Global Methylation of Cytosine is Not Influenced by Age or Diet in Beagle Dogs
Mammalian genomes contain about 5-fold fewer CpG dinucleotides than expected based on random probability. Regions of the genome that have retained "near expected" numbers of CpG dinucleotides are called CpG islands and these CpG islands constitute the 5' ends of approximately half of all genes. This becomes important when one considers that CpG dinucleotides are also the substrate for enzymatic modification of cytosine to 5-methylcytosine. Importantly, age associated changes in gene expressions have been attributed, at least in part, to changes in DNA methylation within the transcriptional control regions of such genes. The purpose of this study was to determine the endogenous DNA methylation in canine lymphocytes and to further determine the role of aging and an antioxidant rich diet on the level of DNA methylation. Twenty four beagle dogs were used in this study. Ten young dogs (aged 2-4 years), 14 dogs greater than 12 years of age on either a control (n=7) or antioxidant enriched food (n=7) for a period of 3 years prior to samples. Lymphocytes were utilized to harvest RNA free DNA and 5 methylcytosine was determined by reversed-phase HPLC. Results for each group were as follows: Young dogs 4.34% ± 0.77; Old control dogs 4.37% ± 0.72; old antioxidant dogs 4.38% ± 0.68 as a percent of cytosine methylated (± SD). There were no significant differences between any of the groups. The data is consistent with levels recorded for other mammalian species. Although there were no differences detected in global methylation it remains possible that regional methylation differences may exist. A redistribution of methylated cytosines within the genome could lead to a changing pattern of gene expression in aging canines without changing the overall level of 5-methylcytosine.