Cancers are caused by changes in the genome, the epigenome, or both. Changes in the epigenome can switch on or off genes involved in cell growth or the immune response. These changes can lead to uncontrolled growth, a hallmark of cancer, or to a failure of the immune system to destroy tumors.

In a type of brain tumor called glioblastoma, doctors have had some success in treating patients with the drug temozolomide, which kills cancer cells by adding methyl groups to DNA. In some cases, methylation has a welcome secondary effect: it blocks a gene that counteracts temozolomide. Glioblastoma patients whose tumors have such methylated genes are far more likely to respond to temozolomide than those with unmethylated genes.

Changes in the epigenome also can activate growth-promoting genes in stomach cancer, colon cancer and the most common type of kidney cancer. In some other cancers, changes in the epigenome silence genes that normally serve to keep cell growth in check.

To compile a complete list of possible epigenomic changes that can lead to cancer, researchers in The Cancer Genome Atlas (TCGA) Network, which is supported by the National Institutes of Health (NIH), are comparing the genomes and epigenomes of normal cells with those of cancer cells. Among other things, they are looking for changes in the DNA sequence and changes in the number of methyl groups on the DNA.Understanding all the changes that turn a normal cell into a cancer cell will speed efforts to develop new and better ways of diagnosing, treating and preventing cancer.