Secreted mutant calreticulins as rogue cytokines in myeloproliferative neoplasms
C. Pecquet, Nicolas Papadopoulos, Thomas Balligand, Ilyas Chachoua, Amandine Tisserand, Gaëlle Vertenoeil, Audrey Nédélec, Didier Vertommen, Anita Roy, Caroline Marty, Harini Nivarthi, Jean‐Philippe Defour, Mira El-Khoury, Eva Hug, Andrea Majoros, Erica Xu, Oleh Zagrijtschuk, Tudor Emanuel Fertig, Daciana Marta, Heinz Gisslinger, Bettina Gisslinger, Martin Schalling, Ilaria Carola Casetti, Elisa Rumi, Daniela Pietra, Chiara Cavalloni, Luca Arcaini, Mario Cazzola, Norio Komatsu, Yoshihiko Kihara, Yoshitaka Sunami, Yoko Edahiro, Marito Araki, Roman Lesyk, Veronika Buxhofer‐Ausch, Sonja Heibl, Florence Pasquier, Violaine Havelange, Isabelle Plo, William Vainchenker, Róbert Královics, Stefan N. Constantinescu
Abstract
Mutant calreticulin (CALR) proteins resulting from a -1/+2 frameshifting mutation of the CALR exon 9 carry a novel C-terminal amino acid sequence and drive the development of myeloproliferative neoplasms (MPNs). Mutant CALRs were shown to interact with and activate the thrombopoietin receptor (TpoR/MPL) in the same cell. We report that mutant CALR proteins are secreted and can be found in patient plasma at levels up to 160 ng/mL, with a mean of 25.64 ng/mL. Plasma mutant CALR is found in complex with soluble transferrin receptor 1 (sTFR1) that acts as a carrier protein and increases mutant CALR half-life. Recombinant mutant CALR proteins bound and activated the TpoR in cell lines and primary megakaryocytic progenitors from patients with mutated CALR in which they drive thrombopoietin-independent colony formation. Importantly, the CALR-sTFR1 complex remains functional for TpoR activation. By bioluminescence resonance energy transfer assay, we show that mutant CALR proteins produced in 1 cell can specifically interact in trans with the TpoR on a target cell. In comparison with cells that only carry TpoR, cells that carry both TpoR and mutant CALR are hypersensitive to exogenous mutant CALR proteins and respond to levels of mutant CALR proteins similar to those in patient plasma. This is consistent with CALR-mutated cells that expose TpoR carrying immature N-linked sugars at the cell surface. Thus, secreted mutant CALR proteins will act more specifically on the MPN clone. In conclusion, a chaperone, CALR, can turn into a rogue cytokine through somatic mutation of its encoding gene.