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Yeast Offers Insight Into Aging and Cancer
Excerpt
by Ed Edelson,
HealthDay
Yeast cells start making a lot of genetic
mistakes as they reproduce in their old age, a finding that may
help explain why the incidence of cancer in humans increases with
age.
It might also someday lead to better
ways of cancer prevention.
It is well known that "old
age is the most powerful carcinogen," with nearly 80 percent
of cancers diagnosed after age 55, and researchers have looked
for molecular reasons why that is so, says study author Daniel
E. Gottschling, a scientist with the Fred Hutchinson Cancer Research
Center in Seattle. His report appears in the Sept. 26 issue of
Science. The new observation "smells a lot like what
happens in humans," he adds.
While yeast cells are a lot simpler
than humans, they follow the same basic principles of reproduction,
with cells dividing to produce new generations of cells that keep
the organism going.
Like humans, yeast cells have a
limited life span. Their life span of 30 or 35 generations of
division is played out in about five days. What happens to yeast
cells is the daughter cells they generate in their later years
begin to have more and defects in their chromosomes, the bodies
that carry genes.
That genetic instability could
erode the built-in mechanisms for controlling cell growth, allowing
the unlimited proliferation that is cancer, Gottschling explains.
"We've never been able to
see a phenomenon in which chromosomal integrity starts breaking
down with age," he says. "It happens as if a switch
is thrown, like something suddenly breaking down in your car."
It is a huge step from yeast to
humans, Gottschling acknowledges, but the basic principles of
cellular reproduction are the same for both. In humans, reproduction
happens as stem lines for various tissues keep reproducing. The
thought is that older human stem cells start making the same kind
of mistakes as seen in yeast.
Because of human complexity, detecting
that process in our cells will be much more difficult, says David
A. Sinclair, an assistant professor of pathology at Harvard Medical
School, who wrote an accompanying editorial. Working with yeast
"could potentially make possible rapid progress in understanding
what happens in aging cells," he says.
One interesting aspect of the study
is that the genetic damage occurs in daughter cells, not in the
older reproducing cells, Sinclair says. "Perhaps old cells
accumulate damaged proteins," he says. "Many studies
have suggested that damaged proteins are the cause of aging. This
study could help validate that theory."
Because yeast cells reproduce rapidly,
with a new generation every two hours, the study led to sleepless
nights for Michael McMurray, the graduate student who made the
observations. He started with 40 "mother" yeast cell
lines grown in 20 Petri dishes.
These were special yeast cells,
engineered to change color when genetic instability appeared.
Several strains of yeast were studied and McMurray's through-the-night
observations showed the color changes appeared in every one of
them at about the 25th cycle, which is late middle age for a yeast
cell.
"Right now we're working like
crazy to figure out how this happens, Gottschling says. "What
causes the breakdown? Once we understand a little bit about the
process and how genes respond, we can think about making the leap
to humans."
More information
An overview of what is being done
to explore the role of aging in human cancer is offered by the
National
Cancer Institute. Go to the American
Cancer Society to learn about early detection and prevention.
Reference
Source 101
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