PROJECT SUMMARY Recombinant interferon-alpha (IFN) remains a highly effective therapy for patients with myeloproliferative neoplasms (MPN). We recently identified that patients with CALR-mutated MPN frequently exhibit normalization of blood counts (i.e. clinical response), but often do not exhibit a decrease in tumor burden (i.e. molecular response), providing an informative model to decipher the mechanisms of therapy-resistance to IFN. Interrogating the molecular impact of IFN on human MPN stem cells may reveal critical insights into mechanisms of therapy-resistance. Thus, we applied our innovative Genotyping of Transcriptomes (GoT) platform – that captures the mutation status and single-cell whole transcriptomes (scRNA-seq) within the same cells – CD34+ cells from serial bone marrow (BM) aspirates from patients with CALR-mutated MPN treated with IFN. Strikingly, we observed that IFN caused major shifts in the differentiation landscapes, distinctly in the mutated and wildtype progenitors: IFN exposure on wildtype cells resulted in a large expansion of lymphoid progenitors, while the mutated cells, in contrast, displayed an expansion of the granulo-monocytic (GM) progenitors (with a less striking expansion of the lymphoid compartment). Our preliminary data indicate that (1) the GM differentiation bias of CALR-mutated stem cells may underlie therapy-resistance, and that (2) the CALR-mutation induced UPR may prime the mutated stem cells toward the GM lineage and play a role in therapy-resistance. To interrogate these hypotheses, we will determine the transcription factor (TF) networks that govern the IFN-induced differentiation shifts by applying a novel single-cell multi-omics platform that captures RNA-seq, chromatin accessibility and somatic genotyping within the same thousands of single cells (GoT-ATAC) to the same IFN-treated cohort (Aim 1a), and by targeting these TF networks in mouse models (Aim 1b). We will define the role of UPR in therapy- resistance in treated CALR-mutated cells through GoT-ATAC and chromatin binding assays (Aim 2a) and by assessing perturbations to the UPR pathways in mouse models (Aim 2b). Finally, we will determine the impact of co-mutations in DNMT3A or ASXL1 in therapy-resistance to IFN in CALR-mutated MPN via application of single-cell multi-omics platforms to clinical samples (Aim 3a) and interrogation of IFN effects on novel mouse models with double mutations (Aim 3b). The project is centered on a conceptually innovative framework in which we superimpose neoplastic and normal hematopoietic development within the same individuals to define how therapy reshapes differentiation topographies, as a function of mutation status and cell identity. This conceptual innovation is enabled by technical innovations in single-cell multi-omics platforms applied to compelling clinical cohorts, coupled with functional assessments in novel mouse models. These studies have the potential to uncover new insights into the mechanisms of...