Prognostic Value of JAK2V617F, Calr and MPL Mutational Status on Outcome and Thrombotic Risk in a Retrospective Cohort of 138 Patients with Essential Thrombocythemia

Background: Life expectancy in Essential Thrombocythemia (ET) patients is superimposed to normal population. The main causes of death are thrombotic events and evolution into myelofibrotic phase or secondary myelodisplasia/acute leukemia. Approximately 50% to 65% of patients with ET carry activating mutations in the Janus kinase 2 gene (JAK2), and an additional 5% in the thrombopoietin receptor gene (MPL) whereas 10 to 20% of patients have mutated calreticulin gene (CALR). Non-mutated JAK2, CALR and MPL ET (triple negative-TN) are about 10%.

Patients and methods : In this study we analysed the prognostic value of JAK2V617F, CALR and MPL mutational status on outcome and thrombotic risk in a retrospective cohort of 138 ET patients defined according to WHO criteria, diagnosed from 1974 to 2013 in a single Italian centre (Turin). JAK2V617F mutation was assessed by Quantitative Real–Time PCR on DNA from peripheral blood or bone marrow samples. JAK2 negative cases were then analyzed for CALR mutations by Gene Scan Analysis in combination with direct sequencing or MPL W515L/K by Allelic Discrimination Real-Time PCR assay. Overall survival (OS), cumulative incidence of myelofibrotic transformations (CI-MT) and thrombosis (CI-TB) were calculated from the date of diagnosis. The between-group comparison for OS was performed with the log-rank test, whereas for CI-MT and CI-TB we using the Gray’s test considering death as competing event and adjusting for presence of cardiovascular risk factors .

Results: Among 138 ET patients, 103 (74.6%) carried the JAK2V617F mutation, 3 (2.2%) carried activating mutations of MPL exon 10, 16 (11.6%) carried mutations of CALR exon 9, and only 16 patients (11.6%) had none of these markers (TN).

An high incidence of elevated haemoglobin levels and/or haematocrit (males >16.5 g/dl or 49% and females >16.0 g/dl or 48%) was significantly associated with JAK2 positivity [13 pts JAK2+ (14.77%) vs 0 pts JAK2- (Fisher test p=0.019)]. Similarly, the CI-TB, analysed with a competing-risk approach, was higher in patients with a JAK2 mutation [(5-year CI-TB: 23.7% JAK2, 0% CALR, 0% MPL, 12.5% TN; P<0.001)].

With a median follow-up of 48 months (IQR: 27-78; range 1-269), we observed 4 (3%) deaths, 9 (7%) myelofibrotic transformations and none leukemic evolution.

The overall survival at 5 years was 98.4% (95%CI: 89.1-99.8) and the cumulative incidence of myelofibrotic transformations was 2.1% (95%IC: 1.0-8.6). The CI-MT was significantly higher in MPL-positive patients [5-year CI-MT: 0% JAK2, 0% CALR, 33.3% MPL, 8.3% TN; (P=0.006)]. Not significant difference on OS was found [5-year OS: 100% JAK2, 100% CALR, 100% MPL, 85.7% TN; (P=0.66)]. We did not include leukemia transformation, because of lack of event.

Conclusion: In our retrospective cohort of ET patients, we found a significant correlation between JAK2 positivity and high risk of thrombosis. We also found a significant incidence of elevated haemoglobin levels and/or haematocrit in JAK2-positive patients. These data support the correlation between JAK2 positivity and cumulative risk of transformation to Polycythemia Vera. The correlation between MPL mutational status and evolution to myelofibrosis found in our study needs to be confirmed in a larger series of patients.