Abstract
Hemophilia B Leyden is a rare form of inherited factor IX deficiency in which patients experience spontaneous postpubertal recovery of factor IX levels. The mutations resulting in this disorder are localized in a 40-nucleotide region encompassing the major transcriptional start site for factor IX. Here we report the further characterization of five cis- acting elements in the factor IX promoter and the effects on protein binding and transcriptional activation of five Leyden mutations (at nucleotides +13, -5, -6, -20, and -26) that occur within the proximal three elements (sites 1 through 3). Bandshift studies using nuclear extracts from four different rat tissues have shown that at least some of the proteins binding to each of the five sites are ubiquitous in nature. The pattern of DNA binding at site 1 suggests that this element plays an important role in mediating the liver-specific expression of factor IX. Additional studies with liver nuclear extracts obtained at several different points in development have shown an increase in DNA binding at sites 1, 4, and 5 between 1 day and 1 week. Using DNase I footprint analysis and competition bandshift studies, we have shown that the binding of nuclear proteins to each of the mutant sites is disrupted to a variable extent. There appears to be some, although reduced, protein binding to all of the mutant oligonucleotides apart from the -26 mutant. In vitro transcription assays have shown that each of the mutations reduces the global proximal promoter activity by approximately 40%. Two double mutant promoters did not show any additional downregulation in the in vitro transcription assay. In experiments designed to assess the relative transcriptional activity mediated from each of the five sites independently, we have tested artificial homopolymer promoters of each site in the in vitro transcription assay. These studies show that sites 4 and 5 are the strongest activators and that transactivation from site 5 is further enhanced by the albumin D site-binding protein. In summary, these investigations show deleterious effects of each of the Leyden mutations tested on the binding of trans-acting factors and also show disruption of transcriptional activation in a functional in vitro transcription assay. Our results also show that cis-acting elements 4 and 5 are the principal activators of this locus.
