Previous studies indicate that transgenic mice that overexpress SIRT1 appear to have beneficial phenotypes that may be relevant in human health, including better glucose tolerance [33,34]. In contrast to the positive effects of increased SIRT1 activity, SIRT1 deficiency impairs metabolism [35]. Therefore, we hypothesize that variants in rs12778366 may lead to overexpression of the protein, especially since this SNP is located in Transcription Factor Binding Site (TFBS). Although rs12778366 is not associated with the SIRT1 protein expression in adipose tissue, lymphoblastoid cell lines and skin (Genevar (GENe Expression VARiation) database) [36], this does not rule out a possible effect of the SNP on protein expression in other tissues i.e. in the lung,

Discussion

carriers of SNP rs12778366 in SIRT1, during a 18 years follow-up study in the general population. Therefore, SIRT1 appears to be an important candidate gene explaining individual differences in human lifespan. There is evidence linking overexpression of Sir2 to extended lifespan in yeast, worms and flies [2–4]. Despite the established role of mammalian SIRT1 in metabolism, genome stability and stress response [30,31], polymorphisms in SIRT1 were not associated with exceptional human longevity in a cross- sectional case-control study [22,24] nor with all-cause mortality in a general population-based cohort [25] and in a group of over 85 years who were followed up until they died [23]. Whereas the last study included only pre-selected old subjects the advantage of our current study is that we did not use a selection criteria based on age, but investigated the whole population in a longitudinal manner. Actually, up till now, only one study in humans showed associations between SIRT1 variants and healthy aging, what the authors defined as being healthy (i.e. normal brain function and verbal fluency test; laboratory findings for hemogram, peripheral smear, urine, electrocytes, chest X-ray, kidney, pulmonary function, echocardiography and ECG were normal) at the age 60 or higher [32]. In the light of recent uncertainty about the role of sirtuins in longevity [1] our results importantly provide new evidence in favor of a role of the gene in longevity.

Strengths and limitations

The major strength of the current study is the longitudinal design. We were able to follow participants for 18 years, which provided a wide time window for evaluating survival in the cohort. A strength of our study is also the number of subjects (n =1,390), sampled from the general population. Additionally, the high follow-up rate is a major strength of the study, since 98.6% of the included subjects could be traced back. A limitation of our study is the limited data on metabolic profile, because the oral glucose tolerance test was performed only in male subjects and was not accompanied by insulin measurements. Furthermore, the study population consisted of white individuals of Dutch descent which limits extrapolation to other ethnic groups.

In summary, this is the first study showing that SIRT1 plays a role in human lifespan in a non-selected general population cohort. The importance of SIRT1 is supported by the association of its polymorphism with long-term survival in the general population. Furthermore, linking SIRT1 polymorphisms to improved glucose tolerance stresses the impact of SIRT1 on metabolism in humans and identifies SIRT1 as a possible candidate for therapeutic purposes.

We have shown an association between the SIRT1 gene and long-term survival among minor allele carriers of SNP rs12778366 in SIRT1 in the total population. Furthermore, the directions of the effect did not change in stratified analyses according to gender, smoking habits and age. Interestingly, stratification according to BMI showed the protective effect of rs1277836 only in overweight/obese subjects. Taking into account the increased mortality per se in obese subjects this finding may shed a new light on obesity-related burdens.

Supporting Information

One of the physiological pathways through which SIRT1 may affect longevity might be glucose homeostasis. Indications suggesting a role of this pathway were backed up by evidence that minor allele carriers of SNP rs12778366 had better glucose tolerance as determined by the oral glucose tolerance test. Interestingly, stratified analysis according to BMI showed that the effect was more pronounced in overweight/obese subjects, whereas in subjects with normal weight only the direction of the

Author Contributions

Prepared the mortality data for analysis: JMV. Obtained the grants for the study: JMV HMB. Discussed results, proposed corrections and approved the final version of the manuscript: SMF JMV HMB. Conceived and designed the experiments: SMF JMV HMB. Performed the experiments: SMF. Analyzed the data: SMF. Wrote the paper: SMF.

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