One of the big mysteries of cancer is how it strikes people who follow all the rules of healthy living – those with healthy diets, who do not smoke, who are not overweight, and have little exposure to known carcinogens. It also occurs in people who may not have a family history of the disease.

New research shows that such cancer that occurs due to random, unpredictable DNA copying errors account for nearly two-thirds of the mutations that cause cancer. Scientists at the Johns Hopkins Kimmel Cancer Center in the United States used a novel mathematical model based on DNA sequencing and epidemiologic data from around the world to examine mutations that drive abnormal cell growth in 32 types of cancers.

The researchers based the study on the premise that it generally takes two or more critical gene mutations for cancer to occur. These mutations can be due to random DNA copying errors, the environment or inherited genes. The mathematical model showed that when critical mutations in pancreatic cancers are added together, 77% of them are due to random DNA copying errors, 18% to environmental factors and the remaining 5% due to heredity. In cancers of the prostate, brain or bone, more than 95% of mutations are due to random copying errors.

However, in lung cancer, 65% of all mutations are due to environmental factors, mostly smoking, and only 35% due to DNA copying errors.

The researchers estimate that 66% cancer mutations across all cancer types result from copying errors, 29% due to to lifestyle or environment and remaining 5% are inherited. These findings do do not change the fact that environmental factors such as smoking must be avoided to decrease the risk of getting cancer. The researchers say their conclusions, published in the journal Science, are in accord with studies showing that about 40% of cancers can be prevented by avoiding unhealthy environments and lifestyles.