Mutant calreticulin knockin mice develop thrombocytosis and myelofibrosis without a stem cell self-renewal advantage

• Published in: Blood Vol. 131; no. 6; pp. 649 - 661
• Main Authors: Li, Juan, Prins, Daniel, Park, Hyun Jung, Grinfeld, Jacob, Gonzalez-Arias, Carlos, Loughran, Stephen, Dovey, Oliver M., Klampfl, Thorsten, Bennett, Cavan, Hamilton, Tina L., Pask, Dean C., Sneade, Rachel, Williams, Matthew, Aungier, Juliet, Ghevaert, Cedric, Vassiliou, George S., Kent, David G., Green, Anthony R.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 08-02-2018
• Subjects: Animals; Calreticulin - genetics; Cell Self Renewal - genetics; Cells, Cultured; Hematopoietic Stem Cells - physiology; Homozygote; Leukocytosis - genetics; Leukocytosis - pathology; Mice; Mice, Inbred C57BL; Mice, Transgenic; Mutation, Missense; Primary Myelofibrosis - genetics; Splenomegaly - genetics; Splenomegaly - pathology; Thrombocythemia, Essential - genetics; Thrombocythemia, Essential - pathology; Thrombocytosis - genetics
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2017-09-806356

Somatic mutations in the endoplasmic reticulum chaperone calreticulin (CALR) are detected in approximately 40% of patients with essential thrombocythemia (ET) and primary myelofibrosis (PMF). Multiple different mutations have been reported, but all result in a +1-bp frameshift and generate a novel protein C terminus. In this study, we generated a conditional mouse knockin model of the most common CALR mutation, a 52-bp deletion. The mutant novel human C-terminal sequence is integrated into the otherwise intact mouse CALR gene and results in mutant CALR expression under the control of the endogenous mouse locus. CALRdel/+ mice develop a transplantable ET-like disease with marked thrombocytosis, which is associated with increased and morphologically abnormal megakaryocytes and increased numbers of phenotypically defined hematopoietic stem cells (HSCs). Homozygous CALRdel/del mice developed extreme thrombocytosis accompanied by features of MF, including leukocytosis, reduced hematocrit, splenomegaly, and increased bone marrow reticulin. CALRdel/+ HSCs were more proliferative in vitro, but neither CALRdel/+ nor CALRdel/del displayed a competitive transplantation advantage in primary or secondary recipient mice. These results demonstrate the consequences of heterozygous and homozygous CALR mutations and provide a powerful model for dissecting the pathogenesis of CALR-mutant ET and PMF.
•Mutant CALR drives ET and MF in knockin mice.•Mutant CALR expression results in expansion of phenotypic HSCs without a self-renewal advantage.