Altering telomerase to prevent breast cancer progression

Institution: University of California, San Francisco
Investigator(s): Elizabeth Blackburn, Ph.D. -
Award Cycle: 1996 (Cycle II) Grant #: 2IB-0077 Award: $44,677
Award Type: IDEA
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease



Initial Award Abstract (1996)
The goal of this proposal is to explore two new strategies for blocking breast cancer progression and tumor maintenance. Both involve altering the action of telomerase in breast cancer cells, but in different ways. These are early experiments designed for the ultimate long-term goal of using these novel strategies clinically, in the prevention and/or treatment of breast cancer.

Several years ago in my laboratory, we discovered the enzyme telomerase, which is the enzyme responsible for creating the DNA which makes up telomeres. Telomeres consist of DNA sequences, repeated over and over at the ends of chromosomes, which bind protein factors and make a "cap" securing the end of every chromosome, preventing it from fraying away. The telomerase enzyme adds short stretches of DNA to the ends (the telomeres) of all chromosomes as cells are dividing. This extra DNA makes up for the loss of DNA from each chromosomal end that is incurred by the natural inability of chromosomal DNA replication to replicate the chromosomal ends. Without replenishments by telomerase, as cells repeatedly divide, the telomeres gradually shrink. When telomeres become too short, cells stop dividing. Thus, for a cell to keep dividing, telomerase is needed in sufficient amounts to keep the chromosomes "topped up". Cancer cells, which divide much more than many normal cells in the body, often activate telomerase. This is thought to contribute to their ability to keep dividing.

We have discovered that telomerase is active in breast cancer cell lines grown in the laboratory. Therefore, we have decided to alter the action of telomerase in breast cancer cells in two very different ways, both designed to stop these tumor cells from dividing.

In the first strategy, inhibitors of telomerase will be used to prevent telomerase from maintaining telomeres. We have made the novel finding that some of the same kinds of drugs that stop the HIV-1 virus from replicating can also inhibit human telomerase, and cause telomere shortening in at least some types of human cells grown in the laboratory. Therefore, the first goal of this proposal is to test whether we can shorten telomeres in human breast cancer cells, using breast cancer cells explanted directly from patient biopsies into short-term primary cultures in the laboratory. Based on previous results with simple organisms, we predict that without telomerase working properly, the telomeres will run down until the cells stop dividing. In this way, we will find out about the role of telomerase in breast cancer progression and tumor maintenance. The ultimate long-term goal is to determine whether telomerase inhibitors will be useful in the prevention and/or treatment of breast cancer.

The second strategy, rather than seeking to shut off telomerase action as the first strategy does, instead actually turns the action of an active telomerase against the breast cancer cells. This active telomerase is one we will have engineered to direct the synthesis of what we term "toxic" telomeric DNA sequences in breast cancer cells. When we do this in simpler organisms, they very quickly cease dividing because of these adverse telomeric sequences.

The planned research explores new strategies for potential treatment of breast cancer and will be greatly augmented by the multidisciplinary approach involving the combined expertise of this laboratory on telomeres and telomerases with those of Dr. Shanaz Dairkee in breast cancer cell explants and cell biology, and Dr. Robert Debs in gene transfer methods.


Final Report (1997)
The goal of this proposal is to explore two new strategies for blocking breast cancer progression and tumor maintenance. The strategies examine different ways to alter the action of an enzyme called telomerase in breast cancer cells. Telomerase keeps chromosomes from degrading during cell division by replacing the telomeres (DNA at end of the chromosomes) and is often turned on in cancer cells. Thus the action of telomerase is thought to contribute to the ability of breast cancer cells to keep dividing. The first strategy involves designing ways to turn telomerase off in cancer cells and the second strategy involves designing ways to selectively kill cells that have telomerase turned on. These are early experiments designed for the ultimate long term goal of using these novel strategies clinically in the prevention and/or treatment of breast cancer.

In progress toward the first strategy during the funding year, inhibitors of telomerase were tested for eventual use in preventing telomerase from maintaining telomeres. An important part of this work has been to remove a previously unrecognized contaminant from the telomerase assay system in the test tube. This contaminant gives a signal that resembles telomerase activity, and during the funding year we have designed ways to distinguish it from authentic telomerase activity. We used a set of 20 novel compounds, which were originally designed to inhibit HIV reverse transcriptase, in tests to see if they inhibit human telomerase. We also tested a different set of compounds, made by a combinatorial chemistry approach, for their effects on human breast cancer telomerase activity. We found that some of these compounds do alter telomerase activity in the test tube. The ultimate long term goal is to determine whether telomerase inhibitors will be useful clinically in the prevention and/or treatment of breast cancer.

In progress toward the second strategy, during the funding year we have carried out the first steps of constructing engineered human telomerase RNA genes for delivery into breast cancer cells in culture. Here the approach is to turn the action of an active telomerase against the breast cancer cells, rather than seeking to shut off telomerase action as the first strategy does. We are in the process of engineering an active telomerase that will cause the breast cancer cells to make what we term "toxic" telomeric DNA sequences. When we do this in simpler organisms, they very quickly cease dividing because of these adverse telomeric sequences.