The study, conducted in breast and prostate cells grown in culture and in tumors formed from human breast cancer cells grafted into mice, suggests that human cancer cells are much more sensitive to disruptions in the telomerase enzyme than had been thought. The finding hints, the researchers say, at a possible new strategy for thwarting human cancers.
In humans, telomerase is inactive in most adult cells, and only active at certain times in others, but it is highly active in cancer cells, and has been suggested as a potential therapeutic target. However, previous studies in human cancer cells have indicated that disrupting telomerase as a means of halting cancer cell replication or inducing cell suicide would require an almost complete loss of normal telomerase activity. And this would require either swamping the enzyme with an overwhelming amount of mutant telomerase or finding a sufficiently potent drug to completely inhibit the enzyme.
But in the current study, the researchers observed that inserting a tiny mutation in the gene coding for a small but critical portion of the telomerase enzyme prompted a dramatic response from cancer cells. The finding suggests a more efficient means of delivering therapy. Most of the human breast and prostate cancer cells grown in culture lost the ability to replicate or they committed suicide, while tumors formed from human breast cancer cells grafted into mice were smaller than those generated from cells that didn't have the mutation. Moreover the response occurred despite the fact that most of the normal telomerase was retained. The researchers induced the response with several different variations of the mutant gene.
"We were quite surprised at how strong the effect was," says the senior author of the study, Elizabeth Blackburn, PhD, UCSF professor of biochemistry and biophysics. "Cancer cells are tough. They usually ignore the signals that tell them to commit suicide. But by spiking the telomerase enzyme with just a little bad telomerase we saw a powerful effect." Blackburn, in 1985, co-discovered the telomerase enzyme.
The study is published in the July 3 issue of Proceedings of the National Academy of Sciences.
The researchers created the mutation in a minute portion of the enzyme's template, a sequence of ribonucleic acid (RNA) that the enzyme synthesizes into deoxyribonucleic acid (DNA) and places on the tips of telomeres, DNA-protein complexes at the ends of chromosomes. Telomeres maintain the integrity of chromosomes and their ability to divide accurately during cell division. However, the tips of telomeres drop off each time a cell divides, and when they are gone a cell stops dividing. (This built-in limit on cell division, known as the Hayflick limit, was discovered in 1961 by UCSF adjunct professor of anatomy Leonard Hayflick, PhD.) The telomerase's ability to spin out DNA from an RNA template - a technique known as reverse transcriptase - to replenish telomeres is essential to the life of dividing cells.
The mutation within the telomerase RNA - which itself is a mere 450 nucleotides - was a tenth the size of a typical gene. Notably, it did not alter most telomeres' lengths. The finding suggests, says Blackburn, that "uncapping" of only one or a few telomeres per cell by the mutant telomerase can trigger a DNA damage response, thereby inducing cell-cycle arrest or inducing cell death in human cancer cells.
The researchers plan to explore the strategy in human cancer cells freshly derived from patients - as opposed to those in the current study, which were grown continuously in the lab dish. (Researchers suspect that cancer cells grown in culture adapt for growth - changing their genetic content - and thus may behave differently than cells extracted directly from patients.)
If the strategy proves viable, one possible therapeutic approach, says Blackburn, would be conducting gene "anti-therapy," as was done in the current study. Another would be using an as yet-to-be-identified "small molecule" drug that could warp telomerase and mimic the mutation in the same portion of the telomerase enzyme as the mutated gene.
In either case, the goal would be to target therapy directly and exclusively at the cancer cells, rather than systemically, thereby avoiding the drawback of many current cancer drugs, which often damage normal tissues.
The fact that telomerase is not activated in the healthy cells of many tissues could help this effort. For instance, in its earliest stages, breast cancer has not spread to the cells of the breast epithelium or the surrounding stroma. These healthy cells do not express telomerase, and therefore would not be vulnerable to a mutated gene or drug that targeted the enzyme in the breast.
In prostate cancer, targeted telomerase therapy also might be possible. Prostate cells have certain activated genes that are inactive in all other cells types. Given this, the mutated telomerase gene theoretically could be placed under the "promoter" region of a prostate-specific gene, which would make its expression only take place in prostate cells. As the first step in prostate cancer treatment is to remove the prostate, any prostate cells remaining in the body could be presumed to be cancerous.
While it's too early to know whether the approach identified by Blackburn's team will bear out, the revelation that human cancer cells are acutely sensitive to mutations in the telomerase enzyme is worth exploring further, she says. "Success will depend on the details and identifying a method of delivery, but it is one of many reasonable targets out there."
Co-authors of the study were Moses M. Kim, BS, an MD, PhD candidate, Melissa A. Rivera, BS, a PhD candidate, and Inna L. Botchkina, BS, a staff research associate, of the UCSF Department of Biochemistry and Biophysics; Refaat Shalaby, PhD, of California Pacific Medical Center and Ann D. Thor, MD, Evanston Northwestern Healthcare.
The study was funded by grants from the Department of Defense, the UCSF Comprehensive Cancer Center, the Steven and Michelle Kirsch Foundation, a UCSF Prostate Cancer Developmental Grant and a SPORE grant from the National Institutes of Health and a Medical Scientist Training Program grant from the NIH.