Recent progress in understanding the genetic susceptibility to osteoporosis.

Family and twin studies have established a genetic contribution to the etiology of osteoporosis. The genes and allelic variants conferring osteoporotic risk are largely undefined, but the number of candidates has increased steadily in recent years (Table I). Osteoporosis is a complex disease, and allelic variation in many other candidate-genes including those that encode growth factors, cytokines, calciotropic hormones, and bone matrix proteins are likely to also play a role and warrant systematic investigation. Most family and association studies to date have focused on the genetic contributions to bone density, a major determinant of bone strength and fracture risk. Bone density is not the only determinant of skeletal fragility, however, and genetic influences on fracture risk are independent of bone density [Cummings et al., 1995]. The microarchitectural properties and overall size and geometry of bone also influence skeletal strength [Bouxsein et al., 1996], and the genetic influences on these phenotypes should be investigated more rigorously. Even fewer studies have assessed the association between candidate-gene variation and the risk of fracture, the most important clinical outcome of osteoporosis. Large-scale molecular epidemiologic studies will be increasingly necessary in the future to quantify the relative, absolute and attributable risks of fracture associated with specific genetic variants. Osteoporosis is a complex, multifactorial disease, and most candidate-gene association studies have had limited statistical power to assess gene-gene and gene-environment interaction. Although gender plays an important role in the development of osteoporosis, genetic studies have almost exclusively focused on women, and have not tested whether gender modifies the association between genetic variation and osteoporotic risk. Therefore, future genetic studies will need to recruit larger samples of individuals including men. Rapid additional progress in our understanding of the molecular basis of osteoporosis can be expected in the near future as ongoing genome-wide linkage [Spotila et al., 1996] and candidate-gene association analyses are completed. Linkage analyses in families at high-risk for rare metabolic bone diseases should also yield important clues to the pathogenesis of osteoporosis. Recent examples are the mapping of loci for both high [Johnson et al., 1997] and low [Gong et al., 1996] bone mass to chromosome 11q and osteopetrosis to chromosome 1p [Van Hul et al., 1997]. Similar ongoing studies in baboons [Rogers and Hixson, 1997] and mice [Beamer et al., 1997] may reveal additional loci whose human homologs contribute to osteoporotic risk. The improved understanding of osteoporosis that will emerge from these genetic studies should lead to better diagnosis of this disease and new treatment and prevention strategies.