Paraoxonase 1 Polymorphisms and Survival

L. Christiansen*, L. Bathum, H. Frederiksen, and K. Christensen

*Correspondence: Dr L. Christiansen, Department of Clinical Biochemistry and Clinical Genetics, Odense University Hospital, Sdr. Boulevard 29, Odense C 5000, Denmark. Tel: +45 6541 2843; Fax: +45 6541 1911; E-mail: lchristiansen@health.sdu.dk

Introduction

The antioxidant enzyme paraoxonase 1 (PON1) is associated with high-density lipoproteins (HDLs) in plasma and has been suggested to be responsible for HDL's ability to prevent oxidative modification of low-density lipoprotein (LDL), thereby conferring protection against coronary heart disease (CHD), one of the main causes of death in the Western world.

Two polymorphisms resulting in amino-acid substitutions, L55M and Q192R, have been described within the coding region of the PON1 gene. Leviev et al identified three common polymorphisms in the promoter region of the PON1 gene and suggested that all three to some extent affected the PON1 level.

Investigating the influence of the two coding sequence polymorphisms on the efficacy of HDLs protection of LDL lipid oxidation, Mackness et al demonstrated that individuals with the PON1 192QQ/55MM genotype were most effective in protecting LDL against lipid oxidation, whereas PON1 192RR/55LL homozygotes were least effective.

Few studies have investigated the impact of PON1 polymorphisms on survival. Heijmans et al found no significant effect of the Q192R and M55L polymorphisms on mortality risk in a 10-year follow-up study of 666 subjects aged 85 years, whereas Bonafe et al reported an increase in the frequency of 55M/192R carriers in 308 centenarians as compared to 579 younger individuals.

In this study, we examined the association between the (cid:1)107C/T, M55L, and Q192R polymorphisms in a sample of Danish subjects aged 47-93 years drawn among participants of three population-based studies. We used the sample both in a cross-sectional design and in a longitudinal follow-up study of 5 years on average. All three polymorphisms were investigated in a total of 1932 individuals, and to confirm positive findings in the initial cohorts, a further 541 subjects aged 93 years were examined.

Materials and Methods

The subjects included in this study were drawn from three major population-based surveys: the Danish 1905 Cohort, which includes all Danes born in 1905; the Longitudinal Study of Ageing Danish Twins (LSADT), including all Danish twins aged 70 years or older; and the Study of Middle-aged Danish Twins (MADT), including Danish twins born between 1931 and 1952. Initially, we genotyped a total of 1932 unrelated individuals (sample 1) previously used for gene-longevity studies. Of these, 724 originated from the 1905 cohort (aged 93 years), and 626 participants were from LSADT and MADT.

DNA was isolated from cheek swabs or blood spots using the QIAamp DNA Mini Kit (Qiagen). The three polymorphisms were detected by a Taqman-based allelic discrimination assay using the following primers and probes: (cid:1)107C/T-sense: 5'-gcccagctagctgcgga-3'; (cid:1)107C/T-antisense: 5'-atcgatgggcgcagaca-3'; (cid:1)107C: 5'-FAM-cgccc- cgcccctccc-3'; 107T: 5'-TET-cccgccccacccctcc-3'; 55M/L-sense: 5'-tttctgttctcttttctggcagaaa-3'; 55M/L-antisense: 5'-gaaaacactcacagagctaatgaaagc-3'; 55M: 5'-FAM-ccattagg- cagtatctccatgtcttcagagcc-3'; 55L: 5'-TET-tccattaggcagtatctc- caagtcttcagagcc-3'; 199Q/R-sense: 5'-ggacctgagcacttttat- ggca-3'; 192Q/R-antisense: 5'-gacaacatacgaccacgctaaacc-3'; 192Q: 5'-FAM-ttinfarction) self-reported in the surveys. The distribution of genotypes did not differ significantly between individuals with and without ischemic heart disease, indicating that the observed effect of the 192Q/R polymorphism on mortality risk is not mediated through its association with ischemic heart disease. No significant associations between mortality risk and the other two polymorphisms were observed in either men or women.There is a nonsignificant increase in the proportion of RR homozygotes in women with ischemic heart disease (11.8%) compared to both men (6.2%) and women (8.4%) with no experience of ischemic heart disease. Information about the cause of death was not obtained in this study, as the Danish Registry of Causes of Death is only updated to year 2000, making it insufficient for the main part of the 5-year follow-up time used in this study. Hence, we cannot directly attribute the reported poorer survival chance of 192RR homozygotic women to an increased susceptibility to CHD and death. Instead, we used self-reported data on ischemic heart disease (angina and/or acute myocardial infarction) to examine whether the increased mortality seen in 192RR women may possibly be caused by an increased susceptibility to development of CHD. We found a slightly higher proportion of RR homozygotes in the disease group compared to the nondisease group (11.8% vs 8.4%) in women. However, this did not reach statistical significance. This observation is in line with the results of the recent meta-analysis conducted by Wheeler et al9 showing that PON 192Q/R was only weakly associated with CHD susceptibility.

Discussion