DNA-dependent protein kinase and DNA repair: relevance to Alzheimer's disease
Alzheimer's Research & Therapy volume 5, Article number: 13 (2013)
The pathological hallmark of Alzheimer's disease (AD), the leading cause of senile dementia, involves region-specific neuronal death and an accumulation of neuronal and extracellular lesions termed neurofibrillary tangles and senile plaques, respectively. One of the biochemical abnormalities observed in AD is reduced DNA end-joining activity. The reduced capacity of post-mitotic neurons for some types of DNA repair is further compromised by aging. The predominant mechanism to repair double-strand DNA (dsDNA) breaks (DSB) is non-homologous end joining (NHEJ), which requires DNA-dependent protein kinase (DNA-PK) activity. DNA-PK is a holoenzyme comprising the p460 kDa DNA-PK catalytic subunit (DNA-PKcs) and the Ku heterodimer consisting of p86 (Ku 80) and p70 (Ku 70) subunits. Ku binds to DNA ends first and then recruits DNA-PKcs during NHEJ. However, in AD brains, reduced NHEJ activity has been reported along with reduced levels of DNA-PKcs and the Ku proteins, indicating a potential link between AD and dsDNA damage. Since age-matched control brains also show a reduction in these protein levels, whether there is a direct link between NHEJ ability and AD remains unknown. Possible mechanisms involving the role of DNA-PK in neurodegeneration, a benchmark of AD, are the focus of this review.
DNA, the hereditary material of all living organisms, is sensitive to damages from oxidation, hydrolysis, and methylation. The living cells are equipped with the ability for efficient DNA repair systems, such as repair base damage (base excision repair, or BER), nucleotide damage (nucleotide excision repair, or NER), single-strand breaks (single-strand break repair), and double-strand breaks (double-strand break repair). Double-strand DNA (DSB) breaks are considered the most lethal form of DNA damage. In eukaryotes, there are two major DSB repair pathways: non-homologous end joining (NHEJ) and homologous recombination (HR). NHEJ is the predominant pathway in higher eukaryotes and is active throughout the cell cycle [1–3], whereas HR is generally limited to S and G2 when a sister chromatid is available as a repair template . The primary role of NHEJ is to resolve DNA double-strand breaks, and data implicate DNA-dependent protein kinase (DNA-PK) as a central regulator of DNA end access .
Alzheimer's disease (AD) is a central nervous system neurodegenerative disease. The pathological presentation of AD, the leading cause of senile dementia, involves regionalized neuronal death and an accumulation of neuronal and extracellular lesions termed neurofibrillary tangles and senile plaques, respectively (reviewed in ). Several independent hypotheses have been proposed to link the pathological lesions and neuronal cytopathology with, among others, apolipoprotein E genotype [7, 8], hyperphosphorylation of cytoskeletal proteins (neurofilaments and Tau) , and amyloid-β metabolism . However, not one of these theories alone is sufficient to explain the diversity of biochemical and pathological abnormalities of AD. There is limited evidence for neuronal loss in most amyloid precursor protein (APP) models, and when neuronal loss was noted, it was modest [11–13].
Cellular damage by oxidative stress has been proposed as a causative factor in pathophysiology of AD and normal aging. Elevated levels of oxidative damage in both nuclear DNA and mitochondrial DNA have been reported in brains of patients with AD , and BER deficiency has been found in post-mortem brains of sporadic patients with AD . However, impaired BER activity found in neuropathological brain regions and in the cerebellum where there is no neuronal death indicates that BER deficiency is not specific to human AD brains . The lack of a difference in BER activity between wild-type and AD model mice brains in any age group  indicates that species-specific mechanisms may be involved in AD progression. Nonetheless, progressive neuronal loss due to cumulative damage to DNA and lack of DNA repair has been hypothesized to contribute to AD and stroke [18, 19]. Moreover, human hereditary syndromes with genetic defects in the DNA repair process manifest in early-onset developmental and progressive neurodegeneration, indicating that defects in DNA damage repair are neuropathological [20, 21].
In the four major DNA repair pathways (double-strand DNA (dsDNA) break repair (HR and NHEJ), NER, BER, and mismatch repair), key proteins, including some with dual functions, participate in DNA damage sensing/repair and apoptosis (Table 1) . The focus of this review is on how DNA-PK activity may be linked to AD and whether a 'cause and effect' scenario emerges from the reported studies.
DNA-dependent protein kinase, a multi-subunit enzyme
DNA-PK is a PI3 kinase family member, and like targets of other members (ATR and ATM) of this family, its preferential targets of phosphorylation are the serines and threonines followed by a glutamine (S-T/Q sites), although other S-T/hydrophobic residues are also phosphorylated . DNA-PK enzyme activity is essential for NHEJ . Although DNA-PK is implicated in a variety of functions from activation of innate immunity  to regulation of gene expression , its primary cellular function is to initiate NHEJ. A multi-subunit enzyme, DNA-PK consists of a catalytic subunit (DNA-PKcs), p460, and a regulatory subunit called Ku. The Ku protein is a heterodimer composed of 70-kDa (Ku70) and 80-kDa (Ku80) subunits and has the capability of binding selectively to specific forms of DNA [25, 26]. In this capacity, it functions as the regulator of DNA-PK that is active in transcription, DNA recombination, and DNA repair [27–29]. Its role in DNA repair was not formally proven until the emergence of studies implicating Ku as the defective factor in cells hypersensitive to DNA-damaging agents .
DNA-dependent protein kinase activators
Ku binds DNA ends in a sequence-independent manner, and in the absence of DNA-PKcs, the extreme DNA terminus is bound in an accessible channel . Ku has strong avidity for DNA with a variety of end structures, such as blunt, over-hanged, hair-pinned, and damaged. Ku can also recognize gaps and nicks in dsDNA, indicating possible roles of DNA-PK in the repair of damage other than DSBs , and is particularly suited to do this since DNA-PKcs assembles onto Ku-bound DNA regardless of end structure . Although DNA-PKcs has innate affinity (itself) for DNA ends (in low salt conditions), Ku is required for targeting DNA-PKcs to damaged DNA in physiologic conditions and in living cells . Although any DSB discontinuity can activate DNA-PK, its activation varies considerably depending on the end structure, and studies show kinase activation in trans (achieved by kinase autophosphrylation) or cis (achieved by specific DNA strand orientation and sequence bias) . Activation by these dual processes represents a potentially powerful mechanism by which DNA-PK protects DNA ends to maintain genome integrity. After the DSB repair, Ku likely remains trapped on the DNA . How Ku gets removed from the DNA is not clear, although a protease-mediated degradation of Ku80 has been speculated .
Cellular and molecular targets of DNA-dependent protein kinase in non-homologous end joining
Many proteins have been listed as excellent in vitro and in vivo DNA-PK targets, but the functional relevance of their phosphorylation by DNA-PK remains mostly unclear. Most of the proteins involved in NHEJ (XRCC4, Ku70, Ku80, Artemis, DNA-PKcs, and XLF) are excellent in vitro and in vivo targets of DNA-PK [2, 37–40]. When a DSB occurs, the Ku heterodimer (Ku80/Ku70) first binds to the broken ends by using Ku80 and then recruits the DNA-PKcs, which is activated upon binding to Artemis nuclease, and the repair process is completed by XRCC4-DNA ligase IV [39, 41] (Figure 1). Physical association of DNA-PKcs and its enzymatic activity are required for Artemis's endonucleolytic activity . In the absence of DNA damage, Artemis is complexed with DNA-PKcs. It has been shown that DNA-PKcs is targeted to Ku-bound DNA; Artemis is released from DNA-PKcs and is rebound again only when the kinase is activated . Artemis itself has both endo- and exonuclease activities . DNA-PKcs strongly suppresses the exonuclease activity of Artemis but allows limited endonucleolytic trimming, likely at regions of transition from single-strand to double-strand [39, 42].
Non-homologous end joining and DNA-dependent protein kinase in neurons
Mature neurons are essentially post-mitotic and do not proliferate, whereas some glial cells can undergo replication especially as a response to stress or damage [43, 44]. Neurons are also among the most metabolically and transcriptionally active cells (reviewed in ), thus making these cells vulnerable to risks that involve DNA damage.
DNA repair pathways in brain have been studied extensively over the last two decades (reviewed in [45, 46]). In mammals, DSB repair uses two mechanisms: HR and NHEJ. NHEJ is the predominant dsDNA repair pathway in mammalian cells . Compared with the HR, NHEJ is considered error-prone and imprecise as it acts at the DNA break sites to restore the chromosomal structural integrity which could come at the expense of one or a few nucleotides. Since most of the higher eukaryote genome is non-coding, error-prone rejoining of DSBs by NHEJ generally has minimal deleterious consequences. However, DSB repair in coding regions can potentially introduce functionally important coding changes. Over time, as in aging, these small errors can accumulate, resulting in genome instability that leads to cellular dysfunction or death. Accordingly, it has been reported that 10% of p53 mutations in human cancers could be attributed to deletions arising from NHEJ sites . NHEJ is also the predominant form of dsDNA repair pathway in post-mitotic neurons  and is critical in the nervous system development since mice deficient in DNA ligase IV, XRCC4, Ku70, and Ku 80, which are participants in the NHEJ event, show massive apoptosis of post-mitotic neurons [46, 50]. Loss of NHEJ activity in the developing brain can be prenatally lethal and, in adults, can lead to neurodegenerative diseases [46, 51, 52]. Mice with defective NHEJ show accelerated aging [53, 54].
DNA-dependent protein kinase and cell-cycle re-entry in neurodegeneration
One of the factors contributing to neurodegeneration is the re-entry of terminally differentiated post-mitotic neurons into the cell cycle because of chronic or acute insults associated with DNA damage and oxidative stress that result in apoptosis [55, 56]. DSB repair capability is critical for neurogenesis during development, and damaged neurons demonstrate this by escaping apoptosis, re-entering the cell cycle, and incorporating into the developing brain, leading to neurodegeneration in mice with low or no ATM activity . While ATM deficiency suppresses the re-entry of post-mitotic neurons into S-phase and protects against apoptosis , it also increases the yield of unrepaired DNA that eventually may be lethal. Neuronal DNA damage is linked to the re-entry of neurons into the cell cycle [56, 58]. When post-mitotic neurons try to re-enter the cell cycle, the very attempt to transcribe a subset of cell cycle-related genes that have not been transcribed for years in the lifetime of a mature neuron may accumulate damaged DNA, which could trigger neuronal apoptosis .
Furthermore, it has been suggested that DNA replication resulted when cell-cycle re-entry preceded neurodegeneration in AD brains . Reactive oxygen/nitrogen species are reported to cause unscheduled and incomplete DNA replication known as 'replication stress' . Thus, inefficient DNA replication posing 'replication stress' in AD pathogenesis leading to genomic instability potentially links Aβ accumulation and erroneous cell-cycle pathways . Obviously, incomplete DNA replication due to DSBs or defective DNA repair systems (or both) would be highly probable in post-mitotic neurons, causing replication stress and subsequently leading to accelerated accumulation of further DNA damages and genomic instabilities [63, 64]. Stalled replication forks collapse to yield one-ended DSBs, or 'double-strand ends', and abnormal DNA replication in post-mitotic neurons may be the source of intracellular increase in DNA content observed in AD brains [60, 65]. It has been shown that DNA-PKcs mutant cells fail to arrest replication following stress . Additionally, studies show that, in response to replication stress-induced DNA damage, DNA-PK phosphorylates replication protein A (RPA) and dissociates RPA:DNA-PK complex [67, 68], thereby inhibiting HR . Thus, reduced DNA-PK activity in a cell could potentially induce replication stress and genome instability.
DNA-dependent protein kinase in Alzheimer's disease and aging
It has also been shown that cells from old mice contain more DSBs than cells from young mice and that the fidelity and efficiency decline significantly during cellular senescence . This event may contribute to age-related genomic instability and aging. DNA-PK plays critical roles in, first, detecting DNA damage and, then, triggering signaling pathways, including programmed cell death . Ku80−/− mice are defective in the NHEJ and telomere maintenance and show premature aging, but surprisingly no human disorder caused by Ku80 deficiency or mutation has been reported [54, 71].Interestingly, Ku80 and DNA-PKcs protein levels as well as the DNA-binding ability of Ku80 are reduced following severe ischemic injury, which causes extensive neuronal death in rabbits . Furthermore, though not significantly different from that of the age-matched controls, Ku-DNA binding is reduced in extracts of post-mortem AD mid-frontal cortex, and this could be attributed to reduced levels of Ku subunits and DNA-PKcs . However, a report from the same laboratory demonstrated that NHEJ is reduced in cortical extracts from brains of AD versus normal subjects and that DNA-PKcs level was significantly lower in the AD brain extracts . Whether other DNA repair systems, especially HR, are altered in the AD brains is not known (Figure 2).
To explain the complexity of AD, a 'two-hit hypothesis' for AD development has been reported; the first hit makes neurons vulnerable and the second hit triggers the neurodegenerative process . The first hit may constitute abnormalities when neurons try to re-enter the cell cycle or oxidative stress, which, if persistent, can create a pro-oxidant environment as encountered in pre-AD and AD cases. In this environment, proteins highly sensitive to redox modulation, including p53, can be compromised . A number of post-mortem studies suggest an involvement of p53 in AD, and high levels of p53 in certain neurons in post-mortem samples from patients with AD have been reported (reviewed in ). DNA-PK activates p53 by phosphorylating the amino-terminal site , and p53 can induce Bax, a pro-apoptotic protein that translocates to the mitochondria and initiates the intrinsic death pathway . Regulation of Bax-mediated neuronal death also reportedly involves Ku70 phosphorylation by DNA-PK . In this regard, reduction in DNA-PKcs levels in AD brains does not seem to be consistent with the role of DNA-PKcs as the trigger for p53-mediated neurodegeneration (Figure 3).
DNA-PK is believed to have little or no effect on p53-dependent cell-cycle arrest. In contrast, there are reports linking p53 phosphorylation by DNA-PK to cellular death machinery (reviewed in ). DNA-PK is also involved in regulating the activities of RNA polymerase I and II via phosphorylation (reviewed in ). Given these important substrates of DNA-PK that are critical players in cell death and gene transcription, it is difficult to pinpoint the exact role(s) of DNA-PKcs and its cofactor (Ku80/Ku70) in AD. Likewise, it would be simplistic to directly link reduced levels of DNA-PK subunits and consequently less proficient NHEJ in AD brains to neurodegeneration. On the other hand, it is attractive to speculate that DNA damage (for example, induced by reactive oxygen species (ROS) downstream of Aβ) in neurons with reduced NHEJ activity, triggering them to re-enter the cell cycle unsuccessfully, could lead to the accumulation of excessive genomic damage and eventually cause neuron death (Figure 3). In either pathway, given that NHEJ is the process involved, the importance of the DNA-PKcs/Ku complex in the development of neurodegenerative pathology may be considerable. The reduced levels of DNA-PKcs and Ku80/Ku70 subunits in post-mortem AD brains may be perceived as upstream events of neuron loss in AD, although further studies to differentiate between cause and consequence are warranted.
DNA-dependent protein kinase and amyloid beta
In a recent study, sublethal levels of aggregated Aβ(25-35) have been shown to inhibit DNA-PK activity in nerve growth factor (NGF)-differentiated PC12 cells . In this study, one of the potential mechanisms appears to be Aβ-induced ROS-mediated degradation of DNA-PKcs. Aβ also induces DNA-PKcs carbonylation, an irreversible oxidative protein modification that may trigger its degradation by proteasomes [83, 84]. DNA-PK activity is also inhibited by H2O2 in cell-free assays, indicating that ROS may directly inhibit DNA-PK activity . On the other hand, Aβ(1-42), which can enter the nucleus of PC12 cells, also downregulates DNA-PK activity, possibly by a mechanism other than the involvement of oxidative stress. In AD cases, a decrease in DNA-PKcs expression in neurons and astrocytes, though not significant, has been reported . Although it is tempting to link AD development to Aβ-induced attenuation of DNA-PK activity and hence to reduced NHEJ activity, it may be argued that this event is a consequence rather than the prime cause, a simultaneous event that could occur independently of Aβ-triggered neurotoxic pathways.
In contrast to other NHEJ and HR factors, all three components of the DNA-PK complex (DNA-PKcs, Ku80, and Ku70) are exceptionally abundant proteins, especially in human cells . Given the complexity of AD, a clear distinction is lacking as to whether the expression of DNA-PK subunits may have been transcriptionally impaired in AD brains because of a hitherto unknown upstream event that is too generic to have a specifically targeted effect on DNA-PK. As for the reduced level of DNA-PKcs, Aβ-induced proteasome-mediated degradation of DNA-PKcs has been proposed [83, 84]. With regard to other NHEJ components as essential as DNA-PK (for example, Artemis), their status in AD remains unexplored. In AD, it is possible that with already-declining NHEJ activity due to the defects in some other components associated with the process, a reduced DNA-PK activity may be consequential or a secondary effect. Therefore, a decline in DNA-PK subunit levels and its kinase activity may, for the time being, serve as biomarkers until future studies, especially in vivo studies, add to substantiate a direct link of DNA-PK to AD. Furthermore, increased HR of substrates in cells that lack DNA-PK [87, 88], co-localization of NHEJ and HR factors at the same DNA lesion , partial rescue of the severe phenotypes associated with deficiency of XRCC4 or ligase IV, by deletion of DNA-PK [90, 91], indicate that, in the absence of DNA-PK and NHEJ, the cells may tend to acquire an alternate path (for example, HR) to repairing DSBs. Interestingly, in cell culture studies, crosstalk between HR repair and NHEJ has been shown to involve ATM, DNA-PK, and ATR, indicating that DNA-PK may participate in HR by co-regulating p53 and RPA . Should such alternate pathway(s) be adversely affected by factors causing AD onset, the neurons devoid of any ability to repair DSBs may be vulnerable to degeneration. Since DNA-PKcs and Ku mutations in humans are not existent [54, 71] and aging brains also exhibit reduced levels of Ku80 and NHEJ activity [73, 74], it remains to be seen whether reduced levels of DNA-PK complex and the enzyme activity do bear any direct relationship to AD (Figure 2). Discerning between normal aging-related attenuation of DNA-PK activity/expression and that in AD cases warrants careful assessment.
Lately, linking genomic lesions during neurodegeneration to transcriptional insufficiency is fast emerging . Nucleolar insensitivity to DSBs has been implicated in neurodegeneration . Nucleolus is the site of ribosomal RNA (rRNA) biogenesis . The RNA polymerase I (Pol I) drives the transcription of rRNA, and continuous Pol I activity is required for nucleolar maintenance. The DNA-PK component Ku has been shown to suppress RNA Pol I transcription in vitro and in P19 stem cells [95–97]. In this context, a reduction in DNA-PK activity and level of expression of its components in AD brains [73, 74] should act as a relief factor for Pol I transcription. Interestingly, while hippocampal neurons have been shown to have AD-associated reduction in nucleolar volume , in subjects with moderate AD pathology without cognitive impairment, nucleolar hypertrophy has been reported in both cortical and hippocampal neurons . Although the latter scenario appears consistent with reduced DNA-PK and Ku levels in AD brains [73, 74], the link remains unclear and needs further research.
Ataxia telangiectasia mutated protein
Ataxia telangiectasia and Rad3-related protein
base excision repair
DNA-dependent protein kinase
DNA-dependent protein kinase catalytic subunit
nucleotide excision repair
non-homologous end joining
- Pol I:
RNA polymerase I
reactive oxygen species
replication protein A
x-ray repair cross-complementing protein 4-like factor
x-ray repair cross-complementing protein 4.
Critchlow SE, Jackson SP: DNA end-joining: from yeast to man. Trends Biochem Sci. 1998, 23: 394-398. 10.1016/S0968-0004(98)01284-5.
Lieber MR: The biochemistry and biological significance of nonhomologous DNA end joining: an essential repair process in multicellular eukaryotes. Genes Cells. 1999, 4: 77-85. 10.1046/j.1365-2443.1999.00245.x.
Pastink A, Eeken JC, Lohman PH: Genomic integrity and the repair of double-strand DNA breaks. Mutat Res. 2001, 480-481: 37-50.
Rothkamm K, Kruger I, Thompson LH, Lobrich M: Pathways of DNA double-strand break repair during the mammalian cell cycle. Mol Cell Biol. 2003, 23: 5706-5715. 10.1128/MCB.23.16.5706-5715.2003.
Kienker LJ, Shin EK, Meek K: Both V(D)J recombination and radioresistance require DNA-PK kinase activity, though minimal levels suffice for V(D)J recombination. Nucleic Acids Res. 2000, 28: 2752-2761. 10.1093/nar/28.14.2752.
Smith MA, Perry G: The pathogenesis of Alzheimer disease: an alternative to the amyloid hypothesis. J Neuropathol Exp Neurol. 1997, 56: 217-10.1097/00005072-199702000-00015.
Corder EH, Saunders AM, Strittmatter WJ, Schmechel DE, Gaskell PC, Small GW, Roses AD, Haines JL, Pericak-Vance MA: Gene dose of apolipoprotein E type 4 allele and the risk of Alzheimer's disease in late onset families. Science. 1993, 261: 921-923. 10.1126/science.8346443.
Roses AD, Saunders AM, Corder EH, Pericakvance MA, Han SH, Einstein G, Hulette C, Schmechel DE, Holsti M, Huang D, Haines JL, Goedert M, Jakes R, Dong LM, Weisgraber KH, Strittmatter WJ: Influence of the susceptibility genes apolipoprotein E-epsilon 4 and apolipoprotein E-epsilon 2 on the rate of disease expressivity of late-onset Alzheimer's disease. Arzneimittelforschung. 1995, 45: 413-417.
Trojanowski JQ, Schmidt ML, Shin RW, Bramblett GT, Rao D, Lee VM: Altered tau and neurofilament proteins in neuro-degenerative diseases: diagnostic implications for Alzheimer's disease and Lewy body dementias. Brain Pathol. 1993, 3: 45-54. 10.1111/j.1750-3639.1993.tb00725.x.
Selkoe DJ: Alzheimer's disease: genotypes, phenotypes, and treatments. Science. 1997, 275: 630-631. 10.1126/science.275.5300.630.
Duyckaerts C, Potier MC, Delatour B: Alzheimer disease models and human neuropathology: similarities and differences. Acta Neuropathol. 2008, 115: 5-38.
Hsiao K, Chapman P, Nilsen S, Eckman C, Harigaya Y, Younkin S, Yang F, Cole G: Correlative memory deficits, Abeta elevation, and amyloid plaques in transgenic mice. Science. 1996, 274: 99-102. 10.1126/science.274.5284.99.
Liu L, Orozco IJ, Planel E, Wen Y, Bretteville A, Krishnamurthy P, Wang L, Herman M, Figueroa H, Yu WH, Arancio O, Duff K: A transgenic rat that develops Alzheimer's disease-like amyloid pathology, deficits in synaptic plasticity and cognitive impairment. Neurobiol Dis. 2008, 31: 46-57. 10.1016/j.nbd.2008.03.005.
Wang J, Xiong S, Xie C, Markesbery WR, Lovell MA: Increased oxidative damage in nuclear and mitochondrial DNA in Alzheimer's disease. J Neurochem. 2005, 93: 953-962. 10.1111/j.1471-4159.2005.03053.x.
Weissman L, Jo DG, Sorensen MM, de Souza-Pinto NC, Markesbery WR, Mattson MP, Bohr VA: Defective DNA base excision repair in brain from individuals with Alzheimer's disease and amnestic mild cognitive impairment. Nucleic Acids Res. 2007, 35: 5545-5555. 10.1093/nar/gkm605.
Jeppesen DK, Bohr VA, Stevnsner T: DNA repair deficiency in neurodegeneration. Prog Neurobiol. 2011, 94: 166-200. 10.1016/j.pneurobio.2011.04.013.
Gredilla R, Weissman L, Yang JL, Bohr VA, Stevnsner T: Mitochondrial base excision repair in mouse synaptosomes during normal aging and in a model of Alzheimer's disease. Neurobiol Aging. 2012, 33: 694-707. 10.1016/j.neurobiolaging.2010.06.019.
Adamec E, Vonsattel JP, Nixon RA: DNA strand breaks in Alzheimer's disease. Brain Res. 1999, 849: 67-77. 10.1016/S0006-8993(99)02004-1.
Love S, Barber R, Wilcock GK: Apoptosis and expression of DNA repair proteins in ischaemic brain injury in man. Neuroreport. 1998, 9: 955-959. 10.1097/00001756-199804200-00001.
Gueven N, Chen P, Nakamura J, Becherel OJ, Kijas AW, Grattan-Smith P, Lavin MF: A subgroup of spinocerebellar ataxias defective in DNA damage responses. Neuroscience. 2007, 145: 1418-1425. 10.1016/j.neuroscience.2006.12.010.
Lee Y, McKinnon PJ: Responding to DNA double strand breaks in the nervous system. Neuroscience. 2007, 145: 1365-1374. 10.1016/j.neuroscience.2006.07.026.
Lees-Miller SP, Meek K: Repair of DNA double strand breaks by nonhomologous end joining. Biochimie. 2003, 85: 1161-1173. 10.1016/j.biochi.2003.10.011.
Chu W, Gong X, Li Z, Takabayashi K, Ouyang H, Chen Y, Lois A, Chen DJ, Li GC, Karin M, Raz E: DNA-PKcs is required for activation of innate immunity by immunostimulatory DNA. Cell. 2000, 103: 909-918. 10.1016/S0092-8674(00)00194-X.
Mo X, Dynan WS: Subnuclear localization of Ku protein: functional association with RNA polymerase II elongation sites. Mol Cell Biol. 2002, 22: 8088-8099. 10.1128/MCB.22.22.8088-8099.2002.
de Vries E, van Driel W, Bergsma WG, Arnberg AC, van der Vliet PC: HeLa nuclear protein recognizing DNA termini and translocating on DNA forming a regular DNA-multimeric protein complex. J Mol Biol. 1989, 208: 65-78. 10.1016/0022-2836(89)90088-0.
Mimori T, Hardin JA: Mechanism of interaction between Ku protein and DNA. J Biol Chem. 1986, 261: 10375-10379.
Carter T, Vancurova I, Sun I, Lou W, DeLeon S: A DNA-activated protein kinase from HeLa cell nuclei. Mol Cell Biol. 1990, 10: 6460-6471.
Jackson SP, MacDonald JJ, Lees-Miller S, Tjian R: GC box binding induces phosphorylation of Sp1 by a DNA-dependent protein kinase. Cell. 1990, 63: 155-165. 10.1016/0092-8674(90)90296-Q.
Satoh MS, Lindahl T: Role of poly(ADP-ribose) formation in DNA repair. Nature. 1992, 356: 356-358. 10.1038/356356a0.
Liang F, Romanienko PJ, Weaver DT, Jeggo PA, Jasin M: Chromosomal double-strand break repair in Ku80-deficient cells. Proc Natl Acad Sci USA. 1996, 93: 8929-8933. 10.1073/pnas.93.17.8929.
Walker JR, Corpina RA, Goldberg J: Structure of the Ku heterodimer bound to DNA and its implications for double-strand break repair. Nature. 2001, 412: 607-614. 10.1038/35088000.
Rathmell WK, Chu G: Involvement of the Ku autoantigen in the cellular response to DNA double-strand breaks. Proc Natl Acad Sci USA. 1994, 91: 7623-7627. 10.1073/pnas.91.16.7623.
Smider V, Rathmell WK, Brown G, Lewis S, Chu G: Failure of hairpin-ended and nicked DNA To activate DNA-dependent protein kinase: implications for V(D)J recombination. Mol Cell Biol. 1998, 18: 6853-6858.
Drouet J, Delteil C, Lefrancois J, Concannon P, Salles B, Calsou P: DNA-dependent protein kinase and XRCC4-DNA ligase IV mobilization in the cell in response to DNA double strand breaks. J Biol Chem. 2005, 280: 7060-7069. 10.1074/jbc.M410746200.
Reddy YV, Ding Q, Lees-Miller SP, Meek K, Ramsden DA: Non-homologous end joining requires that the DNA-PK complex undergo an autophosphorylation-dependent rearrangement at DNA ends. J Biol Chem. 2004, 279: 39408-39413. 10.1074/jbc.M406432200.
Gullo CA, Ge F, Cow G, Teoh G: Ku86 exists as both a full-length and a protease-sensitive natural variant in multiple myeloma cells. Cancer Cell Int. 2008, 8: 4-10.1186/1475-2867-8-4.
Chan DW, Ye R, Veillette CJ, Lees-Miller SP: DNA-dependent protein kinase phosphorylation sites in Ku 70/80 heterodimer. Biochemistry. 1999, 38: 1819-1828. 10.1021/bi982584b.
Goodarzi AA, Yu Y, Riballo E, Douglas P, Walker SA, Ye R, Harer C, Marchetti C, Morrice N, Jeggo PA, Lees-Miller SP: DNA-PK autophosphorylation facilitates Artemis endonuclease activity. EMBO J. 2006, 25: 3880-3889. 10.1038/sj.emboj.7601255.
Ma Y, Pannicke U, Schwarz K, Lieber MR: Hairpin opening and overhang processing by an Artemis/DNA-dependent protein kinase complex in nonhomologous end joining and V(D)J recombination. Cell. 2002, 108: 781-794. 10.1016/S0092-8674(02)00671-2.
Ma Y, Schwarz K, Lieber MR: The Artemis:DNA-PKcs endonuclease cleaves DNA loops, flaps, and gaps. DNA Repair (Amst). 2005, 4: 845-851. 10.1016/j.dnarep.2005.04.013.
DeFazio LG, Stansel RM, Griffith JD, Chu G: Synapsis of DNA ends by DNA-dependent protein kinase. EMBO J. 2002, 21: 3192-3200. 10.1093/emboj/cdf299.
Yannone SM, Khan IS, Zhou RZ, Zhou T, Valerie K, Povirk LF: Coordinate 5' and 3' endonucleolytic trimming of terminally blocked blunt DNA double-strand break ends by Artemis nuclease and DNA-dependent protein kinase. Nucleic Acids Res. 2008, 36: 3354-3365. 10.1093/nar/gkn205.
Korr H: Proliferation of different cell types in the brain. Adv Anat Embryol Cell Biol. 1980, 61: 1-72. 10.1007/978-3-642-67577-5_1.
Ridet JL, Malhotra SK, Privat A, Gage FH: Reactive astrocytes: cellular and molecular cues to biological function. Trends Neurosci. 1997, 20: 570-577. 10.1016/S0166-2236(97)01139-9.
Rao KS: DNA repair in aging rat neurons. Neuroscience. 2007, 145: 1330-1340. 10.1016/j.neuroscience.2006.09.032.
Brooks PJ: DNA repair in neural cells: basic science and clinical implications. Mutat Res. 2002, 509: 93-108. 10.1016/S0027-5107(02)00222-1.
Lieber MR, Ma Y, Pannicke U, Schwarz K: Mechanism and regulation of human non-homologous DNA end-joining. Nat Rev Mol Cell Biol. 2003, 4: 712-720. 10.1038/nrm1202.
Greenblatt MS, Grollman AP, Harris CC: Deletions and insertions in the p53 tumor suppressor gene in human cancers: confirmation of the DNA polymerase slippage/misalignment model. Cancer Res. 1996, 56: 2130-2136.
Rass U, Ahel I, West SC: Defective DNA repair and neurodegenerative disease. Cell. 2007, 130: 991-1004. 10.1016/j.cell.2007.08.043.
Sekiguchi JM, Gao Y, Gu Y, Frank K, Sun Y, Chaudhuri J, Zhu C, Cheng HL, Manis J, Ferguson D, Davidson L, Greenberg ME, Alt FW: Nonhomologous end-joining proteins are required for V(D)J recombination, normal growth, and neurogenesis. Cold Spring Harb Symp Quant Biol. 1999, 64: 169-181. 10.1101/sqb.1999.64.169.
McKinnon PJ, Caldecott KW: DNA strand break repair and human genetic disease. Annu Rev Genomics Hum Genet. 2007, 8: 37-55. 10.1146/annurev.genom.7.080505.115648.
Yang Y, Herrup K: Loss of neuronal cell cycle control in ataxia-telangiectasia: a unified disease mechanism. J Neurosci. 2005, 25: 2522-2529. 10.1523/JNEUROSCI.4946-04.2005.
Smith GC, Jackson SP: The DNA-dependent protein kinase. Genes Dev. 1999, 13: 916-934. 10.1101/gad.13.8.916.
Vogel H, Lim DS, Karsenty G, Finegold M, Hasty P: Deletion of Ku86 causes early onset of senescence in mice. Proc Natl Acad Sci USA. 1999, 96: 10770-10775. 10.1073/pnas.96.19.10770.
Krantic S, Mechawar N, Reix S, Quirion R: Molecular basis of programmed cell death involved in neurodegeneration. Trends Neurosci. 2005, 28: 670-676. 10.1016/j.tins.2005.09.011.
Wersto RP, Cardozo-Pelaez F, Smilenov L, Chan SL, Chrest FJ, Emokpae R, Gorospe M, Mattson MP: Cell cycle activation linked to neuronal cell death initiated by DNA damage. Neuron. 2004, 41: 549-561. 10.1016/S0896-6273(04)00017-0.
Lee Y, Chong MJ, McKinnon PJ: Ataxia telangiectasia mutated-dependent apoptosis after genotoxic stress in the developing nervous system is determined by cellular differentiation status. J Neurosci. 2001, 21: 6687-6693.
McMurray CT: To die or not to die: DNA repair in neurons. Mutat Res. 2005, 577: 260-274. 10.1016/j.mrfmmm.2005.03.006.
Nouspikel T, Hanawalt PC: When parsimony backfires: neglecting DNA repair may doom neurons in Alzheimer's disease. Bioessays. 2003, 25: 168-173. 10.1002/bies.10227.
Yang Y, Geldmacher DS, Herrup K: DNA replication precedes neuronal cell death in Alzheimer's disease. J Neurosci. 2001, 21: 2661-2668.
Shen C, Lancaster CS, Shi B, Guo H, Thimmaiah P, Bjornsti MA: TOR signaling is a determinant of cell survival in response to DNA damage. Mol Cell Biol. 2007, 27: 7007-7017. 10.1128/MCB.00290-07.
Yurov YB, Vorsanova SG, Iourov IY: The DNA replication stress hypothesis of Alzheimer's disease. ScientificWorldJournal. 2011, 11: 2602-2612.
Bester AC, Roniger M, Oren YS, Im MM, Sarni D, Chaoat M, Bensimon A, Zamir G, Shewach DS, Kerem B: Nucleotide deficiency promotes genomic instability in early stages of cancer development. Cell. 2011, 145: 435-446. 10.1016/j.cell.2011.03.044.
Burhans WC, Weinberger M: DNA replication stress, genome instability and aging. Nucleic Acids Res. 2007, 35: 7545-7556. 10.1093/nar/gkm1059.
Chen J, Cohen ML, Lerner AJ, Yang Y, Herrup K: DNA damage and cell cycle events implicate cerebellar dentate nucleus neurons as targets of Alzheimer's disease. Mol Neurodegener. 2010, 5: 60-10.1186/1750-1326-5-60.
Liu S, Opiyo SO, Manthey K, Glanzer JG, Ashley AK, Amerin C, Troksa K, Shrivastav M, Nickoloff JA, Oakley GG: Distinct roles for DNA-PK, ATM and ATR in RPA phosphorylation and checkpoint activation in response to replication stress. Nucleic Acids Res. 2012, 40: 10780-10794. 10.1093/nar/gks849.
Shao RG, Cao CX, Zhang H, Kohn KW, Wold MS, Pommier Y: Replication-mediated DNA damage by camptothecin induces phosphorylation of RPA by DNA-dependent protein kinase and dissociates RPA:DNA-PK complexes. EMBO J. 1999, 18: 1397-1406. 10.1093/emboj/18.5.1397.
Zernik-Kobak M, Vasunia K, Connelly M, Anderson CW, Dixon K: Sites of UV-induced phosphorylation of the p34 subunit of replication protein A from HeLa cells. J Biol Chem. 1997, 272: 23896-23904. 10.1074/jbc.272.38.23896.
Liaw H, Lee D, Myung K: DNA-PK-dependent RPA2 hyperphosphorylation facilitates DNA repair and suppresses sister chromatid exchange. PLoS One. 2011, 6: e21424-10.1371/journal.pone.0021424.
Seluanov A, Mittelman D, Pereira-Smith OM, Wilson JH, Gorbunova V: DNA end joining becomes less efficient and more error-prone during cellular senescence. Proc Natl Acad Sci USA. 2004, 101: 7624-7629. 10.1073/pnas.0400726101.
Pandita TK: The role of ATM in telomere structure and function. Radiat Res. 2001, 156: 642-647. 10.1667/0033-7587(2001)156[0642:TROAIT]2.0.CO;2.
Shackelford DA, Tobaru T, Zhang S, Zivin JA: Changes in expression of the DNA repair protein complex DNA-dependent protein kinase after ischemia and reperfusion. J Neurosci. 1999, 19: 4727-4738.
Shackelford DA: DNA end joining activity is reduced in Alzheimer's disease. Neurobiol Aging. 2006, 27: 596-605. 10.1016/j.neurobiolaging.2005.03.009.
Davydov V, Hansen LA, Shackelford DA: Is DNA repair compromised in Alzheimer's disease?. Neurobiol Aging. 2003, 24: 953-968. 10.1016/S0197-4580(02)00229-4.
Zhu X, Raina AK, Perry G, Smith MA: Alzheimer's disease: the two-hit hypothesis. Lancet Neurol. 2004, 3: 219-226. 10.1016/S1474-4422(04)00707-0.
Liu DG: [Review of neuropathology in the past 10 years in China]. Zhonghua Bing Li Xue Za Zhi. 2005, 34: 550-552. Article in Chinese
Lanni C, Racchi M, Memo M, Govoni S, Uberti D: p53 at the crossroads between cancer and neurodegeneration. Free Radic Biol Med. 2012, 52: 1727-1733. 10.1016/j.freeradbiomed.2012.02.034.
Soubeyrand S, Schild-Poulter C, Hache RJ: Structured DNA promotes phosphorylation of p53 by DNA-dependent protein kinase at serine 9 and threonine 18. Eur J Biochem. 2004, 271: 3776-3784. 10.1111/j.1432-1033.2004.04319.x.
Yee KS, Vousden KH: Complicating the complexity of p53. Carcinogenesis. 2005, 26: 1317-1322. 10.1093/carcin/bgi122.
Liu J, Naegele JR, Lin SL: The DNA-PK catalytic subunit regulates Bax-mediated excitotoxic cell death by Ku70 phosphorylation. Brain Res. 2009, 1296: 164-175.
Hill R, Lee PW: The DNA-dependent protein kinase (DNA-PK): More than just a case of making ends meet?. Cell Cycle. 2010, 9: 3460-3469. 10.4161/cc.9.17.13043.
Cardinale A, Racaniello M, Saladini S, De Chiara G, Mollinari C, de Stefano MC, Pocchiari M, Garaci E, Merlo D: Sublethal doses of beta-amyloid peptide abrogate DNA-dependent protein kinase activity. J Biol Chem. 2012, 287: 2618-2631. 10.1074/jbc.M111.276550.
Grune T, Reinheckel T, Davies KJ: Degradation of oxidized proteins in mammalian cells. FASEB J. 1997, 11: 526-534.
Nystrom T: Role of oxidative carbonylation in protein quality control and senescence. EMBO J. 2005, 24: 1311-1317. 10.1038/sj.emboj.7600599.
Simpson JE, Ince PG, Haynes LJ, Theaker R, Gelsthorpe C, Baxter L, Forster G, Lace GL, Shaw PJ, Matthews FE, Savva GM, Brayne C, Wharton SB, MRC Cognitive Function and Ageing Neuropathology Study Group: Population variation in oxidative stress and astrocyte DNA damage in relation to Alzheimer-type pathology in the ageing brain. Neuropathol Appl Neurobiol. 2010, 36: 25-40. 10.1111/j.1365-2990.2009.01030.x.
Mimori T, Hardin JA, Steitz JA: Characterization of the DNA-binding protein antigen Ku recognized by autoantibodies from patients with rheumatic disorders. J Biol Chem. 1986, 261: 2274-2278.
Allen C, Halbrook J, Nickoloff JA: Interactive competition between homologous recombination and non-homologous end joining. Mol Cancer Res. 2003, 1: 913-920.
Pierce AJ, Hu P, Han M, Ellis N, Jasin M: Ku DNA end-binding protein modulates homologous repair of double-strand breaks in mammalian cells. Genes Dev. 2001, 15: 3237-3242. 10.1101/gad.946401.
Schwartz M, Zlotorynski E, Goldberg M, Ozeri E, Rahat A, le Sage C, Chen BP, Chen DJ, Agami R, Kerem B: Homologous recombination and nonhomologous end-joining repair pathways regulate fragile site stability. Genes Dev. 2005, 19: 2715-2726. 10.1101/gad.340905.
Adachi N, Ishino T, Ishii Y, Takeda S, Koyama H: DNA ligase IV-deficient cells are more resistant to ionizing radiation in the absence of Ku70: implications for DNA double-strand break repair. Proc Natl Acad Sci USA. 2001, 98: 12109-12113. 10.1073/pnas.201271098.
Karanjawala ZE, Adachi N, Irvine RA, Oh EK, Shibata D, Schwarz K, Hsieh CL, Lieber MR: The embryonic lethality in DNA ligase IV-deficient mice is rescued by deletion of Ku: implications for unifying the heterogeneous phenotypes of NHEJ mutants. DNA Repair (Amst). 2002, 1: 1017-1026. 10.1016/S1568-7864(02)00151-9.
Serrano MA, Li Z, Dangeti M, Musich PR, Patrick S, Roginskaya M, Cartwright B, Zou Y: DNA-PK, ATM and ATR collaboratively regulate p53-RPA interaction to facilitate homologous recombination DNA repair. Oncogene. 2012,
Pietrzak M, Rempala G, Nelson PT, Zheng JJ, Hetman M: Epigenetic silencing of nucleolar rRNA genes in Alzheimer's disease. PLoS One. 2011, 6: e22585-10.1371/journal.pone.0022585.
Drygin D, Rice WG, Grummt I: The RNA polymerase I transcription machinery: an emerging target for the treatment of cancer. Annu Rev Pharmacol Toxicol. 2010, 50: 131-156. 10.1146/annurev.pharmtox.010909.105844.
Kanungo J, Wang HY, Malbon CC: Ku80 is required but not sufficient for Galpha13-mediated endodermal differentiation in P19 embryonic carcinoma cells. Biochem Biophys Res Commun. 2004, 323: 293-298. 10.1016/j.bbrc.2004.08.092.
Kuhn A, Stefanovsky V, Grummt I: The nucleolar transcription activator UBF relieves Ku antigen-mediated repression of mouse ribosomal gene transcription. Nucleic Acids Res. 1993, 21: 2057-2063. 10.1093/nar/21.9.2057.
Labhart P: DNA-dependent protein kinase specifically represses promoter-directed transcription initiation by RNA polymerase I. Proc Natl Acad Sci USA. 1995, 92: 2934-2938. 10.1073/pnas.92.7.2934.
Iacono D, O'Brien R, Resnick SM, Zonderman AB, Pletnikova O, Rudow G, An Y, West MJ, Crain B, Troncoso JC: Neuronal hypertrophy in asymptomatic Alzheimer disease. J Neuropathol Exp Neurol. 2008, 67: 578-589. 10.1097/NEN.0b013e3181772794.
The author's research is supported by funds from the National Center for Toxicological Research of the US Food and Drug Administration (FDA). This document has been reviewed in accordance with FDA policy and has been approved for publication. Approval does not signify that the contents necessarily reflect the position or opinions of the FDA, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. The findings and conclusions in this report are those of the author and do not necessarily represent the views of the FDA.
The author declares that she has no competing interests.