Platelet function studies in myeloproliferative neoplasms patients with Calreticulin or JAK2V617F mutation

Background: JAK2V617F and Calreticulin (CALR) mutations are the most frequent molecular causes of Phi-negative myeloproliferative neoplasms (MPN). Patients with CALR mutations are at lower risk of thrombosis than patients with JAK2V617F. We hypothesized that CALR-mutated blood platelets would have platelet function defects that might explain why these patients are at lower risk of thrombosis. Objectives: Our main objective was to explore and compare platelet function depending on the MPN molecular marker. Methods: We analyzed platelet function in 16 patients with MPN with CALR mutations and 17 patients with JAK2V617F mutation and compared them with healthy controls. None of these patients was taking antiplatelet therapy. We performed an extensive analysis of platelet function and measured plasmatic soluble P-selectin and CD40L levels. Results: We observed significant defects in platelet aggregation, surface glycoprotein expression, fibrinogen binding, and granule content in platelets from patients with MPN compared with that in controls. Moreover, soluble CD40L and P-selectin levels were elevated in patients with MPN compared with that in controls, suggesting an in vivo platelet preactivation. Comparison of platelet function between patients with CALR and JAK2V617F MPN revealed only minor differences in platelets from patients with CALR. However, these results need to be interpreted within the context of absence of an inflammatory environment that could impact platelet function during MPN. Conclusions: These results do not support the hypothesis that calreticulin-mutated platelets have platelet function defects that could explain the lower thrombotic risk of patients with CALR

Study of platelet functions of patients carrying CALR and JAK2V617F mutations.

Les néoplasis myéloprolifératives sont des hémopathies malignes caractérisées par la prolifération clonale de lignées hématopoïétiques sous l’influence de mutations initiatrices, ou « drivers », survenant au niveau des cellules souches hématopoïétiques. L’évolution de ces néoplasies est marquée par la survenue de complications, au premier rang desquelles se trouve la thrombose, responsable d’une morbi-mortalité considérable. De nombreuses études ont été menées sur les mécanismes physiopathologies menant aux thromboses, et ont notamment pu révéler les rôles joués par la mutation JAK2V617F dans le développement de celles-ci. La découverte en 2013 des mutations de la Calréticuline, protéine connue jusqu’alors pour ses fonctions de chaperonne au sein du réticulum endoplasmique et de régulatrice de l’homéostasie calcique, a permis une nouvelle approche biologique des NMP. Les données actuelles indiquent que les différentes mutations « drivers », bien que responsables de la même maladie, sont associées à des phénotypes clinico-biologiques différents. En particulier, le risque thrombotique est significativement réduit en cas de mutation CALR comparativement à JAK2V617F Les mécanismes thrombogènes associés à la mutation JAK2V617F ont été explorés et offrent une explication au sur-risque thrombotique qui lui est associé dans cette étude, nous émettons l’hypothèse que les dysrégulations protéique et calcique liées à la mutation de la Calréticuline pourraient perturber les fonctions plaquettaires, notamment hémostatiques, et participer à l’explication de la diminution relative du risque thrombotique des sujets atteints de mutations CALR.Myeloproliferative neoplasms (MPNs) are malignant hemopathies characterized by clonal proliferation of hematopoietic cells, due to the acquisition of driver mutations in hematopoietic stem cells. Three main mutations, ie JAK2V617F, MPL and CALR, are found in more than 90% of MPNs and induce a constitutive activation of the JAK-STAT pathway.The discovery in 2013 of mutations affecting Calreticulin a protein so far mainly known for its function within the endoplasmic reticulum (ER), raised new questions in our understanding of MPNs. Disease evolution of MPNs is marked by the onset of several complications among which thrombosis is the most frequent, being responsible for considerable morbidity and mortality. Surprisingly, although CALR and JAK2V617F act on the same intracellular signaling pathways, the thrombotic risk is significantly reduced in patients carrying CALR mutations compared to JAK2V617F in the case of Essential Thrombocythemia. The reasons of such difference are yet to be investigated. Knowing the key role of Ca2+ in platelet activation, we hypothesized that the potential Ca2+ dysregulation induced by CALR mutations could disturb platelet functions, explaining the relative decrease of thrombotic complications in such patients. In this study, our objective was to explore platelet functions in patients carrying CALR or JAK2 mutations, and free from antiplatelet therapies