Mutations in the thrombopoietin receptor, Mpl, in children with congenitalamegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare disorder of un defined aetiology. The disease presents with severe thrombocytopenia and ab sence of megakaryocytes in the bone marrow. Furthermore, CAMT patients may develop bone marrow aplasia. To obtain more insight into the mechanism unde rlying CAMT, five children were analysed. All patients had increased plasma thrombopoietin (Tpo) levels, indicating a platelet production defect. Bone marrow-derived CD34(+) stem cells from three patients were cultured in an in vitro liquid culture system to study megakaryocytopoiesis. CD34(+) cells from two of the three patients failed to differentiate into megakaryocytes . The lack of megakaryocyte formation could imply that a defect in the c-mp l gene, encoding the Tpo receptor, exists. Sequencing of c-mpl revealed mut ations in four of five patients. Three patients had point mutations and/or a deletion in the coding regions of c-mpl. All point mutations led to an am ino acid substitution or to a premature stop codon. In one patient, a homoz ygous mutation in the last base of intron 10 was found that resulted in los s of a splice site. This study showed that mutations in c-mpl could be the cause of thrombocytopenia in CAMT in the majority of patients. Furthermore, Tpo has been shown to have an anti-apoptotic effect on stem cells. Therefo re, mutations in c-mpl might not only affect megakaryocyte formation but ma y also impair stem cell survival, which could explain the occurrence of bon e marrow failure as final outcome in patients with CAMT.