CALR-mutated cells are vulnerable to combined inhibition of the proteasome and the endoplasmic reticulum stress response

Cancer is driven by somatic mutations that provide a fitness advantage. While targeted therapies often focus on the mutated gene or its direct downstream effectors, imbalances brought on by cell-state alterations may also confer unique vulnerabilities. In myeloproliferative neoplasms (MPN), somatic mutations in the calreticulin (CALR) gene are disease-initiating through aberrant binding of mutant CALR to the thrombopoietin receptor MPL and ligand-independent activation of JAK-STAT signaling. Despite these mechanistic insights into the pathogenesis of CALR-mutant MPN, there are currently no mutant CALR-selective therapies available. Here, we identified differential upregulation of unfolded and ubiquitinylated proteins, the proteasome and the ER stress response in CALR-mutant hematopoietic stem cells and megakaryocyte progenitors. We further found that combined pharmacological inhibition of the proteasome and IRE1-XBP1 axis of the ER stress response preferentially targets Calr-mutated stem and progenitors over wild-type cells in vivo, resulting in an amelioration in the MPN phenotype. Together, these findings leverage loss of proteostasis in Calr-mutant MPN to identify combinatorial dependencies that may be targeted for therapeutic benefit. Overall design: Long-term hematopoietic stem cells of MxCre or Calr 52/del52 MxCre mice were FACS-sorted 14 days after induction of Cre expression following pI:pC injections. Cells were sorted by the following surface markers: lineage negative, CD117 (kit) positive sca-1 positive, CD48 negative CD150 positive cells.