Fork Cleavage-Religation Cycle and Active Transcription Mediate Replication Restart after Fork Stalling at Co-transcriptional R-Loops.

Formation of co-transcriptional R-loops underlies replication fork stalling upon head-on transcription-replication encounters. Here, we demonstrate that RAD51-dependent replication fork reversal induced by R-loops is followed by the restart of semiconservative DNA replication mediated by RECQ1 and RECQ5 helicases, MUS81/EME1 endonuclease, RAD52 strand-annealing factor, the DNA ligase IV (LIG4)/XRCC4 complex, and the non-catalytic subunit of DNA polymerase δ, POLD3. RECQ5 disrupts RAD51 filaments assembled on stalled forks after RECQ1-mediated reverse branch migration, preventing a new round of fork reversal and facilitating fork cleavage by MUS81/EME1. MUS81-dependent DNA breaks accumulate in cells lacking RAD52 or LIG4 upon induction of R-loop formation, suggesting that RAD52 acts in concert with LIG4/XRCC4 to catalyze fork religation, thereby mediating replication restart. The resumption of DNA synthesis after R-loop-associated fork stalling also requires active transcription, the restoration of which depends on MUS81, RAD52, LIG4, and the transcription elongation factor ELL. These findings provide mechanistic insights into transcription-replication conflict resolution.

Context Matters: RNF168 Connects with PALB2 to Rewire Homologous Recombination in BRCA1 Haploinsufficiency.

In this issue of Molecular Cell, Zong et al. (2019) reveal RNF168-driven chromatin ubiquitylation as a key back-up mechanism to sustain homologous recombination (HR) independently of BRCA1. These findings provide new clues to carcinogenesis and cancer therapy in BRCA1 heterozygous mutation carriers.

CtIP-Mediated Fork Protection Synergizes with BRCA1 to Suppress Genomic Instability upon DNA Replication Stress.

Protecting stalled DNA replication forks from degradation by promiscuous nucleases is essential to prevent genomic instability, a major driving force of tumorigenesis. Several proteins commonly associated with the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) have been implicated in the stabilization of stalled forks. Human CtIP, in conjunction with the MRE11 nuclease complex, plays an important role in HR by promoting DSB resection. Here, we report an unanticipated function for CtIP in protecting reversed forks from degradation. Unlike BRCA proteins, which defend nascent DNA strands from nucleolytic attack by MRE11, we find that CtIP protects perturbed forks from erroneous over-resection by DNA2. Finally, we uncover functionally synergistic effects between CtIP and BRCA1 in mitigating replication-stress-induced genomic instability. Collectively, our findings reveal a DSB-resection- and MRE11-independent role for CtIP in preserving fork integrity that contributes to the survival of BRCA1-deficient cells.

No DDRama at chromosome ends: TRF2 takes centre stage.

Telomeres are nucleoprotein structures capping the natural termini of eukaryotic linear chromosomes. Telomeres possess an inherent ability to circumvent the activation of a full-blown DNA damage response (DDR), and hence fusion reactions, by limiting inappropriate double-strand break (DSB) repair and processing activities at eukaryotic chromosome ends. A telomere-specific protein complex, termed shelterin, has a crucial function in safeguarding and securing telomere integrity. Within this complex, TRF2 has emerged as the key player, dictating different states of telomere protection during the replicative lifespan of a cell. How TRF2 prevents activation of DSB repair activities at functional telomeres has now been extensively investigated. In this review we aim at exploring the complex and multi-faceted mechanisms underlying the TRF2-mediated protection of eukaryotic chromosome ends.

MutSß regulates G4-associated telomeric R-loops to maintain telomere integrity in ALT cancer cells.

Up to 15% of human cancers maintain their telomeres through a telomerase-independent mechanism, termed "alternative lengthening of telomeres" (ALT) that relies on homologous recombination between telomeric sequences. Emerging evidence suggests that the recombinogenic nature of ALT telomeres results from the formation of RNA:DNA hybrids (R-loops) between telomeric DNA and the long-noncoding telomeric repeat-containing RNA (TERRA). Here, we show that the mismatch repair protein MutSß, a heterodimer of MSH2 and MSH3 subunits, is enriched at telomeres in ALT cancer cells, where it prevents the accumulation of telomeric G-quadruplex (G4) structures and R-loops. Cells depleted of MSH3 display increased incidence of R-loop-dependent telomere fragility and accumulation of telomeric C-circles. We also demonstrate that purified MutSß recognizes and destabilizes G4 structures in vitro. These data suggest that MutSß destabilizes G4 structures in ALT telomeres to regulate TERRA R-loops, which is a prerequisite for maintenance of telomere integrity during ALT.

RNAi Screening Uncovers a Synthetic Sick Interaction between CtIP and the BARD1 Tumor Suppressor.

Human CtIP is best known for its role in DNA end resection to initiate DNA double-strand break repair by homologous recombination. Recently, CtIP has also been shown to protect reversed replication forks from nucleolytic degradation upon DNA replication stress. However, still little is known about the DNA damage response (DDR) networks that preserve genome integrity and sustain cell survival in the context of CtIP insufficiency. Here, to reveal such potential buffering relationships, we screened a DDR siRNA library in CtIP-deficient cells to identify candidate genes that induce synthetic sickness/lethality (SSL). Our analyses unveil a negative genetic interaction between CtIP and BARD1, the heterodimeric binding partner of BRCA1. We found that simultaneous disruption of CtIP and BARD1 triggers enhanced apoptosis due to persistent replication stress-induced DNA lesions giving rise to chromosomal abnormalities. Moreover, we observed that the genetic interaction between CtIP and BARD1 occurs independently of the BRCA1-BARD1 complex formation and might be, therefore, therapeutical relevant for the treatment of BRCA-defective tumors.

Direct and coordinate regulation of ATP-binding cassette transporter genes by Myc factors generates specific transcription signatures that significantly affect the chemoresistance phenotype of cancer cells.

Increased expression of specific ATP-binding cassette (ABC) transporters is known to mediate the efflux of chemotherapeutic agents from cancer cells. Therefore, establishing how ABC transporter genes are controlled at their transcription level may help provide insight into the role of these multifaceted transporters in the malignant phenotype. We have investigated ABC transporter gene expression in a large neuroblastoma data set of 251 tumor samples. Clustering analysis demonstrated a strong association between differential ABC gene expression patterns in tumor samples and amplification of the MYCN oncogene, suggesting a correlation with MYCN function. Using expression profiling and chromatin immunoprecipitation studies, we show that MYCN oncoprotein coordinately regulates transcription of specific ABC transporter genes, by acting as either an activator or a repressor. Finally, we extend these notions to c-MYC showing that it can also regulate the same set of ABC transporter genes in other tumor cells through similar dynamics. Overall our findings provide insight into MYC-driven molecular mechanisms that contribute to coordinate transcriptional regulation of a large set of ABC transporter genes, thus affecting global drug efflux.

FAN1, a DNA Repair Nuclease, as a Modifier of Repeat Expansion Disorders.

FAN1 encodes a DNA repair nuclease. Genetic deficiencies, copy number variants, and single nucleotide variants of FAN1 have been linked to karyomegalic interstitial nephritis, 15q13.3 microdeletion/microduplication syndrome (autism, schizophrenia, and epilepsy), cancer, and most recently repeat expansion diseases. For seven CAG repeat expansion diseases (Huntington's disease (HD) and certain spinocerebellar ataxias), modification of age of onset is linked to variants of specific DNA repair proteins. FAN1 variants are the strongest modifiers. Non-coding disease-delaying FAN1 variants and coding disease-hastening variants (p.R507H and p.R377W) are known, where the former may lead to increased FAN1 levels and the latter have unknown effects upon FAN1 functions. Current thoughts are that ongoing repeat expansions in disease-vulnerable tissues, as individuals age, promote disease onset. Fan1 is required to suppress against high levels of ongoing somatic CAG and CGG repeat expansions in tissues of HD and FMR1 transgenic mice respectively, in addition to participating in DNA interstrand crosslink repair. FAN1 is also a modifier of autism, schizophrenia, and epilepsy. Coupled with the association of these diseases with repeat expansions, this suggests a common mechanism, by which FAN1 modifies repeat diseases. Yet how any of the FAN1 variants modify disease is unknown. Here, we review FAN1 variants, associated clinical effects, protein structure, and the enzyme's attributed functional roles. We highlight how variants may alter its activities in DNA damage response and/or repeat instability. A thorough awareness of the FAN1 gene and FAN1 protein functions will reveal if and how it may be targeted for clinical benefit.

FAN1-MLH1 interaction affects repair of DNA interstrand cross-links and slipped-CAG/CTG repeats.

FAN1, a DNA structure-specific nuclease, interacts with MLH1, but the repair pathways in which this complex acts are unknown. FAN1 processes DNA interstrand crosslinks (ICLs) and FAN1 variants are modifiers of the neurodegenerative Huntington's disease (HD), presumably by regulating HD-causing CAG repeat expansions. Here, we identify specific amino acid residues in two adjacent FAN1 motifs that are critical for MLH1 binding. Disruption of the FAN1-MLH1 interaction confers cellular hypersensitivity to ICL damage and defective repair of CAG/CTG slip-outs, intermediates of repeat expansion mutations. FAN1-S126 phosphorylation, which hinders FAN1-MLH1 association, is cell cycle-regulated by cyclin-dependent kinase activity and attenuated upon ICL induction. Our data highlight the FAN1-MLH1 complex as a phosphorylation-regulated determinant of ICL response and repeat stability, opening novel paths to modify cancer and neurodegeneration.

FAN1 controls mismatch repair complex assembly via MLH1 retention to stabilize CAG repeat expansion in Huntington's disease.

CAG repeat expansion in the HTT gene drives Huntington's disease (HD) pathogenesis and is modulated by DNA damage repair pathways. In this context, the interaction between FAN1, a DNA-structure-specific nuclease, and MLH1, member of the DNA mismatch repair pathway (MMR), is not defined. Here, we identify a highly conserved SPYF motif at the N terminus of FAN1 that binds to MLH1. Our data support a model where FAN1 has two distinct functions to stabilize CAG repeats. On one hand, it binds MLH1 to restrict its recruitment by MSH3, thus inhibiting the assembly of a functional MMR complex that would otherwise promote CAG repeat expansion. On the other hand, it promotes accurate repair via its nuclease activity. These data highlight a potential avenue for HD therapeutics in attenuating somatic expansion.

Activation of tissue transglutaminase transcription by histone deacetylase inhibition as a therapeutic approach for Myc oncogenesis.

Histone deacetylase (HDAC) inhibitors reactivate tumor suppressor gene transcription; induce cancer cell differentiation, growth arrest, and programmed cell death; and are among the most promising new classes of anticancer drugs. Myc oncoproteins can block cell differentiation and promote cell proliferation and malignant transformation, in some cases by modulating target gene transcription. Here, we show that tissue transglutaminase (TG2) was commonly reactivated by HDAC inhibitors in neuroblastoma and breast cancer cells but not normal cells and contributed to HDAC inhibitor-induced growth arrest. TG2 was the gene most significantly repressed by N-Myc in neuroblastoma cells in a cDNA microarray analysis and was commonly repressed by N-Myc in neuroblastoma cells and c-Myc in breast cancer cells. Repression of TG2 expression by N-Myc in neuroblastoma cells was necessary for the inhibitory effect of N-Myc on neuroblastoma cell differentiation. Dual step cross-linking chromatin immunoprecipitation and protein coimmunoprecipitation assays showed that N-Myc acted as a transrepressor by recruiting the HDAC1 protein to an Sp1-binding site in the TG2 core promoter in a manner distinct from it's action as a transactivator at E-Box binding sites. HDAC inhibitor treatment blocked the N-Myc-mediated HDAC1 recruitment and TG2 repression in vitro. In neuroblastoma-bearing N-Myc transgenic mice, HDAC inhibitor treatment induced TG2 expression and demonstrated marked antitumor activity in vivo. Taken together, our data indicate the critical roles of HDAC1 and TG2 in Myc-induced oncogenesis and have significant implications for the use of HDAC inhibitor therapy in Myc-driven oncogenesis.

A Single-Stranded DNA-Encoded Chemical Library Based on a Stereoisomeric Scaffold Enables Ligand Discovery by Modular Assembly of Building Blocks.

A versatile and Lipinski-compliant DNA-encoded library (DEL), comprising 366 600 glutamic acid derivatives coupled to oligonucleotides serving as amplifiable identification barcodes is designed, constructed, and characterized. The GB-DEL library, constructed in single-stranded DNA format, allows de novo identification of specific binders against several pharmaceutically relevant proteins. Moreover, hybridization of the single-stranded DEL with a set of known protein ligands of low to medium affinity coupled to a complementary DNA strand results in self-assembled selectable chemical structures, leading to the identification of affinity-matured compounds.

FAN1 interaction with ubiquitylated PCNA alleviates replication stress and preserves genomic integrity independently of BRCA2.

Interstrand cross-link (ICL) hypersensitivity is a characteristic trait of Fanconi anemia (FA). Although FANCD2-associated nuclease 1 (FAN1) contributes to ICL repair, FAN1 mutations predispose to karyomegalic interstitial nephritis (KIN) and cancer rather than to FA. Thus, the biological role of FAN1 remains unclear. Because fork stalling in FAN1-deficient cells causes chromosomal instability, we reasoned that the key function of FAN1 might lie in the processing of halted replication forks. Here, we show that FAN1 contains a previously-uncharacterized PCNA interacting peptide (PIP) motif that, together with its ubiquitin-binding zinc finger (UBZ) domain, helps recruit FAN1 to ubiquitylated PCNA accumulated at stalled forks. This prevents replication fork collapse and controls their progression. Furthermore, we show that FAN1 preserves replication fork integrity by a mechanism that is distinct from BRCA2-dependent homologous recombination. Thus, targeting FAN1 activities and its interaction with ubiquitylated PCNA may offer therapeutic opportunities for treatment of BRCA-deficient tumors.

Author Correction: FAN1 interaction with ubiquitylated PCNA alleviates replication stress and preserves genomic integrity independently of BRCA2.

The financial support for this Article was not fully acknowledged. The Acknowledgements should have included the following: This study was in part supported by the Swiss National Foundation Grant No.: 31003A-156023 to Alessandro Sartori.

Cullin3-KLHL15 ubiquitin ligase mediates CtIP protein turnover to fine-tune DNA-end resection.

Human CtIP is a decisive factor in DNA double-strand break repair pathway choice by enabling DNA-end resection, the first step that differentiates homologous recombination (HR) from non-homologous end-joining (NHEJ). To coordinate appropriate and timely execution of DNA-end resection, CtIP function is tightly controlled by multiple protein-protein interactions and post-translational modifications. Here, we identify the Cullin3 E3 ligase substrate adaptor Kelch-like protein 15 (KLHL15) as a new interaction partner of CtIP and show that KLHL15 promotes CtIP protein turnover via the ubiquitin-proteasome pathway. A tripeptide motif (FRY) conserved across vertebrate CtIP proteins is essential for KLHL15-binding; its mutation blocks KLHL15-dependent CtIP ubiquitination and degradation. Consequently, DNA-end resection is strongly attenuated in cells overexpressing KLHL15 but amplified in cells either expressing a CtIP-FRY mutant or lacking KLHL15, thus impacting the balance between HR and NHEJ. Collectively, our findings underline the key importance and high complexity of CtIP modulation for genome integrity.

[Mortality in a cohort of asbestos cement workers in Bari]. / Mortalità di una coorte di lavoratori del cemento amianto a Bari.

The study describes the mortality of 417 workers employed in a asbestos-cement plant, located in Bari, Puglia, Southern Italy. Follow up started on February 1st 1972. The vital status and cause of death were ascertained at 1995. The mortality experience of the Apulian population was used as comparison. Using 90% confidence limits (CLs), a significant increase in mortality was observed in our cohort from: all causes of death (SMR 118, CL 100-139), pneumoconiosis (SMR 14810, CL 10298-20683), all types of cancer (SMR 139, CL 105-181), lung (SMR 191, CL 126-277), pleural (SMR 1560 CL 431-4081) and peritoneum (SMR 1705, CL 303-5367) malignant neoplasms. In our cohort, the discrepancy between observed and expected mortality for lung and pleural cancer occurred 30 years after the first exposure, after 40 years for all neoplasms and peritoneum cancer. Under the Cox regression model, lung cancer SMR showed a curvilinear trend along time since first exposure, the peak being detected at 35 years. Finally, SMRs from our cohort were compared to a previously described cohort including workers from the same plant compensated for asbestosis by INAIL.

TERRA-reinforced association of LSD1 with MRE11 promotes processing of uncapped telomeres.

Telomeres protect chromosome ends from being recognized as sites of DNA damage. Upon telomere shortening or telomere uncapping induced by loss of telomeric repeat-binding factor 2 (TRF2), telomeres elicit a DNA-damage response leading to cellular senescence. Here, we show that following TRF2 depletion, the levels of the long noncoding RNA TERRA increase and LSD1, which binds TERRA, is recruited to telomeres. At uncapped telomeres, LSD1 associates with MRE11, one of the nucleases implicated in the processing of 3' telomeric G overhangs, and we show that LSD1 is required for efficient removal of these structures. The LSD1-MRE11 interaction is reinforced in vivo following TERRA upregulation in TRF2-deficient cells and in vitro by TERRA-mimicking RNA oligonucleotides. Furthermore, LSD1 enhances the nuclease activity of MRE11 in vitro. Our data indicate that recruitment of LSD1 to deprotected telomeres requires MRE11 and is promoted by TERRA. LSD1 stimulates MRE11 catalytic activity and nucleolytic processing of uncapped telomeres.

Functional characterization of the TERRA transcriptome at damaged telomeres.

Telomere deprotection occurs during tumorigenesis and aging upon telomere shortening or loss of the telomeric shelterin component TRF2. Deprotected telomeres undergo changes in chromatin structure and elicit a DNA damage response (DDR) that leads to cellular senescence. The telomeric long noncoding RNA TERRA has been implicated in modulating the structure and processing of deprotected telomeres. Here, we characterize the human TERRA transcriptome at normal and TRF2-depleted telomeres and demonstrate that TERRA upregulation is occurring upon depletion of TRF2 at all transcribed telomeres. TRF2 represses TERRA transcription through its homodimerization domain, which was previously shown to induce chromatin compaction and to prevent the early steps of DDR activation. We show that TERRA associates with SUV39H1 H3K9 histone methyltransferase, which promotes accumulation of H3K9me3 at damaged telomeres and end-to-end fusions. Altogether our data elucidate the TERRA landscape and defines critical roles for this RNA in the telomeric DNA damage response.

A SP1/MIZ1/MYCN repression complex recruits HDAC1 at the TRKA and p75NTR promoters and affects neuroblastoma malignancy by inhibiting the cell response to NGF.

Neuroblastoma is the most common extracranial solid tumor of childhood. One important factor that predicts a favorable prognosis is the robust expression of the TRKA and p75NTR neurotrophin receptor genes. Interestingly, TRKA and p75NTR expression is often attenuated in aggressive MYCN-amplified tumors, suggesting a causal link between elevated MYCN activity and the transcriptional repression of TRKA and p75NTR, but the precise mechanisms involved are unclear. Here, we show that MYCN acts directly to repress TRKA and p75NTR gene transcription. Specifically, we found that MYCN levels were critical for repression and that MYCN targeted proximal/core promoter regions by forming a repression complex with transcription factors SP1 and MIZ1. When bound to the TRKA and p75NTR promoters, MYCN recruited the histone deacetylase HDAC1 to induce a repressed chromatin state. Forced re-expression of endogenous TRKA and p75NTR with exposure to the HDAC inhibitor TSA sensitized neuroblastoma cells to NGF-mediated apoptosis. By directly connecting MYCN to the repression of TRKA and p75NTR, our findings establish a key pathway of clinical pathogenicity and aggressiveness in neuroblastoma.