Targeting and processing of site-specific DNA interstrand crosslinks

Abstract

DNA interstrand crosslinks (ICLs) are among the most cytotoxic types of DNA damage, and thus ICL-inducing agents such as cyclophosphamide, melphalan, cisplatin, psoralen, and mitomycin C have been used clinically as anticancer drugs for decades. ICLs can also be formed endogenously as a consequence of cellular metabolic processes. ICL-inducing agents continue to be among the most effective chemotherapeutic treatments for many cancers; however, treatment with these agents can lead to secondary malignancies, in part due to mutagenic processing of the DNA lesions. The mechanisms of ICL repair have been characterized more thoroughly in bacteria and yeast than in mammalian cells. Thus, a better understanding of the molecular mechanisms of ICL processing offers the potential to improve the efficacy of these drugs in cancer therapy. In mammalian cells, it is thought that ICLs are repaired by the coordination of proteins from several pathways, including nucleotide excision repair (NER), base excision repair (BER), mismatch repair (MMR), homologous recombination (HR), translesion synthesis (TLS), and proteins involved in Fanconi anemia (FA). In this review, we focus on the potential functions of NER, MMR, and HR proteins in the repair of and response to ICLs in human cells and in mice. We will also discuss a unique approach, using psoralen covalently linked to triplex-forming oligonucleotides to direct ICLs to specific sites in the mammalian genome.

Figure 1. Model of ICL repair in mammalian cells

The schematic depicts two modes of ICL processing in mammalian cells. On the left (“G₀/G₁”) is a potential pathway of ICL repair in nonreplicating cells; and on the right (“S”) is a potential pathway of ICL repair in replicating cells

Figure 2. Target duplex and psoralen-modified TFO

(A) Space-filling model of a psoralen-modified TFO bound to its target duplex. The psoralen moiety is shown in yellow and the TFO is shown in red bound to the purine-rich strand of the target duplex (blue) in the major groove. The pyrimidine-rich strand is shown in green. (B) The TFO binding site is shown in red, the psoralen crosslinking site is shown in yellow, and the pyrimidine-rich (py) and purine-rich (pu) strands are depicted in green and blue, respectively. The sequence of the TFO binding site is shown in bold capital letters, the psoralen crosslinking site is underlined (Christensen et al. 2008)

Figure 3. Potential interactions between NER and MMR proteins in processing ICLs

Two general models by which MutSβ (MSH2-3) may participate in ICL processing. In (i), participates in damage recognition of an ICL and acts as an accessory factor to NER proteins for ICL unhooking. This is experimentally supported by the known binding of MutSβ to an ICL and the enhancement of incision and repair synthesis at an ICL. In (ii), MutSβ acts to help recruit and anchor ERCC1-XPF to an ICL. When the ICL is present at a structure such as a blocked replication fork, ERCC1-XPF can cleave near the crosslink. Alternatively, rather than unhooking, a cleavage reaction by ERCC1-XPF could occur during recombinational processing of a broken replication fork. A catalytic role for MLH1-PMS2 is also possible, for example utilizing its nuclease activity in an end-processing reaction