Telomere maintenance genes SIRT1 and XRCC6 impact age-related decline in telomere length, but only SIRT1 is associated with human longevity. Sangkyu Kim, Xiuhua Bi, Malwina Czarny-Ratajczak, Jianliang Dai, David A. Welsh, Leann Myers, Michael A. Welsch, Katie E. Cherry, Jonathan Arnold, Leonard W. Poon, and S. Michal Jazwinski received the article on 27 July 2011, accepted it on 20 September 2011, and published it online on 5 October 2011. Leukocyte telomere length is widely considered a biomarker of human age and in many studies indicative of health or disease. We have obtained quantitative estimates of telomere length from blood leukocytes in a population sample, confirming results of previous studies that telomere length significantly decreases with age. Telomere length was also positively associated with several measures of healthy aging, but this relationship was dependent on age. We screened two genes known to be involved in telomere maintenance for association with the age-related decline in telomere length to identify candidate longevity-associated genes. A single-nucleotide polymorphism located in the SIRT1 gene and another in the 30 flanking region of XRCC6 had significant effects on telomere length. At each bi-allelic locus, the minor variant was associated with longer telomeres, though the mode of inheritance fitting best differed between the two genes. No statistical interaction was detected for telomere length between the SIRT1 and XRCC6 variants or between these polymorphisms and age. The SIRT1 locus was significantly associated with longevity (P < 0.003). The frequency of the minor allele was higher in long-lived cases than in young controls, which coincides with the protective role of the minor variant for telomere length. In contrast, the XRCC6 variant was not associated with longevity. Furthermore, it did not affect the association of SIRT1 with survival. The association of the same variant with longevity was near significant (P < 0.07) in a second population. These results suggest a role of SIRT1 in linking telomere length and exceptional survival. 

Introduction: 

Telomere length shortens with each cell division in most somatic cells, due to the end-replication problem (Harley 1991). Telomere shortening also occurs with cellular senescence and aging in vivo, but its status as a biomarker of human aging is not settled (Shawi and Autexier 2008; Mather et al. 2011). Shortened telomeres are also found in cancer cells, and they are associated with high cancer incidence and mortality, although this association may be related to the presence of the disease and may not be a good predictor of cancer risk (Pooley et al. 2010; Willeit et al. 2010; von Zglinicki 2002). Indeed, longer telomeres appear to be a risk factor for development of cancer according to some studies (Arbeev et al. 2011). Telomere length has been associated with many other factors or conditions, such as oxidative damage (von Zglinicki 2002), UV irradiation (Rochette and Brash 2010), unhealthy lifestyle (Mirabello et al. 2009), and physical and mental functions (Grodstein et al. 2008; Cherkas et al. 2008; Ludlow et al. 2008). Among several age-related decrements, cognitive decline has been associated with decreased telomere length (Devore et al. 2011).

Telomere length can also be maintained through alternative lengthening mechanisms in the absence of telomerase activity, as described in cancer cells (Bryan et al. 1997; Reddel et al. 2001; Reddel and Bryan 2003). These alternative mechanisms include homologous recombination (HR) or non-homologous end joining (NHEJ). In NHEJ, which involves the Ku heterodimer and DNA ligase complex, DNA ends are directly fused without the need for substantial homology (Moore and Haber 1996). Ku consists of Ku70 and Ku80 encoded by XRCC6 and XRCC5, respectively (Critchlow and Jackson 1998; Riha et al. 2006). In normal cells with intact telomeres, Ku promotes genomic stability by repressing intra- or inter-chromatid exchange via the telomeric repeats (Celli et al. 2006). In human somatic cells, induced loss of either Ku subunit results in cell death with substantial telomereNumber of subjects in telomere analyses are in parentheses. b. Nonagenarian in LHAS, centenarian in GCS. c. Years old. d. Years old. e. Louisiana Healthy Aging Study. f. Georgia Centenarian Study. (TRF) lengths were determined by Southern blot analysis (Cawthon 2002, 2009; Pavesi et al., 2009). However, there seems to be large between-study variation in the correlation of the two measures: One unit of T/S corresponds to approximately 610 kb in TRF length, depending on the reference sample used and other experimental conditions.

Telomere length determination: Average telomere length in white blood cells was estimated using a quantitative polymerase chain reaction method (q-PCR), as described (Cawthon, 2002; Terry et al., 2008; Cawthon, 2009) with modifications. Briefly, genomic DNA was extracted from whole blood and quantified using the Picogreen assay (Ahn et al., 1996). To estimate telomere length, two q-PCRs were applied to each sample, a single-copy-gene-specific q-PCR for beta-globin and a telomere-specific q-PCR. A telomere/single-copy gene (T/S) ratio was calculated for each sample using the q-PCR results and normalized to the T/S ratio of a reference sample. The normalized T/S ratio was used as an estimate of the relative telomere length.