RESUMEN
Chromosome instability (CIN) is frequently observed in many tumors. The breakage-fusion-bridge (BFB) cycle has been proposed to be one of the main drivers of CIN during tumorigenesis and tumor evolution. However, the detailed mechanism for the individual steps of the BFB cycle warrants further investigation. Here, we demonstrate that a nuclease-dead Cas9 (dCas9) coupled with a telomere-specific single-guide RNA (sgTelo) can be used to model the BFB cycle. First, we show that targeting dCas9 to telomeres using sgTelo impedes DNA replication at telomeres and induces a pronounced increase of replication stress and DNA damage. Using Single-Molecule Telomere Assay via Optical Mapping (SMTA-OM), we investigate the genome-wide features of telomeres in the dCas9/sgTelo cells and observe a dramatic increase of chromosome end fusions, including fusion/ITS+ and fusion/ITS-. Consistently, we also observe an increase in the formation of dicentric chromosomes, anaphase bridges, and intercellular telomeric chromosome bridges (ITCBs). Utilizing the dCas9/sgTelo system, we uncover many interesting molecular and structural features of the ITCB and demonstrate that multiple DNA repair pathways are implicated in the formation of ITCBs. Our studies shed new light on the molecular mechanisms of the BFB cycle, which will advance our understanding of tumorigenesis, tumor evolution, and drug resistance.
RESUMEN
Telomeres protect chromosome ends through the interaction of telomeric repeats with shelterin, a protein complex that represses DNA damage signaling and DNA repair reactions. The telomeric repeats are maintained by telomerase, which solves the end replication problem. We report that the TTAGGG repeat arrays of mammalian telomeres pose a challenge to the DNA replication machinery, giving rise to replication-dependent defects that resemble those of aphidicolin-induced common fragile sites. Gene deletion experiments showed that efficient duplication of telomeres requires the shelterin component TRF1. Without TRF1, telomeres activate the ATR kinase in S phase and show a fragile-site phenotype in metaphase. Single-molecule analysis of replicating telomeres showed that TRF1 promotes efficient replication of TTAGGG repeats and prevents fork stalling. Two helicases implicated in the removal of G4 DNA structures, BLM and RTEL1, were required to repress the fragile-telomere phenotype. These results identify a second telomere replication problem that is solved by the shelterin component TRF1.
Asunto(s)
Sitios Frágiles del Cromosoma , Replicación del ADN , Telómero/metabolismo , Proteína 1 de Unión a Repeticiones Teloméricas/metabolismo , Animales , Afidicolina , Cromosomas de los Mamíferos/metabolismo , Humanos , Metafase , Ratones , Proteína 1 de Unión a Repeticiones Teloméricas/genéticaRESUMEN
Common fragile sites (CFSs) are genomic regions that are unstable under conditions of replicative stress. Although the characteristics of CFSs that render them vulnerable to stress are associated mainly with replication, the cellular pathways that protect CFSs during replication remain unclear. Here, we identify and describe a role for FANCD2 as a trans-acting facilitator of CFS replication, in the absence of exogenous replicative stress. In the absence of FANCD2, replication forks stall within the AT-rich fragility core of CFS, leading to dormant origin activation. Furthermore, FANCD2 deficiency is associated with DNA:RNA hybrid formation at CFS-FRA16D, and inhibition of DNA:RNA hybrid formation suppresses replication perturbation. In addition, we also found that FANCD2 reduces the number of potential sites of replication initiation. Our data demonstrate that FANCD2 protein is required to ensure efficient CFS replication and provide mechanistic insight into how FANCD2 regulates CFS stability.
Asunto(s)
Sitios Frágiles del Cromosoma , Replicación del ADN , ADN/genética , Proteína del Grupo de Complementación D2 de la Anemia de Fanconi/genética , ARN/genética , Proteína BRCA2/genética , Proteína BRCA2/metabolismo , Línea Celular Transformada , ADN/metabolismo , Anemia de Fanconi , Proteína del Grupo de Complementación A de la Anemia de Fanconi/genética , Proteína del Grupo de Complementación A de la Anemia de Fanconi/metabolismo , Proteína del Grupo de Complementación D2 de la Anemia de Fanconi/metabolismo , Proteínas del Grupo de Complementación de la Anemia de Fanconi/genética , Proteínas del Grupo de Complementación de la Anemia de Fanconi/metabolismo , Fibroblastos/citología , Fibroblastos/metabolismo , Expresión Génica , Inestabilidad Genómica , Herpesvirus Humano 4/genética , Herpesvirus Humano 4/metabolismo , Humanos , Linfocitos/citología , Linfocitos/metabolismo , ARN/metabolismoRESUMEN
Common fragile sites (CFSs) are difficult-to-replicate genomic regions that form gaps and breaks on metaphase chromosomes under replication stress. They are hotspots for chromosomal instability in cancer. Repetitive sequences located at CFS loci are inefficiently copied by replicative DNA polymerase (Pol) delta. However, translesion synthesis Pol eta has been shown to efficiently polymerize CFS-associated repetitive sequences in vitro and facilitate CFS stability by a mechanism that is not fully understood. Here, by locus-specific, single-molecule replication analysis, we identified a crucial role for Pol eta (encoded by the gene POLH) in the in vivo replication of CFSs, even without exogenous stress. We find that Pol eta deficiency induces replication pausing, increases initiation events, and alters the direction of replication-fork progression at CFS-FRA16D in both lymphoblasts and fibroblasts. Furthermore, certain replication pause sites at CFS-FRA16D were associated with the presence of non-B DNA-forming motifs, implying that non-B DNA structures could increase replication hindrance in the absence of Pol eta. Further, in Pol eta-deficient fibroblasts, there was an increase in fork pausing at fibroblast-specific CFSs. Importantly, while not all pause sites were associated with non-B DNA structures, they were embedded within regions of increased genetic variation in the healthy human population, with mutational spectra consistent with Pol eta activity. From these findings, we propose that Pol eta replicating through CFSs may result in genetic variations found in the human population at these sites.
Asunto(s)
Sitios Frágiles del Cromosoma/genética , ADN Polimerasa Dirigida por ADN/metabolismo , ADN Polimerasa Dirigida por ADN/fisiología , Línea Celular , Fragilidad Cromosómica/genética , Fragilidad Cromosómica/fisiología , ADN/genética , Daño del ADN/genética , ADN Polimerasa III/metabolismo , Reparación del ADN/genética , Reparación del ADN/fisiología , Replicación del ADN/fisiología , Variación Genética/genética , Inestabilidad Genómica/genética , Humanos , Antígeno Nuclear de Célula en Proliferación/metabolismoRESUMEN
Fragile X syndrome (FXS) is caused by a CGG repeat expansion in the FMR1 gene that appears to occur during oogenesis and during early embryogenesis. One model proposes that repeat instability depends on the replication fork direction through the repeats such that (CNG)n hairpin-like structures form, causing DNA polymerase to stall and slip. Examining DNA replication fork progression on single DNA molecules at the endogenous FMR1 locus revealed that replication forks stall at CGG repeats in human cells. Furthermore, replication profiles of FXS human embryonic stem cells (hESCs) compared to nonaffected hESCs showed that fork direction through the repeats is altered at the FMR1 locus in FXS hESCs, such that predominantly the CCG strand serves as the lagging-strand template. This is due to the absence of replication initiation that would typically occur upstream of FMR1, suggesting that altered replication origin usage combined with fork stalling promotes repeat instability during early embryonic development.
Asunto(s)
Replicación del ADN , Células Madre Embrionarias/metabolismo , Proteína de la Discapacidad Intelectual del Síndrome del Cromosoma X Frágil/metabolismo , Síndrome del Cromosoma X Frágil/embriología , Sitios Genéticos , Repeticiones de Trinucleótidos , Desarrollo Embrionario/genética , Células Madre Embrionarias/patología , Proteína de la Discapacidad Intelectual del Síndrome del Cromosoma X Frágil/genética , Síndrome del Cromosoma X Frágil/genética , Síndrome del Cromosoma X Frágil/patología , HumanosRESUMEN
In the mammalian genome, certain genomic loci/regions pose greater challenges to the DNA replication machinery (i.e., the replisome) than others. Such known genomic loci/regions include centromeres, common fragile sites, subtelomeres, and telomeres. However, the detailed mechanism of how mammalian cells cope with the replication stress at these loci/regions is largely unknown. Here we show that depletion of FANCM, or of one of its obligatory binding partners, FAAP24, MHF1, and MHF2, induces replication stress primarily at the telomeres of cells that use the alternative lengthening of telomeres (ALT) pathway as their telomere maintenance mechanism. Using the telomere-specific single-molecule analysis of replicated DNA technique, we found that depletion of FANCM dramatically reduces the replication efficiency at ALT telomeres. We further show that FANCM, BRCA1, and BLM are actively recruited to the ALT telomeres that are experiencing replication stress and that the recruitment of BRCA1 and BLM to these damaged telomeres is interdependent and is regulated by both ATR and Chk1. Mechanistically, we demonstrated that, in FANCM-depleted ALT cells, BRCA1 and BLM help to resolve the telomeric replication stress by stimulating DNA end resection and homologous recombination (HR). Consistent with their roles in resolving the replication stress induced by FANCM deficiency, simultaneous depletion of BLM and FANCM, or of BRCA1 and FANCM, leads to increased micronuclei formation and synthetic lethality in ALT cells. We propose that these synthetic lethal interactions can be explored for targeting the ALT cancers.
Asunto(s)
Proteína BRCA1/metabolismo , ADN Helicasas/metabolismo , Replicación del ADN/fisiología , RecQ Helicasas/metabolismo , Homeostasis del Telómero/fisiología , Proteínas de la Ataxia Telangiectasia Mutada/genética , Proteínas de la Ataxia Telangiectasia Mutada/metabolismo , Proteína BRCA1/genética , Línea Celular , Quinasa 1 Reguladora del Ciclo Celular (Checkpoint 1)/genética , Quinasa 1 Reguladora del Ciclo Celular (Checkpoint 1)/metabolismo , ADN Helicasas/genética , Recombinación Homóloga , Humanos , RecQ Helicasas/genética , Telómero/genética , Telómero/metabolismoRESUMEN
Kaposi's sarcoma associated herpesvirus (KSHV) establishes life-long latent infection by persisting as an extra-chromosomal episome in the infected cells and by maintaining its genome in dividing cells. KSHV achieves this by tethering its epigenome to the host chromosome by latency associated nuclear antigen (LANA), which binds in the terminal repeat (TR) region of the viral genome. Sequence analysis of the TR, a GC-rich DNA element, identified several potential Quadruplex G-Rich Sequences (QGRS). Since quadruplexes have the tendency to obstruct DNA replication, we used G-quadruplex stabilizing compounds to examine their effect on latent DNA replication and the persistence of viral episomes. Our results showed that these G-quadruplex stabilizing compounds led to the activation of dormant origins of DNA replication, with preferential bi-directional pausing of replications forks moving out of the TR region, implicating the role of the G-rich TR in the perturbation of episomal DNA replication. Over time, treatment with PhenDC3 showed a loss of viral episomes in the infected cells. Overall, these data show that G-quadruplex stabilizing compounds retard the progression of replication forks leading to a reduction in DNA replication and episomal maintenance. These results suggest a potential role for G-quadruplex stabilizers in the treatment of KSHV-associated diseases.
Asunto(s)
Replicación del ADN/efectos de los fármacos , G-Cuádruplex/efectos de los fármacos , Herpesvirus Humano 8/efectos de los fármacos , Herpesvirus Humano 8/genética , Plásmidos/efectos de los fármacos , Línea Celular , Genoma Viral/efectos de los fármacos , Células HEK293 , Herpesvirus Humano 8/fisiología , Humanos , Porfirinas/farmacología , Origen de Réplica , Secuencias Repetidas Terminales , Latencia del VirusRESUMEN
Eukaryotic genome duplication relies on origins of replication, distributed over multiple chromosomes, to initiate DNA replication. A recent genome-wide analysis of Trypanosoma brucei, the etiological agent of sleeping sickness, localized its replication origins to the boundaries of multigenic transcription units. To better understand genomic replication in this organism, we examined replication by single molecule analysis of replicated DNA. We determined the average speed of replication forks of procyclic and bloodstream form cells and we found that T. brucei DNA replication rate is similar to rates seen in other eukaryotes. We also analyzed the replication dynamics of a central region of chromosome 1 in procyclic forms. We present evidence for replication terminating within the central part of the chromosome and thus emanating from both sides, suggesting a previously unmapped origin toward the 5' extremity of chromosome 1. Also, termination is not at a fixed location in chromosome 1, but is rather variable. Importantly, we found a replication origin located near an ORC1/CDC6 binding site that is detected after replicative stress induced by hydroxyurea treatment, suggesting it may be a dormant origin activated in response to replicative stress. Collectively, our findings support the existence of more replication origins in T. brucei than previously appreciated.
Asunto(s)
Replicación del ADN/genética , ADN Protozoario/genética , Biología Molecular/métodos , Origen de Réplica/genética , Trypanosoma brucei brucei/genética , Proteínas de Ciclo Celular/genética , Proteínas de Ciclo Celular/metabolismo , Cromosomas/genética , Replicación del ADN/efectos de los fármacos , Citometría de Flujo , Genoma de Protozoos/genética , Hidroxiurea/farmacología , Cinética , Estadios del Ciclo de Vida/genética , Inhibidores de la Síntesis del Ácido Nucleico/farmacología , Complejo de Reconocimiento del Origen/genética , Complejo de Reconocimiento del Origen/metabolismo , Reacción en Cadena de la Polimerasa , Proteínas Protozoarias/genética , Proteínas Protozoarias/metabolismo , Factores de Tiempo , Trypanosoma brucei brucei/efectos de los fármacos , Trypanosoma brucei brucei/crecimiento & desarrolloRESUMEN
Cohesin is a protein complex originally identified for its role in sister chromatid cohesion, although increasing evidence portrays it also as a major organizer of interphase chromatin. Vertebrate cohesin consists of Smc1, Smc3, Rad21/Scc1 and either stromal antigen 1 (SA1) or SA2. To explore the functional specificity of these two versions of cohesin and their relevance for embryonic development and cancer, we generated a mouse model deficient for SA1. Complete ablation of SA1 results in embryonic lethality, while heterozygous animals have shorter lifespan and earlier onset of tumourigenesis. SA1-null mouse embryonic fibroblasts show decreased proliferation and increased aneuploidy as a result of chromosome segregation defects. These defects are not caused by impaired centromeric cohesion, which depends on cohesin-SA2. Instead, they arise from defective telomere replication, which requires cohesion mediated specifically by cohesin-SA1. We propose a novel mechanism for aneuploidy generation that involves impaired telomere replication upon loss of cohesin-SA1, with clear implications in tumourigenesis.
Asunto(s)
Aneuploidia , Proteínas de Ciclo Celular/deficiencia , Proteínas Cromosómicas no Histona/deficiencia , Subunidades de Proteína/deficiencia , Telómero/metabolismo , Animales , Carcinógenos , Proteínas de Ciclo Celular/genética , Línea Celular , Cromátides/metabolismo , Proteínas Cromosómicas no Histona/genética , Segregación Cromosómica , Dietilnitrosamina , Fibrosarcoma/inducido químicamente , Fibrosarcoma/genética , Fibrosarcoma/patología , Neoplasias Hepáticas/inducido químicamente , Neoplasias Hepáticas/genética , Neoplasias Hepáticas/patología , Masculino , Metilcolantreno , Ratones , Ratones Noqueados , Neoplasias Experimentales/inducido químicamente , Neoplasias Experimentales/genética , Neoplasias Experimentales/patología , Subunidades de Proteína/genética , Intercambio de Cromátides Hermanas , CohesinasRESUMEN
The temporal order of replication of mammalian chromosomes appears to be linked to their functional organization, but the process that establishes and modifies this order during cell differentiation remains largely unknown. Here, we studied how the replication of the Igh locus initiates, progresses, and terminates in bone marrow pro-B cells undergoing B cell commitment. We show that many aspects of DNA replication can be quantitatively explained by a mechanism involving the stochastic firing of origins (across the S phase and the Igh locus) and extensive variations in their firing rate (along the locus). The firing rate of origins shows a high degree of coordination across Igh domains that span tens to hundreds of kilobases, a phenomenon not observed in simple eukaryotes. Differences in domain sizes and firing rates determine the temporal order of replication. During B cell commitment, the expression of the B-cell-specific factor Pax5 sharply alters the temporal order of replication by modifying the rate of origin firing within various Igh domains (particularly those containing Pax5 binding sites). We propose that, within the Igh C(H)-3'RR domain, Pax5 is responsible for both establishing and maintaining high rates of origin firing, mostly by controlling events downstream of the assembly of pre-replication complexes.
Asunto(s)
Linfocitos B/citología , Replicación del ADN , Cadenas Pesadas de Inmunoglobulina/genética , Animales , Sitios de Unión , Linaje de la Célula , Humanos , Ratones , Factor de Transcripción PAX5/metabolismo , Procesos EstocásticosRESUMEN
Chromosome instability (CIN) is frequently observed in many tumors. The breakage-fusion-bridge (BFB) cycle has been proposed to be one of the main drivers of CIN during tumorigenesis and tumor evolution. However, the detailed mechanisms for the individual steps of the BFB cycle warrants further investigation. Here, we demonstrated that a nuclease-dead Cas9 (dCas9) coupled with a telomere-specific single-guide RNA (sgTelo) can be used to model the BFB cycle. First, we showed that targeting dCas9 to telomeres using sgTelo impeded DNA replication at telomeres and induced a pronounced increase of replication stress and DNA damage. Using Single-Molecule Telomere Assay via Optical Mapping (SMTA-OM), we investigated the genome-wide features of telomeres in the dCas9/sgTelo cells and observed a dramatic increase of chromosome end fusions, including fusion/ITS+ and fusion/ITS-.Consistently, we also observed an increase in the formation of dicentric chromosomes, anaphase bridges, and intercellular telomeric chromosome bridges (ITCBs). Utilizing the dCas9/sgTelo system, we uncovered many novel molecular and structural features of the ITCB and demonstrated that multiple DNA repair pathways are implicated in the formation of ITCBs. Our studies shed new light on the molecular mechanisms of the BFB cycle, which will advance our understanding of tumorigenesis, tumor evolution, and drug resistance.
RESUMEN
Kaposi's sarcoma associated herpesvirus (KSHV), an etiologic agent of Kaposi's sarcoma, Body Cavity Based Lymphoma and Multicentric Castleman's Disease, establishes lifelong latency in infected cells. The KSHV genome tethers to the host chromosome with the help of a latency associated nuclear antigen (LANA). Additionally, LANA supports replication of the latent origins within the terminal repeats by recruiting cellular factors. Our previous studies identified and characterized another latent origin, which supported the replication of plasmids ex-vivo without LANA expression in trans. Therefore identification of an additional origin site prompted us to analyze the entire KSHV genome for replication initiation sites using single molecule analysis of replicated DNA (SMARD). Our results showed that replication of DNA can initiate throughout the KSHV genome and the usage of these regions is not conserved in two different KSHV strains investigated. SMARD also showed that the utilization of multiple replication initiation sites occurs across large regions of the genome rather than a specified sequence. The replication origin of the terminal repeats showed only a slight preference for their usage indicating that LANA dependent origin at the terminal repeats (TR) plays only a limited role in genome duplication. Furthermore, we performed chromatin immunoprecipitation for ORC2 and MCM3, which are part of the pre-replication initiation complex to determine the genomic sites where these proteins accumulate, to provide further characterization of potential replication initiation sites on the KSHV genome. The ChIP data confirmed accumulation of these pre-RC proteins at multiple genomic sites in a cell cycle dependent manner. Our data also show that both the frequency and the sites of replication initiation vary within the two KSHV genomes studied here, suggesting that initiation of replication is likely to be affected by the genomic context rather than the DNA sequences.
Asunto(s)
ADN Viral/biosíntesis , Genoma Viral , Herpesvirus Humano 8/genética , Herpesvirus Humano 8/fisiología , Replicación Viral , Antígenos Virales/genética , Antígenos Virales/metabolismo , Ciclo Celular , Proteínas de Ciclo Celular/genética , Proteínas de Ciclo Celular/metabolismo , Línea Celular Tumoral , Inmunoprecipitación de Cromatina , Replicación del ADN , ADN Viral/química , ADN Viral/genética , Proteínas de Unión al ADN/genética , Proteínas de Unión al ADN/metabolismo , Humanos , Componente 3 del Complejo de Mantenimiento de Minicromosoma , Proteínas Nucleares/genética , Proteínas Nucleares/metabolismo , Complejo de Reconocimiento del Origen/genética , Complejo de Reconocimiento del Origen/metabolismo , Origen de Réplica , Latencia del VirusRESUMEN
When replication forks encounter DNA lesions that cause polymerase stalling, a checkpoint pathway is activated. The ATR-dependent intra-S checkpoint pathway mediates detection and processing of sites of replication fork stalling to maintain genomic integrity. Several factors involved in the global checkpoint pathway have been identified, but the response to a single replication fork barrier (RFB) is poorly understood. We utilized the Escherichia coli-based Tus-Ter system in human MCF7 cells and showed that the Tus protein binding to TerB sequences creates an efficient site-specific RFB. The single fork RFB was sufficient to activate a local, but not global, ATR-dependent checkpoint response that leads to phosphorylation and accumulation of DNA damage sensor protein γH2AX, confined locally to within a kilobase of the site of stalling. These data support a model of local management of fork stalling, which allows global replication at sites other than the RFB to continue to progress without delay.
Asunto(s)
Daño del ADN , Replicación del ADN , Humanos , Fosforilación , Escherichia coli/genética , Genómica , Proteínas de la Ataxia Telangiectasia Mutada/genéticaRESUMEN
When replication forks encounter DNA lesions that cause polymerase stalling a checkpoint pathway is activated. The ATR-dependent intra-S checkpoint pathway mediates detection and processing of sites of replication fork stalling to maintain genomic integrity. Several factors involved in the global checkpoint pathway have been identified, but the response to a single replication fork barrier (RFB) is poorly understood. We utilized the E.coli -based Tus- Ter system in human MCF7 cells and showed that the Tus protein binding to TerB sequences creates an efficient site-specific RFB. The single fork RFB was sufficient to activate a local, but not global, ATR-dependent checkpoint response that leads to phosphorylation and accumulation of DNA damage sensor protein γH2AX, confined locally to within a kilobase of the site of stalling. These data support a model of local management of fork stalling, which allows global replication at sites other than the RFB to continue to progress without delay.
RESUMEN
The organization of mammalian DNA replication is poorly understood. We have produced high-resolution dynamic maps of the timing of replication in human erythroid, mesenchymal, and embryonic stem (ES) cells using TimEX, a method that relies on gaussian convolution of massive, highly redundant determinations of DNA copy-number variations during S phase to produce replication timing profiles. We first obtained timing maps of 3% of the genome using high-density oligonucleotide tiling arrays and then extended the TimEX method genome-wide using massively parallel sequencing. We show that in untransformed human cells, timing of replication is highly regulated and highly synchronous, and that many genomic segments are replicated in temporal transition regions devoid of initiation, where replication forks progress unidirectionally from origins that can be hundreds of kilobases away. Absence of initiation in one transition region is shown at the molecular level by single molecule analysis of replicated DNA (SMARD). Comparison of ES and erythroid cells replication patterns revealed that these cells replicate about 20% of their genome in different quarters of S phase. Importantly, we detected a strong inverse relationship between timing of replication and distance to the closest expressed gene. This relationship can be used to predict tissue-specific timing of replication profiles from expression data and genomic annotations. We also provide evidence that early origins of replication are preferentially located near highly expressed genes, that mid-firing origins are located near moderately expressed genes, and that late-firing origins are located far from genes.
Asunto(s)
Momento de Replicación del ADN , Replicación del ADN , Células Madre Embrionarias , Células Eritroides , Perfilación de la Expresión Génica , Células Madre Mesenquimatosas , Fase S , Diferenciación Celular , ADN/biosíntesis , ADN/genética , Células Madre Embrionarias/citología , Células Madre Embrionarias/metabolismo , Células Eritroides/citología , Células Eritroides/metabolismo , Dosificación de Gen , Humanos , Células Madre Mesenquimatosas/citología , Células Madre Mesenquimatosas/metabolismo , Distribución NormalRESUMEN
Telomere dysfunction causes chromosomal instability which is associated with many cancers and age-related diseases. The non-coding telomeric repeat-containing RNA (TERRA) forms a structural and regulatory component of the telomere that is implicated in telomere maintenance and chromosomal end protection. The basic N-terminal Gly/Arg-rich (GAR) domain of telomeric repeat-binding factor 2 (TRF2) can bind TERRA but the structural basis and significance of this interaction remains poorly understood. Here, we show that TRF2 GAR recognizes G-quadruplex features of TERRA. We show that small molecules that disrupt the TERRA-TRF2 GAR complex, such as N-methyl mesoporphyrin IX (NMM) or genetic deletion of TRF2 GAR domain, result in the loss of TERRA, and the induction of γH2AX-associated telomeric DNA damage associated with decreased telomere length, and increased telomere aberrations, including telomere fragility. Taken together, our data indicates that the G-quadruplex structure of TERRA is an important recognition element for TRF2 GAR domain and this interaction between TRF2 GAR and TERRA is essential to maintain telomere stability.
Asunto(s)
Daño del ADN/genética , ARN/genética , Telómero/metabolismo , Proteína 2 de Unión a Repeticiones Teloméricas/genética , Proteínas de Unión al ADN/genética , G-Cuádruplex/efectos de los fármacos , Humanos , Unión Proteica/genética , ARN/metabolismo , ARN Largo no Codificante/genética , Proteína 2 de Unión a Repeticiones Teloméricas/metabolismo , Factores de Transcripción/genéticaRESUMEN
The telomeric shelterin protein telomeric repeat-binding factor 2 (TRF2) recruits origin recognition complex (ORC) proteins, the foundational building blocks of DNA replication origins, to telomeres. We seek to determine whether TRF2-recruited ORC proteins give rise to functional origins in telomere repeat tracts. We find that reduction of telomeric recruitment of ORC2 by expression of an ORC interaction-defective TRF2 mutant significantly reduces telomeric initiation events in human cells. This reduction in initiation events is accompanied by telomere repeat loss, telomere aberrations and dysfunction. We demonstrate that telomeric origins are activated by induced replication stress to provide a key rescue mechanism for completing compromised telomere replication. Importantly, our studies also indicate that the chromatin remodeler SNF2H promotes telomeric initiation events by providing access for ORC2. Collectively, our findings reveal that active recruitment of ORC by TRF2 leads to formation of functional origins, providing an important mechanism for avoiding telomere dysfunction and rescuing challenged telomere replication.
Asunto(s)
Replicación del ADN/genética , Telómero/metabolismo , Proteína 2 de Unión a Repeticiones Teloméricas/metabolismo , Inestabilidad Genómica , HumanosRESUMEN
Human DNA polymerase delta (Pol δ) forms a holoenzyme complex with the DNA sliding clamp proliferating cell nuclear antigen (PCNA) to perform its essential roles in genome replication. Here, we utilize live-cell single-molecule tracking to monitor Pol δ holoenzyme interaction with the genome in real time. We find holoenzyme assembly and disassembly in vivo are highly dynamic and ordered. PCNA generally loads onto the genome before Pol δ. Once assembled, the holoenzyme has a relatively short lifetime on the genome, implying multiple Pol δ binding events may be needed to synthesize an Okazaki fragment. During disassembly, Pol δ dissociation generally precedes PCNA unloading. We also find that Pol δ p125, the catalytic subunit of the holoenzyme, is maintained at a constant cellular level, indicating an active mechanism for control of Pol δ levels in vivo. Collectively, our studies reveal that Pol δ holoenzyme assembly and disassembly follow a predominant pathway in vivo; however, alternate pathways are observed.
Asunto(s)
ADN Polimerasa III/metabolismo , Genoma Humano , Holoenzimas/metabolismo , Biocatálisis , Línea Celular , Cromatina/metabolismo , Humanos , Antígeno Nuclear de Célula en Proliferación/metabolismoRESUMEN
Telomeres protect chromosome ends from inappropriately activating the DNA damage and repair responses. Primary microcephaly is a key clinical feature of several human telomere disorder syndromes, but how microcephaly is linked to dysfunctional telomeres is not known. Here, we show that the microcephalin 1/BRCT-repeats inhibitor of hTERT (MCPH1/BRIT1) protein, mutated in primary microcephaly, specifically interacts with the TRFH domain of the telomere binding protein TRF2. The crystal structure of the MCPH1-TRF2 complex reveals that this interaction is mediated by the MCPH1 330YRLSP334 motif. TRF2-dependent recruitment of MCPH1 promotes localization of DNA damage factors and homology directed repair of dysfunctional telomeres lacking POT1-TPP1. Additionally, MCPH1 is involved in the replication stress response, promoting telomere replication fork progression and restart of stalled telomere replication forks. Our work uncovers a previously unrecognized role for MCPH1 in promoting telomere replication, providing evidence that telomere replication defects may contribute to the onset of microcephaly.
Asunto(s)
Proteínas de Ciclo Celular/metabolismo , Proteínas del Citoesqueleto/metabolismo , Microcefalia/genética , Telómero/genética , Proteína 2 de Unión a Repeticiones Teloméricas/metabolismo , Aminopeptidasas/genética , Aminopeptidasas/metabolismo , Animales , Sitios de Unión , Calorimetría , Proteínas de Ciclo Celular/química , Proteínas de Ciclo Celular/genética , Proteínas del Citoesqueleto/química , Proteínas del Citoesqueleto/genética , Daño del ADN , Dipeptidil-Peptidasas y Tripeptidil-Peptidasas/genética , Dipeptidil-Peptidasas y Tripeptidil-Peptidasas/metabolismo , Fibroblastos , Células HeLa , Histonas/genética , Histonas/metabolismo , Humanos , Ratones , Mutación , Dominios y Motivos de Interacción de Proteínas , Serina Proteasas/genética , Serina Proteasas/metabolismo , Complejo Shelterina , Telómero/metabolismo , Proteínas de Unión a Telómeros/genética , Proteínas de Unión a Telómeros/metabolismo , Proteína 2 de Unión a Repeticiones Teloméricas/química , Proteína 2 de Unión a Repeticiones Teloméricas/genéticaRESUMEN
Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by mutations in the FMR1 gene and deficiency of a functional FMRP protein. FMRP is known as a translation repressor whose nuclear function is not understood. We investigated the global impact on genome stability due to FMRP loss. Using Break-seq, we map spontaneous and replication stress-induced DNA double-strand breaks (DSBs) in an FXS patient-derived cell line. We report that the genomes of FXS cells are inherently unstable and accumulate twice as many DSBs as those from an unaffected control. We demonstrate that replication stress-induced DSBs in FXS cells colocalize with R-loop forming sequences. Exogenously expressed FMRP in FXS fibroblasts ameliorates DSB formation. FMRP, not the I304N mutant, abates R-loop-induced DSBs during programmed replication-transcription conflict. These results suggest that FMRP is a genome maintenance protein that prevents R-loop accumulation. Our study provides insights into the etiological basis for FXS.