Genotype profiles of loci encoding DNA repair enzymes in newborn and elderly populations: no evidence of association with longevity

1Genetics Department, Westlakes Research Institute, Westlakes Science and Technology Park, CA24 Moor Row, Cumbria, UK; 2Paediatric and Lifecourse Epidemiology Research Group, School of Clinical Medical Sciences (Child Health), Newcastle University, Sir James Spence Institute, Royal Victoria Infirmary, NE1 4LP, Newcastle-upon-Tyne, UK e-mail: craig.wilding@westlakes.ac.uk; fax +44-0-1946- 514057

Received 5 October 2005; accepted in revised form 21 November 2005

Key words: ageing, association study, DNA repair, polymorphism, selection, SNP

Abstract

The comparison of genotype frequencies between neonates and elderly populations can aid in the identi- fication of loci, and polymorphisms within those loci, that affect longevity. Here we have compared genotype frequencies of seven polymorphisms at four loci involved in DNA repair between a cohort of newborns (n=290) and a retired population (average age at sampling 70.02 years; n=430) who have suffered a lifetime of DNA damage from normal, metabolic processes, and on whom selection on DNA repair gene variants may be expected to have acted. No differences in genotype frequencies at the four SNP loci were seen, indicating that there is no evidence of association with longevity in this population. Significant differences in frequency of certain repeat sizes at three microsatellite loci were detected. How- ever, since there is no known functional consequence of these repeat lengths, the action of selection cannot yet be ascribed.

Introduction

over 400 polymorphisms have been identified in 80 of these genes that have been screened for variation (Mohrenweiser et al. 2003). Non-synon- ymous polymorphisms can have functional con- sequences (Mohrenweiser et al. 2003; Savas et al. 2004) and such polymorphisms could therefore alter the ability to repair DNA damage.

Our genetic material is under continual assault from extrinsic genotoxic agents and intrinsic met- abolic processes (Burcham 1999; Marnett 2000; Marnett and Plastaras 2001; Bohr 2002). Thus, as they age, people will have to contend with a lifetime of increasing exposure to DNA damag- ing agents. To cope with this barrage of DNA damage there are a number of DNA repair path- ways that are responsible for its detection and re- pair (Hoeijmakers 2001; Slupphaug et al. 2003). Base excision repair (BER), mismatch repair (MMR), homologous recombination (HR) and non-homologous end-joining (NHEJ) all contrib- ute to the repair of oxidative DNA damage (Slupphaug et al. 2003). Over 115 genes so far have a recognised role in DNA damage recogni- tion and repair (Ruttan and Glickman 2002) and

sure to DNA damaging agents is reflected in age-related increases in somatic mutations in both nuclear (Akiyama et al. 1995; Barnett and King 1995; Morley 1998; Jones et al. 2002) and mitochondrial DNA (Chinnery et al. 2002) and in chromosomal translocations (Ramsey et al. 1995; Lucas et al. 1999; Whitehouse et al. 2005). This increase in mutational events can contribute to the ageing process through cell and tissue degradation, malignancy and an increasing loss of genome integrity (Vijg 2000).

genes of either the BER pathway (XRCC1 R194W, R399Q and a [AC] microsatellite in the n 3¢ UTR of XRCC1), the HR pathway (XRCC3 T241M and a [AC] microsatellite in intron 3 of n XRCC3) or the NHEJ pathway (XRCC4 I134T, and a [GAPyA] microsatellite located 120 kb 5¢ n of XRCC5) between a newborn population and an aged population in order to establish whether polymorphism frequency differences, that may be attributable to selection, can be detected.

defective DNA repair proteins age prematurely (Hasty et al. 2003; Hasty 2005) reflecting the role of genetic determinants in the ageing process. Furthermore, a recent review of population data on DNA repair gene polymorphisms indicates that some common variants reduce DNA repair capacity, thus providing support for the role of DNA repair genes in determining disease suscep- tibility (Ruttan and Glickman 2002). It may therefore be expected that the presence of polymorphisms resulting in sub-optimal repair could effectively decrease lifespan. As a corollary to this, if DNA repair gene polymorphisms do indeed have an impact on life expectancy, then aged populations should present differing geno- type distributions from newborns since there would be an age-related selective effect on indi- viduals with reduced DNA repair capacity and this would manifest as a decreased frequency of sub-optimal DNA repair gene variants in an elderly population.

Materials and methods

Study populations

Umbilical cord blood samples from an unselected newborn population (n=290) were collected un- der the auspices of the North Cumbria Commu- nity Genetics Project (NCCGP), a DNA banking project which collected samples from pregnant mothers and neonates from consecutive pregnan- cies between 1996 and 2003 at the West Cumber- land Hospital, Whitehaven (Chase et al. 1998). Of these 290, 151 (52%) were male. Blood samples were taken, with informed consent, from retired male radiation workers previously employed at the Sellafield nuclear facility (n=430) as part of a separate study (Tawn et al. 2004; Wilding et al. 2005). Mean age at sampling within this group was 70.02 years (range 43.13–89.67 years). The two populations were drawn from the same geo- graphical area and composed predominantly of British Caucasians.