Functional analysis of single nucleotide polymorphisms in the proximal promoter regions of the multidrug transporter genes MRP1/ABCC1 and MRP4/ABCC4
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The ATP-binding cassette (ABC) transporter superfamily consists of 49 members, to which both Multidrug Resistance Protein 1 (MRP1/gene symbol: ABCC1) and MRP4 (ABCC4) belong. Single nucleotide polymorphisms (SNPs) in drug metabolizing genes have been shown to affect individual responses to drugs and toxins. However, the role of SNPs in modulating the activity of drug transporters, such as MRP1 and MRP4, is poorly characterized. The overall goal of my thesis was to determine the effects of SNPs in the promoter regions of human ABCC1 and ABCC4. For MRP1/ABCC1, two proximal promoter SNPs (-275A>G, -260G>C) were identified in the literature and recreated in vitro, and the activity of the mutant ABCC1 promoter constructs was measured in five human cell lines using a dual luciferase assay. The activity of the -275A>G promoter was comparable to the wild-type ABCC1 promoter. On the other hand, the -260G>C substitution decreased ABCC1 promoter activity in HepG2, MCF-7 and HeLa (40 - 60%) cells. A 1706 bp fragment containing the 5’-flanking and untranslated regions of ABCC4 were isolated from two bacterial artificial chromosome clones and six serially deleted ABCC4 promoter reporter constructs generated. Luciferase assays of the basal promoter constructs of ABCC4 in HEK293T cells revealed the presence of one or more negative regulatory regions between -1706 and -876, between -876 and -641, and one or more positive regulatory regions between -641 and -356, and between -356 and -17. Also, the ABCC4 promoter displayed differential activity in MDCKI and LLC-PK1 cells than in HEK293T cells. One SNP (-523G>C) was identified from an online database and its activity tested. However, -523G>C SNP did not cause any significant change in the ABCC4 promoter activity in both HEK293T and HepG2 cells (80 – 130%). In summary, the data obtained suggest that the promoter SNPs studied may affect the transcriptional activity of ABCC1 or ABCC4, but it seems likely that this is not true in all cell types.