Linker histone H1-0 is a specific mediator of the ETV6::RUNX1 transcriptional landscape in preleukemia and B cell acute lymphoblastic leukemia [scRNA-seq]

The chimeric hematopoietic transcription factor ETV6::RUNX1 is the most common oncogenic fusion in pediatric B cell precursor acute lymphoblastic leukemia (BCP-ALL). It induces a clinically silent preleukemic state that requires secondary mutations for progression to full-blown leukemia. ETV6::RUNX1 functions primarily through repression of RUNX1 binding sites. In order to elucidate the characteristic quiescent state of ETV6::RUNX1 expressing cells, we generated preleukemic human induced pluripotent stem cell (hiPSC) models from two healthy donors using CRISPR/Cas9 gene editing. We identified upregulation of linker histone H1-0 in preleukemic hiPSCs, which was preserved upon hematopoietic differentiation and transformation to overt BCP-ALL. ETV6::RUNX1 induces activity of the H1-0 promoter whereas depletion of H1-0 specifically inhibited ETV6::RUNX1 signature genes, indicating its role as a novel key mediator of the quiescent ETV6::RUNX1 transcriptional profile. Single-cell gene expression analysis revealed that H1-0 levels strongly anti-correlate with cellular transcriptional activity, resulting in particularly high expression in quiescent cells during hematopoietic development. Pharmacologically, H1-0 protein levels correspond to susceptibility of BCP-ALL towards histone deacetylase inhibitor (HDACi) treatment and our data indicate efficacy of combinatorial drug treatment using the potent H1-0-inducing HDACi Quisinostat in BCP-ALL expressing high basal H1-0 levels. To delineate effects of ETV6::RUNX1 in hematopoietic cells, we performed scRNA-seq analysis of hematopoietic progenitor cells (HPCs) differentiated from ETV6::RUNX1+ and wild-type hiPSCs.