DEFECTIVE RESPONSE TO THROMBOPOIETIN AND IMPAIRED EXPRESSION OF C-MPLMESSENGER-RNA OF BONE-MARROW CELLS IN CONGENITAL AMEGAKARYOCYTIC THROMBOCYTOPENIA

Congenital amegakaryocytic thrombocytopenia (CAMT) is an uncommon diso rder in newborns and infants, characterized by isolated thrombocytopen ia and megakaryocytopenia in the first year without physical anomalies . The defect of thrombopoiesis is not well understood. Recently, throm bopoietin (TPO), the ligand for the c-mpl receptor, was cloned. Accumu lating evidence from in vitro and in vivo studies indicate that TPO pl ays a key role in the regulation of megakaryocytopoiesis. In this stud y we examined the effect of TPO on megakaryocyte colony formation from a patient with CAMT using a plasma-containing methylcellulose clonal culture. The in vitro results demonstrated a defective response to TPO in megakaryocyte colony formation from bone marrow mononuclear cells (MNC) of the patient, although interleukin-3 (IL-3) but not stem cell factor (SCF) induced only a small number of megakaryocyte colonies, Th ese findings indicated that thrombocytopenia in CAMT could not be corr ected by administration of TPO in vitro. Additionally, clonal cultures containing SCF, IL-3, IL-6 and erythropoietin showed decreased number s of erythroid and myelocytic progenitors in the bone marrow of the pa tient. The serum TPO level measured by enzyme-linked immunosorbent ass ay was significantly higher than that in healthy controls. By PCR, mar row MNC from healthy children and from a patient with essential thromb ocytosis expressed c-mpl mRNA, whereas no c-mpl mRNA was detected in m arrow MNC from the patient with CAMT. There was no difference in the C D34 expression and c-kit mRNA between the CAMT patient and healthy chi ldren, The results of this study suggest that the pathophysiology in C AMT may be a defective response to TPO in haemopoietic cells through i mpaired expression of c-mpl mRNA.