Review

DNA strand breaks, neurodegeneration and aging in the brain

Sachin Katyal, Peter J. McKinnon *

Department of Genetics and Tumor Cell Biology, St. Jude Children’s Research Hospital, Memphis, TN 38105, USA

Mechanisms of Ageing and Development 129 (2008) 483–491

A R T I C L E I N F O

Article history:

Available online 25 March 2008

Keywords:

DNA damage

DNA repair

Nervous system

Aging

ATM

AOA1

SCAN1

A B S T R A C T

Defective responses to DNA single- or double-strand breaks can result in neurological disease,

underscoring the critical importance of DNA repair for neural homeostasis. Human DNA repair-deficient

syndromes are generally congenital, in which brain pathology reflects the consequences of

developmentally incurred DNA damage. Although, it is unclear to what degree DNA strand-break

repair defects in mature neural cells contributes to disease pathology. However, DNA single-strand

breaks are a relatively common lesion which if not repaired can impact cells via interference with

transcription. Thus, this lesion, and probably to a lesser extent DNA double-strand breaks, may be

particularly relevant to aging in the neural cell population. In this review we will examine the

consequences of defective DNA strand-break repair towards homeostasis in the brain. Further, we also

consider the utility of mouse models as reagents to understand the connection between DNA strand

breaks and aging in the brain.

Contents l is ts ava i lab le at ScienceDirec t

Mechanisms of Ageing and Development

journa l homepage: www.e lsev ier .com/ locate /mechagedev

� 2008 Elsevier Ireland Ltd. All rights reserved.

1. Introduction

The mammalian nervous system is formed through continualcycles of proliferation, differentiation and maturation to generatethe large number of cell types required for function (Jacobsen, 1991).Although the human nervous system forms in a matter of months,neural tissues must be functional for decades of life, and the matureneurons bear the brunt of handling a lifetime of potential threats tothe integrity of their DNA. Due to the substantial oxygenrequirement for maintenance of CNS tissue, neurons must copewith oxidative and metabolic stress that can result in DNA strandbreaks (Lombard et al., 2005; Barzilai, 2007; Chen et al., 2007).Accordingly, neurons require efficient DNA strand-break surveil-lance and repair mechanisms to deal with these types of lesions.Human neurological syndromes resulting from defects in DNArepair highlight the importance of multiple repair pathways formaintaining homeostasis in the brain (Rolig and McKinnon, 2000;McKinnon and Caldecott, 2007; Subba Rao, 2007). Hence, indivi-duals who incur genetic mutations that inactivate these repairpathways show accelerated neuronal death, which can manifest asneurodegenerative disease. As most of these inherited syndromesare congenital, less is know about the effects of DNA repairdeficiency during aging. Nonetheless, there are many studiesreporting a link between aging and a decline in DNA repair activity(Intano et al., 2003; Lu et al., 2004; Vijg and Calder, 2004; Imam et al.,

* Corresponding author. Tel.: +1 901 495 2700; fax: +1 901 526 2907.

E-mail address: peter.mckinnon@stjude.org (P.J. McKinnon).

0047-6374/$ – see front matter � 2008 Elsevier Ireland Ltd. All rights reserved.

doi:10.1016/j.mad.2008.03.008

2006; Gorbunova et al., 2007; Rutten et al., 2007; Wilson and Bohr,2007). However, the causal relationship between decreased DNArepair activity, increased mutations and an effect upon aging still hasto be thoroughly evaluated. Clearly, understanding the role of DNArepair and aging in the brain will require suitable model systems andcareful in vivo assessment of how the spatiotemporal changes inDNA repair capacity affects neural homeostasis.

This review will emphasize the requirement for DNA strand-break repair in the context of neural development. We willconsider neurodegenerative diseases that are directly attributableto failure in strand-break responses and the importance of thesepathways in the nervous system. We will also discuss the utility ofmouse models of DNA repair deficiency as an important tool forunderstanding the impact of genomic instability in the brain, andhow more refined genetic manipulation of the mouse will help usbetter understand the links between DNA repair deficiency andage-related disease of the CNS.

2. Neural development

Formation of mature neural tissue requires the expansion anddifferentiation of precursor cells into a variety of neural cell typesthat migrate, organize and stratify into distinct CNS structures.Fig. 1 illustrates the CNS with expanded views of two representa-tive tissues, the retina and cerebellum, illustrating the laminarstructure of these tissues. In many DNA repair syndromes thecerebellum is often a target, and as the cerebellum is responsiblefor motor coordination, ataxia is associated with these syndromes(Frappart and McKinnon, 2006; Lee and McKinnon, 2007).

Fig. 1. The adult mammalian CNS. The mature nervous system contains a myriad of different cell types and tissues. DNA repair processes impact substantially during neural

development leading to defective neurogenesis and development. However, less is known regarding the requirement for DNA repair processes in mature neural cell. Inset

panels are hematoxylin and eosin stained retinal and cerebellar sections that show cell organization in these tissues. The retina is laminar in nature and cell types are stratified

into three distinct nuclear layers: outer, inner and ganglion. The outer nuclear layer contains the photoreceptor (rods and cones) neurons. The inner nuclear layer contains

various signal processing cell types: bipolar, horizontal, amacrine, interplexiform and the Muller glia. The ganglion cells carry the visual signal via its axons through the optic

nerve and project onto the brain. The cerebellum is stratified into three primary layers: inner granule cell layer, the Purkinje cell layer and the molecular layer. Excitory

sensory signals originating from the cerebellum are ultimately transmitted through granule-Purkinje synapses and out of the cerebellum through Purkinje neuron axons to

affect normal control of movement.

S. Katyal, P.J. McKinnon / Mechanisms of Ageing and Development 129 (2008) 483–491484

Although the cerebellum only comprises 10% of the total brainmass, it contains approximately 50% of the neurons in the brain.The cerebellum is primarily composed of three general neuronalpopulations, the granule cells, the Purkinje cells and interneurons,with each type found in distinct cell layers; the inner granule layer,the Purkinje cell layer and interneurons which are foundthroughout the molecular layer and the inner granule layer(Fig. 1). During development, an external germinal layer is presentand as cerebellar development progresses, granule neuronprecursors generate granule cells that migrate inwards andpopulate the IGL as the external granule layer gradually diminishes(Goldowitz and Hamre, 1998; Wang and Zoghbi, 2001). The cellpopulations of the cerebellum are notable because granule cells arethe most numerous neuronal cell types in the brain, while Purkinjecells are amongst the largest neuronal cell type in the brain. Theouter molecular layer consists of interneurons (stellate and basketcells) together with Purkinje cell dendrites and parallel fibersarising from granule cells, making the molecular layer a synapse-rich area (Jacobsen, 1991; Goldowitz and Hamre, 1998). As thecerebellum serves primarily to control sensory–motor function,individuals with cerebellar neurodegenerative disorders, such asspinocerebellar atrophy and ataxia-telangiectasia (A-T), presentwith ataxia (impaired motor coordination) and eye-movementdefects and speech disturbance (dysarthria) (Frappart andMcKinnon, 2006; Limperopoulos and du Plessis, 2006).

While the cerebellum is often affected in diseases associated withDNA strand-break repair, progressive widespread neurodegenera-tion also occurs. In many cases these progressive changes are a laterevent than the effects upon the cerebellum. The likely scenario isthat the cerebellum and perhaps granule cells in particular are verysusceptible during postnatal neurogenesis to DNA damage.Furthermore, as defects in the cerebellum generally present asataxia, an obvious movement disorder, this may cause mildercortical defects to be initially over-looked. As will be discussed later,mice with defective DNA strand-break repair further reveal thecerebellum as a primary target in the nervous system.

3. Repair of DNA strand breaks

DNA strand breaks can occur as either single-strand breaks(SSBs) or double-strand breaks (DSBs) and biochemically distinctpathways repair these lesions. DNA DSBs are repaired by eithernon-homologous end-joining (NHEJ) or homologous recombina-tion (HR), while SSBs are repaired by the DNA SSB repair (SSBR)pathway (Caldecott, 2003; Lieber et al., 2003; West, 2003; Wymanand Kanaar, 2006; Helleday et al., 2007). In the nervous systemDNA strand breaks can arise endogenously from normal cellularmetabolism, during DNA replication or from exogenous agentssuch as ionizing radiation or chemicals in the environment. Indifferentiated neurons that do not divide, DNA DSBs clearly occur

S. Katyal, P.J. McKinnon / Mechanisms of Ageing and Development 129 (2008) 483–491 485

independently of replication and may result from the formation ofadjacent SSBs via oxidative stress.

The choice between HR and NHEJ is related to the availability ofa sister chromatid with which to use as an intact template forrepair. Therefore, HR is restricted to replicating cells, and in thenervous system these are the progenitor populations that give riseto neurons and glia, and are found in the proliferative ventricularzones. Thus, HR promotes error-free DNA DSBR and is the majorDNA repair pathway utilized during early mammalian develop-ment, particularly in tissues that give rise to the nervous system(Orii et al., 2006). A large recombinase complex (composed ofRAD50, RAD51, RAD54, XRCC2 and XRCC3) is recruited to the DNAbreak site in a BRCA2-dependent manner to mediate HR (West,2003; Wyman and Kanaar, 2006; Helleday et al., 2007). Therequirement for an undamaged sister-chromatid restricts usage ofHR to the S and G2 phase of the cell cycle. In contrast, NHEJ utilizesend-processing enzymes to facilitate ligation of incompatible DNAends. Major proteins involved in NHEJ include the Ku heterodimer(Ku70/80), DNA protein kinase (DNA-PKcs) and the XRCC4/LigaseIV/XLF complex (Lees-Miller and Meek, 2003; Lieber et al.,2003; Wyman and Kanaar, 2006; van Gent and van der Burg, 2007).The Ku complex binds to DNA ends and recruits the DNA-PKcs

protein kinase to facilitate efficient DNA ligation by XRCC4/LIG4/XLF. In differentiating and post-mitotic neurons, the repair of DSBsoccurs via NHEJ (Orii et al., 2006).

Coincident with DNA repair is the activation of DNA damage-induced signaling pathways that serve to halt the cell cycle whileDNA repair occurs, or alternatively to activate apoptosis where thedamage is extensive (Shiloh, 2001; Kastan and Bartek, 2004; Wardand Chen, 2004). Amongst the earliest signaling events resultingfrom DNA DSBs is the phosphorylation of histone H2AX (Sedelnikovaet al., 2003). This event serves to retain DSB sensing factors at theDNA break sites, thereby promoting efficient DNA repair. Recruit-ment of the DNA damage sensing Mre11-Rad50-Nbs1 (MRN)complex to the DSBs is important for activating the apical signalingkinase ATM, which effects cell cycle arrest or apoptosis (Carson et al.,2003; Petrini and Stracker, 2003; Uziel et al., 2003; Difilippantonioet al., 2005). In the developing nervous system the activation ofapoptosis after genotoxic stress is often a preferred course as cellreplacement can readily occur from widespread germinal zonesthroughout the embryonic nervous system and at some postnatalstages such as the maturing cerebellum (Lee and McKinnon, 2007).However, defects in many components of the DNA DSB repairmachinery can lead to neuropathology (see later).

In contrast to DNA DSBs, a more common strand break is a DNASSB. Tens of thousands of DNA SSBs are estimated to occur dailywithin a cell due to oxidative stress, and it is this type of lesion thatmay well underpin much of the source of DNA damage in the brain(Caldecott, 2003; Rass et al., 2007). Mammals are oxygen-consuming organisms, and reactive oxygen species (ROS) occurvia normal cellular metabolic processes. ROS production repre-sents the single greatest genotoxin to neurons as ROS-mediatedattack of DNA results in direct DNA SSBs. Resolution of DNA SSBsutilizes a distinct DNA SSBR pathway (Caldecott, 2003; McKinnonand Caldecott, 2007). Detection of SSBs is mediated by poly(ADP-ribose) polymerase (PARP) which acts to recruit the XRCC1scaffolding protein accompanied with a variety of enzymesinvolved in DNA processing (TDP1, APTX, APE1, PKNP), gap-filling(DNA polymerase b) and ligation (ligaseIII) (McKinnon andCaldecott, 2007; Wilson and Bohr, 2007). Depending on thechemistry of the nucleophilic attack on the DNA backbone, DNASSBs incur a multitude of 30- and 50-end-modifications, which mustbe processed by one or many end-processing enzymes prior toDNA, nick resealing (McKinnon and Caldecott, 2007; Wilson, 2007;Wilson and Mattson, 2007). These processing enzymes are

important in neural homeostasis as various neurodegenerativesyndromes arise when germline mutations inactivate theseenzymes. The inability to properly repair or process a SSB canlead to a variety of genotoxic consequences, including interferencewith DNA transcriptional machinery, formation of a DNA DSB uponencounter with the DNA replication machinery, or formation ofeither DNA SSB or DSB as a product of abortive Top1-DNA cleavageintermediates during DNA replication (Saxowsky and Doetsch,2006; el-Khamisy and Caldecott, 2007; Katyal and McKinnon,2007). Notably, SSBs can be converted to DSBs in replicating cellsvia collision with replication forks, where HR is available as anadjunct to SSBR. However, such backup mechanisms do not exist inpost-mitotic neurons, and so the ability of these cells to repair SSBswill be very important for their survival (El-Khamisy et al., 2005;Katyal and McKinnon, 2007).

4. Neurodegenerative disease and DNA strand-break repair

Specific neuropathology is often present in syndromes invol-ving DNA repair deficiency, highlighting the essential need forresponding to genotoxic stress in the nervous system. Asmentioned, the cerebellum is generally susceptible early in diseaseprogression to the effects of DNA repair deficiency. This relativesensitivity may reflect the tremendous levels of neurogenesis thatoccur postnatally in this organ. Furthermore, the fact that neuronsare post-mitotic, have critical higher order functions and havedemanding oxygen requirements makes them particularly sensi-tive to deficiency in resolving genotoxic stress. This section willconsider the basis for the neurological defects in individuals withlesions associated with sensing or repairing DNA strand breaks. Arepresentative list of syndromes associated with defective DNAstrand-break repair is presented in Table 1.

5. Ataxia-telangiectasia and related diseases

Ataxia-telangiectasia is amongst the best studied of the humanneurological syndromes arising from DNA damage response defects.A-T results from mutation of ATM (ataxia-telangiectasia, mutated) alarge nuclear phosphoinositol-3-kinase-related serine/threonineprotein kinase (Shiloh, 2003). DNA DSBs activate ATM resulting ina multi-faceted signaling response involving chromatin remodeling,DNA repair, cell cycle checkpoint activation and initiation ofapoptotic pathways (Shiloh, 2003; Lavin and Kozlov, 2007;Matsuoka et al., 2007). Individuals with A-T develop profoundataxia and are confined to a wheelchair before the first decade of life.Dysarthria, oculomotor apraxia, cerebellar atrophy and variousextra-neurological features such as immunodeficiency, radiosensi-tivity and a predisposition to malignancy also feature in this disease(McKinnon, 2004; Frappart and McKinnon, 2006; McKinnon andCaldecott, 2007). The DNA damage response defects in A-T stronglyaffect the cerebellum as MRI and autopsy analysis reveal widespreadloss of cerebellar Purkinje cell and granule neurons in A-T brains,that are subsequently accompanied by other cerebral and spinaldefects (Frappart and McKinnon, 2006).

Mutation of two members of the MRN complex have also beenlinked to human disease. Patients with hypomorphic mutation ofMre11 develop a syndrome called ataxia-telangiectasia likesyndrome (A-TLD) and show neurological symptoms similar toA-T including ataxia, oculomotor apraxia and dysarthria (Petriniand Stracker, 2003; Taylor et al., 2004). In contrast, hypomorphicmutation of NBS1, lead to Nijmegen breakage syndrome (NBS)which is characterized by immunodeficiency and lymphoidmalignancy and in the nervous system microcephaly rather thanneurodegeneration is present (Weemaes et al., 1981; Demuth andDigweed, 2007). MRE11 has DNA processing activity, while NBS1

Table 1DNA repair syndromes resulting from DNA strand-break deficiency

Disease/syndrome Gene Neurological Extra-neurological Mouse mutant(s) Phenotype References

DNA DSB sensing and repair

Ataxia-telangiectasia ATM Ataxia,

neurodegeneration

Immunological defects Atm�/� Growth retardation,

infertility, immunological

defects

Barlow et al. (1996),

Elson et al. (1996), Xu

and Baltimore (1996),

Herzog et al. (1998),

Spring et al. (2001),

Borghesani et al. (2000)

Telangiectasia,

dysarthria

Malignancy, sterility Radiosensitivity,

malignancy

Seckel syndrome ATR Microcephaly,

mental

retardation

Growth defects Atr�/� Embryonic lethal (<E7.5) Brown and Baltimore (2003)

PCNT Abnormal facial features None reported

Nijmegen breakage

syndrome

NBS1 Microcephaly Immunological defects Nbs1�/� Embryonic lethal (<E7.5) Zhu et al. (2001)

Lymphoid malignancy Nbs1DB/DB Immunological defects,

lymphoid malignancy

Williams et al. (2002)

Nbs1ins-6/D6 Tg-Mx-Cre Defects in lymphogenesis,

marrow, spleen, thymus

Demuth et al. (2004)

Nbs1D6/D6 nestin-Cre Ataxia, cerebellar and

growth defects, microcephaly

Frappart et al. (2005)

Nbs1DC/DC Defective apoptotic response

after IR

Stracker et al. (2007)

Difilippantonio et al. (2005)

Ataxia-telangiectasia-like

disorder

MRE11 Ataxia,

neurodegeneration

dysarthria,

oculomotor

Mild immunological

defects

Mre11�/� Embryonic lethal Xiao and Weaver (1997)

Apraxia Mre11ATLD/ATLD Reduced embryonic viability Theunissen et al. (2003)

LIG4 syndrome LIG4 Microcephaly Immunodeficiency,

lymphoma

LIG4�/� Embryonic lethal (<E16.5):

neuronal

Frank et al. (2000)

Developmental/

growth delay

Apoptosis and defective

lymphogenesis

Barnes et al. (1998)

Fanconi anemia BRCA2 Medulloblastoma Bone marrow and

congenital defects

Brca2loxP/loxPnestin-Cre Defects in neurogenesis,

tumor prone

Frappart et al. (2007)

Human immunodeficiency

with microcephaly

XLF Microcephaly Immunological

defects, infection

Xlf�/� ES cells only Genomic instability,

chromosomal translocations

Zha et al. (2007)

Developmental/

growth delay

None reported

DNA SSB repair

Spinocerebellar ataxia with

axonal neuropathy

TDP1 Ataxia,

neurodegeneration

Hypercholesterolemia Tdp1�/� Mild cerebellar atrophy,

hematopoeitic/intestinal

Katyal et al. (2007)

Muscle weakness Hypoalbuminemia Sensitivity to topoisomerase-1

inhibitors (Topotecan)

Hirano et al. (2007)

Ataxia with oculomotor

apraxia 1

APTX Ataxia,

neurodegeneration

Hypercholesterolemia Aptx�/� Defective resolution of 50-end

abortive ligation intermediates

Ahel et al. (2006)

Oculomotor apraxia Hypoalbuminemia Murine analysis in progress

S. Katyal, P.J. McKinnon / Mechanisms of Ageing and Development 129 (2008) 483–491486

contains specific protein interaction domains (BRCT and FHA) thatmediate critical MRN interactions. Like A-T, cells from A-TLD andNBS patients show substantial radiosensitivity and increasedgenomic instability (Shiloh, 1997; Kobayashi et al., 2004).Interestingly, symptoms pertaining to both A-TLD and NBS seemto make up the breadth of those found in A-T, confirmingbiochemical analyses that the MRN complex is important for ATMactivity. Mre11 and Nbs1 mutations result in defective localizationof MRN and ATM to DSBs, reduced ATM activation and defectiveATM-dependent phosphorylation of downstream substrates (Car-son et al., 2003; Petrini and Stracker, 2003; Uziel et al., 2003;Kitagawa et al., 2004; Difilippantonio et al., 2005). However, most,if not all of the pronounced neurological phenotypes in A-T, A-TLDand NBS can be attributed to DSB processing defects during neuraldevelopment. Further, it is also uncertain if the progressive declineof the nervous system in A-T solely reflects the consequence of a

lack of this protein during development. As yet it is quite unclear asto what extent ATM, MRE11 and NBS1 are functionally importantin the mature brain or as the brain ages.

6. DNA repair deficiency and microcephaly

In addition to neurodegeneration are DNA repair syndromesthat lead to congenital brain development abnormalities resultingin microcephaly (when the brain size is 2 standard deviationsbelow normal). Some individuals with Seckel syndrome, caused byhypomorphic ATR (ATM and Rad3-related) mutations, displaysevere growth retardation, microcephaly, mental retardation and‘‘bird-like’’ facial features (Goodship et al., 2000; O’Driscoll et al.,2004; O’Driscoll and Jeggo, 2006). Like ATM, ATR is a large PI3K-like protein kinase involved in the DNA damage response (Shiloh,2001; Paulsen and Cimprich, 2007). Although ATM and ATR have

S. Katyal, P.J. McKinnon / Mechanisms of Ageing and Development 129 (2008) 483–491 487

similar downstream substrates, ATR is primarily involved inmonitoring DNA replication stress during cell proliferation, such asstalled replication forks (Paulsen and Cimprich, 2007). Therefore,the functional requirement for ATR in terminally differentiated oraging neural cells is unclear. It is conceivable that afterproliferation has ceased ATR is no longer important, with perhapsATM performing genomic surveillance functions.

Mutation of the centrosomal protein, pericentrin, has also beenrecently linked to the pathogenesis of Seckel syndrome (Griffithet al., 2007). These patients, like those with ATR-related Seckelsyndrome, also show pronounced microcephaly. Mutant pericen-trin results in its mislocalization from the centrosome resulting inreplication fork defects. Disruption of this structural chromatinprotein also results in defective DNA repair responses includingthose dependent on ATR (Griffith et al., 2007).

Patients with hypomorphic mutations in the essential NHEJprotein LIG4 show microcephaly, developmental and growth delayand immunological defects including lymphoid malignancy(O’Driscoll et al., 2004). The severity of Lig4 syndrome correlateswith the extent of residual LIG4 activity (Girard et al., 2004).Inactivation of mouse Lig4 results in early embryonic lethalityassociated with widespread neuronal apoptosis and defectivelymphogenesis (Barnes et al., 1998; Gao et al., 1998). Additionally,mutation in another NHEJ factor cernunnos/XLF results in humanimmunodeficiency with microcephaly (HIM), an autosomalrecessive childhood disease characterized by microcephaly,developmental and growth delay, autoimmune defects andrecurrent infection (Ahnesorg et al., 2006; Buck et al., 2006).XLF functions in NHEJ via association with the XRCC4/LIG4complex (Andres et al., 2007; Li et al., 2008). Many other DNArepair-deficient syndromes also show microcephaly includingCockayne’s syndrome in which transcription coupled repair isdefective and also NBS described above, indicating this brain defectprobably occurs as a result of cell loss during development fromvarious types DNA damage.

While the above syndromes involve DNA repair or damageresponse factors, the syndromes are congenital and so the exactrequirements for any of these factors during aging in the brainremain unclear. In fact one unresolved issue in hereditary DNArepair syndromes is determining the portion of the collectivephenotype that is attributable to the DNA repair deficiency duringaging. For example, as mentioned above, is the progressiveneurodegeneration associated with A-T solely a result of devel-opmentally incurred defects, or is there a component of aginginvolved; i.e. is the ATM pathway important in mature neuralcells? Assessing the requirement for ATM and ATR in the maturenervous system will be important for understanding the relativeroles ATM and ATR contribute to the DNA damage response andhow this impacts neural homeostasis. Other DNA damage signalingfactors such as TopBP1 (Kumagai et al., 2006) have also beenimplicated in modulating ATM and ATR function, and again it willbe informative to determine how these factors modulate the DNAdamage response in post-mitotic cells.

Determining the specific requirements for the DNA damageresponse system during the life of neural cells is an area thatwarrants further attention. As discussed later, more refined animalmodels for DNA repair deficiency will be important for under-standing the role of DNA DSB repair in mature neurons. In contrasthowever, DNA SSBs are likely to affect terminally differentiatedneural cells as they occur at much greater frequency than DSBs andare likely to impact transcription (El-Khamisy et al., 2005;Saxowsky and Doetsch, 2006; Katyal and McKinnon, 2007;McKinnon and Caldecott, 2007), perhaps pointing to a greaterneed for activity of this pathway in the aging neural population.Additionally, the potential interrelationships between DNA DSBR

and SSBR (Clements et al., 2004) suggest a more complex scenario,and imply that cross-talk between pathways may also beimportant for neural homeostasis. In the following we considerthe importance of DNA SSBR in the nervous system.

7. DNA SSB repair deficiency in the nervous system

The importance of DNA SSBR in maintaining neural home-ostasis has recently been highlighted by the identification ofdefective repair enzymes as the underlying cause for severalneurodegenerative syndromes. In contrast to DNA DSBs, pheno-types associated with SSBR deficient syndromes are relativelyspecific to the nervous system. Two such diseases are spinocer-ebellar ataxia with axonal neuropathy (SCAN1) and ataxia withoculomotor apraxia (AOA1). These are inherited autosomalrecessive syndromes with neurodegenerative phenotypes similarto A-T, including cerebellar atrophy and ataxia, dysarthria andoculomotor apraxia (only in AOA1) (Date et al., 2001; Moreira et al.,2001; Takashima et al., 2002). While SCAN1 is quite rare, AOA1 isamong the most common autosomal recessive ataxic disorders inJapan and Portugal. AOA1 patients show loss of cerebellar Purkinjeneurons, while MRI analysis of SCAN1 patients has shown reducedcerebellar size (Takashima et al., 2002; Sugawara et al., 2008).SCAN1 and AOA1 result from heritable mutations in the 30-end-processing enzyme tyrosyl DNA phosphodiesterase 1 (TDP1) andthe 50-end-processing enzyme aprataxin (APTX), respectively(Date et al., 2001; Moreira et al., 2001; Takashima et al., 2002).TDP1 is involved in processing of a variety of damaged DNA ends,including 30-phosphoglycolate, 30-Top1 and other types of non-ligateable 30-DNA ends generated after DNA oxidation, DNAreplication or other genotoxic stress (El-Khamisy et al., 2005).APTX is a nucleotide hydrolase and studies using APTX�/� cellshave shown a failure to cleave a 50-adenylate intermediate prior tosealing the nick, implicating a role for APTX in processing abortiveligation intermediates (Ahel et al., 2006; Rass et al., 2007). Whilethe ability of HR to provide a backup repair pathway inproliferative tissues can circumvent SSBR defects, the specificimpact toward the nervous system in defective SSBR syndromesprobably reflects the high oxygen consumption of this tissue andthe associated increased levels of oxidative damage (El-Khamisyet al., 2005; Katyal and McKinnon, 2007; Rass et al., 2007).

8. Retinitis pigmentosa (RP)

Retinitis pigmentosa (RP) is a heterogeneous neurodegenera-tive condition resulting in a gradual loss of photoreceptor neurons,leading to progressive constriction of the visual field and nightblindness. Many RP gene groups and forms exist: includingautosomal recessive, autosomal dominant and X-linked forms.However, mutation of RP2 accounts for the second most frequentcause of X-linked RP (Sharon et al., 2003). RP2 shares homology tonucleotide diphosphate kinases and in vitro oligo nuclease studiesimplicate RP2 as a novel 30–50 exonuclease involved in SSBR. Arecent study describes nuclear relocalization from the membraneof RP2 protein upon exposure to oxidative damage (Yoon et al.,2006). While a number of vision disorders are attributed to retinaldegeneration including RP, age-related macular degeneration andcone-rod dystrophy, links to DNA repair deficiency have not beenmade in these diseases.

9. Utilizing mouse models to understand DNA damage in thenervous system

Human DNA repair syndromes reveal the requirement for DNArepair during neural development, although these are less

S. Katyal, P.J. McKinnon / Mechanisms of Ageing and Development 129 (2008) 483–491488

revealing about repair requirements in the mature nervous system.Detailed analysis of these processes will depend upon suitableexperimental systems that are amenable to experimental manip-ulation. Perhaps the most significant model system that willdirectly expand our understanding of the biology of DNA repair inthe brain is the mouse. Important information has been obtainedusing germline disruption of DNA repair factors, and more recentlytissue-specific gene ablation has helped refine our understandingto a much greater degree (Friedberg and Meira, 2006). However,there are also limitations to the ability of the mouse to recapitulatehuman neurodegenerative disease phenotypes. For example, whileAtm�/�mice have been important for understanding many aspectsof A-T, the substantial neuropathology present in humans is absentin Atm-null animals (Frappart and McKinnon, 2006). The lack ofdiscernable neuropathology suggests some fundamental differ-ences between the response of the nervous system in mice andhumans. It is not certain why the difference is pronounced in thenervous system, when in other physiological compartments thereis equivalency after ATM loss between mouse and man. Similarly,while murine inactivation of either Mre11 or Nbs1 results in earlyembryonic lethality (Xiao and Weaver, 1997; Zhu et al., 2001;Dumon-Jones et al., 2003), hypomorphic murine mutations thatmimic the human gene mutation in these syndromes reflect thecellular and extra-neurological characteristics of the humandisease, but not the neurological symptoms (Kang et al., 2002;Williams et al., 2002; Theunissen et al., 2003; Difilippantonio et al.,2005). Two potential reasons are that the mouse lifespan isinsufficient to manifest age-accumulated stochastic lesions, or thatthe mouse nervous system is more resistant to DNA damage.Nonetheless, important lessons have been learnt from the mouseregarding ATM signaling in the nervous system. Atm�/� mice haveestablished that ATM is required for the induction of neuronalapoptosis selectively in immature, post-mitotic neural cells afterDNA damage (Herzog et al., 1998), suggesting that progressiveneurodegeneration in A-T results from faulty neurons remainingintegrated in neural tissue.

Additionally, Tdp1�/� mice have been created in an effort tounderstand the role of TDP1 in preventing SCAN1. Cerebellarneurons from these mice have deficient DNA SSBR activity afterdamage induced by oxidative stress, ionizing radiation and Top1inhibitors (Hirano et al., 2007; Katyal et al., 2007). Notably, incontrast to mouse models of DNA DSB response deficiency, Tdp1�/�

mice exhibit late-onset cerebellar atrophy, presumably due toaccumulation of DNA lesions as result of neuronal repair deficiency(Katyal et al., 2007). These mice are also extremely sensitive toTop1 inhibitors, particularly in hematopoietic and intestinal tissue.

These knockout mouse models highlight the potential differ-ences in outcome of repair enzyme loss between mouse andhuman, underscoring relevant concerns in considering the mouseas a suitable model for studies of aging in the brain (Zahn et al.,2007). While this may be an issue for generating faithful diseasemodels, the mouse is nonetheless the best currently availablesystem for garnering a comparative understanding of thebiological role of DNA repair pathways in the developing andmature brain. In particular, valuable data has been generated frommouse models in which inactivation of DNA repair factors showprofound tissue-specific effects, particularly within the nervoussystem (Gao et al., 1998; Frappart et al., 2005, 2007; Orii et al.,2006).

10. Refined mouse models to study DNA repair in the nervoussystem

The increasing availability of reagents for tissue-specific geneinactivation has provided a more refined approach with which to

selectively manipulate gene inactivation (Orban et al., 1992;Jonkers and Berns, 2002). Tissue-specific deletion of DNA repairgenes in the nervous system has already provided valuable insightsinto the role of these factors during neural development (Garciaand Mills, 2002; Gaveriaux-Ruff and Kieffer, 2007). A standardapproach to gene deletion in the nervous system via the Cre/LoxPsystem utilizes the Nestin promoter to drive the Cre recombinase.Nestin is expressed at E10.5 a point at which neural development iscommencing. As a result, gene deletion via Nestin-driven Creexpression will result in gene deletion throughout CNS tissue(Bates et al., 1999). For example, while inactivation of Brca2 orNbs1 leads to early embryonic lethality, selective ablation of thesegenes in the nervous system using Nestin-Cre results in a viableanimal, providing important insight into the respective roles ofthese genes in this tissue (Frappart et al., 2005, 2007). A wide arrayof neural-specific Cre lines have been developed which exhibit Crerecombinase activation at specific stages of development of tissues(a comprehensive database is accessible at: http://nagy.msh-ri.on.ca/cre/index.php or www.jax.org). Depending on the spatio-temporal expression pattern of any given gene promoter, Crerecombinase activity can be pan-neuronal, CNS tissue- or cell-typespecific. In addition, inducible tissue-specific Cre lines beendeveloped to allow increased temporal control of Cre expression(Kuhn et al., 1995; Feil et al., 1996; St-Onge et al., 1996; Garcia andMills, 2002).

11. Perspective and conclusion

Neurodegenerative syndromes highlight the importance ofresponding to DNA strand breaks in the nervous system. However,the precise requirement for DNA repair factors during aging of thebrain is not necessarily revealed by the phenotype of human DNArepair syndromes. While progeria is associated with some DNArepair syndromes (e.g. Werner’s), it is not strongly associated withDNA strand-break repair deficiency syndromes. DNA DSB syn-dromes appear to primarily affect developing tissues, and most ifnot all of the neurological phenotype can be attributed to defectsassociated with development. In comparison however, the DNASSBR deficiency syndromes may reflect a greater link to the processof aging. In particular, the high frequency of this lesion and theability of the damage to impact transcription point to this lesion asone that is likely to be important during aging, perhaps in asynergistic manner with other common age-related neurologicaldiseases. The exact requirements for DNA DSBR and SSBR in theaging brain is not clear, but as outlined above mouse models will beextremely important for generating a basic understanding of howvarious repair pathways participate in cellular homeostasis in thebrain.

A general reduction in DNA repair activity has been associatedwith brain aging in humans and rodents raising the possibility thatthis is a normal aspect of aging (Intano et al., 2003; Lu et al., 2004;Imam et al., 2006; Gorbunova et al., 2007; Rao, 2007; Rutten et al.,2007). However, a correlative relationship between repair activityand aging does not necessarily imply that an assayable decrease inenzyme activity would be associated with increased DNA lesions oraccumulation of mutations. Interestingly, relative to the liver thebrain was found to be resistant to accumulated DNA mutations(Dolle et al., 1997; Stuart et al., 2000).

Defective DNA repair is being increasingly linked with diseaseswith striking neuropathology that are associated with aging, suchas Alzheimer’s and Parkinson’s disease (Nouspikel and Hanawalt,2003; Bender et al., 2006; Hegde et al., 2006; Kraytsberg et al.,2006; Shackelford, 2006; Weissman et al., 2007). Studies of DNArepair from brains of Alzheimer’s individuals have shown reducedDNA DSBR during NHEJ that was attributed to reduced DNA-PKcs

S. Katyal, P.J. McKinnon / Mechanisms of Ageing and Development 129 (2008) 483–491 489

levels, while protein levels of the MRN complex have also beenreported to be diminished (Jacobsen et al., 2004; Shackelford,2006). Reduced base excision repair capacity was also found inAlzheimer’s disease brains compared to age-matched controls(Weissman et al., 2007). In Parkinson’s disease (PD) patients,increased DNA damage was also found in the mitochondria insubstantia nigra neurons (a neuronal population lost in PD),implying a relationship between PD and mitochondrial DNAdamage (Bender et al., 2006; Kraytsberg et al., 2006; Reeve et al.,2008).

Other considerations for aging in the nervous system includesthe requirement for DNA repair in neural stem cells, perhaps thosethat potentially contribute towards new neurogenesis in regions ofthe mature brain (Toni et al., 2007; Zhang et al., 2008). Certainly,recent data points to a strong requirement of DNA DSBR in somestem cell populations (Nijnik et al., 2007; Rossi et al., 2007).However, the functional role for stem cells in the aging brain isunclear, as is the requirement for new neurogenesis as acomponent of normal brain function.

While DNA repair syndromes highlight the importance ofmaintaining genomic integrity in the brain, the role of DNAdamage in normal aging processes is less clear. As mentionedpreviously, reports documenting a loss of repair capacity with agesuggest that DNA damage may be involved in aging, althoughmany other reports find no clear defects in DNA repair associatedwith normal aging. Perhaps, assessment of other DNA damageparameters may be more informative for understanding thecontribution of DNA damage to aging? For example, telomeresfeature prominently in eliciting a DNA damage response whendisrupted, and telomere erosion is a consequence of cellularproliferation (Verdun and Karlseder, 2007). Telomeres arestructures present at the end of chromosomes, and serve a criticalfunction to ensure that DNA ends of chromosomes are not detectedas DNA breaks (DePinho and Wong, 2003; Blasco, 2005).Intriguingly, telomere shortening has been found in cells fromindividuals experiencing emotional stresses often associated withaging, suggesting a strong environmental link to maintenance ofgenome integrity (Epel et al., 2004, 2006; Sapolsky, 2004).However, the need for telomere maintenance in non-replicatingtissue such as the nervous system or the potential impact of loss offunction of telomere protection factors has not yet been carefullyassessed in vivo. Many new insights to address these questions willbe forthcoming as new mouse models of DNA repair deficiency aregenerated. These insights will be important guides as we plantherapeutic strategies to potentially alleviate neuropathologyassociated with DNA repair deficiency and aging.

Acknowledgements

We acknowledge research support from the National Institutesof Health (NIH) and the American Lebanese and Syrian AssociatedCharities (ALSAC) of St. Jude Children’s Research Hospital. Weapologize to colleagues whose primary research references couldnot be cited due to space limitations.

References

Ahel, I., Rass, U., El-Khamisy, S.F., Katyal, S., Clements, P.M., McKinnon, P.J., Calde-cott, K.W., West, S.C., 2006. The neurodegenerative disease protein aprataxinresolves abortive DNA ligation intermediates. Nature 443 (7112), 713–716.

Ahnesorg, P., Smith, P., Jackson, S.P., 2006. XLF interacts with the XRCC4-DNA ligaseIV complex to promote DNA nonhomologous end-joining. Cell 124 (2), 301–313.

Andres, S.N., Modesti, M., Tsai, C.J., Chu, G., Junop, M.S., 2007. Crystal structure ofhuman XLF: a twist in nonhomologous DNA end-joining. Mol. Cell 28 (6), 1093–1101.

Barlow, C., Hirotsune, S., Paylor, R., Liyanage, M., Eckhaus, M., Collins, F., Shiloh, Y.,Crawley, J.N., Ried, T., Tagle, D., Wynshaw-Boris, A., 1996. Atm-deficient mice: aparadigm of ataxia-telangiectasia. Cell 86 (1), 159–171.

Barnes, D.E., Stamp, G., Rosewell, I., Denzel, A., Lindahl, T., 1998. Targeted disruptionof the gene encoding DNA ligase IV leads to lethality in embryonic mice. Curr.Biol. 8 (25), 1395–1398.

Barzilai, A., 2007. The contribution of the DNA damage response to neuronalviability. Antioxid. Redox. Signal. 9 (2), 211–218.

Bates, B., Rios, M., Trumpp, A., Chen, C., Fan, G., Bishop, J.M., Jaenisch, R., 1999.Neurotrophin-3 is required for proper cerebellar development. Nat. Neurosci. 2(2), 115–117.

Bender, A., Krishnan, K.J., Morris, C.M., Taylor, G.A., Reeve, A.K., Perry, R.H., Jaros, E.,Hersheson, J.S., Betts, J., Klopstock, T., Taylor, R.W., Turnbull, D.M., 2006. Highlevels of mitochondrial DNA deletions in substantia nigra neurons in aging andParkinson disease. Nat. Genet. 38 (5), 515–517.

Blasco, M.A., 2005. Telomeres and human disease: ageing, cancer and beyond. Nat.Rev. Genet. 6 (8), 611–622.

Borghesani, P.R., Alt, F.W., Bottaro, A., Davidson, L., Aksoy, S., Rathbun, G.A., Roberts,T.M., Swat, W., Segal, R.A., Gu, Y., 2000. Abnormal development of Purkinje cellsand lymphocytes in Atm mutant mice. Proc. Natl. Acad. Sci. U.S.A. 97 (7), 3336–3341.

Brown, E.J., Baltimore, D., 2003. Essential and dispensable roles of ATR in cell cyclearrest and genome maintenance. Genes Dev. 17 (5), 615–628.

Buck, D., Malivert, L., de Chasseval, R., Barraud, A., Fondaneche, M.C., Sanal, O.,Plebani, A., Stephan, J.L., Hufnagel, M., le Deist, F., Fischer, A., Durandy, A., deVillartay, J.P., Revy, P., 2006. Cernunnos, a novel nonhomologous end-joiningfactor, is mutated in human immunodeficiency with microcephaly. Cell 124 (2),287–299.

Caldecott, K.W., 2003. XRCC1 and DNA strand break repair. DNA Repair 2 (9), 955–969.

Carson, C.T., Schwartz, R.A., Stracker, T.H., Lilley, C.E., Lee, D.V., Weitzman, M.D.,2003. The Mre11 complex is required for ATM activation and the G2/Mcheckpoint. EMBO J. 22 (24), 6610–6620.

Chen, L., Lee, H.M., Greeley Jr., G.H., Englander, E.W., 2007. Accumulation ofoxidatively generated DNA damage in the brain: a mechanism of neurotoxicity.Free Radic. Biol. Med. 42 (3), 385–393.

Clements, P.M., Breslin, C., Deeks, E.D., Byrd, P.J., Ju, L., Bieganowski, P., Brenner, C.,Moreira, M.C., Taylor, A.M., Caldecott, K.W., 2004. The ataxia-oculomotor apraxia1 gene product has a role distinct from ATM and interacts with the DNA strandbreak repair proteins XRCC1 and XRCC4. DNA Repair 3 (11), 1493–1502.

Date, H., Onodera, O., Tanaka, H., Iwabuchi, K., Uekawa, K., Igarashi, S., Koike, R.,Hiroi, T., Yuasa, T., Awaya, Y., Sakai, T., Takahashi, T., Nagatomo, H., Sekijima, Y.,Kawachi, I., Takiyama, Y., Nishizawa, M., Fukuhara, N., Saito, K., Sugano, S., Tsuji,S., 2001. Early-onset ataxia with ocular motor apraxia and hypoalbuminemia iscaused by mutations in a new HIT superfamily gene. Nat. Genet. 29 (2), 184–188.

Demuth, I., Digweed, M., 2007. The clinical manifestation of a defective response toDNA double-strand breaks as exemplified by Nijmegen breakage syndrome.Oncogene 26 (56), 7792–7798.

Demuth, I., Frappart, P.O., Hildebrand, G., Melchers, A., Lobitz, S., Stockl, L., Varon, R.,Herceg, Z., Sperling, K., Wang, Z.Q., Digweed, M., 2004. An inducible null mutantmurine model of Nijmegen breakage syndrome proves the essential function ofNBS1 in chromosomal stability and cell viability. Hum. Mol. Genet. 13 (20),2385–2397.

DePinho, R.A., Wong, K.K., 2003. The age of cancer: telomeres, checkpoints, andlongevity. J. Clin. Invest. 111 (7), S9–S14.

Difilippantonio, S., Celeste, A., Fernandez-Capetillo, O., Chen, H.T., Reina San Martin,B., Van Laethem, F., Yang, Y.P., Petukhova, G.V., Eckhaus, M., Feigenbaum, L.,Manova, K., Kruhlak, M., Camerini-Otero, R.D., Sharan, S., Nussenzweig, M.,Nussenzweig, A., 2005. Role of Nbs1 in the activation of the Atm kinase revealedin humanized mouse models. Nat. Cell. Biol. 7 (7), 675–685.

Dolle, M.E., Giese, H., Hopkins, C.L., Martus, H.J., Hausdorff, J.M., Vijg, J., 1997. Rapidaccumulation of genome rearrangements in liver but not in brain of old mice.Nat. Genet. 17 (4), 431–434.

Dumon-Jones, V., Frappart, P.O., Tong, W.M., Sajithlal, G., Hulla, W., Schmid, G.,Herceg, Z., Digweed, M., Wang, Z.Q., 2003. Nbn heterozygosity renders micesusceptible to tumor formation and ionizing radiation-induced tumorigenesis.Cancer Res. 63 (21), 7263–7269.

el-Khamisy, S.F., Caldecott, K.W., 2007. DNA single-strand break repair and spino-cerebellar ataxia with axonal neuropathy-1. Neuroscience 145 (4), 1260–1266.

El-Khamisy, S.F., Saifi, G.M., Weinfeld, M., Johansson, F., Helleday, T., Lupski, J.R.,Caldecott, K.W., 2005. Defective DNA single-strand break repair in spinocer-ebellar ataxia with axonal neuropathy-1. Nature 434 (7029), 108–113.

Elson, A., Wang, Y., Daugherty, C.J., Morton, C.C., Zhou, F., Campos-Torres, J., Leder, P.,1996. Pleiotropic defects in ataxia-telangiectasia protein-deficient mice. Proc.Natl. Acad. Sci. U.S.A. 93 (23), 13084–13089.

Epel, E.S., Blackburn, E.H., Lin, J., Dhabhar, F.S., Adler, N.E., Morrow, J.D., Cawthon,R.M., 2004. Accelerated telomere shortening in response to life stress. Proc. Natl.Acad. Sci. U.S.A. 101 (49), 17312–17315.

Epel, E.S., Lin, J., Wilhelm, F.H., Wolkowitz, O.M., Cawthon, R., Adler, N.E., Dolbier, C.,Mendes, W.B., Blackburn, E.H., 2006. Cell aging in relation to stress arousal andcardiovascular disease risk factors. Psychoneuroendocrinology 31 (3), 277–287.

Feil, R., Brocard, J., Mascrez, B., LeMeur, M., Metzger, D., Chambon, P., 1996. Ligand-activated site-specific recombination in mice. Proc. Natl. Acad. Sci. U.S.A. 93(20), 10887–10890.

S. Katyal, P.J. McKinnon / Mechanisms of Ageing and Development 129 (2008) 483–491490

Frank, K.M., Sharpless, N.E., Gao, Y., Sekiguchi, J.M., Ferguson, D.O., Zhu, C., Manis,J.P., Horner, J., DePinho, R.A., Alt, F.W., 2000. DNA ligase IV deficiency in miceleads to defective neurogenesis and embryonic lethality via the p53 pathway.Mol. Cell 5 (6), 993–1002.

Frappart, P.O., Lee, Y., Lamont, J., McKinnon, P.J., 2007. BRCA2 is required forneurogenesis and suppression of medulloblastoma. EMBO J. 26 (11), 2732–2742.

Frappart, P.O., McKinnon, P.J., 2006. Ataxia-telangiectasia and related diseases.Neuromol. Med. 8 (4), 495–511.

Frappart, P.O., Tong, W.M., Demuth, I., Radovanovic, I., Herceg, Z., Aguzzi, A., Dig-weed, M., Wang, Z.Q., 2005. An essential function for NBS1 in the prevention ofataxia and cerebellar defects. Nat. Med. 11 (5), 538–544.

Friedberg, E.C., Meira, L.B., 2006. Database of mouse strains carrying targetedmutations in genes affecting biological responses to DNA damage version 7.DNA Repair 5 (2), 189–209.

Gao, Y., Sun, Y., Frank, K.M., Dikkes, P., Fujiwara, Y., Seidl, K.J., Sekiguchi, J.M.,Rathbun, G.A., Swat, W., Wang, J., Bronson, R.T., Malynn, B.A., Bryans, M., Zhu, C.,Chaudhuri, J., Davidson, L., Ferrini, R., Stamato, T., Orkin, S.H., Greenberg, M.E.,Alt, F.W., 1998. A critical role for DNA end-joining proteins in both lymphogen-esis and neurogenesis. Cell 95 (7), 891–902.

Garcia, E.L., Mills, A.A., 2002. Getting around lethality with inducible Cre-mediatedexcision. Semin. Cell Dev. Biol. 13 (2), 151–158.

Gaveriaux-Ruff, C., Kieffer, B.L., 2007. Conditional gene targeting in the mousenervous system: insights into brain function and diseases. Pharmacol. Ther. 113(3), 619–634.

Girard, P.M., Kysela, B., Harer, C.J., Doherty, A.J., Jeggo, P.A., 2004. Analysis of DNAligase IV mutations found in LIG4 syndrome patients: the impact of two linkedpolymorphisms. Hum. Mol. Genet. 13 (20), 2369–2376.

Goldowitz, D., Hamre, K., 1998. The cells and molecules that make a cerebellum.Trends Neurosci. 21 (9), 375–382.

Goodship, J., Gill, H., Carter, J., Jackson, A., Splitt, M., Wright, M., 2000. Autozygositymapping of a Seckel syndrome locus to chromosome 3q22. 1-q24. Am. J. Hum.Genet. 67 (2), 498–503.

Gorbunova, V., Seluanov, A., Mao, Z., Hine, C., 2007. Changes in DNA repair duringaging. Nucleic Acids Res. 35 (22), 7466–7474.

Griffith, E., Walker, S., Martin, C.A., Vagnarelli, P., Stiff, T., Vernay, B., Sanna, N.A.,Saggar, A., Hamel, B., Earnshaw, W.C., Jeggo, P.A., Jackson, A.P., O’Driscoll, M.,2007. Mutations in pericentrin cause Seckel syndrome with defective ATR-dependent DNA damage signaling. Nat. Genet..

Hegde, M.L., Gupta, V.B., Anitha, M., Harikrishna, T., Shankar, S.K., Muthane, U.,Subba Rao, K., Jagannatha Rao, K.S., 2006. Studies on genomic DNA topology andstability in brain regions of Parkinson’s disease. Arch. Biochem. Biophys. 449 (1–2), 143–156.

Helleday, T., Lo, J., van Gent, D.C., Engelward, B.P., 2007. DNA double-strand breakrepair: from mechanistic understanding to cancer treatment. DNA Repair 6 (7),923–935.

Herzog, K.H., Chong, M.J., Kapsetaki, M., Morgan, J.I., McKinnon, P.J., 1998. Require-ment for Atm in ionizing radiation-induced cell death in the developing centralnervous system. Science 280 (5366), 1089–1091.

Hirano, R., Interthal, H., Huang, C., Nakamura, T., Deguchi, K., Choi, K., Bhattacharjee,M.B., Arimura, K., Umehara, F., Izumo, S., Northrop, J.L., Salih, M.A., Inoue, K.,Armstrong, D.L., Champoux, J.J., Takashima, H., Boerkoel, C.F., 2007. Spinocer-ebellar ataxia with axonal neuropathy: consequence of a Tdp1 recessive neo-morphic mutation? EMBO J. 26 (22), 4732–4743.

Imam, S.Z., Karahalil, B., Hogue, B.A., Souza-Pinto, N.C., Bohr, V.A., 2006. Mitochon-drial and nuclear DNA-repair capacity of various brain regions in mouse isaltered in an age-dependent manner. Neurobiol. Aging 27 (8), 1129–1136.

Intano, G.W., Cho, E.J., McMahan, C.A., Walter, C.A., 2003. Age-related base excisionrepair activity in mouse brain and liver nuclear extracts. J. Gerontol. 58 (3), 205–211.

Jacobsen, E., Beach, T., Shen, Y., Li, R., Chang, Y., 2004. Deficiency of the Mre11 DNArepair complex in Alzheimer’s disease brains. Brain Res. 128 (1), 1–7.

Jacobsen, M., 1991. Developmental Neurobiology. Plenum Press, New York.Jonkers, J., Berns, A., 2002. Conditional mouse models of sporadic cancer. Nat. Rev.

Cancer 2 (4), 251–265.Kang, J., Bronson, R.T., Xu, Y., 2002. Targeted disruption of NBS1 reveals its roles in

mouse development and DNA repair. EMBO J. 21 (6), 1447–1455.Kastan, M.B., Bartek, J., 2004. Cell-cycle checkpoints and cancer. Nature 432 (7015),

316–323.Katyal, S., el-Khamisy, S.F., Russell, H.R., Li, Y., Ju, L., Caldecott, K.W., McKinnon, P.J.,

2007. TDP1 facilitates chromosomal single-strand break repair in neurons andis neuroprotective in vivo. EMBO J. 26 (22), 4720–4731.

Katyal, S., McKinnon, P.J., 2007. DNA repair deficiency and neurodegeneration. CellCycle 6 (19).

Kitagawa, R., Bakkenist, C.J., McKinnon, P.J., Kastan, M.B., 2004. Phosphorylation ofSMC1 is a critical downstream event in the ATM-NBS1-BRCA1 pathway. GenesDev. 18, 1423–1438.

Kobayashi, J., Antoccia, A., Tauchi, H., Matsuura, S., Komatsu, K., 2004. NBS1and its functional role in the DNA damage response. DNA Repair 3 (8–9),855–861.

Kraytsberg, Y., Kudryavtseva, E., McKee, A.C., Geula, C., Kowall, N.W., Khrapko, K.,2006. Mitochondrial DNA deletions are abundant and cause functional impair-ment in aged human substantia nigra neurons. Nat. Genet. 38 (5), 518–520.

Kuhn, R., Schwenk, F., Aguet, M., Rajewsky, K., 1995. Inducible gene targeting inmice. Science 269 (5229), 1427–1429.

Kumagai, A., Lee, J., Yoo, H.Y., Dunphy, W.G., 2006. TopBP1 activates the ATR-ATRIPcomplex. Cell 124 (5), 943–955.

Lavin, M.F., Kozlov, S., 2007. ATM activation and DNA damage response. Cell Cycle 6(8), 931–942.

Lee, Y., McKinnon, P.J., 2007. Responding to DNA double-strand breaks in thenervous system. Neuroscience 145 (4), 1365–1374.

Lees-Miller, S.P., Meek, K., 2003. Repair of DNA double-strand breaks by non-homologous end joining. Biochimie 85 (11), 1161–1173.

Li, Y., Chirgadze, D.Y., Bolanos-Garcia, V.M., Sibanda, B.L., Davies, O.R., Ahnesorg, P.,Jackson, S.P., Blundell, T.L., 2008. Crystal structure of human XLF/Cernunnosreveals unexpected differences from XRCC4 with implications for NHEJ. EMBO J.27 (1), 290–300.

Lieber, M.R., Ma, Y., Pannicke, U., Schwarz, K., 2003. Mechanism and regulation ofhuman non-homologous DNA end-joining. Nat. Rev. 4 (9), 712–720.

Limperopoulos, C., du Plessis, A.J., 2006. Disorders of cerebellar growth and devel-opment. Curr. Opin. Pediatr. 18 (6), 621–627.

Lombard, D.B., Chua, K.F., Mostoslavsky, R., Franco, S., Gostissa, M., Alt, F.W., 2005.DNA repair, genome stability, and aging. Cell 120 (4), 497–512.

Lu, T., Pan, Y., Kao, S.Y., Li, C., Kohane, I., Chan, J., Yankner, B.A., 2004. Gene regulationand DNA damage in the ageing human brain. Nature 429 (6994), 883–891.

Matsuoka, S., Ballif, B.A., Smogorzewska, A., McDonald 3rd, E.R., Hurov, K.E., Luo, J.,Bakalarski, C.E., Zhao, Z., Solimini, N., Lerenthal, Y., Shiloh, Y., Gygi, S.P., Elledge,S.J., 2007. ATM and ATR substrate analysis reveals extensive protein networksresponsive to DNA damage. Science 316 (5828), 1160–1166.

McKinnon, P.J., 2004. ATM and ataxia-telangiectasia. EMBO Rep. 5 (8), 772–776.McKinnon, P.J., Caldecott, K.W., 2007. DNA strand break repair and human genetic

disease. Annu. Rev. Genom. Hum. Genet. 837–855.Moreira, M.C., Barbot, C., Tachi, N., Kozuka, N., Uchida, E., Gibson, T., Mendonca, P.,

Costa, M., Barros, J., Yanagisawa, T., Watanabe, M., Ikeda, Y., Aoki, M., Nagata, T.,Coutinho, P., Sequeiros, J., Koenig, M., 2001. The gene mutated in ataxia-ocularapraxia 1 encodes the new HIT/Zn-finger protein aprataxin. Nat. Genet. 29 (2),189–193.

Nijnik, A., Woodbine, L., Marchetti, C., Dawson, S., Lambe, T., Liu, C., Rodrigues, N.P.,Crockford, T.L., Cabuy, E., Vindigni, A., Enver, T., Bell, J.I., Slijepcevic, P., Good-now, C.C., Jeggo, P.A., Cornall, R.J., 2007. DNA repair is limiting for haemato-poietic stem cells during ageing. Nature 447 (7145), 686–690.

Nouspikel, T., Hanawalt, P.C., 2003. When parsimony backfires: neglecting DNArepair may doom neurons in Alzheimer’s disease. Bioessays 25 (2), 168–173.

O’Driscoll, M., Gennery, A.R., Seidel, J., Concannon, P., Jeggo, P.A., 2004. An overviewof three new disorders associated with genetic instability: LIG4 syndrome, RS-SCID and ATR-Seckel syndrome. DNA Repair 3 (8–9), 1227–1235.

O’Driscoll, M., Jeggo, P.A., 2006. The role of double-strand break repair—insightsfrom human genetics. Nat. Rev. Genet. 7 (1), 45–54.

Orban, P.C., Chui, D., Marth, J.D., 1992. Tissue- and site-specific DNA recombinationin transgenic mice. Proc. Natl. Acad. Sci. U.S.A. 89 (15), 6861–6865.

Orii, K.E., Lee, Y., Kondo, N., McKinnon, P.J., 2006. Selective utilization of nonho-mologous end-joining and homologous recombination DNA repair pathwaysduring nervous system development. Proc. Natl. Acad. Sci. U.S.A. 103 (26),10017–10022.

Paulsen, R.D., Cimprich, K.A., 2007. The ATR pathway: fine-tuning the fork. DNARepair 6 (7), 953–966.

Petrini, J.H., Stracker, T.H., 2003. The cellular response to DNA double-strand breaks:defining the sensors and mediators. Trends Cell Biol. 13 (9), 458–462.

Rao, K.S., 2007. DNA repair in aging rat neurons. Neuroscience 145 (4), 1330–1340.Rass, U., Ahel, I., West, S.C., 2007. Defective DNA repair and neurodegenerative

disease. Cell 130 (6), 991–1004.Reeve, A.K., Krishnan, K.J., Elson, J.L., Morris, C.M., Bender, A., Lightowlers, R.N.,

Turnbull, D.M., 2008. Nature of mitochondrial DNA deletions in substantia nigraneurons. Am. J. Hum. Genet. 82 (1), 228–235.

Rolig, R.L., McKinnon, P.J., 2000. Linking DNA damage and neurodegeneration.Trends Neurosci. 23 (9), 417–424.

Rossi, D.J., Bryder, D., Seita, J., Nussenzweig, A., Hoeijmakers, J., Weissman, I.L., 2007.Deficiencies in DNA damage repair limit the function of haematopoietic stemcells with age. Nature 447 (7145), 725–729.

Rutten, B.P., Schmitz, C., Gerlach, O.H., Oyen, H.M., de Mesquita, E.B., Steinbusch,H.W., Korr, H., 2007. The aging brain: accumulation of DNA damage or neuronloss? Neurobiol. Aging 28 (1), 91–98.

Sapolsky, R.M., 2004. Organismal stress and telomeric aging: an unexpected con-nection. Proc. Natl. Acad. Sci. U.S.A. 101 (50), 17323–17324.

Saxowsky, T.T., Doetsch, P.W., 2006. RNA polymerase encounters with DNAdamage: transcription-coupled repair or transcriptional mutagenesis? Chem.Rev. 106 (2), 474–488.

Sedelnikova, O.A., Pilch, D.R., Redon, C., Bonner, W.M., 2003. Histone H2AX in DNAdamage and repair. Cancer Biol. Ther. 2 (3), 233–235.

Shackelford, D.A., 2006. DNA end joining activity is reduced in Alzheimer’s disease.Neurobiol. Aging 27 (4), 596–605.

Sharon, D., Sandberg, M.A., Rabe, V.W., Stillberger, M., Dryja, T.P., Berson, E.L., 2003.RP2 and RPGR mutations and clinical correlations in patients with X-linkedretinitis pigmentosa. Am. J. Hum. Genet. 73 (5), 1131–1146.

Shiloh, Y., 1997. Ataxia-telangiectasia and the Nijmegen breakage syndrome:related disorders but genes apart. Annu. Rev. Genet. 31635–31662.

Shiloh, Y., 2001. ATM and ATR: networking cellular responses to DNA damage. Curr.Opin. Genet. Dev. 11 (1), 71–77.

Shiloh, Y., 2003. ATM and related protein kinases: safeguarding genome integrity.Nat. Rev. Cancer 3 (3), 155–168.

S. Katyal, P.J. McKinnon / Mechanisms of Ageing and Development 129 (2008) 483–491 491

Spring, K., Cross, S., Li, C., Watters, D., Ben-Senior, L., Waring, P., Ahangari, F., Lu, S.L.,Chen, P., Misko, I., Paterson, C., Kay, G., Smorodinsky, N.I., Shiloh, Y., Lavin, M.F.,2001. Atm knock-in mice harboring an in-frame deletion corresponding to thehuman ATM 7636del9 common mutation exhibit a variant phenotype. CancerRes. 61 (11), 4561–4568.

St-Onge, L., Furth, P.A., Gruss, P., 1996. Temporal control of the Cre recombinase intransgenic mice by a tetracycline responsive promoter. Nucleic Acids Res. 24(19), 3875–3877.

Stracker, T.H., Morales, M., Couto, S.S., Hussein, H., Petrini, J.H., 2007. The carboxyterminus of NBS1 is required for induction of apoptosis by the MRE11 complex.Nature 447 (7141), 218–221.

Stuart, G.R., Oda, Y., de Boer, J.G., Glickman, B.W., 2000. Mutation frequency andspecificity with age in liver, bladder and brain of lacI transgenic mice. Genetics154 (3), 1291–1300.

Subba Rao, K., 2007. Mechanisms of disease: DNA repair defects and neurologicaldisease. Nat. Clin. Pract. 3 (3), 162–172.

Sugawara, M., Wada, C., Okawa, S., Kobayashi, M., Sageshima, M., Imota, T.,Toyoshima, I., 2008. Purkinje cell loss in the cerebellar flocculus in patientswith ataxia with ocular motor apraxia type 1/early-onset ataxia with ocularmotor apraxia and hypoalbuminemia. Eur. Neurol. 59 (1–2), 18–23.

Takashima, H., Boerkoel, C.F., John, J., Saifi, G.M., Salih, M.A., Armstrong, D., Mao, Y.,Quiocho, F.A., Roa, B.B., Nakagawa, M., Stockton, D.W., Lupski, J.R., 2002.Mutation of TDP1, encoding a topoisomerase I-dependent DNA damage repairenzyme, in spinocerebellar ataxia with axonal neuropathy. Nat. Genet. 32 (2),267–272.

Taylor, A.M., Groom, A., Byrd, P.J., 2004. Ataxia-telangiectasia-like disorder (ATLD)-its clinical presentation and molecular basis. DNA Repair 3 (8–9), 1219–1225.

Theunissen, J.W., Kaplan, M.I., Hunt, P.A., Williams, B.R., Ferguson, D.O., Alt, F.W.,Petrini, J.H., 2003. Checkpoint failure and chromosomal instability withoutlymphomagenesis in Mre11(ATLD1/ATLD1) mice. Mol. Cell 12 (6), 1511–1523.

Toni, N., Teng, E.M., Bushong, E.A., Aimone, J.B., Zhao, C., Consiglio, A., van Praag, H.,Martone, M.E., Ellisman, M.H., Gage, F.H., 2007. Synapse formation on neuronsborn in the adult hippocampus. Nat. Neurosci. 10 (6), 727–734.

Uziel, T., Lerenthal, Y., Moyal, L., Andegeko, Y., Mittelman, L., Shiloh, Y., 2003.Requirement of the MRN complex for ATM activation by DNA damage. EMBOJ. 22 (20), 5612–5621.

van Gent, D.C., van der Burg, M., 2007. Non-homologous end-joining, a sticky affair.Oncogene 26 (56), 7731–7740.

Verdun, R.E., Karlseder, J., 2007. Replication and protection of telomeres. Nature 447(7147), 924–931.

Vijg, J., Calder, R.B., 2004. Transcripts of aging. Trends Genet. 20 (6), 221–224.Wang, V.Y., Zoghbi, H.Y., 2001. Genetic regulation of cerebellar development. Nat.

Rev. Neurosci. 2 (7), 484–491.

Ward, I., Chen, J., 2004. Early events in the DNA damage response. Curr. Top. Dev.Biol. 631–635.

Weemaes, C.M., Hustinx, T.W., Scheres, J.M., van Munster, P.J., Bakkeren, J.A.,Taalman, R.D., 1981. A new chromosomal instability disorder: the Nijmegenbreakage syndrome. Acta Paediatr. Scand. 70 (4), 557–564.

Weissman, L., Jo, D.G., Sorensen, M.M., de Souza-Pinto, N.C., Markesbery, W.R.,Mattson, M.P., Bohr, V.A., 2007. Defective DNA base excision repair in brain fromindividuals with Alzheimer’s disease and amnestic mild cognitive impairment.Nucleic Acids Res. 35 (16), 5545–5555.

West, S.C., 2003. Molecular views of recombination proteins and their control.Nature Rev. 4 (6), 435–445.

Williams, B.R., Mirzoeva, O.K., Morgan, W.F., Lin, J., Dunnick, W., Petrini, J.H., 2002. Amurine model of Nijmegen breakage syndrome. Curr. Biol. 12 (8), 648–653.

Wilson 3rd, D.M., 2007. Processing of nonconventional DNA strand break ends.Environ. Mol. Mutagen. 48 (9), 772–782.

Wilson 3rd, D.M., Bohr, V.A., 2007. The mechanics of base excision repair, and itsrelationship to aging and disease. DNA Repair 6 (4), 544–559.

Wilson 3rd, D.M., Mattson, M.P., 2007. Neurodegeneration: nicked to death. Curr.Biol. 17 (2), R55–R58.

Wyman, C., Kanaar, R., 2006. DNA double-strand break repair: all’s well that endswell. Annu. Rev. Genet. 40363–40383.

Xiao, Y., Weaver, D.T., 1997. Conditional gene targeted deletion by Cre recombinasedemonstrates the requirement for the double-strand break repair Mre11 pro-tein in murine embryonic stem cells. Nucleic Acids Res. 25 (15), 2985–2991.

Xu, Y., Baltimore, D., 1996. Dual roles of ATM in the cellular response to radiationand in cell growth control. Genes Dev. 10 (19), 2401–2410.

Yoon, J.H., Qiu, J., Cai, S., Chen, Y., Cheetham, M.E., Shen, B., Pfeifer, G.P., 2006. Theretinitis pigmentosa-mutated RP2 protein exhibits exonuclease activity andtranslocates to the nucleus in response to DNA damage. Exp. Cell Res. 312 (8),1323–1334.

Zahn, J.M., Poosala, S., Owen, A.B., Ingram, D.K., Lustig, A., Carter, A., Weeraratna,A.T., Taub, D.D., Gorospe, M., Mazan-Mamczarz, K., Lakatta, E.G., Boheler, K.R.,Xu, X., Mattson, M.P., Falco, G., Ko, M.S., Schlessinger, D., Firman, J., Kummerfeld,S.K., Wood 3rd, W.H., Zonderman, A.B., Kim, S.K., Becker, K.G., 2007. AGEMAP: agene expression database for aging in mice. PLoS Genet. 3 (11), e201.

Zha, S., Alt, F.W., Cheng, H.L., Brush, J.W., Li, G., 2007. Defective DNA repair andincreased genomic instability in Cernunnos-XLF-deficient murine ES cells. Proc.Natl. Acad. Sci. U.S.A. 104 (11), 4518–4523.

Zhang, C.L., Zou, Y., He, W., Gage, F.H., Evans, R.M., 2008. A role for adult TLX-positiveneural stem cells in learning and behaviour. Nature.

Zhu, J., Petersen, S., Tessarollo, L., Nussenzweig, A., 2001. Targeted disruption of theNijmegen breakage syndrome gene NBS1 leads to early embryonic lethality inmice. Curr. Biol. 11 (2), 105–109.