By David N. Leff
Telomeres are among the hottest buttons in biotech research today. Scientists who take stock in the potential of telomeres are looking for one of two contrarian dividends: drugs to curb cancer, and ways to prolong life.
Every non-germ-line (somatic) cell in every body has a portfolio of telomeres capping the tips of every chromosome - and these caps dwindle every time a cell divides. A single human chromosome is several hundred million nucleotides long. At birth, its telomeres measure 10 to 15 thousand bases. In octogenarians, that cap is down to half that length.
In 1961, an anatomist named Leonard Hayflick at the University of California-San Francisco (UCSF), where he is now an adjunct professor, discovered nature's built-in limit on the life span of somatic cells. He set their cellular death sentence, now known as the "Hayflick limit," at 50 doublings. At each such cell division, the tip-protecting telomeres on chromosomes shorten until at the posited cut-off, they're nearly gone.
This pitiless process kicks off at birth. During prenatal gestation, the massively dividing embryonic cells are acquiring their mature telomeric build-up. The builder is an enzyme called telomerase, which goes out of business with two exceptions - when the human or lower eukaryotic organism is born. The enzyme stays active to fuel the constant, enormous output of adult immune-system cells, and rears its head to speed the growth of malignant cells.
In an organism's golden years, telomerase takes early retirement - shutting down cell division. Coaxing the enzyme out of retirement is the goal of the venturesome chromosome-capping researchers intent on prolonging life.
That telomerase enzyme already is moonlighting on tumor cells by adding length to their telomeres and promoting cell division, nonstop. So research oncologists are seeking ways to inhibit telomerase in cancer, while gerontologists go for the opposite gold. (See BioWorld Today, March 31, 1999, p. 1.)
Two California cell biologists, Elizabeth Blackburn, at UCSF, and Carol Greider, now at Johns Hopkins University, co-discovered telomerase in 1985. Blackburn, professor of microbiology, immunology, biochemistry and physics at UCSF, is senior author of a paper in the current issue of Science, dated May 5, 2000. Its title: "Template boundary in a yeast telomerase specified by RNA structure." The article's lead author is molecular biologist Yehuda Tzfati, a post-doctoral fellow in Blackburn's lab.
Tiny Template Steers Telomerase Battleship
"Telomerase," Tzfati told BioWorld Today, "is a ribonuclear protein, composed of RNA and at least one protein. RNA is a very puzzling molecule. It's very large, and contains a template, which is used by the RNA to synthesize telomeric DNA repeats on telomeres."
He added: "The catalytic subunit of telomerase is reverse transcriptase - meaning that it can take an RNA molecule and reverse transcribe it - copy it back into DNA. HIV also uses reverse transcriptase to replicate. The RNA component of telomerase is very large, but it contains a short region, which it uses as a template, or guide, to synthesize the telomeric repeats.
"The main finding of our Science paper," he said, "is discovery of a structure in the RNA that participates in the action of telomerase. And it shows that this important role of the enzyme is contributed by its RNA. It's not a property of the protein component.
"Our research addressed the question: 'How does the telomerase protein know where to begin copying the RNA on the RNA template - which is maybe 50 times bigger than the actual fragment it needs to use - and where to stop?' If it doesn't stop exactly at the right place, then sequences that are not desired get added to telomeres. What we discovered is a special structure that tells the protein: 'This is where you need to stop. This is the end of the line.'
"As reported in the paper," Tzfati said, "when we disrupted this structure by making mutations in its base-pairing region, this telomerase now synthesized longer sequences. Its end was beyond the place where it needed to stop. This told us that the protein is more similar to other reverse transcriptases that synthesize longer genomes. So it makes it possible to apply knowledge from this system to other systems, and vice versa."
Tzfati added, "When we disrupted the structure, we incorporated these sequences onto telomeres. And these sequences were bad for the cell and bad for the telomere, because - in yeast - what happened was that yeast cells couldn't maintain their telomeres, and they senesced. Targeting this region," he observed, "might be a way to inhibit the function of the telomeres. Telomerase inhibitors," he said, "are a very desirable thing. So even though the enzyme remained active, and can still synthesize DNA, now the DNA it would polymerize on telomerase would be toxic to the telomere."
In Therapeutic Hairlines: Cancer, HIV
"One of the speculations or ideas we have as to anticancer strategies is that this way of incorporating toxic sequences onto telomeres might be faster than inhibiting telomerase itself, which takes time for telomeres to shorten, and for the cell to respond."
Blackburn said, "As the human version of telomerase appears to have a structural region similar to that in the yeast enzyme, the region could prove to be a target for killing cancer cells, regenerating cells damaged by wear and tear, and possibly for attacking HIV."
But she and her co-authors point out that telomerase and HIV diverge in a key aspect of their reverse transcription mechanism. While HIV draws thousands of bases of RNA through its catalytic site - spinning out long sequences of viral DNA - telomerase draws in only a very small portion of its long RNA sequence to the catalytic site, and copies this template over and over into telomeric DNA, which adds it to the chromosome tips.
"By making just a simple change in telomerase RNA," Blackburn observed, "we can make it act more like HIV. I think this is something one should be thinking about for drug targets."