Epigeneics include any process that alters gene activity without changing the DNA sequence, and leads to modifications that can be transmitted to daughter cells. Many types of epigenetic processes have been identified—they include DNA methylation, alteration in the structure of histone proteins and gene regulation by small noncoding microRNAs.
Many different DNA and histone modifications have been identified to determine the epigenetic landscape. DNA methylation is mainly mediated by DNA-methyl transferase (DNMT), there are two known types of DNMT, namely DNMT1, which preserves preexisting pattern of methylation after cell replication, and DNMT3A/B, so-called “de novo” DNMT, which methylate previously unmethylated DNA. Histone modifications mainly include acetylation, methylation, phosphorylation, and ubiquitination. The acetylation of histones can be mediated by histone acetyltransferases (HATs) and histone deacetyltransferases (HDACs), while Histhone demethylation is performed by two classes of histone demethylases: lysine-specific demethylase (LSD) family proteins (LSD1 and LSD2) and JmjC domain containing histone demethylase (JHDM). Furthermore, enzymes involved in epigenetic modifications can also be governed by miRNAs. For example, miR-34a can directly inhibit the activities of SIRT1 to regulate cholesterol homeostasis.
The accumulated evidence indicates that many genes, diseases, and environmental substances are part of the epigenetics picture. At the FDA, scientists are investigating many drugs that function through epigenetic mechanisms. Drugs that inhibit DNA methylation or histone deacetylation have been studied for the reactivation of tumor suppressor genes and repression of cancer cell growth. Epigenetic inhibitors can also work alone or in combination with other therapeutic agents.
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