Abstract
Myeloproliferative neoplasms (MPN) show dysregulated JAK2 signaling. JAK2 inhibitors provide clinical benefits, but compensatory activation of MAPK pathway signaling impedes efficacy. We hypothesized that dual targeting of JAK2 and ERK1/2 could enhance clone control and therapeutic efficacy. We employed genetic and pharmacologic targeting of ERK1/2 in Jak2V617F MPN mice, cells and patient clinical isolates. Competitive transplantations of Jak2V617F vs. wild-type bone marrow (BM) showed that ERK1/2 deficiency in hematopoiesis mitigated MPN features and reduced the Jak2V617F clone in blood and hematopoietic progenitor compartments. ERK1/2 ablation combined with JAK2 inhibition suppressed MAPK transcriptional programs, normalized cytoses and promoted clone control suggesting dual JAK2/ERK1/2 targeting as enhanced corrective approach. Combined pharmacologic JAK2/ERK1/2 inhibition with ruxolitinib and ERK inhibitors reduced proliferation of Jak2V617F cells and corrected erythrocytosis and splenomegaly of Jak2V617F MPN mice. Longer-term treatment was able to induce clone reductions. BM fibrosis was significantly decreased in MPLW515L-driven MPN to an extent not seen with JAK2 inhibitor monotherapy. Colony formation from JAK2V617F patients’ CD34+ blood and BM was dose-dependently inhibited by combined JAK2/ERK1/2 inhibition in PV, ET, and MF subsets. Overall, we observed that dual targeting of JAK2 and ERK1/2 was able to enhance therapeutic efficacy suggesting a novel treatment approach for MPN.
Introduction
Myeloproliferative neoplasms (MPN) are clonal hematopoietic stem cell disorders characterized by excessive output of mature myeloid cells and inherent risk for leukemic transformation [1]. MPN subtypes show distinct clinical phenotypes; polycythemia vera (PV) primarily characterized by erythrocytosis, essential thrombocythemia (ET) featuring thrombocytosis, and myelofibrosis (MF) typified by progressive bone marrow (BM) fibrosis inducing cytopenias [2]. These MPN subsets share dysregulated JAK2 signaling [3] constitutively activated by somatic mutations in JAK2, thrombopoietin receptor MPL or the chaperone calreticulin (CALR) [4]. JAK2 inhibitors represent a standard treatment in MPN approved for MF and PV [5]. Ruxolitinib, fedratinib or JAK2 inhibitors in development provide benefits including reduction of splenomegaly and symptoms. However, disease-modifying activity of clinical JAK2 inhibitors has remained modest [6].
MAPK pathway signaling including the sequential RAF, MEK, and ERK kinases, is involved in many cancers including hematologic malignancies, and has been suggested for targeting in several disease settings [7,8,9,10,11,12]. In MPN, inhibition of BRAF or MEK has been explored in combination with JAK2 inhibitors in vitro [13,14,15] and RAS/MAPK pathway gene mutations were shown to impact on response to treatment and outcome of MPN patients [16, 17]. It has been shown that MAPK pathway signaling functionally interferes with JAK2 inhibition in MPN, thus limiting the corrective potential of JAK2 inhibitors [18,19,20]. While we have reported that MAPK signaling remains activated upon ruxolitinib therapy via PDGFRα signaling [18], in-depth analyses of global JAK2 signaling have elucidated cell-intrinsic mechanisms of MAPK pathway activation via splicing factor YBX1 [19, 20]. An important role of MAPK pathway activation as a therapeutic target in MPN has been confirmed by the efficacy of combined JAK2 and MEK inhibition in preclinical models and patient cells observed with clinical and preclinical compounds including binimetinib, selumetinib, trametinib, and PD0325901 [18, 19]. These findings highlight that MAPK pathway signaling must be addressed to enhance therapeutic efficacy of JAK2 inhibition in MPN.
Knowledge is scarce about the potential of targeting alternative MAPK pathway components other than BRAF or MEK, which might represent more optimal targets to control MPN. ERK1/2 are distal to MEK1/2 in the MAPK pathway, which could render ERK inhibition less prone for escape from therapeutic inhibition by adaptive signaling changes [7]. ERK1/2 directly act on multiple immediate downstream targets implicated in proliferation and survival, giving them a “switch” function for concerted MAPK pathway-driven effector programs. Importantly, ERK1/2 have been shown to mediate essential functions for hematopoiesis with ERK1/2 ablation inducing cytopenias and decreasing hematopoietic progenitor clones in normal hematopoiesis [21, 22]. We hypothesized that targeting ERK could be advantageous to enhance clone control and increase therapeutic efficacy of JAK2 inhibition in MPN. Thus, we evaluated dual targeting of JAK2 and ERK1/2 as corrective approach in MPN preclinical models, cells and patient clinical isolates using genetic and pharmacologic approaches….
