Identification of Homozygous JAK2V617F Mutation in Unfractioned Blood Samples in a Substantial Proportion of Myeloproliferative Disorders: Long Term Follow-Up Study.

The discovery of the activating V617F mutation in the JAK2 tyrosine kinase gene in patients with myloproliferative disorders (MPD) provides a major breakthrough in the understanding of the pathogenesis, proving clonality and securing diagnosis of these diseases. The JAK2 V617F allele is an acquired somatic disease allele that arises in hematopoietic progenitors and confers a selective growth advantage. The mutation has been traced to a primitive stem cell that is capable of erythroid and myeloid differentiation. The data for the potential involvement of the B and T lymphocytes with the JAK2V617F mutation are still inconsistent. We present the results from the study designed the evaluate the prevalence of the JAK2V617F mutation in MPDs patients in our population and to investigate whether MPD patients that carry the JAK2V617F mutation differ in clinical course and outcome from JAK2V617F negative MPD patients. The study group consisted of 64 living MPD patients diagnosed according to standard WHO criteria for diagnosis of MPD (26 patients were diagnosed as polycythemia vera (PV), 34 as essential thrombocythemia (ET), 6 as idiopatic myelofibrosis (MF) and 8 were classified as atypical MPD) with the median follow-up of 7,4 years. DNA samples were obtained from unfractionated blood samples and the frequency of V617F JAK2 mutation was analyzed by allele-specific PCR assay. The mutant allele burden in mutation positive samples was analyzed by DNA sequencing. Our results showed that the JAK2 V617F mutation was present in 79% of patients with PV (36% were homozygous for the mutation), 58% with ET (11% homozygous), 69% with MF (28% homozygous) and in 33% of patients with atypical MPD. The mutant JAK2V617F allele burden was greater than 95% in two PV patients, which in the presence of 27% lymphocytes in the peripheral blood of the patients indicate lymphocytes involvement with the mutation. The high frequency of observed homozygosity for JAK2V617F in our study group was probably due to the long disease duration (median follow-up 11,4 years) and favors the theory of a time dependent increase in clonal dominance. Correlations of clinical and laboratory features at diagnosis and subsequent follow-up, including incidence of thrombo-hemorrhagic events, disease transformation and survival of JAK2V617F-positive and JAK2V617F-negative patients did not reveal significant differences except for the incidence of thrombotic complications. The JAK2V617F positive group had a higher incidence of thrombotic complications (30%) compared with the JAK2-V617F-negative group (14%, P< 0.05). Although we observed a disparity in the incidence of the JAK2V617F mutation in different MPD entities in our population with respect to the expected frequency of JAK2V617F from the literature, our results confirm the diagnostic significance of the JAK2V617F mutation in MPDs and support the notion that patients with this mutation should be classified in a new entity of MPDs. Identification of homozygous JAK2V617F mutation in unfractionated blood samples in a substantial proportion of long term follow-up MPD patients, together with the identification of the mutant JAK2V617F allele burden greater than 95% in two PV patients, suggests that acquisition of the JAK2V617F mutation arises in early multilineage hematopoietic progenitors and warrants further investigation.