LAUSR

Histone variants are emerging as key regulatory molecules in cancer. Recent data provided first evidence of how transcriptional deregulation and changes in the deposition of histone variants may affect malignant transformation. H2A variants are highly conserved and consist of 8 members. Among all histone variants, macroH2A variants hold a unique C-terminal macro domain, which is approximately twice the size of its H2A-like histone domain. In mammals, there are 3 macroH2A variants encoded by H2AFY (macroH2A1.1, macroH2A1.2) and H2AFY2 (macroH2A2). MacroH2A variants occupy large repressive domains throughout the genome, exerting a repressive role on transcription. So far, the function of macroH2A proteins in hematopoiesis and leukemic transformation is incompletely understood. Here, we report on a functional role of macroH2A histone variants that may influence cell competition in leukemia and normal hematopoietic stem and progenitor cell (HSPC) function. In published gene expression datasets, macroH2A variants H2AFY and H2AFY2 are highly expressed in hematopoietic stem cells (HSCs) compared to mononuclear cells (both p<0.0001****). Moreover, expression of H2AFY and H2AFY2 appeared even higher in acute myeloid leukemia (AML) (p=0.0750 and 0.0037**, respectively) when compared to the expression level in HSCs. This high expression is independent of different genetic subtypes and AML risk groups. To assess for the functional dependency of leukemic cells on the expression of macroH2A variants, we performed an in vitro CRISPR-Cas9 dropout screen focusing on macroH2A variants in MOLM-13 cells. Genetic inactivation of H2AFY but not H2AFY2 resulted in outcompetition of infected cells against non-infected competitors. This finding could be recapitulated in 3 additional AML cell lines (OCI-AML3, THP-1, HL-60), indicating a broader dependency on H2AFY irrespective of the oncogenic background or underlying driver mutation. To validate the functional impact of H2AFY on leukemia development in vivo, we generated a novel conditional knockout mouse model for H2afy and induced leukemia through retroviral infection of sorted Lineage-Sca1+cKit+ (LSK) cells with the oncogene MLL-AF9. Results of these functional studies will be presented. To understand the functional impact of macroH2A variants on normal HSPC function in vivo, we performed competitive repopulation studies. We used conventional knockout mouse models for the 2 H2afy isoforms: macroH2A1.1 and macroH2A1.2 In addition, for inactivation of both isoforms, we used the conditional macroH2A knockout mouse model. H2afy2 was either inactivated by RNAi or conditionally knockout through Mx1-Cre recombinase activation. In brief, whole bone marrow cells (WBMCs) were transplanted into primary recipient mice in a 1:1 competitive manner and compared to the respective wildtype littermate controls. Peripheral blood (PB) chimerism was monitored on a monthly basis 4 weeks post-transplantation for a total of 16 weeks. We observed no significant difference in the recipients of macroH2A1.1 (n=5/5) or macroH2A1.2 (n=10/9), suggesting both macroH2A isoforms are dispensable for intrinsic functions of HSCs. Remarkably, we found macroH2A2-deficient cells (n=6) competed significantly worse than macroH2A2+/+ (n=5) controls in PB (p=0.0043**) and in WBM (p=0.0043**), indicating a requirement for macroH2A2 in maintenance of HSPCs. These findings could be recapitulated by RNA-mediated inactivation of H2afy2 in sorted HSCs. We observed significant reduction of macroH2A2-depleted hematopoiesis after 16 weeks in WBM (p=0.0286*) and PB (p=0.0079**). Importantly, this functional impairment was less pronounced when investigating multipotent progenitor and committed progenitor cell function in spleen colony-forming unit (CFU) assay in vivo or CFU assay in vitro. Taken together, our data indicate distinct requirements for macrohistone variants in leukemic and normal hematopoietic cells. Dependency of leukemic cells but not normal HSPCs on macroH2A may indicate a potential therapeutic index. No relevant conflicts of interest to declare.