Genetics of osteoporosis.

There is clear evidence of genetic modulation of bone phenotype parameters including bone density, quantitative ultrasound, bone size, and bone turnover. At any particular age and phase of life, genetic factors explain about 70% of the variance in bone phenotype after adjustment for major medical and disease factors. Hormonal factors, diet, and lifestyle interact with those genetic factors over time. Common allelic variation in the VDR was the first of several genes and now chromosomal loci to be implicated in the genetic determination of bone phenotype. The VDR polymorphisms have an effect weaker than originally reported, and part of the allelic effects may be mediated by effects on body size and development and even other hormonal regulators such as PTH or insulin. Irrespective of the strength or mechanism of these associations, these initial findings on the VDR stimulated the field of the genetics of osteoporosis with targeted genetic studies and now genome scan approaches. Intronic polymorphisms of the collagen I alpha 1 gene have been shown to be related to bone density and to fracture risk in several studies, although not all findings concur. Common allelic variations have now been associated with bone density for the estrogen receptor, TGF beta receptor, and TGF beta 1, for the insulin-like growth factor-I pathway, for interleukin-4 and -6 and the interleukin-1 receptor antagonist, for calcitonin and the PTH receptors and for apolipoprotein E. Of considerable interest, chromosomal loci, notably 11q 12-13, have now been linked to bone phenotypes in human and mouse studies. The mouse strain studies seem likely to be powerful tools providing insight to important human loci based on the mouse-human chromosomal synteny. Variability of genetic findings across studies seems to be the rule rather than the exception. This variability may relate to interaction of particular loci with specific environmental or even other genetic loci. The importance of genetic heterogeneity, including ethnicity, as well as environmental and hormonal confounders, such as calcium and vitamin D intake, hormonal status and skeletal and body size, will need to be taken into account in future gene search approaches. Genome scans in relation to bone density and fracture end-points will need to account for such important potential confounders in each target population. Interactions between genetic and environmental factors, including lifestyle, have been investigated initially for the VDR polymorphisms in relation to the response of bone density and turnover to calcium intake and treatment with simple vitamin D and active vitamin D compounds. Gene-gene and gene-environment interactions in human and animal models will be critical targets for future research. Further genes with positive and negative effects on bone phenotype are certain to be identified in the near future. Each of these will need to be evaluated in relation to potential environmental modulators in pharmacogenetic models. Understanding the molecular physiology of such gene effects is likely to lead to more specific treatments and to allow the selection of more appropriate and effective treatment options.