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Hypermethylation refers to an epigenetic modification where an increased number of methyl groups are added to a molecule, most commonly DNA or proteins. In the context of DNA, hypermethylation typically occurs at CpG islands, which are regions rich in cytosine-guanine dinucleotides. This process is catalyzed by enzymes called DNA methyltransferases (DNMTs). When CpG islands in promoter regions of genes become hypermethylated, it often leads to gene silencing or reduced gene expression. This is because the added methyl groups can physically block the binding of transcription factors or recruit proteins that condense the chromatin structure, making the DNA inaccessible for transcription.

Hypermethylation plays crucial roles in various biological processes. It is essential for normal development, including processes like X-chromosome inactivation and genomic imprinting. It also acts as a defense mechanism against transposable elements, preventing their uncontrolled movement within the genome. However, aberrant hypermethylation is frequently observed in diseases, particularly in cancer. In tumors, hypermethylation of tumor suppressor gene promoters can lead to their silencing, contributing to uncontrolled cell growth and proliferation.

The role of protein hypermethylation is also being investigated. Similar to DNA methylation, protein methylation can occur at specific amino acid residues and can alter protein function, localization, and interactions. This modification is mediated by protein methyltransferases and demethylases. While the exact mechanisms and consequences of protein hypermethylation are still under extensive research, it is implicated in various cellular processes and disease states. The study of hypermethylation, both in DNA and proteins, is a dynamic field contributing to our understanding of gene regulation and disease pathogenesis.