Many researchers think that tumour growth is fuelled by a subset of 'immortal' cancer stem cells. When they multiply, they produce 'bulk' tumour cells, as well as more stem cells.
Cancer stem cells may explain why cancer can return in patients who first appear to have been successfully treated. Although treatments such as chemotherapy and radiotherapy are effective against bulk tumour cells, they don't appear to target cancer stem cells.
Cancer stem cells have been identified in a number of different types of cancer, but until now, they have not been discovered in melanoma.
The researchers analysed protein molecules on the surface of cells from melanoma samples that had been taken from patients at the Stanford Cancer Centre. They found that between 2.5 and 41 per cent of cells had a protein called CD271 on their surface.
The researchers then transplanted human melanoma cells into mice, some of which received cells with CD271 on their surface while others received cells which lacked CD271.
Cells with CD271 were much more likely to grow into tumours than cells without the protein, suggesting they may have stem cell-like properties.
All but one of the new tumours arising from CD271 cells contained a mixture of CD271-positive and CD271-negative melanoma cells.
This shows that the stem cells could produce bulk tumour cells as well as new stem cells - a classic hallmark of stem cell behaviour.
The team also found that melanoma stem cells lack proteins that are targeted by immunotherapy - a treatment that harnesses the patient's immune system to destroy cancer cells. This helps to explain why some melanomas don't respond to immunotherapy, and could shape the design of future therapies for the disease.
Commenting on the discovery, which is published in the journal Nature, lead researcher Dr Alexander Boiko, a post-doctoral fellow at Stanford, revealed: "These cells lack the traditional melanoma cell surface markers targeted by these treatments. Without wiping out the cells at the root of the cancer, the treatment will fail.
"This could be the reason why we often see melanoma patients relapsing and coming back to the clinic. Our research indicates that it may be more appropriate to also target cells expressing CD271."
Dr Kat Arney, science information manager at Cancer Research UK, said: "Researchers are finding cancer stem cells in many different types of tumour, and many scientists believe they are at the heart of a wide range of cancers.
"Understanding these elusive 'immortal' cells will be the key to developing more effective treatments for cancer in the future, so this research is an important step towards beating melanoma."
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