by ecancer reporter Clare Sansom

Tumours arise through an evolutionary process involving the multiplication of somatic cells that bear mutations in or epigenetic changes to their DNA.

Until recently, it was thought that this process was a linear one involving successive “driver” mutations that each increased the cells’ ability to divide and resist apoptosis.

However, data from cancer genome projects now suggests that tumours contain cell populations or “sub-clones” bearing different driver mutations, contradicting this linear model.

Tumour progression can depend on mutations in sub-clones that both drive the proliferation of those cells and affect the micro-environment and so the growth of cells throughout the tumour.

The former process is termed cell-autonomous tumour proliferation and the latter one non-cell-autonomous proliferation.

Our understanding of sub-clones within tumours and the processes that drive their growth has been hindered by a lack of effective experimental models.

KorneliaPolyak and her colleagues from the Dana-Farber Cancer Institute, Boston, Massachusetts, USA have developed a mouse model of indolent tumours to investigate the effect of specific sub-clones on tumour progression.

They selected a human breast cancer cell line, MDA-MB-468, which forms small tumours with very low growth rates when injected into the mammary fat pads of immunodeficient mice.

These tumours contain a high proportion of actively proliferating cells, but this proliferation is balanced by cell death through both apoptosis and necrosis.

Polyak and her co-workers used this cell line to generate a panel of 18 sub-lines or sub-clones, each of which was characterised by the over-expression of a single secreted factor that was already implicated in tumour progression and known to be over-expressed in breast tumours.

The researchers set up two types of experiments, injecting mice either with cells from a single sub-clone and a larger number from the parenteral cell line, or with cells from all 18 sub-clones, and evaluating the growth and metastasis of the resulting tumours.

The tumours formed from cells including only a single sub-clone varied in morphology and proliferation but none were metastatic.

Only two of the sub-clones, those secreting chemokine (C-C motif) ligand 5 (CCL5) and interleukin 11 (IL-11), produced tumours that grew to a significant extent.

Interestingly, the number of cells expressing CCL5 and IL-11 increased less than that of cells expressing lysyl oxidase homolog 3 (LOXL3), although LOXL3 expression did not drive overall tumour growth.

Interleukin 11 alone was capable of driving significant tumour growth in a non-cell-autonomous fashion: that is, expanding the populations of both the IL-8-expressing cells and the parenteral tumour cells.

This suggests that IL-11 expression is sufficient to drive tumour proliferation.

Tumours that contained populations of each of the 18 sub-clones grew more quickly than any of those that just contained a single sub-clone.

These tumours also displayed leakage from blood and lymphatic vessels, and a high proportion of them were metastatic.

Tumours derived from a sub-clone expressing c-fos-induced growth factor (FIGF) also displayed increased vascular leakage.

The researchers therefore injected mice with a combination of IL-11 and FIGF expressing tumour cells, and found that the resulting tumours displayed this phenotype; they were also fast growing and metastatic.

This suggests that the presence of several interacting sub-clones within a tumour can produce a phenotype that is different from either alone.

Polyak and her co-workers then investigated the mechanism through which interleukin 11 drives tumour growth.

They showed that IL-11 could stimulate tumour proliferation independently of the concentration of its specific receptor, IL1Rα, and that IL-11-driven tumours had significant differences from others in their vasculature and intracellular matrix; this suggested that the increase in growth might be attributable to changes in the micro-environment.

The researchers developed a mathematical model for the interactions between sub-populations of tumour cells and their effect on tumour growth.

A presumed positive effect of the IL-11 secreting clone on the growth rates of all other clones was included in the model.

In the absence of IL-11, the fastest growing clone was predicted to out-compete all other clones, leading to a loss of tumour heterogeneity; in contrast, IL-11 was predicted to stimulate the growth of other clones in a non-cell-autonomous way and thus to maintain tumour heterogeneity.

Tumour heterogeneity was also reduced in tumours that were treated with doxorubicin, a chemotherapeutic agent that is commonly used to treat breast cancer.

These findings prompted a more detailed analysis of IL-11/LOXL3 tumours, which revealed that the faster-growing LOXL3 sub-clone could out-compete the IL-11 sub-clone, leading to a loss of IL-11 secreting cells that could eventually lead to tumour collapse probably through changes to the micro-environment.

Taken together, these results suggest that the most abundant sub-clone within a tumour may not be the one that drives its proliferation, and, therefore, that focusing diagnostics and clinical decision making on the properties of these cells may be misleading.

Reference

​Marusyk, A., Tabassum, D.P., Altrock, P.M., Almendro, A., Michor, F. and Polyak, K. (2014). Non-cell-autonomous driving of tumour growthsupports sub-clonal heterogeneity. Nature, published online ahead of print 30 July 2014.