Incidence and Clinical Features of Calreticulin mutation in Patients with Jak2V617F-Negative Essential Thrombocytosis and Primary Myelofibrosis from Singapore

Background: Slightly less than half of the patients with essential thrombocytosis (ET) and primary myelofibrosis (PMF) do not have specific gene mutations. Recent discovery of CALR mutation in JAK2V617F-negative ET and PMF patients have accounted for 20-25% of ET and PMF patients, and therefore redefining diagnostic approach to ET or PMF. Here we reported the incidence of CALR mutation and clinical features of clinically diagnosed Jak2V617F-negative ET and PMF in Singapore.

Method: We retrospectively identified 140 cases of Jak2V617F-negative ET and PMF from two healthcare institutions in Singapore since 2006. Sanger sequencing was performed for the targeted detection of CALR exon 9 mutations. Clinicopathologic correlation was carried out by assessing relevant clinical data and the CALR mutation status, and compared with JAK2-positive cases (25 cases of ET and 9 cases of PMF).

Results: The Jak2V617-negative ET/PMF patients were ethnic Chinese (76.2%), Malay (13.9%) and others (9.9%). CALR mutations were detected in 32.8% (40/122) of Jak2V617-negative ET and 55.6% (10/18) Jak2V617-negative PMF patients. Higher platelet count was observed in CALR-positive ET patients with median platelet count of 1056.5 x 10⁹/L and 637.5 x 10⁹/L for CALR-positive and -negative patients respectively (p<0.001). CALR-positive ET also required higher doses of hydroxyurea to maintain platelet count below 600 x 10⁹/L (p=0.038). There was no difference in thrombotic events, presence of splenomegaly, presenting WBC count or hemoglobin (Hb) level between CALR-positive and -negative ET. However, when compared to Jak2-positive ET, Jak2-negative ET had a significantly lower incidence of thrombotic events (Jak2-negative ET, 8.2%; Jak2-positive ET, 50.0%, p<0.001), and lower Hb level (p<0.001). The incidence of thrombotic events in CALR-positive ET and PMF were 10.3% and 10.0% respectively. There was also no significant difference in presenting WBC count, platelet count, Hb level, presence of splenomegaly, or thrombotic event between CALR-positive vs -negative PMF. Lactic acid dehydrogenase (LDH) level at diagnosis however, was significantly higher in CALR-positive ET/PMF when compared to CALR-negative group (p=0.011). There was also significantly higher incidence of elevated peripheral blasts of >1% in CALR-positive ET/PMF (12% in CALR-positive and 3.3% in CALR-negative group, p=0.045). Blast transformation occurred in 6% of patients with CALR-positive PMF while only 1.1% in CALR-negative PMF. Besides the two commonly reported mutations (CALR52del, 30.8%; CALR5ins, 33.3%) in our screening of the CALR exon 9 in Jak2V617F-negative ET and PMF patients, 7 other novel mutations (c.1098_1137delinsTTTCTT, c.1102_1104delAAG, c.1122_1155delinsGGGCCAGGCACTTGTCG, c.1129_1153delinsCTTTGCGTTTCTTTT, c.1132_1155delinsTGTCG, c.1139delA, c.1141delG) were also found individually. All mutant CALR proteins possess an altered C-terminus with a longer peptide stretch caused by a disrupted reading frame due to these frameshift mutations.

Conclusion:CALR mutation is common in Jak2V617F-negative PMF and ET in this multiracial Singaporean cohort, and has similar frequency when compared with Western report. It was associated with higher platelet count in ET, higher level of LDH and possibly a higher risk of blast transformation in PMF. 7 novel CALR mutations were identified and more subtypes of CALRmutation variants are expected to be uncovered in the near future.