Genetic determinants of bone mass.

A genetic contribution to bone mass determination was first described in the early 70s. Elucidation of gene contribution to this has since been attempted through studies analyzing associations between bone mass acquisition and/or maintenance and polymorphic variations of several genes. The first to be described was the vitamin D receptor gene (VDR), initially claimed to contribute to almost 75% of the genetic variation in bone mineral density (BMD) in twin and general population studies. Not all of the studies published to date conclude that a clear relationship exists between polymorphic VDR alleles and BMD, and the molecular basis for the VDR gene polymorphisms influence on bone mineralization has not yet been clarified. Since then, other genes with a significant role in bone metabolism such as estradiol receptor, collagen type 1alpha1, TGF-beta1, interleukin-6, calcitonin receptor, alpha2-HS-glycoprotein, osteocalcin, calcium-sensing receptor, interleukin-1 receptor antagonist, beta3-adrenergic receptor, apolipoprotein E, PTH, IGF-I and glucocorticoid receptor have been analyzed. Some polymorphic variations in these genes have been associated in some works with significant differences in BMD, with even more significant contributions when associations of different gene polymorphisms were analyzed. Again, the molecular basis for the contribution of these alleles to bone mass determination has not yet been described. A different approach has been attempted by linkage analysis of loci involved in bone density in pedigrees with low BMD using BMD as a quantitative trait. Recent results do not confirm, in these families, any association with any of the previously reported genes, but rather with other as yet unidentified genes. The genetic contribution to mild variations in the general population, as a result of environmental and endogenous individual influences, probably differs completely from that providing a pathologic BMD.