Presence of Calreticulin Mutations in JAK2-Negative Polycythemia Vera

Calreticulin (CALR) mutations have recently been reported in JAK2- and MPL-negative Myeloproliferative Neoplasms (MPN), particularly essential thrombocythemia (ET) and primary myelofibrosis (PMF).The clinical course of sporadic CALR-mutated patients seems to be more indolent than that of JAK2-mutated patients. In contrast, no CALR mutation has been found in the 647 published cases of Polycythemia Vera (PV) patients tested. Consequently, CALR mutations were considered exclusive to JAK2 and MPL mutations. Since 98% of PV patients harbor a JAK2 mutation (mostly the V617F mutation in exon 14 and more rarely, in exon 12), the absence of CALR mutations in PV seemed logical. Here, we describe two JAK2^V617F-negative PV patients who presented with a CALR mutation at the time of diagnosis.

Patient # 1 had hemoglobin at 168 g/L, hematocrit at 51.3%, and increased red cell mass (RCM) at 128% associated with a normal erythropoietin level. The bone marrow biopsy showed hypercellularity for age, panmyelosis associated with normal megakaryocytes and rare isolated abnormal enlarged forms. Using reticulin stain, no myelofibrosis was noted. Patient # 2 had hemoglobin at 194 g/L, hematocrit at 53% and low erythropoietin level without any dehydration. Both had moderately elevated platelet counts (658 and 575 x10⁹/L respectively) with normal leukocyte counts. They were negative for BCR-ABL. No mutation was found in JAK2 exons 12, 13 and 14 by HRM and allele-specific real-time PCR or in MPL exon 10. Using HRM analysis, CALR mutations were suspected in both patients and confirmed using Sanger sequencing and product sizing analyses: CALR mutations were in both patients type 1 deletions (52-bp deletion; c.1092_1143del). To complete genomic tests made on peripheral blood granulocytes, we performed colony assays in methylcellulose and in collagen, picked single BFU-E colonies grown after 14 days in the presence of erythropoietin, and genotyped each colony individually for CALR. Of the 27 colonies genotyped, 6 had no PCR amplification and 21 harboured the same CALR mutation observed in peripheral blood granulocytes, i.e 52-bp deletion; c.1092_1143del. BFU-E were found heterozygous for CALR, with a mean allele burden of 49%. To our knowledge, these patients are the first cases of CALR-mutated PV to be reported. However, since a biclonalJAK2V617F and CALR MPN case recently reported, we cannot rule out the possibility of a biclonal disease involving a yet unknown mutation associated with a CALR mutation. On the other hand, the presence of a CALR mutation both in peripheral granulocytes and in BFU-E suggests that the CALR mutation plays a role in the polycythemia phenotype. Our observations highlight the fact that in the absence of JAK2 mutation, CALR mutations can also be associated with PV.

In conclusion, our data indicate that testing JAK2-negative PV patients for CALR mutations may be useful.