PS1446 THE ROLE OF THE THROMBOPOIETIN RECEPTOR (MPL) IN JAK2V617F‐POSITIVE MYELOPLROLIFERATIVE NEOPLASMS

Background:Constitutive activation of the Jak/Stat signalling pathway is a hallmark of Myeloproliferative Neoplasms (MPN), with 60–90% of patients carrying the activating JAK2 mutation, JAK2V617F. Several studies suggest expression of a type I cytokine receptor is essential for JAK2V617F‐mediated transformation, but the precise contribution of such receptors is unresolved.Aims:Here we investigate the functional role of Mpl in JAK2V617F‐driven MPN.Methods:Mpl mutants lacking various functional domains were screened for their capacity to support JAK2V617F‐mediated factor independence in BaF/3 cells. Using CRISPR/CAS9 genome editing Mpl‐mutant mice were generated to extend these findings to an in vivo model of JAK2V617F MPN.Results:In vitro studies confirmed that Mpl expression was required for JAK2V617F‐induced transformation, but identified a Mpl truncation mutant retaining the JAK2‐interaction domain, but lacking the C‐terminus (Mpl‐IC36), typically associated with Stat5 binding, to be sufficient to support JAK2V617F‐induced transformation. Interestingly, Stat5‐phosphorylation was maintained. In contrast, deletion of the entire cytoplasmic receptor domain abrogated Stat5 phosphorylation and factor‐independent proliferation. In vivo Mpl‐wt;JAK2V617F mice exhibited the thrombocytosis and erythrocytosis typical of MPN, while Mpl deficiency abrogated thrombocytosis but sustained high red blood cell counts. Although the Mpl‐IC36 receptor was sufficient for JAK2V617F‐induced factor independence in BaF/3 cells, Mpl‐IC36;JAK2V617F mice did not develop thrombocytosis, indeed platelet counts were similar to those in Mpl null mice. This underlines differences that may emerge from in vitro and in vivo systems and highlights the important role of the Mpl cytoplasmic region for JAK2V617F‐mediated signalling.Summary/Conclusion:JAK2V617F requires Mpl to induce transformation. While a Mpl truncation mutant retaining the JAK2‐interaction domain, but lacking the C‐terminus (Mpl‐IC36) was sufficient to induce JAK2V617F‐mediated cytokine independence in BaF/3 cells in vitro, this receptor was insufficient for development of thrombocytosis in JAK2V617F mouse model. These results suggest that the role of Mpl in JAK2V617F‐induced MPN extends beyond a simple scaffold for active JAK2 and that signalling from the Mpl C‐terminal domain is required in vivo.