Methylation is the term used in the chemical sciences to describe the addition of a methyl group to a substrate or the replacement of an atom or group with a methyl group. A methylation is a form of alkylation in which a methyl group, as opposed to a longer carbon chain, substitutes a hydrogen atom. These words are frequently used in the fields of biology, soil science, biochemistry and chemistry.
The Eschweiler-Clarke reaction is a term used in organic chemistry to describe the reductive methylation of amines with formaldehyde when formic acid is present. Amines are modified using this process for both synthetic and analytical applications. The process involves the reductive methylation or alkylation of lysine residues in proteins and can be carried out using sodium borohydride or sodium cyanoborohydride. In the Eschweiler-Clarke reaction, an alkylamine is condensed with formaldehyde in the presence of formic acid to produce the N-methylated product. This reaction serves as a specific example of imine reduction.
Role of Methylation in Biological Systems
The function of DNA methylation in gene silencing is best understood as the result of the attachment of a methyl group (CH3) to the 5′ carbon of cytosine bases, which results in 5-methylcytosine in the promoters of genes and suppresses transcription. De novo methylation, which includes adding a methyl group to unmodified DNA, is referred to as an epigenetic alteration since it modifies DNA chemically rather than changing DNA through a mutation. Methylation alterations, in contrast to mutations, may be reversible. Aside from histone modifications, chromatin-remodelling complexes, and other small non-coding RNAs like miRNAs and siRNAs, epigenetic changes also affect DNA-associated molecules.
Repetitive sequences in the human genome from DNA and RNA viruses or from mRNA and tRNA molecules that are able to reproduce independently of the host genome are protected by DNA methylation. As they lead to genomic instability and oncogene activation, such elements must be silenced via CpG methylation to prevent them from propagating throughout the genome. Three categories, LINES (Long Interspersed Nuclear Elements), SINES (Small Interspersed Nuclear Elements) and LTRS (Long Terminal Repeats), can be used to classify these elements. The Lymphoid-Specific Helicase, also referred to as the “heterochromatin watchdog”, is capable of identifying repetitive sequences.
Methylation in Carcinogenesis
Numerous malignancies have been well-documented to exhibit hypermethylation and de novo methylation-induced promoter suppression, which primarily affect genes in general but sporadically target genes specific to tumours. According to research on more than 1000 CGIS (Cancer Genome Interpreter) from over 100 primary human tumours, on average, 600 CGIS out of an estimated 45,000 scattered throughout the genome were abnormally methylated in malignancies. While some CGI methylation patterns were found to be present in all test tumours, others were found to be highly specific to a particular type of tumour, suggesting that the methylation of particular groups of CGIs may play a role in the development, malignancy, and progression of particular tumour types.
To control gene expression, CGI strands are methylated in a tissue-specific way, but in cancer, they become hypermethylated. De novo methylation was discovered to override the methylation boundaries bordering the CGIS in the E-cad and VHL tumour suppressor genes, causing transcriptional suppression and, ultimately, oncogenesis.