**3. Molecular basis for ataxia telangiectasia syndrome**

A-T is an autosomal recessive genetic disease that affects 1 in every 40,000–100,00 births with an estimated 0.5–1% of the global population being carriers of the illness [100]. Patients confront a variety of clinical manifestations throughout their lives, with the inability to control body movements, or ataxia, being one the earliest to appear [101]. The underlying cause for the ataxia is progressive neurodegeneration, particularly of the cerebellum, which also induces dysarthria (speech difficulties), poor balance, and uncontrolled eye movements. Neurodegeneration involves the gradual disappearance of Purkinje, granular cells and the molecular layer of the cerebellar cortex, and expands to the brain stem and the spinal cord. A-T is also characterized by the presence of vascular abnormalities (telangiectasia) that manifest as red spider-like veins, present mostly in the eyes, but also found in cheeks, ears, neck, and other parts of patients' bodies [102, 103].

In addition to the ataxia and telangiectasia, A-T patients can suffer from a plethora of other clinical symptoms. They have a higher incidence of cancer, diabetes, and show premature aging. They manifest radiosensitivity, sterility, and immunodeficiencies with an elevated risk of developing autoimmune diseases such as arthritis, vitiligo, or immune thrombocytopenia [104]. Authors have also suggested that A-T patients may suffer from prolonged chronic inflammation [94]. Consistent with this, high levels of pro-inflammation cytokines are present in their serum even in the absence of infections [51, 52].

While mutations in other DDR gene can induce similar symptomatology, defective or absent, ATM is the sole genetic cause of A-T. Hundreds of pathogenic mutations have been identified in ATM from A-T patients, many of them altering splicing or causing frameshifts that result in premature termination codons. As a result, ATM is often either missing or containing truncations of different extents in A-T cells. Clinical manifestations correlate with the severity of the mutation, with milder forms of the syndrome appearing in individuals bearing mutations with mild effects on ATM function and vice versa [105].

## **3.1 Neurodegeneration**

The most apparent clinical manifestation of the disease is probably also the most problematic to explain at the molecular level. The question of why mutations in a gene involved in DDR would have specific and discriminating effects in the neural

**75**

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

pleiotropic effects of A-T, except for neurodegeneration [107–109].

system remains to be fully answered [5, 106]. One of the problems in answering this question is that mouse models lacking functional ATM reconstitute most of the

It is clear that during neurodevelopment, rapidly dividing cells—with high energetic demands and increased mitochondria respiration—face increasing levels of threats to the integrity of their genome [110, 111]. High metabolic rates increase ROS, and produce oxidative stress, which combined with the high demand for transcription, may render these cells more susceptible to faulty DNA repair mechanisms [110, 112]. This view is consistent with the high prevalence of neurological problems in patients bearing mutations in DNA repair genes [5, 9]. Authors have proposed a model where different stages during neurodevelopment are more susceptible to mutations in different DNA repair pathways, with HR having major roles during phases of rapid proliferation—when a sister chromatid is readily available—and NHEJ being required during late development when cells undergo differentiation in G1/G0 [9]. This would explain why mutations in HR often result in embryonic lethality; whereas, mutations in some NHEJ genes present neurodevelopment problems such as microcephalia. In this model, single strand lesion repair would be required for post-developmental maintenance of neural tissue.

Cerebellum neurodegeneration in A-T patients also establishes ATM as a require-

ment to maintain neural tissue. The accumulation of unrepaired lesions during development—and beyond—results in degeneration problems later [113]. This is likely to happen at any tissue, but it would affect the neural system in particular, and with greater virulence, due to the longevity of its cells and the subsequent longer exposure to mutagenic agents. This injury build-up would occur progressively,

Supporting this view, there are clear indications that neural A-T cells are under genotoxic stress. Mice cells lacking ATM gradually accumulate DSBs and show depleted levels of oxidized and reduced forms of NAD in cerebellar tissue, a hallmark of cells undergoing high levels of oxidative stress [114]. Interestingly, depletion of NAD levels only occurs in cerebellar tissues, but not in other parts of the brain, indicating that oxidative stress may be particularly acute in the cerebellum. These data are consistent with other studies that found high levels of oxidative stress in the cerebellum and Purkinje cells in particular, which likely explains the higher prevalence of neurodegeneration in the cerebellum than in other parts of the neural system [115]. The reason for the localized high levels of oxidative stress in cerebellar tissue compared to other regions of the neural system is not known, nor is the reason for the lack of a cerebellar degeneration

mimicking the progressive nature of neurodegeneration in A-T patients.

phenotype in mice lacking ATM despite increased levels of oxidative damage.

These studies strongly suggest that the inability to repair damage caused by oxidative stress is the more plausible cause of cerebellar neurodegeneration in A-T and thus, the roles of ATM during the repair of single strand lesions may provide the molecular basis for the disease. The correlation between impaired single strand lesions repair and failure of neural tissue maintenance was further corroborated in mice that showed extensive neuron loss in the cerebellum when XRCC1 expression was selectively prevented in their brain [116]. While ATM mostly acts in DSB repair, it can also play roles during single strand lesion repair. As mentioned before, SSBs can activate ATM and promote BER by the ATM-mediated phosphorylation of XRCC1 (see Section 2.1) [46]. Whether or not, impairment of this DDR branch is related to the neurodegeneration observed in A-T remains to be elucidated.

Authors have also proposed that neurological problems arising in A-T patients may be related to the faulty resolution of R-loops in locations where active transcription is halted due to the presence of DNA lesions [9, 13]. R-loops are hybrids formed by two strands of DNA and one of RNA that are generated in a variety of circumstances and locations and are known to pose a risk to genome stability

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

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

*Neurodevelopment and Neurodevelopmental Disorder*

**3. Molecular basis for ataxia telangiectasia syndrome**

neck, and other parts of patients' bodies [102, 103].

serum even in the absence of infections [51, 52].

mild effects on ATM function and vice versa [105].

mitophagy) and pexophagy—the autophagic degradation of peroxisomes [91–93]. Integration of autophagy pathways as part of DDR repertoire may allow cells in stress to attempt pro-survival pathways first before succumbing to apoptosis. While the complex relationships between DDR and inflammation are beginning to emerge, it is clear that ROS and other types of genomic injuries can elicit a proinflammatory response. As part of DDR, this pro-survival cell response is mediated mostly through ATM and PARP1 [94]. ATM directly binds and phosphorylates IKK-γ (NEMO), the regulatory subunit of the IKK complex that activates NF-kB [41]. Along with PARP1-mediated post-translational modifications, ATM phosphorylation of IKK-γ promotes activation of IKK and subsequent activation of NF-kB [41, 95–97]. Therefore, this critical pro-inflammatory enzyme is under DDR control, where it can function as a transcription factor promoting expression of pro-inflammatory cytokines and DNA repair genes [76, 94, 95, 98]. In addition, ATM is involved in a pro-inflammatory pathway known as senescence-associated secretory phenotype (SASP), a complex mechanism that secretes, among others, pro-inflammatory cytokines [94, 99].

A-T is an autosomal recessive genetic disease that affects 1 in every 40,000–100,00 births with an estimated 0.5–1% of the global population being carriers of the illness [100]. Patients confront a variety of clinical manifestations throughout their lives, with the inability to control body movements, or ataxia, being one the earliest to appear [101]. The underlying cause for the ataxia is progressive neurodegeneration, particularly of the cerebellum, which also induces dysarthria (speech difficulties), poor balance, and uncontrolled eye movements. Neurodegeneration involves the gradual disappearance of Purkinje, granular cells and the molecular layer of the cerebellar cortex, and expands to the brain stem and the spinal cord. A-T is also characterized by the presence of vascular abnormalities (telangiectasia) that manifest as red spider-like veins, present mostly in the eyes, but also found in cheeks, ears,

In addition to the ataxia and telangiectasia, A-T patients can suffer from a plethora of other clinical symptoms. They have a higher incidence of cancer, diabetes, and show premature aging. They manifest radiosensitivity, sterility, and immunodeficiencies with an elevated risk of developing autoimmune diseases such as arthritis, vitiligo, or immune thrombocytopenia [104]. Authors have also suggested that A-T patients may suffer from prolonged chronic inflammation [94]. Consistent with this, high levels of pro-inflammation cytokines are present in their

While mutations in other DDR gene can induce similar symptomatology, defective or absent, ATM is the sole genetic cause of A-T. Hundreds of pathogenic mutations have been identified in ATM from A-T patients, many of them altering splicing or causing frameshifts that result in premature termination codons. As a result, ATM is often either missing or containing truncations of different extents in A-T cells. Clinical manifestations correlate with the severity of the mutation, with milder forms of the syndrome appearing in individuals bearing mutations with

The most apparent clinical manifestation of the disease is probably also the most problematic to explain at the molecular level. The question of why mutations in a gene involved in DDR would have specific and discriminating effects in the neural

**74**

**3.1 Neurodegeneration**

system remains to be fully answered [5, 106]. One of the problems in answering this question is that mouse models lacking functional ATM reconstitute most of the pleiotropic effects of A-T, except for neurodegeneration [107–109].

It is clear that during neurodevelopment, rapidly dividing cells—with high energetic demands and increased mitochondria respiration—face increasing levels of threats to the integrity of their genome [110, 111]. High metabolic rates increase ROS, and produce oxidative stress, which combined with the high demand for transcription, may render these cells more susceptible to faulty DNA repair mechanisms [110, 112]. This view is consistent with the high prevalence of neurological problems in patients bearing mutations in DNA repair genes [5, 9]. Authors have proposed a model where different stages during neurodevelopment are more susceptible to mutations in different DNA repair pathways, with HR having major roles during phases of rapid proliferation—when a sister chromatid is readily available—and NHEJ being required during late development when cells undergo differentiation in G1/G0 [9]. This would explain why mutations in HR often result in embryonic lethality; whereas, mutations in some NHEJ genes present neurodevelopment problems such as microcephalia. In this model, single strand lesion repair would be required for post-developmental maintenance of neural tissue.

Cerebellum neurodegeneration in A-T patients also establishes ATM as a requirement to maintain neural tissue. The accumulation of unrepaired lesions during development—and beyond—results in degeneration problems later [113]. This is likely to happen at any tissue, but it would affect the neural system in particular, and with greater virulence, due to the longevity of its cells and the subsequent longer exposure to mutagenic agents. This injury build-up would occur progressively, mimicking the progressive nature of neurodegeneration in A-T patients.

Supporting this view, there are clear indications that neural A-T cells are under genotoxic stress. Mice cells lacking ATM gradually accumulate DSBs and show depleted levels of oxidized and reduced forms of NAD in cerebellar tissue, a hallmark of cells undergoing high levels of oxidative stress [114]. Interestingly, depletion of NAD levels only occurs in cerebellar tissues, but not in other parts of the brain, indicating that oxidative stress may be particularly acute in the cerebellum. These data are consistent with other studies that found high levels of oxidative stress in the cerebellum and Purkinje cells in particular, which likely explains the higher prevalence of neurodegeneration in the cerebellum than in other parts of the neural system [115]. The reason for the localized high levels of oxidative stress in cerebellar tissue compared to other regions of the neural system is not known, nor is the reason for the lack of a cerebellar degeneration phenotype in mice lacking ATM despite increased levels of oxidative damage.

These studies strongly suggest that the inability to repair damage caused by oxidative stress is the more plausible cause of cerebellar neurodegeneration in A-T and thus, the roles of ATM during the repair of single strand lesions may provide the molecular basis for the disease. The correlation between impaired single strand lesions repair and failure of neural tissue maintenance was further corroborated in mice that showed extensive neuron loss in the cerebellum when XRCC1 expression was selectively prevented in their brain [116]. While ATM mostly acts in DSB repair, it can also play roles during single strand lesion repair. As mentioned before, SSBs can activate ATM and promote BER by the ATM-mediated phosphorylation of XRCC1 (see Section 2.1) [46]. Whether or not, impairment of this DDR branch is related to the neurodegeneration observed in A-T remains to be elucidated.

Authors have also proposed that neurological problems arising in A-T patients may be related to the faulty resolution of R-loops in locations where active transcription is halted due to the presence of DNA lesions [9, 13]. R-loops are hybrids formed by two strands of DNA and one of RNA that are generated in a variety of circumstances and locations and are known to pose a risk to genome stability

[117]. Paused RNA polymerase sites activate ATM, which then elicit a response that includes interactions with spliceosome components that may mediate R-loop resolution [118]. In the cerebellum, the combination of high levels of oxidative stress with high demands of transcription may produce an abnormally high amount of paused transcription sites due to DNA damage. In the absence of ATM, R-loops may not be adequately resolved, eventually creating a scenario incompatible with cell life.
