Note for: ATM kinase: Much more than a DNA damage responsive protein

Note for: ATM kinase: Much more than a DNA damage responsive protein

 (doi: 10.1016/j.dnarep.2015.12.009)

It is a very large protein and post-translational modifications are the mechanism which ATM is regulated. Besides, one protein can either regulate ATM and be a downstream substrate of ATM! ATM has so many cellular roles not only DNA damage sensor. It can sense the redox status within the cell and the environment affects the structure of ATM which later control its activity! Sound to me, both ATM substrates and the proteins that control the ATM might be different in each cell type. 

ATM -- multifunctional protein kinase during past two decades, established and predominant role in DNA damage response, to help in maintaining overall functional integrity of cells; oxidative stress, metabolic syndrome, mitochondrial dysfunction as well as neurodegeneration.

several proteins which might be acting as substrates of ATM.

effective regulatory controls within the ATM-mediated pathways/mechanisms can help in developing better therapeutics.

understanding of ATM-dependent cellular signals could also help in the treatment of variety of other.

these pathways seem to control many critical cellular functions.

Ataxia Telangiectasia (A-T), also referred to as Louis—Bar syndrome, is a rare autosomal recessive disorder in which patients exhibit a wide array of abnormalities such as ataxia, telangiectasia, premature aging, neurodegeneration, insulin resistance, reproductive sterility, increased sensitivity to ionizing radiation, predisposition to cancer and variable immunodeficiencies.

ATM (Ataxia Telangiectasia Mutated) gene that encodes for a multifunctional 370-kDaATM protein kinase.

ATM belongs to the family of phosphatidylinositol kinase like kinases (PI3KK’s) that are serine threonine kinases that phosphorylate their substrates on SQ or TQ motifs. Other members of the family include Ataxia Telangiectasia and Rad3 related (ATR), DNA dependent protein kinase, catalytic subunit (DNA-PKcs), ATX/SMG-1 and Mammalian Target of Rapamycin/FK506 binding protein12-rapamycin associated protein (mTOR/FRAP). The C-terminal catalytic region of these PI3KK’s consists of the FAT domain (named because of being well conserved in FRAP [F]/ATR[A]/TRAAP [T]), phosphoinositide-3-kinase domain (PI3K) and a FAT-C domain. The amino terminal region or non-kinase region is highly variable within the PI3KK family in contrast to the C-terminal catalytic domain that shares significant homology among the members of this class.

Single particle electron microscopy of ATM-DNA complexes has also been carried out to study the conformational changes that ATM undergoes upon binding to DNA. This study revealed that the tip of the arm moves towards the head upon binding to the DNA forming a ring like structure of ∼35 Å diameter. The ring diameter is sufficient to accommodate double helix of DNA having 20 Å diameter.

For a long time since the discovery of ATM, it was primarily thought to be associated with DNA damage response (DDR) and cell cycle checkpoint pathways, as the classical symptoms seen in AT patients are due to defects in these pathways.

suggested localization and functioning of ATM in compartments outside the nucleus.

these extranuclear functions of ATM might help explain some unusual clinical symptoms seen in A-T patients that cannot be attributed to the DDR.

revealed in which ATM has prominent function apart from the well-established DDR pathway.

ATM include oxidative stress, insulin signaling, mitochondrial functioning, and neurogenesis.

is a multifunctional protein kinase that performs diverse physiological functions.

Within minutes of occurrence of DSB’s, mammalian cells accumulate a plethora of repair and signaling proteins at the site of damage. This complex response to double strand breaks, involving multiple cellular pathways, is regulated by ATM in a prominent way, although other pathways are also activated.

An additional group of checkpoint proteins classified as mediators are placed between the sensors and signal transducers and include proteins such as MDC1 (Mediator of DNA Damage Checkpoint Protein1), 53BP1 (Tumor Suppressor p53 Binding Protein 1), among many others. The presence of these hierarchical levels leads to amplification of signals causing activation of multiple cellular pathways. Chromatin remodeling processes such as histone modifications and nucleosome repositioning also take place concurrently for facilitating DNA repair.

Sensors can directly recognize either the damage or the chromatin disturbances, which translate into a cascade of cellular responses and culminates with the help of suitable effectors. These effectors then regulate several pro-survival processes: (a) cell cycle checkpoints, (b) DNA repair pathways, and (c)transcriptional response or cell death. This finely tuned response temporarily arrests the cell cycle and helps cells to either undergo repair or apoptosis depending on the extent of damage.

response to different forms of damage. ATM and DNA-PK mediate the responses to double strand breaks whereas ATR responds to single strand breaks and regions generated at the sites of stalled replication forks.

MRN complex (Meiotic Recombination Protein-11(Mre11)/Rad50/Nijmegen Breakage Syndrome-1(Nbs1)) complex, ATRIP and Ku80 proteins mediate the recruitment of ATM, ATR and DNA-PK, respectively.

there is a conserved mode of recruitment of these PI3KK members to the site of damage via interaction with an evolutionarily conserved carboxy terminal motif of sensor proteins.

The MRN complex (Mre11/Rad50/Nbs1) acts as a sensor and mediates the recruitment of ATM to double strand breaks. It is reported that the C-terminus of NBS1 physically interacts with ATM and recruits it to the damage site.

Mutations in Mre11 cause the A-T-like disorder (ATLD), and mutations in the NBS1 gene have been found to cause Nijmegen breakage syndrome (NBS). ATLD and NBS share some features with A-T (Ataxia Talengiectasia) disorder such as radiation sensitivity, chromosomal instability, and predisposition to cancer.

Nbs1 is also a substrate of ATM for being phosphorylated on various sites, such as theser343 and ser278 moieties. These phosphorylations are critical for the activation of Nbs1 for execution of S-phase checkpoint as well as various other downstream functions. These observations further strengthened the idea that the MRN complex not only mediates downstream functions such as S-phase checkpoint but also acts upstream of ATM and helps in sensing of these double strand breaks.

First these double strand breaks need to be processed to make them accordant for binding of numerous proteins involved in repair and checkpoint signaling. Mre11 forms a heterotetramer withRad50 protein and ropes together the broken DNA ends through its DNA-binding domains present in central and C-terminus regions. it requires the ATPase activity of Walker-A and -B domains present in Rad50.

the importance of the MRN complex for the activation of ATM. They observed that availability of DNA to ATM under in vitro conditions fails to stimulate its kinase activity in the absence of MRN complex. In fact, ATM constitutively exists as a dimer or higher order multimer in untreated/unperturbed cells with the kinase domain of one molecule bound to the internal FAT region of the pairing molecule. the kinase domain of each ATM molecule phosphorylates the other ATM molecule of the dimer at the serine-1981 position present in the FAT domain. This phosphorylation releases ATM molecules from each other’s grip turning them into kinase-active monomers.

provided the earliest evidences suggesting the involvement of autophosphorylation process in activating ATM and observed that ATP can activate it by a mechanism involving autophosphorylation.

employed Quantitative Analysis using Isotope Labeling to find post-irradiation changes in the level of phosphorylation at each of these autophosphorylation sites, and observed a 6-fold increase in Ser(P) 1981, 20-fold increase in Ser(P) 367 and a large increase in Ser(P)1893 and Ser(P) 2996 levels in mammalian cells.

Certain phosphatases play important roles in the regulation of ATM phosphorylation. Dephosphorylation and autophosphorylation complete the circuit of maintaining ATM in inactive or active states. The phosphoprotein phosphatase (PPP) family of serine/threonine protein phosphatases, such as PP2A and PP5, have been shown to regulate autophosphorylation of ATM.

Upon exposure of cells to radiation, phosphorylation-dependent dissociation of PP2A from ATM has been observed and is associated with subsequent loss of its phosphatase activity. above observations it can be assumed that PP2A functions to keep ATM in the unphosphorylated basal state in undisturbed/unirradiated cells. PP5 is another protein phosphatase that plays a role in the regulation of DNA damage induced ATM activation. Moreover PP5 downregulation inhibits the S-phase check-point resulting in radioresistant DNA synthesis. PP5 thus mediates DNA damage induced ATM activation and S-phase checkpoint activation.

Once DNA repair gets completed, the chromatin must return to the undisturbed pre-stress stage. WIP1 migrates to the site of damage and dephosphorylates  g-H2AX after DNA repair is completed. This dephosphorylation prevents the recruitment of downstream proteins such as MDC1, 53BP1, etc., and hence helps in dissolution of IRIF, termination of DNA repair and annulment of cell cycle checkpoints.

the recruitment of ATM to DSB’s via the MRN complex plays a crucial role in ATM activation. Some other factors play an equally important parallel role in the activation of ATM, primarily through its post-translational modifications and changes in DNA conformation.

proposed the ARR(access, repair and restore) model under which lesion accessibility is a prerequisite before any further process such as repair can initiate. Post-translational modifications including phosphorylation, ubiquitination, sumoylation, poly(ADP-ribosylation), acetylation, methylation, etc., play an important role in providing access to numerous proteins to DNA or chromatin.

thus facilitate protein recruitment to the damage site.

Acetylation plays a very important role in the activation of ATM, as well as its downstream signaling events such as phosphorylation of p53 and Chk2, leading to cell cycle arrest.

Histone acetyltransferases (HAT’s) participate in diverse cellular functions such as DNA repair, apoptosis, transcription activation and cell cycle progression.

An important protein that regulates ATM acetylation is the HIV-Tat interactive protein (Tip60) that belongs to the MYST family of acetyltransferases that are well conserved from yeast to humans. Sun et al. established that Tip60 is responsible for activation of ATM’s kinase activity in response to DNA damage via acetylation of lysine-3016 present in the highly conserved FATC domain of ATM.

In the absence of DNA damage, Tip60 is present in a complex with the activating transcription factor-2 (ATF2) and E3 ubiquitin ligase cullin3 (cul3). Cul3 ubiquitinylates Tip60 leading to its degradation. In the event of DNA damage by g-irradiation, Tip60 has been observed to dissociate from ATF2 and cul3 complex, thus enabling the acetylation of ATM at lysine-3016 and its subsequent activation.

DNA damage by UV-radiation, another E3 ubiquitin ligase, i.e., hMDM2, mediates the regulation of Tip60. After recruitment of Tip60 to DSB’s it interacts with histone H3 trimethylated on lysine-9 (H3K9me3) through its chromodomain, a final step required for complete activation of its acetyltransferase activity. Tip60 mediated acetylation plays a significant role as it contributes in chromatin relaxation as well as activation of ATM, hence facilitating the DNA damage response and repair.

hMOF interacts with ATM through its chromodomain. Upon exposure to ionizing radiation, hMOF-dependent and ATM-independent acetylation of histone H4 on lysine-16 was observed whereas no change was observed in the levels of hMOF or in its interaction with ATM.

Thus, hMOF also seems to play an important role in ATM signaling as well as repair process and deserves further investigation.

Poly(ADP-ribosyl)ation is the reaction wherein the poly(ADP-ribose) polymerase (PARP) senses a nick in the sugar–phosphate backbone of DNA with the help of its N-terminus nick-sensing region and catalyses the covalent attachment of ADP-ribose units from NAD+ to many nuclear DNA binding proteins, such as core histones.

PARP also facilitates the removal of RNA polymerase from DNA damage sites preventing any collision with other repair proteins. ALC1 (Amplified in Liver Cancer-1) is another chromatin remodeling enzyme that is recruited to DNA lesions via poly(ADP-ribose) units. ALC1, being a member of SNF2 ATPase, family stimulates nucleosome sliding that facilitates repair by increasing accessibility of DNA ends.PARP is known to be one of the earliest proteins activated in DNA damage signaling. Cells deficient in poly(ADP-ribose) (PAR) synthesis were shown to have impaired DNA damage response and DNA repair. ATM is known to interact directly to these PAR units via its PAR-binding domains and is also poly(ADP-ribosyl)ated by PARP. This association is required for proper activation of ATM’s kinase activity and its further downstream pathways.

is another post-translational modification that assists in the assembly of IRIF and initiation of repair processes at DNA damage sites. Two specific histone methylations, i.e., histone H4 di-methylated at Lys20 (H4-K20me2) and histone H3 methylated at lysine-79 (H3K79me3) are thought to recruit 53BP1, an important mediator of the DNA damage response, to damage sites.

Thus, it is speculated that DSB’s disrupt the nucleosome packaging to reveal the otherwise hidden residues and mediate the recruitment of 53BP1to the damage sites. Further, the inhibition of DOT1L (responsible for methylation of lysine-79 of H3) and histone methyltransferase (HMT) MMSET (responsible for methylation of lysine-20 of H4) prevents the accumulation of 53BP1 at DSB’s.

Arginine methylation of 53BP1and Mre11 in the glycine arginine rich (GAR) motif by protein arginine methyltransferase-1 (PRMT1) is responsible for their nuclear localization, as well as DNA binding. Histone methylation also works cooperatively with another post-translational modification, i.e., ubiquitylation. The B-cell lymphoma and BAL-associated protein (BBAP) is an E3 ligase that selectively mono-ubiquitylates histone H4 at lysine-91. This mono-ubiquitylation has been shown to be necessary for the methylation of H4-K20 and thus recruitment of 53BP1. Ubiquitylation also helps in the accumulation of mediators at the damage site. This is discussed in detail in the next section.

These mediators act as substrates of sensor kinases and hence are phosphorylated at various serine/threonine residues. This dynamic phosphorylation-based activation of mediator proteins helps in the recruitment of other downstream DDR proteins. Once ATM is recruited to the site of DNA damage it phosphorylates histone variant H2AX at serine-139 residue at the DNA double strand break (DSB) to form g-H2AX.

that accumulates at the damage site and is seen as discrete nuclear foci starting as early as one minute after irradiation, with maximal phosphorylation seen by10–30 min. Dephosphorylation of the constitutively phosphorylated tyrosine-142 by tyrosine phosphatase EYA occurs in parallel to the serine-139 phosphorylation, and is necessary for the proper recognition of -H2AX by its sensor MDC-1.

MDC1 helps in the retention of ATM at the damage site via direct interaction with the MRN complex and the subsequent binding of ATM to the C-terminus of NBS. Andegeko et al. reported that ATM exists in two forms, the bound form which binds to the DSB and the unbound form which remains free in the nucleoplasm.

g-H2AX foci span asymmetrically over megabases at the damage site, with the foci density greater near breaks and less far away.

To breaks, whereas in the distal chromatin regions the formation of g-H2AX is MDC-1 independent and is dependent on a soluble pool of ATM leading to formation of a lower density zone away from the break. H2AX and MDC-1 act as scaffold proteins onto which downstream DDR proteins such as BRCA1 complex, 53BP1, etc., accumulate to form IRIF which are described as visual indicators of the DNA repair centres. MDC1 itself is phosphorylated by ATM at various ‘TQXF’ clusters.

 These phosphorylations are very critical for the recruitment of downstream players such as RNF8 (RING finger protein 8). RNF8, a ubiquitin ligase, with an FHA domain at its N-terminus and a RING-finger domain at its C-terminus is recruited to the damage site via direct interaction of its FHA domain with phosphorylated TQXF clusters of MDC1.

RNF8-UBC13 ubiquitylates histones H2AX and H2A and this ubiquitylation was inhibited by RNF8 knockdown. The formation of these polyubiquitin chains at the site of DSB aids in the recruitment of RAP80-BRCA1 complex and 53BP1. demonstrated that  g-H2AX and MDC-1 foci formed properly in RNF-8 depleted cells whereas BRCA1 foci were diminished. This observation implies that RNF8 acts downstream of both g-H2AXand MDC-1 whereas it acts as an important upstream factor for formation of BRCA1 foci.

another ubiquitin ligase, RNF168, is required for the amplification and thus sustenance of DSB associated ubiquitinylation initiated by RNF8. RNF168binds to the ubiquitin chains of H2A and H2AX through its ubiquitin binding domain in an RNF8-dependent manner.

The next protein to join this dynamic assembly of molecules is RAP80 (receptor-associated protein 80) or UIMC1 (ubiquitin-interaction motif containing 1).

the localization of RAP80 at IRIF is actually mediated through the interaction of its UIM domain with K-63 linked ubiquitin chains at the damage site. Rap80 recruits the Abraxas/CCDC98-BRCC36-BRCA1-BARD1 complex of proteins to DNA damage sites [i.e., CCDC98: coiled-coil domain—containing protein, BRCC36: BRCA1/2 containing complex, BARD1: BRCA1associated ring domain protein 1]. Abraxas mediates the interaction of Rap80 with BRCC36 and BRCA1-BARD1.

BRCA1 is recruited to the IRIF in a Rap80 and Abraxas-dependent manner. The BRCA1 protein is associated with hereditary breast and ovarian cancers, and has significant roles in DNA repair, cell cycle checkpoint control and maintenance of genomic stability. BRCA1interacts with BARD, an E3 ubiquitin ligase, via its N-terminal RING domain forming a heterodimeric complex. Abraxas interacts with the BRCA1-BRCT domain through its C-terminal SPTF motif thereby recruiting the BRCA1/BARD1 complex to the DNA damage sites.

Brcc36, a deubiquitinase is also reported to be associated withRap80 and BRCA1. Rnf8/Ubc13 mediates the recruitment of BRCA1 ‘A’ complex containing BRCA1/BARD1, Abraxas, Rap80, and Brcc36 at the IRIF inMDC1-dependent manner.

ATM regulates cell cycle checkpoints by phosphorylation-dependent activation or inactivation of an array of effector molecules in response to DNA damage. Arrests in all three cell cycle phases after ionizing radiation require the activation of ATM protein kinase, as detailed below.

p53 is a pivotal effector protein that is modified at various sites by ATM in response to DNA damage. These modifications help in stabilizing and activating p53, a transcription factor that also activates various proteins downstream to effectively execute the G1-S checkpoint. Under normal conditions p53 is maintained at low levels by Mdm2-mediated ubiquitination and proteasomal degradation,

ATM phosphorylates Mdm2 at ser395which prevents p53 polyubiquitination and subsequent degradation of p53. ATM phosphorylates p53 at serine-15 which stimulates p53 transactivation, but does not influence the stability of p53. ATM also facilitates phosphorylation of serine-20 on p53indirectly via Chk2.

The G1-S checkpoint is enforced in two phases: The first phase of rapid and transient response to DNA damage is ATM-dependent and p53-independent. In response to DNA damage, ATM phosphorylates Chk2 which further phosphorylates and thus inactivates the Cdc25A phosphatase.

persistent phosphorylation of Cdk2 on Thr14/Tyr15, which causes inhibition of Cdk2/cyclin-E complex required for G1-S transition and consequently results in G1-S arrest. The second phase, which is ATM and p53-dependent, is responsible for sustained and delayed response to DNA damage. Upon stabilization, p53 induces overexpression of p21, a Cyclin Dependent Kinase (Cdk) inhibitor capable of silencing Cdk’s (cdk2) required for the G1 to S phase transition.

Thus, ATM regulates the G1-S checkpoint that controls the commitment to allow only the cells with undamaged DNA to pass from G1 phase into S phase.

Thus, ATM regulates the G1-S checkpoint that controls the commitment to allow only the cells with undamaged DNA to pass from G1 phase into S phase.

ATM phosphorylatesBRCA1, FANCD2, Chk2 and Nbs1 for the effective accomplishment of the Intra S-phase checkpoint.

Phosphorylation of BRCA1 at serine-1387 by ATM is required or S-phase arrest in response to ionizing radiation.

ATM also phosphorylates the Fanconi anaemia protein FANCD2on serine-222 in response to ionizing radiation for the activation of the S-phase checkpoint.

Regulation of the S-phase checkpoint is also achieved through phosphorylation of Nbs1 and SMC1 (Structural Maintenance of Chromosomes protein).

Cdc25A inactivation via ATM andChk2 as described for the G1-S phase checkpoint also controls the S-phase checkpoint, as the activity of Cdk2 is required both for G1as well as G2 phase.

The G2-M checkpoint prevents the cell with damaged DNA from entering mitosis and cell division. Under normal unperturbed conditions Cdc25C, a dual-specificity protein phosphatase, removes the inhibitory tyrosine 15 phosphorylation fromCdc2. Consequently, Cdc2 is now free to form a Cdc2-cyclin B complex whose activity is required to facilitate mitotic entry. there is ATM-dependent phosphorylation of Cdc25C on serine-216 via Chk2 which consequently causesCdc25C to bind to 14-3-3Rho protein that facilitates the exclusion of Cdc25C from nucleus. This prevents the action of Cdc25C on its substrate (viz., Cdc2) and thus causes G2-M arrest. found a new protein, NEK11 (NIMA (never in mitosis gene A) - related kinase 11), via large-scale short hairpin RNA (shRNA) library screening, that is involved in the G2-M checkpoint.

BRCA1 regulates key effectors of the G2/M checkpoint via ATM. In response to DNA damage, ATM phosphorylates BRCA1 on serine-1423, which is required to mediate the G2-M checkpoint. Mutation at this serine residue abolishes the G2-M checkpoint. BRCA1 also regulates the expression of Wee1(inhibits Cdc2/cyclinB by tyrosine 15 phosphorylation) and 14-3-3Rho (that sequesters Cdc25C).

BRCT domain and catalyses the ubiquitylation of CtIP via its RING finger domain. The ubiquitylated form of CtIP was shown to interact with chromatin after DNA damage and is involved in the G2-M checkpoint.

p21 (a Cdk inhibitor) inhibitsthe Cdc2/cyclinB1 complex.

Thus, ATM plays an important role in initiating checkpoint pathways in all three of these cell cycle phases through the activation of a series of effector molecules at each checkpoint. Cells lacking ATM function thus exhibit defective or inactive checkpoints.

Dephosphorylation and deubiquitylation are the two major processes required for termination of response at the DSB sites after completion of repair.

the chromatin returns to its pre-stress state. Deubiquitination at the chromatin flanking the DNA breaks occurs during the termination of response, to bring back the chromatin to the pre-stress stage.

the clinical and cellular manifestations of A-T can arise from imbalance in the redox status of the cell, leading to a state of continued oxidative stress. suggested the involvement of ATM in regulating the global cellular response to oxidative stress by acting as either sensor or mediator. Most of the studies that prove the association of ATM with oxidative stress were mainly carried out on ATM-deficient cell lines and tissues. ATM−/−mice showed increased frequency of DNA deletions, as well as increased levels of 8-OHdeoxyguanosine, a marker of oxidative DNA damage. These genetic instabilities reverted to normal levels when the diet of ATM−/−mice was supplemented with the antioxidant NAC (N-acetyl-l-cysteine). They suggested that as NAC counteracts the genetic instability, this could result from oxidative stress in the ATM-deficient mice. Several other studies have confirmed the involvement of ATM in oxidative stress signaling.

ERK1/2(extracellular signal-regulated protein kinase 1 and 2)-p16mediated. ERK1/2 is constitutively activated due to elevated level of ROS in ATM−/−astrocytes. Na Liu et al. . found that many of the markers of oxidative stress, such as malondialdehyde adducts, MnSOD, Cu/Zn SOD and Hsp70, were upregulated in ATM-deficient astrocytes.

ATM functions to maintain redox levels and energy homeostasis in cultured astrocytes. In line with some previously mentioned studies, Liu et al. suggested that oxidatively stressed ATM−/− astrocytes could not provide antioxidant support to the neurons and this eventually resulted in neurodegeneration or neuronal cell death, which is one of the major symptoms of A-T disease. ATM modulates intracellular redox homeostasis by acting as a sensor of ionizing radiation-induced oxidative stress by phosphorylating c jun on Ser63 and Ser73 residues via JNK.

ATM modulates intracellular redox homeostasis by acting as a sensor of ionizing radiation-induced oxidative stress by phosphorylating c jun on Ser63 and Ser73 residues via JNK.

ATM may directly modulate the redox environment of a cell by maintaining optimum levels of antioxidants, either by regulating the genes encoding these antioxidant molecules or by modifying antioxidant enzymes, i.e., at the post-translational level.

ATM was also shown to have extranuclear localization in peroxisomes, a major site of oxidative metabolism, and this further strengthens the role of ATM in direct regulation of oxidative stress signaling. ATM can sense and respond directly to abnormal levels of ROS. ATM acts as a sensor in the oxidative stress signaling cascade by getting directly activated by oxidation.

The hypothesis proposes that ATM deficiency leads to persistent DSB’s that could be otherwise repaired in normal cells. This persistent DNA damage leads to activation of repair enzymes such as PARP which synthesises PAR chain at the site of damage. The substrate for generating PAR is NAD+ and hence this process depletes the stores of cellular NAD+, reducing the antioxidant capacity of cell.

increased ROS can be an outcome of persistent DNA damage existing in ATM-deficient cells.

ATM may regulate global cellular responses to oxidative stress. Guo et al. suggested that oxidants activate ATM through different biochemical mechanisms than that of MRN complex signaling, which is well known in the DNA damage response pathway. When primary human fibroblasts were treated with H2O2, autophosphorylation of ATM was observed at serine-1981 together with phosphorylation of its substrates, viz., p53 and Chk2, whereas no phosphorylation or activation was observed in the case of H2AX and Kap1.

H2O2 can activate ATM even in the absence of DNA damage or phosphorylation of H2AX, thus showing that ATM may be acting as an oxidative stress sensor.

Neurodegeneration is a key aspect of A-T. Hence, understanding the functioning of ATM signaling in the nervous system could provide insight into the neuropathology associated with the A-T condition. neurological defects in A-T patients appear early in life by the age of two years. Symptoms include ataxia of both upper and lower limbs, standing and sitting disability, and impairment of eye and gait head movements. ATM functions in the nervous system have been linked to: (a) its possible role in neurogenesis, (b) regulation of DNA damage and repair in brain, and (c) regulation of the oxidative load in the brain. ATM is involved in survival, proliferation and renewal of NSC’s (Neural Stem Cells).

ATM protein is abundant in neural progenitor cells, but is reduced progressively as these cells differentiate. This suggests that ATM is involved in nervous system maintenance from as early as initial neural differentiation.

Atm−/−neural progenitor cells showed reduced survival and failed to proliferate and differentiate in normal manner. ATM was also found to maintain the genomic stability of neural progenitor cells.

immortalized human neural stem-cell line (ihNSC) capable of differentiating in vitro into the neurons, oligodendrocytes and astrocytes to demonstrate that ATM is required for proper terminal differentiation of neural stem cells.

Neurons are particularly vulnerable to oxidative stress and depend on astrocytes for antioxidant support.

showed that when growth medium conditioned with cerebellar astroglial cells prepared from normal mice was added to ATM-deficient cell cultures, survival was enhanced from 40% to 60%, hence emphasising the ability of astrocytes to provide antioxidant support to Purkinje cells against oxidative stress

Many neurodegenerative diseases are also associated with defective DNA repair efficiency. Neurodegeneration in A-T can be attributed in part to inability to respond to DNA damage.

ATM initiates ap53 and caspase-3 dependent cascade to eliminate neurons that have incurred genomic damage early during neurogenesis. The nervous system of ATM null mice shows a significant defect in ionizing radiation induced apoptosis. Lee et al. demonstrated that cells deficient in ATM could complete neural development even in Ligase IV (DNA repair enzyme) null animals.

Thus, ATM kinase regulates the homeostasis of the nervous system through its DDR and oxidative stress mediated functions, besides playing a pivotal role in neurogenesis

Currently there is no line of therapy for treatment or prevention of A-T. Certain approaches that are being used are only able to slow down the progression of the disease and alleviate some of its symptoms.

Almost 70% of mutations found in A-T patients are protein truncating, and certain mutation targeted therapies achieve expression of functional ATM protein via read-through of premature termination codons.

screened a library of 34,000 compounds and selected two low molecular mass non-aminoglycosides. These read-through compounds could induce expression of functional ATM, as detected by ATM kinase activity assay and colony survival assay for radiosensitivity

Large-scale proteomics, metabolomics and transcriptomic studies have revealed several hundreds of proteins that act as substrate to ATM.

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