Because of the way DNA replication machinery works, it is not possible to replicate an entire chromosome - at each round of cell division, chromosomes lose an estimated 100 base pairs from the 5' end of each DNA strand.
However, cells have turned that potential problem into a positive. The ends of chromosomes, technically known as telomeres, do not code for genes; instead, they consist of repeats of the sequence TTAGGG. Because chromosomes lose an average of 16 such TTAGGG repeats per cell division, telomere length is one way in which cells gauge their age, and after telomere length has fallen below a critical value, further division is not possible, and the cell dies.
Therefore, maintaining telomere length is one way in which cancer cells avoid death. It is well established that inhibiting telomerase, the enzyme that maintains telomere length, can foil cancer cells' aspirations to immortality, and companies including Geron Corp. and CancerVax Corp. are working in that field. Research published in the January 2005 issue of Cancer Cell suggests that the therapeutic approach could receive a boost by simultaneously targeting a related molecule, tankyrase, to hasten the telomere's demise.
With telomerase inhibitors, there is a significant time lag between treatment onset and cell death; in other words, it takes a number of cell cycles for the telomeres to shorten sufficiently to induce a DNA damage response. That leads to the necessity of a fairly long-term continuous treatment - and since cancer cells retain a robust ability to evolve, the potential risk of acquired drug resistance.
"In our experience, it took 20 to 90 cell divisions" for cancer cells to reach crisis once telomerase had been blocked, Hiroyuki Seimiya told BioWorld Today in an email interview. Seimiya is principal investigator at the Japanese Foundation for Cancer Research in Tokyo and lead author of the current Cancer Cell paper, which was co-published with scientists from the University of Tokyo. "We predicted that combination therapy would be a good idea to solve the problem [of treatment length and resistance]. If we could increase the rate of telomere attrition, we could shorten the time period for drug treatment and thus reduce the risk of drug resistance."
The scientists decided to focus their efforts on the enzyme tankyrase. Even in cancer cells, telomeres do not extend indefinitely, and tankyrase plays a role in determining telomere length through its interaction with the protein TRF1, which binds to telomeres to suppress their elongation. Tankyrase chemically modifies TRF1 to inhibit such binding and thus allow telomerase access to the telomeres.
Because tankyrase contains no signal that directs it to the nucleus and excess tankyrase usually will just accumulate in the cytoplasm, the scientists first made a tankyrase with a nuclear localization signal. Expressing that tankyrase in cells led to down-regulation of TRF1 and subsequent telomere elongation by telomerase.
The scientists next treated tumor cells in culture with a telomerase inhibitor, MST-312; that treatment led to gradual telomere shortening and, after about 90 cell divisions, the cells were unable to divide further, though untreated cells kept dividing. Control experiments showed that MST-312 did not directly affect tankyrase. However, overexpressing tankyrase in MST-312-treated cells made them resistant to the effects of telomere inhibition; like untreated cells, such cells kept dividing at least 150 times.
The chemical modification of TRF1 by tankyrase is called poly-ADP ribosylation. The researchers next tested how inhibitors of poly-ADP ribosylation, known as PARP inhibitors, would affect tankyrase and telomere length. The scientists blocked tankyrase's effects on TRF1 with a PARP inhibitor. That treatment reversed the telomerase inhibitor resistance of tankyrase-overexpressing cells and enhanced the effects of telomerase inhibitors in cells with normal amounts of tankyrase by enhancing the binding of TRF1 to DNA.
Importantly, PARP inhibitors shortened the time it took for treatment with telomerase inhibitors to lead to cell death in culture.
"Simultaneous inhibition of telomerase and tankyrase reduced the number of cell division from 90 to mid-30," Seimiya said. As for what length of time that reduced ability to divide might correspond to in patients, Seimiya said that remains to be determined.
He "definitely" wants to take his findings into the clinic, and is looking for commercial partnerships to that end. However, he pointed out that several studies will need to be conducted before tankyrase inhibition is ready. "Since there are many other PARP enzymes, it is important to obtain a specific inhibitor for tankyrase, which would not affect other family enzymes.
"Moreover, we still need to know the consequences of tankyrase inhibition in normal cells, since tankyrase is ubiquitously expressed and present in the cytoplasm, too," he said.