Identification of stem cells enables therapies targeting cells that promote tumour growth

Scientists have discovered direct evidence to support a controversial hypothesis about the growth of cancerous tumours which could revolutionise the diagnosis and treatment of cancer.

The conventional view of cancer is that it results from genetic mutations within ordinary cells that cause them to divide uncontrollably into a tumour that can then spread to other parts of the body. This suggests all cancer cells are created equal with an equal capacity for dividing uncontrollably and an equal tendency to spread. However, three independent studies have now shown this to be a myth.

The scientists found that there is a hierarchy of cancer cells within a solid tumour and at the top of the hierarchy are key "cancer stem cells" that are ultimately responsible for causing a tumour to grow and develop.

Although the existence of cancer stem cells has been postulated for many years, this is the first time that scientists have been able to demonstrate that they exist within solid tumours growing in their natural state, scientists said.

Showing that cancer stem cells exist means that treatments should be focused on killing these cells rather than targeting the wider community of tumour cells, said Luis Parada, of the University of Texas Southwestern Medical Centre in Dallas. "In the past we've tried to get rid of the entire stew of cells within cancer tumours. Now we know that it's a particular bit of the stew that we should try to get rid of," he said.

"Shrinking a tumour by 50 per cent is irrelevant. What you need to know is whether you're targeting the stem cells that allow a tumour to regrow. The good news is that we know what to go after."

The existence of cancer stem cells has been a controversial topic with some specialists rejecting the idea outright. However, in recent years there has been good evidence that they exist for so-called "liquid" tumours, the blood cancers. Now, three independent groups have found direct evidence for cancer stem cells in solid tumours of the brain, skin and digestive system. They have published their findings simultaneously in the journals Nature and Science.

Hugo Snippert, of Utrecht University in the Netherlands, who discovered cancer stem cells in intestinal tumours growing in mice, said the conventional idea of tumours is incorrect.

"Tumour are like caricatures of the tissues from which they were derived. They are composed of different cell types and there is a hierarchy between the types. Like normal tissues have healthy stem cells, tumours have cancer stem cells at the basis of their cellular turnover," Dr Snippert said.

"If we want to treat cancer it is of the utmost importance that the population of cancer stem cells is included in the treatment, otherwise the tumour will grow back."

Dr Parada's work, which was based on studying brain tumours in mice, found that there was a sub-set of tumours cells that grow more slowly than other tumour cells but which allow the tumour continually to replenish itself following treatment with anti-cancer drugs.