Cancer and Aging: New findings in yeast may help reveal why growing older is the greatest carcinogen in humans

SEATTLE, Sept. 25, 2003 -- 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," said Gottschling, also an affiliate professor in the Department of Genome Sciences at the University of Washington.

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," said McMurray, a graduate student in the joint Fred Hutchinson/University of Washington Molecular and Cellular Biology Program. "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.