Background: Human FIX (hFIX) gene transfer into hepatocytes has provided a novel approach for treatment of hemophilia B. To obtain an improved expression of hFIX, the functional hFIX-expressing plasmids with appropriate intron-derived fragments which facilitate transcription and promote an efficient 3'-end formation of mRNAs are required.
Objectives: We aim to evaluate the functions of the heterologous intron-derived fragments intra and extra hFIX-cDNA coding region with respect to the hFIX expression in the hepatocytes and kidney cells.
Materials and methods: HepG2 cells as differentiated hepatocytes and Hek-293T cells as embryonic kidney cells were transfected with the different hFIX-expressing plasmids containing various combinations of the two human beta-globin (hBG) introns within the hFIX-cDNA and Kozak sequence. In the next stage, as a hepatocyte-specific sequence, the rat aldolase B intronic enhancer sequence (rABE), was isolated from the first intron of the rat aldoase B gene and inserted within the upstream CMV promoter (CMVp) and efficacies of the engineered vectors were investigated in the stably-transfected HepG2 cells.
Results: Our data indicate that the intron-less construct and hBG intron-I containing construct are more effective with regard to hFIX expression compared to other constructs in Hek-293 cells. In HepG2 cells, the rABE in combination with CMVp in context of intron-less plasmid induced an increase in total expression of hFIX protein dramatically; ranging from 2.3 to 40 folds increase compared to other constructs. The rABE in combination with CMVp in the hBG intron-I, hBG intron-II, and hBG intron-I,II containing plasmids induced 3.7, 2, and 1.6-fold increase in the total expression of hFIX protein, respectively. The presence of both hBG intronic sequences within the hFIX-cDNA induced a higher secretion level of hFIX than either intron-I or II alone and provided correctly spliced hFIX transcripts in HepG2 and kidney cell lines. The intron-less construct with or without rABE induced the highest hFIX mRNA levels in HepG2 and Hek-293T cells respectively compared to other constructs.
Conclusions: The embryonic kidney cells in addition to the differentiated hepatic cell lines could be successfully targeted by plasmid vectors. The intron-less and hBG intron-I containing plasmids represent a particular interest in producing recombinant hFIX in Hek-293T cells. The synergistic function on the hFIX expression that was achieved by combining the CMVp with the liver-specific rABE would be a useful approach for future designing of the expression cassettes for hepatocyte-mediated gene expression in hemophilia B.
Keywords: Aldolase B intronic enhancer; Beta-globin introns; Hemophilia B; Hepatocyte; Human factor IX (hFIX); Plasmid.