A gene that has for decades been considered a tumor promoter, the PLK1 gene, can also perform the exact opposite function: halting the development of cancer. This finding is published in the journal Nature Communications. The role of PLK1 as a target for powerful drugs must now be reviewed since, depending on the type of tumor to be treated, it might be useful to inhibit it, or it might not. For the time being, the scientists have discovered that the expression of PLK1 in breast tumors can determine a different prognosis, depending on the tumor sub-type.
The PLK1 gene is essential for the division and proliferation of tumor cells. It has been known for years the overexpression of PLK1 is found in a large variety of tumor types, and on occasions this overexpression is associated with poor prognosis. For that reason, PLK1 has for decades been considered an oncogene, a gene that promotes the development of tumors. Plk1 is also a therapeutic target, since inhibiting its activity induces the death of cell tumors. In fact, there are Plk1 inhibitors already at advanced clinical stages.
However, curiously, the oncogenic nature of PLK1 has never been formally demonstrated. In other words, until now, no experiment has been designed to demonstrate that the overexpression of PLK1 does indeed contribute to tumor development.
That was the initial aim of researchers at the start of this joint research project. To this end, they modified the genome of a mouse so that it was possible to overexpress the PLK1 gene at will in these animals.
The first thing they noted was that these animals did not develop any more tumors than the normal mice. They then crossed their mice with others that expressed the oncogenes H-Ras or Her2 in their breast tissue, and hence they developed very aggressive breast tumors. They expected a much greater incidence of cancer, but the result was unexpected: by overexpressing PLK1 together with the oncogenes, the incidence of tumors was reduced drastically.
"That was when we realized that something important was happening," explained one of the lead researchers on this study. "And in effect we have shown that not only does PLK1 not act as an oncogene, but surprisingly it acts as a tumor suppressor".
Intrigued, the researchers consulted the breast cancer databases, in search of a link between the expression of PLK1 and patient prognosis. And they confirmed that "the expression of Plk1 can result in a very different type of prognosis depending on the tumour sub-type", said the author.
In Her2 positive tumors, the expression of PLK1 gives a better prognosis; however, in patients with positive tumors for the expression of the oestrogen receptor (ER+), it's the complete opposite. Not only does this paper describe the novel action of PLK1 as a tumor suppressor, it also identifies the molecular mechanism of how this suppression occurs. "We have seen that the overexpression of PLK1 increases the number of chromosomes in cells, because after they divide, cells cannot correctly segregate their chromosomes", said the lead researcher.
The fact that Plk1 acts as a tumor suppressor could call into question therapeutic strategies based on the inhibition of Plk1. However, the coordinator of the paper, trusts that the inhibition of Plk1 is still a valid and useful option.
"The fact that Plk1 is a tumor suppressor instead of an oncogene does not mean that Plk1 inhibitors will not be effective against cancer", said the author. "Many essential components in cell proliferation can be used as targets against cancer in spite of not having any oncogenic activity, owing to the addition of cancer cells to specific cell processes such as cell division".
The work of these researchers bestows value on the PLK1 gene as an oncological biomarker: "Understanding when PLK1 acts as an oncogene or as a tumor suppressor, and in which types of tumors this happens, is clinically extremely relevant when it comes to using this gene as a therapeutic biomarker", stresses the author.
Oncogene is now a tumor suppressor!
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