Osteoporosis is a common disorder with a strong genetic component. One way in which the genetic component could be expressed is through polymorphism of COLIA1, the gene for collagen type Ialpha1, a bone-matrix protein. We determined the COLIA1 genotypes SS, Ss, and ss in a population-based sample of 1778 postmenopausal women using a polymerase-chain-reaction-based assay. We then related the genotypes to bone mineral density and the occurrence of osteoporotic fractures in these women. As compared with the 1194 women with the SS genotype, the 526 women with the Ss genotype had 2 percent lower bone mineral density at the femoral neck (P=0.003) and the lumbar spine (P=0.02); the 58 women with the ss genotype had reductions of 4 percent at the femoral neck (P= 0.05) and 6 percent at the lumbar spine (P=0.005). These differences increased with age (P=0.01 for modification by age of the effect of COLIA1 on femoral-neck bone density, and P=0.004 for modification of the effect on lumbar-spine bone density). Women with the Ss and ss genotypes were overrepresented among the 111 women who had incident nonvertebral fractures (relative risk per copy of the s allele, 1.5; 95 percent confidence interval, 1.1 to 2.1). The COLIA1 polymorphism is associated with reduced bone density and predisposes women to osteoporotic fractures.
