CpG island methylator phenotypes in aging and cancer

CpG islands are short stretches of CpG rich regions that are frequently associated with the promoter region of genes. Aberrant methylation of CpG islands is one mechanism of inactivating tumor suppressor genes (TSGs) in neoplasia, and there is growing evidence that altered cytosine methylation play important roles in cancer development. However, the differences in global CpG island methylation patterns between normal and cancer cells remain poorly understood. By examining a large number of loci in a series of cancers, global methylation profiles can be constructed. Such studies revealed that in colorectal cancer, there appears to be two types of methylation that are associated with cancer progression: type A (for age-related) methylation, and type C (for cancer-specific) methylation. Initially, type A methylation arises as a function of age in normal colorectal epithelial cells. By affecting genes that regulate the growth and/or differentiation of these cells, such methylation may result in a predisposition state that precedes tumor formation in the colon. Type C methylation, by contrast, was found exclusively in a subset of cancers, which display a CpG island methylator phenotype (CIMP). CIMP is a novel molecular instability pathway that appears to be responsible for most cases of aberrant TSG methylation in colorectal cancer, and which has important interactions with genetic pathways as well. In fact, CIMP+ tumors account for the majority of sporadic colorectal cancers with microsatellite instability, through methylation of the mismatch repair gene hMLH1. This model whereby age-related methylation increases cell-susceptibility to transformation and cancer-specific methylation results in neoplastic progression in a subset of cases may be applicable to many human neoplasms.