Interstrand Crosslink Repair

Chapter 22 describes how different chemical agents react with DNA and form interstrand crosslinks (ICLs) that covalently link the two strands together. This poses a special problem for the cell in that the replication and transcription machineries are unable to progress or unwind the helix at the sites where the two strands are covalently linked to each other. It also presents a challenge for DNA repair since the damaged sites occur in close proximity to each other on opposing strands. Hence it is difficult for one strand to serve as a template for repair of the other strand because they both contain damaged DNA. 

Fanconi anemia (FA) is a rare genetic disease. Patients are highly predisposed to developing a wide variety of different types of cancer. Cells from FA patients are specifically hypersensitive to crosslinking agents. These observations lead to the prediction that FA cells are deficient in the repair of ICLs. While evidence for this 

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Sarkar, S., Davies, A.A., Ulrich, H.D., and McHugh, P.J. (2006) DNA interstrand crosslink repair during Gj involves nucleotide excision repair and DNA polymerase i. EMBO J., 25, 1285-1294. 

Deans AJ, West SC (2011) DNA interstrand crosslink repair and cancer. Nat Rev Cancer 11 467 80... 

Kasparkova, J. Zehnulova, J. Farrell, N. Brabec, V. DNA interstrand crosslinks of novel antitumor trinuclear platinum complex BBR. Conformation, recognition by HMG-domain proteins and nucleotide excision repair. J Biol Chem 2002, 277, 48076-48086. 

If a DNA adduct involves the nitrogen or oxygen atoms involved in base-pairing, and the adducted DNA is not repaired, base substitution can result. Adducts can be small, such as the simple addition of methyl or ethyl groups, or they can be very bulky, owing to reaction with multiringed structures. The most vulnerable base is guanine, which can form adducts at several of its atoms (e.g., N7, C8, O6 and exocyclic N2) (Venitt and Parry, 1984). Adducts can form links between adjacent bases on the same strand (intrastrand cross-links) and can form interstrand crosslinks between each strand of double-stranded DNA. 

Importantly, the ERCCl/XPF structure-specific nuclease has an additional role in the repair of cisplatin adducts besides its function in NER the recombinational repair of interstrand crosslinks (19). ERCCl- or XPF-deficient hamster mutant cell lines are hypersensitive to DNA crosslink agents, much more so than to ultraviolet-induced pyrimide dimers, the classical substrates for NER (20,21). Moreover, co-localization of ERCCl foci and RAD51 foci in response to cisplatin treatment has recently been found and may represent recruitment of ERCCl/XPF to sites of recombination repair (22). Previous studies have shown that BRCAl, involved in homologous recombination repair, also plays a major role in the repair of cisplatin DNA damage (23). 

Interstrand crosslinks pose a considerable challenge for replication, transcription, and repair. The two DNA strands are covalently linked together by the crosslinking agent, and thus cannot be separated. Crosslinks are repaired in cells, however, their... 

Another alkylating drug in clinical use is mitomycin C. As a natural product isolated from Stremtomyces lavendulae, mytomycin C is an aziridine compound, which is closely related to nitrogen mustards. This compound is best known to the DNA repair field as a DNA interstrand crosslinking agent. Mytomycin C has been used to treat breast cancer and cancers of the gastrointestinal tract. 

Cole, R.S. (1973) Repair of DNA containing interstrand crosslinks in Escherichia coli sequential excision and recombination. Proc. Natl. Acad. Sci. USA, 70, 1064-1068. 

Figure 5, Repair of ci DDP-iruiuced DNA-protein crosslinks and DNA inter-strand crosslinks in human fibroblasts during holding in the nondividing state. Stationary cultures were treated with 40 fjjnol cvs-DDP, and after various periods of holding in the nondividing state, crosslinking was measured by alkaline elution (U DNA-protein crosslinks and O, DNA interstrand crosslinks) or by estimation of renaturable DNA in cell lysates (%, DNA interstrand crosslinks).

The equations describing both the cytotoxicities and the alkylating abilities of substituted aniline chloromustards have similar slopes, suggesting that these compounds express their cytotoxicity primarily by alkylation. It is evident that the critical biological alkylation site is DNA, and that cytotoxicity is due to formation of DNA interstrand crosslinks, since the UV4 mutant defective in repair of these lesions (J4) shows marked hypersensitivity to the aniline mustards. 

While three various types of crosslinks can be formed by cisplatin, the intrastrand crosslinks are more common. Most of these crosslinks can be repaired, but at least one type of interstrand crosslink may not induce response of cellular repair enzymes. Fmlher, while intrastrand crosslinks are important in describing the activity of cisplatin, inducing apoptosis also appears to be a factor in the mechanism of cisplatin s anticancer activity. ... 

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