c-mpl Mutations in Congenital Amegakaryocytic Thrombocytopenia: Residual TPO Receptor Activity in Patients of Group CAMT II.

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare inherited bone marrow failure syndrome. Mutations in the gene for the thrombopoietin receptor c-mpl were defined as the molecular cause of CAMT. Extending our sequence analyses from 8 to a total of now 23 CAMT patients we could confirm our hypothesis of genotype-phenotype correlation in CAMT. Seven different nonsense, frame-shift or splice-site mutations, all predicted to lead to a complete loss of function of the thrombopoietin receptor were found in 13 patients belonging to group CAMT I with persistently low platelet counts and a fast progression into pancytopenia. Nine different missense mutations were detected in ten patients of group CAMT II, characterized by a transient increase in platelet counts over 50 nl⁻¹ during the first years of life. Mutations spread nearly over the entire gene with a clear accumulation in exons 2 and 3. Except for two missense mutations in exon 8 and exon 12 coding for the second cytokine receptor homology domain and for the distal half of the intracytoplasmic domain, respectively, all mutations were found in exons 2–5 coding for the first cytokine receptor homology domain of the thrombopoietin receptor. The mutations found in group CAMT I patients were located in exons 2, 3 and in the splice sites between exons 1–3. Missense mutations found in patients of group CAMT II spreaded over exons 3, 4, 5, 8 and 12. Using in vitro assays with hematopoietic progenitors from patients of both patient groups we could demonstrate a residual activity of the thrombopoietin receptor in CAMT II patients: In samples from patients of group CAMT II the addition of TPO led to a significant higher number of total colonies. No difference was found in the number of megakaryocytic colonies. In contrast, TPO did show neither an effect on the total colony number nor on megakaryocytic colony number in samples from patients of group CAMT I. This is a first experimental confirmation of our hypothesis that the type of mutation has a strong influence on the course of the disease and that the transient increase of platelet counts and the delayed development of pancytopenia in CAMT II patients is due to a residual activity of the TPO receptor.