HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY
Since the identification of the cellular homolog of viral oncogene v-mpl (c-MPL) and its ligand thrombopoietin (TPO) as major regulators of megakaryopoiesis and, hence, of circulating platelet numbers, several naturally occurring mutations have been identified in their corresponding genes. Identification of these mutations has come from studies of kindreds diagnosed with familial thrombocythemia or thrombocytopenia. For instance, a germ line mutation in the promoter region of the TPO gene resulting in expression of an unusually stable TPO mRNA transcript has been associated with familial essential thrombocythemia (FET).1 On the other hand, mutations in the c-MPL receptor gene, including those that lead to amino acid substitutions, have been associated with congenital amegakaryocytic thrombocytopenia.2
In a previously reported study, Onishi et al3 described an experimentally induced substitution from Ser498 to Asn498 (Ser498Asn) in c-MPL, which leads to activation of this receptor. Interestingly, an activating mutation has now been identified in a Japanese pedigree with FET, as described by Ding and colleagues (page 4198). A point mutation was identified in the c-MPL gene segment encoding the transmembrane domain, resulting in a shift from Ser to Asn at amino acid position 505 (Ser505Asn). Importantly, this mutation was detected in all 8 family members with thrombocythemia but in none of 8 other unaffected members in this family or in 19 cases of sporadic ET. These findings were augmented by studies of an interkeukin-3 (IL-3)–dependent cell line that was transfected with either wildtype or mutated (Ser505Asn) c-MPL cDNA. Thus, the authors elegantly confirm that Asn505 is an activating mutation with respect to intracellular signaling and cell survival. These authors show that engineered cells carrying the mutated cDNA display IL-3–independent survival and autonomous phosphorylation of signal transducers and activators of transcription 5,6 (Stat5,6). Moreover, autonomous phosphorylation of mitogen-induced extracellular kinase 1/2 (Mek1/2) was demonstrated not only in the engineered cells bearing mutated c-MPL but also and importantly in platelets from affected individuals.
The study by Ding et al is the first to describe a dominant-positive activating mutation in the c-MPL gene in a family with FET. This mutation has not been connected, as yet, with the induction of leukemia. However, it should be noted that mutations in the transmembrane domains of other proteins (eg, c-kit) can increase their ability to dimerize, and it remains to be determined whether the mutation in c-MPL described above leads to a constitutive dimerization of this receptor and, thereby, to autonomous activation. Elucidation of the mechanism by which Asn505-mutated c-MPL activates downstream signals involved in the TPO pathway should shed further light on the effect of this cytokine on the proliferation and survival of megakaryocyte precursors. In this regard, it would be of interest to explore whether megakaryocytes engineered to carry this mutation display an altered proliferation and/or fragmentation potential. In view of reports on the effects of TPO on platelet aggregation and adhesion, it would also be of interest to examine these functions in platelets derived from affected individuals.