Osteoporosis (OMIM166710) is a common skeletal disorder characterized by low bone mass and microarchitectural deterioration of bone tissue with increased susceptibility to fracture. Osteoporosis has a complex etiology and is considered a multifactorial polygenic disease in which genetic determinants are modulated by hormonal, environmental, and nutritional factors. Estrogens are known to play an important role in regulating bone homeostasis and preventing postmenopausal bone loss. They act through binding to two different estrogen receptors (ERs), ER (OMIM133430) and ERß (OMIM601663), which are members of the nuclear receptor superfamily of ligand-activated transcription factors. Different polymorphisms have been described in both the ER and ER ß genes. Although a large number of association studies have been performed, the individual contribution of these polymorphisms to the pathogenesis of osteoporosis remains to be universally confirmed. Moreover, an important aim in future work will be to define their functional molecular consequences and their interaction with the environment in the causation of the osteoporotic phenotype. A further promising application of these polymorphisms comes from their pharmacogenomic implications, with the possibility of providing better guidance for therapeutic regimens, such as estrogen replacement therapy and selective ER modulators. At the moment, no recommendations for population-based screening can be made.

Estrogens exert beneficial effects on the development and maintenance of the skeleton (1-3). These include control of growth plate maturation and closure during longitudinal growth, regulation of cortical and cancellous bone metabolism, acquisition of peak bone mass, and inhibition of bone loss. Recent reports from population-based studies clearly indicate that estrogens are necessary for regulation of skeletal homeostasis not only in women but also in men (4-6). The skeletal effects of estrogen are mediated by its binding to specific estrogen receptors (ERs) localized at the cytosolic and nuclear level. These receptors belong to the nuclear receptor hormone superfamily and are ligand-inducible transcription factors.

Two functional ERs, namely ER and ERß, which are encoded by different genes, have been described so far. The human ER gene (also named ESR1) is located on chromosome 6q25. It comprises eight exons separated by seven intronic regions and spans more than 140 kilobases (7-8). The ERß gene (ESR2) is located on chromosome 14q23-24.1 and is composed of eight exons spanning approximately 40 kilobases (9-10). ERß is smaller than ER but possesses a similar structure and considerable homology in the DNA-binding and ligand-binding domain (11). Both ER isoforms are expressed in osteoblasts, osteoclasts, and bone marrow stromal cells (1,12) and co-localize in adult bone. There is also evidence of age- and gender-specific expression of ERß protein (13). Some studies have suggested that ERß is more ubiquitously expressed with higher levels than ER in trabecular bone (14,15). In contrast, ER predominates in cortical bone (16).

ER appears to be the major receptor mediating estrogen action in bone, and it has a prominent effect on the regulation of bone turnover and the maintenance of bone mass. Although the role of the recently discovered ERß in bone remains unknown, there is a range of molecular evidence from in vitro studies and animal models that points toward a function distinct from that of ER. Mice with null mutations in the ER and ERß genes show distinct skeletal phenotypes. In particular, ERß may have opposing effects than ER in mediating estrogenic action on longitudinal bone growth and bone size, with only a partial and redundant effect on trabecular bone mineral density (BMD) (17-21). Functional ERs are usually homodimers, but ER/ERß heterodimers have been also described (22). Importantly, ERß can inhibit transcriptional activation of ER through formation of these heterodimers, as shown by different in vitro and in vivo experimental studies (23-25). However, in the absence of ER, ERß is able to replace some of its activity (25).

Estrogen receptor
Genetic screening of the ER gene locus has revealed the existence of several polymorphic sites (26-31). An updated view of the ER gene structure with its polymorphic regions is shown in Figure 1. The most widely studied are the PvuII (T397C) and XbaI (C351G) restriction fragment length polymorphisms (RFLPs) in intron I and the (TA) n variable number of tandem repeats (VNTR) within the promoter region of the gene. In different studies, these polymorphisms have been associated with several pathologic conditions such as breast and prostate cancer, osteoporosis, Alzheimer's disease, and cardiovascular diseases (31-37). However, results are still conflicting and the molecular mechanisms by which these polymorphisms influence receptor activity are as yet unclear. PvuII and XbaI RFLPs lie in an intronic and apparently nonfunctional area of the gene and, as would be expected for two polymorphisms separated by 50 base pairs, are in strong linkage disequilibrium. Alleles P and X (absence of restriction sites), as well as alleles p and x (presence of restriction sites), are strongly associated with each other. However, haplotype pX was not observed in the majority of studies, whereas haplotype Px was detected, albeit at a low frequency; this indicates that the disequilibrium is not complete and that either recombination or multiple mutations have occurred between or at these two polymorphic sites.

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Figure 1  Structure of, functional domains of, and described polymorphisms in the  human estrogen receptor gene. Coding exons (E) are indicated with  boxes. TAF, transcriptional activating function.

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Figure 2  Distribution of (TA) n repeat alleles in the human estrogen receptor gene  in different ethnic groups. Top: Major studies in Italian (39) and Dutch (51)  Caucasian populations. Bottom: Major studies in Japanese (75) and Chinese  (118) Asian populations.

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Table 1  Frequency of the combined PvuII-XbaI haplotypes of the human estrogen  receptor gene in different ethnic groups.

Other polymorphisms have recently been described in the ER gene. These include a T262C single nucleotide polymorphism 29 nucleotides downstream from the putative start codon, a CG single nucleotide polymorphism in codon 325 in exon 4, and a G2014A single nucleotide polymorphism in exon 8 (52-54). All three of these single nucleotide polymorphisms are silent, since they do not cause any amino acid change. In one study (52), the G2014A single nucleotide polymorphism in exon 8 was shown to be in linkage disequilibrium with the PvuII polymorphism in intron I.

Estrogen receptor ß
In 1998, Tsukamoto et al. (55) firstly characterized a highly polymorphic (CA) dinucleotide repeat in intron 5 of the human ERß gene in a Japanese population. Subsequently, systematic mutation screening detected five different sequence variants, including two mutations and three polymorphisms (56). The first was a silent T1421C transition in exon 7; the second was a silent G1082A transition within the ligand-binding domain in exon 5; and the third was a A1730G single nucleotide polymorphism in the 3'-untranslated region of exon 8. The functional importance of these polymorphisms has not been clarified, and differences in the distribution of genotypes between Caucasian and Asian populations have been described. More recently, five novel polymorphisms were identified in an African population (57). Three of them (C143T in exon 1, A566T in exon 2, and T1100G in exon 5) are silent polymorphisms, while the other two are expected to change the amino acid sequence of ERß. These include an A105G single nucleotide polymorphism in exon 1 corresponding to an isoleucine-to-valine substitution at amino acid position 3 and a T1057G single nucleotide polymorphism in exon 5 determining a valine-to-glycine substitution at position 320, in helix 4 of the ligand-binding domain. Helix 4 does not participate directly in ligand binding, but it interacts with coactivators. Importantly, in functional in vitro analysis, the presence of the valine in position 320 showed significantly decreased maximal transcriptional activity (57). The characterized polymorphic variants in the ERß gene are summarized in Figure 3.

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Figure 3  Structure of, functional domains of, and described polymorphisms in the  human estrogen receptor ß gene. Coding exons (E) are indicated with  boxes. TAF, transcriptional activating function; UTR, untranslated region.

Osteoporosis is the most prevalent metabolic bone disorder among developed countries. It is defined as a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to nontraumatic fracture (58). Osteoporosis is now considered a major and growing healthcare problem throughout the world. It has been estimated that in the United States, at least 90 percent of all hip and spine fractures among elderly White women and more than 70 percent of those among elderly White men may be attributed to osteoporosis (59). The lifetime risk of fracture in women aged 50 years is approximately 16 percent for hip fractures, 15 percent for wrist fractures, and 32 percent for vertebral fractures (60). Approximately 50 percent of all women will have osteoporosis by the age of 80 years. Conversely, a White man aged 50 years has approximately a 6 percent risk of hip fracture and a 16-25 percent risk of any osteoporotic fracture in his remaining life (61). It has been estimated that costs related to hip fracture will double during the next 25 years (63). Because of the increasing life expectancy of the population, the number of people with osteoporosis will be augmented drastically during the coming years, with huge health implications. Therefore, prevention and treatment of osteoporosis is of major importance for health organizations in all countries.

Osteoporosis affects mainly postmenopausal women, but also men. Osteoporotic fractures, which represent the most relevant clinical aspect of the disease, usually occur in the distal forearm, thoracic and lumbar vertebrae, and proximal femur. Their incidence increases with age and is higher in Whites than in Blacks. Fractures of the hip incur the largest direct cost for health services and mainly occur in the elderly, giving rise to substantial morbidity and mortality. Osteoporotic fractures of the vertebrae and forearm are of less economic significance but also give rise to significant morbidity. Quality of life becomes progressively impaired as the number and severity of vertebral fractures increases. Moreover, future risk of osteoporotic fractures is greatly increased in patients with one or more vertebral fractures.

The major determinant of bone strength and osteoporotic fracture risk is BMD, as assessed by dual photon absorptiometry or dual energy x-ray absorptiometry (63-66). In women, according to World Health Organization criteria, osteoporosis is defined to exist when BMD values fall more than 2.5 standard deviations below the young adult reference mean (63). Many studies have indicated that the risk of fragility fractures increases progressively as BMD declines and that there is a close relation between the prevalence of osteoporosis, as assessed by World Health Organization criteria, and the incidence of fractures (60, 64-66). It has been estimated that the risk of new vertebral fractures increases by a factor of 2.0-2.4 for each standard deviation decrease in BMD, irrespective of the site of measurement (67). Bone strength does not depend only on BMD; bone size and bone quality are other important components. A longer hip axis length is also associated with increased hip fracture risk independently of BMD (68). Several other factors contribute to the risk of osteoporotic fractures, including muscle weakness, impaired vision, and cognitive impairment (69). Trauma, such as a fall, is an important factor as well. Any bone will fracture if subjected to excessive force; however, the lower is BMD the higher is the risk of fracture. A bone mass below average for age can be considered a consequence of inadequate accumulation of bone in young adult life (low peak bone mass) or excessive rates of bone loss or both of these conditions.

Osteoporosis has a complex multifactorial pathogenesis. Although there are several environmental influences on BMD, such as diet (calcium intake and alcohol consumption) and lifestyle factors (smoking and physical exercise), a genetic contribution to the pathogenesis of osteoporosis, accounting for 50-80 percent of the interindividual variability in bone mass, has been recognized (70, 71). There are many candidate genes implicated in the determination of BMD and in the pathogenesis of osteoporosis, spanning from those encoding for cytokines through those encoding for calciotropic hormones and their receptors to the ones encoding for collagenic and noncollagenic proteins of bone. As mediators of estrogen action, the genes encoding ER and ERß have been considered important candidates for the determination of osteoporotic risk. The clinical observation of an osteoporotic phenotype in a man with a disruptive mutation of the ER gene (72), as well as the report of decreased BMD values in mice lacking functional ER (17-20), support the hypothesis of ER as a likely candidate gene for osteoporosis. It is possible that common allelic variants of the ER gene causing differential responsiveness to estrogen exist in the general population and that high estrogen levels can initially overcome the resistance, resulting in a normal phenotype. This compensatory balance could be altered later on by aging or by a condition such as menopause, leading to clinical disorders such as osteo-porosis. Moreover, results from a recent genome-wide BMD linkage analysis in extended pedigrees from the Framingham Study provided evidence for a possible linkage of a region on chromosome 8 containing the ERß gene with lumbar BMD (73).

We appraised the results of studies of the above polymorphisms and osteoporosis. These studies were identified through a Medline (PubMed) search of articles published between January 1990 and June 2004. We combined searches for the keywords "estrogen receptor," "polymorphism," "BMD," and "osteoporosis" to identify relevant studies. We further searched the references of any identified paper to locate additional studies. Results are summarized in Web tables 1, 2, and 3, which are available on the Journal's website.

Estrogen receptor
Both intron I polymorphisms (PvuII and XbaI) and the VNTR TA polymorphism upstream from the ER gene were first associated with BMD variation in the Japanese population (74, 75). Japanese postmenopausal women who were homozygous for the absence of the XbaI restriction site and for the presence of the PvuII restriction site (Px homozygous haplotype), as well as those with a number of 12 TA repeats, were at increased osteoporotic risk, showing the lowest BMD values. Subsequent studies searching for an association between intron I RFLPs and BMD in other Asian populations yielded conflicting results (76-92). No significant associations were observed in studies of Korean (76, 77) or Chinese (78) populations. Other confirmatory studies generally showed an association between the PP and/or xx genotypes and reduced BMD (79, 80, 85, 88-90, 92), but opposite associations were also described (81, 82, 86, 87, 91). The largest study performed to date reported an association between the PX haplotype and reduced BMD in elderly Japanese women (84). Moreover, a single study examined the effect of these polymorphisms in 401 Chinese nuclear families, showing minor but significant effects on BMD variation by linkage tests (83).

Similar observations among White Caucasian subjects showed conflicting results. Studies in Belgian (93, 94), Danish (95), Italian (96), and Australian (97) populations failed to find any significant association between the XbaI and/or PvuII genotypes and BMD or bone accrual (98). Conversely, the pp genotype was associated with the lowest lumbar and total body BMD values in a group of US pre- and perimenopausal women from the Michigan Bone Health Study (99). A subsequent longitudinal evaluation in the same cohort of women demonstrated increased rates of BMD change in subjects with either the xx or the pp genotype (100). The latter observation is in keeping with a recent large-scale analysis carried out among elderly subjects from the Rotterdam Study (51); the investigators described an association of the "px haplotype" with reduced BMD and increased fracture risk, with evidence for an allele dose effect (for allele copy, odds ratio = 2.2). On the other hand, a study of Scottish postmenopausal women showed lower BMD values in subjects with the Px haplotype than in those with the PX and px haplotypes (101). The same haplotype (Px) appeared to be associated with low BMD values in a pedigree study in the Chuvaska population in Russia (102).

Studies on bone turnover and rates of bone loss according to different PvuII and/or XbaI RFLPs also produced conflicting results (74, 77, 79, 81, 93-95, 99, 103, 104).

Data on the relation between the ER polymorphism and BMD in men are limited but still discordant. Positive associations were reported in four studies of elderly men from Thailand (105), Israel (106), Finland (107), and China (108). Similar studies in British (109), Japanese (84), Korean (110), Dutch (51), and North American (111) populations did not detect any significant association. A single study examined the effect of these polymorphisms in adolescent boys, showing evidence for an association between the xx or pp genotype and increased bone density (112).

In summary, the association between intronic PvuII-XbaI RFLPs and osteoporotic risk remains controversial. A recent meta-analysis combining the results of heterogeneous studies published through November 2001 evidenced a protective role of the XX genotype in BMD and fracture risk, with no apparent effect of the PvuII polymorphism (113). Various suggestions have been made to account for these discordant findings, including genotyping error, ethnic or environmental differences among populations, differences in age and menopausal status, the inadequate sample size of many studies, treatments (i.e., estrogen use), study design (population- versus hospital-based case-control studies, cohort studies), and the health-based selection bias, with its tendency to exclude osteoporotic subjects. Even though minor ethnic variations exist in the distribution of the XbaI and PvuII genotypes (114), the degree of disequilibrium may vary among different ethnic groups, since the frequency of the Px haplotype is increased approximately twofold in subjects of Asian ancestry with respect to Caucasian populations (51). The latter observation may explain, at least in part, the observed population specificity in the genotypes predicting low and high BMD. In fact, in the majority of studies performed in populations of Asian origin, the PP and/or xx genotypes were associated with the lowest BMD values, while in Caucasian populations, the pp and/or xx genotypes were shown to confer increased risk of osteoporosis. An interesting additional hypothesis has been recently proposed by Khosla et al. (111), who suggested that the pp or xx genotype may be relatively estrogen-insensitive and that subjects with the P or X allele may benefit more from the protective effects of estrogen on bone than subjects with the p or x allele. Thus, positive or negative associations may be dependent on circulating estrogen levels. A potential confounder in such studies may also be recognized in gene-gene interactions (115).

The original association between the variable number of TA repeats and BMD that was described in the Japanese population (74, 75) was confirmed in most of the studies that followed (39, 51, 101, 116, 117). In two large-scale studies performed in Italian and Dutch populations, a statistically significant correlation between the TA repeat length and fracture risk was also observed. Women with a low number of repeats showed lower BMD and a higher incidence of vertebral fractures in comparison with women with a high repeat genotype, equivalent to a two- to threefold increase in vertebral fracture risk (39,59). A similar association between a low TA repeat number and decreased BMD was observed in pre- and perimenopausal women from the Michigan Bone Health Study but not in young children from the Iowa Bone Development Study (98, 117). By contrast, the opposite association (lowest BMD in women with a high number of repeats) was reported in a sample of Chinese women from Taiwan (118). Finally, no overall association between TA repeat number and BMD was evident in a study on Scottish postmenopausal women (101).

Other polymorphisms within the ER gene (i.e., the exon 8 G2014A and exon 4 codon 325 C/G single nucleotide polymorphisms) have been associated with BMD and osteoporosis in single studies (53, 54, 119, 120).

Estrogen receptor ß
An association between the dinucleotide (CA) n repeat polymorphism and BMD was originally described in the Japanese population (121). Japanese postmenopausal women possessing at least one allele with 26 CA repeats had significantly higher BMD at the lumbar spine compared with those not possessing the 26 CA allele. The same polymorphism was also associated with levels of androgen and sex hormone-binding globulin in premenopausal women of European descent (122), with possible implications for bone turnover. Two other studies recently confirmed the association between the CA repeat polymorphism and BMD but with slightly different results (123, 124). In a cohort of Chinese women from Hong Kong, the 20 CA repeat allele was associated with high BMD in premenopausal women but not in postmenopausal women (124). Scariano et al. (124), in a study of postmenopausal US women from New Mexico, observed a higher BMD in subjects with a low number of CA repeats (CA <25) as compared with those having longer alleles. In contrast, no significant associations between the CA repeat length and longitudinal change in BMD or markers of bone metabolism were detected in this population, indicating that this polymorphism is associated with the attainment of peak bone mass rather than bone loss (124). Importantly, a recent large-scale analysis of the CA repeat polymorphism within the Framingham Study offspring cohort confirmed the significant association between the number of CA repeats and femoral BMD but not spinal BMD (125). The lowest BMD values were observed in subjects who were homozygous for a high number of repeats (CA > 23). In the same cohort, two intronic single nucleotide polymorphisms (rs1256031 and rs1256059) exhibited an association with femoral BMD in men but not in women. When the three significant polymorphisms were analyzed together, haplotype C-23CA-T, with a frequency of 0.09, was significantly associated with lower femoral BMD in both sexes. Two other single nucleotide polymorphisms (rs1256034 and rs944460) with minor allele frequencies (<4 percent) gave no evidence of association (125). Arko et al. (126) examined the RsaI polymorphism in exon 5 of the ERß gene in a small sample of postmenopausal Slovenian women, reporting no association. In a preliminary study in 300 postmenopausal Italian women, Becherini et al. (127) reported no significant effect of the G1082C single nucleotide polymorphism on BMD or vertebral fractures.

Since bone mass and osteoporotic fracture risk are multifactorial traits, there are several possible interactions between ER polymorphisms and effect modifiers such as age, gender, diet, habits, and other environmental factors. Moreover, as with other complex traits, BMD values are likely to be determined by several genes that act collectively, and allelic variants at different genes may have either additive or contrasting effects on bone.

Gene-gene interactions
Investigators from several studies have proposed the existence of a significant gene-gene interaction effect between intron I PvuII-XbaI RFLPs in ER and vitamin D receptor (VDR) gene polymorphisms in the determination of BMD or calcaneal ultrasonic parameters in pre- and postmenopausal women (82, 91, 96, 99, 128-131), as well as in the regulation of skeletal growth (98, 132). The mechanism(s) underlying the described ER-VDR interaction effect on bone mass is not known, and other studies have failed to confirm it (92-94, 97, 104, 108, 133). Other investigators have reported interactions with a polyglutamine tract polymorphism at an ER cotranscriptional activator gene (134), with a VNTR polymorphism at the aromatase gene (135), with the Sp1 binding site polymorphism in the COL1A1 gene (102, 136), and with a Gln223Arg polymorphism of the leptin receptor gene (110).

Gene-environment interactions
Few specific gene-environment interactions have been described for ER gene polymorphisms. An association between the ER exon 8 single nucleotide polymorphism and the Osteoporosis Self-Assessment Tool for Asians, or OSTA index (derived from age and weight), in postmenopausal Asian women was recently proposed (119). Persons with the high-risk genotype were at greater risk of developing osteoporosis, especially with advancing age or decreasing body weight. In a recent 4-year, controlled, randomized exercise intervention trial in middle-aged Finnish men, an interaction between the ERPP or PpPvuII genotype and increased BMD gain during intervention was observed, while there was no BMD change in subjects with the pp genotype (107). Note that there appears to be a gene-environment interaction effect between dietary calcium and VDR genotype, which potentially may prove relevant for the observed ER-VDR gene-gene interaction (115). Given the limited influence of each single polymorphism on the total variation of bone-related traits, all of these gene-environment interactions are extremely difficult to demonstrate, and very large samples are needed.

Associations with different disorders
Interestingly, ER polymorphisms are associated with traits and conditions that are directly or indirectly related to bone metabolism. In particular, the XbaI polymorphism has been found to be significantly associated with upper-body obesity in middle-aged persons (137) and with later age of menarche in girls (138). A similar association with ovulatory dysfunction has been proposed for the G1730A polymorphism in exon 8 of the ERß gene (139), while the ERPvuII RFLP correlated with the onset of natural or surgical menopause in women from the Rotterdam Study cohort (140). Other observations indicated an association between ER intron I RFLPs and height (141) or body mass index (140). In a study on postmenopausal women from Finland, risk of falls (an important factor associated with fracture risk independently of BMD) was higher in women with the PP genotype than in those with the Pp (relative risk = 5.26) or pp (relative risk = 3.84) genotype (143). Moreover, both the PvuII and XbaI RFLPs appeared to influence circulating levels of androstenedione (the highest levels of hormones being found in women with the pp or xx genotype) (144), while the ERß gene CA repeat polymorphism was associated with variations in levels of androgen and sex hormone-binding globulin in premenopausal women (122).

Screening for intron I single nucleotide polymorphisms in the ER gene is commonly carried out by means of polymerase chain reaction (PCR) amplification followed by RFLP analysis; PvuII is used for T397C and XbaI is used for C351G. Recently, van Meurs et al. (51) proposed a direct molecular haplotyping method that consists of simultaneous RFLP digestion with both restriction sites, leading to the identification of four common haplotypes (1 = px, 2 = PX, 3 = Px, and 4 = pX). Molecular methods for determining polymorphic TA repeats upstream from the human ER gene were first described by Sano et al. (75). Briefly, PCR is utilized to amplify the TA dinucleotide repeat using labeled [ 32P]-deoxycytidine triphosphate. Amplified products are then separated and analyzed on denaturing polyacrylamide sequencing gel. The number of TA repeats can be determined by comparing gel bands with a series of TA size standards. More recently, fluorescein-labeled primers have been used and the number of TA repeats from PCR products has been determined automatically (51, 118). Other single nucleotide polymorphisms in the ER gene have been analyzed by either direct sequencing or RFLP analysis (52-54, 119, 120).

For determination of ERß CA VNTR, amplified PCR fragments can be analyzed by denaturing polyacrylamide gel electrophoresis and subjected to autoradiography. The number of CA repeats can be obtained by comparison with previously determined CA size standards. Improvements of this technique have recently been developed utilizing fully automated systems for the electrophoretic separation of the PCR products (123) and fluorescent labeling of DNA fragments (122). Single nucleotide polymorphisms have been analyzed by either direct sequencing or RFLP analysis. In particular, A105G, G1082A, T1100G, and A1730G single nucleotide polymorphisms can be easily detected by RFLP analysis with the FokI, RsaI, Van91I, and AluI restriction endonucleases, respectively (57, 126, 127, 139).

On the basis of the evidence summarized here, testing for polymorphisms of the ER and ERß genes in the general population as part of a population screening program is not presently warranted. The association between these polymorphisms and osteoporotic risk is intriguing but limited to some studies. Additional large-scale longitudinal studies are needed to confirm the reported associations. Moreover, the molecular mechanism underlying the skeletal effect of these polymorphisms remains to be determined.

A promising and interesting field in studies of ER gene polymorphisms comes from their possible pharmacogenomic implications in determining the response to hormone replacement therapy. The positive influence of postmenopausal hormone replacement therapy on bone mass is well established, and its antifracture effect is widely accepted. However, it seems that there are some women who do not respond to hormone replacement therapy (145), and a possible explanation for their less favorable responsiveness to estrogen may be related to ER genotypes. This issue has been investigated in a few studies performed in different ethnic populations, with conflicting results (52, 76, 89, 91, 103, 104, 128, 129, 133, 146, 147). An updated summary of the findings of these pharmacogenomic hormone replacement therapy studies is given in Table 2. Moreover, there is now consistent evidence that the PvuII polymorphism in the ER gene influences individual response to hormone replacement therapy with regard to cholesterol levels and other cardiovascular markers (38, 148, 149).

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Table 2  Findings from pharmacogenomic studies of hormonal replacement therapy  and polymorphisms in the human estrogen receptor gene

A further application of genetic studies on ER gene polymorphisms relates to the possibility of a modulation of the activity of selective ER modulators in different target tissues. In particular, raloxifene represents a potent compound for the prevention and treatment of osteoporosis in postmenopausal women (150) and possibly in men (151). Raloxifene has been shown to bind both ER and ERß, and it exhibits targeted antiestrogenic activity in the breast and uterus while acting as an agonist in bone and liver (152). At present, the possibility of a modulation of raloxifene's effects on bone according to ER genotype has not yet been investigated. However, a recent study of Japanese women treated with tamoxifen (a selective ER modulator indicated for treatment of breast cancer with adjuvant effects on bone) showed an increased gain in BMD and increased suppression of bone turnover in women with the 21 CA repeat allele in intron 5 of the ERß gene with respect to noncarriers, suggesting that this polymorphism might be useful in the prediction of bone response to selective ER modulators (153).

Osteoporosis affects hundreds of millions of patients throughout the world and has a great impact on both individuals and society as a whole. Its pathophysiologic basis includes genetic predisposition and subtle alterations in systemic and local hormone levels, coupled with environmental influences. Identification of the genetic pathways involved will certainly be difficult and represents a great challenge. In the past few years, several loci and genes, including ER and ERß, have been found to be associated with BMD and osteoporotic fractures (115, 154-156). However, the majority of these findings remain inconclusive. Reflecting the complicated inheritance patterns of osteoporosis as a complex disease, these inconsistent findings call for new approaches and strategies that have both sensitivity and robustness to accommodate confounding effects from various sources, such as genetic heterogeneity, population admixture, and gene-environment and gene-gene interactions. Definitions of disease (i.e., analysis of a single trait such as BMD, bone size, or bone quality) and pharmacogenomic interactions in human and animal models will be additional critical targets for future research.

List of References

• Table 1 - Frequency of the combined PvuII-XbaI haplotypes of the human estrogen  receptor gene in different ethnic groups
• Table 2 - Frequency of the combined PvuII-XbaI haplotypes of the human estrogen  receptor gene in different ethnic groups
• Figure 1 - Structure of, functional domains of, and described polymorphisms in the  human estrogen receptor gene
• Figure 2 - Distribution of (TA) n repeat alleles in the human estrogen receptor gene  in different ethnic groups
• Figure 3 - Structure of, functional domains of, and described polymorphisms in the  human estrogen receptor ß gene

Information on osteoporosis

• American Association of Orthopedic Surgeons, Osteoporosis Section
• American Society for Bone and Mineral Research
• Asian Pacific Osteoporosis Foundation
• The Bone and Joint Decade Joint Motion 2000-2010
• Bone and Tooth Society (United Kingdom)
• BoneKey-International Bone and Mineral Society
• European Calcified Tissue Society
• Foundation for Osteoporosis Research and Education
• International Bone and Mineral Society
• International Osteoporosis Foundation
• International Society for Clinical Densitometry
• National Osteoporosis Foundation (United States)
• National Osteoporosis Society (United Kingdom)
• Osteoporosis and Related Bone Diseases National Resource Center, National Institutes of Health
• Società Italiana dell'Osteoporosi, del Metabolismo Minerale, e delle Malattie dello Scheletro (Italy) (last accessed 1/2008)
• Women's Health Matters-Osteoporosis Health Centre, Sunnybrook and Women's College Health Sciences Centre (Canada)

(last accessed all 4/2007)

Information on genetics

• Human Genome Epidemiology Network (HuGENet™)
• Public Health Genetics Unit, University of Cambridge (United Kingdom)
• Online Mendelian Inheritance in Man (OMIM)
• GenAtlas
• GeneCards
• National Center for Biotechnology Information
• Human Genome Mapping Project-Medical Research Council (United Kingdom)

(last accessed all 4/2007)