Why Does Cancer Incidence Increase with Age?

The accumulation of age-associated changes in a biochemical process that helps control genes may be responsible for some of the increased risk of cancer seen in older people.  Scientists have known for years that age is a leading risk factor for the development of many types of cancer, but why aging increases cancer risk remains unclear. Researchers suspect that DNA methylation, or the binding of chemical tags, called methyl groups, onto DNA, may be involved. Methyl groups activate or silence genes, by affecting interactions between DNA and the cell’s protein-making machinery.  Zongli Xu, from the National Institute of Environmental Health Sciences (NIEHS; North Carolina, USA), and colleagues analyzed blood samples from 1,000 women, using a microarray that contained 27,000 specific methylation sites. Nearly one-third of the sites showed increased DNA methylation in association with age. They then looked at three additional data sets from smaller studies that used the same microarray and found 749 methylation sites that behaved consistently across all four data sets. As an additional check, they consulted methylation data from normal tissues and seven different types of cancerous tumors in The Cancer Genome Atlas database. The team observes that DNA methylation appears to be part of the normal aging process and occurs in genes involved in cell development. Cancer cells often have altered DNA methylation, but the researchers were surprised to find that 70-90% of the sites associated with age showed significantly increased methylation in all seven cancer types. The Positing that age-related methylation may disable the expression of certain genes, making it easier for cells to transition to cancer, the study authors write that: “Our findings suggest that as cells acquire methylation at age-related sites, they have a lower threshold for malignant transformation that may explain in part the increase in cancer incidence with age.”