**9. Conclusion**

*DNA - Damages and Repair Mechanisms*

ICLs in the supernatants of Xenopus.

with accumulating evidence for ICL repair in G1.

**8.4 Suppression of NHEJ reduces ICL sensitivity**

translocations in case of joining of incorrect ends.

*anemia pathway of DNA repair*

In response to cross-linking agents, FANCD2 has been exposed to co-localize with Nucleotide Excision Repair component, XPF that affects the solidity of ubiquitylated FANCD2. After replication arrest, FANCD2 has also been shown to co-localize with Rev1 [88]and core complex components of FA i.e. FANCA and FANCG have been shown to be required for Rev1 foci formation [89]. Because of a dynamic ability to play an indecisive role in HDR and upstream process of TLS and NER, FA pathway orchestrates and regulate such repair mechanisms for a suitable removal of ICL damage. In this way, inactivation of FANCD2 affect both nucleolytic incision and translesion synthesis [90]. Recent investigations have examined the role of FA pathway in ICL repair by means of DNA substrates carrying site-specific

Having a DNA substrate containing MMC-like ICL adducts significantly distorts DNA helix. The other study has stated that ICL repair can proceed through replication dependent and independent mechanisms [85]. In nutshell, ICL repair could take place in an absence of DNA replication in *Xenopus* extracts and upon transfection of an ICL- containing plasmid in G1-arrested mammalian cells is consistent

*8.3.1 RUNX poly(ADP-ribosyl)ation and BLM interaction facilitate the Fanconi* 

an impending mechanism for FA pathway inactivation in cancers [91].

Even though Homologous Recombination promotes repair of double strand break in S-phase, an alternative mechanism, Non-homologous end joining (NHEJ) also exist to repair damaged DNA in all phases of the cell cycle. The phenomenon of NHEJ employs a simplest mechanism of splicing to rejoin the free end of DNA. The process involves the binding of KU70-KU80 heterodimers to the free doublestranded ends of DNA, thus allows the binding of DNA-dependent kinase subunit (DNA-PKcs) and initiates the activation of downstream steps [92]. DNA is processed to remove 5′-or 3'-ssDNA tails and the subsequent ends are directly rejoined by the activity of DNA ligase IV-XRCC4. Unlike HRR, in which homologous sequences proofread the repair process, NHEJ generates deletions, insertions and

In past, researches on mice and yeast has stated the notion that human cell lines defective in factors of Non-homologous end joining i.e. KU70, KU80, Ligase, DNA-PKcs or XRCC4, donot exhibit hypersensitivity towards ICL-inducing agents [93]. However, recent analysis has indicated that inhibition of NHEJ pathway in cell lines of FA patients can reduce the toxicity of ICL-inducing agents. For instance, in a knockout model of chicken or nematode, specific FA-like defects can be salvaged by the co-deletion of ligase IV or KU70. Moreover, through simultaneous inhibition

Fanconi anemia is considered as a universal genome maintenance network that orchestrates the repair of DNA interstrand crosslinks (ICL). The tumor suppressors RUNX1 and RUNX3 have been shown to regulate the FA pathway independent of their canonical transcription activities, by controlling the DNA damage dependent chromatin association of FANCD2. RUNX3 usually modifies by PARP-dependent poly(ADP-ribosyl) ation which in turn allows RUNX binding to DNA repair structures lacking transcription-related RUNX consensus motifs. After DNA gets damage, the increased interaction between RUNX3 and BLM facilitates the efficient FANCD2 chromatin localization. The mutations of RUNX-Walker motif in breast cancers have been impaired for DNA damage-inducible PARylation, thus unveiling

**132**

The development of interstrand cross-links play a chief role in the mechanism of significant chemotherapeutic agents. Emerging evidences suggest that these ICL lesions may also be formed by environmental agents and unwanted byproducts of metabolic processes. A better understanding of these lesions could lead to the improvement of supplementary therapeutic agents and strategies. However, despite the efforts of considerable investigations, the mechanism of ICL repair is still an enigma. At the transcriptomic level, proteins involved in a number of repair pathways have been identified. However, the detailed analysis of conditions in which repair should occur is largely elusive. What's clear is that a repair of interstrandcross links in eukaryotes involves multiple factors from NER and HRR pathways. Given the state of activities, it is ostensible that diverse experiments need to be done before we get a vivid picture of this important repair mechanism.
