The MTHFR gene also affects oxidative stress status in human body. The gene codes for an enzyme that play a key role in the folate metabolism [30]. Nucleotide transi- tion (C to T) at nucleotide 677 of MTHFR causes alanine to valine substitution in the N-terminal catalytic domain, leading to 30% and 65% reduction in activity for hetero- zygotes and homozygotes of the variant allele, respec- tively [31]. Reduced activity of MTHFR leads to high levels of blood homocysteine, which is rapidly auto-oxidized, leading to the production of cytotoxic reactive oxygen spe- cies and to endothelial damage [32].

Several studies indicated that gender was a main variable in the genetics of longevity and suggested that men and

Table 1: Frequencies of SOD2 and MTHFR alleles and genotypes in elderly and control groups.

Genotypes and Alleles Control group Elderly group P value N (percentage) N (percentage) -9 SOD2 * TT 42 (31.1) 44 (33.8) TC 61 (45.2) 62 (47.7) CC 32 (23.7) 24 (18.5) 0.576 Allele T 145 (53.7) 150 (57.7) Allele C 125 (46.3) 110 (42.3) 0.355 677 MTHFR * CC 82 (60.7) 77 (59.2) CT 41 (30.4) 40 (30.8) TT 12 (8.9) 13 (10.0) 0.944 Allele C 205 (75.9) 194 (74.6) Allele T 65 (24.1) 66 (25.4) 0.727

* All groups were in Hardy Weinberg equilibrium (P > 0.05).

Table 2: Frequencies of SOD2 and MTHFR alleles and genotypes in elderly and control male subjects

Genotypes and Alleles Control males group Elderly males group P value N (percentage) N (percentage) -9 SOD2 TT 17 (25) 23 (31.5) TC 35 (51.5) 34 (46.6) CC 16 (23.5) 16 (21.9) 0. 691 Allele T 69 (50.7) 80 (54.8) Allele C 67 (49.3) 66 (45.2) 0.495 677 MTHFR CC 42 (61.8) 41 (56.2) CT 21 (30.9) 26 (35.6) TT 5 (7.4) 6 (8.2) 0.795 Allele C 105 (77.2) 108 (74.0) Allele T 31 (22.8) 38 (26.0) 0.528

Ashkenazi males (sample size 150, > 75 years old [19] with SOD2 -9 SNP, while for MTHFR, positive association has been reported in Swiss population (sample size: 104, age criterion > 65 years old) and Ashkenazi women (sam- ple size: 74, age criterion > 75 years old) [19,21]. The dis- crepancy in the finding of the different studies might be due to difference in experimental design, sample size and criteria used in selecting subjects. In addition, the exam- ined polymorphisms/longevity associations might have a population specific component, being affected by the population specific gene pool as well as by gene-environ- ment interaction.

In this study, we hypothesized that the presence of the C allele at -9 position of SOD2 and T allele at position 677 of MTHFR might decrease life span. The data showed no statistically significant difference between the elderly and young groups when comparing genotypic distributions and allelic frequencies of studied SOD2 and MTHFR pol- ymorphisms (Table 1). In agreement with our results, De Benedictis et al., [20] showed that SOD2 variant does not affect individual life expectancy in Italian population (sample size: 109, age criterion > 100 years old). In addi- tion, Brattstrom et al., [22] reported that MTHFR C677T allele is not a strong risk factor for premature death in Ire- land (sample size: 1388, age criterion > 80 years old). In animal models, mice deficient in SOD2 (Sod2-/-) exhibit neonatal lethality in association with dilated cardiomyop- athy and a massive lipid accumulation in the liver [33], while (Sod2+/-) heterozygous mice have increased cancer incidence without affecting aging [34]. Furthermore, SOD isoforms showed no effect on life span in C. elegans [35] and Drosophila [36]. In contrast to our results, positive association has been reported in Danish population (sam- ple size: 1650, age criterion > 92 years old) [37] and

Among the limitations of this study are the sample size and age of recruitments of elderly subjects (≥ 85 years). One hundred and thirty subjects with a mean age of 90.01 years were included in the present study. The population of Jordan was 5.7 million in 2007 and only 4.1% of the total population was individuals of 65 years of age or older [26]. In addition, the mortality rate starts inclining exponentially at fifty year-old getting maximum level in the late seventies indicating that reaching above 85 year-

Table 3: Frequencies of SOD2 and MTHFR alleles and genotypes in elderly and control female subjects

Genotypes and Alleles Control Females group Elderly Females group P value N (percentage) N (percentage) -9 SOD2 TT 25 (37.3) 21 (36.8) TC 26 (38.8) 28 (49.1) CC 16 (23.9) 8 (14.0) 0. 317 Allele T 76 (56.7) 70 (61.4) Allele C 58 (43.3) 44 (38.6) 0.455 677 MTHFR CC 40 (59.7) 36 (63.2) CT 20 (29.9) 14 (24.6) TT 7 (10.4) 7 (12.3) 0.792 Allele C 100 (74.6) 73 (72.3) Allele T 34 (25.4) 28 (27.7) 0.686

old is a rarity (Khoury et al., 1999). Therefore, oldest old people are considered exceptional individuals in Jordan. Moreover, the lack of elderly centers in Jordan makes it very hard to recruit elderly subjects that fit sampling crite- ria. Despite all these obstacles, the sample size of the cur- rent research fall within the range of longevity studies reviewed by Glatt et al., [23] and previous studies that asked the same question in other populations (see discus- sion above). Future studies with a bigger sample size might be more appropriate with this kind of research.

results of this study indicate that SOD -9T/C and MTHFR 677C/T are not important determinant of life span in Jor- danian population.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

OK designed the study, supervised molecular experi- ments, analyzed data and prepared manuscript. EA con- ducted genotyping experiments, performed statistical analysis and participated in recruitment of subjects. AA participated in recruitment of subjects and blood sam- pling.