CALR frameshift mutations in MPN patient-derived iPS cells accelerate maturation of megakaryocytes

Abstract Calreticulin (CALR) mutations are driver mutations in myeloproliferative neoplasms (MPNs), leading to activation of the thrombopoietin receptor, and causing abnormal megakaryopoiesis. Here, we generated patient-derived CALRins5- or CALRdel52-positive induced pluripotent stem (iPS) cells to establish a MPN disease model for molecular and mechanistic studies. We demonstrated myeloperoxidase deficiency in CD15+ granulocytic cells derived from homozygous CALR-mutant iPS cells, rescued by repairing the mutation using CRISPR/Cas9. iPS cell-derived megakaryocytes showed characteristics of primary megakaryocytes such as formation of demarcation membrane system and cytoplasmic pro-platelets protrusions. Importantly, CALR mutations led to enhanced megakaryopoiesis and accelerated megakaryocytic development in a thrombopoietin-independent manner. Mechanistically, our study identified differentially regulated pathways in mutated vs. unmutated megakaryocytes, such as hypoxia signaling, which represents a potential target for therapeutic intervention. Altogether, we demonstrate key aspects of mutated CALR-driven pathogenesis, dependent on its zygosity and found known and novel therapeutic targets, making our model a valuable tool for clinical drug screening in MPNs.