Contribution of mutant HSC clones to immature and mature cells in MDS and CMML, and variations with AZA therapy

•The mutational burden in HSCs is proportionally reflected throughout hematopoietic differentiation in MDS/CMML.•Improved hematopoiesis in response to AZA therapy was associated with increased clonal output from mutant progenitors to mature cells. [Display omitted] Myelodysplastic neoplasms (MDSs) and chronic myelomonocytic leukemia (CMML) are clonal disorders driven by progressively acquired somatic mutations in hematopoietic stem cells (HSCs). Hypomethylating agents (HMAs) can modify the clinical course of MDS and CMML. Clinical improvement does not require eradication of mutated cells and may be related to improved differentiation capacity of mutated HSCs. However, in patients with established disease it is unclear whether (1) HSCs with multiple mutations progress through differentiation with comparable frequency to their less mutated counterparts or (2) improvements in peripheral blood counts following HMA therapy are driven by residual wild-type HSCs or by clones with particular combinations of mutations. To address these questions, the somatic mutations of individual stem cells, progenitors (common myeloid progenitors, granulocyte monocyte progenitors, and megakaryocyte erythroid progenitors), and matched circulating hematopoietic cells (monocytes, neutrophils, and naïve B cells) in MDS and CMML were characterized via high-throughput single-cell genotyping, followed by bulk analysis in immature and mature cells before and after AZA treatment. The mutational burden was similar throughout differentiation, with even the most mutated stem and progenitor clones maintaining their capacity to differentiate to mature cell types in vivo. Increased contributions from productive mutant progenitors appear to underlie improved hematopoiesis in MDS following HMA therapy. Myelodysplastic neoplasms (MDS) and chronic myelomonocytic leukemia (CMML) are driven by progressive acquisition of somatic mutations in hematopoietic stem cells (HSCs). Treatment with hypomethylating agents (HMA) often improves blood counts without decreasing clonal burden. Schnegg-Kaufmann and colleagues investigated whether this reflects a shift in mature cell populations toward those derived from unmutated or less mutated clones. Both before and after HMA treatment, mutational burden in mature cells is similar, suggesting that even highly mutated cells can give rise to mature progeny, indicating that cellular maturation can be improved without eliminating mutant HSCs.