Researchers at UC San Francisco have determined that telomerase - the enzyme brought to fame last year when scientists demonstrated that its insertion into normal cells extended the healthy life span of the cells indefinitely - can wield its power in an unexpected way.
The finding, reported in the current issue of Proceedings of the National Academy of Sciences, could bolster efforts aimed at manipulating the enzyme for therapeutic purposes - whether to prompt cell division in order to replenish stocks of healthy cells, as is needed in bone marrow transplants, or to interrupt the excessive cell proliferation that occurs in cancer cells.
Telomerase synthesizes telomeres, snippets of DNA on the ends of chromosomes that function in part like the plastic tips on the ends of shoelaces, preventing the gene-bearing, threadlike spindles from unraveling. With each cycle of cell division, the tips of telomeres drop off, until the chemical bookends become so eroded that the chromosomes become unstable, a condition that signals cells to stop dividing. This natural control on cell division is known as the Hayflick limit. Cancers develop when cells increasingly ignore or override such signals of chromosome impairment.
Until now, evidence has suggested that the ability of telomerase to extend the life span of cells lay in its capacity to lengthen telomeres.
In the current study, a collaboration by the laboratories of Elizabeth Blackburn, PhD, chair of the Department of Microbiology and Immunology at UCSF and Nobel laureate J. Michael Bishop, MD, chancellor of UCSF, researchers have determined that telomerase can extend the life span of human fibroblast cells without lengthening telomeres. The telomerase enzyme, they discovered, caps the ends of telomeres and, in so doing, somehow contributes to telomere, and thus, chromosome, stability - regardless of telomere length.
"The finding was quite extraordinary," said Blackburn. "Previous evidence has suggested that telomere length is crucial to determining the life span of cells. But in cells in which we activated the telomerase enzyme, we observed cells thriving despite the fact that their telomeres were very short."
Furthermore, the telomeres of these experimental cells were shorter than those in a set of cells lacking the activated enzyme. These comparison cells had predictably stopped dividing once their telomeres had substantially eroded.
"The combination of somewhat shortened telomeres and inactivated telomerase appears to be the devastating blow," said Blackburn. "The telomerase cap is somehow making a contribution to telomere - and thus chromosome - stability."
The finding does not suggest that telomere length is not important to cell life span. Rather, it reveals a new factor contributing to chromosome stability, and offers a new direction for potentially manipulating the enzyme for therapeutic purposes.
Blackburn's co-discovery of the telomerase enzyme in 1985 spawned a whole field of inquiry into the possibility that the enzyme could be activated, or de-activated, to prolong cell life and combat cancer. For while the enzyme is "turned off" in many normal cells in humans, researchers are able to activate it by inserting the gene for its protein component, known as TERT into cells. In the current study, activation of telomerase extended the life span of experimental cells, apparently indefinitely, and reduced the frequency of abnormal chromosomes. Unexpectedly, however, the life span extension occurred under conditions in which the cells' telomeres were very short.
The researchers conducted their study in human fibroblast cells from which critical molecular safety checkpoints had been removed, setting the cells up to ignore warning signs of chromosome instability, and to begin dividing excessively. While not full-fledged cancer cells, since some molecular checkpoints remained in check, these cells, in comparison to the normal ones, were susceptible to excessive proliferation. Adding telomerase removed the last barrier to immortal growth of these precancerous cells, revealing the enzyme to be a cancer-romoting factor in cells already part way along the road to malignancy.
Blackburn's lab had previously observed the phenomenon of cell immortality and shortened telomeres in two yeast species, and she attributes the researchers' recent success in revealing the event in human cells to timing. Telomerase-telomere interaction produces a continuous ebb and flow of telomere length, with telomerase continuously attempting to counter the erosion of telomeres. So while telomeres generally get longer in the company of telomerase, in the current study, the researchers managed to catch the molecular sway at the unusual moment when telomeres were very short and telomerase's role was needed to kick-in to stabilize the chromosome tips, thus revealing telomerase's unique contribution.
"It just happened we caught the two factors in the balance where telomeres were getting shorter even though telomerase was there. The enzyme just wasn't keeping up with the telomere loss," said Blackburn.
"Chromosomes appear to have three or four methods of buttressing themselves," said Blackburn, "and these methods seem to cooperate. If one device is taken away, the chromosome remains stable. But if a second one is taken away, you start to see an effect. This is why the capping role of telomerase isn't revealed until the chromosomes get fairly short. When the chromosomes' telomeres are still very long, they do just fine stabilizing themselves by other means."
The question now, said Blackburn, is how the telomerase works in its newly revealed capacity. "We want to understand what it is about the act of polymerization [when the enzyme replicates the telomeric DNA tips] that contributes to the chromosomes' stability. We'll be looking at mutants in yeast and human cells to try to figure out this mechanism."
The explanation could enable researchers to make the most of the still-theoretical process of replenishing healthy cells via telomerase manipulation. Traditional thinking suggested that it would take a long time (many tens of cell-replication rounds) for telomerase to build up the length of telomeres to an effective length.
"Now we know it could probably be done very quickly, providing a quick fix," said Blackburn. "It would be nice to have a burst of telomerase, where we wouldn't have to wait to build back telomeres."
Being able to quickly replenish telomeres would be particularly helpful in situations where it is necessary to build back a sufficient supply of healthy cells quickly, as during bone marrow transplants.
"Telomerase might be able to replenish telomeres quickly," said Blackburn, "and then enable researchers to turn back off the telomerase enzyme again so wouldn't do any harm and start causing risks of cancer."
The study was led by postdoctoral fellow He Wang, Ph.D., in Blackburn's lab, and Jiyue Zhu, in Bishop's lab.
The UCSF study was funded by the National Institutes of Health, the G.W. Hooper Foundation and the Leukemia Society of America.
Journal
Proceedings of the National Academy of Sciences