production, thus preventing or reducing the age related decay of the aging muscle (Nabben et al., 2008; Mookerjee et al., 2010).

When investigating the association of UCP3 haplotypes (con- sisting of the four SNPs) with HG, in line with the single locus analysis, we found that some haplotypes show different effects on HG performances in both cohorts. In particular, the haplotype AAAG was significantly associated with higher HG scores in MADT Cohort,while in the 1905 samplehaplotypes GAGA and GAAA were associated with lower, and GGAG and GGGG were associated with higher scores. Both the single and haplotypic associations remain significant after correction for multiple testing, therefore they can be considered statistically robust. Accordingly, genetic variants being associated with lower HG at the single-SNP level also show lower HG at the haplotypic level.

Although the candidate SNPs analyzed in this study are not functional, data from the HapMap consortium report a complete LD between the functional variant rs1800849, influencing the HG strength in a southern Italian elderly population (Crocco et al., 2011) and rs11235972 (r2 = 1). A less strong LD exists between rs1800849 and the other SNPs tested (r2 = 0.5 with rs3781907, r2 = 0.23 with rs647126, r2 = 0.22 with rs1685354). Consequently, we may argue that the association observed in this work can be due to a functional linkage of our polymorphisms with the rs1800849, or they could both be in LD with the actual causal variant.

The observed association of rs1685354 with a lower HG in middle-aged individuals and a higher HG in nonagenarians could suggest a possible antagonistic pleiotropic effect of this polymor- phism (or the associated causal variant) on the phenotype. In younger individuals, a more efficient uncoupling process, promot- ing the dissipation of proton gradient, reduces ATP level and so energy availability, which may be associated with a lower muscular functioning in an energy-demanding organism; on the contrary, in a different metabolic scenario, possibly characterized by a loss of muscle mass, a higher expression of uncoupling proteins may replace dysfunctional mitochondria by stimulating the mitochondrial biogenesis, as suggested for UCP2 by Wu et al. (1999), and so represent an adaptation of the aging organism to the new physiological status. In support of this hypothesis is the evidence coming from studies on the regulation of UCP proteins, where it emerges that their activity is modified when facing different physiological and nutritional stimuli (Ricquier et al., 2000). Furthermore, the observation of a correlation between genetic variants harboring higher gene expression of UCP genes

Considering the importance of sarcopenia and loss of muscle mass in the aging process, we tested the possible association of UCP3 single variants with survival in the 1905 Cohort by using AFT models. Taking advantage of the large cohort and follow-up period available in our dataset, we found that UCP3 SNPs were also associated with survival at 10 years in this cohort. Consistently with the previous association results, rs11235972, which nega- tively influenced HG scores, was also associated with a lower survival chance, with rs11235972-A carriers showing higher mortality than non carriers. A borderline significant association with survival was also observed for rs3781907, even if the variant did not show any significant association with HG levels. On the contrary, despite rs1685354 had significant effects on the HG performance, no significant association was detected in terms of survival. A comparison with the classical Cox regression propor- tional hazard models, carried out by using each SNP and sex as covariates, confirmed these results (Table 2SM).

Finally, taking into consideration the modest LD found between SNPs analyzed in this study, we tested the effect of their interaction with respect to survival. Despite the association previously found with HG phenotypes, we could not demonstrate any significant

association with survival for UCP3 haplotypes, probably because of the level of statistical correction needed, and the scarce frequency of some haplotypes in our sample. These considerations, however, did not affect the previous associations found between single polymorphisms with survival in this cohort, rather strengthen the hypothesis of an association with another functional variant in the UCP3 gene region.

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