Johns Hopkins Kimmel Cancer Center scientists report data from a new study providing evidence that random, unpredictable DNA copying “mistakes” account for nearly two-thirds of the mutations that cause cancer (Science 2017;355(6331):1330-1334). Their research is grounded on a novel mathematical model based on DNA sequencing and epidemiologic data from around the world.
“It is well-known that we must avoid environmental factors such as smoking to decrease our risk of getting cancer. But it is not as well-known that each time a normal cell divides and copies its DNA to produce two new cells, it makes multiple mistakes,” said Cristian Tomasetti, PhD, Assistant Professor of Biostatistics at the Johns Hopkins Kimmel Cancer Center and the Johns Hopkins Bloomberg School of Public Health. “These copying mistakes are a potent source of cancer mutations that historically have been scientifically undervalued, and this new work provides the first estimate of the fraction of mutations caused by these mistakes.”
“We need to continue to encourage people to avoid environmental agents and lifestyles that increase their risk of developing cancer mutations. However, many people will still develop cancers due to these random DNA copying errors, and better methods to detect all cancers earlier, while they are still curable, are urgently needed,” explained Bert Vogelstein, MD, Co-Director of the Ludwig Center at the Johns Hopkins Kimmel Cancer Center.
The researchers say their conclusions are in accord with epidemiologic studies showing that approximately 40 percent of cancers can be prevented by avoiding unhealthy environments and lifestyles. But among the factors driving the new study, is that cancer often strikes people who follow all the rules of healthy living—nonsmoker, healthy diet, healthy weight, little or no exposure to known carcinogens—and have no family history of the disease, prompting the pained question, “Why me?”
Tomasetti and Vogelstein believe the answer to this question rests in random DNA copying errors.
Current and future efforts to reduce known environmental risk factors, they say, will have major impacts on cancer incidence in the U.S. and abroad. But the new study confirms that too little scientific attention is given to early detection strategies that would address the large number of cancers caused by random DNA copying errors.
In a previous study authored by Tomasetti and Vogelstein, the pair reported that DNA copying errors could explain why certain cancers in the U.S., such as those of the colon, occur more commonly than other cancers, such as brain cancer (Science 2015;347(6217):78-81).
In the new study, the researchers addressed a different question: What fraction of mutations in cancer are due to these DNA copying errors?
To answer this question, the scientists took a close look at the mutations that drive abnormal cell growth among 32 cancer types. They developed a new mathematical model using DNA sequencing data from The Cancer Genome Atlas and epidemiologic data from the Cancer Research UK database.
According to the researchers, it generally takes two or more critical gene mutations for cancer to occur. In a person, these mutations can be due to random DNA copying errors, the environment or inherited genes. Knowing this, Tomasetti and Vogelstein used their mathematical model to show, for example, that when critical mutations in pancreatic cancers are added together, 77 percent of them are due to random DNA copying errors, 18 percent to environmental factors, such as smoking, and the remaining 5 percent to heredity.
In other cancer types, such as those of the prostate, brain, or bone, more than 95 percent of the mutations are due to random copying errors.
Lung cancer, they note, presents a different picture: 65 percent of all the mutations are due to environmental factors, mostly smoking, and 35 percent are due to DNA copying errors. Inherited factors are not known to play a role in lung cancers.
Looking across all 32 cancer types studied, the researchers estimate that 66 percent of cancer mutations result from copying errors, 29 percent can be attributed to lifestyle or environmental factors, and the remaining 5 percent are inherited.
Tomasetti explained these random DNA copying errors will only get more important as societies face aging populations, prolonging the opportunity for our cells to make more and more DNA copying errors. And because these errors contribute to a large fraction of cancer, Vogelstein noted people with cancer who have avoided known risk factors should be comforted by their findings. “It's not your fault,” said Vogelstein. “Nothing you did or didn't do was responsible for your illness.”