**7. Evidence of clustered mutation influencing repair pathways**

APOBEC (apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like) enzymes and translesional DNA synthesizing enzyme are found to be associated with these events. There is literature available on this context. APOBEC enzymes are cytidine deaminase that is responsible for C → T transitions [53, 54]. H3K36me3 chromatin is normally protected from such somatic mutations, it is tri-methylation at the 36th lysine residue of the histone H3 protein, it relishes this protection from somatic mutation because of increased activity in canonical mismatch repair machinery at its locations. However, exposure to some carcinogens results in increased activity of a non-canonical, error-prone, mismatch repair pathway involving (POLH) DNA polymerase eta, which results in a relative increased mutation rate in H3K36me3-marked regions. This explains that some factors act as carcinogens not because they increase the mutation rate but because they relocate mutations to the more important regions of the genome. These environmental factors include alcohol, ionizing radiations, UV radiations etc. [6].

There are other evidence stating clustered mutations are driven by break induced replication (BIR) like mechanisms, which is associated with homologous recombination. Tremendous progress in whole genome analysis revealed that BIR is likely the mechanism of multiple genomic rearrangements in humans that give clustered mutation. To the date, there is no clear understanding of how BIR transforms from a beneficial pathway aimed at rescuing cells into a dangerous mechanism with high destabilizing potential [55].

These events are very common to cancer cells. They serve as source for catastrophically higher rate of mutational events giving rise to sustainable amount of genomic instability. And as mentioned several times before genomic instability is the prime mechanism for the cancer cell to hold control over cellular machinery for uncontrolled division these events are very specific to cancer cells and a proper process of these events has not yet been elucidated [51]. But we can clearly see the potential role of DNA repair systems in these clustered mutational events. Through clustered mutations the cancer cell tries to exhaust DNA repair pathways. Repair pathways are meant to repair the DNA at a specific rate, and they are designed to tackle a limited burden. When mutation rate become overwhelming for them, their fidelity exhaust and that is the opportunity cancer cells create to accumulate mutation [53].

Cancer cells first changes the expression and regulation of the DNA repair systems by either epigenetic modifications, mutating its coding sequence or regulatory sequence. This in turn gives error prone DNA repair system for clustered mutation. Again, the clustered mutation also exhausts the DNA repair systems leaving no chance for fixing the genomic instability taking place in the cell. There still lies a debate on how these catastrophic mutational processes occur. But there is proof that they are indebted to faulty repair systems for their birth [54].

## **8. Cancer therapy targeting DNA repair pathways**

Presently there are a few chemotherapeutic drugs and some of them are even in phase 2 or phase 3 trials. This itself emphasize the crucial role DNA repair pathways and proves that it is an important chemotherapeutic target. Basically, DNA repair inhibitors are used as chemotherapeutic drug they make already fragile DNA repair system of cancer to collapse leading to destruction of cellular homeostasis ultimately leading to cancer death [13, 26].

#### **8.1 MGMT inhibition**

The *O*6-methylguanine-DNA methyltransferase (MGMT), a DNA repair protein that removes alkyl group, was the target of the earliest attempt to develop a DNA repair inhibitor. MGMT is the most widely studied DNA repair mechanism [56].

In 1970s, nitrosoureas was introduced as a chemotherapeutic drug for glioblastoma and other malignant gliomas, it alkylates DNA at various positions on guanine, subsequently causing single- or double-strand damage which chemosensitiz cells to more damage by other drugs. Scientists quickly learned that something could reverse the DNA damage that they inflicted, that was MGMT. After some time, a potent MGMT inhibitor was used along with nitrosoureas but it did not work [26]. Although compromising MGMT fell short of expectations in chemo sensitizing tumors to alkylating agents, it was continued to be studied. There was evidence that in different cancer it is manipulated in different ways. But still there is not any effective drug involving this [57].

#### **8.2 PARP inhibitors**

The PARP is a nucleus specific enzyme that detects single strand breaks that are being formed spontaneously or during BER and binds to that position on the DNA strand. It then undergoes a structural change and begin synthesizing a polymeric adenosine diphosphate ribose (poly ADP-ribose) chain, which acts as signal for the other DNA-repairing enzymes. Three members of that family have roles in DNA repair, with PARP1 being the most important. It took a lot of time for PARP to be recognized as target for chemotherapeutic drug. First PARP inhibitor (PARPi) entered clinical trials, as a chemosensitizer like MGMT inhibitors. But its capacity as single agents to treat BRCA-deficient cell lines from germline breast cancers proved later. Olaparib was the first PARP inhibitor for ovarian cancer. Today, there are number of PARP inhibitors in clinical trials for not only breast cancer but also for other cancer types [58]. PARP's clinical efficacy on BRCA-deficient tumors is one of the most effective drug findings. PARPi function includes binding to PARP and inhibiting its function until next round of DNA replication, then accumulation of unrepaired SSBs will automatically get converted to DSB. Cells that are missing both alleles of BRCA 1, BRCA2 or PALB2 have no efficient HR functionality, which leaves repairs in the hands of NHEJ, its limited ability to repair extensive DSB damage leads to tumor cell death specifically because the cells with non-compromised HR can tackle these breaks very easily (**Figure 3**) [26]. That is why these are used as add on for effective cancer treatment. However, the effect of PARP inhibition is not as simple as it seems, there is lot more complexity to it like PARP's interactions with other proteins and PARP trapping [59].

PARP not only works with BER, but it also activates XRCC in HR pathway and is involved in a regulatory feedback loop with BRCA1. It also appears to inhibit the NHEJ pathway by inactivating DNA-PKcs and ATM's checkpoint activity. Moreover, it has a role in inflammation that proves its involvement in transcriptional regulation and many other biological functions associated to cancer. As mentioned earlier,

**201**

*Genomic Instability and DNA Repair in Cancer DOI: http://dx.doi.org/10.5772/intechopen.95736*

of manipulating NHEJ, etc. [60].

*Mechanism of action of PARP inhibitors.*

**9. Conclusion**

**Figure 3.**

**Acknowledgements**

**Conflict of interest**

BD is thankful to UGC, New Delhi for fellowship.

The authors declare no conflict of interest.

cancers are notoriously clever when it comes to combat their survival, then they come up with new methods to the stress imparted to it by PARP inhibitors. Till date a lot of instances have been proved such as reverse mutation in BRCA, various ways

Defects in the repair system assure genomic instability, this fuels disorderliness required for cancer to survive, sustain and evolve; that is why hereditary deficiencies in them makes the individual more susceptible to cancer. There are only some repair genes known to be exploited by cancer, a more extensive search of potential points might give a perspicuous picture. Researchers has been into understanding and finding cure to cancer since decades; but still till date we do not have a conclusion. This refers to its multiple techniques, different hierarchical steps and several process that it applies for its successful survival. DNA repair system is one of its basic targets, so cancer wangle it very well to establish its existence. It applies different mechanism from simple mutations to clustered mutations to various epigenetic changes just to assure a compromised repair system. A very elaborate venture of these changes can give us insight into generation of genomic instability by suppressing DNA repair in cancer. This information can help us get the much-sought effective treatment. Therapies targeting DNA repair genes already available are example to this.

*Genomic Instability and DNA Repair in Cancer DOI: http://dx.doi.org/10.5772/intechopen.95736*

*DNA - Damages and Repair Mechanisms*

mately leading to cancer death [13, 26].

not any effective drug involving this [57].

**8.1 MGMT inhibition**

**8.2 PARP inhibitors**

**8. Cancer therapy targeting DNA repair pathways**

Presently there are a few chemotherapeutic drugs and some of them are even in phase 2 or phase 3 trials. This itself emphasize the crucial role DNA repair pathways and proves that it is an important chemotherapeutic target. Basically, DNA repair inhibitors are used as chemotherapeutic drug they make already fragile DNA repair system of cancer to collapse leading to destruction of cellular homeostasis ulti-

The *O*6-methylguanine-DNA methyltransferase (MGMT), a DNA repair protein that removes alkyl group, was the target of the earliest attempt to develop a DNA repair inhibitor. MGMT is the most widely studied DNA repair mechanism [56]. In 1970s, nitrosoureas was introduced as a chemotherapeutic drug for glioblastoma and other malignant gliomas, it alkylates DNA at various positions on guanine, subsequently causing single- or double-strand damage which chemosensitiz cells to more damage by other drugs. Scientists quickly learned that something could reverse the DNA damage that they inflicted, that was MGMT. After some time, a potent MGMT inhibitor was used along with nitrosoureas but it did not work [26]. Although compromising MGMT fell short of expectations in chemo sensitizing tumors to alkylating agents, it was continued to be studied. There was evidence that in different cancer it is manipulated in different ways. But still there is

The PARP is a nucleus specific enzyme that detects single strand breaks that are being formed spontaneously or during BER and binds to that position on the DNA strand. It then undergoes a structural change and begin synthesizing a polymeric adenosine diphosphate ribose (poly ADP-ribose) chain, which acts as signal for the other DNA-repairing enzymes. Three members of that family have roles in DNA repair, with PARP1 being the most important. It took a lot of time for PARP to be recognized as target for chemotherapeutic drug. First PARP inhibitor (PARPi) entered clinical trials, as a chemosensitizer like MGMT inhibitors. But its capacity as single agents to treat BRCA-deficient cell lines from germline breast cancers proved later. Olaparib was the first PARP inhibitor for ovarian cancer. Today, there are number of PARP inhibitors in clinical trials for not only breast cancer but also for other cancer types [58]. PARP's clinical efficacy on BRCA-deficient tumors is one of the most effective drug findings. PARPi function includes binding to PARP and inhibiting its function until next round of DNA replication, then accumulation of unrepaired SSBs will automatically get converted to DSB. Cells that are missing both alleles of BRCA 1, BRCA2 or PALB2 have no efficient HR functionality, which leaves repairs in the hands of NHEJ, its limited ability to repair extensive DSB damage leads to tumor cell death specifically because the cells with non-compromised HR can tackle these breaks very easily (**Figure 3**) [26]. That is why these are used as add on for effective cancer treatment. However, the effect of PARP inhibition is not as simple as it seems, there is lot more complexity to it

like PARP's interactions with other proteins and PARP trapping [59].

PARP not only works with BER, but it also activates XRCC in HR pathway and is involved in a regulatory feedback loop with BRCA1. It also appears to inhibit the NHEJ pathway by inactivating DNA-PKcs and ATM's checkpoint activity. Moreover, it has a role in inflammation that proves its involvement in transcriptional regulation and many other biological functions associated to cancer. As mentioned earlier,

**200**

**Figure 3.** *Mechanism of action of PARP inhibitors.*

cancers are notoriously clever when it comes to combat their survival, then they come up with new methods to the stress imparted to it by PARP inhibitors. Till date a lot of instances have been proved such as reverse mutation in BRCA, various ways of manipulating NHEJ, etc. [60].
