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Modest Implication of Interleukin-6 Promoter Polymorphisms in Longevity

L. Christiansen, L. Bathum, K. Andersen-Ranberg, B. Jeune, K. Christensen

Department of Epidemiology, Institute of Public Health, University of Southern Denmark, Odense, Denmark

Department of Clinical Biochemistry and Clinical Genetics, Odense University Hospital, Odense, Denmark

The multifunctional interleukin-6 has been suggested to contribute to a chronic low-grade inflammatory status, thereby conferring susceptibility to age-related pathological conditions as well as functional decline and increased mortality. Several polymorphisms have been identified in the interleukin-6 promoter, but investigation of the effect of these on interleukin-6 levels and disease susceptibility has led to contradictory results. This study investigates the significance of the three single-point polymorphisms (597G/A, 572G/C and 174G/C) and the AT-stretch polymorphism (373(A)n(T)m) in ageing by comparing the frequency of each single polymorphism separately as well as the entire promoter haplotype in a total of 1710 Danish subjects ranging in age from 47 to 100 years. We found a modest but significant increase in the frequency of interleukin-6 174GG homozygotes with age suggesting that this genotype is advantageous for longevity.

Interleukin-6; Polymorphisms; Haplotype; Ageing; Longevity

1. Introduction

Ageing has been suggested to be associated with a chronic low-grade inflammatory activity that seems to be implicated in the pathology of several age-related diseases, consequently leading to an increased mortality (Bruunsgaard et al., 2001; Harris et al., 1999).

Inter-individual variations in IL-6 expression may be a result of base variations located in the promoter region of the IL-6 gene. Hence, putative genetic variants could partly confer susceptibility to diseases accompanied by alterations in IL-6 levels.

The multifunctional cytokine interleukin-6 (IL-6; MIM# 147620) is one of the inflammatory markers that are suspected to play a role in this low-grade inflammation. IL-6 is over-expressed in many of the stress conditions that are characteristic features of the ageing process. Furthermore, IL-6 levels are generally believed to be highly correlated with age and increase considerably during life (Ershler and Keller, 2000; Bruunsgaard et al., 1999; Cohen et al., 1997). It has been proposed that this increase is a risk factor for development of age-related diseases, such as rheumatoid arthritis, osteoporosis, Alzheimer's disease, cardiovascular diseases, and type 2 diabetes (Pradhan et al., 2001; Ershler and Keller, 2000; Licastro et al., 2000; Bonaccorso et al., 1998; Robak et al., 1998).

Three single-point polymorphisms (597G/A, 572G/C, and 174G/C) and an AT-stretch polymorphism (373(A)n(T)m) have been identified in the IL-6 promoter (Georges et al., 2001; Terry et al., 2000; Fishman et al., 1998). The potential significance of the 174G/C polymorphism for both the IL-6 level and disease susceptibility has been extensively investigated in a large number of studies involving a variety of diseases. The results have, however, been remarkably conflicting (Nauck et al., 2002; Humphries et al., 2001; Rauramaa et al., 2000).

One possible explanation for the inconsistent findings on the association of the 174G/C polymorphism with disease phenotypes is the restriction of looking at the polymorphism in isolation. In support of this, Terry et al. (2000) have demonstrated that the IL-6 gene promoter polymorphisms are not independent, and that the differences in the haplotype, rather than the 174G/C genotype, may be determinative for the IL-6 expression level.

2. Materials and Methods

2.1. Subjects

The subjects included in this study were randomly selected participants from four large nation-wide surveys conducted at University of Southern Denmark, Odense, Denmark: the Danish 1905 Cohort, which includes all Danes born in 1905 (Nybo et al., 2001); the longitudinal study of ageing Danish twins (LSADT), including all Danish twins age 70 or older (Christensen et al., 2002; Skytthe et al., 2002); the study of middle-aged Danish twins (MADT), including Danish twins born betweenThere was no difference in the 572G/C genotype distribution between age groups (results not shown). Statistics showed that the 373(A)n(T)m polymorphism was investigated in <568 individuals aged 70 years and 579 individuals aged ≥93 years. Four different 373(A)n(T)m alleles were detected in the Danish population: (A)(T)8-12, (A)(T)9-11, (A)(T)10-10, and (A)(T). There was no overall difference in the genotype distribution between the younger individuals and the group of 93-year-old people. However, the frequency of the (A)(T) allele decreased slightly with age from 47.8% in the youngest group to 43.8% in the elderly (results not significant, P>. Due to conflicting reports on the implication of the 174G/C polymorphism in ageing and disease, it was relevant to investigate whether the 174C allele was only displayed when embedded in a specific haplotype.

Results showed that there are 16 possible haplotypes when considering the polymorphisms 572G/C, 373(A)n(T)m, and 174GC. But only ten haplotypes were present in the Danish population. The estimated frequencies of the six most common haplotypes identified in four of the seven age-groups were summarized in Table 2. The 373(A)(T) allele was almost exclusively found together with the 174C allele, whereas other 373 alleles were more diversely distributed but mainly found together with the 174G allele. There was no statistically significant difference in haplotype distribution between the two age groups.