Mutations in the calreticulin gene (CALR) represented by deletions and insertions in exon 9 inducing a 21/12 frameshift are associated with a significant fraction of myeloproliferative neoplasms (MPNs). The mechanisms by which CALR mutants induce MPN are unknown. Here, we show by transcriptional, proliferation, biochemical, and primary cell assays that the pathogenic CALR mutants specifically activate the thrombopoietin receptor (TpoR/MPL). No activation is detected with a battery of type I and II cytokine receptors, except granulocyte colony-stimulating factor receptor, which supported only transient and weak activation. CALR mutants induce ligand-independent activation of JAK2/STAT/phosphatydylinositol-39-kinase (PI3-K) and mitogen-activated protein (MAP) kinase pathways via TpoR, and autonomous growth in Ba/F3 cells. In these transformed cells, no synergy is observed between JAK2 and PI3-K inhibitors in inhibiting cytokine-independent proliferation, thus showing a major difference from JAK2V617F cells where such synergy is strong. TpoR activation was dependent on its extracellular domain and its N-glycosylation, especially at N117. The glycan binding site and the novel C-terminal tail of the mutant CALR proteins were required for TpoR activation. A soluble form of TpoR was able to prevent activation of full-length TpoR provided that it was N-glycosylated. By confocal microscopy and subcellular fractionation, CALR mutants exhibit different intracellular localization from that of wild-type CALR. Finally, knocking down either MPL/TpoR or JAK2 in megakaryocytic progenitors from patients carrying CALR mutations inhibited cytokine-independent megakaryocytic colony formation. Taken together, our study provides a novel signaling paradigm, whereby a mutated chaperone constitutively activates cytokine receptor signaling.
Institut National du Cancer (PLBIO2015), Agence Nationale de la Recherche, (ANR-13-JVSV1-
GERMPN-01), Institut National de la Sant´e et de la Recherche M´edicale (INSERM), la Ligue Nationale contre le Cancer (´equipe labellis´ee HR 2013, 2016), Ludwig Institute for Cancer Research, Salus Sanguinis Foundation, the Action de Recherche Concert´ee project ARC10/15–027 of the Universit´e catholique de Louvain, the Fondation contre le Cancer, the PAI Programs BCHM61B5, and Belgian Medical Genetics Initiative, Cliniques universitaires St-Luc, Brussels, Austrian Science Fund (FWF: project numbers
F2812-B20 and F4702-B20) and A*STAR BMRC.