Effect of caloric restriction on Hprt lymphocyte mutation in aging rats

Caloric restriction (CR) reduces tumor incidence and retards aging in laboratory animals, including non-human primates. Because of the relationships among mutation, disease susceptibility, and aging, we investigated whether or not CR affects the accumulation of somatic cell mutations in aging animals. Starting at approximately 2 months of age, male CD rats (Harlan Sprague-Dawley-derived) were placed on different levels of dietary intake: ad libitum (AL) feeding, and 90% (10% CR), 75% (25% CR) and 60% (40% CR) of the total calories consumed by AL animals. At 3, 6, 12, and 24 months after the beginning of CR, Hprt mutant frequencies (MFs) were determined. The MFs measured in spleen lymphocytes from AL and CR rats sacrificed at 3 months of dietary restriction were similar for all dietary groups. However, the MFs at 6, 12, and 24 months of CR were significantly higher in AL-fed rats compared with animals on 40% CR: (4.5+/-0.4)x10(-6) versus (3.3+/-0.3)x10(-6) (P=0.032) in 6 months CR rats; (10.3+/-2.3)x10(-6) versus (7.3+/-1.2)x10(-6) in 12 months CR rats (P=0.04), and (18.3+/-3.2)x10(-6) versus (7.8+/-1.0)x10(-6) (P=0.001) in 24 months CR rats. In addition, rats receiving 25% CR for 24 months had a MF, (10.7+/-2.0)x10(-6), between the 40% CR and AL rats. Multiplex PCR of the Hprt gene in mutant clones from 12 and 24 months 40% CR rats and the corresponding AL rats detected deletions in 42% of CR mutants and 19% of AL mutants. Because of the difference in Hprt MF in the two groups, the estimated MF associated with deletions in CR rats was similar to the deletion MF in AL rats. This observation implies that the lower MF in CR rats is due to a reduction in smaller Hprt mutations (i.e. base substitutions and frameshifts). The pattern of smaller Hprt mutations from AL rats suggests that many were produced by reactive oxygen species (ROS). The results indicate that CR reduces the accumulation of spontaneous somatic cell mutation in aging rats, especially those caused by base substitutions and frameshifts.