Photo by Todd McNaught
A drop of Lake Union water seems an unlikely source of clues to the workings of a cancer cell.
Yet a genetic trick first discovered in an obscure, pond-dwelling microbe could yield insight into many aggressive tumors. In fact, say researchers from the Basic Sciences and Human Biology divisions, it may even form the basis for new anti-cancer therapies.
A study led by postdoctoral fellow Dr. Hisashi Tanaka has revealed that seemingly unrelated cell types may share a pathway for a process called gene amplification, essentially a "more is better" strategy in which chromosomes are reengineered to contain dozens or even hundreds of copies of genes that normally exist as single entities.
For tumors, the extra gene copies provide malignant cells with a way to outpace growth of their healthy neighbors or to evade killing by chemotherapy. For the microscopic pond creature, Tetrahymena, gene amplification allows the organism to cycle through its normal developmental stages.
The new understanding of how gene amplification may be triggered in human cells could allow researchers inhibit the process, which spells a poor prognosis for many cancers. It also could lead to techniques for surveying human chromosomes to predict where such potentially harmful rearrangements might occur.
The findings, which appeared June 25 in the Proceedings of the National Academy of Sciences, come from the labs of Dr. Stephen Tapscott of the Human Biology Division and Dr. Meng-Chao Yao in Basic Sciences. Tanaka is a Dual Mentor Program fellow working with Tapscott and Yao. Dr. Barbara Trask, director of Human Biology, was a co-author.
Gene amplification is one of many biological tricks organisms use to ramp up the expression of genes whose products are needed in excess. Some organisms, such as Tetrahymena, do this in a controlled fashion as part of their normal course of development, when rapid growth of some types of cells is required.
Cancer cells, though, corrupt the process to their advantage, Tapscott said. Such deregulated genetic upheaval is one aspect of what is known as genome instability, a hallmark of cancer cells in which typically static and orderly chromosomes swell and shrink with destabilizing gaps, duplications and other rearrangements.
"It's long been recognized that cancer cells amplify parts of their genomes," he said. "Often this is one of the later events in the cancer process that gives a cell a survival or growth advantage. In cancers like neuroblastoma, for example, amplification of a gene called N-myc correlates with poor survival."
Clues in a microbe
Amplification of genes that are targets of anti-cancer drugs allows cancer cells to develop resistance to chemotherapy. A classic example, Tapscott said, is amplification of a human gene known as MDR, which stands for multiple drug resistance.
Despite this knowledge, it took studies on a curious single-celled organism to yield insight into how genes become amplified in human cells. Work on the process in Tetrahymena - a microbe that undertakes a variety of unusual genetic activities - was initiated more than 20 years ago, when Yao was a postdoctoral fellow at Yale University in New Haven, Conn.
"When I was studying gene amplification in Tetrahymena, it was really more of a curiosity," he said. "Gene amplification in frog eggs had also been discovered, but it was much easier to figure out the mechanism in a single-celled organism. Hisashi's work on this phenomenon in mammalian cells is a wonderful example of how research on unusual phenomena in model systems may link to problems in cancer."
In Tetrahymena and human cancer cells, amplified DNA takes the form of large gene segments arranged in head-to-head or tail-to-tail repeats, which, like phrases such as "madam I'm Adam," are called palindromes.
Yao discovered that in Tetrahymena, the trigger for gene amplification is a break in the DNA double helix that occurs next to a short, naturally occurring palindrome in the chromosome. Through a series of chromosomal acrobatics, extended palindromes that consist of up to hundreds of copies of a single gene can form, enabling cells to drive high-level output of a gene's product.
Five years ago, Tanaka, a gastrointestinal surgeon who was completing his doctoral degree in surgical oncology in Kyoto, Japan, contacted Yao about initiating a project to test whether human cells might use similar mechanics to amplify genes. His interest was sparked by his studies on human cancers.
"I realized genome instability is a common feature of many cancers, and I was curious about its mechanisms, which aren't clearly understood. I wanted to develop a system to study this, and Meng-Chao had worked out how it occurs in Tetrahymena."
Tanaka found that as in Tetrahymena, cultured mammalian cells that suffer breaks in the DNA double helix adjacent to short inverted repeats subsequently form the large palindromes characteristic of gene amplification.
Trask said Tanaka is the first to come up with a plausible mechanism for human gene amplification, a phenomenon first observed in chromosomes of cancer cells 20 years ago.
"Much of what we've known previously about human gene amplification is from cytogenetic studies (viewing chromosomes under a microscope)," she said. "Thanks to Hisashi's work, we now have molecular evidence to explain how such rearrangements might occur."
The team next hopes to examine the process in a variety of human tumors. Ultimately, it may be possible to develop agents that inhibit the earliest steps of gene amplification as a form of cancer therapy.
Yao noted that the human genome is filled with short, inverted repeats that could serve as "hot spots" for the initiation of gene amplification.
"If we can survey the genome for the locations of these inverted repeats, we might be able to predict which regions are at risk for amplification of cancer-causing genes," he said.
Tapscott said what intrigues him most is one of the more fundamental aspects of the research.
"What fascinates me is that this is a process that's been conserved from Tetrahymena to mammals," he said. "Presumably, it wasn't just conserved to create cancer. Rather, it seems likely that human cancer cells subvert a normal process that might play an important role in development."
How to address that mystery? It might be yet another opportunity for the humble Tetrahymena - or one of many model organisms under study - to show off its experimental prowess.
Having pair of mentors 'makes science more interesting'
The center's dual-mentor program provides opportunities for graduate students and postdocs to conduct interdisciplinary research under the guidance of faculty from two different areas of study.
Applicants must identify two mentors from different fields of research who will contribute meaningfully to the training program.
Dr. Hisashi Tanaka, a fellow working with Dr. Meng-Chao Yao from Basic Sciences and Dr. Stephen Tapscott from Human Biology, said that having two mentors with different approaches to research was essential to the success of his recently published work. Yao's expertise is in fundamental chromosome biology, while Tapscott, a practicing neurologist studying neural development and disorders, has extensive experience manipulating cultured human cells
"It's made science much more interesting to have two mentors with different views," Tanaka said.
For Yao and Tapscott, serving as mentors in the program has sparked research that each would have been unlikely or ill equipped to tackle on his own.