Epigenetics is a relatively new field of genetics and the term itself is only 60 years old, originally referring to a more broad concept of complex organism development. The epigenome is the pattern of chemical alterations on chromosomes, which result in changes in gene expression (the phenotype) by condensing the chromosome or affecting the binding of transcription factors but does not change the underlying DNA sequence (the genotype). These alterations have been found in recent years to be inherited in some form and are likely affected by the environment of the womb during embryonic development. The full extent of this process of the mechanism of the affect of epigenetics on disease is still being studied.
Epigenetic modifications include DNA methylation and histone acetylation, but is sometimes thought to encompass RNA modifications leading to the degradation of the material before the protein is translated. This latter is defined better under the umbrella of gene silencing.
Chromatin condensation, a more tightly wound chromosomal configuration, closes the DNA off from transcription factor binding, essentially shutting off the genes located on that portion of the chromosome. The chromatin is wrapped around proteins, called histones, to save space in the cell. Altering the amino acids on the histone proteins via histone acetyltransferase (HAT) enzymes can alter the structure of the histone-DNA interaction, promoting the condensed heterochromatin. This is thought to be the process that keeps differentiated cells differentiated.DNA methylation is accomplished by the DNA methyltransferase enzyme, which recognizes CpG islands, strings of cytosines (C) and guanines (G) where “p” stands for the phosphate between the two nucleotides. The methyltransferase attaches a methyl group to cytosine, resulting in blocks of methylation when there is a long string of CpGs. This DNA methylation is absent during embryonic development because of the high level of replication and gene expression. The methylation is thought to help the DNA condense, cutting off access to transcription factors. In fact, altered methylation patterns are thought to play a role in the development of some cancers; cancers tend to result from increased or inappropriate gene expression.
The use of epigenetics in the cells is, essentially, to shut off unnecessary genes. Once a cell has differentiated to become a liver cell, for example, it no longer needs cardiac-specific genes to be transcribed. The cellular machinery saves its activity for needed processes. However, these patterns are not identical in different people. Genetic imprinting is a process of inheriting genetic patterns from one parent over the other. It is now thought that there are two inherited genomes the DNA sequence and the epigenome. A complexity though is that recent studies have found that the two are inherited independently of one another, which may explain differences in identical twins.
