**2.4 Apoptosis and senescence**

*Neurodevelopment and Neurodevelopmental Disorder*

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

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,

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].

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

**2.2 Cell cycle progression regulation**

and subsequent halt of the cell cycle.

the next cell cycle stage [7].

**72**

**2.3 Transcription regulation**

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