**7. ICL recognition and repair in proliferating S-phase**

The repair of ICL faces several complications during the S phase. The data exhibits the formation of double stranded breaks by interaction with ICL causing agents [59]. The ICL induced Double stranded breaks can be repaired by HR rather than non-homologous end joining (NHEJ) method [68]. This brings to the conclusion that ICL-induced DSBs are linked with DNA replication forks. NER indicates ICLs in *S. cerevisiae* and NER function is important for ICL repair. So, all NER-mutants exhibit hyper sensitivity to the ICL causative agents. Contrarily, the cells deficient in *XPF*- as well as *ERCC1*- show immense hypersensitivity to the ICL agents (mitomycin C & nitrogen mustard) in mammals. The product of *XPF* as well as *ERCC1* make up an endonuclease which is hetero-dimeric in nature identifies and incise the single stranded branched structures [69]. Moreover, MUS81-EME1 along with XPF-ERCC1, the homologous structure specific endonucleases are also keen in repairing the ICL lesions [70]. MUS81-EME1 is notable for its binding with the double-stranded branched structures, flaps at 3′ end, as well as Holliday junctions [71]. Either of the two XPF-ERCC1 and MUS81-EME1 are responsible for ICL-induced double strand formation. Since, a multitude of nucleases are recognized recently being the key players in ICLs incision, the mechanism underlying the process need to be explored. We abridge the current knowledge about the ICL repair mechanism in S phase. HR repairs the ICLs induced DSBs. An experiment conducted in *S. cerevisiae*, gives an outline of hypersensitivity against ICL causative agents in *rad51*, *rad52, rad54*, *rad59*, as well as *mre11* mutants but not in case of *yku70* mutants. The hypersensitivity of *rad52 yku70* double mutants to ICLs is at par with that of rad52 mutants [72]. The HR deficient strains show the increase in accumulated DSBs successively on treating with ICL inducers as there lacks an ability to cure DSB which means that NHEJ is not a pre-requisite to remediate DSBs stimulated by ICLs. The mammals follow the same process in their cells. The HR deficient cells depict hypersensitivity against ICLs like cells having mutated paralogs of *RAD51*, *RAD54*, *RAD54B*, along with *BRCA2*, while it is not observed in cells deficient in NHEJ [73]. It significantly highlights the role of HR in repairing DSBs and re-initiating the halted replication forks of DNA. Fanconi anemia (FA) genes are key players in the remediation of ICL in eukaryotes. The proper role of FA gene products in biochemical reactions are still not identified properly, but are notable for their control of HR at the replication forks of DNA [74].

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**Figure 1.**

*Interstrand Crosslink Repair: New Horizons of DNA Damage Repair*

**8. Interstrand crosslinks lesion repair mechanisms**

Lesions in interstrand crosslinks epitomize an arduous challenge in genome maintenance pathways due to the compromise of genomic information present on both strands. Therefore, an application of non-damaged strand as a template for accurate repair in straightforward cut and patch mechanism is not feasible. In this regard, ICL repair employs the concerted and synchronized interaction of dynamics from numerous mechanisms of DNA damage repair, including NER, homologous recombination, mismatch repair, translesion synthesis, ataxia telangiectasia, Rad3 related and Fanconi anemia pathway. **Figure 1** illustrates the schematic mechanism of ICL repair [75].

*Schematic of ICL repair mechanism. (1) After the formation of ICLsin the cells, UHRF1 is recruited through its SRA domain immediately. (2) Single replication fork reaches at ICL. (3) Then Replication machinery is transversed through ICL by the help of FANCM/MHF complex and allowes the ICL for later repair. (4), (5) On an alternate basis FANCS or BRCA1 allows the unloading of CMG helicase complex, when second replication fork arrives at ICL. (6) Then replicative polymerase reaches at −1 position of ICL, leaving X shaped similar to the transverse mechanism. (7) Then ATR allows the phosphorylation of FANCD2/FANCI complex at multiple sites and meanwhile FA core complex mono-ubiquitinate at FANCD2/FANCI complex at K561 and K523 respectively. (8) The complex is then recruited to ICL at the replication fork. (9), (10) This ubiquitinated complex recruits SLX4/XPF on ICL in order to unhook the ICL. (11) Afterwards, CtlP an MRN complex resect the double strand breaks and BRCA2 facilitates the formation of RAD51 filament on single stranded DNA generated by resection. (12) Then Polζ carry out the polymerization step through the unhooked ICL. (13) Rad51 then facilitates the invasion of strand with subsequent extension of the other strand. (14) Lastly SLX4 and nucleases resolve the Holliday junction (15) and NER repair proteins remove the damaged nucleotides.*

*DOI: http://dx.doi.org/10.5772/intechopen.97551*

*Interstrand Crosslink Repair: New Horizons of DNA Damage Repair DOI: http://dx.doi.org/10.5772/intechopen.97551*
