The risk of thrombosis in essential thrombocythemia is associated with the type of CALR mutation: A multicentre collaborative study

OBJECTIVES: In patients with essential thrombocythemia (ET), after the JAK2V617F driver mutation, mutations in CALR are common (classified as type-1, 52-bp deletion, or type-2, 5-bp insertion). CALR mutations have generally been associated with a lower risk of thrombosis. This study aimed to confirm the impact of CALR mutation type on thrombotic risk. METHODS: We retrospectively investigated 983 ET patients diagnosed in Spanish and Polish hospitals. RESULTS: With 7.5 years median follow-up from diagnosis, 155 patients (15.8%) had one or more thrombotic event. The 5-year thrombosis free survival (TFS) rate was 83.8%, 91.6% and 93.9% for the JAK2V617F, CALR-type 1 and CALR-type 2 groups, respectively (p = 0.002). Comparing CALR-type 1 and CALR-type 2 groups, TFS for venous thrombosis was lower in CALR-type 1 (p = 0.046), with no difference in TFS for arterial thrombosis observed. The cumulative incidence of thrombosis was significantly different comparing JAK2V617F vs. CALR-type 2 groups but not JAK2V617F vs. CALR-type 1 groups. Moreover, CALR-type 2 mutation was a statistically significant protective factor for thrombosis with respect to JAK2V617F in multivariate logistic regression (OR: 0.45, p = 0.04) adjusted by age. CONCLUSIONS: Our results suggest that CALR mutation type has prognostic value for the stratification of thrombotic risk in ET patients.