4. Discusssion

Our results confirm that higher circulating IL-6 levels strongly predict future mortality in a large community-based sample of old- er EA and AA adults from CHS. By screening a panel of inflamma- tion and stress-response candidate genes, we demonstrate that SNPs tagging common functional polymorphisms of IL6R and PARP1 are associated with circulating IL-6 levels among Euro- pean–American CHS participants. Though no SNPs contained with- in our candidate gene panel reached the experiment-wide significance threshold of association with longevity in our screen- ing analysis, it is noteworthy that the minor allele of PARP1 rs1805415, which was significantly associated with higher base- line IL-6 levels (an important longevity endo-phenotype), was nominally associated with decreased longevity in CHS EA. In con- trast, there was no evidence of association between IL6R genotype and longevity in CHS EA. The PARP1 rs1805415 variant showed a similar pattern of association with longevity in one of two inde- pendent replication longevity case-control samples.

3.3. Candidate gene SNP associations with longevity in CHS

At an experiment-wide significance threshold (FDR < 0.05), none of the candidate gene SNPs were significantly associated with longevity in either CHS EA or AA (Supplemental Tables 2 and 3). Nonetheless, it is notable that the tagSNP with the strongest evi- dence of association with longevity in EA was rs1805415, the PARP1 variant associated with higher IL-6 concentration. The odds ratio for each additional copy of the rs1805415 minor allele was 0.66 (95%CI 0.55–0.81; p = 0.001). In a time-to-event analysis among the entire CHS EA cohort, each additional copy of the PARP1 rs1805415 variant allele was associated with a 1.11-fold increased risk in all-cause mortality (95%CI 1.02–1.18; p = 0.01). Adjustment for major risk factors at the baseline examination (age, sex, hyper- tension, diabetes, cigarette smoking, diagnosis of cancer or clinical CVD) did not alter these results (hazard ratio = 1.12; 95%CI 1.03– 1.22; p = 0.006). On the other hand, adjustment for baseline plasma IL-6 levels reduced the relative risk of death associated with the rs1805415 variant allele to 1.05 (95%CI 0.98–1.14; p = 0.18). There was no evidence of association between longevity or all-cause mortality and any of the IL6R tag SNPs associated with IL-6 levels.

PARP-1 is a nuclear enzyme that catalyzes the post-transla- tional poly(ADP-ribosyl)ation of target proteins in response to DNA damage and appears to have diverse roles in various aging-re- lated process such as DNA repair, transcription, chromatin remod- eling, mitosis, telomere maintenance, apoptosis, and inflammation (Beneke and Bürkle, 2007). Based on HapMap data, PARP1 rs1805415 is in strong linkage disequilibrium (r2 > 0.9) with an ex- tended haplotype of least 13 other common PARP1 SNPs among Europeans; these include rs1136410, which encodes a non-synon- ymous Val762Ala amino acid substitution. The Val762Ala poly- morphism is located within the catalytic domain of PARP-1 and has been associated with reduced enzymatic activity and increased cancer susceptibility among Europeans and Asians (Lockett et al.,

3.4. Replication of PARP-1 longevity result

PARP1 rs1805415 showed an association with decreased risk of longevity in the Ashkenazi Jewish longevity cases and controls

PARP activity in peripheral blood mononuclear cells has been correlated with maximal life span in mammalian species (Grube

and Bürkle, 1992), and also reported to be higher in immortalized lymphocytes from centenarians than from shorter-lived controls (Muiras et al., 1998). PARP-1 also activates various pro-inflamma- tory pathways by interacting with NFjB, a key regulator of innate immunity (Csiszar et al., 2008). PARP1 genotype was recently re- ported to be associated with plasma C-reactive protein, another well-characterized inflammation and stress-responsive marker associated with mortality risk in older adults (Reiner et al., 2008). The same PARP1 variant associated with higher IL-6 levels was associated with higher CRP levels in CHS. Therefore, PARP1 genotype may have pleiotropic effects on various inflammatory and other aging-related traits, thereby representing an important link between inflammatory responses to environmental or cellular stress, aging, and mortality in older adults.

cannot distinguish which SNP is the true functional variant(s), nor exclude the possible contribution of rare variants within our candidate genes. Due to multiple testing issues and the observed heterogeneity in PARP1 – longevity association results between studies, additional validation of in other samples of older adults and analysis of other aging-related phenotypes in older adults is warranted. Given that PARP-1 inhibitors are currently under inves- tigation for the treatment of various chronic inflammatory condi- tions (de la Lastra et al., 2007), PARP1 genotype may ultimately prove to have potential clinical or pharmacogenomic utility for a wide range of inflammatory and aging-related disorders.

Acknowledgments