By David N. Leff
Every cell in a newborn boy's body — with one major exception — comes into the world sitting on death row.
That is, every somatic cell has a finite life span, measured by the number of times it divides before senescence sets in. And this ticking time bomb relates to its body's aging process.
"The child is father of the man" in more ways than the poet who wrote those lines two centuries ago could have imagined.
Just before the birth of that child, an enzyme called telomerase goes out of business. All during the nine months of gestation, that enzyme protected the rapidly dividing cells of fetal growth from genetic damage during division. It did so by capping the tips of their chromosomes with a chain of repeated terminal DNA sequences (TTAGGG) called telomeres. Once that fetus turns into an infant, the ends of its chromosomes start the lifelong process of shedding the tips of those tips. A small segment of telomeric DNA is lost each time a mortal cell divides.
The one exception applies to the boy's — and man's — fast-dividing reproductive germline cells, notably the sperm. This immortal cell line conceptually constitutes the future man's future contribution to his next-generation fetus. Girls complete their complement of future eggs before being born, so their germline chromosomes don't need this tip-end shielding by telomerase.
"The shortening of telomeres is believed to prevent cells from growing continuously," observed biochemist and molecular biologist Gregg Morin, of Geron Corp., in Menlo Park, Calif. "As telomeres get too short and enter senescence," he continued, "the cells stop dividing."
He pointed out that "a single chromosome is several hundred million nucleotides long. And its telomeres have 10 to 15 kilobases at birth. In octogenarians, that gets down to five or six kilobases And all the experiments that we and others have done," Morin added, "indicate that telomere-length shortening correlates very well with the Hayflick limit," which posits that a human cell will divide only about 50 times before dying out.
"Telomeres," explained Geron's director of cell biology, Calvin Harley, "can be envisioned as 'molecular clocks' that regulate the life span of cells and affect the aging process, while telomerase can be envisioned as the 'key' that 'rewinds' the telomere clocks."
It has not escaped the notice, and efforts, of researchers these past 10 years or so, that if one could wake telomerase out of its dormancy in somatic cells, and restore its activity of re-capping the ends of chromosomes, this might well give aging cells a longer lease on life, and health.
Morin and Harley are senior co-authors of a paper in the December 1997 issue of Nature Genetics, titled: "Reconstitution of human telomerase with the template RNA component hTR and the catalytic protein subunit hTRT."
What this means, Morin told BioWorld Today, "is that for the first time Geron has produced telomerase activity in normal, mortal human cells, using the gene for the human telomerase catalytic protein that we cloned recently." (See BioWorld Today, Aug. 15, 1997, p. 1.)
Against Cancer, Inhibit; Against Aging, Activate
Harley added: "This is the first demonstration of reconstitution of the enzyme from its RNA and protein." And he pointed out: "Having discovered the two key components that are responsible for telomerase activity — human telomerase reverse transcriptase protein [hTRT] and human telomerase RNA component [hTR], Geron is now in a stronger position to identify chemical compounds that regulate the activity of the enzyme."
That activity has an up-side and a down-side.
"If you could extend the life span of human cells," Morin hypothesized, "you should be able radically to alleviate some age-related diseases."
On the other hand, inhibiting unwanted telomerase reactivation, a hallmark of cancer, involves an all-out drug-screening program at Geron. The company and its collaborators have found the enzyme at work in a broad spectrum of malignancies, but not expressed in most normal tissues.
"Because telomerase is required for cancer cells to keep proliferating," Harley said, "we are working to discover small-molecule anticancer drugs designed to inhibit it. Such compounds are expected to lead to the death of tumor cells through resumed telomere shortening."
To show that their catalytic protein is the only additional component needed to restore telomerase activity in normal human somatic cells, the researchers transfected the recombinant hTRT protein with four human cell lines and recorded the enzyme active in each. The protein consists of 1,133 amino acids.
"We expressed the gene's RNA sequence in rabbit reticulocyte systems," Morin recounted. "These are blood extracts that contain all the materials necessary for recognizing, transcribing and translating proteins." His team is also exploring the use of E. coli as a host organism.
Geron Scales Up For Drug Screening
In Geron's ongoing anticancer drug discovery program, Morin explained, "We are going after any inhibitor that prevents the telomerase activity from being assembled within the tumor cell. One can inhibit an activity directly, or inhibit expression of the gene, or assembly of the enzyme from its two components, or so on.
"To do those kinds of experiments," he pointed out, "we basically need to have a source of the components, then design experiments to look for compounds that will inhibit whatever one of those functions we want to block."
He and his team have been doing it for awhile with native enzyme, purified from cells. "So now that we have the ability to reconstitute the enzyme in vitro," he continued, "once we get large amounts of it, we will be able to do the assays in new and better ways, looking at different inhibitory functions or features of the enzyme, which we couldn't readily do with the old method. And we're also setting up new kinds of screens, rather than just the activity kind.
"That was cancer," Morin concluded. "Now, with the ability to put the catalytic protein back into the cell and restore telomerase activity, we'll be able to confirm our hypothesis, with future experiments, as to whether telomerase activity in telomere maintenance is playing an important role in aging." *