MPL p.S204P Is a Recurrent Mutation in Essential Thrombocythemia
Authors: AL Assaf Carla , Papadopoulos Petros , Guttierez Laura , Smits Sanne , Graux Carlos , Emmerechts Jan , Lierman Els , Devos Timothy , Michaux Lucienne , Vandenberghe Peter 
Published: October 2019

Abstract
Introduction: The JAK2 p.V617F, MPL p.W515K/L and CALR indels occur in a mutually exclusive pattern in 80-90% of cases with Essential Thrombocythemia (ET), but the driver mutations are unknown in the remaining 10-20%. In this study we aimed to identify driver mutations in the latter group of triple negative (TN) ET by exome sequencing of 10 such cases.
Results: We found 27 somatic variants, including indels, in 6 out of 10 TN ET patients (range: 1-10 mutations/case; mean: 2,7 mutations/case), none of which were recurrent. In one case, we found a MPL c.610T>C (p.S204P) mutation, which is located in the extracellular domain of the MPLreceptor. By Sanger sequencing of MPL exon 4 in 20 additional TN ET cases, an additional patient with the MPL S204P mutation was identified. Moreover, this mutation was previously reported in one case with idiopathic myelofibrosis1.
In order to study the effect of this mutation on the function of MPL, we produced stable Ba/F3 cell lines expressing MPL S204P, MPL W515K or MPL WT, and assessed the dependence of their growth on exogenous thrombopoietin (TPO). Only MPL W515K transduced Ba/F3 cells proliferated in the absence of TPO, but growth of MPL S204P Ba/F3 and of MPL WT Ba/F3 could be rescued by exogenous TPO, indicating the proper surface expression and the functionality of the transduced receptors. The levels of phospho-JAK2 and phospho-STAT5 were low in cytokine-deprived MPL S204P cells but increased upon TPO stimulation. In contrast, phospho-JAK2 and phospho-STAT5 were detectable in MPL W515K transduced Ba/F3 in the absence of cytokines as assessed by Western blotting. Culture of MPL S204P transduced Ba/F3 in the presence of TPO over a range of concentrations (0,01-10 ng/ml) yielded growth curves comparable with MPL WT transduced Ba/F3.
Using flow cytometry, we also explored cell surface marker expression on peripheral blood platelets from the two MPL S204P ET patients. Data were compared with healthy donors or ET patients with JAK2 or CALR mutations. MPL S204P ET platelets displayed higher expression of CD61 than platelets from healthy donors or from JAK2 or CALR mutated ET (p<0,01). In addition, there was a trend for higher expression of KIT, CD36 and CD42b on platelets from the MPL S204P ET cases. Moreover, following platelet activation through the protease activated receptor 1, the degranulation response of platelets from MPL S204P ET was decreased in comparison with JAK2 or CALR mutated ET.
Conclusion: The MPL S204P mutation is a recurrent mutation in TN ET, with a frequency of 7% (2/30) in this series, but this mutation does not induce TPO-independent growth nor increased TPO-sensitivity in Ba/F3 cells. However, preliminary phenotypic and functional evidence supports the notion that MPL S204P platelets display specific characteristics as compared with JAK2 or CALR mutated ET. The mechanisms by which the MPL S204P mutation influences megakaryopoiesis and platelet function remain to be elucidated.
1. Williams DM, et al. Phenotypic variations and new mutations in JAK2 V617F-negative polycythemia vera, erythrocytosis, and idiopathic myelofibrosis. Exp Hematol 2007; 35: 1641.

Disclosures
Graux: Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees.