Genetic determinants of bone mineral content at the spine and radius: a twin study

The heritability of bone mineral density, ultrasound of the calcaneus and hip axis length: a study of postmenopausal twins

Population based studies have demonstrated that having a first degree relative with a hip fracture is predictive of future hip fractures. Postmenopausal bone mineral density (BMD), ultrasound of calcaneus and hip axis length are associated with hip fracture, with the association for ultrasound and hip axis length being independent of BMD. The aim of this study was to determine the genetic component of these three important risk factors. We performed a classical twin study using 500 normal female twins, 128 identical and 122 non-identical pairs, aged 50 to 70 years. We measured bone mineral density at multiple sites, hip axis length (distance from the inner rim of the acetabulum to the greater trochanter), broadband ultrasound attenuation and velocity of sound of the calcaneus. Bone density had a strong genetic component at all sites with estimates of heritability ranging from 0.46 to 0.84. Hip axis length and velocity of sound had major genetic components with estimates of 0.62 and 0.61 respectively, which remained virtually unchanged after adjustment for bone mineral density. Broadband ultrasound attenuation had a moderate genetic component with an estimate of 0.53, which was reduced further to 0.45 after adjustment for BMD. In summary, all three bone measurements, which are independently associated with hip fracture, are independently heritable. This study suggests that a combination of different genetic factors acting on the structure, dimensions and density of bone may explain the importance of family history as a risk factor for hip fracture.

Determinants of hip axis length in women aged 10-89 years: a twin study

Hip axis length (HAL), a measure of femoral geometry, has been shown to predict hip fracture in white women over the age of 67 years, independently of bone mineral density at the femoral neck. A cross-sectional study of 304 pairs of female twins [176 monozygous (MZ) and 128 dizygous (DZ)], aged between 10 and 89 years, was performed to examine the influence of age, constitutional, lifestyle, and genetic factors on HAL. HAL was calculated from dual energy X-ray absorptiometry scans of the proximal femur using an automated technique with an Hologic QDR-1000W. Lean mass, fat mass, height, and weight were also measured. Maximum mean HAL was achieved by the age of 15 years. After this age there was no discernible dependency of mean HAL on age. Using within-pair differences, after adjusting for height there were no other independent constitutional or lifestyle predictors. Cross-sectionally, after adjustment for height, MZ and DZ correlations were 0.79 (95% CI: 0.73-0.84) and 0.54 (95% CI: 0.39-0.68), respectively, and independent of age. The MZ correlation exceeded the DZ correlation (p < 0.001). The best-fitting model apportioned 79% (SE 7%) of variation in height-adjusted HAL to additive genetic factors. There was marginal evidence that an environmental influence shared by twins explained 31% (SE 16%) of height-adjusted variance (p = 0.07), in which case the genetic variance was reduced to 51% (SE 15%). Adjustment for height had reduced the magnitude of total variance by 26%, and 95% of this reduction was in the additive genetic component. Applying a previously described theoretical model, approximately 10% of the increased risk of hip fracture associated with a maternal history of hip fracture could be attributed to the genetic factors determining HAL. We conclude that, in women, adult HAL is achieved by midadolescence. After adjustment for height, which is itself largely under genetic influence, other genetic factors appear to play the predominant role in explaining variation in HAL.

Polymorphisms of the vitamin D receptor gene do not predict quantitative ultrasound of the calcaneus or hip axis length

Quantitative ultrasound of the calcaneus and hip axis length are independent predictors of hip fracture and have a major genetic component. Polymorphisms of the vitamin D receptor gene (VDR) have been associated with variations in bone density in a number of studies. The aim of this study was to examine the role of VDR on other parameters associated with the risk of fracture. One hundred and eighty-nine pairs of healthy female dizygous twins were genotyped and had calcaneal ultrasound (broadband ultrasound attenuation and velocity of sound) and hip axis length measurements performed. Twin analysis using intraclass correlation coefficients and intrapair differences failed to find an association between the VDR polymorphisms and hip axis length or calcaneal ultrasound. Analysing the twins as a population, irrespective of twinning, also failed to find any association. The search for alternative genes influencing bone fragility should continue as a research priority.

Segregation analysis and variance components analysis of bone mineral density in healthy families

Bone mineral density (BMD) was measured in 1992-93 in 129 nuclear families, including 258 parents and 183 children, and was analyzed for familial resemblance factors. BMD measurements were adjusted on weight and age. Segregation analysis rejected the monogenic hypothesis and exhibited a strong polygenic component. Variance components analysis was then used to estimate the parameters of a multivariate normal model including an additive polygenic component, a common environment factor, and a residual specific to each individual. The genetic component was independent of sex and age. The common environmental factor was not significant. The variance of the residual specific factor appeared to be a quadratic function of age, reaching its minimum value at 26.4 years. Consequently, the maximum value for heritability (ratio of genetic variance to total variance) is observed at this age (h2 = 0.84). According to this model, the correlation between two relatives is a function of the ages of each individual in the pair.

Bone density determinants in elderly women: a twin study

This cross-sectional twin study examined the influence of constitutional, lifestyle, and genetic factors on bone mineral density (BMD) in elderly women. BMD, at the lumbar spine, femoral neck, Ward's triangle, total hip, and total forearm, total body bone mineral content (BMC), and lean mass and fat mass were measured using dual energy X-ray absorptiometry in 69 volunteer female twin pairs (37 monozygotic [MZ], 32 dizygotic [DZ]) aged 60-89 years. Height and weight were measured. Medical history and lifetime tobacco and alcohol use were determined by questionnaire. In terms of within-pair differences, lean mass was independently associated with BMD at all sites. In contrast, fat mass was not associated with BMD at any site once allowance had been made for lean mass. Lifetime tobacco use was independently associated with BMD at the lumbar spine, total hip, and forearm. Total body BMC was independently predicted by lean mass, fat mass, tobacco use, and alcohol consumption. Age and the above independently predictive body composition and lifestyle factors accounted for 20-33% of variation in BMD. After allowing for these covariates, MZ and DZ correlations were consistent with about 75% of residual variation in BMD at the nonforearm sites being determined by genetic factors. For total body BMC, the covariates explained 75% of total variation, and genetic factors 76% of the residual variation. Therefore, at the proximal femur and lumbar spine, after taking into account the relation of BMD with lean mass and smoking, genetic factors appear to play a substantial role in explaining variation in BMD in elderly women.

Vitamin D receptor gene polymorphisms do not predict bone turnover and bone mass in healthy premenopausal women

Bone mineral density (BMD) is under strong genetic control. Polymorphisms at the vitamin D receptor (VDR) gene have been recently suggested to account for up to 75% of this genetic effect. We analyzed these polymorphisms, i.e., that of BsmI, TaqI, and ApaI restriction enzymes by PCR of the DNA in 189 healthy premenopausal women aged 31 to 57 years. For the BsmI polymorphism they were 17% BB homozygotes, 51% Bb heterozygotes, and 32% bb homozygotes, genotype frequencies that are very similar to those previously reported in other Caucasian populations of north European ancestry. Women in the three genotypes for any of the three polymorphisms were matched for age and did not differ in body weight, height, physical activity, nor smoking habits. We found no relationship between the genotype for any of the three polymorphisms nor bone formation and resorption rate assessed by five specific biochemical markers of bone turnover nor with BMD measured at the spine, proximal femur, forearm, and whole body by dual-energy X-ray absorptiometry (DXA). We concluded that these polymorphisms are not predictive of bone turnover nor BMD in a sample of healthy premenopausal women drawn from the French population.

Genetic lineage, bone mass, and physical activity in mice

Five strains of inbred mice were found to have variations in bone mass although they were similar in body weight. Two of these strains, C57BL/6J and A/J, were studied in greater detail and the former had more bone in both femur and tibia. The increased bone mass was associated with larger quadriceps muscles in the C57BL/6J animals when measured at 18 weeks of age. Activities of animals from these two strains were studied over 24 h periods using a cage with an ultrasonic movement detector and automatic counter. The C57BL/6J animals were more active than the A/J mice. The male C57BL/6J mice tended to have larger testicles and higher testosterone levels than the A/J animals, whereas the female A/J animals had larger ovaries and higher oestradiol levels. As both male and female C57BL/6J animals were more active, it was concluded that the sex hormone differences between the two strains was not responsible for either the changes in bone mass or physical activity. These findings indicate that in the mouse, activity is in part genetically determined. We have hypothesized that this, in turn, could affect muscle mass and secondarily, bone size and strength. If these results can be applied to humans, it would suggest that differences at birth between individuals are important for bone mass in later life and that muscle mass and activity are in part genetically influenced. If this was the case, then muscle mass and strength could be a factor in bone mass and one of the goals in prevention and treatment of osteoporosis should be directed toward preservation and/or augmentation of muscle strength.

Osteoporosis in rheumatoid arthritis. A monozygotic co-twin control study

OBJECTIVE

To quantify the magnitude and distribution of osteoporosis in rheumatoid arthritis (RA).

METHODS

Bone mineral density (BMD) was measured by dual x-ray absorptiometry, in a monozygotic co-twin control study.

RESULTS

BMD was reduced at most skeletal sites in the twin with RA compared with the co-twin (lumbar spine 4.6%, femoral neck 9.7%, total body 5.7%). Differences in lean soft tissue (5.6% for total body) correlated with differences in BMD between twins at multiple sites.

CONCLUSION

Osteoporosis in RA is generalized and may be related to loss of mobility or muscle mass associated with the disease.

The contribution of vitamin D receptor gene alleles to the determination of bone mineral density in normal and osteoporotic women

Bone mass and its mineral content are under genetic control. The vitamin D receptor (VDR) gene has been shown to be a major locus for genetic effects on bone mineral density (BMD), and polymorphisms in this gene accounted for a large proportion of genetic variance in BMD in an Australian population. In this study, we investigated whether similar associations are present in a North American population. We studied 139 normal healthy women (age 53.2 +/- 14.5, mean +/- SD) and 43 severely osteoporotic postmenopausal women (age 65.8 +/- 5.9). In the 127 of them with complete genetic studies, the distribution of genotypes, determined by polymerase chain reaction on leukocyte DNA samples, agreed closely with that in the Australian population. BMD was strongly related to age and weight, and, thus was adjusted for these parameters prior to genetic analysis. We found that age modulated the effect of VDR genotypes on femoral neck BMD (FN-BMD) (TaqI, p = 0.036; BsmI, p = 0.118; ApaI, p = 0.041) such that the effect of genotype was greatest among younger (premenopausal) women and declined with age so that there was no discernible difference by age 70. Among the younger women, a high FN-BMD was associated with the TT (or aa or bb) genotype while low FN-BMD was associated with the tt (or AA or BB) genotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Can we detect women with low bone mass using clinical risk factors?

The magnitude of osteoporosis, the established relationship between low bone mass and the risk of fracture, and the availability of preventive treatment suggest that the early detection of women with low bone mass is justified. The feasibility of population screening using bone mass measurements remains controversial. Another approach is the use of clinical risk factors to detect women at high risk. However, several studies have demonstrated that the assessment of risk factor status does not appear to be an efficient tool for the identification of perimenopausal women with low bone mass. The poor performance of the prediction models might be explained in part by unmeasured factors, especially genetic factors, which are an important determinant of bone mass. On the other hand, the clinical usefulness of clinical risk factors needs to be more precisely evaluated, especially in the detection of women at high risk for hip fracture.

Vitamin-D-receptor-gene polymorphisms and change in lumbar-spine bone mineral density

Common vitamin-D-receptor (VDR) gene allelic variants predict bone mineral density. We analysed VDR alleles and rate of change of lumbar-spine bone mineral density over 18 months in 72 elderly subjects. 9 BB homozygotes lost bone mineral density but 26 homozygotes for the alternative genotype (bb) did not (mean change -2.3 [SE 1.0] vs 0.9 [0.7]% per year, p < 0.05), irrespective of calcium intake. Among 37 heterozygotes (Bb), however, change in bone mineral density correlated with calcium intake (r = 0.35, p < 0.03). This association between a genetic marker and rate of bone loss in the elderly suggests that the effect of calcium intake on maintenance of bone mass could relate to VDR gene polymorphisms.

Investigation of the relationship between osteoporosis and the collagenase gene by means of polymorphism of the 5'upstream region of this gene

Osteoporosis is a slowly progressing disease resulting from an imbalance between bone accretion and degradation. As interstitial collagenase is a key enzyme in the degradation of bone matrix, we investigated a possible relationship between the collagenase gene and osteoporosis. Analysis of an amplified genomic DNA fragment from -524 to +52 by denaturing gradient gel electrophoresis and sequencing allowed us to detect three dimorphic sites upstream of base -300, one of them leading to a BanI restriction site. None of the sites could be directly associated with osteoporosis. The allele frequencies of the three dimorphic sites were estimated. The interallelic ratios were high, thus providing new useful genetic markers for linkage analysis. When comparing these ratios in osteoporotic and nonosteoporotic subjects, no significant differences could be observed.

Environmental and genetic factors affecting bone mass. Similarity of bone density among members of healthy families

OBJECTIVE

To evaluate the relative importance of environmental and genetic factors in the determination of bone mineral density (BMD) and to quantify the risk of low BMD in healthy young adults in relation to the BMD of their parents.

METHODS

Dual-energy x-ray absorptiometry study of a series of 129 nuclear families (441 subjects), including 183 children over age 15, was performed. Correlation of BMD in children with BMD in their parents was studied in a linear model, taking into account environmental factors. Logistic regression was used to quantify the relative risk of lower BMD according to the parents' BMD level.

RESULTS

BMD was significantly correlated with weight, height, and body mass index (BMI) in all family members, and with parents' alcohol consumption and with physical activity in fathers and sons. The BMD of the children correlated with that of their parents (r = 0.27). The child's BMI, his/her father's BMD and daily calcium intake, and his/her mother's BMD, BMI, and body fat accounted for 41.4% of the variance in the child's BMD. A son had a 3.8 times higher risk of having a low BMD if his father had a low BMD, and a daughter had a 5.1 times higher risk if her mother had a low BMD.

CONCLUSION

The BMD of children in healthy families was related to the BMD of their parents as well as to environmental factors, confirming the contribution of genetic inheritance in the determination of bone density in young adults, especially in girls.

Total body mineral mass measured with dual photon absorptiometry in healthy children

Using dual photon absorptiometry, bone mineral content (BMC) and bone mineral density (BMD) of the total body and the lumbar spine were assessed in 97 healthy, Caucasian children aged 3-14 years. Excellent correlations were found between BMC and BMD on the one hand and age, body height and body weight on the other. No differences were found between boys and girls. There was a strong correlation between lumbar spine measurement as compared to those of the total body. Regression equations for total body and the different parts of the skeleton were calculated with either BMC or BMD as the dependent variable, and age, body height and body weight as independent variables. High variation coefficients were obtained in these multiple regressions, except for the head. For total body BMC and total body BMD, growth charts were constructed using Tanner and Whitehouse data on body height and body height and body weight. Conclusions. The increase in total body mineral content is an important feature of normal growth. Normal data for BMC and BMD in childhood are essential for bone mineralisation abnormalities in paediatric patients.

Osteoporosis and genetic influence: a three-generation study

We have studied 27 triads of mother, daughter and grandmother for possible genetic influence on distal and proximal forearm bone density, measured by single photon absorptiometry. We found a significant correlation of bone density at the proximal forearm between the mothers and grandmothers (r = 0.499, P < 0.01). There was also a weak correlation between proximal forearm bone densities of mothers and daughters (r = 0.327, P < 0.1). Significant correlations were found between the three generations for grip strength, pedometry, height and triceps skinfold thickness. There was also significant correlation between mother and grandmother for alcohol intake. There was no correlation for contraceptive pill use, smoking, dietary calcium intake, body weight or body mass index. The study concludes that, although there are similarities in bone mineral content between the three generations, genetic factors cannot be conclusively proven to be the major determinant of bone density. Lifestyle and environmental factors may have a bearing on achieving the peak bone mass and subsequent development of osteoporosis.

Vitamin D receptor alleles and bone physiology

The vitamin D endocrine system is central to the control of bone and calcium homeostasis. The active hormonal form of vitamin D, 1,25 dihydroxyvitamin D (calcitriol), the circulating level of which is tightly regulated, acts through a specific receptor to mediate its genomic actions on almost every aspect of calcium homeostasis. Because of its transactivation function, it is possible that a small difference in vitamin D receptor level could be amplified into a biologically significant alteration in physiological setpoint. The recent finding that polymorphisms in the vitamin D receptor gene are predictive of bone density (Morrison et al., Nature 367:284-287, 1994) is the first example of an allelic effect in such a homeostatically controlled system. This raises the possibility that such central operators may exist in other regulatory pathways, and could explain a large part of the observed "normal" population distribution that exists for all physiological parameters.

Family history of osteoporosis and bone mineral density at the axial skeleton: the Rancho Bernardo Study

To determine whether a family history of osteoporosis identifies individuals with low bone mineral density (BMD), we studied 1477 white elderly (aged 60-89 years), noninstitutionalized ambulatory men (n = 600) and women (n = 877) from the Rancho Bernardo, California cohort. Family history data on biologic parents and full sisters were obtained by questionnaire. BMD of the lumbar spine and hip was measured using dual-energy x-ray absorptiometry. After adjustment for age, body mass index, history of cigarette smoking, thiazide use, and estrogen use, men and women with a family history of osteoporosis had lower BMD than those with a negative family history. In men, a positive family history was associated with lower BMD at the hip (p = 0.01), whereas in women a significant association was observed for the spine (p = 0.02). BMD decreased in a stepwise fashion with an increasing number of family members with a history of osteoporosis. Analysis of the effect of parental history of osteoporosis on BMD showed a significant relation between paternal (but not maternal) history and lumbar spine BMD in both sexes and a significant relation between maternal (but not paternal) history and hip BMD only in men. The relative risk of having categoric osteopenia was highest in those whose fathers had a history of osteoporosis (RR 2.16, 95% CI = 1.38-3.37). A similar association was found for subjects with fractures. These results were not explained by differential awareness of family history in individuals with known osteoporosis, because the prevalence of family history was unrelated to personal history of osteoporosis in men and only weakly related in women.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation between vitamin D receptor allele and bone mineral density

The risk of osteoporotic fracture (frequently occurring in the vertebrae, femur, or hip) is inversely related to the extent of bone mineral density. Evaluation of the gene coding for the vitamin D receptor (VDR) for the hormonally active form of vitamin D3, 1 alpha, 25(OH)2-vitamin D3, indicates the presence of two allelic variants. In a study of 250 Caucasian twins and a second study of 311 unrelated healthy women, it was shown that the common allelic variants of the VDR gene can be used to predict differences in bone mineral density. These results may enable evaluation and prediction of those individuals at risk for early onset of osteoporosis.

Lumbar vertebral and femoral neck bone mineral density are higher in postmenopausal women with the alpha 2HS-glycoprotein 2 phenotype

alpha 2HS-glycoprotein (AHSG) is a plasma protein which becomes concentrated in the organic matrix of bone. The two most common alleles, AHSG*1 and AHSG*2, give rise to three common phenotypes. A recent report showed that a group of postmenopausal white North American women with different AHSG phenotypes differed significantly with respect to their oestrogen status. We have studied variations in bone mineral density, measured by DEXA, and levels of sex hormones and biochemical markers of bone metabolism in a group of 88 post-menopausal women unselected as to their health status. Lumbar vertebral and femoral neck bone mineral density (BMD), and the free oestradiol index were all significantly higher (P < 0.05) in women with the AHSG 2 phenotype. Values of these three parameters were lowest in the AHSG 1 phenotype and intermediate in the AHSG 2-1 phenotype. Because the differences in BMD between the AHSG 2 and 1 phenotypes represent at least a 40% difference in fracture risk, the AHSG phenotype may be of some clinical relevance as a risk factor for osteoporosis.

Familial comparison of bone mineral density at the proximal femur and lumbar spine

Familial resemblance of bone mineral density (BMD) was studied in the lumbar spine and three regions of the proximal femur in 41 biological mother-daughter (M-D), 42 mother-son (M-S), 24 mother-grandmother (M-G) pairs and 18 mother-grandmother-daughter (M-G-D) triads. Children were placed into three maturity categories based on an assessment of secondary sex characteristics and growth velocities. Two sets of standardized BMD Z-scores were derived for the children based on either their chronological age or their maturational status. These scores were compared with maternal Z-scores derived from age-specific norms. Similar comparisons were made between the Z-scores of the mothers and grandmothers. For all three regions of the proximal femur and for the total AP lumbar spine the correlations between Z-score values were similar and significant (P < 0.05) between the M-G and M-D pairs ranging from 0.41 to 0.57. In general, the familial correlations improved when maturity-status based Z-scores were used for comparison. The absolute BMD values measured in the grandmothers and the three maturity groups of the children--expressed as a percentage of the BMD of the mothers--showed that at the neck and the trochanteric regions of the proximal femur the late-pubescent girls and boys had a significantly (P < 0.05) greater bone density than their mothers (115-123%), whereas at the AP spine these groups averaged only 88% of their mothers BMD. This site differential was not apparent when comparing the post-menopausal grandmothers with the pre-menopausal mothers (80% at both sites). Three generation comparisons demonstrated a strong familial resemblance in bone mineral density. The value of incorporating maturity-based versus chronological-based parameters for comparison with adult measures in studies that involve growing children at different stages of development was also demonstrated.

Prediction of bone density from vitamin D receptor alleles

Bone density achieved in early adulthood is the major determinant of risk of osteoporotic fracture. Up to 60% of women suffer osteoporotic fractures as a result of low bone density, which is under strong genetic control acting through effects on bone turnover. Here we show that common allelic variants in the gene encoding the vitamin D receptor can be used to predict differences in bone density, accounting for up to 75% of the total genetic effect on bone density in healthy individuals. The genotype associated with lower bone density was overrepresented in postmenopausal women with bone densities more than 2 standard deviations below values in young normal women. The molecular mechanisms by which bone density is regulated by the vitamin D receptor gene are not certain, although allelic differences in the 3' untranslated region may alter messenger RNA levels. These findings could open new avenues to the development and targeting of prophylactic interventions. It follows that other pathophysiological processes considered to be subject to complex multifactorial genetic regulation may also be modulated by a single gene with pleiotropic transcriptional actions.

Generalized osteoarthritis associated with increased insulin-like growth factor types I and II and transforming growth factor beta in cortical bone from the iliac crest. Possible mechanism of increased bone density and protection against osteoporosis

OBJECTIVE

To investigate whether growth factors stored in bone might explain the increased bone density and resistance to osteoporosis in generalized osteoarthritis.

METHODS

Levels of insulin-like growth factor (IGF) types I and II and transforming growth factor beta (TGF beta) were measured in extracts of cortical bone from the iliac crest obtained at necropsy from subjects with or without osteoarthritis of the hands.

RESULTS

Concentrations of IGF-I, IGF-II, and TGF beta were significantly higher in extracts of bone powder from subjects in the osteoarthritis group than in extracts from subjects in the control group.

CONCLUSION

The results suggest that the increased bone density and resistance to osteoporosis in patients with osteoarthritis may be associated with increased skeletal concentrations of IGF-I, IGF-II, and TGF beta and may reflect a generally increased biosynthetic activity of osteoblasts in these patients.

Cross-mating study on bone mass in the spontaneously osteoporotic mouse (SAM-P/6)

Genetic control of bone mass in the spontaneously osteoporotic mouse (SAM-P/6) was examined by classical cross-mating experiments, in which SAM-P/2 with a high bone mass was used as a control. Femoral bone mass corrected by size (= bone density) was assayed photometrically. F1 hybrids exhibited intermediate values between those of the two parental strains, i.e. higher than SAM-P/6 and lower than SAM-P/2. F2 hybrids showed a more widely distributed pattern with a mean value quite similar to that of F1 hybrids. The values for backcrosses were between those of F1 and the respective parental strains. They did not show segregations expected in cases of a single-gene control. Numerical analysis of data distribution in each generation suggested that strain-specific bone mass in these strains of mice is inherited as a polygenic characteristic and controlled by a relatively small number of genes.

Heritable and life-style determinants of bone mineral density

Familial resemblance in bone mineral density at five skeletal sites was measured among 160 adult members of 40 families. Each family included a postmenopausal mother, one premenopausal daughter, one son, and the children's father. Similarities in selected life-style factors thought to influence bone density, such as physical activity, smoking, alcohol use, and diet, were also evaluated. Bone density was measured by dual-energy (total body, femoral neck, and lumbar spine) or single-photon (radius and os calcis) absorptiometry. Correlation coefficients between the midparent Z score and offspring Z scores of bone mineral density ranged from 0.22 to 0.52 among daughters and from 0.27 to 0.58 among sons. Adjustment of bone density for age, height, weight, and significant life-style or environmental factors yielded heritability estimates for the five skeletal sites between 0.46 and 0.62. That is, 46-62% of variance in bone density was attributable to heredity. Most estimates derived from the group of daughters were similar to those from the sons. These observations provide support for a significant contribution of heredity to bone density. However, an individual's life-style may account for a potentially large proportion of the nonheritable variance in bone density.

Peak bone mass and osteoporosis prevention

The incidence of osteoporotic fractures increases with advancing age. Despite advances in therapy, reversal of bone loss in established osteoporosis remains problematic and deformities and disability due to fractures often persist. Therefore the logical approach to osteoporosis treatment is preventive. Risk of fracture is determined largely by bone density, which is the end result of peak value achieved at skeletal maturity and subsequent age- and menopause-related bone loss. Thus the determinants of peak bone density and bone loss require full characterization. Environmental and lifestyle factors are important determinants of bone density, particularly physical activity and diet. For example, muscle strength and physical fitness predict bone density, so that regular moderate exercise may help maintain bone mass but probably does not reverse loss. Long-term calcium intake appears to be important for achievement and maintenance of peak bone density, especially in males. Smoking and excessive alcohol intake are deleterious to bone mass. Cultural norms in diet, lifestyle and physical activity obviously have an impact on bone density. Genetic factors have a strong role in determining the wide range in 'normal' peak bone mass. Moreover we have found strong genetic determinants of rates of change of bone mass in the lumbar spine and similar trends for sites in the femoral neck. We have shown previously that genetic factors influence bone turnover indices, particularly osteocalcin. Investigating these relationships with restriction fragment length polymorphisms, we have identified variants of the vitamin D receptor gene which predict osteocalcin levels and presumably bone turnover.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in axial bone density with age: a twin study

Bone mineral density in adult life, which is an important determinant of fracture risk, is determined by peak adult bone density, achieved in early adulthood and subsequent rates of change during adult life. Cross-sectional twin and family studies indicate that the majority of population variation in bone density may be explained by genetic factors. Although there is evidence for a genetic effect on peak bone mass, it is unknown whether there is a genetic effect on rates of changes in bone density with age. Changes in lumbar spine and femoral neck bone density determined by dual-photon absorptiometry (Lunar DP3) were examined in a cohort of monozygotic (MZ, n = 21, 3 male and 18 female pairs, median age, range, 46; 24-75 years) and dizygotic twins (DZ, n = 19; 43, 25-65 years). The median follow-up was 3 years (range 1.1-5.5 years), with each subject having at least two and up to four bone density assessments. In these twins, genetic factors determine variation in rates of change (% change/year) in lumbar spine bone density, rMZ = 0.93 and rDZ = 0.51, p < 0.02 (one tailed), and Ward's triangle, rMZ = 0.60, rDZ = 0.11, p < 0.05 (one tailed). Model-fitting analysis was also consistent with a genetic effect on rates of change in bone density at the trochanteric site, although such an effect was not shown at the femoral neck. These data demonstrate, for the first time, the possible existence of genetic determinants of rates of change in bone mineral density in adults.

Adolescent osteoporosis disclosing familial osteopenia

The observations of familial juvenile osteoporosis, presumably of genetic origin are exceptional. The authors report the observation of a 16-year old adolescent suffering from osteoporosis, confirmed by histomorphometry and decrease in bone density (lumbar vertebrae 0.79 g/cm2 and femoral neck 0.88 g/cm2: LUNAR DPX). We prescribed fluorine and calcium therapy. Lumbar bone density increases by 11% and bone density of the thighbone neck by 7.6%. We cannot rule out growth as a factor in the changes observed, given that the propositus is only 16. A densitometric investigation performed in 4 of his 12 brothers shows a decrease in the lumbar bone mineral content (from 61 to 94% expressed as Z score). A genotypic origin seems to be conceivable, especially since no other cause could be considered (endocrinal, alimentary...). On the other hand, there is no argument in favour of osteogenesis imperfecta disease. The bone densitometry is a useful diagnostic means to detect familial forms of osteoporosis.

Is heritability a risk factor for postmenopausal osteoporosis?

We investigated heritability as a risk factor for the development of osteoporosis in two randomly selected populations of postmenopausal women and their premenopausal daughters. We determined the familial resemblance in bone mass at three sites; the distal forearm, lumbar spine, and proximal femur, premenopausally and with increasing maternal postmenopausal age. We also examined the bone mass of daughters in relation to mothers with and without osteoporotic fractures. Peak bone mass among premenopausal siblings was significantly correlated at all sites (r = 0.30-0.42, p less than 0.001). The same levels of resemblance were found between early postmenopausal mothers and premenopausal daughters. There was no significant difference in bone mass at any skeletal site between daughters of women with either peripheral or spinal fractures and daughters of women without fractures. We also examined familial resemblance with four biochemical markers of bone turnover (fasting urinary calcium and hydroxyproline, both corrected for creatinine, serum alkaline phosphatase, and plasma bone Gla protein). A generally significant resemblance were seen in premenopausal siblings (r = 0.25-0.39, hydroxyproline NS), but not between premenopausal daughters and postmenopausal mothers. We conclude that peak bone mass is hereditary in the distal forearm, lumbar spine, and proximal femur, but the mother-daughter resemblance explains only about 16% of the variability in daughters' bone mass. Furthermore, daughters of women with a moderate state of osteoporotic fractures are not substantially at an increased risk of having a low peak bone mass compared to the daughters of women without fractures.

Epidemiology of osteoporosis

Osteoporosis is a major public health problem through its association with age-related fractures. Although fracture risk at any skeletal site depends upon a complex interaction between bone strength and trauma, recent epidemiologic studies confirm that bone density is currently the best single predictor of future fracture. The increasing burden of osteoporotic fractures urgently requires effective preventive strategies aimed at maximizing peak bone density, preventing excessive bone loss, and reducing the risk of falls.

Familiality and partitioning the variability of femoral bone mineral density in women of child-bearing age

The contributions of polygenic loci and environmental factors to femoral bone mineral density (BMD) in g/cm2) variability were estimated in modified family sets consisting of women of child-bearing age. Femoral BMDs were measured in 535 women who were members of 137 family sets consisting minimally of an index, her sister, and unrelated female control. The family set could also include multiple sisters and first cousins. Women included in these family sets were all between 20 and 40 years of age to minimize the cohort effects of maturation and menopause on measures of BMD. BMDs were measured at three femoral sites using dual photon densitometry. Values were regressed on age and Quetelet Index which explained 13-15% of the variability in BMD (dependent on site). Subsequent variance components analysis on the residuals indicated that unmeasured polygenic loci accounted for substantial additional variability: 67% for femoral neck, 58% for Wards triangle, and 45% for trochanter. These results suggest that polygenic loci account for approximately half of the variability in maximal femoral BMD.

Principal components analysis of regional bone density in black and white women: relationship to body size and composition

Black and white women in the United States differ with respect to bone mass and the risk of developing osteoporosis. It has been suggested that greater body size among U.S. blacks may contribute to greater bone density in this group. It is not known whether the fat or lean component contributes more to this relationship. Bone density was measured at seven sites in 161 normal black and white women using single and dual photon absorptiometry. The first principal component accounted for 73% of the variance in the sample and constitutes an index of skeletal mass. The second principal component added another 10% and contrasts the axial and appendicular sites. Both the regional bone densities and the first principal component showed significantly greater bone densities for blacks; adjustment for body size reduced bone mass differences by approximately 50%. Body composition analysis done on a subset of these women indicated that the fat component of body mass may be the more important factor in its effect on bone mass.

Genetic determinants of bone mass in adult women: a reevaluation of the twin model and the potential importance of gene interaction on heritability estimates

We estimated genetic effects on bone density in pre- and postmenopausal twins and critically considered the assumptions of the twin model. Bone mass in the radius, lumbar spine, and hip, anthropometric measurements, usual calcium and caffeine intake, tobacco and alcohol use, number of pregnancies and live births, menstrual history, usual physical activity, and medical history were measured in a volunteer sample of 171 twin pairs [124 monozygotic (MZ) and 47 dizygotic (DZ)], aged 25-80, free of diseases known to affect bone mass or mineral metabolism. At all skeletal sites, MZ intraclass correlations exceeded DZ correlations for both pre- and postmenopausal women, yielding highly significant estimates of heritability for bone mass. Adjustments for height, age, and environmental characteristics did not reduce the heritability estimates. However, many of these estimates were unrealistically high, suggesting some violation(s) of the assumptions of the twin model. Thus, the familial resemblance in bone mass is due primarily to genetic effects at all skeletal sites and at all ages, although the importance of genetic effects is diminished with aging, as evidenced by increasing within-MZ pair variability in older women. Because of failures in the assumptions of the twin model, however, particularly the greater MZ environmental similarity and the probability of gene interaction, heritability estimates are probably too high and require cautious interpretation.

Sex differences in peak adult bone mineral density

Osteoporotic fractures are more common in women than men. Although accelerated bone loss following the menopause is recognized as of major importance, it is generally considered that a lower peak adult bone mass in females also contributes to their increased risk of osteoporosis in later life. To examine potential sex differences in peak adult bone mass we studied 29 pairs of dizygotic twins of differing within-pair sex in whom the female twin was premenopausal (mean age 37 years, range 21-55). Bone mineral density (BMD, g/cm2) was measured at the lumbar spine and femoral neck by dual-photon absorptiometry; 22 pairs also had BMD measured in the distal and 21 pairs in the ultradistal radius by single-photon absorptiometry. There was no significant difference in usual dietary calcium intake or tobacco consumption between the twin pairs. Consistent with accepted dogma, BMD at both radial sites were higher (+27%) in the males than their female cotwins. In contrast, there was no sex difference (male versus female) in BMD (mean +/- SEM) in the femoral neck (0.96 +/- 0.02 versus 0.97 +/- 0.03), and surprisingly, the females had a greater lumbar spine BMD than their male cotwins (1.19 +/- 0.03 versus 1.26 +/- 0.03, p less than 0.05). This difference was observed despite the fact that the males were taller (p = 0.033). If the femoral neck BMD values in the females were corrected for this difference in BMI, their values (0.99 +/- 0.03 g/cm2) were significantly higher than those in their male cotwin (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of genetic and environmental influences on peak bone density

Risk of osteoporotic fracture in later life relates to both age and menopause-related bone loss but also to peak bone density achieved in early adulthood. Several studies have shown that genetic influences make a major contribution to variance in adult bone density, but environmental factors such as dietary calcium and physical activity also contribute a large proportion of observed variance in bone density. Previous hypotheses have suggested that the effect of certain environmental factors, such as hormonal and dietary influences, may be permissive to development of peak bone mass. Consideration of the evidence for the interaction between environmental influences, such as physical activity and nutrition, and genotype leads us to propose that environmental factors interact to allow or prevent full expression of bone density genotype. This expansion of the 'threshold' hypothesis can include the effects of sex, physical activity and dietary calcium in a model that allows more systematic study of the determinants of peak bone density and thereby more rational intervention to augment bone density in early adulthood.

Familial resemblance of radial bone mass between premenopausal mothers and their college-age daughters

The influences of heredity and environmental factors on radial bone mass were evaluated in 84 premenopausal mothers with their biological daughters (ages 18-22). Mid- and distal radial bone mineral content (BMC) and density (BMD) were assessed using single-photon absorptiometry. As a group, the daughters (mean age 18.6 years) had 5-10% less bone mass at both the distal and midradial sites than their mothers (mean age 44.2 years). Familial resemblance estimates showed significant relationships between mothers and daughters for mid- and distal BMC and BMD after considering the influence of body mass index (BMI). Daughters with a maternal family history of osteoporosis had 6-7% lower but nonsignificant values of mid- (P = 0.086) and distal BMC (P = 0.075) compared to values of women with a negative family history, whereas mothers with a positive family history had 3-4% lower (NS) values of distal and mid-BMC compared to those of mothers with a negative family history after adjustment for BMI. Multiple regression analyses showed BMI to be the most important determinant of the bone values of the mothers, and both BMI and dietary calcium intake were found to be significant for the daughters. The findings of this study suggest that hereditary contributions from the mothers play an overwhelmingly critical role in the accrual of bone mass by their daughters by ages 18-22, but that environmental influences on bone consolidation during the premenopausal decades may be more important in promoting optimal (peak) bone mass and thereby may help to delay the postmenopausal onset of osteoporotic fractures.