Type 1 vs type 2 calreticulin mutations and thrombotic risk in essential thrombocythemia patients: analysis of a monocentric series

Background: Recent advances in BCR-ABL1-negative myeloproliferative neoplasms (MPN) have highlighted the prevalence of mutations in the calreticulin (CALR) gene. This discovery enables for the first time molecular information, in addition to JAK2 V617F, to be used in the majority of MPN patients as an affirmative variable to discriminate neoplastic from reactive myeloproliferation. CALR mutations were reported in about 25% of essential thrombocythemia (ET) and in 35% of primary myelofibrosis (PMF) patients, not associated with JAK2 or MPL gene mutations. Specifically, two variants account for 85% of the CALR mutations in both ET and PMF: L367fs*46, type 1 and K385fs*47, type 2. Methods: In a cohort of 213 adult patients with a WHO 2008-based diagnosis of ET, baseline clinical/molecular characteristics and outcome measures (thrombosis, hemorrhages, death, overall and event-free survival) were evaluated. The JAK2 V617F mutation was detected by allele-specific PCR and confirmed by direct Sanger sequencing. CALR and MPL mutations were assessed by direct Sanger sequencing of exon 9 and 10, respectively. Results: In our ET series we identified mutations of CALR gene in 41 patients (19.3%): in particular, there were 20 (48.8%) and 15 cases (36.6%) of type 1 and 2 mutation, respectively, and 6 cases (14.6%) carried other distinct variants: D373fs*54, E372fs*50, E372fs*48, K368fs*51, K368fs*50 and L385fs*48, all detected in only one patient. Their main clinical and laboratory features at diagnosis are reported in Table 1. We then compared the three subgroups of CALR-mutated ET patients considering thrombotic risk. After a median follow-up of 11.8 years (range 1.9-31.2), we globally described 11 thrombotic events, in particular 5 arterial and 6 venous thrombosis. Of these, 10 cases (90.9%) happened in the type 1 mutation group, only one case (9.1%) in patients with a type 2 mutation, and none in those bearing other CALR mutations. Conclusions: With the limitations due to the retrospective nature of the present study, our data showed a different thrombotic rate in type 1 and 2 mutation groups, being the former higher than the latter. This was associated with an older age at diagnosis, but there was only a slight different distribution in the thrombotic risk according to either conventional (considering age >60 years, history of thrombosis and extreme thrombocytosis (platelet count >1500 x109/l) as risk factors) or IPSET-thrombosis score.