Type 2 calreticulin mutations activate ATF6 to promote BCL-xL-mediated survival in myeloproliferative neoplasms

The majority of calreticulin (CALR) mutations in myeloproliferative neoplasms (MPNs) are classified as either type 1, a 52 base-pair deletion (CALRdel52), or type 2, a 5 base-pair insertion (CALRins5). Both are gain-of-function (GOF) mutations that generate an identical mutant C-terminal tail, which mediates the binding to and activation of the thrombopoietin receptor MPL. We recently reported that despite this shared GOF, CALRdel52 but not CALRins5 mutations cause loss of calcium binding function, leading to activation of and dependency on the IRE1a/XBP1 pathway of the unfolded protein response (UPR). This led us to ask whether CALRins5 mutations activate and depend on a different UPR pathway, and whether this is likewise mediated by a mutation type-specific loss-of-function (LOF). Here, we show that CALRins5 mutations lead to activation of the ATF6 pathway of the UPR due to loss of CALR chaperone function. This LOF is caused by interference of the CALRins5 mutant C-terminus with key chaperone residue H170. Further, we show that CALRins5 cells are partially dependent on ATF6 for cytokine-independent growth, and identify BCL-xL as a transcriptional target of ATF6 that promotes type 2 CALR mutant cell survival.