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Defining the requirements for the pathogenic interaction between mutant calreticulin and MPL in MPN

Shannon Elf, Nouran S. Abdelfattah, April J. Baral, Danielle Beeson, Jeanne F. Rivera, Amy Ko, Natalie Florescu, Gabriel Birrane, Edwin Chen and Ann Mullally

Key points

  • The binding of mutant calreticulin to MPL can be uncoupled from MPL activation.

  • The lectin activity but not the chaperone functionality of CALRMUT is required for cytokine independent growth.

Abstract

Mutations in calreticulin (CALR) are phenotypic drivers in the pathogenesis of myeloproliferative neoplasms (MPN). Mechanistic studies have demonstrated that mutant CALR binds to the thrombopoietin receptor MPL, and that the positive electrostatic charge of the mutant CALR C-terminus is required for mutant CALR-mediated activation of JAK-STAT signaling. Here we demonstrate that although binding between mutant CALR and MPL is required for mutant CALR to transform hematopoietic cells, binding alone is insufficient for cytokine independent growth. We further show that the threshold of positive charge in the mutant CALR C-terminus influences both binding of mutant CALR to MPL and activation of MPL signaling. We find that mutant CALR binds to the extra-cellular domain of MPL and that three tyrosine residues within the intracellular domain of MPL are required to activate signaling. With respect to mutant CALR function, we show that its lectin-dependent function is required for binding to MPL and for cytokine independent growth while its chaperone and polypeptide binding functionalities are dispensable. Together, our findings provide additional insights into the mechanism of the pathogenic mutant CALR-MPL interaction in MPN.

  • Submitted August 8, 2017.
  • Accepted December 18, 2017.