Cancer cells, on the other hand, have found a way to
mobilize telomerase continually so they can quickly
double and redouble while keeping the telomeres intact.
Some cancer cell lines in use today have been growing
for almost half a century, their telomeres kept long
and healthy by the readily available telomerase.
"Normal somatic cells keep most of the telomerase away
from the DNA, whereas cancer cells let all of it go,"
said Collins, a member of the Department of Molecular
& Cell Biology and part of the campus's Health Sciences
Initiative. "This should help the transformed (cancer)
cells use telomerase much more efficiently."
Though Wong and Collins have yet to discover how cancer
cells mobilize telomerase, finding a way to round them
up again could stop the unbridled growth of cancer cells.
They now are searching for the proteins and signals
involved in telomerase shuttling around the cell nucleus.
An alternative approach taken by most telomerase researchers
and a number of biotech firms is to search for ways
to turn off telomerase to stop cancerous growth.
The UC Berkeley researchers also see a potential medical
benefit in boosting telomerase in patients receiving
chemotherapy, or in people with HIV who experience continual,
rapid turnover of immune cells, which exhausts the limited
number of divisions allotted to immune cells.
"Activation of telomerase, perhaps by gene therapy,
could have a lot of benefit for these people," Collins
said. "Combining telomerase activation with chemotherapy
is an approach we would like to try."
One group of people who could benefit from more telomerase
are those with a rare, predominantly inherited disease
called dyskeratosis congenita (DKC). In 1999, Collins
and her colleagues reported that some cases of DKC are
caused by a shortage of telomerase. Because at each
cell division the telomeres get shorter, cells that
continually renew themselves, such as blood, skin and
other epithelial cells, reach the end of their growth
capacity quicker without telomerase to refresh their
telomeres.
As a result, those with DKC develop skin lesions and
anemia, are prone to life-threatening infections, and
frequently die from bone marrow failure. Those who survive
into their twenties or thirties often die of cancer.
Telomeres were recognized as chromosome end caps in
the early 20th century, but it wasn't until 1982 that
research at UC Berkeley proved that they protected the
chromosome ends from improper rearrangements and generally
unhealthy habits. That work by UC Berkeley professor
Elizabeth Blackburn, now at UC San Francisco, showed
that telomeres, which are comprised of the same short
sequence of base pairs repeated over and over, are there
to protect valuable DNA from a flaw in the DNA replication
process. The machinery that reads and duplicates DNA
can't go all the way to the end of the chromosome, so
a few base pairs the building blocks of DNA are
lost from the ends each round. During a single cell
division, human chromosomes may lose 50-250 base pairs
from each end, out of a starting total of 4-15,000 base
pairs per telomere.
In most mammalian cells, part of the length of telomeres
are expendable, giving cells a fixed life span they
divide perhaps 35 to 50 times, and that's it. After
that, they just sit around or die a natural death. The
Blackburn lab's discovery of telomerase in 1984 provided
the answer to how some cells keep dividing indefinitely.
In yeast, for example, telomerase is always around,
replacing lost telomere tips and allowing yeast to divide
forever.
Though in most human cells telomerase production is
turned off, the enzyme is turned on in cells that turn
over frequently blood cells, skin cells and others
that line surfaces (epithelial cells), and cancer cells.
The enzyme refreshes the telomere ends and allows more
cell divisions.
Collins got interested in telomerase once it was found
to be not merely protein, but a complex of protein and
RNA a ribonucleoprotein. Initially, the RNA was thought
to be merely a template that the protein used it to
make the DNA that it subsequently added to the chromosome
end. Collins and her graduate students at UC Berkeley
have shown in many experiments over the past few years
that the RNA is much more than a template.
"The RNA is not just specialized as an internal template,
the non-template region of RNA is critical for catalysis,"
she said. "People used to think that functional RNA
was just a relic of an ancestral 'RNA World' left over
in the evolution to protein-based enzymes, but recently
there has been a recognition that RNA plays an important
role in ribonucleoprotein enzymes with recently evolved
roles: a modern-day 'RNP World.'"
Most of the ribonucleoprotein in cells is in the form
of the ribosome, an ancient complex that translates
RNA into protein. Like telomerase, it is built in the
nucleus, inside a haven called the nucleolus.
In searching for where telomerase hangs out in the nucleus,
Wong noticed that telomerase is not always in the same
place, at least in cultured human fibroblasts. During
most of the cell cycle, telomerase is attached to the
outer region of the nucleolus, based on localization
studies of the telomerase reverse transcriptase protein
tagged with green fluorescent protein. But just before
cell division, when one would expect telomerase to be
active at the telomeres, it suddenly appears throughout
the nucleus.
Looking at cancer cells, however, Wong and Collins found
that telomerase was everywhere throughout the cells'
entire cell cycle it was never sequestered in the
nucleolus. When Wong and graduate student Leonard Kusdra
infected a normal cell with simian virus 40 a standard
way to induce cancer telomerase was all over the resulting
transformed cell.
Finally, when Wong irradiated a normal cell and a transformed,
cancerous cell to induce chromosome breaks, she found
that telomerase was taken up and sequestered into the
nucleolus.
These results explain one puzzle of telomerase: If it
can add telomeric repeat units to chromosome ends, why
doesn't it also add repeats to the ends of breaks that
occur frequently in DNA? This would interfere with the
ability of the cell to repair breaks, and quickly lead
to genomic instability and death.
Based on this new research, cells evidently sweep up
telomerase when DNA damage occurs to prevent such interference.
Collins and her laboratory colleagues continue to explore
the activity of telomerase and other RNA enzymes, concentrating
on how they're assembled and how they're regulated.
