Myeloproliferative neoplasm-driving Calr frameshift promotes the development of pulmonary hypertension in mice

Frameshifts in the Calreticulin (CALR) exon 9 provide a recurrent driver mutation of essential thrombocythemia (ET) and primary myelofibrosis among myeloproliferative neoplasms (MPNs). Here, we generated knock-in mice with murine Calr exon 9 mimicking the human CALR mutations, using the CRISPR-Cas9 method. Knock-in mice with del10 [Calr^{del10/WT (wild−type)} mice] exhibited an ET phenotype with increases of peripheral blood (PB) platelets and leukocytes, and accumulation of megakaryocytes in bone marrow (BM), while those with ins2 (Calr^ins2/WT mice) showed a slight splenic enlargement. Phosphorylated STAT3 (pSTAT3) was upregulated in BM cells of both knock-in mice. In BM transplantation (BMT) recipients from Calr^del10/WT mice, although PB cell counts were not different from those in BMT recipients from Calr^WT/WT mice, Calr^del10/WT BM-derived macrophages exhibited elevations of pSTAT3 and Endothelin-1 levels. Strikingly, BMT recipients from Calr^del10/WT mice developed more severe pulmonary hypertension (PH)—which often arises as a comorbidity in patients with MPNs—than BMT recipients from Calr^WT/WT mice, with pulmonary arterial remodeling accompanied by an accumulation of donor-derived macrophages in response to chronic hypoxia. In conclusion, our murine model with the frameshifted murine Calr presented an ET phenotype analogous to human MPNs in molecular mechanisms and cardiovascular complications such as PH.