In conclusion, carriers of combinations of variants that result in decreased IIS signaling are associated with reduced body height and improved old age survival, with the effects on survival being confined to females. The most significant polymorphism analyzed was the GH1 SNP, as female carriers of this variant were significantly smaller and exhibited a significant benefit in old age survival. Sex differences in GH/IGF-1 signaling may explain the observed sexual dimorphism in the association between reduced IIS activity and longevity.For the polymorphisms affecting the oxidative stress response, we analyzed them as independent covariates in a left-censored Cox proportional hazards model (Table 6) for both males and females. However, we did not find any association between a composite oxidative stress response (OSR) score and relative mortality risk in the combined cohorts of the Leiden 85-plus Study. The estimates were 1.11 for women (P for linear trend = 0.14) and 1.03 for men (P for linear trend = 0.72) for each increase. 

However, reduced GH/IGF-1 signaling may have adverse effects, especially in old age, as increased induction of apoptosis is associated with growth factor withdrawal throughout adulthood. The relationship between IIS signaling and longevity in mammals is complex and controversial (Rincon et al., 2004). Here, we show in a prospective study of older people from the population that serum GH concentration and pulsatile GH secretion rates fall steadily, with an estimated 14% per decade in men, a phenomenon known as somatopause (Iranmanesh et al., 1991). The reduced GH secretion is believed to contribute to many age-related diseases, such as loss of muscle mass, increased adiposity, reduced bone mineral density, and a decline in cognition (reviewed in Bartke et al., 2003). In these parameters too, sex differences have been found. Premenopausal women are significantly better protected than men from the negative impact of increasing age on GH levels (reviewed in Giustina & Veldhuis, 1998). Our results suggest that in females but not in males, the beneficial effects of reduced cumulative exposure to GH/IGF-1 outweigh the possible detrimental effects of lower GH/IGF-1 levels at old age.

The strengths of this study are several-fold. First, the analyzed polymorphisms were selected based on known effects on gene expression. Second, the effects on longevity were detected in a prospective analysis, which contained a high number of events (deaths) during the time of follow-up. Third, these effects were detected despite the fact that gene-by-environment interactions are important and human studies are uncontrolled with respect to environmental exposure. 

A first weakness of the study is that the results have not been replicated in another cohort of elderly, as cohorts of similarly aged people are not readily available. A second weakness is that we cannot completely exclude the possibility that differences in birth cohorts have confounded our results. However, such problems are of greater importance in cross-sectional studies than in prospective studies, whereas allele frequencies were not statistically different between cohort 87 (birth cohorts 1883-1902) and cohort 97 (birth cohorts 1912-1914). A third drawback is related to our selection of analyzed polymorphisms based on published effects on gene expression. This approach implies that we may have missed other existing (but not yet published) functional polymorphisms and that therefore our analyses are not exhaustive.

Of the polymorphisms analyzed, the association with body height and longevity is most significant for the GH1 SNP (in female carriers), although a similar trend was observed for two of the other polymorphisms tested (the IGF1 CA repeat and the IRS1 SNP). This could be caused by differences in the effects of the genetic variants on gene expression or protein function or by differences in the impact of these effects. Because the IIS pathway is a hierarchical network, effects of variation in IIS components may depend on their position within the hierarchy. We speculate that effects of variation in regulatory components that arose relatively late in evolution, such as GH1, may be of greater importance for human lifespan than variation in core components.

In conclusion, we present evidence that reduced IIS activity is associated with longevity in females but not in males. Of the analyzed polymorphisms, results were most pronounced for the GH1 SNP. The detected associations with longevity are in line with corresponding associations with body height, and the observed sex difference can be explained by biologically determined sex differences in the regulation of the GH/IGF-1 axis.

One of the mechanisms postulated for the life-extending effect of reduced signaling via the GH/IGF-1 axis is increased resistance to oxidative stress. In mice, administration of GH or IGF-1 in vitro as well as in vivo significantly suppresses enzymes implicated in the antioxidative stress response (Brown-Borg & Rakoczy, 2003). We, therefore, looked at the effect of polymorphisms in genes implicated in the antioxidative stress response on longevity. One mechanism by which forkhead transcription factor FOXO3a (also known as FKHR-L1), the closest mammalian homologue of C. elegans DAF-16, protects quiescent cells from oxidative stress is increasing the expression of manganese superoxide dismutase (SOD2) (the homozygous wild-type allele carriers, the latter being the reference group.

Table 2: Polymorphisms from the oxidative stress response were selected based on their previously described functionality and expected effect on the oxidative stress response pathway.

Acknowledgments: