Skewed ratio between type 1 and type 2 calreticulin mutations in essential thrombocytosis patients with concomitant Janus kinase 2 V617F mutation

Detection of somatic mutations in cardinal driver genes is a strong argument for diagnosis in classical Philadelphia-negative myeloproliferative neoplasms (MPNs). Driver mutations in Janus kinase 2 (JAK2), calreticulin (CALR), and thrombopoietin receptor (MPL), are generally considered mutually exclusive, but several reports have suggested that they coexist in a small subgroup of patients. In this study, we retrospectively searched for CALR mutations in 136 suspected MPN patients with low allelic burden (≤5%) JAK2 V617F. Fifteen patients with concomitant JAK2 V617F and CALR mutations were identified, of whom 10 were diagnosed with essential thrombocytosis (ET). More than 50 different indel mutations in exon 9 of CALR have been reported, with type 1 (52 bp deletion) and type 2 (5 bp insertion) accounting for more than 80% of CALR-mutated MPN cases. Type 1 is generally considered the most common mutation, but, interestingly, our double-mutated ET patients seem to have an inversed ratio between type 1 and type 2 CALR mutations. Our findings support the possibility of coexisting JAK2 V617F and CALR mutations and stress the importance of further molecular screening in MPN patients with low allele frequencies of JAK2 V617F.