Heritability of bone mineral density

OBJECTIVE

To evaluate the influence of genetic background as a determinant of peak bone mass.

PATIENTS AND METHODS

We compared lumbar spine bone mineral density in 175 girls with a bone age of 16 years or older and in their premenopausal mothers. We also investigated the influence of a family history of osteoporosis on lumbar spine bone mineral density in 275 women and their 559 daughters.

RESULTS

In the 175 mother-daughter pairs, heritability (h2) was significantly different from 0 (P < 0.0001) for lumbar spine bone mineral density (h2 = 53%; 95% confidence interval [95% CI] = 28.5-77.6%), bone mineral content (h2 = 62.3%; 95% CI = 37.7-86.8%), bone mineral density adjusted for body mass index (h2 = 56%; CI = 31.5-80.5%), and bone mineral content adjusted for body mass index (h2 = 68.2%; CI = 43.6-92.7%). However, the heritability estimations lacked accuracy, as shown by the wide 95% CIs. Osteopenia and osteoporosis were found in 16.4% and 1% of the mothers, respectively. In the subgroup defined by osteopenia or osteoporosis in the mother, lumbar spine bone mineral density was significantly higher in the daughters than in the mothers (0.994 +/- 0.095 g/cm2 versus 0.895 +/- 0.098 g/cm2; P < 0.0001), whereas the opposite was true in the subgroup defined by normal bone mass in the mothers (1.068 +/- 0.110 g/cm2 versus 1.109 +/- 0.098 g/cm2; P = 0.0003). Nevertheless, lumbar spine bone mineral density was significantly lower in the daughters of low-bone-mass women than in those of normal-bone-mass women (0.994 +/- 0.009 g/cm2 versus 1.069 +/- 0.012 g/cm2; P = 0.0006). These findings suggest a role of genetic factors inherited from the father and also indicate that bone mass gains during adulthood contribute to achievement of the optimal peak bone mass. In the family history study, bone mass was lower in the subjects with a family history of osteoporosis (123 of the 559 daughters, Z-scores normalized for height, weight, and pubertal status: bone mineral density Z-score, -0.054 +/- 1.104; bone mineral content Z-score, -0.014 +/- 1.079; 58 of the 275 mothers: bone mineral density, 1.048 +/- 0.107 g/cm2; bone mineral content, 43.3 +/- 6.8 g) than in those without a family history of osteoporosis (436 daughters, bone mineral density Z-score, 0.006 +/- 0.981; bone mineral content Z-score, -0.007 +/- 0.985; 217 mothers: bone mineral density, 1.070 +/- 0.127 g/cm2; bone mineral content, 43.8 +/- 6.7 g); however, none of these differences were statistically significant.

CONCLUSION

Our findings challenge the currently popular concept of marked bone mass heritability but are consistent with early genetic influences on lumbar spine bone mass. Thus, optimization of the peak bone mass acquired during growth may help to prevent osteoporosis.

Evolution of lumbar bone mineral content during adolescence and adulthood: a longitudinal study in 395 healthy females 10-24 years of age and 206 premenopausal women

In a longitudinal study of 395 normal 10- to 24-year-old female volunteers, 105 of whom were initially premenarcheal, lumbar bone mineral density (BMD) and content (BMC) were measured by dual-energy X-ray absorptiometry (DXA) at inclusion and after a 2-year interval. The mean age of menarche was 13.1 +/- 1.1 years (n = 395). In a multiple regression analysis the BMD and BMC relative gains were highly correlated with the height and weight relative gains and with the time since menarche (r = 0.91 and r = 0.93, respectively). The mean relative annual increments in body height, in L2-4 vertebral height, in BMD and in BMC peaked respectively at 1.5, 1.0, 0.6 and 0.7 years before menarche. The four perimenarcheal years, beginning with the first pubertal clinical signs, are essential for bone acquisition, since 46.7% of adult BMC is acquired during this period. Two years after menarche, BMC is 85% of the adult value. Seven years after menarche no further significant variation in BMC is observed. In 206 menstruating women 27-47 years old, a DXA lumbar measurement was also performed after a 4-year interval. There was a small but significant increase of 0.3%/year in BMD and 0.7%/year in BMC, contrasting with the results in the young population. This could be explained by a volumetric expansion with aging, which is supported by a small increase in L2-4 area (0.4%/year). In conclusion, this longitudinal study on the lumbar site emphasizes the importance of the pre- and perimenarcheal period, when half of lumbar adult BMC is acquired. This suggests that greater attention must be paid to this period regarding nutrition and physical activity.

Comparison and cross-calibration of DXA systems: ODX-240 and Sophos L-XRA versus Hologic QDR-4500, for spinal bone mineral measurement. Translation of a reference database

Replacement of dual-energy X-ray densitometry equipment may be necessary in time as a result of upgrading systems or new equipment. The lack of standardization in bone mineral density (BMD) measurements is known. Standardization efforts have been made for several years by the European Union under its organization COMAC-BME (Comité d'Actions Concertés-BioMedical Engineering) and by the International Committee for Standards in Bone Measurement. Cross-calibration is generally considered to be the result of linear regression between the measurements obtained with two densitometers. A major disadvantage of the regression method is the noncompatibility of the two formulae of calibration (Y versus X and X versus Y). In this study we considered cross-calibration in terms of a structural model that produced circular equations when, for example, three systems were cross-calibrated. Cross-calibration in this study was calculated from the measurement of the lumbar BMD of a population of 204 patients, with Hologic QDR-4500, ODX-240 and Sophos L-XRA systems. In vitro accuracy and short-term reproducibility of the three systems were studied. Using the structural calibration equation we transformed a reference database for L2-4 BMD obtained from a population of 983 French females, aged 11-47 years, on an ODX-240 to a reference database for a Hologic QDR-4500. A new young adult reference was obtained and consequently a new evaluation of the T-score for the Hologic QDR-4500.

Bone mineral acquisition during adolescence and early adulthood: a study in 574 healthy females 10-24 years of age

Low bone mass is known to be associated with an increased risk of fractures. Osteoporosis prevention by maximizing bone mass will be crucial and requires a better knowledge of bone mass acquisition during adolescence. Bone mass was assessed in 574 healthy volunteer females aged 10-24 years. Spine bone mineral density (BMD) in anteroposterior (AP L2-4) and lateral (LAT L3) views was measured using dual-energy X-ray absorptiometry (DXA) and AP bone mineral content (BMC) was calculated. At the same time, spine AP-BMD (L2-4) was evaluated in 333 normal menstruating women, aged 27-47 years. Bone values, osteocalcin and IGF-1 serum concentrations were correlated with chronological age, skeletal age, pubertal stages and time after menarche. In this cross-sectional study, AP- and LAT-BMD and BMC increased dramatically between skeletal ages 10 and 14 or until the first year after menarche. Between 14 and 17 skeletal years of age, AP-BMD and BMC increased moderately, whereas LAT-BMD remained unchanged. After skeletal age 17, or the fourth year after menarche, there was no significant increase in BMD or BMC, and their values did not differ from those of menstruating women. A serum osteocalcin peak was observed at skeletal ages 11-12 or at stage P3, whereas IGF-1 peaked at 13-14 skeletal years of age or at P4 and the first year after menarche. Eighty-six per cent of the adult bone mass of the spine is acquired before skeletal age 14 or the second year after menarche; therefore osteoporosis prevention programs will be particularly effective before that age.