Most Cancer Mutations Are the Result of Bad Luck, Study Says

It's a question that's long been debated among medical experts: How many cancers come from lifestyle habits and inherited genes and how many are from sheer bad luck? A study out today says that plain old chance—in the form of random mutations (mistakes cells make when dividing and copying their own DNA)—is what leads to most of the mutations that lead to cancer.

The researchers used genome sequencing to look at the mutations responsible for abnormal cell growth in 17 types of cancer and compared it to data collected from cancer patients in 69 countries, including the US. They sought to assign the proportions of mutations to one of three factors: cell division, lifestyle and environment, and heredity.

They found that, on average, 66 percent of the differences in cancer rates among different tissues were due to random mutations, 29 percent could be attributed to lifestyle and environment, and 5 percent were inherited. There was a strong correlation between random mutations and cancer in all, but it ranged widely for different types. They found that random copying errors were responsible for 77 percent of pancreatic cancers and more than 95 percent of brain, bone, and prostate cancers, but only 35 percent of lung cancers (the other 65 percent there was due to lifestyle/environment).

The research team, from Johns Hopkins Medicine, said the paper helps explain why some people who "do everything right"—that is, eat a healthy diet, maintain a healthy weight, don't smoke, and have no family history—get cancer. It could be particularly comforting for parents of children with cancer who might blame their genes or the environment they provided.

The findings, published in Science, are consistent with previous studies suggesting that about 43 percent of cancers could be prevented by minimizing unhealthy lifestyle factors (like smoking and eating processed red meat) and environmental factors (like exposure to asbestos and radiation).

The authors wanted to be clear that their paper does NOT mean that prevention is pointless—so sorry, but diet, exercise, and avoiding carcinogens are still important. That's because multiple mutations are needed for cancer to develop and exposing yourself to carcinogens would up the chance of mutations. As senior author Bert Vogelstein, oncology professor at Johns Hopkins Medicine, said at a press briefing: "Mutations are unavoidable, and cancers to some extent are unavoidable. It doesn't mean that we should add to that by smoking or exposure to other noxious influences."

Some necessary context: A January 2015 study by the same Johns Hopkins team got people up in arms after saying that more cancers are caused by random mutations than by hereditary or environmental factors. The researchers found that the more often cells of a certain tissue divide, the more likely it is that the tissue will develop cancer, and said that two-thirds of the difference in cancer rates among different tissues comes from those differences in cell division.

Critics of that paper, which was also published in Science, didn't like that it was only based on US cancer patients and that it didn't look at breast, cervical, or prostate cancers, which have known environmental causes. (The current study did include these three cancers.) They also took issue with the fact that the researchers were suggesting that avoiding carcinogens like cigarette smoke and ultraviolet light wouldn't help much, which oncologists thought was irresponsible. Finally, lifestyle and environment can affect cell division rates, so, critics said, it's all part of the same mess.

The study provides some comfort, but the authors say it also points out that experts pay too little attention to early detection strategies for cancers that result from random mutations. These cases could only become more prevalent as humans live longer. But shocker: This new paper is already getting criticism for some of the same reasons as the first one did, namely that it doesn't take into account that lifestyle and environment can affect cell division rates, but also because the paper makes conclusions about cancer as a whole, even though cancer is really a collection of many distinct diseases. This debate won't be solved tomorrow, but in the meantime you can be sure that your doctor still wants you to keep up with the healthy shit.

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