Platelets from Calreticulin mutated essential thrombocythemia patients are less reactive than JAK2 V617F mutated platelets

Abstract

Both JAK2V617F and calreticulin (CALR) mutated essential thrombocythemia (ET) patients have different clinical characteristics, with lower thrombosis risk in patients with CALR mutations. To elucidate the mechanism for this lower risk we studied platelet function in ET patients with either JAK2V617F or a CALR mutation. Platelet activation state was similar in ET and healthy controls at baseline using P‐selectin and PAC1 flow cytometry analysis. However, CALR mutated platelets were significantly less activated following ADP stimulation, compared to control or JAK2 mutated platelets (P < .001). In live‐cell imaging of platelet attachment to immobilized fibrinogen by Interference Reflection Microscopy (IRM), the number of attached CALR mutated platelets was lower compared to control and JAK2 mutated platelets, with lower fractions of platelets achieving the fully spread state (90%, 78% and 54% of adherent cells for control, JAK2 and CALR mutated subjects, respectively). Compared to controls, ET patients, regardless of the mutation type, had increased numbers of immature platelets (IP) and leukocyte platelet aggregates (LPA), as well as plasma sP‐selectin. These were all correlated with the platelet count and not to the state of platelet activation. We also found that intracellular free Ca2+ was increased in resting ET compared to control platelets. Note, CALR had a more dispersed localization in activated ET platelets compared to healthy controls, and mutated CALR interact physically with TpoR in CALR mutated platelets. We hypothesize that defects in platelet activation and spreading in CALR mutated patients can explain, at least in part, the lower thrombotic tendency in CALR mutated ET patients.

Myeloproliferative neoplasms and pregnancy: Overview and practice recommendations

“…Additionally, in an international collaborative study, arterial thrombosis in ET was predicted by age > 60 years, thrombosis history, cardiovascular risk factors including tobacco use, hypertension, diabetes mellitus, leukocytosis >11 × 10 9 /L, JAK2V617F mutation; male gender predicted venous thrombosis, while a lower risk of thrombosis was noted in patients with extreme thrombocytosis greater than a million/μL 18 . Presence of CALR mutation in ET patients confers a lower risk for thrombotic events which may be explained by defects in platelet activation not only in response to ADP stimulation but also impaired interaction with fibrinogen, compared to JAK2 mutated patients or those without MPN 23 …”

“…18 Presence of CALR mutation in ET patients confers a lower risk for thrombotic events which may be explained by defects in platelet activation not only in response to ADP stimulation but also impaired interaction with fibrinogen, compared to JAK2 mutated patients or those without MPN. 23 It is to be noted that current thrombotic risk stratification models in MPN have limited applicability in determining pregnancy complications. In this regard, some have suggested classifying pregnant ET patients as low risk in the absence of prior thrombotic events or inherited thrombophilia, age < 35 years, and platelet count less than a million/μL, whereas highest risk was conferred by history of obstetric complications, or maternal thrombosis or hemorrhage, particularly a recent thrombotic event.…”

Kidney Dysfunction Is Associated with Thrombosis and Disease Severity in Myeloproliferative Neoplasms: Implications from the German Study Group for MPN Bioregistry

“…The presence of the JAK2V617F mutation has been associated with an increased risk of thrombosis in ET [34,35], and the mechanism involved enhanced activation of platelets, leukocytes, and endothelial cells as well as prothrombotic soluble factors [36][37][38]. In our overall cohort, JAK2V617F was not associated with kidney dysfunction; however, CALR mutations were significantly less frequent in eGFR3 (Table S1).…”

Acquired Gray Platelet Syndrome Associated with Primary Myelofibrosis

“…Morphologically unusual platelets with an abnormal function may be observed in CALR-mutated MPNs because thrombopoietin-independent megakaryopoiesis through the thrombopoietin receptor has been reported to be activated by mutant CALR ( 13 ). Another study reported that CALR-mutated platelets were less activated following ADP stimulation ( 14 ). They speculated that the result could explain the lower risk of thrombosis in CALR-mutated ET patients compared with JAK2-mutated ET patients.…”