Myeloproliferative neoplasms (MPNs) are a group of chronic blood cancers characterised by excess production of mature blood cells accompanied by an increased risk of thrombosis and progression to marrow fibrosis (PMF) and acute myeloid leukaemia (AML). A mutation in the JAK2 tyrosine kinase (V617F) drives ~70% of MPNs. In order to find direct targets of JAK2-STAT signalling that underpin the pathophysiology of polycythaemia vera (PV) and essential thrombocythaemia (ET), we undertook ChIP-Seq for pSTAT5 and pSTAT3 in human cell lines, HEL and SET2, which carry the JAK2-V617F mutation. We found ~690 robust pSTAT5-occupied sites in promoters and enhancers of known and novel target genes including a novel intronic enhancer of the BCL2L1 gene, which encodes the pro-survival protein, BCL-XL. There was a marked overlap of pSTAT5-occupied sites in erythroid versus megakaryotye environments suggesting a commonality of targets. There was also a strong overlap between pSTAT5 and pSTAT3-occupied sites suggesting redundancy. We undertook RNA-seq in the cell lines following inhibition of JAK-STAT signalling with ruxolitinib and two STAT5 inhibitors. There was an overlap between JAK-STAT target genes in human MPN cell lines and EPO-induced genes in the mouse 1. They encode for genes involved in cell survival, proliferation, epigenetic regulation and down regulation of cytokine signalling. This gene list provides new insights into the biology of MPNs, and provides a source of potential new biomarkers and drug targets. We have validated some of these target genes in primary samples from patients with PV and ET, and functionally validated some using gene editing in cell lines and/or mice.
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