Screening JAK2 Exon 12 in JAK2 V617F Negative Patients with Myeloproliferative Disorders Reveals a New Splice Site Mutation

Background. The identification of the somatic mutation V617F in exon 14 of the Janus kinase 2 gene (JAK2) has simplified the diagnosis of many patients affected with typical chronic myeloproliferative diseases (MPDs). In patients without the V617F the molecular basis of MPD are still unclear but, recently, mutations in exon 12 of the JAK2 gene have been identified in a minority of patients, associated with a selective increase in erythropoiesis resulting in polycythemia vera (PV) or idiopathic erythrocytosis (IE) (Scott et al, NEJM 2007).

Aim. To determine the JAK2 exon 12 mutational status in a group of JAK2 V617F negative patients with PV or IE.

Methods. Genomic DNA was extracted from peripheral blood leukocytes from 85 MPD patients (PV or IE) included in the present study. All the samples were tested for JAK2 V617F mutation by allele specific polymerase chain reaction (ASO-PCR) and those negative for V617F mutant allele were subjected to real time quantitative PCR (RQ-PCR) using hybridization probes. Subsequently, all JAK2 V617F negative samples by both ASO-PCR and RQ-PCR where subjected to direct sequencing to exclude JAK2 exon 12 mutations.

Results. JAK2 V617F was positive in 78 (91.7%) of the 85 patients, however, in two of these patients the V617F mutation was only detected upon subjecting genomic DNA to RQ-PCR which revealed low levels of the mutant allele, 2.65 % and 3.98%. Analysis of the JAK2 exon 12 in the seven JAK2 V617F negative patients detected three previously described mutations: a duplication (V536-1546) and two deletions (H539-K540del+542K and R541-E543 delinsK)and a previously unreported splice site mutation detected in intron 12 (IVS12nt6 T-C). To our knowledge this was the first description of intronic mutations in the JAK2 gene. No mutations were detected in the remaining 3 (3.6%) patients.

Conclusions. In this cohort study JAK2 mutations were observed in 82 (96,5%), of the 85 patients, of these 78 (95.12%) had the V617F allele. In two cases V617F was detected at low levels (2.65% and 3.98%) only by RQ-PCR, highlighting the need for sensitive techniques to detect somatic mutations. One of the patients, a young male with erythrocytosis and low serum Epo levels, revealed a previously unreported splicing mutation (IVS12 nt6: T-C). This study illustrates the heterogeneity at the DNA level in PV patients which may assist in better understanding the genotype and phenotype relationship in MPD patients and assist in further delineating the role of JAK2 in these disorders.