Air Pollution and Cancer

 Researchers at the Francis Crick Institute in London have shown that air pollution does not cause cell damage directly but 'wakes up' old damaged cells. This transforms our understanding of how tumours develop and it may now be possible to develop drugs that stop cancers forming.

The classical view of cancer is that it starts with a healthy cell, which over time acquires more and more mutations in its genetic code, or DNA, until it reaches a point where is becomes a cancer and grows uncontrollably. However it is also known that cancerous mutations are also found in seemingly healthy tissue, and many substances known to cause cancer, including air pollution, don't seem to damage people's DNA.

Research has now produced evidence of a different idea. The damage is already there in our cell's DNA, as we grow and age, but there needs to be a trigger to make it cancerous. The research focused on exploring why non-smokers get lung cancer. While the vast majority of lung cancers are caused by smoking, one in ten cases is due to air pollution.

Focusing on a form of pollution called particulate matter 2.5 (known as PM2.5), which of far smaller than the diameter of a human hair, they carried out a series of detailed human and animal experiments which showed the following.

• Places with higher levels of air pollution had more lung cancers not caused by smoking.
• Breathing in PM2.5 leads to the release of a chemical alarm (interleukin-1-beta) in the lungs.
• This causes inflammation and activates cells in the lungs to help repair any damage.
• But around one in every 600,000 cells in the lungs of a 50-year-old already contains potentially cancerous mutations.
• These are acquired as we age but appear completely healthy until they are activated by the chemical alarm and become cancerous.

And crucially, the researchers were able to stop cancers forming in mice exposed to air pollution by using a drug that blocks the alarm signal.

People who had never smoked but developed lung cancer often had no idea why. To give them clues about how this might work is important, since 99% of people in the world live in places where air pollution exceeds the WHO guidelines, so it impacts everyone.

The results also showed that mutations alone are not always enough to cause cancer. It can need another element. We even need to rethink how smoking causes cancer - is it just the known DNA damage caused by the chemicals in tobacco or is the smoke causing inflammation too? The idea that mutated DNA is not enough to cause cancers and that they need another trigger to grow was first proposed by Isaac Berenblaum in 1947.

Source: Air pollution cancer breakthrough will rewrite the rules. By James Gallagher, Health and science correspondent, BBC News. https://www.bbc.co.uk/news/health-62797777