**2. ATM roles during DDR**

ATM and ATR are two kinases belonging to the protein phosphatidylinositol-3-kinase-like kinases (PIKK) family that function as the chief regulators of DDR [3, 11, 13]. Together, they coordinate all pathways implicated in DDR to offer an adequate and timely response proportionate to the type and extent of the genomic injury. Recently, DNA-PKcs, another member of the PIKK family, has also been found playing more substantial roles in regulating DDR than initially thought, albeit to a lesser extent that ATM and ATR [17].

ATM is a very large kinase of 3056 amino acids and a molecular weight of 350.6 kD that resides in the nucleus as inactive homodimers. Upon DNA damage infliction, phosphorylation of a critical ATM residue disrupts dimerization, prompting monomers to undergo further phosphorylation to achieve full kinase activation [18–20]. Active ATM monomers phosphorylate substrates on serine or threonine residues followed by glutamine (S/TQ ), and a significant amount of ATM substrates contain clusters of S/TQ sites in short stretches of the protein [21]. These so-called SCD domains can be used to mine the proteome for putative ATM targets [22–24]. Using mass spectrometry, a high-throughput screen for proteins phosphorylated following DNA damage found 686 putative DDR targets and the final number is estimated to surpass a thousand proteins [25]. These large numbers showcase the complexity of DDR, and the need for an orchestrated coordination of all pathways involved. Some of the most important direct ATM targets are CHK2 and p53, two downstream effectors that modulate pivotal DDR pathways like cell cycle progression regulation, DNA repair, or apoptosis [26–28].

## **2.1 DNA repair**

ATM is not only activated by different kinds of DNA damage but can also actively participate in several DNA repair mechanisms and coordinate their activities with other DDR-related pathways [7, 29]. During DSB repair, ATM

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*The Role of the DNA Damage Response in Ataxia-Telangiectasia Syndrome*

plays crucial roles in the early end-processing events, signal amplification, and recruitment of other DNA repair proteins to the sites of damage [3, 13]. ATM functions in homologous recombination (HR) and nonhomologous end-joining (NHEJ), the two pathways entrusted by cells to repair DSBs. Whereas, NHEJ is active throughout the cell cycle, its function is mostly limited to G0/G1 as S/G2 phases prefer the more accurate HR, a mechanism that uses sister chromatids only present during those phases as repair templates. The first sensor of DSBs is PARP1, which in addition to binding breaks, also adds branches of poly-(ADP)-ribose to proteins post-translationally [30]. This so-called PARylation process activates and recruits several DNA repair proteins to the sites of damage [31]. One of them is the MRN complex—made up of MRE11, RAD50, and NBS1—that binds and activates ATM [32, 33]. Interaction with PARP1 and NBS1, thus, activates and recruits ATM to DSB sites, where it phosphorylates several downstream targets and effectors to amplify DDR signaling. For instance, ATM phosphorylates histone variant H2AX, which promotes MDC1 binding to the chromatin surrounding DSB [34–36]. Once there, ATM-mediated phosphorylation of MDC1 promotes its binding to MRN, and recruitment of more ATM to phosphorylate more H2AX, further spreading DDR

Although the complete process remains to be fully elucidated, it is clear that ATM is also involved in the decision-making process that selects either HR or NHEJ to repair a DSB [37, 38]. A crucial step in this process is the extent of end resection that takes place at DSB [39]. ATM directly phosphorylates CtIP and BRCA1, two HR proteins required for resection initiation and binding of RAD51 to ssDNA ends, respectively [40–42]. Once formed, RAD51 coated 3′ ssDNA ends steer repair toward HR by initiating strand invasion into the sister chromatid. Intriguingly, ATM phosphorylates p53BP1 and promotes its recruitment to sites of DNA damage [43]. Phosphorylated p53BP1 has opposing roles to CtIP and BRCA1, and favors the formation of p53BP1 containing complexes at DSB that counteract HR in favor of NHEJ repair [38, 44]. ATM also influences NHEJ by mediating DNA-PKcs phosphorylation and subsequent recruitment of Artemis, an end-processing nuclease, to

Although ATM is mostly activated by DSBs, recent data suggest that some lesions that are usually repaired by BER can also activate ATM and that ATMdependent phosphorylation events can regulate BER [46]. Following base damage, BER requires the sequential action of DNA glycosylases—to remove damaged bases and create apyrimidinic or apurinic (AP) sites, PARP1—to PARylate the AP site, and endonucleases that will generate an SSB at the AP site [47]. These events can lead to ATM activation and the ATM-dependent phosphorylation of CHK2 [46]. Upon activation, CHK2 phosphorylates XRCC1, a BER protein required for sealing

DDR is capable of modulating DNA repair pathways through multiple effectors. For instance, both ATR and p53 regulate NER through quite distinct mechanisms. While ATR phosphorylates XPA, one of the earliest respondents to pyrimidine photodimers and other bulky lesions, DDR-dependent phosphorylation of p53 acts by upregulating expression of NER genes and recruiting XPC and TFIIH to sites of damage [7, 48–52]. ATR also regulates ICLR through the phosphorylation of several members of the Fanconi anemia group, a set of proteins that in combination with NER and HR, repair DNA cross-linkage damage [53–55]. Other examples of DDRsignaling-dependent regulation of DNA repair mechanisms include the upregulation of BER through the stimulatory binding of p53 to BER proteins, the promotion of HR that ensues after disruption of the p53-RPA complex by ATM, ATR and DNA-PKcs phosphorylation, and the PIKK-dependent phosphorylation of Werner

syndrome and Bloom syndrome proteins involved in DSB repair [56–60].

*DOI: http://dx.doi.org/10.5772/intechopen.84902*

signaling [13].

DSB sites [45].

the nick and completing the repair.
