Activation of the thrombopoietin receptor by mutant calreticulin in CALR-mutant myeloproliferative neoplasms

• Published in: Blood Vol. 127; no. 10; pp. 1307 - 1316
• Main Authors: Araki, Marito, Yang, Yinjie, Masubuchi, Nami, Hironaka, Yumi, Takei, Hiraku, Morishita, Soji, Mizukami, Yoshihisa, Kan, Shin, Shirane, Shuichi, Edahiro, Yoko, Sunami, Yoshitaka, Ohsaka, Akimichi, Komatsu, Norio
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 10.03.2016
• Subjects: Calreticulin - genetics; Calreticulin - metabolism; Cell Line, Tumor; HEK293 Cells; Hematologic Neoplasms - genetics; Hematologic Neoplasms - metabolism; Hematologic Neoplasms - mortality; Humans; Induced Pluripotent Stem Cells - metabolism; Induced Pluripotent Stem Cells - pathology; Janus Kinase 2 - metabolism; Myeloproliferative Disorders - genetics; Myeloproliferative Disorders - metabolism; Myeloproliferative Disorders - pathology; Neoplasm Proteins - genetics; Neoplasm Proteins - metabolism; Phosphorylation; Protein Structure, Tertiary; Receptors, Thrombopoietin - genetics; Receptors, Thrombopoietin - metabolism; Thrombopoiesis - genetics; Thrombopoietin - metabolism
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2015-09-671172

Recurrent somatic mutations of calreticulin (CALR) have been identified in patients harboring myeloproliferative neoplasms; however, their role in tumorigenesis remains elusive. Here, we found that the expression of mutant but not wild-type CALR induces the thrombopoietin (TPO)-independent growth of UT-7/TPO cells. We demonstrated that c-MPL, the TPO receptor, is required for this cytokine-independent growth of UT-7/TPO cells. Mutant CALR preferentially associates with c-MPL that is bound to Janus kinase 2 (JAK2) over the wild-type protein. Furthermore, we demonstrated that the mutant-specific carboxyl terminus portion of CALR interferes with the P-domain of CALR to allow the N-domain to interact with c-MPL, providing an explanation for the gain-of-function property of mutant CALR. We showed that mutant CALR induces the phosphorylation of JAK2 and its downstream signaling molecules in UT-7/TPO cells and that this induction was blocked by JAK2 inhibitor treatment. Finally, we demonstrated that c-MPL is required for TPO-independent megakaryopoiesis in induced pluripotent stem cell–derived hematopoietic stem cells harboring the CALR mutation. These findings imply that mutant CALR activates the JAK2 downstream pathway via its association with c-MPL. Considering these results, we propose that mutant CALR promotes myeloproliferative neoplasm development by activating c-MPL and its downstream pathway. •Mutant CALR induces TPO-independent growth in the human megakaryocytic cell line UT-7/TPO.•Mutant CALR binds to the TPO receptor, inducing phosphorylation of JAK2 and activating downstream signaling.