Cancer stem cell

A cancer stem cell is a hypothetical type of stem cell which would form tumours while having stem cell properties such as self-renewal and the ability to differentiate into multiple cell types. A theory suggests such cells persist in tumors as a distinct population and cause relapse and metastasis by giving rise to new tumours. Development of specific therapies targeted at cancer stem cells holds hope for improvement of survival and quality of life of cancer patients, especially for sufferers of metastatic disease.

Existing cancer treatments were mostly developed on animal models, where therapies able to promote tumor shrinkage were deemed effective. However, animals could not provide a complete model of human disease. In particular, in mice, whose life spans do not exceed two years, tumor relapse is exceptionally difficult to study.

The efficacy of cancer treatments are, in the initial stages of testing, often measured by the amount of tumour mass they kill off. As cancer stem cells would form a very small proportion of the tumour, this may not necessarily select for drugs that act specifically on the stem cells. The theory suggests that conventional chemotherapies kill differentiated or differentiating cells, which form the bulk of the tumor but are unable to generate new cells. A population of cancer stem cells, which gave rise to it, could remain untouched and cause a relapse of the disease.

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Evidence for cancer stem cells

Opponents of the paradigm do not deny the existence of cancer stem cells as such. Cancer cells must be capable of continuous proliferation and self-renewal in order to retain the many mutations required for carcinogenesis, and to sustain the growth of a tumor since differentiated cells cannot divide indefinitely (constrained by the Hayflick Limit). However, it is debated whether such cells represent a minority. If most cells of the tumor are endowed with stem cell properties there is no incentive to focusing on a specific subpopulation. There is also debate on the cell of origin of these cancer stem cells - whether they originate from stem cells that have lost the ability to regulate proliferation, or from more differentiated population of progenitor cells that have acquired abilities to self-renew (which is related to the issue of stem cell plasticity).

The first conclusive evidence for cancer stem cells was published in 1997 in Nature Medicine. Bonnet and Dick^[1] isolated a subpopulation of leukaemic cells that express a specific surface marker CD34, but lacks the CD138 marker. The authors established that the CD34⁺/CD138^- subpopulation is capable of initiating tumors in NOD/SCID mice that is histologically similar to the donor. (Matsui, 2004)

In cancer research experiments, tumor cells are sometimes injected into an experimental animal to establish a tumor. Disease progression is then followed in time and novel drugs can be tested for their ability to inhibit it. However, efficient tumor formation requires thousands or tens of thousands of cells to be introduced. Classically, this has been explained by poor methodology (i.e. the tumor cells lose their viability during transfer) or the critical importance of the microenvironment, the particular biochemical surroundings of the injected cells. Supporters of the cancer stem cell paradigm argue that only a small fraction of the injected cells, the cancer stem cells, have the potential to generate a tumor. In human acute myeloid leukemia the frequency of these cells is less than 1 in 10,000.^[1]

Further evidence comes from histology, the study of tissue structure of tumors. Many tumors are very heterogeneous and contain multiple cell types native to the host organ. Heterogeneity is commonly retained by tumor metastases. This implies that the cell that produced them had the capacity to generate multiple cell types. In other words, it possessed multidifferentiative potential, a classical hallmark of stem cells.^[1]

The existence of leukaemic stem cells prompted further research into other types of cancer. Cancer stem cells have recently been identified in several solid tumours, including breast cancer,^[2] brain cancer^[3] and colon cancers.^[4]

Mechanistic and mathematical models

Once the pathways to cancer are hypothesized, it is possible to develop predictive mathematical biology models,^[5] e.g., based on the cell compartment method. For instance, the growths of the abnormal cells from their normal counterparts can be denoted with specific mutation probabilities. Such a model has been employed to predict that repeated insult to mature cells increases the formation of abnormal progeny, and hence the risk of cancer.^[6] Considerable work needs to be done, however, before the clinical efficacy of such models is established.

Where do cancer stem cells come from?

This is still an area of ongoing research. Logically, the smallest change (and hence the most likely mutation) to produce a cancer stem cell would be a mutation from a normal stem cell. Also, in tissues with a high rate of cell turnover (such as the skin or GI epithelium, where cancers are common), it can be argued that stem cells are the only cells that live long enough to acquire enough genetic abnormalities to become cancerous. However it is still possible that more differentiated cancer cells (in which the genome is less stable) could acquire properties of 'stemness'.

It is likely that in a tumour there are several lines of stem cells, with new ones being created and others dying off as a tumour grows and adapts to its surroundings.^[7] Hence, tumour stem cells can constitute a 'moving target', making them even harder to treat.

Implications on Cancer Treatment

The existence of cancer stem cells have several implications in terms of future cancer treatment and therapies. These include disease identification, selective drug targets, prevention of metastasis, and development of new strategies in fighting cancer.

Normal somatic stem cells are naturally resistant to chemotherapeutic agents - they have various pumps (such as MDR) that pump out drugs, DNA repair proteins and they also have a slow rate of cell turnover (chemotherapeutic agents naturally target rapidly replicating cells). Cancer stem cells, being the mutated counterparts of normal stem cells, may also have similar functions which allows them to survive therapy. These surviving cancer stem cells then repopulates the tumour, causing relapse. By selectively targeting cancer stem cells, it would be possible to treat patients with aggressive, non-resectable tumours, as well as preventing the tumour from metastasizing. The hypothesis infers that if the cancer stem cells are eliminated, the cancer would simply regress due to differentiation and cell death.

There has also been a lot of research into finding specific markers which may distinguish cancer stem cells from the bulk of the tumour (as well as from normal stem cells), with some success.^[8] Proteomic and genomic signatures of tumours are also being investigated. Success in these area would enable faster identification of tumour subtypes, as well as enable personalized medicine in cancer treatments by using the right combination of drugs and/or treatments to efficiently eliminate the tumour.

Cancer stem cell pathways

A normal stem cell may be transformed into a cancer stem cell through disregulation of the proliferation and differentiation pathways controlling it. Scientists working on cancer stem cells hope to design new drugs targeting these cellular mechanisms. The first findings in this area were made using haematopoietic stem cells (HSCs) and their transformed counterparts in leukemia, the disease whose stem cell origin is most strongly established. However, these pathways appear to be shared by stem cells of all organs.

Bmi-1

The Polycomb group transcriptional repressor Bmi-1 was discovered as a common oncogene activated in lymphoma^[9] and later shown to specifically regulate HSCs.^[10] The role of Bmi-1 has also been illustrated in neural stem cells.^[11] The pathway appears to be active in cancer stem cells of pediatric brain tumors.^[12]

Notch

The Notch pathway has been known to developmental biologists for decades. Its role in control of stem cell proliferation has now been demonstrated for several cell types including haematopoietic, neural and mammary^[13] stem cells. Components of the Notch pathway have been proposed to act as oncogenes in mammary^[14] and other tumors.

Sonic hedgehog and Wnt

These developmental pathways are also strongly implicated as stem cell regulators.^[15] Both Sonic hedgehog(SHH) and Wnt pathways are commonly hyperactivated in tumors and are required to sustain tumor growth. However, the Gli transcription factors that are regulated by SHH take their name from gliomas, where they are commonly expressed at high levels. A degree of crosstalk exists between the two pathways and their activation commonly goes hand-in-hand.^[16] This is a trend rather than a rule. For instance, in colon cancer hedgehog signalling appears to antagonise Wnt.^[17]

Sonic hedgehog blockers are available, such as cyclopamine. There is also a new water soluble cyclopamine that may be more effective in cancer treatment. There is also DMAPT, a water soluble derivative of parthenolide that targets AML (leukemia) stem cells, and possibly other cancer stem cells as in myeloma or prostate cancer. A clinical trial of DMAPT is to start in England in late 2007 or 2008. Furthermore, GRN163L was recently started in trials to target myeloma stem cells. If it is possible to eliminate the cancer stem cell, than a potential cure may be achieved if there are no more cancer stem cells to repopulate a cancer.

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