Breast cancer: An elitist sport

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Researchers have found that a subset of cells that comprise just a small proportion of a tumor are actually able to initiate tumorigenesis.

Is tumour formation a 'first-past-the-post' situation, in which all cells have an equal ability to become tumorigenic and cells race to see which accumulate the correct set of mutations first, or is it more elitist? Perhaps only a few cells possess this extraordinary ability. Michael Clarke and colleagues, reporting in Proceedings of the National Academy of Sciences, have found that a subset of cells that comprise just a small proportion of a tumour are actually able to initiate tumorigenesis.

Human breast cancer cells were isolated from nine patients — one from the primary tumour and the others from metastatic pleural effusions — and were used fresh, from frozen samples, or following passage through mice. These cells were heterogeneous with respect to cell-surface markers, so the authors investigated whether the markers could distinguish between tumorigenic and non-tumorigenic cells. The cells were separated by flow cytometry according to whether they were positive or negative for the adhesion markers CD44 and CD24, and 200,000–800,000 cells were injected into the mammary fat pads of mice. After 12 weeks, tumours had formed at the site of injection in all mice with CD44⁺ and CD24^-/low cells, but in no mice with CD44^- and CD24⁺ cells (although 2 of 12 mice injected with CD24⁺ cells contained small growths, possibly due to contamination with a different cell type).

Cancer cells were found not to express lineage markers, so the elimination of lineage⁺ cells removed normal human cells such as leukocytes and fibroblasts, leaving only lineage^- cancer cells. Of these, 11–35% (depending on tumour type) are CD44⁺CD24^-/low. Interestingly, fewer of these cells need to be injected to generate tumours — they seem to have a 10–50-fold enrichment in tumorigenic activity. But can this cell population be further defined in terms of this activity?

The ESA marker has previously been used to distinguish epithelial and mesothelial cells, and the isolation of ESA⁺CD44⁺CD24^-/lowlineage^- cells from three tumours led to further enrichment of tumorigenic ability — as few as 200 cells were able to generate tumours after 4–6 months, compared with the 2,000 CD44⁺CD24^-/lowlineage^- cells or 20,000 unsorted tumour cells that were needed to establish tumours.

The difference in ability was not due to changes in the cell cycle, as the ESA⁺CD44⁺CD24^-/lowlineage^- cells had the same cell-cycle distribution as the other lineage^- cells. These cell populations were also morphologically and histologically indistinguishable when assessed before injection, but when the injection sites were examined 6 months after injection, those with CD44⁺CD24^-/lowlineage^- cells contain palpable tumours and malignant cells, and those with CD44⁺CD24⁺lineage^- cells contain no tumours and only normal mouse epithelial cells.

Tumours normally contain a heterogeneous mix of cells, so are the CD44⁺CD24^-/lowlineage^- cells similar to stem cells in their ability to give rise to diverse cell types? Either 200 ESA⁺CD44⁺CD24^-/lowlineage^- cells or 1,000 CD44⁺CD24^-/lowlineage^- cells were injected into mice, and the tumours that formed were analysed by flow cytometry. Both cell types were able to generate heterogeneous expression patterns of the ESA, CD44 and CD24 markers, which resembled the tumours from which they were orginally derived. So, the tumorigenic cells were able to generate both tumorigenic CD44⁺CD24^-/lowlineage^- cells and diverse non-tumorigenic cells.

Clarke and colleagues have therefore identified a subpopulation of tumorigenic breast cancer cells that resemble stem cells in their ability to proliferate and give rise to diverse cell types. This discovery is important for both basic and clinical research, as the molecular pathways that are involved in breast cancer development will be easier to characterize and therapeutics could be developed that target proteins specifically expressed by this cell population.

Emma Greenwood

References

Al-Hajj, M., Wicha, M. S., Benito-Hernandez, A., Morrison, S. J. & Clarke, M. F. Prospective identification of tumorigenic breast cancer cells. Proc. Natl Acad. Sci. USA 100, 3983–3988 (2003) | Article | PubMed |
Dick, J. E. Breast cancer stem cells revealed. Proc. Natl Acad. Sci. USA 100, 3547–3549 (2003) | Article | PubMed |

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