摘要：

Be- cause EPO is a relatively specific erythroid proliferative and differentiating,' as well as apoptosis-preventing6 hematopoi- etic growth factor, this finding was intuitively satisfying. The abnormal behavior of erythroid progenitors was found to be present in virtually all patients with PV; however, some patients with essential thrombocythemia, which is also char- acterized by clonal proliferation of myeloid cells,7 had sim- ilar in vitro behavior of their erythroid progenitors. Differ- ent culture systems produced conflicting data regarding the independence or hypersensitivity of PV erythroid progeni- tor~.~,~ Polycythemia vera is an example of a orimarv polycythe- mia, a condition in which the defect is intrinsic to the cells and not driven by external stimuli such as increased EPO production, cobalt, or other factors."." Other types of pri- mary polycythemias have been described in recent years. These are nonacquired primary polycythemic disorders, which have been noted to display both familial (dominantI2 or re~essive'~) and sporadic in~idence.'~ Because these con- ditions are associated with normal or decreased EPO con- centration~,'','~ the molecular cloning of the EPO receptor made it possible to test the hypothesis that primary polycy- themias were caused by a mutation of the EPO receptor gene. Early studies showed that erythroid progenitors in PV and other primary polycythemias displayed no major ab- normalities in the number of EPO receptors present on pro- genitor cells, or in their affinity for ligand.'' However, be- cause relatively minor modifications in the mouse EPO receptor, such as point mutations, have been shown in transfected mouse hematopoietic cell lines to have a pro- found effect on the signal transduction necessary for prolif- eration of erythroid progenitor^'^.'^ as well as on the modu- lation of apoptosis,2° the presence of a more subtle abnormality in the EPO receptor became attractive as a pos- sible explanation for the primary acquired and congenital primary polycythemias. In fact, in Friend virus infected mouse, a mutation that changes arginine to cysteine at posi- tion no. 129 in the extracellular domain has been found to render the EPO receptor constitutively active without its li- gand,'8-21 whereas deletion of the cytoplasmic C-terminal p~rtion'~.~' of the mouse EPO receptor increases EPO me- diated proliferation and cell survival. Unfortunately, the search for these mutations in PV has failed to reveal any abnormality of the EPO recept~r.'~.~~ However, two different mutations of the EPO receptor have been found in cases of congenital p~lycythemia.~~, ~~ The first, a substitution of proline for serine at position no. 488, was found to be insufficient on its own to cause the polycy- themia phenotype.22 In the second case, in a large multi- generation Finnish family a mutation that generates a stop codon in the cytoplasmic region of the receptor was found to be linked with the polycythemia phenotype.24 The effect of this mutation on the function of the EPO receptor has not yet been determined.25 The functional relevance of EPO receptor mutations can be determined by establishing link- age between the polycythemic phenotype and the EPO re- ceptor polymorphism. Linkage analysis showed that a poly- morphism, approximately 500 nucleotides upstream from the AUG translation start codon of the EPO re~eptor,~~.~~ was found to be present in a Finnish family. Several other autosomal dominantly inherited polycythemias studied in our laboratory failed to display such linkage27 (Sokol, Prchal, unpublished, 1993). More recently we have found that this previously uncharacterized polymorphic region ac- tually consists of two separate and distinct polymorphic sites close to each other.28 One consists of GGAA repeats, whereas the other consists of repeats of the dinucleotide GA. Both of these polymorphisms are encompassed in the PCR product studied in the previous reports on the 5' EPO recep- tor p~lymorphism.~~, ~~ Correct linkage analysis requires that EPO receptor polymorphisms found in this genomi

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