New findings in yeast may reveal why growing older is the greatest carcinogen in humans

Graduate student camped out in the lab, sleepless in Seattle, to collect the data

Scientists at Fred Hutchinson Cancer Research Center have made a landmark discovery in yeast that may hold the key to revealing why growing older is the greatest cancer-risk factor in humans. Their findings appear in the Sept. 26 issue of Science.

Senior author Daniel Gottschling, Ph.D., a member of Fred Hutchinson’s Basic Sciences Division, and first author Michael McMurray, a graduate student in Gottschling’s laboratory, have found striking similarities between humans and simple baker’s yeast with regard to the changes their genes undergo as they age.

“While yeast don’t get cancer, they do have one of the major hallmarks of malignancy, which is genetic instability,” Gottschling said. “We found a similar thing in yeast that has been seen in humans: genetic instability shoots up dramatically in the middle to late stage of life.”

When yeast cells hit the equivalent of late-middle age, the Fred Hutchinson researchers discovered they experience a sudden, 200-fold surge in the production of genetic changes typically manifested as loss of heterozygosity, or LOH, a condition characterized by missing or mutated chromosomes. This finding suggests that the yeast Saccharomyces cerevisiae, a simple, single-celled organism, may be an ideal model for understanding the complexities of age-related cancer development in humans.

“Yeast gives us, for the first time, the potential for not only understanding the principles of what’s going on mechanistically but also which molecules might be relevant to the process of age-related cancer development,” Gottschling said.

Aging indeed is a potent carcinogen. Consider these statistics from the American Cancer Society: Nearly 80 percent of cancers are diagnosed after age 55. After reaching late-middle age, men face a 50 percent chance of developing cancer and women have a 35 percent chance. No one knows why cancer typically surfaces later in life, although a multitude of scientific theories abound. “This finding may provide scientists with a new tool to test those theories,” Gottschling said.

To determine whether yeast could be used as a model to help explain the abrupt increase in human-cancer risk, the researchers tracked the life cycles of multiple yeast strains. Most yeast cells survive for about 30 or 35 generations of cell division. Each generation is represented by a mother cell’s production of a new daughter cell, or yeast bud. The yeast cells were genetically manipulated to turn color if they started showing genetic instability. In every strain of yeast studied, genetic mistakes started happening at the equivalent of late-middle age.

“In following the life history of the cells, we found it takes about 25 generations, or cell divisions, to see an LOH event,” Gottschling said. “After that, the genetic instability just starts happening like crazy. We think a switch of some kind is being thrown, because it’s happening in virtually all of the new offspring at the same time.”

Even among the longest-lived yeast that were genetically manipulated to go through 50 to 60 generations of cell division before dying, the evidence of DNA damage surfaced, like clockwork, right around the 25th generation. “This tells us that life span operates on its own clock; it is independent of genetic instability. Living longer doesn’t necessarily mean you have fewer genetic mistakes. It just means you somehow live longer with more of them,” Gottschling said.

As such, the researchers surmise that genetic instability isn’t related to how close cells are to death, but how far they are from birth – how many times they’ve divided.

The discovery that an age-dependent switch is somehow activated to trigger genomic instability could have major scientific consequences, Gottschling said. “This helps us to simplify. It gives us a place to focus to try and understand the causal event at the onset of cancer development.” If researchers can determine the molecular mechanics that trip the switch, they one day may be able to develop drugs or gene-replacement methods to prevent the switch from being thrown in the first place.

The researchers’ findings also may lead to a better understanding of the role of stem cells in cancer development, a subject of intense scientific interest. In tracking the life span of the mother-yeast cells, which are largely analogous to stem cells in humans, they found that the mothers retained their genetic integrity as they aged – only their daughters inherited chromosomal defects.

“If you think of mother cells as stem cells, then the discovery that the offspring of aging mother-yeast cells have an increased rate of genomic instability fits with the idea that age-associated effects on stem cells could relate to the age-associated increase in cancer,” McMurray said. “The theories about mutation, stem cells and cancer that have been floating around for years may now have some correlates in the microbial world. This might point to a fundamental relationship between cellular aging and genomic instability and, in particular, how aging cells manifest that instability.”

The fact that aging mother cells are protected from age-induced genetic instability also has evolutionary implications, McMurray said. “In yeast genetics, people historically have thought of the mother cell as being the trash bin that accumulates all the genetic bad stuff so that the daughters could be protected. But we found the opposite. The mother remains protected, which preserves her chance to produce more normal daughters.”

If this evolutionary process is biologically conserved in human stem cells, Gottschling said, “It could explain a lot of the age-induced diseases that happen in people.”

So if cancer is an inherent consequence of aging, are lifestyle interventions to prevent the disease – such as eating right, not smoking and getting enough physical activity – merely an exercise in futility?

“People should still keep eating their broccoli,” Gottschling said. “Our yeast were on a diet equivalent to steak and potatoes. We had the mother cells growing in a very rich, nutrient-dense environment. They were, in essence, pigging out the whole time. We’d like to do similar experiments in which we put the yeast on a ’lean and mean’ diet to see if we could delay the switch that triggers the genetic instability,” he said. “Yeast promises to be an excellent model system for testing various environmental factors, such as caloric restriction, to get at the mechanisms of cancer initiation.”

Yeast also has been an indispensable scientific tool for unraveling the mysteries of how cells divide. The lowly microbe, best known for its supporting role in baking bread and brewing beer, in 2001 gained new respect when Fred Hutchinson’s president and director, Lee Hartwell, Ph.D., received the Nobel Prize in physiology or medicine for using brewer’s yeast to uncover the genetic mechanisms of cell division. He shared the award with British researchers Timothy Hunt and Sir Paul Nurse.

“Yeast cells have been an informative model system for human cells, revealing many conserved aspects of cell biology. If this discovery – a genetic instability that accompanies mother-cell aging in yeast – turns out to apply to human stem cells as well, it would revolutionize our concepts of how cancer arises and how aging occurs,” Hartwell said.

Gottschling’s work was funded by a four-year Senior Scholar grant from the Ellison Medical Foundation Aging Program. Established by Larry Ellison, president of Oracle software, the foundation supports basic biomedical research on understanding aging processes and age-related diseases and disabilities. McMurray’s work was supported by training grants from the National Science Foundation and National Institutes of Health.

SIDEBAR
GRADUATE STUDENT CAMPED OUT FOR WEEKS IN THE LAB, SLEEPLESS IN SEATTLE, COLLECTING THE YEAST DATA FOR STUDY

“Sleepless in Seattle” isn’t just a movie. For the past few years it’s been a frequent way of life for graduate student Michael McMurray. To gather the data upon which the Sept. 26 Science paper’s findings are based, he spent many bleary-eyed weeks camping out in the laboratory of his mentor, Daniel Gottschling, Ph.D., and taking catnaps on a couch in his office at Seattle’s Fred Hutchinson Cancer Research Center.

“He knew what he was getting into but he did it anyway, because he was curious,” Gottschling said of McMurray’s unique experiment, which was elegantly simple from a scientific perspective but, admittedly, somewhat grueling to execute.

To see if yeast could be used as a model organism to help explain the abrupt, age-related increase in human-cancer risk, McMurray needed to closely track the life spans of 40 “mother” yeast cells that were arranged in pairs within 20 petri plates.

He kept the round, shallow plates stacked in an incubator and brought them out every couple of hours to check them under a microscope for the emergence of new “daughter” cells, or buds. Most mother cells produce 30 to 35 daughters before expiring. The whole process, from birth to death, takes about five days.

“When I began I really didn’t know how long it would take – how fast or slow the yeast mothers would divide before finally pooping out,” McMurray said. “Toward the end of the life cycle the yeast started slowing down and so I’d get longer breaks, which allowed me to go home and shower and come right back.”

As the buds sprouted, McMurray would gently pull them away from the mother cells with a needle-like instrument and line them up in a row in another area of the petri plates so he could easily keep track of each new generation. The buds, which soon grew into small colonies, were genetically manipulated to turn bright red or dark brown if they started showing signs of genetic instability.

About three-quarters of the way through the life of each mother cell, colored colonies appeared, signaling the presence of genes gone awry.

“At first it was a shot in the dark to see if we’d see anything this way. We didn’t have any expectations that it would work, but it was definitely worth it to go through the whole process,” said McMurray, who had to repeat the experiment multiple times on various strains of yeast to collect enough data for analysis, resulting in many near-sleepless days and nights.

“It takes a lot of practice to be able to do this kind of work under sleep-deprivation conditions,” he said. “But the crucial information easily could have been missed if we hadn’t done it this way.”

The Fred Hutchinson Cancer Research Center, home of two Nobel Prize laureates, is an independent, nonprofit research institution dedicated to the development and advancement of biomedical research to eliminate cancer and other potentially fatal diseases. Fred Hutchinson receives more funding from the National Institutes of Health than any other independent U.S. research center. Recognized internationally for its pioneering work in bone-marrow transplantation, the center’s four scientific divisions collaborate to form a unique environment for conducting basic and applied science. Fred Hutchinson, in collaboration with its clinical and research partners, the University of Washington and Children’s Hospital and Regional Medical Center, is the only National Cancer Institute-designated comprehensive cancer center in the Pacific Northwest and is one of 39 nationwide. For more information, visit the center’s Web site at www.fhcrc.org.