Myeloproliferative neoplasms with calreticulin mutations exhibit distinctive morphologic features

Objectives: Calreticulin (CALR) mutations are present in 50% to 85% of JAK2/MPL wild-type (wt) myeloproliferative neoplasms (MPNs). The histopathologic features of CALRmutated MPNs are unknown. Methods: We identified 71 patients with essential thrombocythemia (ET), primary myelofibrosis (PMF), and post-essential thrombocythemia myelofibrosis (post-ET MF) with available CALR status. CALR was assessed using capillary electrophoresis followed by Sanger sequencing confirmation. CALR status was correlated with histopathologic features. Results: The megakaryocytes of CALR-mutated PMF more often were hyperchromatic (20/21) compared with CALRwt cases (10/14) (P =.05). CALR-mutated ET showed more megakaryocytic clustering (7/7) compared with CALR-wt cases (5/9) (P=.03). Megakaryocytes of CALR-mutated post-ET MF (8/8) had a predominance of convoluted nuclei compared with CALR-wt cases (2/4) (P =.03). CALR mutations were more frequent in post-ET MF compared with ET (P =.04). Conclusions: CALR-mutated MPNs have a higher frequency of megakaryocytic aberrancies compared with CALR-wt cases. Patients with CALR-mutated ET appear to be more likely to develop myelofibrosis compared with patients with wt CALR.