The discovery of a key molecule linked to the immortalization of human tumor cells provides an important new target for anti-cancer drug design. Researchers led by Dr. Robert A. Weinberg have isolated and cloned the gene for the long-sought catalytic subunit of human telomerase, a molecule believed to play a major role in the transition from normal to cancerous growth.
"The telomerase enzyme is an ideal target for chemotherapy because this enzyme is active in about 90 percent of human tumors, but inactive in most normal cells," said Dr. Weinberg, the Daniel K. Ludwig Cancer Research Professor and an American Cancer Society Research Professor in the Department of Biology, as well as a founding member of the Whitehead Institute for Biomedical Research. "Pharmaceutical companies have screened thousands of compounds to find agents capable of blocking telomerase. Now that we know the identity of the catalytic subunit, drug development should move much faster."
Dr. Weinberg, principal authors Drs. Matthew Meyerson and Christopher Counter, and their colleagues from the Whitehead Institute, Massachusetts General Hospital, Merck Research Laboratories and McMaster University in Ontario describe the new telomerase subunit gene, hEST2 (human Ever Shorter Telomeres 2), in the August 22 issue of Cell. They report that this telomerase gene is expressed at high levels in primary human tumors (11 of 11 tumor samples studied, including two breast tumors and four ovarian tumors), but undetectable in most normal human tissues, including breast, ovary, heart, brain, placenta, liver, skeletal muscle and prostate.
The normal function of telomerase in the body is to help maintain the ends of chromosomes in reproductive cells (cells that produce eggs and sperm) and in certain immature progenitor cells that give rise to other body tissues. Telomerase activity is not detectable in most mature cells.
Switching off telomerase during development can be likened to setting a stopwatch. This stopwatch keeps track of the number of cell divisions that occur in any one population of cells over a person's lifetime. Normal cells have a finite replication potential; they can divide only so many times and then they die. In contrast, cancer cells divide and multiply without limit.
Evidence collected by many laboratories indicates that the clock or counting mechanism relies on specialized bits of DNA, called telomeres, at the ends of each human chromosome. In the absence of telomerase enzyme, these specialized end-structures grow shorter with each round of cell division. Eventually, the shortening process reaches a critical stage; the chromosomes become unstable and any further cell division leads to cell death. (This limited allowance for cell divisions may be a significant factor in normal human aging.)
"Cancer cells find a way of switching telomerase activity on, which gives them a tremendous competitive advantage," Dr. Weinberg said. "They have the potential for continuous reproduction in the body or in cell culture -- they become immortalized. We want to learn how cancer cells regenerate telomerase function and, at a more fundamental level, how all cells switch telomerase off and on."
The telomerase enzyme is a complex structure containing multiple proteins and an RNA molecule (RNA is a chemical cousin of DNA and, in this case, acts as a template for the production of new telomere segments). Previously, researchers in other laboratories had identified the genes responsible for producing human telomerase RNA and one telomerase-associated human protein, but careful studies of these genes revealed that neither could explain the regulation of telomerase activity: expression of the genes does not correlate with observed levels of telomerase activity in normal or cancerous cells.
Finding the critical catalytic subunit that actually transcribes telomerase's RNA template into DNA proved to be an elusive goal. "The main difficulty in isolating telomerase proteins has been that even when telomerase genes are turned on full blast, the quantity of telomerase in a human cell is vanishingly small," Dr. Weinberg said.
To overcome this problem, the Weinberg lab developed a genetic ploy using yeast. They created novel yeast mutants with specific defects in their ability to replicate telomeres. Shortly thereafter, both the Whitehead researchers and a team led by Drs. Joachim Lingner and Thomas Cech of the University of Colorado at Boulder and Dr. Victoria Lundblad of the Baylor College of Medicine in Houston independently converged upon the same discovery: a key protein in the telomerase complex. "Our studies and those of the Colorado/Baylor group proved without doubt that the new protein was the catalytic subunit of yeast telomerase," Dr. Counter said.
Using the sequences of the yeast telomerase protein and a comparable telomerase protein from a single-celled protozoan (discovered by the Colorado/Baylor group), the Whitehead researchers quickly isolated the human gene. The order of the building blocks of all three proteins is remarkably similar.
In addition, the Whitehead studies revealed a strong association between the presence of the hEST2 message and telomerase activity; both are present in immortal transformed cells and absent in mortal normal cells. "This suggests that hEST2 expression might underlie the activation of telomerase that occurs during cellular immortalization," Dr. Meyerson said. "Blocking hEST2 expression or activity could halt or slow the progression of malignant tumors."
In fact, say Drs. Meyerson and Counter, the foundation for an anti-telomerase drug may already exist. The three known catalytic subunits of telomerase (from humans, yeast and a protozoan, respectively) belong to the same general family of molecules as the reverse transcriptase enzyme produced by the HIV virus that causes AIDS. Several of the most commonly used AIDS drugs, including AZT, are reverse transcriptase inhibitors.
"The beauty of this finding is that we already know a great deal about the structure of reverse transcriptase inhibitors," Dr. Counter said. "We have a good starting point for developing anti-telomerase drugs."
This work was supported in part by the National Cancer Institute of the United States, the National Cancer Institute of Canada, the Damon Runyon-Walter Winchell Cancer Research Foundation, the MIT-Merck Postdoctoral Fellowships Program, the Howard Hughes Medical Institute, and the Human Frontier Science Program.
Drs. Meyerson and Counter are postdoctoral fellows in the Weinberg lab. Of the Cell paper's 13 authors, Elinor Ng Eaton, Dr. Philipp Steiner, Stephanie Dickinson Caddle, Liuda Ziaugra and Dr. Roderick L. Beijersbergen are also affiliated with the Whitehead Institute.
A version of this article appeared in MIT Tech Talk on August 27, 1997.