DNA repair deficiency and neurological disease. Nat Rev Neurosci. 2009;10:100-112. [PMID: 19145234 DOI: 10.1038/nrn2559]

Should I stay or should I go: VCP/p97-mediated chromatin extraction in the DNA damage response

The ordered assembly of DNA repair factors on chromatin has been studied in great detail, whereas we are only beginning to realize that selective extraction of proteins from chromatin plays a central role in the DNA damage response. Interestingly, the protein modifier ubiquitin not only regulates the well-documented recruitment of repair proteins, but also governs the temporally and spatially controlled extraction of proteins from DNA lesions. The facilitator of protein extraction is the ubiquitin-dependent ATPase valosin-containing protein (VCP)/p97 complex, which, through its segregase activity, directly extracts ubiquitylated proteins from chromatin. In this review, we summarize recent studies that uncovered this important role of VCP/p97 in the cellular response to genomic insults and discuss how ubiquitin regulates two intuitively counteracting activities at sites of DNA damage.

Chromatin regulation of DNA damage repair and genome integrity in the central nervous system

With the continued extension of lifespan, aging and age-related diseases have become a major medical challenge to our society. Aging is accompanied by changes in multiple systems. Among these, the aging process in the central nervous system is critically important but very poorly understood. Neurons, as post-mitotic cells, are devoid of replicative associated aging processes, such as senescence and telomere shortening. However, because of the inability to self-replenish, neurons have to withstand challenge from numerous stressors over their lifetime. Many of these stressors can lead to damage of the neurons' DNA. When the accumulation of DNA damage exceeds a neuron's capacity for repair, or when there are deficiencies in DNA repair machinery, genome instability can manifest. The increased mutation load associated with genome instability can lead to neuronal dysfunction and ultimately to neuron degeneration. In this review, we first briefly introduce the sources and types of DNA damage and the relevant repair pathways in the nervous system (summarized in Fig. 1). We then discuss the chromatin regulation of these processes and summarize our understanding of the contribution of genomic instability to neurodegenerative diseases.

DNA repair abnormalities leading to ataxia: shared neurological phenotypes and risk factors

Since identification of mutations in the ATM gene leading to ataxia-telangiectasia, enormous efforts have been devoted to discovering the roles this protein plays in DNA repair as well as other cellular functions. Even before the identification of ATM mutations, it was clear that other diseases with different genomic loci had very similar neurological symptoms. There has been significant progress in understanding why cancer and immunodeficiency occur in ataxia-telangiectasia even though many details remain to be determined, but the field is no closer to determining why the nervous system requires ATM and other DNA repair genes. Even though rodent disease models have similar DNA repair abnormalities as the human diseases, they have no consistent, robust neuropathological phenotype making it difficult to understand the neurological underpinnings of disease. Therefore, it may be useful to reassess the neurological and neuropathological characteristics of ataxia-telangiectasia in human patients to look for potential commonalities in DNA repair diseases that result in ataxia. In doing so, it is clear that ataxia-telangiectasia and similar diseases share neurological features other than merely ataxia, such as length-dependent motor and sensory neuropathies, and that the neuroanatomical localization for these symptoms is understood. Cells affected in ataxia-telangiectasia and similar diseases are some of the largest single nucleated cells in the body. In addition, a subset of these diseases also has extrapyramidal movements and oculomotor apraxia. These neurological and neuropathological similarities may indicate a common DNA repair related pathogenesis with very large cell size as a critical risk factor.

Functional and molecular defects of hiPSC-derived neurons from patients with ATM deficiency

Loss of ataxia telangiectasia mutated (ATM) kinase, a key factor of the DNA damage response (DDR) pathway, causes the cancer predisposing and neurodegenerative syndrome ataxia-telangiectasia (A-T). To investigate the mechanisms of neurodegeneration, we have reprogrammed fibroblasts from ATM-null A-T patients and normal controls to pluripotency (human-induced pluripotent stem cells), and derived from these neural precursor cells able to terminally differentiate into post-mitotic neurons positive to >90% for β-tubulin III+/microtubule-associated protein 2+. We show that A-T neurons display similar voltage-gated potassium and sodium currents and discharges of action potentials as control neurons, but defective expression of the maturation and synaptic markers SCG10, SYP and PSD95 (postsynaptic density protein 95). A-T neurons exhibited defective repair of DNA double-strand breaks (DSBs) and repressed phosphorylation of ATM substrates (e.g., γH2AX, Smc1-S966, Kap1-S824, Chk2-T68, p53-S15), but normal repair of single-strand breaks, and normal short- and long-patch base excision repair activities. Moreover, A-T neurons were resistant to apoptosis induced by the genotoxic agents camptothecin and trabectedin, but as sensitive as controls to the oxidative agents. Most notably, A-T neurons exhibited abnormal accumulation of topoisomerase 1-DNA covalent complexes (Top1-ccs). These findings reveal that ATM deficiency impairs neuronal maturation, suppresses the response and repair of DNA DSBs, and enhances Top1-cc accumulation. Top1-cc could be a risk factor for neurodegeneration as they may interfere with transcription elongation and promote transcriptional decline.

Nbn gene inactivation in the CNS of mouse inhibits the myelinating ability of the mature cortical oligodendrocytes

Nijmegen Breakage Syndrome (NBS) is a recessive genetic disorder characterized by immunodeficiency, elevated sensitivity to ionizing radiation, chromosomal instability, microcephaly, and high predisposition to malignancies. Since the underlying molecular mechanisms of the NBS microcephaly are still obscure, thus our group previously inactivated the Nbn gene in the central nervous system (CNS) of mice by nestin-Cre targeting gene system, and generated Nbn(CNS-del) mice. Interestingly, the newborn Nbn(CNS-del) mice exhibit obvious microcephaly, which is accompanied by severe ataxia and balance deficiency. In this study presented here, we report that Nbn-deficiency induces the enhanced apoptosis of the mature oligodendrocytes at postnatal day 7, which further affects the myelination of the nerve fibers of cerebrum and corpus callosum.The distinct regulatory roles of Ataxia telangiectasia mutated (ATM) signaling and protein kinase B(Akt)/the mammalian target of Rapamycin (AKT/mTOR) signaling are responsible for the enhanced apoptosis of the Nbn-deficient oligodendrocytes. In addition, a series of transcriptional factors including histonedeacetylase (HDAC), zinc finger protein 191 (ZFP-191) and myelin sheath regulatory factor (MRF) play distinct roles in regulating the myelination of the Nbn-deficient oligodendrocytes. Based on these results, it concludes that ATM-Chk2-P53-P21 signaling pathway and the AKT/mTOR signaling pathway are both responsible for the enhanced apoptosis of the Nbn-deficient oligodendrocytes. HDAC, ZFP-191, and MRF are also involved in the pathogenesis of the hypomyelination of the Nbn-deficient oligodendrocytes.

Baiji genomes reveal low genetic variability and new insights into secondary aquatic adaptations

The baiji, or Yangtze River dolphin (Lipotes vexillifer), is a flagship species for the conservation of aquatic animals and ecosystems in the Yangtze River of China; however, this species has now been recognized as functionally extinct. Here we report a high-quality draft genome and three re-sequenced genomes of L. vexillifer using Illumina short-read sequencing technology. Comparative genomic analyses reveal that cetaceans have a slow molecular clock and molecular adaptations to their aquatic lifestyle. We also find a significantly lower number of heterozygous single nucleotide polymorphisms in the baiji compared to all other mammalian genomes reported thus far. A reconstruction of the demographic history of the baiji indicates that a bottleneck occurred near the end of the last deglaciation, a time coinciding with a rapid decrease in temperature and the rise of eustatic sea level.

Causative novel PNKP mutations and concomitant PCDH15 mutations in a patient with microcephaly with early-onset seizures and developmental delay syndrome and hearing loss

We report on a 1-year-old boy with microcephaly with a simplified gyral pattern, early-onset seizures, congenital hearing loss and a severe developmental delay. Trio-based whole-exome sequencing identified candidate compound heterozygous mutations in two genes: c.163G>T (p.Ala55Ser) and c.874G>A (p.Gly292Arg) in polynucleotide kinase 3'-phosphatase gene (PNKP), and c.195G>A (p.Met65Ile) and c.1210A>C (p.Ser404Arg) in PCDH15. PNKP and PCDH15 mutations have been reported in autosomal recessive microcephaly with early-onset seizures and developmental delay syndrome, and Usher syndrome type 1F, respectively. Our patient showed neurological features similar to reported cases of both syndromes that could be explained by the observed mutations in both PNKP and PCDH15, which therefore appear to be pathogenic in this case.

Pot1a prevents telomere dysfunction and ATM-dependent neuronal loss

Genome stability is essential for neural development and the prevention of neurological disease. Here we determined how DNA damage signaling from dysfunctional telomeres affects neurogenesis. We found that telomere uncapping by Pot1a inactivation resulted in an Atm-dependent loss of cerebellar interneurons and granule neuron precursors in the mouse nervous system. The activation of Atm by Pot1a loss occurred in an Atr-dependent manner, revealing an Atr to Atm signaling axis in the nervous system after telomere dysfunction. In contrast to telomere lesions, Brca2 inactivation in neural progenitors also led to ablation of cerebellar interneurons, but this did not require Atm. These data reveal that neural cell loss after DNA damage selectively engages Atm signaling, highlighting how specific DNA lesions can dictate neuropathology arising in human neurodegenerative syndromes.

Aberrant topoisomerase-1 DNA lesions are pathogenic in neurodegenerative genome instability syndromes

DNA damage is considered to be a prime factor in several spinocerebellar neurodegenerative diseases; however, the DNA lesions underpinning disease etiology are unknown. We observed the endogenous accumulation of pathogenic topoisomerase-1 (Top1)-DNA cleavage complexes (Top1ccs) in murine models of ataxia telangiectasia and spinocerebellar ataxia with axonal neuropathy 1. We found that the defective DNA damage response factors in these two diseases cooperatively modulated Top1cc turnover in a non-epistatic and ATM kinase-independent manner. Furthermore, coincident neural inactivation of ATM and DNA single-strand break repair factors, including tyrosyl-DNA phosphodiesterase-1 or XRCC1, resulted in increased Top1cc formation and excessive DNA damage and neurodevelopmental defects. Notably, direct Top1 poisoning to elevate Top1cc levels phenocopied the neuropathology of the mouse models described above. Our results identify a critical endogenous pathogenic lesion associated with neurodegenerative syndromes arising from DNA repair deficiency, indicating that genome integrity is important for preventing disease in the nervous system.

CDK5-mediated phosphorylation of p19INK4d avoids DNA damage-induced neurodegeneration in mouse hippocampus and prevents loss of cognitive functions

DNA damage, which perturbs genomic stability, has been linked to cognitive decline in the aging human brain, and mutations in DNA repair genes have neurological implications. Several studies have suggested that DNA damage is also increased in brain disorders such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. However, the precise mechanisms connecting DNA damage with neurodegeneration remain poorly understood. CDK5, a critical enzyme in the development of the central nervous system, phosphorylates a number of synaptic proteins and regulates dendritic spine morphogenesis, synaptic plasticity and learning. In addition to these physiological roles, CDK5 has been involved in the neuronal death initiated by DNA damage. We hypothesized that p19INK4d, a member of the cell cycle inhibitor family INK4, is involved in a neuroprotective mechanism activated in response to DNA damage. We found that in response to genotoxic injury or increased levels of intracellular calcium, p19INK4d is transcriptionally induced and phosphorylated by CDK5 which provides it with greater stability in postmitotic neurons. p19INK4d expression improves DNA repair, decreases apoptosis and increases neuronal survival under conditions of genotoxic stress. Our in vivo experiments showed that decreased levels of p19INK4d rendered hippocampal neurons more sensitive to genotoxic insult resulting in the loss of cognitive abilities that rely on the integrity of this brain structure. We propose a feedback mechanism by which the neurotoxic effects of CDK5-p25 activated by genotoxic stress or abnormal intracellular calcium levels are counteracted by the induction and stabilization of p19INK4d protein reducing the adverse consequences on brain functions.

Structural basis of SOSS1 complex assembly and recognition of ssDNA

The SOSS1 complex comprising SOSSA, SOSSB1, and SOSSC senses single-stranded DNA (ssDNA) and promotes repair of DNA double-strand breaks (DSBs). But how SOSS1 is assembled and recognizes ssDNA remains elusive. The crystal structure of the N-terminal half of SOSSA (SOSSAN) in complex with SOSSB1 and SOSSC showed that SOSSAN serves as a scaffold to bind both SOSSB1 and SOSSC for assembly of the SOSS1 complex. The structures of SOSSAN/B1 in complex with a 12 nt ssDNA and SOSSAN/B1/C in complex with a 35 nt ssDNA showed that SOSSB1 interacts with both SOSSAN and ssDNA via two distinct surfaces. Recognition of ssDNA with a length of up to nine nucleotides is mediated solely by SOSSB1, whereas neither SOSSC nor SOSSAN are critical for ssDNA binding. These results reveal the structural basis of SOSS1 assembly and provide a framework for further study of the mechanism governing longer ssDNA recognition by the SOSS1 complex during DSB repair.

Living with xeroderma pigmentosum: comprehensive photoprotection for highly photosensitive patients

Xeroderma pigmentosum (XP) is a rare autosomal recessive disease of deoxyribonucleic acid (DNA) repair with ultraviolet (UV) radiation sensitivity and a 10 000-fold increased risk of skin cancer. Symptoms include: freckle-like pigmentation in sun-exposed skin before age 2 years, severe burns after minimal sun exposure (50% of patients) and damage to exposed surfaces of the eyes with loss of vision and ocular cancer. About 25% of patients develop a progressive neurodegeneration. The combination of an inherited inability to repair UV-induced DNA damage and environmental exposure to UV must occur for cutaneous and ocular symptoms to develop. There is no cure for XP, but many of its manifestations may be reduced or prevented through consistent UV protection; thus XP serves as a model for sun protection of patients with marked photosenstivity. Sun protective clothing including hats, sunglasses and face shields, sun screen lotions and avoidance of environmental sources of UV are cornerstones of prevention of skin and eye damage and cancer. Although XP is a serious disease with the potential for limitation of life expectancy, XP patients can live active lives while at the same time avoiding UV.

Interdependence of Bad and Puma during ionizing-radiation-induced apoptosis

Ionizing radiation (IR)-induced DNA double-strand breaks trigger an extensive cellular signaling response that involves the coordination of hundreds of proteins to regulate DNA repair, cell cycle arrest and apoptotic pathways. The cellular outcome often depends on the level of DNA damage as well as the particular cell type. Proliferating zebrafish embryonic neurons are highly sensitive to IR-induced apoptosis, and both p53 and its transcriptional target puma are essential mediators of the response. The BH3-only protein Puma has previously been reported to activate mitochondrial apoptosis through direct interaction with the pro-apoptotic Bcl-2 family proteins Bax and Bak, thus constituting the role of an “activator” BH3-only protein. This distinguishes it from BH3-only proteins like Bad that are thought to indirectly promote apoptosis through binding to anti-apoptotic Bcl-2 family members, thereby preventing the sequestration of activator BH3-only proteins and allowing them to directly interact with and activate Bax and Bak. We have shown previously that overexpression of the BH3-only protein Bad in zebrafish embryos supports normal embryonic development but greatly sensitizes developing neurons to IR-induced apoptosis. While Bad has previously been shown to play only a minor role in promoting IR-induced apoptosis of T cells in mice, we demonstrate that Bad is essential for robust IR-induced apoptosis in zebrafish embryonic neural tissue. Moreover, we found that both p53 and Puma are required for Bad-mediated radiosensitization in vivo. Our findings show the existence of a hierarchical interdependence between Bad and Puma whereby Bad functions as an essential sensitizer and Puma as an essential activator of IR-induced mitochondrial apoptosis specifically in embryonic neural tissue.

SETX sumoylation: A link between DNA damage and RNA surveillance disrupted in AOA2

Senataxin (SETX) is a putative RNA:DNA helicase that is mutated in two distinct juvenile neurological disorders, AOA2 and ALS4. SETX is involved in the response to oxidative stress and is suggested to resolve R loops formed at transcription termination sites or at sites of collisions between the transcription and replication machineries. R loops are hybrids between RNA and DNA that are believed to lead to DNA damage and genomic instability. We discovered that Rrp45, a core component of the exosome, is a SETX-interacting protein and that the interaction depends on modification of SETX by sumoylation. Importantly, we showed that AOA2 but not ALS4 mutations prevented both SETX sumoylation and the Rrp45 interaction. We also found that upon replication stress induction, SETX and Rrp45 co-localize in nuclear foci that constitute sites of R-loop formation generated by transcription and replication machinery collisions. We suggest that SETX links transcription, DNA damage and RNA surveillance, and discuss here how this link can be relevant to AOA2 disease.

Role of 53BP1 in the regulation of DNA double-strand break repair pathway choice

The p53-binding protein 1 (53BP1) is a well-known DNA damage response (DDR) factor, which is recruited to nuclear structures at the site of DNA damage and forms readily visualized ionizing radiation (IR) induced foci. Depletion of 53BP1 results in cell cycle arrest in G2/M phase as well as genomic instability in human as well as mouse cells. Within the DNA damage response mechanism, 53BP1 is classified as an adaptor/mediator, required for processing of the DNA damage response signal and as a platform for recruitment of other repair factors. More recently, specific 53BP1 contributions to DSB repair pathway choice have been recognized and are being characterized. In this review, we have summarized recent advances in understanding the role of 53BP1 in regulating DNA DSBs repair pathway choice, variable diversity joining [V(D)J] recombination and class-switch recombination (CSR).

DNA damage and oxidative injury are associated with hypomyelination in the corpus callosum of newborn Nbn(CNS-del) mice

Nijmegen breakage syndrome (NBS), caused by mutation of the Nbn gene, is a recessive genetic disorder characterized by immunodeficiency, elevated sensitivity to ionizing radiation, chromosomal instability, microcephaly, and high predisposition to malignancies. To explore the underlying molecular mechanisms of NBS microcephaly, Frappart et al. previously inactivated Nbn gene in the central nervous system (CNS) of mice by the nestin-Cre targeting gene system and generated Nbn(CNS-del) mice. Here we first report that Nbn gene inactivation induces the defective proliferation and enhanced apoptosis of the oligodendrocyte precursor cells (OPCs), contributing to the severe hypomyelination of the nerve fibers of the corpus callosum. Under conditions of DNA damage and oxidative stress, the distinct regulatory roles of ATM-Chk2 signaling and AKT/mTOR signaling are responsible for the defective proliferation and enhanced apoptosis of the Nbn-deficient OPCs. In addition, specific HDAC isoforms may play distinctive roles in regulating the myelination of the Nbn-deficient OPCs. However, brain-derived neurotrophic factor and nerve growth factor stimulation attenuates the oxidative stress and thereby increases the proliferation of the Nbn-deficient OPCs, which is accompanied by upregulation of the AKT/mTOR/P70S6K signaling pathway. Taken together, these findings demonstrate that DNA damage and oxidative stress resulting from Nbn gene inactivation are associated with hypomyelination of the nerve fibers of corpus callosum.

Maintaining genome stability in the nervous system

Active maintenance of genome stability is a prerequisite for the development and function of the nervous system. The high replication index during neurogenesis and the long life of mature neurons highlight the need for efficient cellular programs to safeguard genetic fidelity. Multiple DNA damage response pathways ensure that replication stress and other types of DNA lesions, such as oxidative damage, do not affect neural homeostasis. Numerous human neurologic syndromes result from defective DNA damage signaling and compromised genome integrity. These syndromes can involve different neuropathology, which highlights the diverse maintenance roles that are required for genome stability in the nervous system. Understanding how DNA damage signaling pathways promote neural development and preserve homeostasis is essential for understanding fundamental brain function.

A SUMO-dependent interaction between Senataxin and the exosome, disrupted in the neurodegenerative disease AOA2, targets the exosome to sites of transcription-induced DNA damage

Senataxin (SETX) is an RNA/DNA helicase implicated in transcription termination and the DNA damage response and is mutated in two distinct neurological disorders: AOA2 (ataxia oculomotor apraxia 2) and ALS4 (amyotrophic lateral sclerosis 4). Here we provide evidence that Rrp45, a subunit of the exosome, associates with SETX in a manner dependent on SETX sumoylation. We show that the interaction and SETX sumoylation are disrupted by SETX mutations associated with AOA2 but not ALS4. Furthermore, Rrp45 colocalizes with SETX in distinct foci upon induction of transcription-related DNA damage. Our results thus provide evidence for a SUMO-dependent interaction between SETX and the exosome, disrupted in AOA2, that targets the exosome to sites of DNA damage.

The need to accessorize: molecular roles of HTLV-1 p30 and HTLV-2 p28 accessory proteins in the viral life cycle

Extensive studies of human T-cell leukemia virus (HTLV)-1 and HTLV-2 over the last three decades have provided detailed knowledge on viral transformation, host–viral interactions and pathogenesis. HTLV-1 is the etiological agent of adult T cell leukemia and multiple neurodegenerative and inflammatory diseases while HTLV-2 disease association remains elusive, with few infected individuals displaying neurodegenerative diseases similar to HTLV-1. The HTLV group of oncoretroviruses has a genome that encodes structural and enzymatic proteins Gag, Pro, and Env, regulatory proteins Tax and Rex, and several accessory proteins from the pX region. Of these proteins, HTLV-1 p30 and HTLV-2 p28 are encoded by the open reading frame II of the pX region. Like most other accessory proteins, p30 and p28 are dispensable for in vitro viral replication and transformation but are required for efficient viral replication and persistence in vivo. Both p30 and p28 regulate viral gene expression at the post-transcriptional level whereas p30 can also function at the transcriptional level. Recently, several reports have implicated p30 and p28 in multiple cellular processes, which provide novel insight into HTLV spread and survival and ultimately pathogenesis. In this review we summarize and compare what is known about p30 and p28, highlighting their roles in viral replication and viral pathogenesis.

Interaction of FUS and HDAC1 regulates DNA damage response and repair in neurons

Defects in DNA repair have been extensively linked to neurodegenerative diseases, but the exact mechanisms remain poorly understood. We found that FUS, an RNA/DNA-binding protein that has been linked to amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration, is important for the DNA damage response (DDR). The function of FUS in DDR involved a direct interaction with histone deacetylase 1 (HDAC1), and the recruitment of FUS to double-stranded break sites was important for proper DDR signaling. Notably, FUS proteins carrying familial ALS mutations were defective in DDR and DNA repair and showed a diminished interaction with HDAC1. Moreover, we observed increased DNA damage in human ALS patients harboring FUS mutations. Our findings suggest that an impaired DDR and DNA repair may contribute to the pathogenesis of neurodegenerative diseases linked to FUS mutations.

The distinct signaling regulatory roles in the cortical atrophy and cerebellar apoptosis of newborn Nbn-deficient mice

Human Nijmegen breakage syndrome, caused by the hypomorphic mutation of Nbn gene, is a hereditary instability disease, characterized by chromosomal instability, immunodeficiency, radiosensitivity, cancer predisposition and microcephaly. To study the roles of Nbn protein in microcephaly, Nbn gene was specifically deleted in the central nervous system of mice by nestin-Cre targeting gene system (Frappart et al. in Nat Med 11:538-544, 2005). Strikingly, newborn Nbn-deficient mice exhibit the evident microcephalic cerebellum, which contributes to severe ataxia and balance deficiency. In this study, we first report that PI3K/AKT/mTOR signaling pathway that performs neurotrophic-protecting role in neuronal growth is significantly inhibited in newborn Nbn-deficient cortex and cerebellum. In addition, JNK signaling and ATR signaling are likely to converge to regulate the cerebellar apoptosis of newborn Nbn-deficient mice.

Nbn and atm cooperate in a tissue and developmental stage-specific manner to prevent double strand breaks and apoptosis in developing brain and eye

Nibrin (NBN or NBS1) and ATM are key factors for DNA Double Strand Break (DSB) signaling and repair. Mutations in NBN or ATM result in Nijmegen Breakage Syndrome and Ataxia telangiectasia. These syndromes share common features such as radiosensitivity, neurological developmental defects and cancer predisposition. However, the functional synergy of Nbn and Atm in different tissues and developmental stages is not yet understood. Here, we show in vivo consequences of conditional inactivation of both genes in neural stem/progenitor cells using Nestin-Cre mice. Genetic inactivation of Atm in the central nervous system of Nbn-deficient mice led to reduced life span and increased DSBs, resulting in increased apoptosis during neural development. Surprisingly, the increase of DSBs and apoptosis was found only in few tissues including cerebellum, ganglionic eminences and lens. In sharp contrast, we showed that apoptosis associated with Nbn deletion was prevented by simultaneous inactivation of Atm in developing retina. Therefore, we propose that Nbn and Atm collaborate to prevent DSB accumulation and apoptosis during development in a tissue- and developmental stage-specific manner.

Fate mapping for activation-induced cytidine deaminase (AID) marks non-lymphoid cells during mouse development

The Aicda gene encodes Activation-Induced cytidine Deaminase (AID), an enzyme essential for remodeling antibody genes in mature B lymphocytes. AID is also responsible for DNA damage at oncogenes, leading to their mutation and cancer-associated chromosome translocation in lymphoma. We used fate mapping and AID^GFP reporter mice to determine if AID expression in the mouse extends beyond lymphocytes. We discovered that AID^cre tags a small fraction of non-lymphoid cells starting at 10.5 days post conception (dpc), and that AID^GFP+ cells are detectable at dpc 11.5 and 12.5. Embryonic cells are tagged by AID^cre in the submandibular region, where conditional deletion of the tumor suppressor PTEN causes squamous papillomas. AID^cre also tags non-lymphoid cells in the embryonic central nervous system. Finally, in the adult mouse brain, AID^cre marks a small fraction of diverse neurons and distinct neuronal populations, including pyramidal cells in cortical layer IV.

Genetic causes of microcephaly and lessons for neuronal development

The study of human developmental microcephaly is providing important insights into brain development. It has become clear that developmental microcephalies are associated with abnormalities in cellular production, and that the pathophysiology of microcephaly provides remarkable insights into how the brain generates the proper number of neurons that determine brain size. Most of the genetic causes of 'primary' developmental microcephaly (i.e., not associated with other syndromic features) are associated with centrosomal abnormalities. In addition to other functions, centrosomal proteins control the mitotic spindle, which is essential for normal cell proliferation during mitosis. However, the brain is often uniquely affected when microcephaly genes are mutated implying special centrosomal-related functions in neuronal production. Although models explaining how this could occur have some compelling data, they are not without controversy. Interestingly, some of the microcephaly genes show evidence that they were targets of evolutionary selection in primates and human ancestors, suggesting potential evolutionary roles in controlling neuronal number and brain volume across species. Mutations in DNA repair pathway genes also lead to microcephaly. Double-stranded DNA breaks appear to be a prominent type of damage that needs to be repaired during brain development, yet why defects in DNA repair affect the brain preferentially and if DNA repair relates to centrosome function, are not clearly understood.

Brain and induced pluripotent stem cell-derived neural stem cells as an in vitro model of neurodegeneration in ataxia-telangiectasia

The ataxia telangiectasia mutated (ATM) kinase is a key transducer of the cellular response to DNA double strand breaks and its deficiency causes ataxia-telangiectasia (A-T), a pleiotropic genetic disorder primarily characterized by cerebellar neuropathy, immunodeficiency and cancer predisposition. While enormous progress has been achieved in elucidating the biochemical and functional regulation of ATM in DNA damage response, and more recently in redox signalling and antioxidant defence, the factors that make neurons in A-T extremely vulnerable remain unclear. Given also that ATM knockout mice do not recapitulate the central nervous system phenotype, a number of human neural stem cell (hNSC) model systems have been developed to provide insights into the mechanisms of neurodegeneration associated with ATM dysfunction. Here we review the hNSC systems developed by us an others to model A-T.

Sen1p contributes to genomic integrity by regulating expression of ribonucleotide reductase 1 (RNR1) in Saccharomyces cerevisiae

Gene expression is a multi-step process which requires recruitment of several factors to promoters. One of the factors, Sen1p is an RNA/DNA helicase implicated in transcriptional termination and RNA processing in yeast. In the present study, we have identified a novel function of Sen1p that regulates the expression of ribonucleotide reductase RNR1 gene, which is essential for maintaining genomic integrity. Cells with mutation in the helicase domain or lacking N-terminal domain of Sen1p displayed a drastic decrease in the basal level transcription of RNR1 gene and showed enhanced sensitivity to various DNA damaging agents. Moreover, SEN1 mutants [Sen1-1 (G1747D), Sen1-2 (Δ1-975)] exhibited defects in DNA damage checkpoint activation. Surprisingly, CRT1 deletion in Sen1p mutants (Sen1-1, Sen1-2) was partly able to rescue the slow growth phenotype upon genotoxic stress. Altogether, our observations suggest that Sen1p is required for cell protection against DNA damage by regulating the expression of DNA repair gene RNR1. Thus, the misregulation of Sen1p regulated genes can cause genomic instability that may lead to neurological disorders and premature aging.

ATM-deficient human neural stem cells as an in vitro model system to study neurodegeneration

Loss of ATM kinase, a transducer of the DNA damage response and redox sensor, causes the neurodegenerative disorder ataxia-telangiectasia (A-T). While a great deal of progress has been made in elucidating the ATM-dependent DNA damage response (DDR) network, a key challenge remains in understanding the selective susceptibility of the nervous system to faulty DDR. Several factors appear implicated in the neurodegenerative phenotype in A-T, but which of them plays a crucial role remains unclear, especially since mouse models of A-T do not fully mirror the respective human syndrome. Therefore, a number of human neural stem cell (hNSC) systems have been developed to get an insight into the molecular mechanisms of neurodegeneration as consequence of ATM inactivation. Here we review the hNSC systems developed by us an others to model A-T.

DNA repair mechanisms in dividing and non-dividing cells

DNA damage created by endogenous or exogenous genotoxic agents can exist in multiple forms, and if allowed to persist, can promote genome instability and directly lead to various human diseases, particularly cancer, neurological abnormalities, immunodeficiency and premature aging. To avoid such deleterious outcomes, cells have evolved an array of DNA repair pathways, which carry out what is typically a multiple-step process to resolve specific DNA lesions and maintain genome integrity. To fully appreciate the biological contributions of the different DNA repair systems, one must keep in mind the cellular context within which they operate. For example, the human body is composed of non-dividing and dividing cell types, including, in the brain, neurons and glial cells. We describe herein the molecular mechanisms of the different DNA repair pathways, and review their roles in non-dividing and dividing cells, with an eye toward how these pathways may regulate the development of neurological disease.

Somatic alpha-synuclein mutations in Parkinson's disease: hypothesis and preliminary data

Alpha-synuclein (SNCA) is crucial in the pathogenesis of Parkinson's disease (PD), yet mutations in the SNCA gene are rare. Evidence for somatic genetic variation in normal humans, also involving the brain, is increasing, but its role in disease is unknown. Somatic SNCA mutations, arising in early development and leading to mosaicism, could contribute to PD pathogenesis and yet be absent or undetectable in DNA derived from peripheral lymphocytes. Such mutations could underlie the widespread pathology in PD, with the precise clinical outcome dependent on their type and the timing and location of their occurrence. We recently reported a novel SNCA mutation (c.150T>G, p.H50Q) in PD brain-derived DNA. To determine if there was mosaicism for this, a PCR and cloning strategy was used to take advantage of a nearby heterozygous intronic polymorphism. No evidence of mosaicism was found. High-resolution melting curve analysis of SNCA coding exons, which was shown to be sensitive enough to detect low proportions of 2 known mutations, did not reveal any further mutations in DNA from 28 PD brain-derived samples. We outline the grounds that make the somatic SNCA mutation hypothesis consistent with genetic, embryological, and pathological data. Further studies of brain-derived DNA are warranted and should include DNA from multiple regions and methods for detecting other types of genomic variation. © 2013 Movement Disorder Society

TET proteins: on the frenetic hunt for new cytosine modifications

Epigenetic genome marking and chromatin regulation are central to establishing tissue-specific gene expression programs, and hence to several biological processes. Until recently, the only known epigenetic mark on DNA in mammals was 5-methylcytosine, established and propagated by DNA methyltransferases and generally associated with gene repression. All of a sudden, a host of new actors—novel cytosine modifications and the ten eleven translocation (TET) enzymes—has appeared on the scene, sparking great interest. The challenge is now to uncover the roles they play and how they relate to DNA demethylation. Knowledge is accumulating at a frantic pace, linking these new players to essential biological processes (e.g. cell pluripotency and development) and also to cancerogenesis. Here, we review the recent progress in this exciting field, highlighting the TET enzymes as epigenetic DNA modifiers, their physiological roles, and their functions in health and disease. We also discuss the need to find relevant TET interactants and the newly discovered TET–O-linked N-acetylglucosamine transferase (OGT) pathway.

Aag DNA glycosylase promotes alkylation-induced tissue damage mediated by Parp1

Alkylating agents comprise a major class of front-line cancer chemotherapeutic compounds, and while these agents effectively kill tumor cells, they also damage healthy tissues. Although base excision repair (BER) is essential in repairing DNA alkylation damage, under certain conditions, initiation of BER can be detrimental. Here we illustrate that the alkyladenine DNA glycosylase (AAG) mediates alkylation-induced tissue damage and whole-animal lethality following exposure to alkylating agents. Aag-dependent tissue damage, as observed in cerebellar granule cells, splenocytes, thymocytes, bone marrow cells, pancreatic β-cells, and retinal photoreceptor cells, was detected in wild-type mice, exacerbated in Aag transgenic mice, and completely suppressed in Aag^−/− mice. Additional genetic experiments dissected the effects of modulating both BER and Parp1 on alkylation sensitivity in mice and determined that Aag acts upstream of Parp1 in alkylation-induced tissue damage; in fact, cytotoxicity in WT and Aag transgenic mice was abrogated in the absence of Parp1. These results provide in vivo evidence that Aag-initiated BER may play a critical role in determining the side-effects of alkylating agent chemotherapies and that Parp1 plays a crucial role in Aag-mediated tissue damage.

Alkylating agents are genotoxic chemicals that induce both toxic and mutagenic DNA damage through addition of an alkyl group to DNA. Alkylating agents are routinely and successfully used as chemotherapeutic therapies for cancer patients, with one major disadvantage being the significant toxicity induced in non-tumor tissues. Accordingly, identifying factors that modify susceptibility to alkylation-induced toxicity will provide valuable information in designing cancer therapeutic regimens. This study used mouse genetic experiments to investigate whether proteins important in the base excision repair pathway modulate susceptibility to alkylating agents. In addition to whole-animal toxicity at high doses, treatment of mice with alkylating agents resulted in severe damage to numerous tissues including the cerebellum, retina, bone marrow, spleen, thymus, and the pancreas. We illustrate that the DNA glycosylase Aag can actually confer, rather than prevent, alkylation sensitivity at both the whole-animal and tissue level; i.e., Aag transgenic animals are more susceptible than wild type, whereas Aag-deficient animals are less susceptible than wild type to alkylation-induced toxicity. Further genetic experiments show that the Aag-mediated alkylation sensitivity is dependent on Parp1. Given that we observe a wide range of human AAG expression among healthy individuals, this and other base excision repair proteins may be important factors modulating alkylation susceptibility.

Double-strand break repair by homologous recombination in primary mouse somatic cells requires BRCA1 but not the ATM kinase

Homology-directed repair (HDR) is a critical pathway for the repair of DNA double-strand breaks (DSBs) in mammalian cells. Efficient HDR is thought to be crucial for maintenance of genomic integrity during organismal development and tumor suppression. However, most mammalian HDR studies have focused on transformed and immortalized cell lines. We report here the generation of a Direct Repeat (DR)-GFP reporter-based mouse model to study HDR in primary cell types derived from diverse lineages. Embryonic and adult fibroblasts from these mice as well as cells derived from mammary epithelium, ovary, and neonatal brain were observed to undergo HDR at I-SceI endonuclease-induced DSBs at similar frequencies. When the DR-GFP reporter was crossed into mice carrying a hypomorphic mutation in the breast cancer susceptibility gene Brca1, a significant reduction in HDR was detected, showing that BRCA1 is critical for HDR in somatic cell types. Consistent with an HDR defect, Brca1 mutant mice are highly sensitive to the cross-linking agent mitomycin C. By contrast, loss of the DSB signaling ataxia telangiectasia-mutated (ATM) kinase did not significantly alter HDR levels, indicating that ATM is dispensable for HDR. Notably, chemical inhibition of ATM interfered with HDR. The DR-GFP mouse provides a powerful tool for dissecting the genetic requirements of HDR in a diverse array of somatic cell types in a normal, nontransformed cellular milieu.

RIF1 is essential for 53BP1-dependent nonhomologous end joining and suppression of DNA double-strand break resection

The appropriate execution of DNA double-strand break (DSB) repair is critical for genome stability and tumor avoidance. 53BP1 and BRCA1 directly influence DSB repair pathway choice by regulating 5' end resection, but how this is achieved remains uncertain. Here we report that Rif1(-/-) mice are severely compromised for 53BP1-dependent class switch recombination (CSR) and fusion of dysfunctional telomeres. The inappropriate accumulation of RIF1 at DSBs in S phase is antagonized by BRCA1, and deletion of Rif1 suppresses toxic nonhomologous end joining (NHEJ) induced by PARP inhibition in Brca1-deficient cells. Mechanistically, RIF1 is recruited to DSBs via the N-terminal phospho-SQ/TQ domain of 53BP1, and DSBs generated by ionizing radiation or during CSR are hyperresected in the absence of RIF1. Thus, RIF1 and 53BP1 cooperate to block DSB resection to promote NHEJ in G1, which is antagonized by BRCA1 in S phase to ensure a switch of DSB repair mode to homologous recombination.

Effects of expression level of DNA repair-related genes involved in the NHEJ pathway on radiation-induced cognitive impairment

Cranial radiation therapy can induce cognitive decline. Impairments of hippocampal neurogenesis are thought to be a paramountly important mechanism underlying radiation-induced cognitive dysfunction. In the mature nervous system, DNA double-strand breaks (DSBs) are mainly repaired by non-homologous end-joining (NHEJ) pathways. It has been demonstrated that NHEJ deficiencies are associated with impaired neurogenesis. In our study, rats were randomly divided into five groups to be irradiated by single doses of 0 (control), 0 (anesthesia control), 2, 10, and 20 Gy, respectively. The cognitive function of the irradiated rats was measured by open field, Morris water maze and passive avoidance tests. Real-time PCR was also used to detect the expression level of DNA DSB repair-related genes involved in the NHEJ pathway, such as XRCC4, XRCC5and XRCC6, in the hippocampus. The influence of different radiation doses on cognitive function in rats was investigated. From the results of the behavior tests, we found that rats receiving 20 Gy irradiation revealed poorer learning and memory, while no significant loss of learning and memory existed in rats receiving irradiation from 0-10 Gy. The real-time PCR and Western blot results showed no significant difference in the expression level of DNA repair-related genes between the 10 and 20 Gy groups, which may help to explain the behavioral results, i.e. DNA damage caused by 0-10 Gy exposure was appropriately repaired, however, damage induced by 20 Gy exceeded the body's maximum DSB repair ability. Ionizing radiation-induced cognitive impairments depend on the radiation dose, and more directly on the body's own ability to repair DNA DSBs via the NHEJ pathway.

Senataxin, defective in the neurodegenerative disorder ataxia with oculomotor apraxia 2, lies at the interface of transcription and the DNA damage response

The neurodegenerative disorder ataxia with oculomotor apraxia 2 (AOA-2) is caused by defects in senataxin, a putative RNA/DNA helicase thought to be involved in the termination of transcription at RNA polymerase pause sites. RNA/DNA hybrids (R loops) that arise during transcription pausing lead to genome instability unless they are resolved efficiently. We found that senataxin forms distinct nuclear foci in S/G(2)-phase human cells and that the number of these foci increases in response to impaired DNA replication or DNA damage. Senataxin colocalizes with 53BP1, a key DNA damage response protein, and with other factors involved in DNA repair. Inhibition of transcription using α-amanitin, or the dissolution of R loops by transient expression of RNase H1, leads to the loss of senataxin foci. These results indicate that senataxin localizes to sites of collision between components of the replisome and the transcription apparatus and that it is targeted to R loops, where it plays an important role at the interface of transcription and the DNA damage response.

Microcephaly

True microcephaly (head circumference ≤-3SD), either primary (present at birth) or secondary (of postnatal onset) results from an imbalance between progenitor cell production and cell death that lead to a reduced number of neuronal and glial cells within the brain, resulting in reduced brain growth. Primary non-syndromal microcephalies are recessive disorders resulting from abnormal control of mitotic spindle and cell cycle kinetics in progenitor cells. Microcephaly is also a frequent sign of defects in DNA double- and/or single-strand break repair and in nucleotide excision repair, in which it often is associated with general growth impairment. In these etiologies, cognitive functions are reasonably well preserved despite severe reduction in brain volume. Neuronal migration defects are often associated with secondary microcephaly, as are anomalies of telencephalic cleavage. Secondary microcephalies are often associated with increased neuronal death, and can be associated with metabolic disorders such as serine deficiency or thiamine pyrophosphate transporter deficiency. Microcephaly can be associated with hundreds of syndromal congenital anomalies, including many chromosomal disorders. Genetic etiologies of developmental microcephalies are reviewed.

Progressive cerebellar atrophy and polyneuropathy: expanding the spectrum of PNKP mutations

We present a neurodegenerative disorder starting in early childhood of two brothers consisting of severe progressive polyneuropathy, severe progressive cerebellar atrophy, microcephaly, mild epilepsy, and intellectual disability. The cause of this rare syndrome was found to be a homozygous mutation (c.1250_1266dup, resulting in a frameshift p.Thr424GlyfsX48) in PNKP, identified by applying homozygosity mapping and whole-genome sequencing. Mutations in PNKP have previously been associated with a syndrome of microcephaly, seizures and developmental delay (MIM 613402), but not with a neurodegenerative disorder. PNKP is a dual-function enzyme with a key role in different pathways of DNA damage repair. DNA repair disorders can result in accelerated cell death, leading to underdevelopment and neurodegeneration. In skin fibroblasts from both affected individuals, we show increased susceptibility to apoptosis under stress conditions and reduced PNKP expression. PNKP is known to interact with DNA repair proteins involved in the onset of polyneuropathy and cerebellar degeneration; therefore, our findings explain this novel phenotype.

Neurogenesis requires TopBP1 to prevent catastrophic replicative DNA damage in early progenitors

The rapid proliferation of progenitors during neurogenesis requires a stringent genomic maintenance program to ensure transmission of genetic fidelity. However the essential factors that govern neural progenitor genome integrity are unknown. Here we report that conditional inactivation of mouse TopBP1, a protein linked to DNA replication, and a key activator of the DNA damage response kinase ATR (ataxia telangiectasia and rad3-related) is critical for maintenance of early-born neural progenitors. During cortical development TopBP1 prevented replication-associated DNA damage in Emx1-progenitors which otherwise resulted in profound tissue ablation. Notably, disrupted neurogenesis in TopBP1-depleted tissues was substantially rescued by inactivation of p53 but not of ATM. Our data establish that TopBP1 is essential for preventing replication-associated DNA strand breaks, but is not essential per se for DNA replication. Thus, TopBP1 is crucial for maintaining genome integrity in the early progenitors that drive neurogenesis.

Ccdc94 protects cells from ionizing radiation by inhibiting the expression of p53

DNA double-strand breaks (DSBs) represent one of the most deleterious forms of DNA damage to a cell. In cancer therapy, induction of cell death by DNA DSBs by ionizing radiation (IR) and certain chemotherapies is thought to mediate the successful elimination of cancer cells. However, cancer cells often evolve to evade the cytotoxicity induced by DNA DSBs, thereby forming the basis for treatment resistance. As such, a better understanding of the DSB DNA damage response (DSB–DDR) pathway will facilitate the design of more effective strategies to overcome chemo- and radioresistance. To identify novel mechanisms that protect cells from the cytotoxic effects of DNA DSBs, we performed a forward genetic screen in zebrafish for recessive mutations that enhance the IR–induced apoptotic response. Here, we describe radiosensitizing mutation 7 (rs7), which causes a severe sensitivity of zebrafish embryonic neurons to IR–induced apoptosis and is required for the proper development of the central nervous system. The rs7 mutation disrupts the coding sequence of ccdc94, a highly conserved gene that has no previous links to the DSB–DDR pathway. We demonstrate that Ccdc94 is a functional member of the Prp19 complex and that genetic knockdown of core members of this complex causes increased sensitivity to IR–induced apoptosis. We further show that Ccdc94 and the Prp19 complex protect cells from IR–induced apoptosis by repressing the expression of p53 mRNA. In summary, we have identified a new gene regulating a dosage-sensitive response to DNA DSBs during embryonic development. Future studies in human cancer cells will determine whether pharmacological inactivation of CCDC94 reduces the threshold of the cancer cell apoptotic response.

Radiation therapy and most chemotherapies elicit cancer cell death through the induction of excessive DNA damage. However, cancer cells can harbor genetic defects that confer resistance to these therapies. To identify cellular components whose targeted therapeutic inactivation could potentially enhance the sensitivity of treatment-resistant cancer cells to DNA–damaging therapies, we have chosen an unbiased genetic approach in live whole zebrafish embryos to identify genes that normally protect cells from the lethal effects of DNA damage. This approach has yielded the discovery of a novel radioprotective gene called ccdc94. Upon inactivation of ccdc94, cells become more sensitive to radiation-induced cell death. Our further analysis revealed that the Ccdc94 protein functions in the Prp19 complex, which is known to regulate gene expression and repair of damaged DNA. We found that this complex normally represses radiation-induced cell death by inhibiting the expression of the p53 gene, a critical mediator of DNA damage–induced cell death. Future experiments that inactivate Ccdc94 and Prp19 complex proteins in human cancer cells will determine if inactivation of this complex represents a novel therapeutic strategy that could increase p53 expression to enhance sensitivity to DNA damaging therapies in chemo- and radio-resistant cancer cells.

Playing the end game: DNA double-strand break repair pathway choice

DNA double-strand breaks (DSBs) are highly toxic lesions that can drive genetic instability. To preserve genome integrity, organisms have evolved several DSB repair mechanisms, of which nonhomologous end-joining (NHEJ) and homologous recombination (HR) represent the two most prominent. It has recently become apparent that multiple layers of regulation exist to ensure these repair pathways are accurate and restricted to the appropriate cellular contexts. Such regulation is crucial, as failure to properly execute DSB repair is known to accelerate tumorigenesis and is associated with several human genetic syndromes. Here, we review recent insights into the mechanisms that influence the choice between competing DSB repair pathways, how this is regulated during the cell cycle, and how imbalances in this equilibrium result in genome instability.

JNK Activation by Up-Regulation of iNOS on Cholesterol Accumulation Limits Neurogenesis and Induces Region-Specific DNA Damage Responses in the Subventricular Zone of NPC Mice

Abstract Aims: We explore the region-specific impact of nitric oxide (NO) on adult neural stem cell (aNSC) niches with regard to neurogenesis and NSC damage and investigate the underlying mechanisms in Niemann-Pick disease type C (NPC) mice. Results: Among the two anatomical stem-cell niches of the brain, subventricular zone (SVZ)-derived aNSCs enhanced c-Jun N-terminal kinase (JNK) activity because of excessive NO production by the cholesterol accumulation. Activated JNK interacts with γH2AX, a marker for DNA damage; however, almost none of the aNSCs in the dentate gyrus (DG) showed either JNK signaling activation or abundant DNA damage. SVZ-derived aNSCs were protected from DNA damage by the treatment of Nω-nitro-L-arginine methyl ester (L-NAME), a NO synthase (NOS) inhibitor, both in vitro and in vivo. We also observed that U18666A, an inducer of cholesterol accumulation, increased inducible NOS expression, JNK activation, and DNA damage in the wild type (WT)-aNSCs. Interestingly, we found that endogenous cholesterol efflux transporters and their regulator were less activated in the SVZ than in the DG, in both WT and NPC mice. This result explains the high vulnerability of SVZ-derived aNSCs to the cholesterol imbalance as observed in NPC mice. Innovation and Conclusion: In this study, we demonstrated that the SVZ-derived aNSCs might be major targets of NPC. Significantly, aNSCs showed different responses depending on their anatomical origins due to dissimilarities in their cholesterol transporting system and NO-dependent JNK activation. These findings can contribute to the understanding of the region-specific nature of the two SVZ and DG neurogenic niches. Antioxid. Redox Signal. 00, 000-000.

Mammalian neurogenesis requires Treacle-Plk1 for precise control of spindle orientation, mitotic progression, and maintenance of neural progenitor cells

The cerebral cortex is a specialized region of the brain that processes cognitive, motor, somatosensory, auditory, and visual functions. Its characteristic architecture and size is dependent upon the number of neurons generated during embryogenesis and has been postulated to be governed by symmetric versus asymmetric cell divisions, which mediate the balance between progenitor cell maintenance and neuron differentiation, respectively. The mechanistic importance of spindle orientation remains controversial, hence there is considerable interest in understanding how neural progenitor cell mitosis is controlled during neurogenesis. We discovered that Treacle, which is encoded by the Tcof1 gene, is a novel centrosome- and kinetochore-associated protein that is critical for spindle fidelity and mitotic progression. Tcof1/Treacle loss-of-function disrupts spindle orientation and cell cycle progression, which perturbs the maintenance, proliferation, and localization of neural progenitors during cortical neurogenesis. Consistent with this, Tcof1(+/-) mice exhibit reduced brain size as a consequence of defects in neural progenitor maintenance. We determined that Treacle elicits its effect via a direct interaction with Polo-like kinase1 (Plk1), and furthermore we discovered novel in vivo roles for Plk1 in governing mitotic progression and spindle orientation in the developing mammalian cortex. Increased asymmetric cell division, however, did not promote increased neuronal differentiation. Collectively our research has therefore identified Treacle and Plk1 as novel in vivo regulators of spindle fidelity, mitotic progression, and proliferation in the maintenance and localization of neural progenitor cells. Together, Treacle and Plk1 are critically required for proper cortical neurogenesis, which has important implications in the regulation of mammalian brain size and the pathogenesis of congenital neurodevelopmental disorders such as microcephaly.

Novel telomerase-increasing compound in mouse brain delays the onset of amyotrophic lateral sclerosis

Telomerase is expressed in the neonatal brain, in distinct regions of adult brain, and was shown to protect developing neurons from apoptosis. Telomerase reactivation by gene manipulation reverses neurodegeneration in aged telomerase-deficient mice. Hence, we and others hypothesized that increasing telomerase expression by pharmaceutical compounds may protect brain cells from death caused by damaging agents. In this study, we demonstrate for the first time that the novel compound AGS-499 increases telomerase activity and expression in the mouse brain and spinal cord (SC). It exerts neuroprotective effects in NMDA-injected CD-1 mice, delays the onset and progression of the amyotrophic lateral sclerosis (ALS) disease in SOD1 transgenic mice, and, after the onset of ALS, it increases the survival of motor neurons in the SC by 60%. The survival of telomerase-expressing cells (i.e. motor neurons), but not telomerase-deficient cells, exposed to oxidative stress was increased by AGS-499 treatment, suggesting that the AGS-499 effects are telomerase-mediated. Therefore, a controlled and transient increase in telomerase expression and activity in the brain by AGS-499 may exert neuroprotective effects.

Requirement of mouse BCCIP for neural development and progenitor proliferation

Multiple DNA repair pathways are involved in the orderly development of neural systems at distinct stages. The homologous recombination (HR) pathway is required to resolve stalled replication forks and critical for the proliferation of progenitor cells during neural development. BCCIP is a BRCA2 and CDKN1A interacting protein implicated in HR and inhibition of DNA replication stress. In this study, we determined the role of BCCIP in neural development using a conditional BCCIP knock-down mouse model. BCCIP deficiency impaired embryonic and postnatal neural development, causing severe ataxia, cerebral and cerebellar defects, and microcephaly. These development defects are associated with spontaneous DNA damage and subsequent cell death in the proliferative cell populations of the neural system during embryogenesis. With in vitro neural spheroid cultures, BCCIP deficiency impaired neural progenitor's self-renewal capability, and spontaneously activated p53. These data suggest that BCCIP and its anti-replication stress functions are essential for normal neural development by maintaining an orderly proliferation of neural progenitors.

ATR maintains select progenitors during nervous system development

The ATR (ATM (ataxia telangiectasia mutated) and rad3-related) checkpoint kinase is considered critical for signalling DNA replication stress and its dysfunction can lead to the neurodevelopmental disorder, ATR-Seckel syndrome. To understand how ATR functions during neurogenesis, we conditionally deleted Atr broadly throughout the murine nervous system, or in a restricted manner in the dorsal telencephalon. Unexpectedly, in both scenarios, Atr loss impacted neurogenesis relatively late during neural development involving only certain progenitor populations. Whereas the Atr-deficient embryonic cerebellar external germinal layer underwent p53- (and p16(Ink4a/Arf))-independent proliferation arrest, other brain regions suffered apoptosis that was partially p53 dependent. In contrast to other organs, in the nervous system, p53 loss did not worsen the outcome of Atr inactivation. Coincident inactivation of Atm also did not affect the phenotype after Atr deletion, supporting non-overlapping physiological roles for these related DNA damage-response kinases in the brain. Rather than an essential general role in preventing replication stress, our data indicate that ATR functions to monitor genomic integrity in a selective spatiotemporal manner during neurogenesis.