DNA Damage
Nature has found numerous and sophisticated ways to address DNA damage, whatever the source and whatever the organism; both eukaryotes and prokaryotes have many known DNA damage repair mechanisms in response to oxidation, base hydrolysis, and UV exposure, to name a few. Paradoxically, some DNA damage is good and some DNA damage repair is bad. It turns out non-differentiated cells, such as stem or cancer cells, progress through certain stages of the cell division cycle faster than specialized cells like neurons and red blood cells (Figure 1)1. Due to this timing differential, chemical and radiation treatments have a higher probability of damaging the DNA in cancer cells at susceptible points in the cell cycle, resulting in selective cell death. In the case of cancer, DNA damage can be a very good thing and DNA damage repair can reduce the effectiveness of chemotherapy.
Serendipitously discovered, cisplatin (1) is one of the most common and effective chemotherapies. Platinum binds the purines in DNA and distorts the double helix structure; if the timing is right, damage repair processes will route the cancer cell to cell death when they encounter the cisplatin-DNA adduct. Within 2-3 hours of intravenous injection, cisplatin accumulates in the cell nucleus, undergoes ligand exchange to form a platinum-aquo species, and binds to DNA. Within 6-8 hours, DNA damage repair mechanisms identify the distortion of the DNA structure and begin repair processes. Approximately 2/3 of the Pt-DNA adducts are removed within 21 hours2. Thus, a cancer cell must cycle through the cell division checkpoints and be routed to cell death within this ~20 hour period.
The amazing coincidence that platinum ligand exchange rates are on the timescale of cell division in certain cancers is the reason cisplatin is an effective chemotherapeutic agent. However, the type and stage of cancer dictate the rate at which the cancer cell advances through the cell division cycle, and this is highly variable between different individuals3. Since the effectiveness of cisplatin comes from gaming the cancer cell cycle, can exploiting the different ligand exchange rates of the transition metals game the system even more to reach cancers outside the range of cisplatin? The inherent ligand exchange rates of the transition metals highlighted in Figure 3 span orders of magnitude and will encompass the cell division rate of any given cancer (Note: the ligand exchange rate of cisplatin is around 10-3 s-1)4. The chemistry that cisplatin undergoes is not special or unique to platinum. The only requirement for an effective chemotherapy drug is that it binds to DNA, distorting its structure enough to be reported as DNA damage by the cell repair mechanisms. Since many transition metals have been shown to bind to DNA, it only remains to see if matching the ligand exchange rate with the rate of progression through the cancer cell cycle can create more effective, customized drugs. Doctors could then prescribe personalized chemotherapies using the full range of ligand exchange rates from the different transition metals to treat the specific cancer growth rates of individual patients.
DNA Damage Repair
Of course, drug delivery is only half the battle in chemotherapy. Resistance to chemotherapy is often due to the functioning of DNA damage repair mechanisms in cancer cells. Cisplatin is deactivated by the nucleotide excision repair (NER) system comprising some ~20 proteins which recognize and then excise the distorted cisplatin-DNA adduct. Cancer cell lines in which NER is impaired are much more sensitive to cisplatin, while increased NER activity leads to cisplatin resistance5. To make chemotherapy more effective, can we change the rules of the game by delaying NER long enough for a cancerous cell to be preferentially routed to cell death? It is important to specifically target the part of NER that cuts out the damaged DNA section; the recognition mechanism of NER must remain functional in order for the cisplatin-DNA adduct to be registered as DNA damage and induce cell death. Of the many proteins involved in NER, it is well established that a dimer of the proteins XPF-ERCC1 cleaves the 5’ side of the DNA to release the cisplatin-DNA adduct (Figure 4)6. Disruption of the protein ERCC1 is known to increase sensitivity to cisplatin treatment, therefore this protein would be a good place to start in the effort to disrupt NER. A selective, yet effective, method of delay could be through a small molecule capture agent covalently coordinated to a chemotherapy drug that spatially blocks the DNA damage repair mechanism. An obvious target region of the XPF-ERCC1 dimer for a capture agent is the amino acid sequence CRSLMHH segment of the XPF sequence (see inset of Figure 4). A molecule with high affinity for the sulfurs and nitrogens in the cysteine, methionine, and histidines of this segment could be designed to geometrically snap selectively onto the XPF-ERCC1 dimer. The concentrations of DNA and any cancer drug would be orders of magnitude higher than the concentration of the NER proteins, therefore an effective Your browser may not support display of this image. chemotherapy drug with a coordinated capture agent could nearly quantitatively yet reversibly inhibit NER for the desired length of time.
As the scientific community learns more about DNA damage repair, we will undoubtedly be able to better address complex systems such as cancer. The timing of cisplatin-induced DNA damage within the cancer cell cycle was a lucky break, but I predict chemotherapy can be tailored through the utilization of traditional inorganic chemistry to synchronize treatment to cancer growth rates. Using crystal structures of DNA damage repair proteins to design capture agents that delay excision of chemotherapy-DNA adducts could make cancer treatment even more effective. With greater molecular understanding and design, we can change the game, and achieve more successful, more predictable outcomes.
1. Figure modified from Rang, H.P. and Dale, M.M., (1991) Pharmacology; Longman Group UK Limited, Second Edition, pp. 781-789.
2. Wang, D., and Lippard, S.J., (2005) Cellular processing of platinum anticancer drugs, Nature Revs. 4, 307-320.
3. For illustrative purposes, certain strains of Human Papillomavirus (HPV) have a culture doubling time of 14 hours while other strains of Leukemia (HL-60) have a doubling time of 48 hours.
Isaka, K., et. al, (2004) Establishment of a HPV and p53-mutation-negative human cell line (CA) derived from a squamous carcinoma of the uterine cervix , Gynecologic Oncology 92, 15-21; Akman, S. A., et. al, (1981) Growth Inhibition by Thymidine of Leukemic HL-60 and Normal Human Myeloid Progenitor Cells, Cancer Research 41, 2141-2146.
4. Figure modified from Reedijk, J., (1999) Why does cisplatin reach guanine-N7 with competing S-donor ligands available in the cell? Chem. Rev. 99, 2499-2510.
5. Masters, J.R.W., and Koberle, B., (2003) Curing metastatic cancer: lessons from testicular germ-cell tumours, Nature 3, 517-525; Selvakumaran, M., Pisarcik, D.A., Bao, R., Yeung, A.T., and Hamilton, T.C., (2003) Enhanced cisplatin cytotoxicity by disturbing the nucleotide excision repair pathway in ovarian cancer cell lines, Cancer Res. 63, 1311-1316.
6. Li, Q., Gardner, K., Zhang, L., Tsang , B., Bostick-Bruton, F., and Reed, E., (1998) Cisplatin induction of ERCC-1 mRNA expressions in A2780/CP70 human ovarian cancer cells, J. Biol. Chem. 273, 23419-23425.
