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Abstract: Bone mineral density (BMD) is a complex trait that is the single best predictor of the risk of osteoporotic fractures. Candidate gene and genome-wide association studies have identified genetic variations in approximately 30 genetic loci associated with BMD variation in humans. α-Actinin-3 (ACTN3) is highly expressed in fast skeletal muscle fibres. There is a common null-polymorphism R577X in human ACTN3 that results in complete deficiency of the α-actinin-3 protein in approximately 20% of Eurasians. Absence of α-actinin-3 does not cause any disease phenotypes in muscle because of compensation by α-actinin-2. However, α-actinin-3 deficiency has been shown to be detrimental to athletic sprint/power performance. In this report we reveal additional functions for α-actinin-3 in bone. α-Actinin-3 but not α-actinin-2 is expressed in osteoblasts. The Actn3 −/− mouse displays significantly reduced bone mass, with reduced cortical bone volume (−14%) and trabecular number (−61%) seen by microCT. Dynamic histomorphometry indicated this was due to a reduction in bone formation. In a cohort of postmenopausal Australian women, ACTN3 577XX genotype was associated with lower BMD in an additive genetic model, with the R577X genotype contributing 1.1% of the variance in BMD. Microarray analysis of cultured osteoprogenitors from Actn3 −/− mice showed alterations in expression of several genes regulating bone mass and osteoblast/osteoclast activity, including Enpp1, Opg and Wnt7b. Our studies suggest that ACTN3 likely contributes to the regulation of bone mass through alterations in bone turnover. Given the high frequency of R577X in the general population, the potential role of ACTN3 R577X as a factor influencing variations in BMD in elderly humans warrants further study. [Copyright &y& Elsevier]

Abstract: Deficiency of the fast-twitch muscle protein α-actinin-3 due to homozygosity for a nonsense polymorphism (R577X) in the ACTN3 gene is common in humans. α-Actinin-3 deficiency (XX) is associated with reduced muscle strength/power and enhanced endurance performance in elite athletes and in the general population. The association between R577X and loss in muscle mass and function (sarcopenia) has previously been investigated in a number of studies in elderly humans. The majority of studies report loss of ACTN3 genotype association with muscle traits in the elderly, however, there is some indication that the XX genotype may be associated with faster muscle function decline. To further explore these potential age-related effects and the underlying mechanisms, we examined the effect of α-actinin-3 deficiency in aging male and female Actn3 knockout (KO) mice (2, 6, 12, and 18months). Our findings support previous reports of a diminished influence of ACTN3 genotype on muscle performance in the elderly: genotype differences in intrinsic exercise performance, fast muscle force generation and male muscle mass were lost in aged mice, but were maintained for other muscle function traits such as grip strength. The loss of genotype difference in exercise performance occurred despite the maintenance of some “slower” muscle characteristics in KO muscles, such as increased oxidative metabolism and greater force recovery after fatigue. Interestingly, muscle mass decline in aged 18month old male KO mice was greater compared to wild-type controls (WT) (−12.2% in KO; −6.5% in WT). These results provide further support that α-actinin-3 deficient individuals may experience faster decline in muscle function with increasing age. [Copyright &y& Elsevier]