Genetic influence on postmenopausal bone loss falls with age

By Lauretta Ihonor

The effect of genetics on bone loss rates in postmenopausal women reduces until the age of 65 years, after which time its influence disappears completely, according to results from a twin-based study.

Furthermore, the extent to which genetics impact bone loss varies markedly between different bone sites in the body, say Timothy Spector (King's College London, UK) and colleagues.

They explain that up to 70% of spinal bone mineral density (BMD) loss is heritable, whereas this figure is only 40% for hip BMD loss.

"The difference between hip and spine regions might be, in part, explained by degenerative changes in lumbar spine or potential differences between genetic effects on trabecular versus cortical bones," the authors suggest.

The study, published in the Journal of Bone and Mineral Research, involved 2716 female twins aged 36 years or older. All had at least two dual-energy X-ray absorptiometry (DXA) scans of the hip and lumbar spine performed at intervals of 4 years or more.

The intra-class correlation coefficient (ICC; defined as the difference between BMDs for different age groups) was assessed for monozygotic (MZ) and dizygotic (DZ) twin pairs. ICC values were then used in regression models to obtain estimates of heritability.

Spector et al explain that BMD differences between MZ and DZ twin pairs were used to determine heritability because "any greater similarity between MZ twins than DZ twins is attributed to greater sharing of genetic influences."

The authors add: "Higher ICC in MZ twins compared to DZ twins would indicate a significant genetic influence on the bone loss."

The calculated heritability estimates showed that in women aged 40-45 years, genetics contributed to around 40% and 70% of BMD changes in the hip and lumbar spine regions, respectively. These figures fell with increasing age, reaching 0% in women aged over 65 years.

These heritability estimates remained the same even after adjustment for the bone-related effects of hormone replacement therapy.

Spector and co-authors conclude that the "findings warrant consideration in future genetic and epigenetic bone studies."

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