Differential roles for DNA polymerases eta, zeta, and REV1 in lesion bypass of intrastrand versus interstrand DNA cross-links

Suppression of Rev3, the catalytic subunit of Pol{zeta}, sensitizes drug-resistant lung tumors to chemotherapy

Platinum-based chemotherapeutic drugs are front-line therapies for the treatment of non-small cell lung cancer. However, intrinsic drug resistance limits the clinical efficacy of these agents. Recent evidence suggests that loss of the translesion polymerase, Polζ, can sensitize tumor cell lines to cisplatin, although the relevance of these findings to the treatment of chemoresistant tumors in vivo has remained unclear. Here, we describe a tumor transplantation approach that enables the rapid introduction of defined genetic lesions into a preclinical model of lung adenocarcinoma. Using this approach, we examined the effect of impaired translesion DNA synthesis on cisplatin response in aggressive late-stage lung cancers. In the presence of reduced levels of Rev3, an essential component of Polζ, tumors exhibited pronounced sensitivity to cisplatin, leading to a significant extension in overall survival of treated recipient mice. Additionally, treated Rev3-deficient cells exhibited reduced cisplatin-induced mutation, a process that has been implicated in the induction of secondary malignancies following chemotherapy. Taken together, our data illustrate the potential of Rev3 inhibition as an adjuvant therapy for the treatment of chemoresistant malignancies, and highlight the utility of rapid transplantation methodologies for evaluating mechanisms of chemotherapeutic resistance in preclinical settings.

RAD18-mediated ubiquitination of PCNA activates the Fanconi anemia DNA repair network

The DNA damage–activated E3 ubiquitin ligase RAD18 promotes repair of interstrand DNA cross-links by ubiquitylating PCNA and recruiting FANCL to chromatin.

The Fanconi anemia (FA) network is important for the repair of interstrand DNA cross-links. A key event in FA pathway activation is the monoubiquitylation of the FA complementation group I (FANCI)–FANCD2 (ID) complex by FA complementation group L (FANCL), an E3 ubiquitin ligase. In this study, we show that RAD18, another DNA damage–activated E3 ubiquitin ligase, also participates in ID complex activation by ubiquitylating proliferating cell nuclear antigen (PCNA) on Lys164, an event required for the recruitment of FANCL to chromatin. We also found that monoubiquitylated PCNA stimulates FANCL-catalyzed FANCD2 and FANCI monoubiquitylation. Collectively, these experiments identify RAD18-mediated PCNA monoubiquitination as a central hub for the mobilization of the FA pathway by promoting FANCL-mediated FANCD2 monoubiquitylation.

Rad18-mediated translesion synthesis of bulky DNA adducts is coupled to activation of the Fanconi anemia DNA repair pathway

Fanconi anemia (FA) is a cancer susceptibility syndrome characterized by sensitivity to DNA-damaging agents. The FA proteins (FANCs) are implicated in DNA repair, although the precise mechanisms by which FANCs process DNA lesions are not fully understood. An epistatic relationship between the FA pathway and translesion synthesis (TLS, a post-replication DNA repair mechanism) has been suggested, but the basis for cross-talk between the FA and TLS pathways is poorly understood. We show here that ectopic overexpression of the E3 ubiquitin ligase Rad18 (a central regulator of TLS) induces DNA damage-independent mono-ubiquitination of proliferating cell nuclear antigen (PCNA) (a known Rad18 substrate) and FANCD2. Conversely, DNA damage-induced mono-ubiquitination of both PCNA and FANCD2 is attenuated in Rad18-deficient cells, demonstrating that Rad18 contributes to activation of the FA pathway. WT Rad18 but not an E3 ubiquitin ligase-deficient Rad18 C28F mutant fully complements both PCNA ubiquitination and FANCD2 activation in Rad18-depleted cells. Rad18-induced mono-ubiquitination of FANCD2 is not observed in FA core complex-deficient cells, demonstrating that Rad18 E3 ligase activity alone is insufficient for FANCD2 ubiquitylation. Instead, Rad18 promotes FA core complex-dependent FANCD2 ubiquitination in a manner that is secondary to PCNA mono-ubiquitination. Taken together, these results demonstrate a novel Rad18-dependent mechanism that couples activation of the FA pathway with TLS.

Translesion DNA synthesis polymerases in DNA interstrand crosslink repair

DNA interstrand crosslinks (ICLs) are induced by a number of bifunctional antitumor drugs such as cisplatin, mitomycin C, or the nitrogen mustards as well as endogenous agents formed by lipid peroxidation. The repair of ICLs requires the coordinated interplay of a number of genome maintenance pathways, leading to the removal of ICLs through at least two distinct mechanisms. The major pathway of ICL repair is dependent on replication, homologous recombination, and the Fanconi anemia (FA) pathway, whereas a minor, G0/G1-specific and recombination-independent pathway depends on nucleotide excision repair. A central step in both pathways in vertebrates is translesion synthesis (TLS) and mutants in the TLS polymerases Rev1 and Pol zeta are exquisitely sensitive to crosslinking agents. Here, we review the involvement of Rev1 and Pol zeta as well as additional TLS polymerases, in particular, Pol eta, Pol kappa, Pol iota, and Pol nu, in ICL repair. Biochemical studies suggest that multiple TLS polymerases have the ability to bypass ICLs and that the extent ofbypass depends upon the structure as well as the extent of endo- or exonucleolytic processing of the ICL. As has been observed for lesions that affect only one strand of DNA, TLS polymerases are recruited by ubiquitinated proliferating nuclear antigen (PCNA) to repair ICLs in the G0/G1 pathway. By contrast, this data suggest that a different mechanism involving the FA pathway is operative in coordinating TLS in the context of replication-dependent ICL repair.

A novel link to base excision repair?

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.