeterminate potential and clonal cytopenias of indeterminate potential. In all cases, ASXL1 mutations are associated with more aggressive disease biology and resistance to treatment. Mutations in ASXL1 broadly dysregulate the hematopoietic system, opening chromatin at genes associated with differentiation and self-renewal, predisposing to malignant transformation. However, in spite of this, the specific role of ASXL1 at different phases of hematopoiesis remains unknown. Indeed, the development of therapeutic approaches for ASXL1-mutant malignancies will require a nuanced understanding of the role of ASXL1 in directing normal blood development to maximize on target effects and minimize toxicity.
ASXL1 mutations are commonly identified in myeloid disorders with dysplasia. In the neutrophil lineage, morphologic dysplasia is associated with nuclear-cytoplasmic dyssynchrony, where neutrophils demonstrate differences in nuclear and cytoplasmic differentiation (i.e. hypolobated nuclei or hypogranular cytoplasm). Given its associated with dysplasia, we hypothesized that ASXL1 plays a fundamental role in neutrophil maturation. To investigate this, we performed single cell RNA sequencing (scRNA-seq) on lineage depleted bone marrow from MX-1 Cre/Asxl1FL/FL mice (Asxl1KO) or cre negative littermate controls (Asxl1WT). This analysis revealed a loss of multi-lineage differentiation potential in response to Asxl1 deletion with the most prominent effects noted in myeloid differentiation. Although the neutrophil-primed granulocyte-macrophage progenitors appeared relatively normal, a differentiation block was identified at the transition between promyelocytes and myelocytes. Specifically, Asxl1KO mice demonstrated a failure to normally upregulate specific granule constituents. Although key differentiation-associated transcription factors are present in the appropriate precursor populations, they appear to require normal Asxl1 function to effectively initiate transcription of specific granule genes. This is the first description of a crucial role for Asxl1 in terminal neutrophil differentiation. Furthermore, the failure to effectively upregulate specific granule genes in Asxl1 deficient mice may provide a mechanistic explanation for the dysplasia-associated hypogranular neutrophils present in dysplastic disorders with mutant ASXL1.

Disclosures
Druker: Vivid Biosciences: Membership on an entity's Board of Directors or advisory committees, Other: Stock options; Beat AML LLC: Other: Service on joint steering committee; GRAIL: Equity Ownership, Other: former member of Scientific Advisory Board; CureOne: Membership on an entity's Board of Directors or advisory committees; Beta Cat: Membership on an entity's Board of Directors or advisory committees, Other: Stock options; Monojul: Other: former consultant; ALLCRON: Membership on an entity's Board of Directors or advisory committees; Amgen: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Aptose Biosciences: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Patient True Talk: Consultancy; The RUNX1 Research Program: Membership on an entity's Board of Directors or advisory committees; Novartis: Other: PI or co-investigator on clinical trial(s) funded via contract with OHSU., Patents & Royalties: Patent 6958335, Treatment of Gastrointestinal Stromal Tumors, exclusively licensed to Novartis, Research Funding; Pfizer: Other: PI or co-investigator on clinical trial(s) funded via contract with OHSU., Research Funding; Merck & Co: Patents & Royalties: Dana-Farber Cancer Institute license #2063, Monoclonal antiphosphotyrosine antibody 4G10, exclusive commercial license to Merck & Co; Dana-Farber Cancer Institute (antibody royalty): Patents & Royalties: #2524, antibody royalty; OHSU (licensing fees): Patents & Royalties: #2573, Constructs and cell lines harboring various mutations in TNK2 and PTPN11, licensing fees ; Cepheid: Consultancy, Honoraria; Burroughs Wellcome Fund: Membership on an entity's Board of Directors or advisory committees; Blueprint Medicines: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; ICON: Other: Scientific Founder of Molecular MD, which was acquired by ICON in Feb. 2019; Gilead Sciences: Other: former member of Scientific Advisory Board; Celgene: Consultancy; Pfizer: Research Funding; Aileron Therapeutics: #2573, Constructs and cell lines harboring various mutations in TNK2 and PTPN11, licensing fees , Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Patents & Royalties, Research Funding; Bristol-Myers Squibb: Other: PI or co-investigator on clinical trial(s) funded via contract with OHSU., Research Funding.