Sequential Evaluation of the Proportion of Granulocyte JAK2 (V617F) Mutant Alleles in Chronic Myeloproliferative Disorders.
Authors: Passamonti Francesco , Rumi Elisa , Pietra Daniela , Elena Chiara , Arcaini Luca , Boveri Emanuela , Pascutto Cristiana , Lazzarino Mario , Cazzola Mario 
Published: October 2019

Abstract
A somatic gain-of-function mutation of the Janus kinase 2 (JAK2) gene is found in most patients with polycythemia vera (PV) and in about half of those with essential thrombocythemia (ET) or chronic idiopathic myelofibrosis (CIMF). In this study, we investigated the time course of the proportion of mutant alleles during the clinical course of the disease, and its relationship with clinical phenotype and treatment. Using a quantitative real-time polymerase chain reaction (qRT-PCR)-based allelic discrimination assay for the detection of the V617F mutation, we studied 522 patients diagnosed according to WHO criteria; 289 subjects were at diagnosis and 233 were at follow-up. Sequential studies were performed in 113 patients. Of these, 42 individuals were JAK2 (V617F)-negative at the first evaluation (6 with PV, 20 with ET and 16 with CIMF), and all of them remained negative after a median interval of 20 months (range 6–43 months). The remaining 71 patients carried JAK2 (V617F): 31 had PV, 13 had ET, 20 had CIMF and 7 had post-PV myelofibrosis. A statistically significant increase in the proportion of mutant alleles between the two time points was found only in PV (P=0.02). Taking into account the effect of time on the proportion of JAK2 (V617F) mutant alleles, we studied the slope between sequential assessments. The slope calculation was based on the difference between sequential assessments and the time elapsed. Values obtained in patients who received phlebotomy or anti-platelet therapy exclusively were considered as associated with the natural history of the disease. Median values for slopes were positive in all conditions: the highest increase was found in the PV subgroup (0.33% increase in the proportion of mutant alleles per month) while the lowest increment was observed in ET (0.08% increase in the proportion of mutant alleles per month). This observation suggests that the rate of clonal expansion of JAK2 (V617F)-positive cells is much slower in ET than in PV. This conclusion is also supported by findings in patients belonging to the follow-up subgroup: in 42 of 43 ET patients the proportion of mutant alleles did not exceed 50% after a median disease duration of 5.8 years. In order to investigate the impact of cytoreductive therapy on the time course of the proportion of JAK2 (V617F) mutant alleles, factorial ANOVA was performed using cytoreductive therapy and diagnosis as independent factors, and linear slope of the proportion of JAK2 (V617F) mutant alleles as a dependent variable. The interaction between diagnosis and treatment was also included in the model. Being on cytoreductive treatment was associated with a lower increase in the proportion of JAK2 mutant alleles (p=0.03). We also studied five JAK2 (V617F)-positive patients who needed sequential diagnostic reevaluation for clinical reasons. Three patients went through different stages of myelofibrosis and two patients with PV developed myelofibrosis. All these patients showed significant increases in the proportion of JAK2 (V617F) mutant alleles. In conclusion, JAK2 (V617F)-positive and JAK2 (V617F)-negative myeloproliferative disorders are likely distinct nosological entities. Within JAK2 (V617F)-positive conditions, the proportion of mutant alleles is relatively stable in ET while it tends to increase in PV. Cytoreductive agents may reduce the rate of clonal expansion of JAK2 (V617F)-positive cells.