Novel signals in mutant calreticulin enhance its secretion in myeloproliferative neoplasms

Abstract

Calreticulin is an endoplasmic reticulum resident chaperone that is involved in many physiological roles including protein folding and calcium homeostasis. Recently, frameshift mutations have been documented in the exon 9 of the calreticulin (CALR) gene in myeloproliferative neoplasms. These mutations result in the substitution of the negatively charged sequence, in the carboxy-terminal domain, with positively charged and hydrophobic residues, which leads to the activation of the JAK/STAT pathway via direct interaction with the thrombopoietin receptor (MPL). Moreover, the loss of the endoplasmic retention signal (KDEL) facilitates its secretion to the extracellular medium, where it acts as a rogue cytokine. In this work, we aim to gain more understanding of the molecular mechanism behind calreticulin mutant secretion. To this end, we performed in silico studies where we analyzed the newly generated sequence of CALR mutants, and we show that this sequence is highly enriched in secretion signals containing mainly arginine. By using two online prediction tools we predict that mutating these signals into alanines significantly reduced the secretion of the mutants. Interestingly, introducing these signals to the wild type CALR enhances its secretion despite the presence of the KDEL signal. Altogether, these results challenge our current understanding of the molecular mechanisms behind protein secretion and open new avenues towards the development of novel treatment strategies for CALR-associated MPNs by targeting the predicted signals with small molecules.