Contribution of a common variant in the promoter of the 1-α-hydroxylase gene (CYP27B1) to fracture risk in the elderly

Abstract

CYP27B1 encodes mitochondrial 1α-hydroxylase, which converts 25-hydroxyvitamin D to its active 1,25-dihydroxylated metabolite. We tested the hypothesis that common variants in the CYP27B1 promoter are associated with fracture risk. The study was designed as a population-based genetic association study, which involved 153 men and 596 women aged 65-101 years, who had been followed for 2.2 years (range 0.1-5.5) between 1999 and 2006. During the follow-up period, the incidence of fragility fractures was ascertained. Bone ultrasound attenuation (BUA) was measured in all individuals, as were serum 25-hydroxyvitamin D and PTH concentrations; 86% subjects had vitamin D insufficiency. Genotypes were determined for the -1260C>A (rs10877012) and +2838T>C (rs4646536) CYP27B1 polymorphisms. A reporter gene assay was used to assess functional expression of the -1260C>A CYP27B1 variants. The association between genotypes and fracture risk was analyzed by Cox's proportional hazards model. We found that genotypic distribution of CYP27B1 -1260 and CYP27B1 +2838 polymorphisms was consistent with the Hardy-Weinberg equilibrium law. The two polymorphisms were in high linkage disequilibrium, with D' = 0.96 and r² = 0.94. Each C allele of the CYP27B1 -1260 polymorphism was associated with increased risk of fracture (hazard ratio = 1.34, 95% CI 1.03-1.73), after adjustment for age, sex, number of falls, and BUA. In transient transfection studies, a reporter gene downstream of the -1260(A)-containing promoter was more highly expressed than that containing the C allele. These data suggest that a common but functional variation within the CYP27B1 promoter gene is associated with fracture risk in the elderly.

Fig. 1

Cumulative probability of fracture stratified by CYP27B1 –1260 genotype. Probability of fracture over time (y) is expressed for each genotype (AA, short dashed line; AC, long dashed line; CC, solid line)

Fig. 2

CYP27B1 promoter activity in vitro. HEK293 cells were transfected with a luciferase reporter gene with the 1.3-kb CYP27B1 promoter containing either the –1260C or the –1260A allele. Luciferase expression in each case was normalized for an internal transfection control (renilla) and expressed relative to activity from cells transfected with the empty reporter plasmid (pGL3-basic). Data are mean ± SD of three experiments, each performed in triplicate wells. CYP27B1 –1260C allele, white bars; CYP27B1 –1260A allele, solid bars. Significance was determined by Student’s t-test