Research has shown that cancer cells are not all the same. Within a malignant tumor or among the circulating cancerous cells of a leukemia, there can be a variety of types of cells. The stem cell theory of cancer proposes that among all cancerous cells, a few act as stem cells that reproduce themselves and sustain the cancer, much like normal stem cells normally renew and sustain our organs and tissues. In this view, cancer cells that are not stem cells can cause problems, but they cannot sustain an attack on our bodies over the long term.
The idea that cancer is primarily driven by a smaller population of stem cells has important implications. For instance, many new anti-cancer therapies are evaluated based on their ability to shrink tumors, but if the therapies are not killing the cancer stem cells, the tumor will soon grow back (often with a vexing resistance to the previously used therapy). An analogy would be a weeding technique that is evaluated based on how low it can chop the weed stalks—but no matter how low the weeks are cut, if the roots aren’t taken out, the weeds will just grow back.
Another important implication is that it is the cancer stem cells that give rise to metastases (when cancer travels from one part of the body to another) and can also act as a reservoir of cancer cells that may cause a relapse after surgery, radiation or chemotherapy has eliminated all observable signs of a cancer.
One component of the cancer stem cell theory concerns how cancers arise. In order for a cell to become cancerous, it must undergo a significant number of essential changes in the DNA sequences that regulate the cell. Conventional cancer theory is that any cell in the body can undergo these changes and become a cancerous outlaw. But researchers at the Ludwig Center observe that our normal stem cells are the only cells that reproduce themselves and are therefore around long enough to accumulate all the necessary changes to produce cancer. The theory, therefore, is that cancer stem cells arise out of normal stem cells or the precursor cells that normal stem cells produce.
Thus, another important implication of the cancer stem cell theory is that cancer stem cells are closely related to normal stem cells and will share many of the behaviors and features of those normal stem cells. The other cancer cells produced by cancer stem cells should follow many of the rules observed by daughter cells in normal tissues. Some researchers say that cancerous cells are like a caricature of normal cells: they display many of the same features as normal tissues, but in a distorted way. If this is true, then we can use what we know about normal stem cells to identify and attack cancer stem cells and the malignant cells they produce. One recent success illustrating this approach is research on anti-CD47 therapy.