The Transcription Factor Nf-E2 Is Overexpressed in Patients with Polycythemia Vera.

Despite recent advances in characterizing molecular markers for the diagnosis of polcycythemia vera (PV), the aberrations leading to disease development remain unknown. We therefore used expression profiling to identify candidate genes involved in the pathophysiology of PV. RNA from purified granulocytes of 40 PV patients was analyzed by hybridizing individual samples to a pool of 50 healthy controls. Of the 7,496 genes represented in the cDNA array, 253 were upregulated more than 1.5 fold in PV compared to healthy controls (p< 0.01, FDR corrected). Promoters for 26 of the 253 genes overexpressed in PV are regulated by members of the Sp1 family of transcription factors. We have therefore hypothesized that altered activity of one or several Sp1-like transcription factors may contribute to the molecular etiology of PV. Here we report that one of the Sp1 target genes identified, the transcription factor NF-E2, is overexpressed in 37 of the 40 PV patients (92.5%) assayed by microarray. NF-E2 overexpression was confirmed by Northern Blot and quantitative RT-PCR analysis. Transcription factor overexpression varies from 2.3 to 40 fold, with a median increase of 7 fold in PV patients compared to healthy controls. The NF-E2 protein is readily detected in PV granulocytes by Western Blot whereas it is undetectable in healthy control cells. Immunohistochemistry revealed that in PV bone marrow, NF-E2 is overexpressed in megakaryocytes as well as erythroid and granulocytic precursors. Several published observations suggest that NF-E2 is an exceptionally promising candidate in the molecular etiology of PV. Firstly, the transcription factor is expressed in hematopoietic precursors as well as in erythroid, megakaryocytic and granulocytic cells, those lineages affected in PV. Secondly, Sayer et al. have shown that overexpression of NF-E2 in fetal liver cells leads to the development of Epo-independent erythroid colonies, analogous to the endogenous erythroid colonies (EECs) observed in PV patients. Furthermore, ectopic expression of NF-E2 results in the spontaneous emergence of morphologically mature erythroid cells in the absence of Epo and can reprogram monocytic cells towards erythroid and megakaryocytic differentiation. These data support the hypothesis that the concentration of an individual transcription factor can control lineage commitment. We thus propose that in PV patients elevated concentrations of NF-E2 alter the physiological transcription factor balance leading to an overproduction of erythroid and, in select patients, megakaryocytic cells/platelets. In this model the level of NF-E2 overexpression determines both the severity of erythrocytosis and the concurrent presence or absence of thrombocytosis.