Chapter 3 - Relevance of Peak Bone Mass to Osteoporosis and Fracture Risk in Later Life

Abstract

Osteoporosis is highly influenced by genetic factors. Bone mineral density (BMD) has also been shown to be highly heritable, as are other risk factors for osteoporotic fracture such as bone quality, femoral neck geometry and bone turnover. Susceptibility to osteoporosis is mediated, in all likelihood, by multiple genes each having small effect and a number of different approaches are being employed to identify the genes involved. Study methods include linkage studies in both humans and experimental animals as well as candidate gene and gene expression studies. Linkage studies have identified multiple quantitative trait loci (QTL) for regulation of BMD and, along with twin studies, have indicated that the effects of these loci on BMD are site-dependent and sex-specific. On the whole, the genes responsible for BMD regulation at these QTL have not been identified. Most studies have used the candidate gene approach, based on what is known of bone metabolism. The vitamin D receptor gene (VDR), the collagen type I alpha I gene (COLIA1) and estrogen receptor gene (ER) alpha have been widely investigated and found to play a role in regulating BMD. A recent meta-analysis suggests, however, that VDR plays no significant role, and the effects of the other 2 genes are modest--probably accounting for less than 3% of the genetic contribution to BMD. Cost-effective large scale genetic testing is becoming available and lends itself to combining large multi-national populations for candidate gene analysis, meta-analyses, DNA pooling studies and gene expression studies.