Stem Cell Elimination

DEFINITION

There are at least two theories that try to explain tumour formation and growth. The first is the stochastic model, which assumes that all cancer cells have a low but equal tumour forming capacity; the second is the cancer stem cell or tumour initiating model. This theory suggests that there is a small population of cells within a tumour responsible for its generation and maintenance, and that these cells are also capable of forming new tumours. All other cells in the tumour lack this capacity. Hence there is a cellular hierarchy within a tumour, with all cells ultimately derived from this small population of cancer stem cells. In recent years, more and more publications have provided evidence in support of the cancer stem cell/tumour initiating cell hypothesis, and this report will summarise some of the literature and techniques used in the analysis of cancer stem cells. From here on, this model will be referred to as the cancer stem cell model, but this is interchangeable with the tumour initiating cell model.

The term ‘cancer stem cell’ and its definition – “a cancer stem cell is a cell within a tumour that possesses the capacity to selfrenew and to cause the heterogeneous lineages of cancer cells that comprise the tumour” – were determined by a group of non-stem cell and stem cell scientists at a scientific meeting in 2006 (1). This term describes the function of these cells well, but unintentionally causes some confusion, as it suggests that a cancer stem cell is derived from a stem cell (2). Rather, a cancer stem cell is any cell which has acquired the characteristics of a stem cell within a tumour and meets the criteria in the definition above. WHY STUDY CANCER STEM CELLS? It is acknowledged that while existing cancer therapies reduce the bulk of the rapidly proliferating and differentiating cells within a tumour, they may not have an effect on the cancer stem cells, which are possibly more dormant or resistant to treatment. The ability to reduce tumour mass, while allowing these cells to survive, may explain why many tumours recur following treatment (see Figure 1). Identifying agents that can remove the cancer stem cells from a tumour may provide an effective strategy in the treatment of cancer, and such agents may be curative rather than palliative.