An incomplete trafficking defect to the cell-surface leads to paradoxical thrombocytosis for human and murine MPL P106L

The mechanisms behind the hereditary thrombocytosis induced by the thrombopoietin (THPO) receptor MPL P106L mutant remain unknown. A complete trafficking defect to the cell-surface has been reported, suggesting either weak constitutive activity or non-conventional THPO-dependent mechanisms. Here we report that the thrombocytosis phenotype induced by MPL P106L belongs to the paradoxical group, where low MPL levels on platelets and mature megakaryocytes lead to high serum THPO levels, while weak but not absent MPL cell-surface localization in earlier megakaryocyte progenitors allows response to THPO by signaling and amplification of the platelet lineage. Megakaryocyte progenitors from patients showed no spontaneous growth, responded to THPO and megakaryocytes expressed MPL on their cell surface at low levels, whereas their platelets did not respond to THPO. Transduction of MPL P106L in CD34 + cells showed that this receptor was more efficiently localized at the cell-surface on immature than on mature MKs, explaining a proliferative response to THPO of immature cells and a defect in THPO clearance in mature cells. In a retroviral mouse model performed in Mpl -/- mice, MPL P106L could induce a thrombocytosis phenotype with high circulating THPO levels. Furthermore, we could select THPO-dependent cell lines with more cell surface MPL P106L localization that was detected by flow cytometry and [ 125 I]-THPO binding. Altogether these results demonstrate that MPL P106L is a receptor with an incomplete defect in trafficking, which induces a low but not absent localization of the receptor on cell surface and a response to THPO in immature MK cells.