A key weapon in the body's immune system armoury may play a major role in triggering cancer, new research suggests.
Scientists were surprised to find that the family of virus-fighting proteins produce gene defects linked to several cancers.
The discovery was especially unexpected because one of the main functions of the molecules is to protect DNA from attack by viruses.
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The body's immune system could trigger cancer
The proteins, known as APOBEC cytidine deaminases, cause clusters of mutations that can outnumber all others associated with cancer, say the researchers.
They were found to account for more than two-thirds of some bladder, cervical, breast, head and neck, and lung tumour mutations.
All cancers are ultimately the result of mistakes in the genetic code that remain uncorrected. These can have a number of causes, including toxic chemicals and radiation.
The new evidence, from a study of a million mutations in 2,680 cancer samples, suggests APOBEC proteins may be one important trigger.
In some samples nearly 70% of mutations could be traced to the molecules.
The scientists wrote in the journal Nature Genetics: "Here we have developed a single detailed hypothesis - that APOBEC cytidine deaminases are a significant source of mutagenesis in human cancer genomes."
Lead researcher Dr Dmitry Gordenin, from the US National Institute of Environmental Health Sciences, said the proteins may have caused "many mutations" across the human genome, or genetic code book.
It was possible APOBEC-triggered mutations and viral infection were linked, the scientists added.
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Know Your Body, Spot Cancer Symptoms
A lump or swelling anywhere on your body needs checking out(01 of10)
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AXA’s research found that 79% of people were able to correctly identify breast lumps as a potential indicator of cancer. But a lump or swelling in any part of the body, including the armpit, neck, abdomen, groin or chest area, is worth having checked by a doctor. (credit:Alamy)
Talk about your toilet habits(02 of10)
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Diarrhoea or changes in bowel habits are most likely to be caused by a stomach bug or eating something that disagrees with you. But if you’re noticing changes that have lasted more than a few days, for example if your bowel movements are looser for three weeks or more, or you notice any blood when you’ve been to the toilet, then make an appointment to get it checked out. (credit:Alamy)
Sores and ulcers should disappear quickly – investigate them if they don’t(03 of10)
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A lot of people get mouth ulcers when their immune system is low or they’re stressed. Generally they’re nothing to worry about and, as the lining of the mouth regenerates itself every couple of weeks, shouldn’t last long. But any ulcer that hasn’t healed after three weeks merits attention from your doctor or dentist. The same goes for any sore or spot that lasts for several weeks without healing – get it checked by a doctor. (credit:Alamy)
Difficult passing urine – not just an inevitable consequence of age (04 of10)
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Many men find it more difficult to pass urine as they get older, needing to go more often or urgently or being unable to go when they need to. These problems are usually caused by an enlarged prostate, which is a common condition that is not in itself cause for concern. But occasionally these symptoms can be a sign of prostate cancer – either way, men experiencing these symptoms should see their GP. Similarly, while urinary tract infections are the most likely cause of women having pain or difficulty passing urine, this should pass relatively quickly. If it doesn’t, then any sudden urges to pass urine or the need to go more often should be discussed with your doctor. (credit:Alamy)
Lost weight without dieting?(05 of10)
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It’s natural for most people’s weight to fluctuate over time. But if you haven’t instigated any changes in your diet or exercise regime and have obviously lost weight, then talk to your doctor. And if you’re experiencing heavy night sweats you should seek medical advice – these don’t always have a sinister cause, and can be brought about by certain infections or medications, but they’re worth checking. (credit:Alamy)
Coughing up blood needs to be checked out(06 of10)
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If you’ve coughed up any blood, you should see your doctor, regardless of the amount of blood or frequency. It can be a sign of lung cancer, so needs to be checked out. (credit:Alamy)
Coughs and sore throats(07 of10)
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Most of us will experience coughs or croaky voices at some point, normally when we’ve had a cold. But as with many other changes to your body, anything that hasn’t gone away after three weeks or so should be investigated. (credit:Alamy)
Educate yourself on what to look out for(08 of10)
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AXA’s research found women were more likely than men to identify key cancer warning signs, including breast lumps, changes in bowel habits and irregular moles. But for both men and women, ensuring you’re aware of symptoms to keep an eye out for is important. Knowledge is power: understanding what you’re looking for means you can any changes checked out quickly. (credit:Alamy)
Know your own body(09 of10)
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AXA’s research found only 6% of men and 3% of women check their bodies daily for anything unusual. But understanding what’s normal for your own body is essential if you’re to spot when anything has changed. If you do notice changes that are persisting for a long time, or causing you pain and discomfort, then see your GP. (credit:Alamy)
Don’t put off seeing the doctor!(10 of10)
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A sizeable 61% of people AXA spoke to admitted they’d delayed seeing their doctor when they spotted changes that could be potential flags for cancer. But early detection of any problems can make a huge difference if any treatment is then needed. Similarly, if changes are harmless your doctor will be able to reassure you. Overall, the sooner you go to see your GP, the better. (credit:Alamy)
Cervical and head and neck cancers, both strongly associated with human papillomavirus (HPV) infection, had especially large numbers of APOBEC mutations.
The discovery could lead to new ways to avoid cancer, the researchers believe.
Co-author Dr Steven Roberts, a member of the same NIEA team, said: "We hope that determining the environment link to these mutations will lead to viable cancer prevention strategies."
A second study published in Nature Genetics shows that a genetic abnormality linked to cancer is associated with type 2 diabetes.
People with type 2 diabetes were already known to have a higher risk of some cancers, especially lymphoma and leukaemia.
The research showed a connection with mutations called clonal mosaic events (CMEs) which result in extra or missing copies of large chunks of DNA.
People with CMEs have a 10-fold increased risk of blood cancers.
The study showed that CMEs were four times more common in people with type 2 diabetes than in the general population.
"This finding may partly explain why people with type 2 diabetes are more likely to get blood cancers," said study leader Professor Philippe Froguel, from Imperial College London. "It could have profound clinical implications."
It may be useful for doctors to test for CMEs in patients with diabetes to identify those at highest risk of cancer, said the researchers.