Scientists have discovered that cancers are fueled by small populations of cancer stem cells. These cells are resistant to current therapies and are thought to drive cancer relapse and metastasis, which are the main cause of death in cancer patients. The exciting findings published this week in the journals Nature and Science could lead to revolutionary new strategies for cancer treatment.

Cancer is the second cause of death in the US and Europe, and despite an increase in cancer survival in some cancers due to prevention and early diagnosis, the survival rate for patients with cancers in advanced stages has not changed significantly in the past decades.

After a tumor is removed surgically or by chemo and radiotherapy, it often grows back (relapse) and spreads to other parts of the body (metastasis). Scientists have believed for many years that a small population of cancerous stem cells is resistant to therapy and responsible for tumor growth, including during relapse and metastasis- this is called 'cancer stem cell hypothesis'.

During the past 15 years, several research groups have described cancer stem cells in many types of cancer, and transplantation experiments, in which cells from biopsies of cancer patients are injected into mice, have shown that such cells could generate new tumors. However, these studies did not provide direct evidence for the existence of cancer stem cells. "This manipulation of tumors could potentially bring pitfalls and stronger evidence from unperturbed tumors were needed," said Dr. Driessens from the Université Libre de Bruxelles in Belgium.

Now researchers from three independent groups were able to 'see' cancer stem cells labeled with fluorescent markers promoting tumor growth in the brain, skin and digestive system of mice. "Our finding confirms that cancer stem cells really exist as it was suggested but not formally proven so far by grafting experiments," said Dr. Driessens, leading author in the study that identified cancer stem cells in skin.

Cancer stem cells consist of only about 1-3% of all cells in a tumor. So why is their discovery so important? Cancer stem cells could be the source of the most aggressive cancers with a poor prognostic. "This the first time researchers have traced the cell of origin within different tumors. Because cancers are proving to be so complex, we don't yet know how relevant this research in mice is to humans, but it gives us new insights into how cancers might develop and why they can sometimes grow back after therapy." explains Dr Michaela Frye, a Cancer Research UK scientist based at the University of Cambridge (UK).

"Anticancer treatments should not only be evaluated on their efficacy on the bulk tumor but also specifically for the effect on cancer stem cells, since these cells could be more resistant to chemo and radiotherapies," adds Dr. Driessens. These discoveries therefore open the way for the development of new therapies targeting cancer stem cells, which could revolutionize the treatment of cancer.