Introduction: Although many studies have addressed the relationship between multiple individual polymorphisms in the vitamin D receptor (VDR) gene and bone health, few have analyzed this data in terms of haplotypes. We performed a meta-analysis of studies with data on the BsmI, ApaI, and TaqI polymorphisms in order to (a) estimate haplotype frequencies, (b) determine linkage disequilibrium (LD), and (c) estimate the magnitude of the association between haplotypes and osteoporosis/bone mineral density (BMD).
Methods: Haplotypes were inferred using the expectation-maximization algorithm (EM); log-linear models were used to determine association with osteoporosis; and regression analysis with variance components was used to determine association with BMD.
Results: Our results indicate that the most common haplotype for the VDR gene, regardless of ethnicity, is baT, followed by BAt and bAT in Caucasians, and bAT and BaT in Asians. This indicates strong LD between the BsmI and TaqI polymorphisms. We demonstrate a gain in power when considering the haplotypes rather than the individual polymorphisms separately, i.e., although BsmI, ApaI, and TaqI were not significantly associated with osteoporosis on their own, the haplotypes Bat and BAt were significantly associated, with an OR of approximately 4.
Conclusion: We have applied haplotype analysis to the VDR polymorphisms and bone measures. We also highlight a number of methodologic issues, including linkage disequilibrium, the robustness of the EM algorithm in this context, and the potential for exploring effect modification.