Two thirds of cancers that develop in adults are due primarily to “bad luck”, a new US study has claimed.
A statistical model that measures the proportion of cancer incidence has been created by scientists at the Johns Hopkins Kimmel Cancer Center, in Baltimore, MD, and published in Science.
It found that two thirds of adult cancer incidence across tissues develop primarily thanks to bad luck, when random gene mutations occur. The remaining third is down to environmental factors and inherited genes.
“All cancers are caused by a combination of bad luck, the environment and heredity, and we’ve created a model that may help quantify how much of these 3 factors contribute to cancer development,” explained Dr Bert Vogelstein, professor of oncology at the Johns Hopkins University School of Medicine.
“Cancer-free longevity in people exposed to cancer-causing agents, such as tobacco, is often attributed to their ‘good genes’, but the truth is that most of them simply had good luck.”
Dr Cristian Tomasetti, an assistant professor of oncology at the Johns Hopkins University School of Medicine and Bloomberg School of Public Health, added: “If two thirds of cancer incidence across tissues is explained by random DNA mutations that occur when stem cells divide, then changing our lifestyle and habits will be a huge help in preventing certain cancers, but this may not be as effective for a variety of others.
“We should focus more resources on finding ways to detect such cancers at early, curable stages.”
Drs Tomasetti and Vogelstein searched scientific literature for information on the cumulative total number of divisions of stem cells among 31 tissue types during an average individual’s lifetime.
Cancer develops when tissue-specific stem cells make random mutations, when one chemical letter in DNA is incorrectly swapped for another during the replication process in cell division. The more these mutations accumulate, the higher the risk that cells will grow unchecked.
However, said Dr Vogelstein, the contribution of these random mistakes to cancer incidence, in comparison to the contribution of hereditary or environmental factors, was not previously known.
To ascertain the information, they charted the number of stem cell divisions in 31 tissues and compared these rates with the lifetime risks of cancer in the same tissues among Americans.
From this, Tomasetti and Vogelstein determined the correlation between the total number of stem cell divisions and cancer risk to be 0.804. The closer the value is to 1, the more that stem cell divisions and cancer risk are correlated.
Using statistical theory, they then calculated how much of the variation in cancer risk can be explained by the number of stem cell divisions, which is 0.804 squared – approximately 65%.
Then they divided the types of cancers they studied into2 groups and statistically calculated which types had an incidence predicted by the number of stem cell divisions.
From that, they found 22 cancer types were largely explained by the “bad luck” factor of random DNA mutations during cell division, while the remaining 9 had incidences higher than predicted by “bad luck” and were believed to include environmental or inherited factors.
“We found that the types of cancer that had higher risk than predicted by the number of stem cell divisions were precisely the ones you’d expect, including lung cancer, which is linked to smoking; skin cancer, linked to sun exposure; and forms of cancers associated with hereditary syndromes,” says Vogelstein.
“This study shows that you can add to your risk of getting cancers by smoking or other poor lifestyle factors.
“However, many forms of cancer are due largely to the bad luck of acquiring a mutation in a cancer driver gene regardless of lifestyle and heredity factors. The best way to eradicate these cancers will be through early detection, when they are still curable by surgery,” adds Vogelstein.
Science 2 January 2015
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