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

Decreased endosteal formation during cortical bone modelling in SAM-P/6 mice with a low peak bone mass

Inter-strain differences in bone mass and density during growth were followed in three strains of mice: SAM-P/2, SAM-R/1 and SAM-P/6 (a murine model of senile osteoporosis, Matsushita et al., Am J Pathol 1986;125:276-283). Photometrically, the inter-strain disparities first appeared in mice at about age 28 days and increased until age 60 days. During this period, tetracycline labelling revealed significant strain differences regarding rate of the appositional formation at the endosteal surface but not at the periosteal surface. The order coincided with results of the photometrical assay, that is, highest in SAM-P/2, followed by SAM-R/1 and SAM-P/6, respectively. Therefore, strain differences, especially the osteopenic state of SAM-P/6, occur, at least in part, by disparities in endosteal formation rates during cortical bone modelling.

The peak bone mass concept

Peak bone mass (PBM), i.e. the bone mass developed after growth in early adult life, is a major determinant for bone mass in the senescence, resp. for the risk of osteoporosis. Individual differences among young adults are so important that a person with a high PBM has a relatively low risk for osteoporosis, even if its annual bone loss is fast. PBM is conditioned by genetic, hormonal, and nutritional factors, and by physical activity, the latter two offering possible impact for preventive measures. Preservation of PBM through the early, resp. premenopausal life, can be favoured by adequate nutrition and physical activity, both together being capable of postponing the appearance of osteoporosis by several years. But various parts of the skeleton decrease constantly through life, starting already in the twenties; even when absorptiometry shows no loss of bone density until menopause, resp. during early adult life, there is a constant decrease of the mechanical properties of the bone with age. Therefore, the development of an optimal PBM is probably more effective in preventing osteoporosis than the measures for preservation of bone.

Reduced bone mass in daughters of women with osteoporosis

To determine whether premenopausal daughters of women with postmenopausal osteoporosis have lower bone mass than other women of the same age, we measured the bone mineral content of the lumbar spine and femoral neck and midshaft, using dual-photon absorptiometry, in 25 postmenopausal women with osteoporotic compression fractures and in 32 of their premenopausal daughters; we then compared the results with those in normal controls. As compared with normal postmenopausal women, women with osteoporosis had lower bone mineral content in the lumbar spine, femoral neck, and femoral midshaft by 33, 24, and 15 percent, respectively (P less than 0.001 for each comparison by the one-tailed t-test). As compared with normal premenopausal women, the daughters of women with osteoporosis had lower bone mineral content at these sites by 7, 5, and 3 percent, respectively (P = 0.03, 0.07, and 0.15, respectively, by the one-tailed t-test). In terms of a standardized score, we calculated that the mean (+/- SEM) relative deficits in bone mineral content in the daughters of women with osteoporosis were 58 +/- 18 percent (lumbar spine) and 34 +/- 16 percent (femoral neck) of the relative deficits in their mothers. We conclude that daughters of women with osteoporosis have reduced bone mass in the lumbar spine and perhaps in the femoral neck; this reduction in bone mass may put them at increased risk for fractures. We also conclude that postmenopausal osteoporosis may result partly from a relatively low peak bone mass rather than from excessive loss of bone.