Mutant Calreticulin Requires Both Its Mutant C-terminus and the Thrombopoietin Receptor for Oncogenic Transformation

Abstract

Somatic mutations in calreticulin (CALR) are present in approximately 40% of patients with myeloproliferative neoplasms (MPN) but the mechanism by which mutant CALR is oncogenic remains unclear. Here, we demonstrate that expression of mutant CALR alone is sufficient to engender MPN in mice and recapitulates the disease phenotype of CALR-mutant MPN patients. We further show that the thrombopoietin receptor, MPL is required for mutant CALR-driven transformation through JAK-STAT pathway activation, thus rendering mutant CALR-transformed hematopoietic cells sensitive to JAK2 inhibition. Finally, we demonstrate that the oncogenicity of mutant CALR is dependent on the positive electrostatic charge of the C-terminus of the mutant protein, which is necessary for physical interaction between mutant CALR and MPL. Together, our findings elucidate a novel paradigm of cancer pathogenesis and reveal how CALR mutations induce MPN.

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“…The effect of ruxolitinib on mice with CALR mutations also supports the idea that mutant CALR autonomously activates the JAK-STAT signaling cascade and induces sustained thrombocytosis with increased numbers of megakaryocytes. Our observations, together with previous reports [4][5][6][7], indicate that the CALR mutation is sufficient to augment megakaryocytic cell growth and cause ET, and that CALR mutations, like the JAK2 mutation, play a driver role in MPNs.…”

Loss of pleckstrin-2 reverts lethality and vascular occlusions in JAK2V617F-positive myeloproliferative neoplasms

“…More recent studies further confirmed that patients with calreticulin or myeloproliferative leukemia virus oncogene (MPL) mutations, the other 2 major genetic abnormalities in Philadelphia-chromosome-negative MPNs (10), also involve activation of the JAK/STAT pathway (11). These reports underscore the JAK/STAT pathway in the pathogenesis of MPNs and as a valid target for therapy.…”

Molecular determinants of pathogenesis and clinical phenotype in myeloproliferative neoplasms

“…43 Differences in cytosolic calcium mobilization have been reported with the 52 base pair deletion, 44 suggesting that this may be one mechanism by which mutant CALR exerts its effect, and expression of the mutant protein does appear to be particularly restricted to megakaryocytes on immunohistochemical evaluation of bone marrow specimens. 45 More recently, it has been shown that CALR mutations can impart TPO-independence in both cell lines 46,47 and retroviral mouse models, 48,49 in a MPL-and JAK2-dependent manner, mimicking the effect of activating MPL mutations. This has been shown to be mediated by direct binding of MPL by the N domain of CALR, a phenomenon that uniquely occurs in the presence of the mutated C-terminus, 48,49 leading to autocrine activation of MPL, JAK2 dimerization and downstream STAT5 and ERK phosphorylation.…”

“…45 More recently, it has been shown that CALR mutations can impart TPO-independence in both cell lines 46,47 and retroviral mouse models, 48,49 in a MPL-and JAK2-dependent manner, mimicking the effect of activating MPL mutations. This has been shown to be mediated by direct binding of MPL by the N domain of CALR, a phenomenon that uniquely occurs in the presence of the mutated C-terminus, 48,49 leading to autocrine activation of MPL, JAK2 dimerization and downstream STAT5 and ERK phosphorylation. 46,49 It is therefore clear that the inappropriate activation of JAK2 signaling is common to the three main phenotypic driver mutations (i.e., those in CALR, MPL and JAK2 itself) and plays an important role in disease pathogenesis in each case, in keeping with the clinical efficacy of JAK2 inhibition irrespective of the presence of the mutations in JAK2.…”