Somatic mutations of calreticulin in a Brazilian cohort of patients with myeloproliferative neoplasms

Essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (PMF) are Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) characterized by increased myeloid proliferation. The gain of function induced by the Janus kinase 2 mutation, JAK2 V617F , has been reported in most PV and in more than half of ET and PMF cases. 1 However, the presence of different disease phenotypes and the absence of the JAK2 mutation in some MPNs suggests that additional genetic lesions or/and aberrant signaling pathways may be involved in the pathogenesis of these diseases. 1,2 In December 2013, somatic mutations in the calreticulin (CALR) gene were identified in ET and PMF patients by two independent groups 3,4 and confirmed by others. 5–9 CALR mutations have been reported as mutually exclusive with JAK2 and MPL mutations and may be present in 56–88% of JAK2/MPL-negative cases. 3,4 A recent paper reported a patient that had both mutations, JAK2 V617F and a CALR exon 9 mutation , simultaneously. 10 Over thirty different CALR mutations in exon 9 have been described, but the most frequent mutations (about 80%) may be classified as type-1 [L367fs*46; deletion of 52 base pairs (bp)] and type-2 (K385fs*47; insertion of 5 bp). 3,4 The functional changes induced by the mutations are still not completely elucidated, but the overexpression of the type 1 CALR mutation in Ba/F3 cells (an IL3 dependent cell line) leads to cytokine-independent cell growth and STAT5 activation. 4 Gene expression signature studies also indicate that JAK2 and CALR mutations share mechanisms of malignant transformation , reaffirming a central role of the JAK/STAT signaling pathway in the pathogenesis of MPN. 11 The aim of the present study was to characterize the prevalence of CALR mutations and the clinical and labora-torial characteristics of CALR-mutated patients in a Brazilian cohort of MPNs. Seventy-three MPN patients were included in the study (ET = 32, PV = 20, PMF = 21). Patients' characteristics are described in Table 1. Peripheral blood samples were collected , submitted to hemolysis, and DNA was extracted by the phenol/chloroform method. All samples were investigated for JAK2 and CALR mutations. The JAK2 V617F mutation and CALR exon 9 mutations were verified as previously described. 12,13 CALR mutations were classified as type-1 (deletion of 52 bp), type-2 (insertion of 5 bp) or others (Figure 1). In our cohort, CALR mutations were found in 20 patients (13 ET and 7 PMF; Table 1) and were mutually …