In conclusion, our study found that telomere shortening occurs with age and that function ability decreased with telomere shortening. The SIRT1 and XRCC6 SNPs had significant effects on telomere length, with rs7896005 in SIRT1 having significant effects on longevity. The effects of the SNPs on telomere length were independent of age, and the effects of the SIRT1 and XRCC6 were independent of each other.We consider different pathways of maintenance, involving either SIRT1 or XRCC6, even though these genes seem to overlap each other in some of their functions. As for rs132793, this SNP lies between XRCC6 and NHP2L1 (Online Resource 3); see Materials and Methods for details. Upstream of NHP2L1 (farther downstream of XRCC6) lies a gene called MEI1, and we found that a few SNPs in MEI1 are also in high LD with rs132793 (not shown). This gene is expressed specifically in the testis and is required for normal meiotic chromosome synapsis (Libby et al. 2003; Sato et al. 2006). Because we didn't see any sex-specific phenotypes of rs132793, we rule out MEI1 as a gene responsible for the effects of rs132793 described in this article. SNP rs132793 is in high LD with four SNPs in or near XRCC6. Of these, according to our database search, rs132770 in the 50 promoter region of XRCC6 has regulatory potential. It is located 30 nucleotides upstream of the start of the first exon, and the locus is part of several transcription factor binding sites, with the profile of predicted binding factors and their affinities substantially varying depending on the alleles present. The possibility that SIRT1 and XRCC6 are in different pathways for telomere maintenance could explain the lack of any statistical interaction between the two genes observed in this study. However, we cannot rule out the possibility that our study was not powered enough to detect such interactions due to either insufficient sample sizes or suboptimal statistical modeling. In this regard, we note the high variability of T/S values throughout the age range (Fig. 1), which resulted in low R2 values in all the models of linear regression. Thus, the rate of telomere shortening is likely to vary among different individuals of the same age group, and this suggests additional factors, such as individual-specific environments, are in operation. It is also important to acknowledge the imperfect measure of global telomere shortening that leukocyte telomere length provides. Among the factors contributing to this limitation is the superposition of the state of the leukocyte population itself, which can undergo periodic fluctuations. A statistically significant effect of size was seen, but GCS was not, and this is reflected in its near significance (P = 0.0696). The genetic architecture of the two populations differs, as evidenced even by the LD profile in the SIRT1 gene (Online Resource 2). Thus, the complex genetic and environmental nature of exceptional longevity must be taken into account. This complexity includes potential gene-gene and gene-environment interactions, as seen in other studies (Jazwinski et al. 2010; Bergman et al. 2007). This remains an area open to further investigation.

Our study suggests that telomere shortening occurs, at least in part, in an age-independent manner, and that SIRT1 and XRCC6 define two pathways impinging on this telomere decline. Obviously, operation of multiple genetic pathways for telomere maintenance adds flexibility and complexity to the age-dependent default rate of telomere attrition. One can imagine that these pathways may act as modulators of environmental factors or conditions that bear the potential to afflict telomere integrity leading to genomic instability.