Calreticulin Gene Mutations in Suspicion of Myeloproliferative Neoplasms: A Single Institution Experience

Recently, Klampfl et al. and Nangalia et al. reported recurrent somatic mutations in the calreticulin (CALR) gene in patients with essential thrombocythemia or primary myelofibrosis with nonmutated JAK2 and MPL. All CALR mutations described to date are either deletions or insertions, and occur in exon 9 and result in a frameshift leading to a loss of KDEL sequence (endoplasmic reticulum retention motif) and multiple calcium biding sites with a new basic (instead of acidic) C terminal region.

The aim of this work is to know how far the CALR mutations can fill the molecular diagnostic gap for myeloproliferative neoplasms (MPNs) left by JAK2 and MPL mutations.

The study was approved by the local ethics committee.

Our study population consists of 155 patients with platelet counts > 500 G/L sampled between 2012 and 2013 for suspicion of MPNs. Only JAK2 V617F negative patients were included.

Analyses were performed using genomic DNA isolated from peripheral blood. Patients were screened for CALR mutations using a High Resolution Melting (HRM) and then the precise mutations were characterized by Sanger sequencing which is a new approach for this test.

A total of 21 (13.5%) patients with CALR mutations were detected with 8 distinct variants.

Among CALR mutations, type 1 (52 basepair deletion, c.1092_1143del) and type 2 (5 basepair insertion, c.1154_1155insTTGTC) were the most common (respectively 7 (33%) and 7 (33%) cases). Three patients had 2 concurrent mutations (insertion+substitution / Deletion+substitution / deletion +insertion). We found a deletion that has not been reported so far to our knowledge (c.1125_1147del). Sequencing is ongoing for 4 patients.

Compared with negative patients, CALR-mutated patients demonstrated markedly higher platelet counts (median count: 820 vs 543 G/L).