Most cases of cancer are the result of ‘bad luck’ rather than unhealthy lifestyles, diet or even inherited genes, claim scientists. For two out of three cancer victims, the cumulative effect of random mistakes in genes is to blame for the disease rather than poor choices about how they lived their lives or ‘chose’ their parents.
Previously experts have estimated that 30-40 per cent of cancer cases would be avoided given a better lifestyle, but there has been no similar calculation about whether the remainder can be prevented. US researchers have now made the first attempt to quantify the proportion of cancer cases that are unavoidable.
They looked at the random mistakes, or mutations, that occur in DNA when cells divide during an average person’s lifetime. The more of these mutations that accumulate, the higher the risk the cells will grow unchecked – the hallmark of cancer. The actual contribution of these random mistakes to the development of the disease was previously unknown.
Professor Bert Vogelstein, from Johns Hopkins University School of Medicine in the US, said ‘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,’
Although lifestyle can add to the ‘bad luck’ factor for some specific cancers, the researchers call for more effort to go into early detection when surgery increases the chances of cure.
Almost 332,000 people were diagnosed with cancer in the UK in 2011. The biggest risk factor for the disease is simply getting older - this is because there’s more time for the cells in our body to accumulate genetic damage.
The US researchers developed a statistical model to measure the number of divisions of self-renewing stem cells that occur in an average lifetime in 31 different tissues.
These results were compared with the lifetime incidence of cancer in the same tissues. A strong correlation was seen between a particular tissue’s stem cell division rate and its likelihood of developing cancer. The more often cells divide, the more likely it is that letters of their genetic code will become renewed with a mistake, leading to an increased cancer risk. Overall, the study found that random mutations due to stem cell division could explain around 65 per cent of cancer incidence.
Prof Vogelstein said ‘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 three factors contribute to cancer development.
‘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.’
Scientists have known for more than a century that some tissue types give rise to cancer millions of times more often than others, but why this should be so has not been clear. The new study, published in the journal Science (must credit), suggests the answer chiefly lies in the number of times a tissue’s stem cells divide and replicate their DNA.
‘Bad luck’ mutations that occur when one chemical letter in DNA is wrongly swapped for another during cell replication largely explained 22 of the 31 cancer types studied. The remaining nine had incidence rates higher than predicted by bad luck, presumably due to the influence of 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,’ said Prof Vogelstein.
He pointed out that human colon - or large intestine - tissue underwent four times more stem cell divisions than small intestine tissue, with colon cancer being much more prevalent than small intestine cancer.
Co-author biomathematician Dr Cristian Tomasetti, also from Johns Hopkins University, said ‘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.’
Experts say not smoking, cutting down on alcohol, keeping to a healthy weight, avoiding sunburn and being more active can help reduce the risk of certain cancers.
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