Cellular response to DNA interstrand crosslinks: the Fanconi anemia pathway

Abstract

Interstrand crosslinks (ICLs) are a highly toxic form of DNA damage. ICLs can interfere with vital biological processes requiring separation of the two DNA strands, such as replication and transcription. If ICLs are left unrepaired, it can lead to mutations, chromosome breakage and mitotic catastrophe. The Fanconi anemia (FA) pathway can repair this type of DNA lesion, ensuring genomic stability. In this review, we will provide an overview of the cellular response to ICLs. First, we will discuss the origin of ICLs, comparing various endogenous and exogenous sources. Second, we will describe FA proteins as well as FA-related proteins involved in ICL repair, and the post-translational modifications that regulate these proteins. Finally, we will review the process of how ICLs are repaired by both replication-dependent and replication-independent mechanisms.

Keywords: DNA repair; FANCD2; FANCI; Genomic instability; Phosphorylation; SUMO; UHRF1; Ubiquitination.

Fig. 1

Schematic representation of the chemical structure of the main crosslinking agents and the ICLs they form. a Mechlorethamine (nitrogen mustard), b mitomycin C, c cisplatin, d psoralen, e BCNU (nitrosourea), f diepoxybutane, g aldehydes [acetaldehyde, acrolein and crotonaldehyde (R = CH₃)] and h nitric oxide. Crosslinking agents are shown in red

Fig. 2

Structures of various ICLs. a B-DNA and the ICLs formed by b cisplatin, c psoralen, d BCNU and e acetaldehyde and crotonaldehyde viewed from the major groove (left) or the minor groove (right). The crosslinked bases and the crosslinking agents are shown in red. Structures taken from PDB, accession numbers: B-DNA (1-BNA) [140], cisplatin (1A2E) [13], psoralen (204D) [21], BCNU (2MH6) [23], acetaldehyde (2HMD) [29]

Fig. 3

Diagram of the main posttranslational modification events involved in the activation of response to ICLs. Once the ICL is detected it triggers the ATR/Chk1 pathway leading to the phosphorylation of several components of the FA core complex. ATR and potentially other kinases phosphorylate FANCI and FANCD2 (1) priming the complex for its monoubiquitination. These phosphorylation events lead then to the monoubiquitination of the FANCD2/FANCI complex by FANCL/UBE2T (2), which promotes its recruitment onto chromatin and the action of the effector proteins. On the other hand, the dosage of the FANCD2/FANCI complex on chromatin can be regulated through SUMOylation-dependent polyubiquitination mediated by PIAS1/4, UBC9 and RNF4 (3, 4). Finally, these events can be reversed by the action of a hypothetical deubiquitinase (5), SENP6 (6), the deubiquitinating enzyme USP1/UAF1 complex (7) as well as putative phosphatases (8) still unidentified

Fig. 4

Schematic of ICL repair. 1 UHRF1 is recruited to ICLs through its SRA domain shortly after ICLs are formed in the cell. 2 Single replication fork arrives at the ICL. 3 FANCM/MHF complex mediates the traverse of the replication machinery through the ICL, which allows the replication fork to proceed, and leaves the ICL for later repair. 4, 5 Alternatively, BRCA1 (FANCS) facilitates the unloading of the CMG helicase complex when the second replication fork arrives at the ICL. 6 The replicative polymerase proceeds to the −1 position of the ICL, which leaves an X-shaped structure similar to the traverse mechanism. 7 ATR phosphorylates FANCD2/FANCI complex at multiple sites and FA core complex monoubiquitinates FANCD2/FANCI complex at K561 and K523, respectively. 8 FANCD2/FANCI is recruited to the ICL at the replication fork. 9, 10 Ubiquitinated FANCD2/FANCI complex recruits SLX4/XPF to ICL to unhook the ICL. 11 CtIP and the MRN complex resect the double-strand break ends generated by the incision in the previous step, and BRCA2 facilitates Rad51 filament formation on the ssDNA generated by the resection. 12 Polζ polymerizes new strand of DNA through the unhooked ICL. 13 Rad51 facilitates the strand invasion, which allows extension of the other strand. 14 SLX4 and nucleases resolve the double Holliday junction. 15 NER repair proteins remove the damaged nucleotide