ATM function and telomere stability

Close Ties between the Nuclear Envelope and Mammalian Telomeres: Give Me Shelter

The nuclear envelope (NE) in eukaryotic cells is essential to provide a protective compartment for the genome. Beside its role in connecting the nucleus with the cytoplasm, the NE has numerous important functions including chromatin organization, DNA replication and repair. NE alterations have been linked to different human diseases, such as laminopathies, and are a hallmark of cancer cells. Telomeres, the ends of eukaryotic chromosomes, are crucial for preserving genome stability. Their maintenance involves specific telomeric proteins, repair proteins and several additional factors, including NE proteins. Links between telomere maintenance and the NE have been well established in yeast, in which telomere tethering to the NE is critical for their preservation and beyond. For a long time, in mammalian cells, except during meiosis, telomeres were thought to be randomly localized throughout the nucleus, but recent advances have uncovered close ties between mammalian telomeres and the NE that play important roles for maintaining genome integrity. In this review, we will summarize these connections, with a special focus on telomere dynamics and the nuclear lamina, one of the main NE components, and discuss the evolutionary conservation of these mechanisms.

Cellular Senescence as a Brake or Accelerator for Oncogenic Transformation and Role in Lymphatic Metastasis

Cellular senescence-the irreversible cell cycle arrest driven by a variety of mechanisms and, more specifically, the senescence-associated secretory phenotype (SASP)-is an important area of research in the context of different age-related diseases, such as cardiovascular disease and cancer. SASP factors play both beneficial and detrimental roles in age-related disease progression depending on the source of the SASPs, the target cells, and the microenvironment. The impact of senescence and the SASP on different cell types, the immune system, and the vascular system has been widely discussed. However, the impact of replicative or stress-induced senescence on lymphatic biology and pathological lymphangiogenesis remains underexplored. The lymphatic system plays a crucial role in the maintenance of body fluid homeostasis and immune surveillance. The perturbation of lymphatic function can hamper normal physiological function. Natural aging or stress-induced premature aging influences the lymphatic vessel structure and function, which significantly affect the role of lymphatics in tumor dissemination and metastasis. In this review, we focus on the role of senescence on lymphatic pathobiology, its impact on cancer, and potential therapeutic interventions to manipulate the aged or senescent lymphatic system for disease management.

Oxidative Damage Induced Telomere Mediated Genomic Instability in Cells from Ataxia Telangiectasia Patients

Our cellular genome is susceptible to cytotoxic lesions which include single strand breaks and double strand breaks among other lesions. Ataxia telangiectasia mutated (ATM) protein was one of the first DNA damage sensor proteins to be discovered as being involved in DNA repair and as well as in telomere maintenance. Telomeres help maintain the stability of our chromosomes by protecting the ends from degradation. Cells from ataxia telangiectasia (AT) patients lack ATM and accumulate chromosomal alterations. AT patients display heightened susceptibility to cancer. In this study, cells from AT patients (called as AT -/- and AT +/- cells) were characterized for genome stability status and it was observed that AT -/- cells show considerable telomere attrition. Furthermore, DNA damage and genomic instability were compared between normal (AT +/+ cells) and AT -/- cells exhibiting increased frequencies of spontaneous DNA damage and genomic instability markers. Both AT -/- and AT +/- cells were sensitive to sodium arsenite (1.5 and 3.0 μg/ml) and ionizing radiation-induced (2 Gy, gamma rays) oxidative stress. Interestingly, telomeric fragments were detected in the comet tails as revealed by comet-fluorescence in situ hybridization analysis, suggestive of telomeric instability in AT -/- cells upon exposure to sodium arsenite or radiation. Besides, there was an increase in the number of chromosome alterations in AT -/- cells following arsenite treatment or irradiation. In addition, complex chromosome aberrations were detected by multicolor fluorescence in situ hybridization in AT -/- cells in comparison to AT +/- and normal cells. Telomere attrition and chromosome alterations were detected even at lower doses of sodium arsenite. Peptide nucleic acid - FISH analysis revealed defective chromosome segregation in cells lacking ATM proteins. The data obtained in this study substantiates the role of ATM in telomere stability under oxidative stress.

Discovery of Therapeutics Targeting Oxidative Stress in Autosomal Recessive Cerebellar Ataxia: A Systematic Review

Autosomal recessive cerebellar ataxias (ARCAs) are a heterogeneous group of rare neurodegenerative inherited disorders. The resulting motor incoordination and progressive functional disabilities lead to reduced lifespan. There is currently no cure for ARCAs, likely attributed to the lack of understanding of the multifaceted roles of antioxidant defense and the underlying mechanisms. This systematic review aims to evaluate the extant literature on the current developments of therapeutic strategies that target oxidative stress for the management of ARCAs. We searched PubMed, Web of Science, and Science Direct Scopus for relevant peer-reviewed articles published from 1 January 2016 onwards. A total of 28 preclinical studies fulfilled the eligibility criteria for inclusion in this systematic review. We first evaluated the altered cellular processes, abnormal signaling cascades, and disrupted protein quality control underlying the pathogenesis of ARCA. We then examined the current potential therapeutic strategies for ARCAs, including aromatic, organic and pharmacological compounds, gene therapy, natural products, and nanotechnology, as well as their associated antioxidant pathways and modes of action. We then discussed their potential as antioxidant therapeutics for ARCAs, with the long-term view toward their possible translation to clinical practice. In conclusion, our current understanding is that these antioxidant therapies show promise in improving or halting the progression of ARCAs. Tailoring the therapies to specific disease stages could greatly facilitate the management of ARCAs.

iTRAQ-based proteomic analysis reveals potential osteogenesis-promoted role of ATM in strontium-incorporated titanium implant

The present study aims to reveal the osteogenic roles played by DNA damage response biomarkers through implementing isobaric tags for relative and absolute quantitation (iTRAQ) technique. First, sandblasted large-grit double acid-etched (SLA) titanium implant and strontium-incorporated (SLA-Sr) titanium implant were used for inserting in the tibiae of rats. iTRAQ technique was used to detect protein expression changes and identify differentially expressed proteins (DEPs). In total, 19,343 peptides and 4280 proteins were screened out. Among them, 91 and 138 DEPs were identified in the SLA-Sr group after implantation for 3 and 7 days, respectively. Ataxia-telangiectasia mutated (ATM) protein up-regulated on the 3rd day showed a trend of further up-regulation on the 7th day. Moreover, functional enrichment analyses were also conducted to explore the biological function of DEPs during the initial stage of osseointegration in vivo, which revealed that the biological functions of the DEPs on the 7th day were mainly related to "mismatch repair" and "mitotic G1 DNA damage checkpoint." Analysis of the Reactome signaling pathway showed that ATM was associated with TP53's regulation and activation. Finally, DNA damage repair related genes were selected for validation at mRNA and protein expression levels. Real-time reverse transcription-polymerase chain reaction and immunohistochemistry validation results demonstrated that mRNA expression level of ATM was higher in SLA-Sr group. In conclusion, SLA-Sr titanium implant could initiate DNA damage repair by activating expression levels of ATM. This study was striving to reveal new faces of better osseointegration and shedding light on the biological function and underlying mechanisms of this important procedure.

Dysfunction of cerebellar microglia in Ataxia-telangiectasia

Ataxia-telangiectasia (A-T) is a multisystem autosomal recessive disease caused by mutations in the ATM gene and characterized by cerebellar atrophy, progressive ataxia, immunodeficiency, male and female sterility, radiosensitivity, cancer predisposition, growth retardation, insulin-resistant diabetes, and premature aging. ATM phosphorylates more than 1500 target proteins, which are involved in cell cycle control, DNA repair, apoptosis, modulation of chromatin structure, and other cytoplasmic as well as mitochondrial processes. In our quest to better understand the mechanisms by which ATM deficiency causes cerebellar degeneration, we hypothesized that specific vulnerabilities of cerebellar microglia underlie the etiology of A-T. Our hypothesis is based on the recent finding that dysfunction of glial cells affect a variety of process leading to impaired neuronal functionality (Song et al., 2019). Whereas astrocytes and neurons descend from the neural tube, microglia originate from the hematopoietic system, invade the brain at early embryonic stage, and become the innate immune cells of the central nervous system and important participants in development of synaptic plasticity. Here we demonstrate that microglia derived from Atm^-/- mouse cerebellum display accelerated cell migration and are severely impaired in phagocytosis, secretion of neurotrophic factors, and mitochondrial activity, suggestive of apoptotic processes. Interestingly, no microglial impairment was detected in Atm-deficient cerebral cortex, and Atm deficiency had less impact on astroglia than microglia. Collectively, our findings validate the roles of glial cells in cerebellar attrition in A-T.

Role of histone acetyltransferases MOF and Tip60 in genome stability

The accurate repair of DNA damage specifically the chromosomal double-strand breaks (DSBs) arising from exposure to physical or chemical agents, such as ionizing radiation (IR) and radiomimetic drugs is critical in maintaining genomic integrity. The DNA DSB response and repair is facilitated by hierarchical signaling networks that orchestrate chromatin structural changes specifically histone modifications which impact cell-cycle checkpoints through enzymatic activities to repair the broken DNA ends. Various histone posttranslational modifications such as phosphorylation, acetylation, methylation and ubiquitylation have been shown to play a role in DNA damage repair. Recent studies have provided important insights into the role of histone-specific modifications in sensing DNA damage and facilitating the DNA repair. Histone modifications have been shown to determine the pathway choice for repair of DNA DSBs. This review will summarize the role of important histone acetyltransferases MOF and Tip60 mediated acetylation in repair of DNA DSBs in eukaryotic cells.

Autism-Associated Vigilin Depletion Impairs DNA Damage Repair

Vigilin (Vgl1) is essential for heterochromatin formation, chromosome segregation, and mRNA stability and is associated with autism spectrum disorders and cancer: vigilin, for example, can suppress proto-oncogene c-fms expression in breast cancer. Conserved from yeast to humans, vigilin is an RNA-binding protein with 14 tandemly arranged nonidentical hnRNP K-type homology (KH) domains. Here, we report that vigilin depletion increased cell sensitivity to cisplatin- or ionizing radiation (IR)-induced cell death and genomic instability due to defective DNA repair. Vigilin depletion delayed dephosphorylation of IR-induced γ-H2AX and elevated levels of residual 53BP1 and RIF1 foci, while reducing Rad51 and BRCA1 focus formation, DNA end resection, and double-strand break (DSB) repair. We show that vigilin interacts with the DNA damage response (DDR) proteins RAD51 and BRCA1, and vigilin depletion impairs their recruitment to DSB sites. Transient hydroxyurea (HU)-induced replicative stress in vigilin-depleted cells increased replication fork stalling and blocked restart of DNA synthesis. Furthermore, histone acetylation promoted vigilin recruitment to DSBs preferentially in the transcriptionally active genome. These findings uncover a novel vigilin role in DNA damage repair with implications for autism and cancer-related disorders.

Chk1 promotes non-homologous end joining in G1 through direct phosphorylation of ASF1A

The cell-cycle phase is a major determinant of repair pathway choice at DNA double strand breaks, non-homologous end joining (NHEJ), or homologous recombination (HR). Chk1 responds to genotoxic stress in S/G2 phase, but here, we report a role of Chk1 in directly promoting NHEJ repair in G1 phase. ASF1A is a histone chaperone, but it promotes NHEJ through a pathway independent of its histone-chaperone activity. Chk1 activated by ataxia telangiectasia mutated (ATM) kinase on DNA breaks in G1 promotes NHEJ through direct phosphorylation of ASF1A at Ser-166. ASF1A phosphorylated at Ser-166 interacts with the repair protein MDC1 and thus enhances MDC1's interaction with ATM and the stable localization of ATM at DNA breaks. Chk1 deficiency suppresses all steps downstream of MDC1 following a DNA break in G1, namely histone ubiquitination, 53BP1 localization to the DNA break, and NHEJ. Thus, ASF1A phosphorylation by Chk1 is essential for DNA break repair by NHEJ in G1.

Exome Sequencing Analysis Identifies Rare Variants in ATM and RPL8 That Are Associated With Shorter Telomere Length

Telomeres are important for maintaining genomic stability. Telomere length has been associated with aging, disease, and mortality and is highly heritable (∼82%). In this study, we aimed to identify rare genetic variants associated with telomere length using whole-exome sequence data. We studied 1,303 participants of the Erasmus Rucphen Family (ERF) study, 1,259 of the Rotterdam Study (RS), and 674 of the British Heart Foundation Family Heart Study (BHF-FHS). We conducted two analyses, first we analyzed the family-based ERF study and used the RS and BHF-FHS for replication. Second, we combined the summary data of the three studies in a meta-analysis. Telomere length was measured by quantitative polymerase chain reaction in blood. We identified nine rare variants significantly associated with telomere length (p-value < 1.42 × 10^–7, minor allele frequency of 0.2–0.5%) in the ERF study. Eight of these variants (in C11orf65, ACAT1, NPAT, ATM, KDELC2, and EXPH5) were located on chromosome 11q22.3 that contains ATM, a gene involved in telomere maintenance. Although we were unable to replicate the variants in the RS and BHF-FHS (p-value ≥ 0.21), segregation analysis showed that all variants segregate with shorter telomere length in a family. In the meta-analysis of all studies, a nominally significant association with LTL was observed with a rare variant in RPL8 (p-value = 1.48 × 10⁻⁶), which has previously been associated with age. Additionally, a novel rare variant in the known RTEL1 locus showed suggestive evidence for association (p-value = 1.18 × 10^–4) with LTL. To conclude, we identified novel rare variants associated with telomere length. Larger samples size are needed to confirm these findings and to identify additional variants.

Extrachromosomal Circular DNA: Current Knowledge and Implications for CNS Aging and Neurodegeneration

Still unresolved is the question of how a lifetime accumulation of somatic gene copy number alterations impact organ functionality and aging and age-related pathologies. Such an issue appears particularly relevant in the broadly post-mitotic central nervous system (CNS), where non-replicative neurons are restricted in DNA-repair choices and are prone to accumulate DNA damage, as they remain unreplaced over a lifetime. Both DNA injuries and consecutive DNA-repair strategies are processes that can evoke extrachromosomal circular DNA species, apparently from either part of the genome. Due to their capacity to amplify gene copies and related transcripts, the individual cellular load of extrachromosomal circular DNAs will contribute to a dynamic pool of additional coding and regulatory chromatin elements. Analogous to tumor tissues, where the mosaicism of circular DNAs plays a well-characterized role in oncogene plasticity and drug resistance, we suggest involvement of the "circulome" also in the CNS. Accordingly, we summarize current knowledge on the molecular biogenesis, homeostasis and gene regulatory impacts of circular extrachromosomal DNA and propose, in light of recent discoveries, a critical role in CNS aging and neurodegeneration. Future studies will elucidate the influence of individual extrachromosomal DNA species according to their sequence complexity and regional distribution or cell-type-specific abundance.

GSE4 peptide suppresses oxidative and telomere deficiencies in ataxia telangiectasia patient cells

Ataxia telangiectasia (AT) is a genetic disease caused by mutations in the ATM gene but the mechanisms underlying AT are not completely understood. Key functions of the ATM protein are to sense and regulate cellular redox status and to transduce DNA double-strand break signals to downstream effectors. ATM-deficient cells show increased ROS accumulation, activation of p38 protein kinase, and increased levels of DNA damage. GSE24.2 peptide and a short derivative GSE4 peptide corresponding to an internal domain of Dyskerin have proved to induce telomerase activity, decrease oxidative stress, and protect from DNA damage in dyskeratosis congenita (DC) cells. We have found that expression of GSE24.2 and GSE4 in human AT fibroblast is able to decrease DNA damage, detected by γ-H2A.X and 53BP1 foci. However, GSE24.2/GSE4 expression does not improve double-strand break signaling and repair caused by the lack of ATM activity. In contrast, they cause a decrease in 8-oxoguanine and OGG1-derived lesions, particularly at telomeres and mitochondrial DNA, as well as in reactive oxygen species, in parallel with increased expression of SOD1. These cells also showed lower levels of IL6 and decreased p38 phosphorylation, decreased senescence and increased ability to divide for longer times. Additionally, these cells are more resistant to treatment with H202 and the radiomimetic-drug bleomycin. Finally, we found shorter telomere length (TL) in AT cells, lower levels of TERT expression, and telomerase activity that were also partially reverted by GSE4. These observations suggest that GSE4 may be considered as a new therapy for the treatment of AT that counteracts the cellular effects of high ROS levels generated in AT cells and in addition increases telomerase activity contributing to increased cell proliferation.

Telomere repeat-binding factor 2 binds extensively to extra-telomeric G-quadruplexes and regulates the epigenetic status of several gene promoters

The role of the telomere repeat-binding factor 2 (TRF2) in telomere maintenance is well-established. However, recent findings suggest that TRF2 also functions outside telomeres, but relatively little is known about this function. Herein, using genome-wide ChIP-Seq assays of TRF2-bound chromatin from HT1080 fibrosarcoma cells, we identified thousands of TRF2-binding sites within the extra-telomeric genome. In light of this observation, we asked how TRF2 occupancy is organized within the genome. Interestingly, we found that extra-telomeric TRF2 sites throughout the genome are enriched in potential G-quadruplex-forming DNA sequences. Furthermore, we validated TRF2 occupancy at several promoter G-quadruplex motifs, which did adopt quadruplex forms in solution. TRF2 binding altered expression and the epigenetic state of several target promoters, indicated by histone modifications resulting in transcriptional repression of eight of nine genes investigated here. Furthermore, TRF2 occupancy and target gene expression were also sensitive to the well-known intracellular G-quadruplex-binding ligand 360A. Together, these results reveal an extensive genome-wide association of TRF2 outside telomeres and that it regulates gene expression in a G-quadruplex-dependent fashion.

The Role of Telomerase and Telomeres in Interstitial Lung Diseases: From Molecules to Clinical Implications

Telomeres are distal chromosome regions associated with specific protein complexes that protect the chromosome against degradation and aberrations. Telomere maintenance capacity is an essential indication of healthy cell populations, and telomere damage is observed in processes such as malignant transformation, apoptosis, or cell senescence. At a cellular level, telomere damage may result from genotoxic stress, decreased activity of telomerase enzyme complex, dysfunction of shelterin proteins, or changes in expression of telomere-associated RNA such as TERRA. Clinical evidence suggests that mutation of telomerase genes (Tert/Terc) are associated with increased risk of congenital as well as age-related diseases (e.g., pneumonitis, idiopathic pulmonary fibrosis (IPF), dyskeratosis congenita, emphysema, nonspecific interstitial pneumonia, etc.). Thus, telomere length and maintenance can serve as an important prognostic factor as well as a potential target for new strategies of treatment for interstitial lung diseases (ILDs) and associated pulmonary pathologies.

Loci for human leukocyte telomere length in the Singaporean Chinese population and trans-ethnic genetic studies

Genetic factors underlying leukocyte telomere length (LTL) may provide insights into telomere homeostasis, with direct links to disease susceptibility. Genetic evaluation of 23,096 Singaporean Chinese samples identifies 10 genome-wide loci (P < 5 × 10-8). Several of these contain candidate genes (TINF2, PARP1, TERF1, ATM and POT1) with potential roles in telomere biology and DNA repair mechanisms. Meta-analyses with additional 37,505 European individuals reveals six more genome-wide loci, including associations at MPHOSPH6, NKX2-3 and TYMS. We demonstrate that longer LTL associates with protection against respiratory disease mortality [HR = 0.854(0.804-0.906), P = 1.88 × 10-7] in the Singaporean Chinese samples. We further show that the LTL reducing SNP rs7253490 associates with respiratory infections (P = 7.44 × 10-4) although this effect may not be strongly mediated through LTL. Our data expands on the genetic basis of LTL and may indicate on a potential role of LTL in immune competence.

MYBL2 Supports DNA Double Strand Break Repair in Hematopoietic Stem Cells

Myelodysplastic syndromes (MDS) are a heterogeneous group of diseases characterized by blood cytopenias that occur as a result of somatic mutations in hematopoietic stem cells (HSC). MDS leads to ineffective hematopoiesis, and as many as 30% of patients progress to acute myeloid leukemia (AML). The mechanisms by which mutations accumulate in HSC during aging remain poorly understood. Here we identify a novel role for MYBL2 in DNA double-strand break (DSB) repair in HSC. In patients with MDS, low MYBL2 levels associated with and preceded transcriptional deregulation of DNA repair genes. Stem/progenitor cells from these patients display dysfunctional DSB repair kinetics after exposure to ionizing radiation (IR). Haploinsufficiency of Mybl2 in mice also led to a defect in the repair of DSBs induced by IR in HSC and was characterized by unsustained phosphorylation of the ATM substrate KAP1 and telomere fragility. Our study identifies MYBL2 as a crucial regulator of DSB repair and identifies MYBL2 expression levels as a potential biomarker to predict cellular response to genotoxic treatments in MDS and to identify patients with defects in DNA repair. Such patients with worse prognosis may require a different therapeutic regimen to prevent progression to AML.Significance: These findings suggest MYBL2 levels may be used as a biological biomarker to determine the DNA repair capacity of hematopoietic stem cells from patients with MDS and as a clinical biomarker to inform decisions regarding patient selection for treatments that target DNA repair.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/20/5767/F1.large.jpg Cancer Res; 78(20); 5767-79. ©2018 AACR.

Regulation of cellular senescence via the FOXO4-p53 axis

Forkhead box O (FOXO) and p53 proteins are transcription factors that regulate diverse signalling pathways to control cell cycle, apoptosis and metabolism. In the last decade both FOXO and p53 have been identified as key players in aging, and their misregulation is linked to numerous diseases including cancers. However, many of the underlying molecular mechanisms remain mysterious, including regulation of ageing by FOXOs and p53. Several activities appear to be shared between FOXOs and p53, including their central role in the regulation of cellular senescence. In this review, we will focus on the recent advances on the link between FOXOs and p53, with a particular focus on the FOXO4‐p53 axis and the role of FOXO4/p53 in cellular senescence. Moreover, we discuss potential strategies for targeting the FOXO4‐p53 interaction to modulate cellular senescence as a drug target in treatment of aging‐related diseases and morbidity.

Chromosome Healing Is Promoted by the Telomere Cap Component Hiphop in Drosophila

The addition of a new telomere onto a chromosome break, a process termed healing, has been studied extensively in organisms that utilize telomerase to maintain their telomeres. In comparison, relatively little is known about how new telomeres are constructed on broken chromosomes in organisms that do not use telomerase. Chromosome healing was studied in somatic and germline cells of Drosophila melanogaster, a nontelomerase species. We observed, for the first time, that broken chromosomes can be healed in somatic cells. In addition, overexpression of the telomere cap component Hiphop increased the survival of somatic cells with broken chromosomes, while the cap component HP1 did not, and overexpression of the cap protein HOAP decreased their survival. In the male germline, Hiphop overexpression greatly increased the transmission of healed chromosomes. These results indicate that Hiphop can stimulate healing of a chromosome break. We suggest that this reflects a unique function of Hiphop: it is capable of seeding formation of a new telomeric cap on a chromosome end that lacks a telomere.

Genomic profiling of Acute lymphoblastic leukemia in ataxia telangiectasia patients reveals tight link between ATM mutations and chromothripsis

Recent developments in sequencing technologies led to the discovery of a novel form of genomic instability, termed chromothripsis. This catastrophic genomic event, involved in tumorigenesis, is characterized by tens to hundreds of simultaneously acquired locally clustered rearrangements on one chromosome. We hypothesized that leukemias developing in individuals with Ataxia Telangiectasia, who are born with two mutated copies of the ATM gene, an essential guardian of genome stability, would show a higher prevalence of chromothripsis due to the associated defect in DNA double-strand break repair. Using whole-genome sequencing, fluorescence in situ hybridization and RNA sequencing, we characterized the genomic landscape of Acute Lymphoblastic Leukemia (ALL) arising in patients with Ataxia Telangiectasia. We detected a high frequency of chromothriptic events in these tumors, specifically on acrocentric chromosomes, as compared with tumors from individuals with other types of DNA repair syndromes (27 cases total, 10 with Ataxia Telangiectasia). Our data suggest that the genomic landscape of Ataxia Telangiectasia ALL is clearly distinct from that of sporadic ALL. Mechanistically, short telomeres and compromised DNA damage response in cells of Ataxia Telangiectasia patients may be linked with frequent chromothripsis. Furthermore, we show that ATM loss is associated with increased chromothripsis prevalence in additional tumor entities.

Telomere-associated aging disorders

Telomeres are dynamic nucleoprotein-DNA structures that cap and protect linear chromosome ends. Several monogenic inherited diseases that display features of human premature aging correlate with shortened telomeres, and are referred to collectively as telomeropathies. These disorders have overlapping symptoms and a common underlying mechanism of telomere dysfunction, but also exhibit variable symptoms and age of onset, suggesting they fall along a spectrum of disorders. Primary telomeropathies are caused by defects in the telomere maintenance machinery, whereas secondary telomeropathies have some overlapping symptoms with primary telomeropathies, but are generally caused by mutations in DNA repair proteins that contribute to telomere preservation. Here we review both the primary and secondary telomeropathies, discuss potential mechanisms for tissue specificity and age of onset, and highlight outstanding questions in the field and future directions toward elucidating disease etiology and developing therapeutic strategies.

Critical role of the POT1 OB domain in maintaining genomic stability

Oligonucleotide/oligosaccharide-binding (OB) domain-containing proteins have been identified as critical for telomere maintenance, DNA repair, transcription and other DNA metabolism processes. Protection of telomere 1 (POT1), a telomere binding protein, has an OB domain like single-strand binding protein (SSB1). In this issue of Oncogene, Gu et al. present evidence that POT1, like SSB1, is required to maintain genomic stability. This work, in conjunction with results from previous investigators, highlights the importance of POT1 in telomere metabolism. Inactivation of POT1 telomere protective functions in mouse models lacking p53 expression in the breast epithelium unleashes a torrent of DNA damage responses (DDRs) at the telomeres, culminating in karyotypic alterations with massive arrays of telomere fusions. Therefore, POT1 is not only required to promote telomere homeostasis, but also plays an essential role in maintaining a stable genome.

DNA damage-induced inflammation and nuclear architecture

Nuclear architecture and the chromatin state affect most-if not all- DNA-dependent transactions, including the ability of cells to sense DNA lesions and restore damaged DNA back to its native form. Recent evidence points to functional links between DNA damage sensors, DNA repair mechanisms and the innate immune responses. The latter raises the question of how such seemingly disparate processes operate within the intrinsically complex nuclear landscape and the chromatin environment. Here, we discuss how DNA damage-induced immune responses operate within chromatin and the distinct sub-nuclear compartments highlighting their relevance to chronic inflammation.

Pluripotent Stem Cells and DNA Damage Response to Ionizing Radiations

Pluripotent stem cells (PSCs) hold great promise in regenerative medicine, disease modeling, functional genomics, toxicological studies and cell-based therapeutics due to their unique characteristics of self-renewal and pluripotency. Novel methods for generation of pluripotent stem cells and their differentiation to the specialized cell types such as neuronal cells, myocardial cells, hepatocytes and beta cells of the pancreas and many other cells of the body are constantly being refined. Pluripotent stem cell derived differentiated cells, including neuronal cells or cardiac cells, are ideal for stem cell transplantation as autologous or allogeneic cells from healthy donors due to their minimal risk of rejection. Radiation-induced DNA damage, ultraviolet light, genotoxic stress and other intrinsic and extrinsic factors triggers a series of biochemical reactions known as DNA damage response. To maintain genomic stability and avoid transmission of mutations into progenitors cells, stem cells have robust DNA damage response signaling, a contrast to somatic cells. Stem cell transplantation may protect against radiation-induced late effects. In particular, this review focuses on differential DNA damage response between stem cells and derived differentiated cells and the possible pathways that determine such differences.

Detecting ATM-dependent chromatin modification in DNA damage response

Loss of function or mutation of the ataxia-telangiectasia mutated gene product (ATM) results in inherited genetic disorders characterized by neurodegeneration, immunodeficiency, and cancer. Ataxia-telangiectasia mutated (ATM) gene product belongs to the PI3K-like protein kinase (PIKKs) family and is functionally implicated in mitogenic signal transduction, chromosome condensation, meiotic recombination, cell-cycle control, and telomere maintenance. The ATM protein kinase is primarily activated in response to DNA double strand breaks (DSBs), the most deleterious form of DNA damage produced by ionizing radiation (IR) or radiomimetic drugs. It is detected at DNA damage sites, where ATM autophosphorylation causes dissociation of the inactive homodimeric form to the activated monomeric form. Interestingly, heat shock can activate ATM independent of the presence of DNA strand breaks. ATM is an integral part of the sensory machinery that detects DSBs during meiosis, mitosis, or DNA breaks mediated by free radicals. These DNA lesions can trigger higher order chromatin reorganization fuelled by posttranslational modifications of histones and histone binding proteins. Our group, and others, have shown that ATM activation is tightly regulated by chromatin modifications. This review summarizes the multiple approaches used to discern the role of ATM and other associated proteins in chromatin modification in response to DNA damage.

Genomic integrity and the ageing brain

DNA damage is correlated with and may drive the ageing process. Neurons in the brain are postmitotic and are excluded from many forms of DNA repair; therefore, neurons are vulnerable to various neurodegenerative diseases. The challenges facing the field are to understand how and when neuronal DNA damage accumulates, how this loss of genomic integrity might serve as a 'time keeper' of nerve cell ageing and why this process manifests itself as different diseases in different individuals.

Multifunctional role of ATM/Tel1 kinase in genome stability: from the DNA damage response to telomere maintenance

The mammalian protein kinase ataxia telangiectasia mutated (ATM) is a key regulator of the DNA double-strand-break response and belongs to the evolutionary conserved phosphatidylinositol-3-kinase-related protein kinases. ATM deficiency causes ataxia telangiectasia (AT), a genetic disorder that is characterized by premature aging, cerebellar neuropathy, immunodeficiency, and predisposition to cancer. AT cells show defects in the DNA damage-response pathway, cell-cycle control, and telomere maintenance and length regulation. Likewise, in Saccharomyces cerevisiae, haploid strains defective in the TEL1 gene, the ATM ortholog, show chromosomal aberrations and short telomeres. In this review, we outline the complex role of ATM/Tel1 in maintaining genomic stability through its control of numerous aspects of cellular survival. In particular, we describe how ATM/Tel1 participates in the signal transduction pathways elicited by DNA damage and in telomere homeostasis and its importance as a barrier to cancer development.

Telomerase enzymatic component hTERT shortens long telomeres in human cells

Telomere lengths are tightly regulated within a narrow range in normal human cells. Previous studies have extensively focused on how short telomeres are extended and have demonstrated that telomerase plays a central role in elongating short telomeres. However, much about the molecular mechanisms of regulating excessively long telomeres is unknown. In this report, we demonstrated that the telomerase enzymatic component, hTERT, plays a dual role in the regulation of telomere length. It shortens excessively long telomeres and elongates short telomeres simultaneously in one cell, maintaining the optimal telomere length at each chromosomal end for efficient protection. This novel hTERT-mediated telomere-shortening mechanism not only exists in cancer cells, but also in primary human cells. The hTERT-mediated telomere shortening requires hTERT's enzymatic activity, but the telomerase RNA component, hTR, is not involved in that process. We found that expression of hTERT increases telomeric circular DNA formation, suggesting that telomere homologous recombination is involved in the telomere-shortening process. We further demonstrated that shelterin protein TPP1 interacts with hTERT and recruits hTERT onto the telomeres, suggesting that TPP1 might be involved in regulation of telomere shortening. This study reveals a novel function of hTERT in telomere length regulation and adds a new element to the current molecular model of telomere length maintenance.

c-Myc quadruplex-forming sequence Pu-27 induces extensive damage in both telomeric and nontelomeric regions of DNA

Quadruplex-forming DNA sequences are present throughout the eukaryotic genome, including in telomeric DNA. We have shown that the c-Myc promoter quadruplex-forming sequence Pu-27 selectively kills transformed cells (Sedoris, K. C., Thomas, S. D., Clarkson, C. R., Muench, D., Islam, A., Singh, R., and Miller, D. M. (2012) Genomic c-Myc quadruplex DNA selectively kills leukemia. Mol. Cancer Ther. 11, 66-76). In this study, we show that Pu-27 induces profound DNA damage, resulting in striking chromosomal abnormalities in the form of chromatid or chromosomal breaks, radial formation, and telomeric DNA loss, which induces γ-H2AX in U937 cells. Pu-27 down-regulates telomeric shelterin proteins, DNA damage response mediators (RAD17 and RAD50), double-stranded break repair molecule 53BP1, G2 checkpoint regulators (CHK1 and CHK2), and anti-apoptosis gene survivin. Interestingly, there are no changes of DNA repair molecules H2AX, BRCA1, and the telomere maintenance gene, hTERT. ΔB-U937, where U937 cells stably transfected with deleted basic domain of TRF2 is partially sensitive to Pu-27 but exhibits no changes in expression of shelterin proteins. However, there is an up-regulation of CHK1, CHK2, H2AX, BRCA1, and survivin. Telomere dysfunction-induced foci assay revealed co-association of TRF1with γ-H2AX in ATM deficient cells, which are differentially sensitive to Pu-27 than ATM proficient cells. Alt (alternating lengthening of telomere) cells are relatively resistant to Pu-27, but there are no significant changes of telomerase activity in both Alt and non-Alt cells. Lastly, we show that this Pu-27-mediated sensitivity is p53-independent. The data therefore support two conclusions. First, Pu-27 induces DNA damage within both telomeric and nontelomeric regions of the genome. Second, Pu-27-mediated telomeric damage is due, at least in part, to compromise of the telomeric shelterin protein complex.

T-cell-specific deletion of Mof blocks their differentiation and results in genomic instability in mice

Ataxia telangiectasia patients develop lymphoid malignancies of both B- and T-cell origin. Similarly, ataxia telangiectasia mutated (Atm)-deficient mice exhibit severe defects in T-cell maturation and eventually develop thymomas. The function of ATM is known to be influenced by the mammalian orthologue of the Drosophila MOF (males absent on the first) gene. Here, we report the effect of T-cell-specific ablation of the mouse Mof (Mof) gene on leucocyte trafficking and survival. Conditional Mof(Flox/Flox) (Mof (F/F)) mice expressing Cre recombinase under control of the T-cell-specific Lck proximal promoter (Mof(F/F)/Lck-Cre(+)) display a marked reduction in thymus size compared with Mof(F/F)/Lck-Cre(-) mice. In contrast, the spleen size of Mof(F/F)/Lck-Cre(+) mice was increased compared with control Mof(F/F)/Lck-Cre(-) mice. The thymus of Mof(F/F)/Lck-Cre(+) mice contained significantly reduced T cells, whereas thymic B cells were elevated. Within the T-cell population, CD4(+)CD8(+) double-positive T-cell levels were reduced, whereas the immature CD4(-)CD8(-) double-negative (DN) population was elevated. Defective T-cell differentiation is also evident as an increased DN3 (CD44(-)CD25(+)) population, the cell stage during which T-cell receptor rearrangement takes place. The differentiation defect in T cells and reduced thymus size were not rescued in a p53-deficient background. Splenic B-cell distributions were similar between Mof(F/F)/Lck-Cre(+) and Mof(F/F)/Lck-Cre(-) mice except for an elevation of the κ light-chain population, suggestive of an abnormal clonal expansion. T cells from Mof(F/F)/Lck-Cre(+) mice did not respond to phytohaemagglutinin (PHA) stimulation, whereas LPS-stimulated B cells from Mof(F/F)/Lck-Cre(+) mice demonstrated spontaneous genomic instability. Mice with T-cell-specific loss of MOF had shorter lifespans and decreased survival following irradiation than did Mof(F/F)/Lck-Cre(-) mice. These observations suggest that Mof plays a critical role in T-cell differentiation and that depletion of Mof in T cells reduces T-cell numbers and, by an undefined mechanism, induces genomic instability in B cells through bystander mechanism. As a result, these mice have a shorter lifespan and reduced survival after irradiation.

Histone modifications and DNA double-strand break repair after exposure to ionizing radiations

Ionizing radiation exposure induces highly lethal DNA double-strand breaks (DSBs) in all phases of the cell cycle. After DSBs are detected by the cellular machinery, these breaks are repaired by either of two mechanisms: (1) nonhomologous end joining (NHEJ), which re-ligates the broken ends of the DNA and (2) homologous recombination (HR), that makes use of an undamaged identical DNA sequence as a template to maintain the fidelity of DNA repair. DNA DSB repair must occur within the context of the natural cellular DNA structure. Among the major factors influencing DNA organization are specific histone and nonhistone proteins that form chromatin. The overall chromatin structure regulates DNA damage responses since chromatin status can impede DNA damage site access by repair proteins. During the process of DNA DSB repair, several chromatin alterations are required to sense damage and facilitate accessibility of the repair machinery. The DNA DSB response is also facilitated by hierarchical signaling networks that orchestrate chromatin structural changes that may coordinate cell-cycle checkpoints involving multiple enzymatic activities to repair broken DNA ends. During DNA damage sensing and repair, histones undergo posttranslational modifications (PTMs) including phosphorylation, acetylation, methylation and ubiquitylation. Such histone modifications represent a histone code that directs the recruitment of proteins involved in DNA damage sensing and repair processes. In this review, we summarize histone modifications that occur during DNA DSB repair processes.

Chromatin modifications and the DNA damage response to ionizing radiation

In order to survive, cells have evolved highly effective repair mechanisms to deal with the potentially lethal DNA damage produced by exposure to endogenous as well as exogenous agents. Ionizing radiation exposure induces highly lethal DNA damage, especially DNA double-strand breaks (DSBs), that is sensed by the cellular machinery and then subsequently repaired by either of two different DSB repair mechanisms: (1) non-homologous end joining, which re-ligates the broken ends of the DNA and (2) homologous recombination, that employs an undamaged identical DNA sequence as a template, to maintain the fidelity of DNA repair. Repair of DSBs must occur within the natural context of the cellular DNA which, along with specific proteins, is organized to form chromatin, the overall structure of which can impede DNA damage site access by repair proteins. The chromatin complex is a dynamic structure and is known to change as required for ongoing cellular processes such as gene transcription or DNA replication. Similarly, during the process of DNA damage sensing and repair, chromatin needs to undergo several changes in order to facilitate accessibility of the repair machinery. Cells utilize several factors to modify the chromatin in order to locally open up the structure to reveal the underlying DNA sequence but post-translational modification of the histone components is one of the primary mechanisms. In this review, we will summarize chromatin modifications by the respective chromatin modifying factors that occur during the DNA damage response.

Senescence regulation by the p53 protein family

p53, a guardian of the genome, exerts its tumor suppression activity by regulating a large number of downstream targets involved in cell cycle arrest, DNA repair, apoptosis, and cellular senescence. Although p53-mediated apoptosis is able to kill cancer cells, a role for cellular senescence in p53-dependent tumor suppression is becoming clear. Mouse studies showed that activation of p53-induced premature senescence promotes tumor regression in vivo. However, p53-mediated cellular senescence also leads to aging-related phenotypes, such as tissue atrophy, stem cell depletion, and impaired wound healing. In addition, several p53 isoforms and two p53 homologs, p63 and p73, have been shown to play a role in cellular senescence and/or aging. Importantly, p53, p63, and p73 are necessary for the maintenance of adult stem cells. Therefore, understanding the dual role the p53 protein family in cancer and aging is critical to solve cancer and longevity in the future. In this chapter, we provide an overview on how p53, p63, p73, and their isoforms regulate cellular senescence and aging.

Induced pluripotent stem cells from ataxia-telangiectasia recapitulate the cellular phenotype

Pluripotent stem cells can differentiate into every cell type of the human body. Reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) therefore provides an opportunity to gain insight into the molecular and cellular basis of disease. Because the cellular DNA damage response poses a barrier to reprogramming, generation of iPSCs from patients with chromosomal instability syndromes has thus far proven to be difficult. Here we demonstrate that fibroblasts from patients with ataxia-telangiectasia (A-T), a disorder characterized by chromosomal instability, progressive neurodegeneration, high risk of cancer, and immunodeficiency, can be reprogrammed to bona fide iPSCs, albeit at a reduced efficiency. A-T iPSCs display defective radiation-induced signaling, radiosensitivity, and cell cycle checkpoint defects. Bioinformatic analysis of gene expression in the A-T iPSCs identifies abnormalities in DNA damage signaling pathways, as well as changes in mitochondrial and pentose phosphate pathways. A-T iPSCs can be differentiated into functional neurons and thus represent a suitable model system to investigate A-T-associated neurodegeneration. Collectively, our data show that iPSCs can be generated from a chromosomal instability syndrome and that these cells can be used to discover early developmental consequences of ATM deficiency, such as altered mitochondrial function, that may be relevant to A-T pathogenesis and amenable to therapeutic intervention.

Post-translational modifications of TRF1 and TRF2 and their roles in telomere maintenance

Telomeres, heterochromatic structures, found at the ends of linear eukaryotic chromosomes, function to protect natural chromosome ends from nucleolytic attack. Human telomeric DNA is bound by a telomere-specific six-subunit protein complex, termed shelterin/telosome. The shelterin subunits TRF1 and TRF2 bind in a sequence-specific manner to double-stranded telomeric DNA, providing a vital platform for recruitment of additional shelterin proteins as well as non-shelterin factors crucial for the maintenance of telomere length and structure. Both TRF1 and TRF2 are engaged in multiple roles at telomeres including telomere protection, telomere replication, sister telomere resolution and telomere length maintenance. Regulation of TRF1 and TRF2 in these various processes is controlled by post-translational modifications, at times in a cell-cycle-dependent manner, affecting key functions such as DNA binding, dimerization, localization, degradation and interactions with other proteins. Here we review the post-translational modifications of TRF1 and TRF2 and discuss the mechanisms by which these modifications contribute to the function of these two proteins.

ATM regulates proteasome-dependent subnuclear localization of TRF1, which is important for telomere maintenance

Ataxia telangiectasia mutated (ATM), a PI-3 kinase essential for maintaining genomic stability, has been shown to regulate TRF1, a negative mediator of telomerase-dependent telomere extension. However, little is known about ATM-mediated TRF1 phosphorylation site(s) in vivo. Here, we report that ATM phosphorylates S367 of TRF1 and that this phosphorylation renders TRF1 free of chromatin. We show that phosphorylated (pS367)TRF1 forms distinct non-telomeric subnuclear foci and that these foci occur predominantly in S and G2 phases, implying that their formation is cell cycle regulated. We show that phosphorylated (pS367)TRF1-containing foci are sensitive to proteasome inhibition. We find that a phosphomimic mutation of S367D abrogates TRF1 binding to telomeric DNA and renders TRF1 susceptible to protein degradation. In addition, we demonstrate that overexpressed TRF1-S367D accumulates in the subnuclear domains containing phosphorylated (pS367)TRF1 and that these subnuclear domains overlap with nuclear proteasome centers. Taken together, these results suggest that phosphorylated (pS367)TRF1-containing foci may represent nuclear sites for TRF1 proteolysis. Furthermore, we show that TRF1 carrying the S367D mutation is unable to inhibit telomerase-dependent telomere lengthening or to suppress the formation of telomere doublets and telomere loss in TRF1-depleted cells, suggesting that S367 phosphorylation by ATM is important for the regulation of telomere length and stability.

The role of MOF in the ionizing radiation response is conserved in Drosophila melanogaster

In Drosophila, males absent on the first (MOF) acetylates histone H4 at lysine 16 (H4K16ac). This acetylation mark is highly enriched on the male X chromosome and is required for dosage compensation in Drosophila but not utilized for such in mammals. Recently, we and others reported that mammalian MOF, through H4K16ac, has a critical role at multiple stages in the DNA damage response (DDR) and double-strand break repair pathways. The goal of this study was to test whether mof is similarly required for the response to ionizing radiation (IR) in Drosophila. We report that Drosophila mof mutations in males and females, as well as mof knockdown in SL-2 cells, reduce post-irradiation survival. MOF depletion in SL-2 cells also results in an elevated frequency of metaphases with chromosomal aberrations, suggesting that MOF is involved in DDR. Mutation in Drosophila mof also results in a defective mitotic checkpoint, enhanced apoptosis, and a defective p53 response post-irradiation. In addition, IR exposure enhanced H4K16ac levels in Drosophila as it also does in mammals. These results are the first to demonstrate a requirement for MOF in the whole animal IR response and suggest that the role of MOF in the response to IR is conserved between Drosophila and mammals.

ATM protein kinase: the linchpin of cellular defenses to stress

ATM is the most significant molecule involved in monitoring the genomic integrity of the cell. Any damage done to DNA relentlessly challenges the cellular machinery involved in recognition, processing and repair of these insults. ATM kinase is activated early to detect and signal lesions in DNA, arrest the cell cycle, establish DNA repair signaling and faithfully restore the damaged chromatin. ATM activation plays an important role as a barrier to tumorigenesis, metabolic syndrome and neurodegeneration. Therefore, studies of ATM-dependent DNA damage signaling pathways hold promise for treatment of a variety of debilitating diseases through the development of new therapeutics capable of modulating cellular responses to stress. In this review, we have tried to untangle the complex web of ATM signaling pathways with the purpose of pinpointing multiple roles of ATM underlying the complex phenotypes observed in AT patients.

The emerging role of matrix metalloproteases of the ADAM family in male germ cell apoptosis

Constitutive germ cell apoptosis during mammalian spermatogenesis is a key process for controlling sperm output and to eliminate damaged or unwanted cells. An increase or decrease in the apoptosis rate has deleterious consequences and leads to low sperm production. Apoptosis in spermatogenesis has been widely studied, but the mechanism by which it is induced under physiological or pathological conditions has not been clarified. We have recently identified the metalloprotease ADAM17 (TACE) as a putative physiological inducer of germ cell apoptosis. The mechanisms involved in regulating the shedding of the ADAM17 extracellular domain are still far from being understood, although they are important in order to understand cell-cell communications. Here, we review the available data regarding apoptosis during mammalian spermatogenesis and the localization of ADAM proteins in the male reproductive tract. We propose an integrative working model where ADAM17, p38 MAPK, protein kinase C (PKC) and the tyrosine kinase c-Abl participate in the physiological signalling cascade inducing apoptosis in germ cells. In our model, we also propose a role for the Sertoli cell in regulating the Fas/FasL system in order to induce the extrinsic pathway of apoptosis in germ cells. This working model could be applied to further understand constitutive apoptosis in spermatogenesis and in pathological conditions (e.g., varicocele) or following environmental toxicants exposure (e.g., genotoxicity or xenoestrogens).

IFI16 induction by glucose restriction in human fibroblasts contributes to autophagy through activation of the ATM/AMPK/p53 pathway

Background

Glucose restriction in cells increases the AMP/ATP ratio (energetic stress), which activates the AMPK/p53 pathway. Depending upon the energetic stress levels, cells undergo either autophagy or cell death. Given that the activated p53 induces the expression of IFI16 protein, we investigated the potential role of the IFI16 protein in glucose restriction-induced responses.

Methodology/Principal Findings

We found that glucose restriction or treatment of human diploid fibroblasts (HDFs) with the activators of the AMPK/p53 pathway induced the expression of IFI16 protein. The induced levels of IFI16 protein were associated with the induction of autophagy and reduced cell survival. Moreover, the increase in the IFI16 protein levels was dependent upon the expression of the functional ATM protein kinase. Importantly, the knockdown of the IFI16 expression in HDFs inhibited the activation of the ATM/AMPK/p53 pathway in response to glucose restriction and also increased the survival of HDFs.

Conclusions/Significance

Our observations demonstrate a role for the IFI16 protein in the energetic stress-induced regulation of autophagy and cell survival. Additionally, our findings also indicate that the loss of IFI16 expression, as found in certain cancers, may provide a survival advantage to cancer cells in microenvironments with low glucose levels.

Introduction to telomeres and telomerase

Telomeres are ends of chromosomes that play an important part in the biology of eukaryotic cells. Through the coordinated action of the telomerase and networks of other proteins and factors, the length and integrity of telomeres are maintained to prevent telomere dysfunction that has been linked to senescence, aging, diseases, and cancer. The tools and assays being used to study telomeres are being broadened, which has allowed us to derive a more detailed, high-resolution picture of the various players and pathways at work at the telomeres.

Impact of G₂ checkpoint defect on centromeric instability

Centromeric instability is characterized by dynamic formation of centromeric breaks, deletions, isochromosomes and translocations, which are commonly observed in cancer. So far, however, the mechanisms of centromeric instability in cancer cells are still poorly understood. In this study, we tested the hypothesis that G(2) checkpoint defect promotes centromeric instability. Our observations from multiple approaches consistently support this hypothesis. We found that overexpression of cyclin B1, one of the pivotal genes driving G(2) to M phase transition, impaired G(2) checkpoint and promoted the formation of centromeric aberrations in telomerase-immortalized cell lines. Conversely, centromeric instability in cancer cells was ameliorated through reinforcement of G(2) checkpoint by cyclin B1 knockdown. Remarkably, treatment with KU55933 for only 2.5 h, which abrogated G(2) checkpoint, was sufficient to produce centromeric aberrations. Moreover, centromeric aberrations constituted the major form of structural abnormalities in G(2) checkpoint-defective ataxia telangiectasia cells. Statistical analysis showed that the frequencies of centromeric aberrations in G(2) checkpoint-defective cells were always significantly overrepresented compared with random assumption. As there are multiple pathways leading to G(2) checkpoint defect, our finding offers a broad explanation for the common occurrence of centromeric aberrations in cancer cells.

Survival responses to oxidative stress and aging

Oxidative stress is recognized as an important environmental factor in aging; however, because reactive oxygen species (ROS) and related free radicals are normally produced both intra- and extracellularly, air-living organisms cannot avoid the risk of oxidative stress. Consequently, these organisms have evolved various anti-oxidant systems to prevent ROS, scavenge free radicals, repair damaged components and adaptive responses. This review will focus on the repair and adaptive response to oxidative stress, and summarize the changes of these systems as a result aging and their relationship to premature aging.

S phase entry causes homocysteine-induced death while ataxia telangiectasia and Rad3 related protein functions anti-apoptotically to protect neurons

A major phenotype seen in neurodegenerative disorders is the selective loss of neurons due to apoptotic death and evidence suggests that inappropriate re-activation of cell cycle proteins in post-mitotic neurons may be responsible. To investigate whether reactivation of the G1 cell cycle proteins and S phase entry was linked with apoptosis, we examined homocysteine-induced neuronal cell death in a rat cortical neuron tissue culture system. Hyperhomocysteinaemia is a physiological risk factor for a variety of neurodegenerative diseases, including Alzheimer's disease. We found that in response to homocysteine treatment, cyclin D1, and cyclin-dependent kinases 4 and 2 translocated to the nucleus, and p27 levels decreased. Both cyclin-dependent kinases 4 and 2 regained catalytic activity, the G1 gatekeeper retinoblastoma protein was phosphorylated and DNA synthesis was detected, suggesting transit into S phase. Double-labelling immunofluorescence showed a 95% co-localization of anti-bromodeoxyuridine labelling with apoptotic markers, demonstrating that those cells that entered S phase eventually died. Neurons could be protected from homocysteine-induced death by methods that inhibited G1 phase progression, including down-regulation of cyclin D1 expression, inhibition of cyclin-dependent kinases 4 or 2 activity by small molecule inhibitors, or use of the c-Abl kinase inhibitor, Gleevec, which blocked cyclin D and cyclin-dependent kinase 4 nuclear translocation. However, blocking cell cycle progression post G1, using DNA replication inhibitors, did not prevent apoptosis, suggesting that death was not preventable post the G1-S phase checkpoint. While homocysteine treatment caused DNA damage and activated the DNA damage response, its mechanism of action was distinct from that of more traditional DNA damaging agents, such as camptothecin, as it was p53-independent. Likewise, inhibition of the DNA damage sensors, ataxia-telangiectasia mutant and ataxia telangiectasia and Rad3 related proteins, did not rescue apoptosis and in fact exacerbated death, suggesting that the DNA damage response might normally function neuroprotectively to block S phase-dependent apoptosis induction. As cell cycle events appear to be maintained in vivo in affected neurons for weeks to years before apoptosis is observed, activation of the DNA damage response might be able to hold cell cycle-induced death in check.

MOF and histone H4 acetylation at lysine 16 are critical for DNA damage response and double-strand break repair

The human MOF gene encodes a protein that specifically acetylates histone H4 at lysine 16 (H4K16ac). Here we show that reduced levels of H4K16ac correlate with a defective DNA damage response (DDR) and double-strand break (DSB) repair to ionizing radiation (IR). The defect, however, is not due to altered expression of proteins involved in DDR. Abrogation of IR-induced DDR by MOF depletion is inhibited by blocking H4K16ac deacetylation. MOF was found to be associated with the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), a protein involved in nonhomologous end-joining (NHEJ) repair. ATM-dependent IR-induced phosphorylation of DNA-PKcs was also abrogated in MOF-depleted cells. Our data indicate that MOF depletion greatly decreased DNA double-strand break repair by both NHEJ and homologous recombination (HR). In addition, MOF activity was associated with general chromatin upon DNA damage and colocalized with the synaptonemal complex in male meiocytes. We propose that MOF, through H4K16ac (histone code), has a critical role at multiple stages in the cellular DNA damage response and DSB repair.

Tumor suppression by p53: making cells senescent

Cellular senescence is a permanent cell cycle arrest and a potent tumor suppression mechanism. The p53 tumor suppressor is a sequence-specific transcription factor and acts as a central hub sensing various stress signals and activating an array of target genes to induce cell cycle arrest, apoptosis, and senescence. Recent reports showed that restoration of p53 induces premature senescence and tumor regression in mice with hepatocarcinomas or sarcomas. Thus, p53-mediated senescence is capable of eliminating cancer cells in vivo. p63 and p73, two homologues of p53, have similar function in cell cycle arrest and apoptosis. However, the role of p63 and p73 in cellular senescence is elusive. In this review, we will discuss how p53 regulates senescence and future studies about p53 family members in senescence.

Multiple roles of BRIT1/MCPH1 in DNA damage response, DNA repair, and cancer suppression

Mammalian cells are frequently at risk of DNA damage from both endogenous and exogenous sources. Accordingly, cells have evolved the DNA damage response (DDR) pathways to monitor and assure the integrity of their genome. In cells, the intact and effective DDR is essential for the maintenance of genomic stability and it acts as a critical barrier to suppress the development of cancer in humans. Two central kinases for the DDR pathway are ATM and ATR, which can phosphorylate and activate many downstream proteins for cell cycle arrest, DNA repair, or apoptosis if the damages are irreparable. In the last several years, we and others have made significant progress to this field by identifying BRIT1 (also known as MCPH1) as a novel key regulator in the DDR pathway. BRIT1 protein contains 3 breast cancer carboxyl terminal (BRCT) domains which are conserved in BRCA1, MDC1, 53BP1, and other important molecules involved in DNA damage signaling, DNA repair, and tumor suppression. Our in vitro studies revealed BRIT1 to be a chromatinbinding protein required for recruitment of many important DDR proteins (ATM, MDC1, NBS1, RAD51, BRCA2) to the DNA damage sites. We recently also generated the BRIT1 knockout mice and demonstrated its essential roles in homologous recombination DNA repair and in maintaining genomic stability in vivo. In humans, BRIT1 is located on chromosome 8p23.1, where loss of hetero-zigosity is very common in many types of cancer. In this review, we will summarize the novel roles of BRIT1 in DDR, describe the relationship of BRIT1 deficiency with cancer development, and also discuss the use of synthetic lethality approach to target cancers with HR defects due to BRIT1 deficiency.