Osteoprotegerin Gene Polymorphism and the Risk of Osteoporosis and Vascular Disease

Lorenz C. Hofbauer and Michael Schoppet

Departments of Internal Medicine and Gastroenterology, Endocrinology and Metabolism (L.C.H.) and Cardiology (M.S.), Philipps-University, Baldingerstrasse, D-35033 Marburg, Germany

Address all correspondence and requests for reprints to: Lorenz C. Hofbauer, M.D., Division of Gastroenterology, Endocrinology and Metabolism, Department of Internal Medicine, Philipps-University, Baldingerstrasse, D-35033 Marburg, Germany. E-mail: . hofbauer{at}post.med.uni-marburg.de

We used to think our fate was in the stars. Now we know, in large measure, our fate is in our genes.

[James D. Watson, 1989]

A positive family history of osteoporosis is a strong predictor of low bone mineral density (BMD) and osteoporotic fractures. This notion has an important clinical impact when assessing an individual’s risk of developing osteoporosis. Several genetic studies conducted in monozygotic twins have highlighted the influence of genetic factors on bone metabolism and have suggested that 50–70% of the variability of the bone phenotype is genetically determined (1). Interestingly, the largest genetic influence was reported at skeletal sites that were mainly composed of trabecular bone, such as the lumbar spine.

However, unlike osteogenesis imperfecta or osteopetrosis, which are characterized by a single gene defect that results in a defined alteration of bone cell biology and a distinct clinical phenotype, no such linear relationship exists for osteoporosis. Osteoporosis represents a polygenic disease that is largely modified by environmental, nutritional, and behavioral factors. Furthermore, important aspects of the skeletal development, including the size and shape of bones, accumulation of peak bone mass, and the rate of bone loss are determined by a variety of pleiotropic factors that regulate bone cell biology through multiple skeletal and extraskeletal pathways. Moreover, the risk of suffering an osteoporotic fracture is also determined by the concurrent presence and severity of other polygenic disorders such as cardiovascular, neurological, and ophthalmologic diseases that directly or indirectly predispose to, or protect against, falls.

Various candidate genes have been implicated to account for the genetic basis of osteoporosis, including hormones and their receptors, cytokines and growth factors, bone matrix proteins, and others (Table 1Go). Overall, polymorphisms in the genes encoding vitamin D receptor (VDR), estrogen receptor-{alpha}, and collagen type I{alpha}1 have been evaluated in detail and were found to be variably associated with biochemical markers of bone turnover, BMD, and the risk of osteoporotic fractures (2, 3).


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Table 1. Candidate genes for osteoporosis based on polymorphism studies

 
Receptor activator of nuclear factor-{kappa}B ligand (RANKL) is a critical cytokine for osteoclast differentiation and activation and an essential regulator of osteoblast-osteoclast cross-talks (4). RANKL activates its receptor RANK, which is located on osteoclastic lineage cells, and this interaction is prevented by osteoprotegerin (OPG), which acts as an endogenous receptor antagonist and blocks the effects of RANKL (4). While RANKL enhances bone resorption and bone loss and promotes osteoporosis, OPG has opposite effects (5). In animal studies of knockout and transgenic mice, bone mass was positively correlated with the amount of the OPG gene product (5, 6), thus underlining the pivotal role of OPG in the regulation of bone mass. These studies suggest that OPG (and RANKL) are obvious candidate genes for osteoporosis. Of interest, the promoter region of the human OPG gene (7) contains various binding sites that are able to mediate the stimulation of OPG gene expression by TGF-ß, bone morphogenetic protein-2, and the osteoblast-specific transcription factor Cbfa1, or the inhibition by PTH (4).

In this issue of JCEM, Arko et al. (8) report that polymorphisms in the promoter region of the human OPG gene may contribute to the genetic regulation of BMD at the lumbar spine. A total of 103 women from Slovenia with postmenopausal osteoporosis were evaluated. In addition to clinical, biochemical, and radiological assessment, the OPG gene promoter region was screened for sequence variations using PCR, single-strand conformation polymorphism analysis, and DNA sequencing, and four polymorphisms were discovered in the promoter region: 209 G->A, 245 T->G, 889 C->T, and 950 T->C. Women with polymorphisms 209 G->A or 245 T->G had a lower BMD at the lumbar spine. All women were either homozygotic (GG/TT) or heterozygotic (GA/TG) at these two sites (209/245), and the genotype distribution was identical. Interestingly, the mutated 209/245 haplotypes GA/TG were associated with a lower BMD at the lumbar spine as compared with the nonmutated GG/TT, but not at any other skeletal sites. These findings are, in part, supported by a recent study conducted in Danish women and men. In a more comprehensive study, Langdahl et al. (9) analyzed the promoter region and the five exons of the human OPG gene and identified a total of 12 polymorphisms, including the 245 T->G and the 950 T->C substitutions reported by Arko et al. (8) as well as polymorphisms 163 A->G (promoter), 1181 G->C (exon 1), and 6890 A->C (intron 4). Further analysis of these five polymorphisms in 595 men and women confirmed that the G allele of 245 T->G was present in 12.4% of patients with osteoporotic fractures, but only in 6.5% of normal controls, and that a carrier of the G allele had an odds ratio of 2.00 for an osteoporotic vertebral fracture (9). Similarly, the G allele of 163 A->G was more frequent in patients with osteoporotic fractures with an odds ratio of 1.44 for an osteoporotic vertebral fracture. While the C allele of the 1181 G->C polymorphism was less frequent in patients with osteoporosis as compared with normal subjects, the 950 T->C and the 6890 A->C substitutions were equally distributed between patients with osteoporosis and normal subjects. Interestingly, none of these five polymorphisms was associated with changes of biochemical markers of bone turnover or BMD in this study (9). A third genetic study on OPG polymorphism conducted on 511 Irish women also reported the presence of 950 T->C in the promoter region and 1181 G->C in exon 1 of the OPG gene (10). In contrast to the study by Langdahl et al. (9), the C allele of the 1181 G->C polymorphism was slightly, but nonsignificantly, associated with a low BMD in women (10). The 950 T->C substitution of the OPG gene promoter as well as VDR and COLL I{alpha}1 polymorphisms were not associated with BMD in this population (10). Finally, Wuyts et al. (11) reported a weak association of a 445 C->T polymorphism of the OPG gene with Paget’s disease of bone. However, this polymorphism has not been assessed in osteoporosis (8, 9, 10).

Interestingly, one of the OPG promoter polymorphisms (950 T->C) that was not associated with osteoporosis in any of these three studies (8, 9, 10) has been reported to be associated with vascular morphology and function (12). Healthy subjects with a CC genotype displayed an increased intima-media thickness of their common carotid artery and a reduced maximal forearm blood flow following ischemia (12), indicating that the CC genotype carriers may be at increased risk of cardiovascular diseases. Intriguingly, the RANKL-OPG cytokine system has been linked previously to the concurrent development of osteoporosis and vascular disease (13), and OPG gene polymorphisms may represent a potential genetic basis for this association.

Taken together, OPG gene polymorphisms, notably the 245 T->G substitution in the promoter region, may affect the risk of osteoporotic fractures. Clearly, OPG polymorphism studies enrolling a larger number of patients from different ethnic backgrounds are required to confirm these data. In addition, details on the impact of polymorphisms on RNA splicing as well as OPG protein structure and function should help to clarify the significance and mechanism how OPG regulates bone metabolism and vascular function in humans.

Acknowledgments

Footnotes

Abbreviations: BMD, Bone mineral density; OPG, osteoprotegerin; RANKL, receptor activator of NF-{kappa}B ligand; VDR, vitamin D receptor.

Received July 9, 2002.

Accepted July 11, 2002.

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