Quantitative genetics of circulating molecules associated with bone metabolism: a review.

This paper reviews recent advances in the studies of various biochemical factors (biomarkers) involved in bone metabolism and remodeling. The collected data in this area suggest the existence of complex and multilevel relationships between calciotropic hormones, various cytokines and growth factors. The paper summarizes the data on the magnitude of the familial and genetic effects on the interindividual variation in circulating levels of many of these biomarkers. The majority of the cited heritability estimates are well above 20%, reaching up to 80% for some cytokines (e.g., TNFalpha and VEGF). These estimates point to potential targets for the identification of novel quantitative trait loci involved in the control of the respective molecules variation. This information is of particular importance, because the available data on the association between specific genes/polymorphisms and the respective circulating molecules variation is still very limited. The paper also provides recent findings on the genetics of co-variation between the circulating levels of various biomarkers. It shows that only in a few instances, such as for example, between IGF-I and IGFBP-3, and IGFBP-1 and leptin, significant and substantial genetic (and environmental) correlations were found. It appears that despite the prominent strong genetic effects on variation of each of the numerous biomarkers, the pleiotropic effects are rather limited. We consider briefly some important new data obtained using the gene expression approach and microarray technique. The data, for instance, indicate that the genetic effects on bone metabolism appear to be an open system, which can be activated or modulated by external factors such as drugs, e.g., PTH. Extensive molecular genetic studies in this area are both timely and imperative to detect the specific genes affecting variation (and co-variation) of the circulating factors associated with bone metabolism.