Variability of the SIRT3 gene, human silent information regulator Sir2 homologue, and survivorship in the elderly

Abstract

The human sirtuin 3 (SIRT3) gene encodes a putative mitochondrial NAD-dependent deacetylase (SIRT3) which belongs to the evolutionary conserved family of sirtuin 2 proteins. Studies in model organisms have demonstrated that SIR2 genes control lifespan, while no data are available regarding a possible role of SIRT3 in human longevity. By analysing the genotype-specific survival function relevant to the G477T marker of SIRT3, we found that in males the TT genotype increases (p=0.0272), while the GT genotype decreases (p=0.0391) survival in the elderly. Since SIRT3 lies in a chromosomal region (11p15.5) where four genes potentially associated with longevity are located (HRAS1, Insulin-like Growth Factor 2, Proinsulin, and Tyrosine Hydroxylase) we tested for linkage-disequilibrium between G477T alleles and alleles of the above genes. The disequilibrium was not significant in any case, thus suggesting that SIRT3 itself, or a gene strictly linked to SIRT3, may have a role in human longevity.

Introduction

The human sirtuin 3 (SIRT3) gene codes for a mitochondrial protein (SIRT3) (Schwer et al., 2002, Onyango et al., 2002) which is homologous to the yeast protein Sir2p (silent information regulator 2 protein) the founding member of a large family of NAD+ dependent protein deacetylases named sirtuins (Smith et al., 2000). The SIR2 gene is a key determinant in yeast lifespan: indeed null mutations of SIR2 shorten while an extra-copy of SIR2 extends lifespan, probably through chromatin silencing in the ribosomal DNA repeats (Kaeberlein et al., 1999, Sinclair and Guarente, 1997). Also in C. elegans a SIR2 homologue, sir-2.1, seems to modulate lifespan through a mechanism actually unknown, but probably associated with the insulin signalling pathway (Tissenbaum and Guarente, 2001). The role played by Sir2p as NAD+ dependent protein deacetylase links cellular metabolism, transcriptional silencing and ageing (Guarente, 2000) and it has been proposed that SIR2 genes may regulate ageing in many species, possibly by coordinating the pace of ageing to the metabolic rate (Guarente and Kenyon, 2000, Guarente, 2001).

The SIRT3 gene (www.ncbi.nlm.nih.gov/omim: MIM 604481; contig NT_035113) lies at the telomeric terminal on 11p15.5 chromosome. Allelic association studies carried out in people older than 100 years showed that a relationship exists between longevity and polymorphism of four genes located in this region. In particular, by analysing an STR marker of the Tyrosine Hydroxylase (TH) gene, as well as the haplotypes defined by this marker and RFLPs of Proinsulin (INS) and Insulin-like Growth Factor 2 (IGF2) genes, significant age-related variations of the allelic pool had been observed (De Luca et al., 2001, Tan et al., 2002); similar age-related variations were also found for the 3′VNTR marker of the HRAS1 gene (Bonafè et al., 2002). The HRAS1 gene (contig NT_035113) lies at 0.665 Mb from the telomere, while the genes IGF2, INS and TH lie at 2.112, 2.140, and 2.145 Mb, respectively, from the telomeric terminal and are comprised within the contig NT_028310. Therefore, within a short piece of chromosome, five genes potentially involved in longevity are located.

The effect exerted on yeast and worm lifespan by sirtuin 2 genes, as well as the location of SIRT3 in the 11p15.5 chromosomal region, prompted us to investigate whether variants of this gene were associated with longevity. Since polymorphisms of SIRT3 were not known, we first searched for variability in the SIRT3 region encoding for a core domain that contains short motifs of conserved amino acids (Frye, 1999, Frye, 2000). After the identification of a SNP marker in exon 3, we used this marker in searching for possible modifications of the genetic pool which may occur as the population ages and survival selection operates.