Type 1 versus Type 2 calreticulin mutations in essential thrombocythemia: a collaborative study of 1027 patients.

CALR (calreticulin) trails JAK2 as the second most mutated gene in essential thrombocythemia (ET). Mutant CALR in ET is a result of frameshift mutations, caused by exon 9 deletions or insertions; type-1, 52-bp deletion (p.L367fs*46), and type-2, 5-bp TTGTC insertion (p.K385fs*47) variants constitute more than 80% of these mutations. The current study includes a total of 1027 patients divided into test (n = 402) and validation (n = 625) cohorts. Among the 402 ET patients in the test cohort, 227 (57%) harbored JAK2, 11 (3%) Myeloproliferative leukemia virus oncogene (MPL), and 114 (28%) CALR mutations; 12% were wild-type for all three mutations (i.e., triple-negative). Among the 114 patients with CALR mutations, 51 (45%) displayed type-1 and 44 (39%) type-2 variants; compared to mutant JAK2, both variants were associated with higher platelet and lower hemoglobin and leukocyte counts. However, male sex was associated with only type-1 (P = 0.005) and younger age with type-2 (P = 0.001) variants. Notably, platelet count was significantly higher in type-2 vs. type-1 CALR-mutated patients (P = 0.03) and the particular observation was validated in the validation cohort that included 111 CALR-mutated ET patients (P = 0.002). These findings, coupled with the recent demonstration of preferential expression of mutant and wild-type CALR in megakaryocytes, suggest differential effects of CALR variants on thrombopoiesis.