**2.2 Cell cycle progression regulation**

One of the most dangerous threats of DNA damage is the possibility of spreading to daughter cells during cell duplication. To prevent this, DDR is capable of halting cell cycle progression at any point during the cell cycle [61]. A series of overlapping mechanisms ensure that cells attempt DNA repair before progressing to the next cell cycle stage [7].

ATM is in charge of preventing lesions produced during G1/G0 to enter S phase, which is particularly important for some of the most common DNA injuries like oxidative damage. Since G1/G0 duration is usually longer than other cell cycle phases, exposure to ROS and other mutating agents is also higher in these stages, and so is the appearance of related damage. ATM acts in conjunction with CHK2 and p53 to block G1/S transition by inhibiting CDK2, the cycle-dependent kinase that along with Cyclin E, triggers S-phase entry [62]. CDK2 inhibition is achieved by two overlapping mechanisms that have ATM at their apex. On one hand, ATM phosphorylation of CHK2 triggers phosphorylation of CDC25A, a phosphatase required for CDK2 activation and promoting entry into S-phase [63, 64]. On the other hand, ATM-dependent activation of p53 induces upregulation of p21, which acts as a CDK2 inhibitor [65].

Replicative stresses during S-phase trigger the activation of the Intra-S-phase checkpoint to ensure that replicative stress and other types of damages do not persist in the following cell cycle stages. ATR, not ATM, is the PIKK responsible for halting the cell cycle at this stage through the activation of the intra-S-phase [61]. During this checkpoint ATR, CHK1, and p53 act together and in overlapping ways to phosphorylate CDK2, which renders it unable to form an active CDK2/cyclin A complex [63, 66]. The final result is DNA synthesis termination, premature stalling, and subsequent halt of the cell cycle.

The concerted action of ATR, CHK1, and p53 also controls the G2/M transition to ensure that no cell enters mitosis with lingering DNA damage from previous phases [67–69]. The importance of this checkpoint is highlighted by the presence of multiple overlapping and complementary mechanisms actively working together to inhibit CDK1/CyclinB1, the complex required to trigger entry into mitosis [66]. CDK1 phosphorylation has an inhibitory effect and thus, is the primary target of several of these mechanisms. After ATR-mediated activation, CHK1 phosphorylates CDC25C, a phosphatase required for CDK1 activation. Phosphorylated CDC25C binds to the 14-3-3 complex, which promotes its transport to the cytoplasm, effectively preventing CDK1 activation [70]. Active CHK1 also phosphorylates and activates WEE1, a kinase that promotes inhibitory phosphorylation of CDK1 [71]. Furthermore, ATR phosphorylates PLK1 and inhibits its role as WEE1 inhibitor, while p53 upregulates GADD45, which binds and further inhibits CDK1/CyclinB1 complex [72, 73]. Importantly, ATM also play roles in this combined effort to keep CDK1/CyclinB1 inhibited, as it can phosphorylate PLK1 and promote CHK1-mediated CDC25C phosphorylation [73, 74].

Finally, the Intra-M checkpoint is the last opportunity to prevent the transmission of damage to daughter cells. ATM and CHK1 govern this checkpoint through two distinct mechanisms that act sequentially during mitosis progression. First, inhibitory phosphorylation of PLK1 by CHK1 prevents it from acting during spindle formation and halts the cell cycle [74]. At a later point, ATR-mediated phosphorylation of Aurora B stimulates the inhibitory effect that this enzyme exerts over cytokinesis and delays exits of mitosis if the damage is detected [75].

## **2.3 Transcription regulation**

Activation of DDR induces substantial changes to the transcriptome to equip cells with necessary tools and time to articulate a proper response. While the overall effect

**73**

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

induced during DDR by ATM and p53 upregulation of p21 [84].

contributing to the regulation of dNTP levels by DDR kinases.

as DNA damage events and activate DDR.

of DDR activation is an attenuation of global transcription and translation, many genes involved in DDR pathways must be upregulated instead [76]. For example, upregulation of XPC and other NER genes follows DDR activation, and as previously noted, DDR-mediated blocking of cell cycle progression is dependent on the induction of certain genes, namely p21 [77]. DDR exerts its influence on gene transcription through the action of several transcription factors that act as downstream effectors of DDR signaling. Some of the most important examples are p53 and BRAC1, AP-1, or E2F1. For instance, BRAC1 and p53 upregulate XPC during DDRmediated NER upregulation; whereas, AP-1 induces the expression of XPF and XPG during the same process [76]. Other examples are p53 and AP-1 serving as transcription factors for MLH1 and MSH2—two mismatch repair genes—and E2F1 and AP-1 influencing the expression of BER components XRCC1 and APEX1, respectively.

Paramount for DDR is its ability to trigger apoptosis when DNA damage is too extensive and incompatible with genome stability. Both ATM and ATR can promote apoptosis through the phosphorylation of p53, the chief regulator of apoptosis during DDR [78–80]. p53 can trigger apoptosis by playing dual roles as transcription factor activator and anti-apoptotic protein inhibitor. In the presence of unrepairable damage, p53 upregulates pro-apoptotic genes like PUMA or BAX, while binding and inhibiting anti-apoptotic proteins like BCL2 [81, 82]. In addition to apoptosis, extensive DNA damage can also induce senescence, a metabolic state that causes irreversible growth arrest [83]. Among other mechanisms, senescence can be

ATM and ATR also integrate into DDR several other pathways that are essential

Dysfunctional telomeres can also activate ATM and ATR and elicit a response that includes halting the cell cycle and induction of senescence [88]. Telomere dysfunction can arise when errors in the Shelterin complex render telomeres unprotected. Loss of protection at telomeres can also occur by the natural attrition of telomere length experienced during DNA replication in cells that do not express telomerase [89]. In both cases, DNA ends at telomeres can be mistakenly recognized

Recently, activation of autophagy has emerged as another tool that DDR can use to fight severe DNA damage. While autophagy was initially thought to be exclusively activated in response to cellular damage or starvation, there is clear evidence that DNA damage can also trigger autophagy [90]. For instance, the action of mTOR—the main autophagy inhibitor—can be repressed either in an ATM or PARP1 dependent manner following DNA damage, effectively promoting autophagy [7]. Consistent with this, in response to ROS mediated damage, ATM can induce selective degradation of mitochondria by autophagy (also known as

to provide an adequate and proportionate response to all kind of injuries. For instance, no proper DDR can occur without the upregulation of dNTP for DNA repair [85]. This upregulation requires tight control, as excessive dNTP production can lead to increased mutation frequency [86]. In the presence of DNA damage, DDR kinases regulate RNR—the kinase that catalyzes rate-limiting step during dNTP production—at multiple levels. For instance, p53 regulates the expression levels of RNR; whereas, ATM phosphorylation increases the stability of RNR [87]. In addition, ATR signaling inhibits degradation of some RNR subunits, further

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

**2.4 Apoptosis and senescence**

**2.5 Other DDR pathways**

*The Role of the DNA Damage Response in Ataxia-Telangiectasia Syndrome DOI: http://dx.doi.org/10.5772/intechopen.84902*

of DDR activation is an attenuation of global transcription and translation, many genes involved in DDR pathways must be upregulated instead [76]. For example, upregulation of XPC and other NER genes follows DDR activation, and as previously noted, DDR-mediated blocking of cell cycle progression is dependent on the induction of certain genes, namely p21 [77]. DDR exerts its influence on gene transcription through the action of several transcription factors that act as downstream effectors of DDR signaling. Some of the most important examples are p53 and BRAC1, AP-1, or E2F1. For instance, BRAC1 and p53 upregulate XPC during DDRmediated NER upregulation; whereas, AP-1 induces the expression of XPF and XPG during the same process [76]. Other examples are p53 and AP-1 serving as transcription factors for MLH1 and MSH2—two mismatch repair genes—and E2F1 and AP-1 influencing the expression of BER components XRCC1 and APEX1, respectively.
