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TELOMERE CRISIS IS A CRUCIAL STAGE IN BREAST CANCER


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http://www.lbl.gov/Science-Articles/Archive/LSD-telomere-crisis.html


Contact: Paul Preuss, (510) 486-6249, paul_preuss@lbl.gov


BERKELEY, CA -- Telomere crisis is an important early event in the
development of breast cancer, and its occurrence can be identified with
precision, according to recent findings by a team of scientists at the
Department of Energy's Lawrence Berkeley National Laboratory and the
University of California at San Francisco. Their report is now available
through advance online publication of Nature Genetics.

Joe Gray, director of Berkeley Lab's Life Sciences Division and a
professor of laboratory medicine and radiation oncology at UCSF, is one
of the paper's lead authors, with Koei Chin and Britt Marie Ljung of
UCSF; Carlos Ortiz de Solorzano, Paul Yaswen, and Martha Stampfer of
Berkeley Lab; and Stephen J. Lockett from the National Cancer Institute.

In the breast, cells in a milk-collecting duct occasionally proliferate
excessively due to development of a regulatory defect. Gray and his
colleagues postulate that this results in a lesion called "usual ductal
hyperplasia."

"The chromosomes in these growing cells lose a hundred or so base pairs
of DNA every time they divide," Gray explains, "because the usual DNA
replication processes don't copy DNA all the way out to the ends of the
chromosomes. This erodes the DNA sequences that interact with proteins
to form structures called telomeres, which protect the chromosome ends."

Eventually the DNA ends erode so much they can no longer protect the
chromosomes. When this happens the chromosomes become unstable, and
damage-control mechanisms kick in that kill the unstable cells. This
process, known as "telomere crisis," normally protects against
inappropriate long-term cell growths like cancer.

Gray and his colleagues believe that "very rarely, the chromosome
instability activates a specialized DNA-replication complex, telomerase,
which can restore telomeres. Cells in which telomerase is activated can
then proliferate indefinitely to form the next stage of cancer, known as
'ductal carcinoma in situ.'" Should the cancer progress further, it next
invades other parts of the breast and may escape to other organs.

Not all cancer researchers agree that telomere crisis in hyperplasia,
followed by reactivation of telomerase, leads to carcinoma in situ --
and thence, sometimes, to invasive cancer; they assign cancer to other
causes. Partly the disagreement arises because sequential events can't
be followed in individual tissue samples from living subjects.

"In human studies, the order from normal ducts, to ductal hyperplasia,
to ductal carcinoma in situ, to invasive cancer is just association,"
says Gray, "because we can't look at the same tissue all the way through
the crisis."

Therefore the researchers compared the assumed sequence of events in
tissue with what happened when they induced a culture of human mammary
epithelial cells, HMEC, derived from normal breast tissue, to undergo
telomere crisis and immortalization. Says Gray, "With HMEC in vitro we
can follow the progression all the way through crisis, compare this to
what we observe in actual tissue specimens from patients, and see if
they are similar."

Using 3-D confocal microscopy and working first with breast-cancer
tissue samples, at each stage the researchers assessed genomic
instability and such correlated features as the amount of DNA content,
signs of rearranged chromosomes, and the number of copies of genes known
to play a role in cancer. These measures increased, on average, from the
hyperplasia stage to the invasive cancer stage.

They also measured mean telomere lengths of cells at each stage. They
found that mean telomere length decreased from normal tissue to
carcinoma in situ, and decreased even more in invasive cancers.

When they looked at cultured human mammary cells, the researchers found
a remarkably similar series. To induce telomeric crisis and subsequent
immortalization in these cells, they introduced a known breast cancer
gene into the culture and examined progressive generations of cells.

Telomere length decreased steadily. Genome instability and evidence of
rearranged chromosomes were low before telomere crisis -- just as in the
tissue samples of usual ductal hyperplasia -- and highest during the
crisis, as in the samples of ductal carcinoma in situ. Instability then
decreased, and changes in genome complexity leveled off, as in invasive
cancer tissues -- where critically short telomeres are presumably
maintained by reactivated telomerase.

The mammary cell culture studies also confirmed that the probability of
successful passage through the telomere crisis is low -- most cells
damaged because of shortened telomeres can't evade cell death. In fact,
women with usual ductal hyperplasia are only slightly more at risk of
developing invasive cancer. When carcinoma in situ does form, it is
probably from a single cell that has managed to reactivate telomerase.

"Our research establishes two things," Gray says. "First is that
telomere crisis does appear to play an important role in the development
of most breast cancers. Second is where it occurs: at the transition
from hyperplasia to carcinoma in situ."

These findings suggest that people at higher risk of developing cancer
can be identified in advance by measuring telomerase activity, genome
instability, and other signals in the clinic.

The findings also point to possible ways of stopping cancer by derailing
transition through the telomere crisis (rare as successful transition
is): by using drugs that maintain the cell's damage-control mechanisms,
for example, or that prevent telomerase reactivation or that poison
cells in which telomerase is already active. Some of these possible
preventive agents are already being tested.

"In situ ****yses of genome instability in breast cancer," by Koei Chin,
Carlos Ortiz de Solorzano, David Knowles, Arthur Jones, William Chou,
Enrique Garcia Rodriguez, Wen-Lin Kuo, Britt-Marie Ljung, Karen Chew,
Kenneth Myambo, Monica Miranda, Sheryl Krig, James Garbe, Martha
Stampfer, Paul Yaswen, Joe W. Gray, and Stephen J. Lockett, will appear
in Nature Genetics and is currently online at http://www.nature.com/ng/.

Berkeley Lab is a U.S. Department of Energy national laboratory located
in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California. Visit our website at
http://www.lbl.gov.