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Is Cancer the Next of Kin to the Developing Foetus?

Foetus

Mutations in the PAX genes lead to developmental abnormalities of organs and tissues in which they are switched on.

By Mike Eccles

A gene that is important for the development of the foetus may hold new clues to how cancer cell division gets out of control, and guide the identification of new targets for cancer therapy.

Genes that are critically involved in the development of the foetus can sometimes go out of control, and when that happens they may become associated with cancer. While this idea is not new, it has been borne out by a multitude of examples over the past few decades of cancer research. Understanding its significance may provide answers to stop many cancers in their tracks.

The PAX genes (PAX1 to PAX9) are an example of a small family of developmental genes that are active in cancer. These genes are important because of their essential role in the development of certain organs and tissues, such as the brain, in species as diverse as humans and fruitflies. Mutations in the PAX genes lead to developmental abnormalities of organs and tissues in which they are switched on.

In the early 1990s my group was one of the first to report that PAX genes are abnormally active in human cancer tissues. The significance of expression of PAX genes in cancer was not apparent at that time. Since then my lab has shown that PAX genes are important for cancer cell growth and survival. Despite this knowledge, understanding exactly how and why PAX genes are able to regulate cancer cell growth and survival has remained elusive until recently.

We have now found that PAX8 controls the process by which cancer cells undergo cell division, implying that PAX8 is required for the continuing growth of cancer cells. The cancer cell types in which PAX8 exerts its control are those where PAX8 is usually a prerequisite for the foetal development of the associated normal tissues, and includes ovarian cancer, brain cancer, kidney cancer and thyroid cancer.

Cancer cells frequently have specific changes that lead to the loss of control of cell division. A surprising finding from our research is that although cancer cells harbour many loss-of-control changes within them, with each change enhancing cancer cell growth, suppression of PAX8 activity in cancer is like performing the “force-quit” on your computer in order to shut down a wayward software program; it forces the cancer cells to exit the cell division cycle and undergo cell death.

This finding is significant because we believe it is the first example where the activity of a developmental control factor normally required for tissue patterning in the foetus is directly linked to the cell division cycle in cancer cells.

This was the thesis topic of PhD student Grace Li’s doctoral studies of PAX genes in cancer. She began her investigation by carrying out a survey of PAX gene activity in 27 human cancer cell lines. This survey showed that PAX8 was clearly the most consistently active PAX gene in several panels of kidney, ovarian and thyroid cancer cell lines.

However, the PAX8 activity in each cancer cell line was accompanied by the activity of two or three other PAX genes as well, and these were not the same PAX genes that were active each time. Furthermore, while PAX8 activity was high, the activity of the other PAX genes accompanying PAX8 was quite variable.

To evaluate the relative importance of the three or four active PAX genes in the cancer cells, Grace inhibited the activity of each of the PAX genes in each of the cancer cell lines in turn. It turned out that PAX8 gene activity was always important in the cancer cell lines, and it was only when PAX8 was inhibited that the cancer cells unambiguously stopped dividing and underwent cell death.

The next step for Grace was to determine how PAX8 activity was important in cancer cells. To investigate this question, Grace employed microarray technology in order to query the activities of more than 47,000 other genes in the cancer cells simultaneously, with or without PAX8 inhibition at the same time. Biological pathway analysis software then identified the RB1-E2F1 pathway, which regulates cell division. This explains why the cancer cells stopped dividing when PAX8 was inhibited.

Indeed, Grace found the E2F1 gene itself was an excellent candidate for direct regulation by PAX8 activity in cells. She showed that the PAX8 protein physically attached to the regulatory region of the E2F1 gene, and strongly promoted E2F1 activity in the cancer cells.

Another surprising finding in her research was that PAX8 also physically attached to the RB1 protein, and was directly responsible for maintaining the level of RB1 protein in cancer cells.

Therefore it turns out that PAX8 was intimately associated with two of the most important factors that regulate cancer cell division.

Based on these discoveries Grace devised a model of what we think happens in cancer cells where PAX8 is active. She used this model to predict that PAX8 would be necessary for cancer cell division and growth, and that PAX8 would link cell division in cancer cells to cell maturation.

In a second prediction, the model suggests that PAX8 should regulate whether cancer cells take on characteristics of high potency foetal-like cells capable of extraordinary cell expansion, or alternatively take on the characteristics of more terminal cells of lower potency and with somewhat less capacity for cellular expansion.

The first of these predictions of Grace’s model has just recently been verified in spectacular fashion. In a paper by researchers at Harvard University and Massachusetts Institute of Technology, which appeared soon after the publication of our work, PAX8 was identified as an important “lineage-survival” gene in ovarian cancer. Of the 11,194 genes they investigated to find genes that were essential for ovarian cancer cell survival, they found that PAX8 was the lead hit.

These new data therefore strongly support the notion that PAX8 is central to the growth and survival pathways involved in at least kidney and ovarian cancer, and possibly other cancer types as well.

So where to now? An area of interest for my group is to identify novel therapeutic targets for the treatment of ovarian and kidney cancer. However, it is most likely that PAX8 is not in itself a good drug target in cancer because the PAX8 protein resides within the nucleus of cells, which is difficult to access using most present-day drugs.

However, PAX8 activity is generally thought to be controlled by specific growth factors and their pathways, so we are now working towards the identification of factors that could modify the activity of PAX8 in cancer cells.

In addition, we are interested in investigating the second prediction of Grace’s model. During embryogenesis, PAX8 is expressed in the developing thyroid gland among other tissues, as well as in the developing Mullerian duct, which gives rise to tissues of the female sex organs in adults.

What exactly does PAX8 do in the development of these tissues? Well, we know that PAX8 regulates some genes, but could it also be involved in cell division and cell maturation in these tissues?

These and other questions remain to be solved, and will undoubtedly lead to further fascinating discoveries.

Professor Mike Eccles is NZICRT Chair in Cancer Pathology in the Department of Pathology, Dunedin School of Medicine, the University of Otago.