Involvement of the TPO/Mpl pathway in the pathogenesis of inherited thrombocytopenia.

HELLER PAULA G; GLEMBOTSKY ANA C; LAGUNA MARÍA S; MARTA ROSANA F; MOLINAS FELISA C

Ginebra

Congreso; XXIst Congress of the Internacional Society on Thrombosis and Haemostasis; 2007

Introduction: Inherited thrombocytopenias (IT) comprise heterogeneous disorders characterized by impaired platelet production. Some of the underlying genetic defects have been identified although in many cases the mechanisms leading to thrombocytopenia remain unknown. Considering the key role of thrombopoietin (TPO) and its receptor in megakaryopoiesis, we sought to determine whether abnormalities in this pathway contribute to the pathogenesis of IT. Methods: We studied 20 patients belonging to 7 families: I, X-linked thrombocytopenia (WASP mutation); II, familial platelet disorder with predisposition to acute myelogenous leukemia (Blood 2005;105:4664); III, MYH9-related disorder; IV and V, autosomal dominant macrothrombocytopenia; VI, autosomal dominant thrombocytopenia; VII, unclassified, and 12 patients with immune thrombocytopenic purpura (ITP). TPO levels were measured by ELISA; platelet Mpl expression was analyzed by flow cytometry, Western blot and RT-PCR; TPO-induced tyrosine phosphorylation of platelet proteins and potentiation of platelet aggregation were determined. Data were analyzed by Student t test and Kruskall-Wallis test. Results: TPO levels were higher in IT and ITP vs controls, 99.2 (0-333) and 65.7 (15-264) vs 0 (0-32.1) pg/ml, respectively, p<0.0001. Decreased Mpl expression was found in some IT pedigrees (II, VI and VII); Mpl/isotype ratio by flow cytometry was lower in IT vs controls, 1.35 ± 0.34 vs 1.70 ± 0.27, p=0.01, while Mpl/b3 integrin ratio by Western blot was lower in IT vs ITP, 56.3 ± 38% vs 137.4 ± 49%, p=0.0002. Patients with decreased Mpl levels had reduced or absent response to TPO in vitro. Low Mpl transcripts were found in patients with reduced Mpl levels, suggesting decreased transcription. Conclusions: Decreased Mpl levels could be involved in the pathogenesis of some IT. The difference found between IT and ITP could be a useful diagnostic tool in certain cases. Whether low Mpl levels will limit the response to treatment with TPO-mimetics remains to be established.