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

<jats:title>Abstract</jats:title><jats:sec><jats:title>Objectives</jats:title><jats:p>In patients with essential thrombocythemia (ET), after the <jats:italic>JAK2</jats:italic>V617F driver mutation, mutations in <jats:italic>CALR</jats:italic> are common (classified as type 1, 52‐bp deletion or type 2, 5‐bp insertion). <jats:italic>CALR</jats:italic> mutations have generally been associated with a lower risk of thrombosis. This study aimed to confirm the impact of <jats:italic>CALR</jats:italic> mutation type on thrombotic risk.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>We retrospectively investigated 983 ET patients diagnosed in Spanish and Polish hospitals.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>With 7.5 years of 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 <jats:italic>JAK2</jats:italic>V617F, <jats:italic>CALR</jats:italic>‐type 1 and <jats:italic>CALR</jats:italic>‐type 2 groups, respectively (<jats:italic>P</jats:italic> = .002). Comparing <jats:italic>CALR</jats:italic>‐type 1 and <jats:italic>CALR</jats:italic>‐type 2 groups, TFS for venous thrombosis was lower in <jats:italic>CALR</jats:italic>‐type 1 (<jats:italic>P</jats:italic> = .046), with no difference in TFS for arterial thrombosis observed. The cumulative incidence of thrombosis was significantly different comparing <jats:italic>JAK2</jats:italic>V617F vs <jats:italic>CALR</jats:italic>‐type 2 groups but not <jats:italic>JAK2</jats:italic>V617F vs <jats:italic>CALR</jats:italic>‐type 1 groups. Moreover, <jats:italic>CALR‐</jats:italic>type 2 mutation was a statistically significant protective factor for thrombosis with respect to <jats:italic>JAK2</jats:italic>V617F in multivariate logistic regression (OR: 0.45, <jats:italic>P</jats:italic> = .04) adjusted by age.</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>Our results suggest that <jats:italic>CALR</jats:italic> mutation type has prognostic value for the stratification of thrombotic risk in ET patients.</jats:p></jats:sec>