Genetics of fracture: challenges and opportunities

Volumetric bone mineral density (vBMD), bone structure, and structural geometry among rural South Indian, US Caucasian, and Afro-Caribbean older men

Peripheral quantitative computed tomography (pQCT) provides biomechanical estimates of bone strength. Rural South Indian men have reduced biomechanical indices of bone strength compared to US Caucasian and Afro-Caribbean men. This suggests an underlying higher risk of osteoporotic fractures and greater future fracture burden among the rural South Indian men.

INTRODUCTION

Geographical and racial comparisons of bone mineral density (BMD) have largely focused on DXA measures of areal BMD. In contrast, peripheral quantitative computed tomography (pQCT) measures volumetric BMD (vBMD), bone structural geometry and provides estimates of biomechanical strength. To further understand potential geographical and racial differences in skeletal health, we compared pQCT measures among US Caucasian, Afro-Caribbean, and rural South Indian men.

METHODS

We studied men aged ≥ 60 years enrolled in the Mobility and Independent Living among Elders Study (MILES) in rural south India (N = 245), Osteoporotic Fractures in Men Study (MrOS) in the US (N = 1148), and the Tobago Bone Health Study (N = 828).

RESULTS

The BMI (kg/m²) of rural South Indian men (21.6) was significantly lower compared to the US Caucasians (28) and Afro-Caribbean men (26.9). Adjusting for age, height, body weight, and grip strength; rural South Indian men compared to US Caucasians had significantly lower trabecular vBMD [- 1.3 to - 1.5 standard deviation (SD)], cortical thickness [- 0.8 to - 1.2 SD]; significantly higher endosteal circumference [0.5 to 0.8 SD]; but similar cortical vBMD. Afro-Caribbean men compared to US Caucasians had similar trabecular vBMD but significantly higher cortical vBMD [0.9 to 1.2 SD], SSIp [0.2 to 1.4 SD], and tibial endosteal circumference [1 SD], CONCLUSIONS: In comparison to US Caucasians, rural South Indian men have reduced bone strength (lower trabecular vBMD) and Afro-Caribbean men have greater bone strength (higher cortical vBMD). These results suggest an underlying higher risk of osteoporotic fractures and greater future fracture burden among rural South Indian men.

Prospective cohort study of the risk factors for stress fractures in Chinese male infantry recruits

Objective

To determine potential risk factors that could predict stress fractures over an 8-week basic military training in Chinese male infantry recruits.

Methods

Recruits from three infantry units enrolled in this prospective study. At baseline, demographic data, personal history of stress fractures, mean duration of weekly exercise and smoking history were recorded on questionnaires and blood samples taken for analysis of bone turnover biomarkers and genetic factors.

Results

Of the 1516 male recruits who volunteered to participate in the study, 1398 recruits provided data for analysis. In total, 189 stress fracture cases were observed (incidence rate: 13.5%) during the 8-week training period. Recruits with stress fractures had a significantly higher incidence of prior fracture history and lower exercise level prior to enrolment compared with those without stress fractures. A significant difference in both allelic frequency and genotypic distribution of the growth differentiation factor 5 (GDF5) gene rs143383 polymorphism was observed between recruits with and without stress fractures. However, no difference in serum bone turnover biomarkers was detected between groups.

Conclusion

This prospective, cohort study indicates that fracture history, lower exercise level and GDF5 rs143383 may be predictive risk factors for stress fractures in Chinese male infantry recruits.

Contribution of Quadriceps Weakness to Fragility Fracture: A Prospective Study

The association between muscle weakness and fracture is not well understood. This study sought to examine the contribution of muscle strength at baseline and change in muscle strength to the observed risk of fragility fracture in older people. The study involved 595 men and 1066 women aged 60+ years (median 69 years) who had been followed for a median of 11 years (range, 4 to 22 years). Quadriceps isometric muscle strength (MS) measured at baseline and biennially was adjusted for height. Femoral neck bone mineral density (FNBMD) was measured by DXA. Low-trauma fracture was ascertained from X-ray reports and interview. The relationship between baseline MS and serial MS and fracture assessed by time-invariant and time-variant Cox's regression models was expressed as hazard ratio (HR) and 95% confidence interval (CI). During the follow-up period, 282 (26%) women and 89 (15%) men sustained a fragility fracture. From age 60 years, women lost 0.28 kg/m (1.6%) of MS per year, whereas men lost 0.39 kg/m (1.5%) of MS per year. In the time-variant model, using serial MS, each 1 SD (4.7 kg/m) lower MS was associated with a 27% increase in the risk of fracture in women (HR 1.27; 95% CI, 1.11 to 1.43); and 46% increase in men (HR 1.46; 95% CI, 1.22 to 1.75). After adjusting for FNBMD, age and prior fracture, history of fall and smoking, HR per SD of lower MS was 1.13 (95% CI, 0.99 to 1.28) for women and 1.35 (95% CI, 1.18 to 1.64) for men. These data indicate that muscle weakness is an independent determinant of fracture risk in men, but not in women. This sex difference suggests that apart from mechanical load effect of muscle on bone, there are other muscle-bone interactions that need to be investigated in future studies. The accuracy of fracture risk prediction for men may be improved by incorporating muscle strength.

Tryptophan hydroxylase 1 (Tph-1)-targeted bone anabolic agents for osteoporosis

Tryptophan hydroxylase 1 (Tph-1), the principal enzyme for peripheral serotonin biosynthesis, provides a novel target to design anabolic agents for osteoporosis. Here, we present a design, synthesis of a novel series of ursolic acid derivatives under the guidance of docking technique, and bioevaluation of the derivatives using RBL2H3 cells and ovariectomized (OVX) rats. Of the compounds, 9a showed a potent inhibitory activity on serotonin biosynthesis. Further investigations revealed that 9a, as an efficient Tph-1 binder identified by SPR (estimated KD: 6.82 μM), suppressed the protein and mRNA expressions of Tph-1 and lowered serotonin contents in serum and gut without influence on brain serotonin. Moreover, oral administration of 9a elevated serum level of N-terminal propeptide of procollagen type 1 (P1NP), a bone formation marker, and improved bone microarchitecture without estrogenic side effects in ovariectomized rats. Collectively, 9a may serve as a new candidate for bone anabolic drug discovery.

Prevention of Osteoporosis and Bone Fragility

The importance of optimal bone growth in childhood and adolescence has been recognized as one of the key strategies in osteoporotic fracture prevention. Low birth size, poor childhood growth, and low peak bone mass at the cessation of growth have been linked to the later risk of osteoporosis and hip fracture. Formerly, the focus was merely on maximizing bone mineral accrual because a high peak bone mineral mass may prevent attainment of a critical “fracture threshold” associated with age-related bone loss and osteoporosis. More recently, the focus has shifted away from bone mineral accrual—as measured by dual-energy X-ray absorptiometry (DXA)—toward the optimization of bone strength. This is partly because of the advances in bone imaging that have enabled estimation of bone strength beyond bone mass. In this review, we briefly describe long-bone growth and structural development and our abilities to assess bone properties by medical imaging tools. In addition, we summarize the evidence of factors contributing to skeletal growth, bone fragility, and the development of strong, healthy bones.

The pathogenesis, treatment and prevention of osteoporosis in men

Testosterone stimulates longitudinal and appositional growth during childhood, whereas estrogen induces epiphysial closure. During adulthood, testosterone continues to stimulate periosteal growth, whereas estrogen is important for the maintenance of trabecular bone mass and structure. In males, testosterone is aromatized to estradiol. Both free and bioavailable plasma levels of testosterone and estradiol decrease with age in males, and fracture risk is associated with low estradiol levels. Testosterone may increase muscle mass and prevent fractures related to falls. Younger hypogonadal males should be treated with testosterone to attain peak bone mass and increase bone mineral density (BMD). Older hypogonadal males should be treated in cases of osteoporosis, reduced muscle strength and increased risk of falling. Secondary hyperparathyroidism caused by calcium and vitamin D insufficiency may reduce bone mass and strength and increase fracture risk and should be avoided. Since calcium supplementation has been associated with an increased risk of cardiovascular complications and renal stones, the dose should be tailored to the habitual daily calcium intake. Lifestyle-related risk factors (smoking, alcohol consumption, lack of physical activity and low body weight) should be addressed. The antifracture efficacy of antiresorptive and anabolic treatment for osteoporosis has not been documented in larger randomized controlled studies. However, changes in BMD and bone markers suggest similar effects in males and females of bisphosphonates (alendronate, risedronate, ibandronate, zoledronic acid), nasal calcitonin, denosumab and teriparatide (parathyroid hormone [1-34]). The antiresorptive drugs should be used in males with BMD T-score less than -2.5 and one or more risk factors, or with hip and vertebral fractures. It seems appropriate to recommend a higher cut-off T-score (e.g. less than -1.0 standard deviation [SD]) in glucocorticoid-induced osteoporosis and in patients receiving androgen deprivation therapy because of the fast initial bone loss. Anabolic treatment should be used in more severe spinal fracture cases, including glucocorticoid-induced osteoporosis.

Genetic predisposition for femoral neck stress fractures in military conscripts

BACKGROUND

Stress fractures are a significant problem among athletes and soldiers and may result in devastating complications or even permanent handicap. Genetic factors may increase the risk, but no major susceptibility genes have been identified. The purpose of this study was to search for possible genetic factors predisposing military conscripts to femoral neck stress fractures.

RESULTS

Eight genes involved in bone metabolism or pathology (COL1A1, COL1A2, OPG, ESR1, VDR, CTR, LRP5, IL-6) were examined in 72 military conscripts with a femoral neck stress fracture and 120 controls. The risk of femoral neck stress fracture was significantly higher in subjects with low weight and body mass index (BMI). An interaction between the CTR (rs1801197) minor allele C and the VDR C-A haplotype was observed, and subjects lacking the C allele in CTR and/or the C-A haplotype in VDR had a 3-fold higher risk of stress fracture than subjects carrying both (OR = 3.22, 95% CI 1.38-7.49, p = 0.007). In addition, the LRP5 haplotype A-G-G-C alone and in combination with the VDR haplotype C-A was associated with stress fractures through reduced body weight and BMI.

CONCLUSIONS

Our findings suggest that genetic factors play a role in the development of stress fractures in individuals subjected to heavy exercise and mechanical loading. The present results can be applied to the design of future studies that will further elucidate the genetics of stress fractures.

Physical activity is the strongest predictor of calcaneal peak bone mass in young Swedish men

SUMMARY

In a highly representative sample of young adult Swedish men (n = 2,384), we demonstrate that physical activity during childhood and adolescence was the strongest predictor of calcaneal bone mineral density (BMD), and that peak bone mass was reached at this site at the age of 18 years.

INTRODUCTION

The purpose of the present study was to determine if physical activity during growth is associated with peak calcaneal BMD in a large, highly representative cohort of young Swedish men.

METHODS

In this study, 2,384 men, 18.3 +/- 0.3 (mean +/- SD) years old, were included from a population attending the mandatory tests for selection to compulsory military service in Sweden. BMD (g/cm(2)) of the calcaneus was measured using dual-energy X-ray absorptiometry. Training habits were investigated using a standardized questionnaire.

RESULTS

Regression analysis (with age, height, weight, smoking, and calcium intake as covariates) demonstrated that history of regular physical activity was the strongest predictor and could explain 10.1% of the variation in BMD (standardized beta = 0.31, p < 0.001). A regression model with quadratic age effect revealed maximum BMD at 18.4 years.

CONCLUSIONS

We found that history of physical activity during growth was the strongest predictor of peak calcaneal BMD in young men.

Possible association of combined vitamin D receptor and estrogen receptor genotypes and low bone mineral density in Jordanian postmenopausal women

The genetics of osteoporosis has been extensively studied over the last 20 years. Many of the studies have been aimed at identifying possible risk factors and possible association with low bone mineral density (BMD). Vitamin D receptor (VDR) and estrogen receptor (ER) gene polymorphisms were the first to be studied. Some studies have shown a possible association for individual VDR and ER or combined VDR and ER genotypes in some populations, and others showed lack of such an association. This study is aiming at identification of a possible association with low BMD in Jordanian postmenopausal women. We used restriction fragment length polymorphisms (RFLPs) to study four polymorphisms in the VDR gene and two polymorphisms in the ER gene. Our sample was composed of 100 normal controls and 120 samples from patients with symptomatic vertebral fractures. The results showed a possible association of the ppxx genotype with low BMD in controls and patients and an association of the AaBbTT genotype with high BMD in control subjects. The AABBTT, AABBTTFF, and AABBTTFFPPXX genotypes showed a possible association with low BMD in patients. Further studies are needed to confirm the last finding since it could be an important predictor of low BMD in the Jordanian population.

Trabecular bone mechanical properties in patients with fragility fractures

Fragility fractures are generally associated with substantial loss in trabecular bone mass and alterations in structural anisotropy. Despite the high correlations between measures of trabecular mass and mechanical properties, significant overlap in density measures exists between individuals with osteoporosis and those who do not fracture. The purpose of this paper is to provide an analysis of trabecular properties associated with fragility fractures. While accurate measures of bone mass and 3-D orientation have been demonstrated to explain 80% to 90% of the variance in mechanical behavior, clinical and experimental experience suggests the unexplained proportion of variance may be a key determinant in separating high- and low-risk patients. Using a hierarchical perspective, we demonstrate the potential contributions of structural and tissue morphology, material properties, and chemical composition to the apparent mechanical properties of trabecular bone. The results suggest that the propensity for an individual to remodel or adapt to habitual damaging or nondamaging loads may distinguish them in terms of risk for failure.

Pharmacogenetics of osteoporosis and the prospect of individualized prognosis and individualized therapy

PURPOSE OF REVIEW

Description of recent progress in genetics and pharmacogenetics of osteoporosis.

RECENT FINDINGS

Osteoporosis and its consequence of fragility fracture are characterized by highly complex phenotypes, which include bone mineral density, bone strength, bone turnover markers, and nonskeletal traits. Recent developments in the genome-wide studies using high-throughput single-nucleotide polymorphisms have yielded reliable findings. Four genome-wide studies have identified 40 single-nucleotide polymorphisms in various chromosomes that were modestly associated with either bone mineral density or fracture risk. Clinical response, including adverse reactions, to antiosteoporosis therapy (such as bisphosphonates and selective estrogen receptor modulators) is highly variable among treated individuals. Candidate gene studies have found that common polymorphic variations within the collagen I alpha 1 and vitamin D receptor genes were associated with variability in response to antiosteoporosis treatment. Moreover, a recent genome-wide study identified four single-nucleotide polymorphisms that were associated with bisphosphonate-related osteonecrosis of the jaw with relative risk being between 10 and 13.

SUMMARY

The evaluation of osteoporosis and fracture risk is moving from a risk stratification approach to a more individualized approach, in which an individual's absolute risk of fracture is evaluable as a constellation of the individual's environmental exposure and genetic makeup. Therefore, the identification of gene variants that are associated with osteoporosis phenotypes or response to therapy can eventually help individualize the prognosis, treatment and prevention of fracture and its adverse outcomes.

Long-term effects of progestins on bone quality and fractures

The effects of progestins on the quality of bone and their influence on the risk of fractures are reviewed. Data discussed are based on experimental studies in vivo that generally lasted for longer than one year. Information is given on the background of osteoporosis and on several means of inducing changes in bone quality. In young women who start using oral contraceptives based on progestins alone shortly after pubertal development, a significant decrease in bone quality has been documented. World Health Organization experts have concluded that this is not a real argument for restrictions on the use of these contraceptives. In postmenopausal women, no evidence has been found for a bone-protective or an estrogen-antagonistic effect of progestins. A wide range of estrogens have been used that have shown positive effects on bone, which are not antagonized by progestins. The therapeutic use of high-dose megestrol acetate may result in marked negative effects on bone, leading to severe osteoporosis, possibly due to the inherent glucocorticoid activity of this progestin. Other pharmacotherapeutic agents that can be used in postmenopausal therapy, and that clearly have beneficial effects on bone, are discussed.

Molecular genetic studies of gene identification for osteoporosis: a 2004 update

This review summarizes comprehensively the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of December 2004. It is intended to constitute a sequential update of our previously published review covering the available data up to the end of 2002. Evidence from candidate gene association studies and genome-wide linkage studies in humans, as well as quantitative trait locus mapping animal models are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. An important extension of this update is incorporation of functional genomic studies (including DNA microarrays and proteomics) on osteogenesis and osteoporosis, in light of the rapid advances and the promising prospects of the field. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.

A genetic approach to fracture epidemiology in childhood

The purpose of this report is to provide a review of both childhood fracture epidemiology and known heritable causes for fracture predisposition to the Medical Geneticist, who is frequently consulted to assess children with multiple or unexplained fractures for a physiologic etiology. A detailed knowledge of the clinical and laboratory evaluation for osteogenesis imperfecta (OI) and other single-gene disorders is obviously essential to complete a useful evaluation of such children. The experienced clinician will immediately recognize that single gene disorders represent only a small fraction of these patients. In infants, non-accidental trauma (NAT) unfortunately is the likely explanation for the fracture pattern, but in some infants, and certainly in older children with recurrent fractures, no medical explanations can be found. Recent studies in which bone mineral density (BMD) has been associated with genetic variation at a number of candidate genes are promising but these studies are too premature yet to be used clinically. Nonetheless, we do expect that in the future whole-genome approaches in conjunction with key clinical and epidemiological variables may be combined through an informatics approach to create better predictors of fracture susceptibility for these populations of patients.

Gene polymorphisms involved in the regulation of bone quality

Osteoporotic fractures in subjects at advanced age constitute a tremendous and growing problem. Established lifestyle risk factors can explain only a modest proportion of the liability to osteoporotic fractures. Bone mineral density (BMD) is considered the best established risk factor for osteoporotic fractures. The importance of genetic factors in the quality of bone is substantial, but no consensus exists yet on the genes that are involved. However, concomitant diseases, balance disorders and lifestyle habits are more important for fractures in elderly subjects. The abundance of common sequence variations, so-called polymorphisms, in the human genome and their high frequency in the population have made them targets to explain variation in the risk. Some genes have been identified that appear to be involved in the regulation of bone mass and in the pathogenesis of osteoporosis. Among these are those coding for the two estrogen receptors (ERalpha and ERbeta), the androgen receptor (AR) and the vitamin D receptor (VDR). In addition, enzymes involved in the biogenesis of estrone and estradiol have attracted attention as well as polymorphisms in the regulatory region of the type I collagen gene, COLIA1, affecting the binding site for the transcription factor Specificity protein 1 (Sp1). Although evidence suggests that the quality of bone is determined to a large extent by genetic factors, research so far has not been able to unequivocally identify genes involved in this matter. Over the last years a large number of studies have pointed to the variability in many genes and their relation with BMD, bone-related symptoms or specific therapies. The findings emphasize the complexity of the genetics of bone mass and bone loss.

LRP5 gene polymorphisms and idiopathic osteoporosis in men

Mutations in the low-density lipoprotein receptor-related protein 5 gene (LRP5) have demonstrated the role of LRP5 in bone mass acquisition. LRP5 variants were recently reported to contribute to the population-based variance in vertebral bone mass and size in males. To investigate whether LRP5 variants are implicated in idiopathic male osteoporosis, we studied 78 men with low BMD (<2.5 T score or < -2 Z score) aged less than 70 years (mean +/- SD: 50 +/- 16 years) in whom secondary causes of osteoporosis had been excluded and 86 controls (51 +/- 10 years). Genotypes and haplotypes were based on LRP5 missense substitutions in exons 9 (c.2047G > A, p.V667M) and 18 (c.4037C > T, p.A1330V), and their association with osteoporosis evaluated after adjustment for multiple clinical and environmental variables using logistic regression. The presence of osteoporosis was significantly associated with LRP5 haplotypes (P = 0.0036) independent of age (P = 0.006), weight (P = 0.004), calcium intake (P = 0.002), alcohol (P = 0.005) and tobacco (P = 0.004) consumption. Accordingly, the odds ratio for osteoporosis was 3.78 (95% CI 1.27-11.26, P < 0.001) in male carriers of haplotype 3 (c.2047A-4037T, n = 20 cases and 12 controls) versus homozygous carriers of haplotype 1 (c.2047G-4037C, n = 42 cases and 61 controls). In conclusion, these data indicate beyond a significant role for environmental factors, an association between LRP5 variants and idiopathic osteoporosis in males, pointing to a role of LRP5 in this disease.

[Partnership between academic research and industry to study a new anti-osteoporotic drug]

The activity of the osteoclast, the cell responsible for bone resorption, is subjected to different regulation factors. Amongst these, those issued from the matrix, particularly released minerals such as calcium, are determinants. We have shown that variations in calcium concentration in the medium regulates resorption activity and duration of the osteoclast lifespan. The development of a new therapeutic agent, strontium ranelate, has shown very interesting clinical effects reliant on the stimulation of bone formation activity by osteoblasts and modulation of bone resorption activity. From our knowledge regarding osteoclast physiology, in particular calcium signaling pathways, and the control of different osteoclast cellular models, a consequent collaboration was formed between our laboratory and Servier in order to elaborate on the effects of strontium ranelate on the osteoclast. In several years, this collaboration has been further enriched by other collaborators in order to better understand this mechanism. It has also been shown that strontium likely interacts with the calcium-sensing receptor and that the pathways of intracellular signaling pathways activated by calcium and strontium ranelate via this receptor are different. In fact, within the scope of this collaboration with Servier, exchanges with other academic laboratories were initiated and collaboration on numerous techniques became possible. Then, it has been possible to confirm the presence of the calcium-sensing receptor on the osteoclasts and to demonstrate its role in the molecular events associated with strontium ranelate's effects on the osteoclast.

Association of ANKH gene polymorphisms with radiographic hand bone size and geometry in a Chuvasha population

We performed a family-based association study to test the hypothesis that genetic variation at the human orthologue of the mouse progressive ankylosis gene (ANKH) is involved in determining bone size (BS) and bone geometry (BG). The study population comprised 126 nuclear families with 574 adult Chuvashian individuals living in small villages in the Russian Federation. Quantitative bone traits were determined by analyzing plain hand radiographs. Familial correlations for all studied traits revealed a high degree of heritability in this ethnically homogeneous population. Three simple tandem repeat (STR) polymorphisms, one intragenic and two flanking markers, as well as six single nucleotide polymorphisms (SNPs) were tested. The SNPs were detected by re-sequencing experiments and covered ANKH exons with their flanking splice sites and the promoter region. We used three different transmission disequilibrium tests (TDTs) and obtained multiple significant association signals for all investigated bone traits. Alleles of several markers located at different positions of the ANKH locus, including the promoter, consistently revealed the association. The bone traits tested are closely related to bone fragility suggesting a role for ANKH in osteoporosis.

Maternal history of osteoporosis and femur geometry

Most studies that have examined the role of skeletal factors in the relationship between an individual's family history of fracture or osteoporosis and their fracture risk have focused on bone density. In this study, we expanded the scope of skeletal factors to include geometric properties (subperiosteal width, section modulus, cortical thickness, and buckling ratio) in addition to areal bone mineral density (BMD). We compared these skeletal factors at the femur neck and shaft by self-reported maternal history of osteoporosis (OP HX) from 5334 non-Hispanic whites, ages > or =20 years in the Third National Health and Nutrition Examination Survey (NHANES III, 1988-94). A total of 213 men and 315 women reported a positive OP HX (e.g., their biological mother had sustained a hip fracture after age 50 years or had a physician's diagnosis of osteoporosis). Differences in bone density and geometry by OP HX were examined after adjusting for potential confounding variables. Several bone parameters differed significantly by OP HX in both sexes at the femur neck, but none differed at the femur shaft. At the neck, those with a positive OP HX had values that differed by approximately 3% to 4% (lower for BMD, bone mineral content (BMC), cross-sectional area, and cortical thickness; higher for buckling ratios) from those with a negative OP HX (P < 0.05). The magnitude of these relationships was similar in both sexes, but differences were greater in younger versus older adults. In conclusion, both men and women with a positive maternal history of osteoporosis may be at greater risk of femur neck fracture owing to thinner cortices and lower BMC, which in turn results in potentially greater cortical instability (buckling ratio) at this skeletal site.

Invited Review: Pathogenesis of osteoporosis

Patients with fragility fractures may have abnormalities in bone structural and material properties such as larger or smaller bone size, fewer and thinner trabeculae, thinned and porous cortices, and tissue mineral content that is either too high or too low. Bone models and remodels throughout life; however, with advancing age, less bone is replaced than was resorbed within each remodeling site. Estrogen deficiency at menopause increases remodeling intensity: a greater proportion of bone is remodeled on its endosteal (inner) surface, and within each of the many sites even more bone is lost as more bone is resorbed while less is replaced, accelerating architectural decay. In men, there is no midlife increase in remodeling. Bone loss within each remodeling site proceeds by reduced bone formation, producing trabecular and cortical thinning. Hypogonadism in 20-30% of elderly men contributes to bone loss. In both sexes, calcium malabsorption and secondary hyperparathyroidism increase remodeling: more bone is removed from an ever-diminishing bone mass. As bone is removed from the endosteal envelope, concurrent bone formation on the periosteal (outer) bone surface during aging partly offsets bone loss and increases bone's cross-sectional area. Periosteal apposition is less in women than in men; therefore, women have more net bone loss because they gain less on the periosteal surface, not because they resorb more on the endosteal surface. More women than men experience fractures because their smaller skeleton incurs greater architectural damage and adapts less by periosteal apposition.

Overview on the effects of progestins on bone

In view of the fact that fractures are the clinically relevant events, risk factors for fractures are discussed first. Bone mineral density (BMD) appears to be a much less important risk factor for the most severe hip fractures than the risk of falling. No results of experimental studies on hormones and fractures at advanced age are available. An overview of the effects of progestins on bone is given. Effects of progestins on bone have been studied by in vitro experiments using cell lines and by more relevant clinical observations. Prospective studies have been conducted following the use of progestins contained in oral contraceptives, alone or in combination with oestrogens; long-term contraception by injection of depot preparations; so-called "add-back" hormonal therapy attempting to reverse the adverse effects of gonadotropin releasing hormone agonists on bone and after different regimens of hormone replacement therapy (HRT) in postmenopausal women. From the data there are no indications that the various progestins, used in clinical practice, have either a bone-protective or an oestrogen antagonistic activity. Progestins do not add or subtract much of the protective action of oestrogens on the bones.

Association of polymorphisms of the estrogen receptor alpha gene with bone mineral density and fracture risk in women: a meta-analysis

The contribution of genetic polymorphisms to bone mineral density (BMD) and fracture risk in women is a controversial topic. We evaluated the effect of the XbaI and PvuII polymorphisms of the estrogen receptor a to BMD and fracture risk in a meta-analysis, including published data and additional information from investigators. Five thousand eight hundred thirty-four women from 30 study groups were analyzed with fixed and random effects models. The PvuII polymorphism was not associated with BMD at any skeletal site examined and 95% CIs exclude effects over 0.015 g/cm2 for both the femoral neck and the lumbar spine. Conversely, XX homozygotes (women carrying two copies of the gene variant without an XbaI restriction site) consistently had higher BMD than other subjects. The magnitude of the effect was similar in the femoral neck and lumbar spine (0.014 g/cm2 [95% CI, 0.003-0.025] and 0.015 g/cm2 [95% CI, 0.000-0.030], respectively; no between-study heterogeneity for either). Total body BMD was also significantly higher in XX homozygotes (by 0.039 g/cm2 and 0.029 g/cm2 compared with Xx and xx, respectively). Available data on fractures suggested a protective effect for XX (odds ratio [OR], 0.66 [95% CI, 0.47-0.93] among 1591 women), but not PP (OR, 0.93 [95% CI, 0.72-1.18] among 2,229 women). In summary, we have found that XX homozygotes may have higher BMD and also a decreased risk of fractures when compared with carriers of the x allele, whereas the PvuII polymorphism is not associated with either BMD or fracture risk.

Discoveries, drugs and skeletal disorders

Bone turnover, in which cells of the osteoclast lineage resorb bone and cells of the osteoblast lineage deposit bone, normally occurs in a highly regulated manner throughout life. Perturbations to these processes underlie skeletal disorders, such as osteoporosis, which are common, chronic and disabling, and increase with age. On the basis of empirical observations or on understanding of the endocrinology of the skeleton, excellent bone-resorption inhibitors, but few anabolic agents, have been developed as therapeutics for skeletal disorders. However, powerful new genomic and genetic tools are uncovering new loci that regulate the activity of both osteoclasts and osteoblasts, and these hold great promise for future drug development.

Pathogenesis of bone fragility in women and men

There is no one cause of bone fragility; genetic and environmental factors play a part in development of smaller bones, fewer or thinner trabeculae, and thin cortices, all of which result in low peak bone density. Material and structural strength is maintained in early adulthood by remodelling; the focal replacement of old with new bone. However, as age advances less new bone is formed than resorbed in each site remodelled, producing bone loss and structural damage. In women, menopause-related oestrogen deficiency increases remodelling, and at each remodelled site more bone is resorbed and less is formed, accelerating bone loss and causing trabecular thinning and disconnection, cortical thinning and porosity. There is no equivalent midlife event in men, though reduced bone formation and subsequent trabecular and cortical thinning do result in bone loss. Hypogonadism contributes to bone loss in 20-30% of elderly men, and in both sexes hyperparathyroidism secondary to calcium malabsorption increases remodelling, worsening the cortical thinning and porosity and predisposing to hip fractures. Concurrent bone formation on the outer (periosteal) cortical bone surface during ageing partly compensates for bone loss and is greater in men than in women, so internal bone loss is better offset in men. More women than men sustain fractures because their smaller skeleton incurs greater architectural damage and adapts less effectively by periosteal bone formation. The structural basis of bone fragility is determined before birth, takes root during growth, and gains full expression during ageing in both sexes.

Role of genetics in osteoporosis

Osteoporosis is a disease characterized by fragile bones and high susceptibility to low-trauma fractures. It is a serious health problem, especially in elderly women. Bone mineral density (BMD) has been employed most commonly as the index for defining and studying osteoporosis. BMD has high genetic determination, with heritability ranging from 50 to 90%. Various gene-mapping approaches have been applied to identify specific genes underlying osteoporosis, largely using BMD as the study phenotype. We review here the genetic determination of osteoporosis as defined by BMD and discuss a fundamental issue we encounter in genetic research in osteoporosis: the choice of phenotype(s) to study. We briefly summarize and discuss advantages and disadvantages of various approaches used in genetic studies of osteoporosis. Finally, we review and discuss the current status for mapping and identification of genes for osteoporosis. We focus on linkage studies in humans and quantitative trait loci mapping in mice to supplement the already extensive reviews of association studies made by many investigators for candidate genes.

From Wolff's law to the Utah paradigm: insights about bone physiology and its clinical applications

Efforts to understand our anatomy and physiology can involve four often overlapping phases. We study what occurs, then how, then ask why, and then seek clinical applications. In that regard, in 1960 views, bone's effector cells (osteoblasts and osteoclasts) worked chiefly to maintain homeostasis under the control of nonmechanical agents, and that physiology had little to do with anatomy, biomechanics, tissue-level things, muscle, and other clinical applications. But it seems later-discovered tissue-level mechanisms and functions (including biomechanical ones, plus muscle) are the true key players in bone physiology, and homeostasis ranks below the mechanical functions. Adding that information to earlier views led to the Utah paradigm of skeletal physiology that combines varied anatomical, clinical, pathological, and basic science evidence and ideas. While it explains in a general way how strong muscles make strong bones and chronically weak muscles make weak ones, and while many anatomists know about the physiology that fact depends on, poor interdisciplinary communication left people in many other specialties unaware of it and its applications. Those applications concern 1.) healing of fractures, osteotomies, and arthrodeses; 2.) criteria that distinguish mechanically competent from incompetent bones; 3.) design criteria that should let load-bearing implants endure; 4.) how to increase bone strength during growth, and how to maintain it afterwards on earth and in microgravity situations in space; 5.) how and why healthy women only lose bone next to marrow during menopause; 6.) why normal bone functions can cause osteopenias; 7.) why whole-bone strength and bone health are different matters; 8.) why falls can cause metaphyseal and diaphyseal fractures of the radius in children, but mainly metaphyseal fractures of that bone in aged adults; 9.) which methods could best evaluate whole-bone strength, "osteopenias" and "osteoporoses"; 10.) and why most "osteoporoses" should not have bone-genetic causes and some could have extraosseous genetic causes. Clinical specialties that currently require this information include orthopaedics, endocrinology, radiology, rheumatology, pediatrics, neurology, nutrition, dentistry, and physical, space and sports medicine. Basic science specialties include absorptiometry, anatomy, anthropology, biochemistry, biomechanics, biophysics, genetics, histology, pathology, pharmacology, and cell and molecular biology. This article reviews our present general understanding of this new bone physiology and some of its clinical applications and implications. It must leave to other times, places, and people the resolution of questions about that new physiology, and to understand the many devils that should lie in its details. (Thompson D'Arcy, 1917).