Abstract
In this chapter, the best-understood epigenetic mark, cytosine methylation of genomic DNA, will be introduced. DNA methylation is performed by DNA methyltransferases (DNMTs) and primarily result in 5-methylcytosine (5mC) within CpGs. It is a prominent epigenetic mechanisms, which has an impact on genome stability, gene expression and development. In most cases, DNA methylation leads to the formation of heterochromatin and subsequent gene silencing. Coordinated DNA methylation and its recognition via methylation-sensitive DNA-binding proteins have a large impact on health, such as genetic imprinting, i.e., the expression of a gene in a parent-of-origin-specific manner. In contrast, aberrant DNA methylation is a well-established marker of diseases, such as cancer. This can lead to the inactivation of tumor suppressor genes, a disturbance in genomic imprinting and genomic instabilities through reduced heterochromatin formation on repetitive sequences. In addition, we will learn about the main function of the transcription factor CTCF which mediates intra- and interchromosomal contacts. In this way, CTCF stabilizes 3D complexes of chromatin loops. CTCF-mediated loops at some 100 developmentally regulated loci provide a mechanistic explanation of genomic imprinting in health.