By David N. Leff
True or false? There is no scientific definition of the process we know as aging.
That's true, averred bio-organic chemist Danith Ly, at the Scripps Research Institute in La Jolla, Calif. "We don't have any clue, any marker, as to what is aging. How can you guess at a person's age unless you look at his or her skin, hair, face, etc.?"
Dry and wrinkled skin, gray hair or bald head and wizened facial features are visible hallmarks of advanced years. But one in 10 million babies begin developing these visible stigmata of accelerated superannuation around the first birthday. Heart disease and other geriatric ailments follow, and these children die in their earliest teens - of old age.
"They are victims of a rare syndrome called progeria," observed Ly, adding, "An infant with progeria is comparable to a normal person in his 80s or 90s. Such unfortunate children could never live to the reproductive stage, so evolution gets rid of them early - which makes their disease so rare." He and his colleagues at Scripps are comparing progeria genes to those of individuals at various stages of maturity and senility.
Ly, a postgraduate fellow, is first author of a research paper in the issue of Science dated March 31, 2000, and titled: "Mitotic misregulation and human aging." Its co-senior authors are immunochemist Richard Lerner, president of Scripps, and genomicist Peter Schultz, at the Novartis Research Foundation in San Diego.
"Gene misregulation," Ly explained, "is when the protein level is either under-expressed or over-expressed. In a normal system the level must be tightly regulated at a certain set-point in order for the body to function normally."
He and his co-authors studied gene changes during aging, at the level of transcription expression, in fibroblast cell lines from 10 individuals in three stages of life - normal young, middle-aged, very old - plus children with progeria.
Young, Old, In Between - And Progeria
"We had three persons represented in the very old cohort, " Ly recounted, "three in progeria, two each in middle-aged and young. We were only looking for the ones with genes that are common to all. Meaning, that are unique to the aging process, not the phenotype that an individual has. The idea was to eliminate individual variation.
"These fibroblasts," he continued, "were donated through the NIH's National Institute of Aging, which cultured them. I think they were obtained during clinical examinations. They needed only a small puncture of the skin, from which to culture the fibroblasts.
"We chose to follow fibroblasts," Ly observed, "because they are the foundation of connective tissues, bone, growth and resorption - everything that is under our skins, and, reflecting the visual appearance of aging."
The team scanned the 6,800 known human gene sequences deposited in the gene banks, and singled out 61 that clearly correlated with aging. "We used the DNA microarray," Ly recounted. "These are small snippets of DNA gene sequences, which we put on a chip, containing those 6,800 genes. We looked at the expressed messenger RNA level from these cell lines to see which ones were missing. So based on that, we could determine which genes are no longer expressed at the normal level, and which ones are up-expressed.
"Most of those genes," he went on, "we could classify into two groups: One is in the ECM - extracellular matrix - which gives age-related structures like bone formation, or the skin's elasticity, for example. The other is mitosis - the machinery of cell division.
"I think the conclusion or take-home lesson of this study," Ly suggested, "is the result of misregulation of genes. This happens because of breakdowns in the mitotic machinery, and that breaking down could be the result of the cell-cycle checkpoint. We saw a number of genes that were involved in a checkpoint, and those were down-regulated."
Checking Up On The Checkpoint
Lerner commented, "Aging is predominantly a disease of mismanagement of cell-division checkpoints." Ly compared this event to "having a police car patrolling a road. If there is some defect in the DNA, then the police officer would stop the driver, to make sure that everything is corrected. Then he or she would allow it to go through. But if that police officer is no longer there - which is the purpose of the checkpoint - then anything could pass through. And you could imagine if the cell-division cycle of the cell divides a couple of times without a checkpoint to make sure that everything is correct, then everything would spiral downward.
"So the entire basis behind the cell-cycle checkpoint," he pointed out, "is to ensure that the chromosome is dividing properly between cell divisions, and make sure all the proper genes are expressed properly.
"In the dividing cell," Ly explained, "somehow the DNA gets damaged or is not aligned properly during cell division. If the checkpoint is no longer on duty, then all the damage would go right on through. Maybe a checkpoint would stop the cell cycle, or stop the cell. And it would recruit all the DNA repair genes to come in and fix it. But if the damage is too severe, then it would lead to apoptosis - programmed cell death.
"Now this data," Ly summed up, "gives the research community a hypothesis to go after in terms of aging. Because before, when we studied aging, we had no way of knowing where to go, how to begin. Right now we have a hypothesis that DNA checkpoints are the major component that causes this mitotic machinery breakdown, and that leads to misregulation of aging genes. The next step in this field is either to prove it or disprove it."
Meanwhile, he allowed, "Since progeria is a very close mimic of natural aging, what we would like to do is go in and clone its gene. And then we are going to see if animal models have the same effect. Because right now with the result of this paper, we have a marker, we know what aging looks like. We have a hypothesis, that checkpoint is the major player. We'd like to knock out that player to see what the ballgame would look like.
"Also, now we can genetically go in and try to renormalize the misregulations. So maybe our best bet," Ly concluded, "is to live to the same age that we are now, but with less disease in that aging process."