Researchers in the Basic Sciences Division 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.
Dr. Dan Gottschling and graduate student Michael McMurray 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 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.
Life cycle of a cell
Aging indeed is a potent carcinogen. 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 help researchers to hone in on 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.
That observation 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."
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.
Sleepless in Seattle
"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 for his study, he spent many bleary-eyed weeks camping out in the laboratory, taking catnaps on a couch in the office of his mentor, Dr. Dan Gottschling.
"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 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. Each experiment took 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 cell division 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-every couple of hours-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."