Analysis of genetic polymorphisms in the transforming growth factor-β1 gene and the risk of Alzheimer's disease

Abstract.

Alzheimer's disease (AD) is a complex disease involving several genetic and environmental components. Genetic studies have yet to identify all the genes involved in the pathogenesis of AD. Transforming growth factor-β1 (TGF-β1) is a candidate gene for AD. It is a multifunctional cytokine whose overexpression has been shown to promote the deposition of amyloid-β peptide. The goal of this study was to investigate the association of three polymorphisms in TGF-β1 with the risk of AD. Two of the polymorphisms are located in the 5' region at positions –800 (G→A) and –509 (C→T), and the third is in exon 5 at codon 263 (Thr→Ile). We screened DNA samples from 428 sporadic, late-onset patients and 421 controls by PCR-based assays. There was no statistically significant difference in genotype or allele frequency distributions between cases and controls for the –800 or codon 263 polymorphisms (P=0.38 and P=0.60, respectively). The overall genotype distribution at the –509 site was significantly different between cases and controls. (P=0.017). The frequency of the –509/TT genotype was significantly higher in AD patients than controls (P=0.015). We further tested whether this polymorphism may alter the regulation of the TGF-β1 gene using dual luciferase reporter assay. We subcloned the 5' flanking region, which contained the –509 C/T polymorphic sites, in front of the firefly luciferase reporter gene in pGL-3 basic vector and co-transfected with the pRL-CMV vector containing Renilla luciferase gene as a control for transfection efficiency in COS-1 cells. The activity of each promoter allele was directly measured by the ratio of firefly luciferase activity to Renilla luciferase activity. The –509 T allele was associated with marginally higher transcriptional activity of TGF-β compared with the –509 C allele (P=0.051). These data suggest that the –509 polymorphism of TGF-β1 may be modestly associated with the risk of AD. However, these data should be interpreted with caution as the differences associated with the –509 alleles in both the genetic association and the transfection studies were modest.