Concerning the HLA-B and HLA-DQB1 alleles, no differ- ences were observed among elders and young controls nor when compared by gender. The high resolution typing of HLA-DQB1 showed that in the elderly group the most fre- quent subtypes were HLA-DQB1*0302, DQB1*0402 and DQB1*0501 whereas in the young group were the HLA- DQB1*0302, DQB1*0301, DQB1*0402 and DQB1*0201. Finally, the generic typing of the HLA-B locus showed that the more frequent alleles in both groups were HLA-B*35 and HLA -B*15. A decreased ten- dency in the HLA-B*14 frequency in the elder group was also observed when compared to that of the control group, although this was not statistically significant.

The aim of this study was to evaluate the class I, class II α HLA genotypes as well as the TNF- -308 polymorphism with successful ageing in Mexican Mestizo.

Results and Discussion

Of the 71 elders, 38 (53.5%) were females and 33 (46.5%) males. Results of the observed and expected anti- gen and genotype frequencies for HLA-B, -DRB1 and - DQB1 loci were consistent with those predicted by the Hardy-Weinberg equilibrium. It is worth mentioning that the frequencies identified in our population are similar from those reported in previous studies performed in Mexican Mestizo populations [10].

The analysis of the TNF- -308 promoter polymorphism showed a diminished frequency of the TNF1 allele accom- panied by an increased one in the TNF2 allele in elders. The heterozygote genotype TNF1/2 was more common in the elderly group (9.02%) than in the control group α (1.5%) (Table 1). Serum levels of TNF- showed a slight increase in the elderly group although this difference was not statistically significant (data not shown). Interest- ingly, it was found that 58% of the TNF2 individuals were α HLA-DR3 and HLA-DR4. The -308A TNF- polymor- phism has been shown to vary between individuals. These variants have been associated with certain HLA-DR alleles, α. e.g., HLA-DR2 subjects are considered low TNF- produc-

Gene frequencies at the HLA-DRB1 locus were not statis- tically significant different among groups. The most fre- quent allele in both, young subjects and elders, was the HLA-DRB1*04. The high resolution typing revealed that the more frequent alleles in the elderly group were HLA- DRB1*0802, DRB1*0701, DRB1*0404 and DRB1*0407, whereas HLA-DRB1*0802, DRB1*0701, DRB1*1101 and

Table 1: TNF-α -308 polymorphism in elderly and young adults.

Healthy elders (n = 72) Young controls (n = 198) Statistics gf (n = 144) gf (n = 110) P OR CI 95% TNF1 0.909 0.985 NS - - TNF2 0.090 0.015 0.001 0.33 0.14–0.73

gf = gene frequency; OR = Odds ratio; CI = Confidence interval; NS: non significant.

ers while those bearing HLA-DR3 and HLA-DR4 alleles produce high levels of this cytokine [13,14,17-19]

Certainly, a great diversity of genes and factors influence successful ageing. Single nucleotide polymorphisms located in cytokine gene promoters have been demon- strated to affect the binding of transcription factors and, hence, its gene expression. Genetic variants that deter- mine an increased production of anti-inflammatory cytokines or a decreased one of pro-inflammatory cytokines have been associated with successful ageing, suggesting a role in the control of the inflammatory state in the attainment of increased life span [12,26,27].

The TNF2 genotype has been related to an increased cytokine transcription rate [20]. Furthermore, it has been proposed that individuals who are heterozygous for this polymorphism possess an optimal inflammatory response that protects them against age-related neurode- generation [21]. Although the inflammatory process has been related to chronic illnesses, incapacity, and death, it is interesting that in healthy old people this pro-inflam- matory phenotype may be involved in the remodeling of the cytokine network which contributes to the successful ageing process [22]. Notwithstanding, lack of such associ- ation has also been reported [23,24].

Moreover, the study of polymorphic genes on the X chro- mosome do not have to be procrastinated for there are some – albeit relatively old – evidence that they are critical in the genetic regulation of the immune response and thus could be of utmost importance for conditioning the life span expectancy [28-30].

Conclusion

Longevity studies must ideally include healthy centenari- ans, as they represent the extreme of human old age. Most studies carried out on centenarians come from either Europe or Asiatic countries such as Japan. It is precisely in these regions that the largest numbers of centenarians can be found. This is closely related on the one hand, with the social, cultural, and economic conditions of developed countries and, on the other, with biological factors includ- ing the long history of their ethnic groups, which has per- mitted a natural selection of genes favouring longevity [25]. By contrast, the ethnic group known as Mexican Mestizo is, in evolutionary terms, a recent one barely 500 years old. This lapse is certainly not enough to establish an allele selection or, even, modify completely the structure of previous haplotypes by the genetic admixture process that could account to change certain HLA clusters of Mex- ican natives. On the other hand, infectious diseases imported by conquerors indeed, have influenced the selection of HLA repertoire in our region. The cultural and socio-economic conditions of our geographical zone (Mexico and Central America) have contributed to a marked increase in life expectancy, particularly over the last 50 years, even though this is still far removed from that of developed countries. It is worth mentioning that the mean age of our study group is at least 10 years greater than the current life expectancy for Mexico and, hence, they represent the oldest population that have enjoyed successful ageing within our country.

Methods

Subjects