A novel link to base excision repair?

Abstract

DNA interstrand crosslinks (ICLs) can arise from reactions with endogenous chemicals, such as malondialdehyde - a lipid peroxidation product - or from exposure to various clinical anti-cancer drugs, most notably bifunctional alkylators and platinum compounds. Because they covalently link the two strands of DNA, ICLs completely block transcription and replication, and, as a result, are lethal to the cell. It is well established that proteins that function in nucleotide excision repair and homologous recombination are involved in ICL resolution. Recent work, coupled with a much earlier report, now suggest an emerging link between proteins of the base excision repair pathway and crosslink processing.

Figure 1

Examples of DNA Crosslinking Agents. The chemical structure of the indicated agent is shown in the top row. The bottom row depicts the nature of the ICL. (i) Nitrogen mustards form a 1,3 ICL between the N7-positions of two G residues on opposite strands within a 5'-d(GNC) sequence. Mitomycin C (MMC; ii) and cisplatin (iii) generate ICLs between juxtapose Gs of 5'-d(GC). (iv) [SC1]Psoralen can produce ICLs between the T residues of 5'-d(TA) complementary sequences upon exposure to UVA. The frequency of ICLs for cisplatin is around 5–10% of the total adducts, about 5% for nitrogen mustards, less than 10% for MMC, and 20–30% for psoralen. See for further details.

Figure 2

Conventional Mechanisms for ICL Repair. Cells cope with ICLs differently depending on the stage of cell cycle and mode of ICL recognition. a) G1 phase repair. ICL recognition is mediated by XPC–HR23B dependent functions, or perhaps by a block to transcription (not shown). The ERCC1–XPF endonuclease is required for crosslink “unhooking”, in which incisions are placed on either side of the linked base on one strand of the duplex. Further processing, perhaps by the SNM1A nuclease, converts the incision product to a gapped structure that can be filled-in by a translesion DNA synthesis (TLS) polymerase. This step completes the first stage of ICL repair and produces an intact duplex with a crosslink remnant that is a substrate for classic NER, which carries out the second and final stage of ICL repair. b) S phase repair. Recognition as a consequence of collision of the ICL with a progressing replication fork is followed by cleavages by MUS81–EME1 and ERCC1–XPF which unhook the crosslink. Additional processing as in G1 phase results in lesion bypass [with the aid of Fanconi anemia (FA) proteins] to generate a crosslink remnant. Classic NER excises and replaces the segment of DNA harboring the remnant, before the replication fork is reestablished by homologous recombination.

Figure 3

BER Pathway. The five enzymatic steps of BER are indicated to the left, and the major proteins involved in executing these steps are shown to the right. In brief, a DNA glycosylase (e.g. MPG or NEIL1) cleaves the bond that links the substrate base (filled diamond) to the sugar phosphate backbone, creating an AP site intermediate (filled circle). The AP site is typically incised by AP endonuclease 1 (APE1) 5' to the lesion to create a strand break with a 3'-hydroxyl group and a 5'-abasic residue (deoxyribose phosphate, dRP). Polymerase β (POLβ) then replaces the missing nucleotide (thicker line) and excises the 5'-dRP group. The remaining nick is sealed by a complex comprising X-ray repair cross-complementing 1 (XRCC1) and DNA ligase 3 (LIG3α). Shown is short-patch (or single nucleotide) BER, but there are alternative BER-related responses, such as long-patch BER, which involves the incorporation of multiple nucleotides and the formation of a 5'-flap DNA intermediate that requires processing by flap-endonuclease 1 (FEN1) (see, for further details).

Figure 4

Potential Involvement of BER Proteins in ICL Resolution. NEIL1 might initiate a classic BER response by excising (i) a psoralen monoadduct (MA) prior to the commencement of Stage I of ICL repair or (ii) the crosslink remnant (CR) formed at the completion of the first stage of ICL repair (see also Figure 2). This excision (glycosylase) activity would generate an abasic site (filled circle) that would be processed by APE1 and the remaining BER proteins (Figure 3). The precise involvement of MPG is less clear, but evidence suggests participation after ICL formation in a structural, non-enzymatic capacity.