Essential thrombocythemia (ET) is characterized by bleeding tendency, thrombotic complications and qualitative platelet defects. All those abnormalities are likely to contribute to the excessive mortality rate of this disease. Among many specific morphological, biochemical and metabolic platelet defects, a complete loss of platelet responsiveness to epinephrine is the most frequent in these patients. Since an abnormal platelet fibrinolytic activity was suggested to contribute to bleeding tendency, our initial goal was to see if platelet fibrinolysis activity is impaired in patients with ET. 22 patients were enrolled into the study (17 females, 5 males age 57.2±13.0.). This group included: 6 untreated patients, 9 treated with anagrelide, 4 with hydroxyurea, and 3 treated with a combination of both drugs. 8 thrombotic complications and 2 clinically significant bleeding episodes occurred. The control group consisted of 6 females age 33.3±9.9. Platelet count was 777±333 x109/L and 257±70 x109/L p<0.001, for the ET and the control group, respectively. Platelet activation was studied using flow cytometry to detect CD62P expression on the platelet surface. There was no difference between the control and the ET group. Platelet aggregation was measured using adenosine diphosphate and epinephrine as agonists. While 90% individuals from control group responded to epinephrine, in 45% of patients, a lack of epinephrine-induced platelet aggregation was detected. In this 45% of patients a statistically significantly lower expression of CD62P on CD61 positive cells was observed (mean value 1.90% versus 3.82%). Further, urokinase-type plasminogen activator (uPA) concentration was evaluated in plasma and in platelet lysates. Concentration of uPA was statistically significantly higher in patient plasma as compared to the control group (0.049±0.030 versus 0.029±0.015 ng/ml, p<0.05). Mean uPA concentration measured in platelet lysates was similar in both groups (ET 0.114±0.060ng/109 platelets, control group 0.110±0.042 ng/109 platelets). However, in platelets lysates from three patients, an extremely high uPA concentration was detected (more than 0.194 ng/109 platelets). One of these patients had an episode of hypermenorrhea and the second presented a thigh hematoma after a minor trauma. In both of those patients platelet count was above 600 x 109/L. In addition, in one patient very low uPA activity in platelet lysates was observed (0.030 ng/109 platelets). In this patient’s medical history, two episodes of DVT occurred. To evaluate platelet fibrinolytic activity, uPA activity was assessed by means of casein zymography. Zymograms have show that similar fibrinolytic activity was present both in patients and the control group and the activity of fibrinolytic enzymes was inhibited by AEBSF, an inhibitor of serine proteases. From the data gathered we concluded that uPA concentration is significantly higher in ET patient plasma as compared to the control group. However, platelet fibrinolytic activity, measured as uPA activity and uPA concentration in platelets lysates obtained from ET patients is not impaired as was initially presumed. Because of the small sample size, we could not precisely assess the clinical importance of extremely high or extremely low uPA activity in platelets lysates. All these data need further evaluation in a larger group of patients.

Author notes

Disclosure: No relevant conflicts of interest to declare.