Joseph Costello, of the Ludwig Institute for Cancer Research in Melbourne, Australia, suggests that a typical human tumor cell carries about 600, and as many as 4,500, abnormal methylations in DNA sequences that are frequently associated with controlling gene activity. Although a clear causal link between methylation and cancer exists for only a handful of tumors, some researchers speculate that abnormal methylation or gene-silencing could prove as important as mutations in causing cancer. It is not surprising that gene silencing gone awry may have such dire consequences. When it works normally, gene silencing acts as a master coordinator of normal development. Silencing is passed from generation to generation of dividing cells, conferring a molecular memory of the genes that cells turn off as they acquire more specialized functions.
Gene silencing also seems to help protect genomes from foreign invaders.
A spate of new studies is revealing gene silencing's far-ranging effects, adding a new layer of complexity to our understanding of the way genes work. The research is also offering new ways of manipulating genes to advance basic science, improve agricultural products, and treat disease. Researchers have implicated genes that control silencing in several genetic diseases such as Rett's syndrome, one of the major causes of mental retardation in women. In addition, new studies are bolstering the link between cancer and aberrant gene silencing. The mottling exhibited by some of these kernels indicates partial silencing of the color gene.