Bhuyan, G. S.; Yan, F.; Nguyen, M. N. T.; Zou, X.; Gullapalli, V.; Vaughan, L.; Stonehouse, O.; Hampton, H. R.; Shen, S.; Truong, P.; Dissanayake, R.; Ghodousi, E. S.; Joshi, S.; Koch, F. C.; Chung, H. M.; Zanini, F.; Vafaee, F.; Huang, Y.; Thoms, J. A. I.; Faridani, O.; Jolly, C. J.; Pimanda, J. E.

Myelodysplastic neoplasms (MDS) and related myeloid neoplasms such as chronic myelomonocytic leukaemia (CMML) are clonal haematopoietic stem cell disorders characterised by ineffective and dysplastic haematopoiesis. They are associated with peripheral cytopaenias, variable increases in immature blasts, and a risk of progression to acute myeloid leukaemia. Hypomethylating agents (HMA) can improve blood counts and reduce blasts, but responses are usually limited. Epigenetic rewiring of haematopoietic stem and progenitor cells (HSPC) by HMA enhances hematopoietic output but is influenced by clonal mosaicism, which requires tracking of response at the single cell level to achieve full understanding. We developed SCIMETAR-seq for single-cell interrogation of DNA methylation, target amplicons, and mRNA in FACS-indexed HSPC, then deployed SCIMETAR-seq on CD34+ HSPC from longitudinal HMA-treated patient BM in vitro and in vivo. HMA-induced LINE-1 (L1) demethylation was positively correlated with cell cycling; being lowest in quiescent HSC and highest in erythrocyte progenitors. Erythrocyte progenitor frequencies were particularly increased by HMA exposure. SRSF2 p.P95 genotype did not influence HMA-induced L1 demethylation but was enriched into cells with a CMP immunophenotype, which were transcriptionally biased away from MEP towards granulocytic progenitors. Despite a lack of L1 demethylation in quiescent HSC/MPP after 7 days of HMA treatment in vivo, their transcriptomes were enriched for TNF-, TGF{beta}- and WNT-signaling, suggesting that extrinsic factors secreted by other BM cells in response to HMA mediates reprogramming of quiescent HSC during HMA therapy in vivo.