The influence of an insulin-like growth factor I gene promoter polymorphism on hip bone geometry and the risk of nonvertebral fracture in the elderly: the Rotterdam Study.

UNLABELLED: The absence of the wildtype allele of a promoter polymorphism of the IGF-I gene is associated with increased risk (1.5; 95% CI, 1.1-2.0) of fragility fracture in women (n = 4212) but not in men (n = 2799). An approximation of hip bone geometry (from DXA) suggested the polymorphism is associated with bone strength and stability in gender-specific ways.

INTRODUCTION: Previously, we found a CA-repeat promoter polymorphism in the insulin-like growth factor I (IGF-I) gene associated with IGF-I levels and BMD in postmenopausal women, but the relationship with fractures is unclear. In this large population-based study of elderly men and women, we examined the association between this IGF-I promoter polymorphism with parameters of bone geometry and the occurrence of fractures.

MATERIAL AND METHODS: Within the Rotterdam Study, a prospective population-based cohort, the IGF-I polymorphism was analyzed in relation to incident nonvertebral fractures in 2799 men and 4212 women followed on average for 8.6 years. Furthermore, we estimated structural parameters of hip bone geometry indirectly from DXA outputs of the femoral neck in 2372 men and 3114 women. We studied neck width, cortical thickness, and the cortical buckling ratio and the section modulus as indexes of bone stability and bending strength.

RESULTS: Women heterozygotes and noncarriers of the allele had, respectively, 1.2 (95% CI, 1.0-1.5) and 1.5 (95% CI, 1.1-2.0) increased risk of having a fragility fracture at older age compared with homozygotes for the 192-bp allele (p trend = 0.0007). In men, fracture risk was not influenced by the polymorphism. Compared with homozygotes for the 192-bp allele, noncarrier males had approximately 1% narrower femoral necks and 2.2% lower section moduli (p trend < 0.05). Noncarrier females had 1.7% thinner cortices and 1.6% higher buckling ratios (p trend < 0.05) but no significant differences in femoral neck widths and section moduli. In women with low body mass index, genotype differences in bone strength (section modulus) and fracture risk were accentuated (p interaction = 0.05). The genotype-dependent differences in hip bone geometry did not fully explain the genotype-dependent differences in fracture risk.

CONCLUSIONS: The CA-repeat promoter polymorphism in the IGF-I gene is associated with the risk for fragility fracture at old age in women and with bone structure in both genders.