September 16, 2010
Yeast Studies Hint at New Ways to Exploit Cancer Cells’ Excess Genes
Genetic mutations that supercharge a cellular garbage disposal may explain why cancer cells can thrive even as their genetic material multiplies out of control, suggests new research by Howard Hughes Medical Institute investigator Angelika Amon. Though performed in yeast cells, the work may one day point to new strategies for finding novel cancer drugs.
“We hope in the long run that the genes we find mutated in our yeast cells could very well be the same key genes responsible for the survival of cancer cells in humans,” says Amon, who is at the Massachusetts Institute of Technology.
“At the cellular level, with cancer cells, having crazy numbers of chromosomes seems to be associated with this unrestricted growth and proliferation ”
The research, which was published online in the journal Cell on September 16, 2010, helps resolve a long-standing conundrum in the field of cancer biology. In some 90 percent of human cancers, individual tumor cells contain too many chromosomes, a condition called aneuploidy. Instead of the normal complement of 46 human chromosomes, cancer cells often bulge with 60, 100, or more. Three years ago, Amon and her team published a paper in Science describing the devastating impact of these extra chromosomes in otherwise normal cells: The cell produces too many proteins. “The extra proteins start clogging up various cellular systems, which causes stress for the cells. They get sick,” says Amon.
And yet, with cancer cells, aneuploidy seems to invigorate growth rather than impede it – just the opposite of what would be expected based on Amon’s previous studies. Cells proliferate rapidly, and this supercharged growth ultimately turns fatal.
“At the level of the whole organism, having a single extra chromosome is highly detrimental,” says Amon, pointing out that the best-known human condition caused by aneuploidy is Down syndrome. In Down syndrome, individuals carry three copies of chromosome 21 instead of the usual two copies. Most other aneuploidy conditions almost invariably cause spontaneous abortions early during pregnancy.
“We know aneuploidy causes a lot of problems,” says Amon. “But at the cellular level, with cancer cells, having crazy numbers of chromosomes seems to be associated with this unrestricted growth and proliferation. The question we asked is, ‘How is this possible?’”
To answer that question, Amon and her colleagues first developed a technique to grow budding yeast cells of the species Saccharomyces cerevisiae that contained extra chromosomes. The team then performed an experiment in evolution: They let the colonies of aneuploid yeast cells grow unperturbed. “For a long time, they grow slowly, but eventually some begin growing more quickly,” Amon says. “The quick-growing strain then takes over the culture and continues to grow.”
Amon and her colleagues focused on these thriving aneuploid strains, reasoning that they had acquired random genetic mutations that conferred some advantage. Using new sequencing technologies that rapidly read out the entire genome of yeast cells, Amon and her team identified 43 genetic mutations in the fast-growing strains that were not seen in normal yeast cells.
Most of these mutations appeared in only one strain, meaning that the mutation only conferred an advantage to cells with one particular type of aneuploidy. But a few of the mutations appeared across several strains of aneuploid yeast, suggesting they conferred a more general survival advantage. “Immediately we thought that these mutations must help cells deal with being in an aneuploid state,” Amon says.
Amon quickly zoomed in on a mutation that appeared in 4 of 12 fast-growing aneuploid strains. The mutation caught her attention because it occurred in a gene that helps cells deal with excess, misshapen, or otherwise wayward proteins – something that might be very beneficial for a cancer cell. In particular, the gene, called UBP6, removes a tag that otherwise marks a protein for destruction. This tag, called ubiquitin, gets added by the cell to unwanted proteins, which then get trafficked into the cellular garbage disposal – a structure called the proteasome. But at the opening of the proteasome, the enzyme made by the UBP6 gene will sometimes de-tag a protein, rescuing it from destruction and slowing down protein disposal in the cell.
In the fast-growing aneuploid yeast cells, though, a mutation in UBP6 rendered the enzyme useless. Proteins were not de-tagged, and the proteasome revved up, chopping up proteins faster than usual. “The mutation accelerates the degradation of proteins, making the garbage disposal more active,” Amon says.
Working with Harvard cell biologist Steven Gygi, Amon then quantified the amount of protein made by three types of yeast cells: Normal cells; cells with an extra chromosome; and cells with an extra chromosome and a mutation that de-activated UBP6. She found that aneuploid yeast cells do indeed make more proteins than normal. But aneuploid yeast cells with the UBP6 mutation had a protein composition that was more similar to normal cells again Amon concluded that the UBP6 mutation helps correct the protein imbalance created by extra chromosomes. Correcting this imbalance appears to help some aneuploid yeast cells proliferate more quickly - an important trait they share with human cancer cells.
And, curiously, an anti-cancer drug already on the market acts on the very pathway Amon’s work uncovered. Velcade (bortezomib) was approved by the Food and Drug Administration in 2003 to fight the blood cancer multiple myeloma and is being tested against other cancers. It works by interfering with the cellular garbage disposal, the proteasome.
“Our data raise the interesting possibility that cancer cells rely on the proteasome for survival because it helps the cells deal with the adverse effects of aneuploidy,” says Amon. “So if you inactivate the proteasome, aneuploid cells are probably much more likely to die than regular cells, which is probably how Velcade works. Cancer cells really need the proteasome to be working hard.”
Amon is now trying to replicate her results in aneuploid mouse cells; she is also searching for compounds that selectively kill aneuploid cells while leaving normal cells undamaged. “If we can find such a compound, it would be an incredible new direction for our research,” she says.