ETV6 Represses Inflammatory Response Genes and Regulates HSPC Function During Stress Hematopoiesis in Mice.

ETS Variant 6 (ETV6) encodes an essential transcriptional repressor abundantly expressed in hematopoietic stem and progenitor cells (HSPCs), where it is required for adult hematopoiesis. Heterozygous pathogenic germline ETV6 variants are associated with Thrombocytopenia 5 (T5), a poorly understood genetic condition predisposing to thrombocytopenia and hematologic malignancies. To elucidate how germline ETV6 variants impact the HSPC compartment and contribute to disease, we generated a knock-in mouse model harboring an Etv6R355X loss-of-function variant, which is equivalent to the T5-associated variant ETV6R359X. Under homeostatic conditions, all HSPC subpopulations are present in the bone marrow (BM) of Etv6R355X/+ mice; however, these animals display subtle shifts in the proportions and/or numbers of specific progenitor subtypes. To examine whether the Etv6R355X/+ mutation impacts HSPC function, we carried out serial competitive transplantation and observed that Etv6R355X/+ lineage-sca1+cKit+ (LSK) cells exhibit significantly impaired reconstitution compared to Etv6+/+ LSK cells with near complete failure to repopulate irradiated-recipients by the tertiary transplant. Mechanistic studies incorporating CUT&RUN, ATAC-Seq and Hi-C identify ETV6 binding at inflammatory gene loci, including those within the TNF signaling pathway, in Etv6+/+ HSPCs, mouse BM-progenitor-derived HPC5 cells, and human CD34+ cells. Further, single-cell RNA-Seq of BM cells isolated post-competitive transplantation reveals upregulation of inflammatory genes in Etv6R355X/+ compared to Etv6+/+ progenitors. Corroborating these findings, Etv6R355X/+ HSPCs produce significantly more TNF than Etv6+/+ cells post-transplantation. From these studies, we conclude that ETV6 is required to repress inflammatory gene expression in HSPCs under conditions of hematopoietic stress and this mechanism may be critical to sustain HSPC function.