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The term epigenetics was first coined by Doctor C. H. Waddington in 1942 to define interactions between “genes and their products, which bring the phenotype into being.”1  Today, the study of genome modifications that allow differential gene expression without changing the DNA sequence defines the field of “epigenetics”.2  One type of epigenetic modification – chromatin remodeling modification – permits regulation in the degree of relaxation of chromatin affecting DNA packing in nucleosomes, facilitating or hindering the transcription and replications of DNA.3 

DNA methylation is another type of epigenetic modification. In humans, methylation occurs primarily at the 5′ carbon of cytosine when followed by a guanine (CpGs). Methylation occurs through the addition of a covalently attached methyl group.4  CpG islands (CpGi) are areas with higher-than-normal densities of CpG motifs and are mostly located upstream or near gene promoters.5  Recent research points to a correlation between DNA methylation levels and chromatin packaging, where the recruitment of proteins capable of remodeling chromatin may depend on the presence or absence of methyl groups in DNA.6  It is generally accepted that DNA methylation affects gene expression, even though the exact mechanisms are unknown.7,8  However, DNA methylation in some portions of the genome is transient and dynamic, changing according to exposure to the environment. Extensive research has focused on the impact of environmental changes such as diet, lifestyle, physical exercise, and pollution on the epigenome.9–11  As a result of the response of DNA methylation to environmental changes, monozygotic (identical) twins can show differences in their DNA methylation patterns that increase with age.12,13  Studying such differences permits twin differentiation,14  which is not possible using standard DNA typing methods as their genomic DNA is virtually identical. Several studies have shown success at predicting age by studying patterns of DNA methylation in specific regions of the human genome.12,15–19 

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