Telomerase and Telomeres in Polycythemia Vera and Essential Thrombocythemia.

Polycythemia Vera (PV) and Essential Thrombocythemia (ET) are chronic myeloproliferative disorders (CMPD) with a relatively favorable prognosis and a long natural history. They originate from a multipotent progenitor cell with dominance of the transformed clone over normal hematopoiesis. However, the heterogeneity of these diseases with respect to clonal development from a common progenitor has been established, and no specific molecular or cytogenetic markers are so far considered useful prognostic indicators. In the current study we correlate telomerase activity (TA) and telomeres [terminal restriction fragment (TRF)] lengths in granulocytes (PMN) and mononuclear cells (MNC), with the results of the clonality status, as determined by investigation of X chromosome inactivation patterns (XCIPs), from 22 informative female patients with ET and PV. All patients were in chronic phase without immature myeloid cells in the peripheral blood. After testing for heterozygosity at the HUMARA locus PMN expressed a monoclonal XCIP in 9 out of 13 subjects with ET (69%), and in 6 out of 9 with PV (67%). MNC were always polyclonal. The amount of TA for each reaction was expressed in Total Product Generated (TPG) units. ΔTRF length was calculated by subtracting the telomer length of PMNs from that of MNCs and expressed as kilobases. TA and ΔTRF results were blindly compared with those from XCIP analysis. Monoclonal PMN displayed a TA ranging between 47.6 and 212.5 TPG units (mean 118.2 ± 45.5 SD), whereas polyclonal PMN had a TA between 1.0 and 33.3 TPG units (mean 14.4 ± 11.9 SD) (p<0.001). MNC had a TA similar to that of polyclonal PMN. Mean ΔTRF of clonal PMNs was 4.6 kilobases, while that of polyclonal PMNs was 0.44 kilobases (p<0.001). PMNs from normal control subjects had a mean ΔTRF of 0.1 kilobases, similar to that of polyclonal PMNs (p= n.s.). The results of our study clearly show the existence of two distinct subgroups of CMPD, with polyclonal or monoclonal granulopoiesis, each of them correlating with a peculiar pattern of TA and telomere lengths. Although none of the patients analyzed showed any difference in clinical characteristics with respect to the parameters considered, we believe that TA and ΔTRF analysis and their relationship with clonality status may represent an useful tool for further investigation, to predict survival in larger cohorts of patients with PV and ET, over an extended period of follow up.