'Gang of 4' genes drives spread of breast cancer 11/4/07. By the Howard Hughes Medical Institute Studies of human tumour cells implanted in mice show that abnormal activation of four genes drives the spread of breast cancer to the lungs.

Howard Hughes Medical Institute researchers have found that four aberrant genes work together to promote the growth of primary breast tumours. Cooperation among the four genes also enables cancerous cells to escape into the bloodstream and penetrate through blood vessels into lung tissues.

Although shutting off these genes individually can slow cancer growth and metastasis, the researchers found that turning off all four together had a far more dramatic effect on halting cancer growth and metastasis.

Metastasis occurs when cells from a primary tumour break off and invade another organ. It is the deadliest transformation that a cancer can undergo, and therefore researchers have been looking for specific genes that propel metastasis.

Background: Cancer as a genetic disease

The researchers, led by Joan Massagué at the Memorial Sloan-Kettering Cancer Center, also found that they could reduce the growth and spread of human breast tumors in mice by simultaneously targeting two of the proteins produced by these genes, using drugs already on the market. The researchers are exploring clinical testing of combination therapy with the drugs – cetuximab (trade name Erbitux) and celecoxib (Celebrex) – to treat breast cancer metastasis.

In an earlier study, Massagué and colleagues had identified 18 genes whose abnormal activity is associated with breast cancer’s ability to spread to the lungs. In the new study published in Nature, they focused on four of these genes. These genes, which code for proteins called epiregulin, COX2, and matrix metalloproteinases 1 and 2, were already known to help regulate growth and remodeling of blood vessels.

The researchers silenced various combinations of the four genes in human breast cancer cells that had metastasised to the lung, and then tested these cells in mice. To silence the four genes, the scientists used a technique called RNA interference, in which RNA molecules are tailored to suppress expression of target genes.

"We found that depriving aggressive metastatic tumour cells of these genes decreased both their ability to grow large aggressive tumours in the mouse mammary gland and also the ability to release cells from these tumours into the circulation," said Massagué. "The remarkable thing was that while silencing these genes individually was effective, silencing the quartet nearly completely eliminated tumour growth and spread."

Microscopic analysis of blood vessel structure in the tumours revealed that knocking down all four genes greatly reduced growth of the tangle of blood vessels typically seen in tumours. Further experiments revealed that the tumour blood vessels that did form allowed fewer cancer cells to escape into circulation.

The researchers next explored how loss of the four abnormal genes affected the metastatic capability of the cells in the lung. They injected cells deficient in the four genes directly into the circulatory system of mice. "When these cells reached the lung capillaries, they just got stuck there," said Massagué. "We concluded that metastatic cells use these same genes to loosen up cells in capillaries, so that the cells can penetrate the lung tissue to grow there.

Two drugs already on the market act directly on proteins produced by the genes Massagué’s group had been studying. Cetuximab is an antibody that blocks the action of epiregulin and is used to treat advanced colorectal cancer. Celecoxib is an inhibitor of COX2 that is used as an anti-inflammatory, and is being tested in clinical trials against many types of cancer. The researchers also tested whether cetuximab and celecoxib would work effectively in concert to reduce metastasis in mice.

"We found that the combination of these two inhibitory drugs was effective, even though the drugs individually were not very effective," said Massagué. "This really nailed the case that if we can inactivate these genes in concert, it will affect metastasis," he said.

Massagué said that while clinical trials of the drug combination are being discussed, "there are already treatments to diminish the chance of metastasis in breast cancer, so such trials would have to be designed very carefully to understand how and whether the new drug combination would be of additional benefit."

Adapted from a news release by the Howard Hughes Medical Institute.

Image: Breast cancer cells, courtesy of Annie Cavanagh

Further reading

Gupta GP et al. Mediators of vascular remodelling co-opted for sequential steps in lung metastasis. Nature 2007;446(7137):765-70. Abstract