Functional analysis of single amino-acid mutations in the thrombopoietin-receptor Mpl underlying congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare disorder that presents with severe thrombocytopenia and absence of megakaryocytes in the bone marrow. The disease may develop into bone marrow aplasia. Genetic defects in the gene encoding the thrombopoietin (Tpo) receptor, MPL, are the cause of this disease. In a previous study, we identified four missense mutations in CAMT patients, predicting Arg102Pro, Pro136His, Arg257Cys and Pro635Leu. To investigate whether these mutations result in defective Tpo-binding and/or signalling, full-length wildtype and mutant MPL were transduced into K562 cells. Expression levels and the ability to activate the mitogen-activated protein kinase, Janus kinase-signal transducer and activator of transcription and phosphoinositide-3 kinase pathways upon Tpo-binding were studied. The results predicted that MPL carrying the P136H or P635L mutation was not properly expressed, whereas the R102P and R257C mutations resulted in impaired signal transduction. Our results indicate that a severe clinical course may be expected when these mutations lead to absent Mpl expression or signalling in CAMT patients with missense mutations.