Vitamin D receptor genotype is associated with radiographic osteoarthritis at the knee

Osteoporosis and osteoarthritis are age-related disorders of the skeleton with genetic components. Low bone density is a risk factor for osteoporotic fracture while osteoarthritis is associated with increased bone density. The 1,25-dihydroxyvitamin D3 receptor (VDR) gene locus was previously found to be associated with bone density. We therefore studied the relationship between radiographic osteoarthritis at the knee and VDR genotype in a population-based sample (n = 846), using molecular haplotyping of anonymous intragenic DNA polymorphisms. Radiographic osteoarthritis was defined using the Kellgren score, which is based on the assessment of osteophytes and joint space narrowing (JSN). We show that one VDR haplotype allele is significantly overrepresented in individuals with knee osteoarthritis and associated with a 2.27-fold increased relative risk (95% confidence interval 1.46, 3.52). Adjustment for bone density at the femoral neck did not change these results, indicating that the association is not mediated by bone density. The association appeared to be largely explained by the presence of osteophytes rather than JSN. Our results indicate a role of the VDR gene in the pathogenesis of osteophytes while linkage disequilibrium with another nearby gene, i.e., the collagen type IIa1 gene encoding the most abundant protein in cartilage, might contribute to the association.