STUDY OF THE THROMBOPOIETIN RECEPTOR IN ESSENTIAL THROMBOCYTHEMIA

Essential thrombocythemia (ET) is a myeloproliferative disorder associ ated with megakaryocytic hyperplasia and thrombocytosis. In this disea se, in vitro autonomous growth of megakaryocytic colonies has been dem onstrated by various investigators. This phenomenon is impaired by the inhibition of the thrombopoietin/c-mpl pathway. In order to evaluate the potential role of mutations of the receptor gene in the origin of this autonomous growth, we compared the expression of c-mpl mRNA isofo rms in platelets derived from ET patients and normal subjects, Overlap ping c-mpl PCR fragments derived from four ET patients were sequenced to search for small mutations, In the 10 ET and five normal samples we studied, relative expression of the c-mpl isoforms was identical, New variants of Mpl-P and K isoforms, Mpl-P2 and K2 were detected, Clonin g of these isoforms indicated that they are produced by alternative sp licing of exon 9 sequences shared by Mpl-P and K. Their predicted amin o acid sequence would be deleted by 24 aminoacids, upstream of the WSS WS box of the second domain of c-mpl. Two sequence variations, leading to DNA restriction polymorphisms, were present in the extracellular a nd Mpl-K intracytoplasmic domains. Both were present in normal and ET samples, excluding mutations of c-mpl as a cause of ET.