Researchers at Johns Hopkins Kimmel Cancer Center have demonstrated that random, unpredictable DNA copying mistakes account for nearly two-thirds of the mutations that cause cancer. The results were reported in Science.

Using a new mathematical model, the researchers analyzed DNA sequencing data from the Cancer Genome Atlas and epidemiologic data from the Cancer Research UK database to determine how many critical mutations were caused by environmental factors, heredity, and random copying errors.

The results differed by cancer type. They found, for example, that for pancreatic cancer, 77% of mutations are from random DNA copying errors, 18% from environmental factors (e.g., smoking), and 5% from heredity. For cancers such as prostate, brain, or bone, more than 95% of mutations are from random copying errors. Lung cancer trended in a different direction: 65% of all mutations are from environmental factors (e.g., smoking) and 35% from random copying errors; inherited factors did not play a role in lung cancer. Across all 32 cancer types in the study, the researchers estimated that 66% of mutations were from copying errors, 29% from lifestyle or environment, and 5% were inherited.

The researchers said that their findings are supported by other epidemiologic studies that show that approximately 40% of cancers can be prevented with environmental or lifestyle measures. They said that their study’s findings provide rationale for why people who follow all healthy living recommendations and have no family history of cancer may still develop the disease. They advised that random DNA copying errors become more relevant in aging populations, which have more opportunity for cells to make those errors.

For oncology nurses, the findings reinforce the importance of patients adhering to appropriate screening guidelines, even when healthy lifestyle or lack of family history may make patients feel that screening isn’t necessary.

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