1017 – INVESTIGATING THE MECHANISMS OF IFNALPHA THERAPY IN JAK2V617F AND CALR MUTATED MYELOPROLIFERATIVE NEOPLASMS

Abstract

Classical <i>BCR-ABL</i>-negative myeloproliferative neoplasms (MPN) are acquired clonal disorders of hematopoietic stem cells (HSC) leading to the hyperplasia of one or several myeloid lineages. They are caused by recurrent mutations in 3 three main genes: <i>JAK2,</i> mainly <i>JAK2</i>^V617F, calreticulin <i>(CALR</i>) and thrombopoietin receptor (<i>MPL</i>). Most therapies have limited effect on the malignant clone, except the interferon alpha (IFN) that has demonstrated some efficacy induction of a molecular remission. However, since its mechanism of action is still largely unknown, we have followed two strategies: First, we studied how the IFN affects the different MPN hematopoietic cell compartment by following a longitudinal observational cohort of 50 <i>JAK2^V617F</i> and <i>CALR^mut</i> patients treated with IFN for 4-5 years. At 4-month intervals, we performed the clonal architecture in early and late hematopoietic progenitors and in mature cells. We also used a mathematical model to infer the behavior of IFN-targeted mutated HSC from those of mutated progenitors and mature cells. Our results predict that IFN slowly exhausts the mutated HSC by inducing their differentiation into progenitors and mature cells. In addition, we show that this effect was increased with high IFN doses and in patients with homozygous <i>JAK2</i>^V617F or very low proportion of heterozygous <i>JAK2</i>^V617F and <i>CALR</i>^mut HSC. Second, we searched to improve IFN therapy and focused on PML, an IFNα downstream effector whose targeting by arsenic (AS) was implicated in acute promyelocytic leukemia eradication. In a JAK2^V617F mouse model, the combination improved hematological and molecular responses at the level of early progenitors leading to disease-initiating cell eradication. The effects of this combination required PML and were associated with features of senescence. Collectively, these studies identify HSC exhaustion as a mechanism of IFN disease eradication, may help to stratify patients for IFN therapy and show that a combination with AS will improve its clinical efficacy.