© Geriatric Times. All rights reserved.

Telomeres and Aging

by John Medina, Ph.D.

Geriatric Times

November/December 2000

Vol. I

Issue 4

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The ticking of a countdown stopwatch is a chilling metaphor, often used by modern literary types to describe the aging process and, by implication, our own mortality. Variations of that metaphor-the ticking crocodilian time-bomb nemesis of Captain Hook in the story of Peter Pan, a tolling bell, a beating telltale heart-all point to the incessant intrusion of time into our lives. That the cessation of such ticking only ends in death is the depressing reality of such art.

One of the reasons why the metaphor gives even cold, hard scientists like me the shivers is that this allegorical ticking has a non-allegorical biological correlate. And that biological correlate also has to do with the aging process. In this column, a space devoted to understanding senescing phenomenon at the molecular level, we are going to talk about this correlate. I am going to introduce you to the ticking world of the telomere, a world we will visit numerous times throughout the life of this column. The "telomere," a term that really describes a region at the end of a chromosome, is one of the clearest examples of how molecular mechanisms contribute to the aging process.

Let's Start at the Beginning: A Very Good Place to Start

Before we get into the more biochemical aspects of the telomeric role in aging, it might be instructive to view a few background pieces of data. These data led to the suspicion that the ends of the chromosome may play a key role as important regulators of cell death and, aggregately, as parts of the aging process itself.

One of the first key findings came from cell culture experiments. It was first observed many years ago that human cells placed in a petri dish divide only for a limited amount of time (each division appropriately called a doubling). Then they undergo a process known formally as senescence. In senescence, the cells are still alive, but they have stopped dividing. In fact, they are becoming dysfunctional, probably through a series of biochemical alterations not observed when they were replicatively active.

Eventually, the cells will die. Deleterious changes over time are one of the definitional hallmarks of the aging process, and this cell culture phenomenon has been used as a model to study aging at the molecular/cellular level.

Such cell doubling followed by senescence and cell death were puzzling observations, however. In the body, cells divide for two reasons: either to maintain homeostatic function or in response to an injury. This doubling can occur frequently (certain intestinal cells divide every three days) or hardly at all (certain endothelial cells will divide once in the lifetime of an individual). Yet, whether one places fibroblasts or endothelial cells in culture, they all exhibit a finite amount of doublings, and then they grow old and die.

The obvious question then became, what stops the cells from dividing? And then, why do they die? Besides sharing similar terminology, does the experience of senescence contribute anything to our understanding of the aging process? It was known that as certain types of cells undergo senescence, striking changes begin to show up in their DNA. These changes appear to contribute to some form of genomic instability-chromosomes begin to fragment, the cell starts to die.

Hints that genomic instability might contribute to the phenomenon of senescence led scientists to study the overt structure of chromosomes in some detail. Work from several directions gave strong hints that the ends of chromosomes held special structures, and that these structures might provide important clues to the process.

It was noted, for example, that chromosomal ends were of uneven lengths. As you know, DNA is a double helix, and this result showed that one strand of the double helix was actually longer than the other. Such structures are known as staggered ends. That gave researchers two immediate clues as to what was going on, and then another puzzle to solve.

The first clue came from replication studies, where a chromosome duplicates itself just prior to division. It turns out that human cells do not have the ability to replicate the staggered ends of their chromosomes. There are a number of reasons for this, including the fact that the copying mechanism the cells employ, an enzyme complex known as DNA polymerase, gets in the way of the final replication events at the end of a chromosome. While too complex to detail here, the upshot of this inadequacy is that with every round of replication, the chromosome tip gets shorter and shorter.

Sort of sounds like the ticking of a clock, doesn't it?

The second clue was equally intriguing. Exposed staggered DNA ends act like time bombs inside a cell. If a cell recognizes a staggered end, it will immediately attempt to repair it. Unfortunately, the repair quickly becomes deregulated, and the chromosome becomes damaged.

As if a derailed repair process weren't enough, renegade fusions begin to occur if staggered ends of a chromosome are directly exposed to the inner space of a nucleus. Fusions are where one strand of the dangling DNA unhealthily attempts to bind to other chromosomal regions.

Staggered ends can even lead to rogue forms of recombination, where the cell acts like a sperm cell or an egg and attempts to cut and repaste its genetic material from one chromosome to the next. Given enough time, the aggregate fragmentation will result in cell death. Staggered ends are a disaster, and no chromosome should have one exposed.

The puzzle was this: all chromosomes have staggered ends, yet they don't normally undergo such dramatic dysfunction. Why don't they? Why isn't the life of a normal cell filled with attempts to wall off such powerful self-destructing tendencies?

Perhaps some kind of masking event was occurring that hides the staggered ends from the potent forces that could destroy the chromosome, thus allowing cell survival. During cell division, unmasking might transiently occur to allow duplication of the chromosomal DNA. Afterward, the mask could be reassembled, allowing cell survival through another doubling.

This idea turned out to have a great deal of merit. Scientists looking at the detailed structure of chromosomal termini found an extraordinary thing: most chromosomal ends contained redundant sequences of nucleotides.

As you recall from high school, nucleotides are the subunit building blocks upon which a DNA molecule is constructed. Since chromosomes are just long strands of DNA, chromosomes are really just long strands of nucleotides (a typical human chromosome can have more than 100 million nucleotides). As you further recall, there are a total of four different nucleotides available to the DNA molecules, symbolized by the letters A, G, T and C. A typical gene is made of over 1,000 of these nucleotides in a specified arrangement.

The redundant sequences found at the ends of chromosomes were much, much smaller than a typical gene, although no less specific. The sequences were made of six-letter repeats-TTAGGG. In humans, there were found to be as many as 50 to 100 such repeats at a chromosome's very end. In reference to its terminal placement, the entire structure was termed a telomere.

There is a reason for having these repeats, these telomeric sequences. Their presence actually allows the chromosomal end to kind of bend over onto itself, sort of like a paper clip. As shown in the Graphic accompanying this article, this bending creates a complex looped structure at the very end of the chromosome. Because of this loop, proteins can bind in a regular fashion, creating very stable termini. This looped structure-complete with its proteins-hides the dangling ends, keeping them from being exposed to the cell. In other words, a protective mask is created, and the idea mentioned above was confirmed.

What This Has To Do With the Aging Process

The summary of this research showed that human cells learned to solve a potentially disastrous instability problem at the ends of their chromosomes, even while those ends whittled away over time. This does very little to answer the question of cell doublings, however. And even if it did, how could the presence of masks and ever-shrinking chromosomal ends possibly be connected to an aging process?

The key result that allowed scientists to begin answering this question came from examining the lengths of the chromosome tips themselves. It was found that the tips had to have a certain number of telomeres-in other words, to have a certain length-in order to make the masking paper clip. If there were not enough telomeres on the ends of the chromosomes-if the chromosomes were too short-the paper clip structure would not form. Without that structure, no mask could be created, of course, and the staggered ends would thus be exposed. As you know, exposed staggered ends would mean the death of the cell.

All of a sudden, things began to make sense. The cell culture data could actually be explained in biochemical terms, and telomeres would vault to the very front of the effort to understand the molecules of aging. The insight could be divided into three steps.

1) Cells replicate in a dish. With every round of replication, their tips grow shorter. The mask reforms with every round of replication for a time, and the cells live.

2) A critical length is eventually reached whereby there are not enough telomeres to form the mask. Eventually, there is a round of replication in which the ends of the chromosomes do not make the paper clip structure. The staggered ends now lie exposed, and the cell begins accumulating damage. Replication ceases.

3) Eventually, enough chromosomal damage takes place that the cell begins to lose critical functions, and eventually, the cell dies.

What Conclusions Can We Draw From These Data?

These data do not explain everything that is observed as cultured cells pass through time, of course. And they certainly do not provide a complete explanation of the aging process as the entire human organism passes through time (most of the above-described work was done only in human fibroblasts, for example). Much more research needs to be conducted before the role of telomeres can be properly integrated into the overarching explanations of human aging.

What these data do provide is a molecular foothold whereby certain aspects of aging may be understood biochemically. And that, of course, is very valuable. Such research has even had spin-offs, most notably in cancer research.

We will return to the telomere story in future columns because, as you can imagine, it is a successful and quite powerful focus of current effort. This introduction serves primarily to explain how genes and aging processes co-exist, and to characterize the uncomfortably persistent, almost literary ticking of the smallest countdown stopwatch ever discovered.

Dr. Medina, a former faculty member at the University of Washington School of Medicine, is now founding director and CEO of the Talaris Research Institute, a brain research center devoted to the science of early learning.