Hereditary Thrombocytosis: The Thrombopoietin Receptor c-MPL Can Activate Its Downstream Pathway without Full Glycosylation and Membrane Expression
Authors: Stockklausner Clemens , Echner Nicole , Klotter Anne-Christine , Kuhlee Isabelle Nadine , Kulozik Andreas E 
Published: October 2019

Abstract
Abstract 2374
Thrombopoiesis is tightly regulated by the interaction between thrombopoietin (TPO) and its receptor c-Mpl. Receptor binding also leads to the clearance of TPO from the plasma thus establishing a negative feedback loop. However, it is still an open question how the receptor activates its downstream pathway. Alternative models posit that ligand binding either results in receptor dimerization in the plasma membrane or leads to conformational change of preformed receptor dimers.
Several mutations in the TPO and the c-Mpl receptor genes have been linked to either hereditary thrombocytopenia or thrombocytosis. We focused on mutations in the extracellular part of the c-Mpl receptor, where ligand binding and receptor dimerization occur. Mutated homozygous c-Mpl R102P and compound heterozygous R102P/F104S receptors cause severe hereditary thrombocytopenia. In contrast, the homozygous c-Mpl P106L mutation was found in patients with hereditary thrombocytosis. We now addressed the question of how the disparate phenotype of mutations in the same domain of the c-Mpl receptor can be explained. We first functionally analyzed and compared normal with mutated R102P, F104S and P106L c-Mpl receptors in transfected HeLa and BA/F3 cells and found that the normal and the F104S c-Mpl receptors are glycosylated normally by the Golgi apparatus and reach the plasma membrane. In contrast, the R102P and P106L mutated receptors are not fully glycosylated, do not reach the plasma membrane and are atypically distributed in the ER.
Functional analysis of the TPO/c-Mpl signaling pathway in BA/F3 cells showed decreased phosphorylation of Stat3, Stat5 and Erk1/2 with the R102P and F104S mutants when compared to normal. By contrast, TPO/c-Mpl signaling was up-regulated in cells transfected with the P106L-mutated receptor. Moreover, the P106L mutant, but not the other mutant receptors, enhanced ligand-independent growth of transfected BA/F3 cells.
Despite of their opposite function, the TPO plasma levels of patients carrying both, homozygous R102P and P106L mutations were elevated 10 to 20-fold compared to normal and heterozygous individuals. This finding, together with their impaired glycosylation and inability to reach the plasma membrane, suggests that these mutants do not bind and internalize their ligand. TPO binding and degradation thus requires the receptor to be expressed at the plasma membrane, whereas, surprisingly, c-Mpl P106L activated its signaling pathway in a ligand independent fashion. Correct receptor processing and function can thus be separated. This indicates that TPO binding is required for regulation but that the constitutive activation of c-Mpl P106L is a likely direct consequence of premature receptor dimerization in the ER, auto-phosphorylation and subsequent activation of downstream targets.

Disclosures:
No relevant conflicts of interest to declare.