By David N. Leff

One way or another, every dividing cell in everyone¿s body has a dicey future. For one thing, a given cell can divide only about 50 times before going out of business. For another, it runs an uncertain risk of becoming cancerous.

That 50-time cellular death sentence is known as the ¿Hayflick limit,¿ so called after its discoverer, anatomist Leonard Hayflick. Every chromosome in the human genome is capped at both ends by a stuttering stretch of DNA repeat sequences known as telomeres. Every time the cell divides, it loses 10 segments of these terminal telomeres, until the last one is toast, and the Hayflick grim-reaper verdict kicks in. (See BioWorld Today, May 8, 2000, p. 1.)

The molecule that creates and grows these telomeres during fetal development is the enzyme telomerase. After an infant is born, it switches hats and executes the piecemeal demise of those telomeres. If something upsets this enzyme¿s postnatal function, the telomeres revert to their prenatal stance, and hang in there indefinitely. Thus the cell becomes immortal ¿ in a less lyrical word, malignant.

Molecular oncologists in industry and academe have fixated on this anticancer concept in efforts to turn telomerase around once more, and allow the cell to resume its telomere-whittling gig. One recent advance in this field is reported in the April 1, 2001, issue of Cancer Research. Its title: ¿Ribozyme cleavage of telomerase mRNA sensitizes breast epithelial cells to inhibitors of topoisomerase.¿ Calvin Harley, chief scientific officer of Geron Corp. in Menlo Park, Calif., is a co-author.

¿Because the enzyme telomerase is responsible for allowing cells to divide indefinitely, to be immortal,¿ Harley told BioWorld Today, ¿if we can block or inhibit its production, we can cause the cancer cells to age, and ultimately die ¿ after they¿ve divided a certain number of times. That¿s how telomerase monotherapy might work.

¿The synergistic, added benefit of combining telomerase inhibition with other anticancer drugs that damage DNA,¿ he continued, ¿likely involves a role of DNA repair.¿

Ribozymes To In Vitro Rescue

¿When a cell has telomerase,¿ Harley went on, ¿it¿s more resistant to anticancer drugs. We showed it specifically for doxorubicin, a topoisomerase inhibitor, with our in vitro experiments on breast cancer cells or immortalized cell lines.¿ (Topoisomerase converts one DNA structure to another.)

As reported in their journal article, Harley and his co-authors utilized a novel ribozyme ¿ a nonprotein RNA biocatalyst ¿ to degrade the messenger RNA precursor of telomerase¿s protein subunit. ¿It is the first time, I believe,¿ he said, ¿that anyone has targeted the messenger RNA component of the telomerase catalytic subunit.

¿Our target cells,¿ Harley recounted, ¿were two different breast-cancer lines. In one case, it was a natural cell line, isolated from a breast tumor biopsy. The other was a normal, standard mammary epithelial cell line that had been immortalized, and is telomerase-positive. The transformed cells showed many changes in growth characteristics, and they¿ll also grow as human xenografts in immunocompromised mice. So in that sense they were transformed. The results were that we could achieve much higher levels of tumor killing when the telomerase ribozyme was combined with a DNA-damaging agent such as doxorubicin. And it provides additional evidence that a telomerase inhibitor could be synergistic with existing anticancer drugs.

¿We transduced or transfected the cells with a gene construct that makes a ribozyme,¿ Harley said. ¿The RNA enzyme can be engineered to directly cleave any specific mRNA that you target.

¿In this particular case,¿ Harley continued, ¿we chose a critical region of hTERT RNA [which stands for human telomerase reverse transcriptase¿]. It¿s the only reverse transcriptase that nonvirally infected human cells have. Human cells in fetal development have hTERT turned on. In normal body tissues its gene is turned off, but hTERT is abnormally reactivated in cancer. Interestingly, it¿s very close to the telomere, which sits on the short arm of chromosome 5, and its proximity to the telomere may or may not be a coincidence. We don¿t know yet. It has a telomerase-specific motif next to the catalytic region of the gene.¿

Harley described a threefold therapeutic strategy his team is now pursuing. ¿We have certain proprietary oligonucleotide strategies for targeting telomerase for cancer,¿ he related. ¿They involve focusing on three areas:

1. ¿Direct inhibition of the enzyme, through small molecules or oligonucleotides;

2. ¿Telomerase vaccine strategy for teaching the immune system to kill telomerase-positive tumor cells. That may be the product opportunity closest to the clinic. It¿s looking very good;

3. ¿Directing telomerase promoter-activated toxicity, which utilizes the regulatory region of the hTERT gene, turned on in tumor cells. It¿s a suicide-gene or oncolytic viral strategy, which can kill tumor cells specifically where the telomerase promoter is turned on, but not normal cells where it¿s turned off.¿

Putative Clinical/Marketing Partnership

¿The ribozyme strategy,¿ Harley observed, ¿which is really the focus of this research paper, is a possibility, but at Geron we¿re not doing ribozyme research ourselves. As we move toward the clinical implications, we are interested in possible applications of the ribozyme technology, which we may explore in the future with companies that are focusing on that,¿ such as Ribozyme Pharmaceuticals Inc. in Boulder, Colo.

That firm¿s vice president of research, Nissam Usmann, Harley recalled, ¿actually phoned me after this paper came out. He asked the status of clinical studies with our ribozymes, and our level of interest in telomerase.¿

¿Cal told me a little bit about it on the phone Friday, in a three-minute conversation.¿ Usmann told BioWorld Today. ¿The gist of it was,¿ he added, ¿that Geron used a ribozyme to inhibit the telomerase RNA fragment directly, and had a very nice antiproliferative effect in a cancer model.

¿We, of course,¿ Usmann remarked, ¿believe it¿s a very interesting finding, since we are also developing ribozymes for oncology. Telomerase is certainly becoming a more and more interesting target, and we are pleased that other researchers, both academic and in biotechnology, are using ribozymes to downregulate the expression of important targets in cancer. So we¿re very excited about this result.¿

¿Geron has the expertise in the field of telomerase,¿ Usmann went on. ¿We would provide the drug ¿ the ribozyme ¿ to downregulate the telomerase. How one would commercially set that up would have to be worked out later,¿ he concluded. ¿It¿s pretty straightforward.¿ n

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