Recently, Temerinac and colleagues1 found that the polycythemia rubra vera receptor-1 (PRV-1) m-RNA is increased in granulocytes of patients with polycythemia vera (PV) and in some cases of essential thrombocythemia (ET). We have previously described a sensitive qualitative method for PRV-1 investigation, based on PRV-1 m-RNA amplification by reverse transcriptase-polymerase chain reaction (RT-PCR).2 By using this approach, we found that PRV-1 m-RNA is increased in 95% of patients with PV and in all patients with ET, while it is undetectable in secondary erythrocythosis (SE) and thrombocythosis (ST).2 In other published studies, the PRV-1 m-RNA has been quantitatively evaluated,3-8 and the percentage of PRV-1-positive ET patients varies widely among the mentioned studies, ranging from 16%3 to 100%.7 Up to date we have investigated by RT-PCR 75 patients with ET, 27 patients with ST, and 20 healthy individuals. Clinical and laboratory data of patients are shown in Table 1. Our results show that 71 of 75 ET patients are PRV-1 positive (95%), while all ST and healthy subjects are PRV-1 negative. These findings are in contrast with other reports and may result from the use of different methodological procedures in both granulocyte purification and PRV-1 evaluation (RT-PCR instead of real-time PCR). In a recent technical report, Palmqvist et al9 compared the quantification of PRV-1 m-RNA in whole-blood leukocytes and in selected granulocytes. The authors clearly demonstrated that the detection of PRV-1-positive patients in ET increases by examining purified granulocytes (26% and 46% in unfractionated cells and granulocytes, respectively).9 Thus, the low percentage of PRV-1-positive patients reported in some studies can be explained by the evaluation of RNA extracted by unfractionated nuclear cell population. Moreover, Jelinek et al10 reported that PRV-1 m-RNA is more rapidly degraded than control m-RNAs when blood samples are stored for several hours before using. In our assay, granulocyte separation is performed within 2 hours of blood collection; the hypotonic lysis of red cells, reported in other studies,1,5 is not required; and granulocyte pellets are stored in TRIZOL (Invitrogen, Paisley, Scotland) at -80°C until PRV-1 analysis. In this way, the PRV-1 m-RNA integrity is assured.
Finally, it has been demonstrated that treatment with interferon can induce a significant decrease of PRV-1 m-RNA at real-time PCR in PV patients.11 In our series, only a small proportion (28%) of patients received cytoreductive therapy at the time of examination. Interestingly, a high percentage of patients evaluated by Liu et al3 received chemotherapy at the time of PRV-1 examination (58%), and the PRV-1 positivity reported is very low (16%). In contrast, in the study of Florensa et al6 the percentage of treated patients is low (29%), and PRV-1 m-RNA is increased in 59% of cases. So, further studies are necessary to assess whether in ET patients the cytoreductive treatment could affect the PRV-1 expression level.
In conclusion, we retain that a qualitative evaluation of PRV-1 can be a feasible diagnostic tool for ET.
