Seed and soil – understanding how cancers spread
Cancer is a growing problem - and because we're all living longer, we're more likely to develop the disease at some point in our lifetime. Although we've got a lot better at treating the disease in recent years, it does become a lot more tricky once it has spread through the body - a process called metastasis, which is the main cause of 9 out of 10 deaths from cancer.
So there's understandably a lot of interest in understanding why cancer spreads, and how we can stop it, and in a paper in this week's issue of the journal Nature, Ilaria Malanchi and her colleagues have made an important step forward.
Cancer spreads when cancer cells break away from a tumour - the primary tumour - and travel around the bloodstream or lymphatic system, looking for new places to start growing, forming secondary tumours. The researchers were intrigued by the fact that although many cancer cells set off around the body from a primary tumour, only a very small number of them actually form secondary tumours. It's a bit like a dandelion throwing off thousands of tiny seeds, but only a tiny handful of them actually landing somewhere where they can grow.
In this new paper, the researchers have discovered that it's a bit of both. The scientists were studying an animal model of breast cancer that spreads to the lungs, and found that secondary tumours in the lung could only be started by a small group of cancer stem cells - these are the 'immortal' cells thought to be at the heart of many cancers, and make up less than 5 per cent of the cells coming from a tumour. So, there's definitely something special about these cells. But still, not all the available cancer stem cells went on to form secondary tumours, so there must be something special about the places they land too.
The scientists homed in on a protein called periostin, which is produced by supporting cells - known as stromal cells - found in the places where healthy stem cells grow, and also in places where secondary tumours grow. The scientists found periostin in the stromal cells in secondary tumours in the lungs, but when they looked at normal, healthy lung tissue, they didn't see any periostin -and, importantly, they didn't see periostin in the lungs of mice who had breast cancer that hadn't yet started spreading.
When the scientists looked at mice with cancer that had been genetically engineered to lack periostin, they found a massive reduction in the number of secondary tumours, proving that it's extremely important for helping cancers to spread.
The scientists think that the cancer stem cells give out some kind of signal that makes supporting stromal cells produce periostin, which in turn causes the cancer cells to switch on processes that make them settle down and grow. Effectively, the scientists think that the cancer stem cells are wandering around the body, shouting out a message to stromal cells. Some of the stromal cells hear this message and respond, making a suitable environment for the cancer stem cells to bed down and take root.
This early research is very exciting because the processes the team have uncovered could be at the heart of many different types of cancer that spread.