The Dual Role of DNA Hypermethylation and Hypomethylation in Colorectal and Gastric Tumorigenesis: Mechanisms and Non-Invasive Biomarker.

DNA methylation is a key epigenetic modification catalyzed by DNA methyltransferases (DNMTs) and predominantly occurs at cytosine-phosphate-guanine (CpG) islands, which are often located in gene promoter regions. Hypermethylation of CpG islands within gene promoters can silence tumor suppressor gene expression, thereby disrupting normal cellular functions, including maintenance of genomic stability and regulation of cell growth, and contributing to tumor initiation and progression. In contrast, global hypomethylation may promote genomic instability and oncogene activation. This review discusses the molecular mechanisms underlying DNA methylation and evaluates its functional and clinical significance in colorectal and gastric cancers, with emphasis on its potential application as a noninvasive biomarker for diagnosis.