S122: mutant idh1 hijacks differentiation programs in myeloid progenitor cells to impair granulocytic differentiation.

Background:Isocitrate dehydrogenase 1 (IDH1) mutations are common genetic lesions in acute myeloid leukemia (AML) and are considered as early events during pathogenesis despite rarely being detected in clonal hematopoiesis. Clinically relevant mutations in IDH1 are always heterozygous and result in neomorphic enzymatic activity leading to the production of the oncometabolite (R)-2-hydroxyglutarate which has been associated with DNA and histone hypermethylation. However, the intricate molecular alterations resulting from IDH1 mutations and how they contribute to malignant transformation are only poorly understood. Aims: The aims of this study were to investigate the cellular and molecular aberrations elicited by mutant Idh1 in hematopoiesis in order to better understand the mechanisms driving IDH1-mutant (pre-)leukemias. Methods: We established an inducible mouse model expressing the Idh1-R132H mutation from the endogenous gene locus together with a YFP-reporter. Expression of a tamoxifen-inducible Cre was driven by the Scl enhancer (Scl-CreERT), resulting in activation of Idh1-R132H and YFP expression in hematopoietic stem and progenitor cells (HSPC). Lineage negative/ YFP+ cells were transplanted into lethally irradiated mice. Blood and bone marrow cells from fully chimeric Idh1-WT and Idh1-R132H mice were used for flow cytometry analyses, scRNA-sequencing (10X platform), and ex vivo/in vivo functional assays. Results: Analysis of scRNA-seq data of normal human and murine bone marrow revealed dynamic regulation of IDH1/Idh1 expression in hematopoiesis with the highest expression in myeloid progenitor (MP) cells. Two transcript variants (TV) of Idh1 encoding for the same protein are found in the murine genome. Specific upregulation of Idh1-TV2 was observed in MPs isolated from C57BL/6 mice. In Idh1-R132H (Idh1-mut) mice, this upregulation strongly correlated with a significant expansion of MPs (CD55-CMPs, Ly6C+GMPs), a significant loss of granulocytes and a significant increase in Ly6Chigh monocytes. Adoptive transfer experiments confirmed that Idh1-mut CD55-CMPs produce less granulocytes than the WT controls, suggesting that Idh1-mut MPs exhibit a cell-intrinsic defect in granulopoiesis. Treatment of Idh1-mut CD55-CMPs with a mutant-specific inhibitor significantly reduced the aberrant production of Ly6Chigh monocytes in an ex vivo differentiation assay. Further, scRNA-seq revealed that late neutrophil progenitors are depleted in Idh1-mut mice and show reduced expression of the granulocyte transcription factor Cebpe. To test the relevance of our findings for AML patients, we compared DNA methylation data from IDH1-mut AML samples with data from normal human hematopoietic cell types. This revealed epigenetic scars indicating granulocytic lineage commitment in 9/16 IDH1-mut samples, while none of the IDH2-mut samples (n=29) tested revealed such scars. This suggested that granulocytic differentiation defects might contribute to leukemogenesis in IDH1-mut AML. Summary/Conclusion: In the present study, we identified cellular defects in the myeloid progenitor cell compartment of Idh1-mut mice. These effects were mediated by a physiologic upregulation of Idh1 expression, which resulted in downregulation of Cebpe in neutrophil progenitors and impaired granulopoiesis. Identification of granulocytic epigenetic scars in IDH1-mut AML samples suggest granulocytic differentiation defect might also play a role in leukemogenesis in humans. In summary, for the first time, we present molecular and cellular data that explain the association of IDH1 mutations with myeloid neoplasms. Keywords: Mouse model, Acute myeloid leukemia, Myeloid differentiation, Epigenetic