ERK2 Substrate Binding Domains Perform Opposing Roles in Pathogenesis of a JAK2V617F-Driven Myeloproliferative Neoplasm

The interaction between ERK2 and its substrates is critically mediated by two domains, the common docking (CD) D-domain and DEF-binding pocket (DBP) domain. Previous studies have suggested that ERK2 is not only necessary to drive hematopoietic and myelo-erythroid development, but it is also important for the pathogenesis of hematological cancers, as revealed by recurrent ERK2 somatic mutations in many types of lymphoma and leukemia. Here we show that the activation of ERK2 in JAK2V617F-driven myeloproliferative neoplasm (MPN) enhance polycythemia vera (PV) progression from erythrocytosis to myelofibrosis when ERK2-DBP binding capacity is disabled. Conversely, targeting the ERK2 D-domain preserves ERK2 catalytic function while reducing outgrowth and proliferation of human and murine MPN cell lines.

To determine whether ERK2 binding domains play a role in the pathogenesis of JAK2V617F-driven MPN, we generated an ERK2-Y261A (Erk2 ^Y261A) mutant knockin mouse model in which the ERK2-DBP domain was inactivated, but ERK2 kinase activity and D-domain function was preserved. We observed splenomegaly in Erk2 ^Y261A/Y261A, characterized by a 10-fold expansion of CD44+/Ter119+ primitive erythroblasts and Ter119+CD44+/ FSC ^hi immature erythroid progenitors. We then transplanted Jak2 ^V617F-expressing ERK2 wildtype (WT), knockout (KO) or DBP hematopoietic progenitors (HSPCs) into irradiated, immunodeficient mice (Rag2 ^−/−Il2rg ^−/−) and observed disease resembling human PV, with elevated RBC counts, hematocrit, and hemoglobin levels. ERK2-DBP mutant recipients exhibited mild but sustained erythrocytosis with a prevalence of circulating GFP+ monocytes and neutrophils. At 12-weeks post-transplantation, ERK2-DBP mutant recipients had significant splenic burden and decreased bone marrow cellularity relative to WT and KO recipients. Reticulin staining confirmed progression towards myelofibrosis with interstitial infiltration in both the BM and the spleen of ERK2 DBP mutant recipients.

To understand the molecular basis of ERK-DBP domain in JAK2V617F-driven MPN, we performed in vitro colony forming unit assay to assess the clonogenic potential of JAK2V617F-expressing HSPCs in cytokine supplemented-methylcellulose supporting myeloid or erythroid progenitor development. Ectopic JAK2V617F expression markedly reduced colony formation by both ERK2 wildtype and knockout HSPCs. On the other hand, JAK2V617F expression did not reduce colony formation by ERK2-DBP HSPCs. These differences were not evident in methylcellulose cultures that supported erythroid development, suggesting that disabling ERK-DBP binding capacity primarily affects the myeloid lineage. Furthermore, we found that JAK2V617F increased senescence associated β-galactosidase (SA-βGa) activity in ERK2-WT HSPCs. SA-βGal activity was modestly affected by ERK deficiency but was strongly attenuated by the ERK2-DBP mutation, supporting the notion that ERK2-DBP domain promotes oncogene-induced senescence (OIS). Previously, we identified transcription factor, Early growth response 1 (Egr1) which interacts with ERK2-DBP domain and plays a critical role as a tumor suppressor in myeloid neoplasms. We observed that JAK2V617F expression failed to suppress colony formation and senescence by Egr1-deficient HSPCs. The ability of EGR1 to restore these effects is abrogated by inactivating the DEF motif (EGR1-Y252A). Together, this data demonstrates that the interaction between ERK2-DBP and Egr1 is required for JAK2V617F-mediated colony suppression and senescence induction.

Finally, to determine whether ERK-D domain acts to promote progression by targeting substrates distinct from those that bind to the DBP domain, we developed an inhibitor (#76) of the ERK-D domain. Indeed, 76 spared ERK phosphorylation and EGR1 induction but impaired RSK phosphorylation, a critical D-domain substrate involved in cancer progression. Moreover, 76 suppressed colony formation by JAK2V61F-expressing HSPCs akin to the active site MEK/ERK inhibitor, U0126. By altering the 4-ethoxybenylidene moiety of 76, we were able to enhance anti-proliferative effects of the parent compound against human MPN cell lines, SET-2, UKE-1, and Ku812. Our findings identify ERK2-domain specific roles in the pathogenesis of JAK2V617F driven MPN and supports a novel therapeutic approach to targeting JAK2 and MAPK dependent MPN.