UCSF Study Reveals Mechanism of Telomerase Enzyme That Could Lead to Target for Cancer Therapy, Cell Regeneration

UC San Francisco researchers have discovered a region in the telomerase enzyme
that they say could prove to be a target for killing cancer cells and
regenerating damaged cells, and could also lead to a possible target for
attacking HIV.

The enzyme—brought to popular fame two years ago by studies showing that it
could be manipulated in cell culture to increase the life span of cells - has
the capacity to replenish the tips of chromosomes, known as telomeres, which
lose their final fragments with each cycle of cell division. When activated,
telomerase replenishes telomeres by copying the RNA folded within it into
telomeric DNA and assembling it on the ends of the chromosomes.

Telomeres maintain the stability of chromosomes in numerous ways and, much like
chemical bookends, prevent them from unwinding. But in a fine illustration of
nature’s ingenuity, they also play a role in regulating the life span of a
cell, because the tips of telomeres drop off chromosomes with each cycle of
cell division.1 As the telomeres gradually shorten, the probability that they
will signal the cell to stop dividing gradually increases.  As a result, after
many cycles of cell division have occurred, all cells in a population have

While the telomerase enzyme has the ability to replenish the telomeres on the
ends of chromosomes, it is inactive in many adult human tissues. However, it is
active when massive amounts of cell division are underway - as in self-renewing
adult cells of the immune system, during the development of an embryo, and in

In the UCSF study, reported in the May 5 issue of Science, the researchers
determined that a small structure within the RNA molecule of yeast telomerase
controls the precision with which the enzyme carries out its key function—
spinning out the repeated sequences of telomeric DNA that bind the ends of

When they disrupted this small region of RNA, the enzyme began to spin out
telomeres uncontrollably, until they stalled out like a car run into a ditch.
This result occurred in culture and in the yeast cells themselves. Cell death
soon ensued.

As the human version of telomerase appears to have a structural region that is
similar to that examined in the yeast enzyme, the region could prove to be a
target for killing cancer cells, says the senior author of the study, Elizabeth
Blackburn, PhD, UCSF professor of microbiology and immunology and biochemistry
and biophysics, who co-discovered telomerase in 1985. (The function of the
human version of the region has not been determined.) Moreover, she says, it
could prove a target for regenerating cells that have been damaged through
injury or wear and tear.

Telomerase and other so-called reverse transcriptases, including HIV, both
contain RNA enfolded in protein. As such, they are known as ribonucleoproteins.
Most enzymes, by contrast, contain only protein.

Much of the excitement regarding the current finding stems from the fact that,
to date, research on telomerase and HIV has focused only on the protein
components, as they make up a central part of the enzymes’ active sites.

“This discovery represents the first time anybody has shown a mechanistic role
for a structure of RNA in the action of telomerase, and we think this is
probably a universal kind of feature of telomerase,” says Blackburn.

The finding also adds weight to recent evidence that the RNA component of HIV,
traditionally disregarded in retroviral therapies as passive, plays an integral
role in HIV replication, and it therefore should be closely re-examined as a
possible target for therapy, says Blackburn.

“This discovery should turn researchers’ spotlights back to the RNA components
of both telomerase and HIV,” she says.

The discovery may also offer a glimpse into the evolutionary past. Some
researchers have believed that during an earlier period in evolutionary history
the RNA molecule in telomerase might have deferred functional power to the
protein component.

The discovery that the seemingly archaic RNA retains a key mechanistic role in
telomerase function builds support for the theory that telomerase—and
reverse transcriptases such as HIV—represent an intermediate step in the
evolution of enzymes from strictly RNA sequences to strictly protein sequences,
says Blackburn.

“Such a direct function for the RNA structure in the enzymatic action of
telomerase supports an evolutionary scheme in which RNA enzymes acquired
protein components evolving into ribonucleoprotein enzymes,” she says. “The RNA
components then gradually lost their functional roles in catalysis and were
subsequently dispensable.”

Telomerase and the other reverse transcriptase enzymes function by synthesizing
copies of DNA from the RNA folded within their protein. Nearly all organisms
have an enzyme that can stimulate the conversion of DNA, which contains an
organism’s genetic code, or genes, into messenger RNA, the first step on the
road to developing protein. Only the reverse transcriptases do the opposite.

But telomerase and HIV diverge in a key aspect of their 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
one segment, known as the template, over and over into telomeric DNA, which it
then assembles and adds to the ends of chromosomes.

Until now, researchers have not understood what specifies the enzyme’s template
boundaries, preventing unbridled spinning of telomeric DNA onto the ends of
chromosomes. In the current study, the researchers determined that the
limitation on DNA synthesis is controlled by a small segment of RNA nestled up
adjacent to the downstream end of the RNA template. The region, made up of base
pairs of RNA that are “zipped up” together within a larger segment of RNA, acts
as a boundary for the replication process.

When the researchers altered this RNA region, the buffer “unzipped,” providing
a long strip of RNA - up one side of the zipper—for the enzyme to continue
converting into DNA. With free reign, the enzyme drew more and more of the
ribonucleotides into its active site, until it synthesized so much telomeric
DNA that eventually, says Blackburn, the telomerase RNA may have bunched up,
halting telomere synthesis and causing cell death.

Such abnormal, almost ceaseless, replication, says Blackburn, is reminiscent of
the behavior of normal reverse transcriptases such as those found in
retroviruses like HIV.

“Our finding in telomerase gives strength to the importance of looking at the
RNA component of HIV,” says Blackburn, “because by making just a simple change
in telomerase RNA we can make it act more like HIV, and this suggests that HIV
is actually like telomerase—when it acts in cells to make new viruses, it
really is acting together with its RNAs. I think this is something one should
be thinking about for drug targets.”

The finding adds weight to a recent UCSF study led by Tristram Parslow, MD,
PhD, professor of pathology, and Thomas James, PhD, professor and chair of
pharmaceutical chemistry, who reported (Nature Structural Biology, vol. 5, p.
432, 1998) that a different “zipped up” region of RNA base pairs is essential
for replication of HIV. When the base pairs were mutated, HIV lost its ability
to infect a cell.

Co-authors of the study were Yehuda Tzfati, PhD, postdoctoral researcher, Tracy
B. Fulton, graduate student and Jagoree Roy, PhD, postdoctoral researcher, all
of the UCSF Department of Microbiology and Immunology.

The study was funded by the National Institutes of Health.

1. This built-in limit on cell division is known as the “Hayflick limit” and
was discovered in 1961 by Leonard Hayflick, PhD, an adjunct professor of
anatomy at UCSF before any one knew about the molecular nature of telomeres or
telomerase was discovered.