In addition to actual alterations in DNA sequence, gene expression can be altered by changes to the DNA and chromatin that do not change the sequence. Since these changes do not alter the sequence of the DNA in the genes, they are termed epigenetic changes. Two types of epigenetic changes are described below.
Methylation
In this alteration, some nucleotides in the DNA are modified by the addition of a methyl (-CH3) group to the base. Methylation of DNA is associated with the inactivation of that particular region of DNA. Abnormal DNA methylation patterns have been seen in cancer cells. Like the changes described, methylation alters the expression of the affected genes.
Acetylation
In this epigenetic change, the histone proteins around which the DNA is wound become modified by the addition of acetyl (-CH3CHO) groups. This alteration leads to a loosening of the DNA:histone interaction and is associated with increased gene expression. The modification of the processes of addition and removal of acetyl groups to DNA is an active area of cancer treatment research.
More on epigenetic changes and cancer treatments.
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