Role of DNA secondary structures in fragile site breakage along human chromosome 10

DNA fragility at topologically associated domain boundaries is promoted by alternative DNA secondary structure and topoisomerase II activity

CCCTC-binding factor (CTCF) binding sites are hotspots of genome instability. Although many factors have been associated with CTCF binding site fragility, no study has integrated all fragility-related factors to understand the mechanism(s) of how they work together. Using an unbiased, genome-wide approach, we found that DNA double-strand breaks (DSBs) are enriched at strong, but not weak, CTCF binding sites in five human cell types. Energetically favorable alternative DNA secondary structures underlie strong CTCF binding sites. These structures coincided with the location of topoisomerase II (TOP2) cleavage complex, suggesting that DNA secondary structure acts as a recognition sequence for TOP2 binding and cleavage at CTCF binding sites. Furthermore, CTCF knockdown significantly increased DSBs at strong CTCF binding sites and at CTCF sites that are located at topologically associated domain (TAD) boundaries. TAD boundary-associated CTCF sites that lost CTCF upon knockdown displayed increased DSBs when compared to the gained sites, and those lost sites are overrepresented with G-quadruplexes, suggesting that the structures act as boundary insulators in the absence of CTCF, and contribute to increased DSBs. These results model how alternative DNA secondary structures facilitate recruitment of TOP2 to CTCF binding sites, providing mechanistic insight into DNA fragility at CTCF binding sites.

Intrinsic deletion at 10q23.31, including the PTEN gene locus, is aggravated upon CRISPR-Cas9-mediated genome engineering in HAP1 cells mimicking cancer profiles

The CRISPR-Cas9 system is a powerful tool for studying gene functions and holds potential for disease treatment. However, precise genome editing requires thorough assessments to minimize unintended on- and off-target effects. Here, we report an unexpected 283-kb deletion on Chromosome 10 (10q23.31) in chronic myelogenous leukemia-derived HAP1 cells, which are frequently used in CRISPR screens. The deleted region encodes regulatory genes, including PAPSS2, ATAD1, KLLN, and PTEN We found that this deletion was not a direct consequence of CRISPR-Cas9 off-targeting but rather occurred frequently during the generation of CRISPR-Cas9-modified cells. The deletion was associated with global changes in histone acetylation and gene expression, affecting fundamental cellular processes such as cell cycle and DNA replication. We detected this deletion in cancer patient genomes. As in HAP1 cells, the deletion contributed to similar gene expression patterns among cancer patients despite interindividual differences. Our findings suggest that the unintended deletion of 10q23.31 can confound CRISPR-Cas9 studies and underscore the importance to assess unintended genomic changes in CRISPR-Cas9-modified cells, which could impact cancer research.

Insights into common fragile site instability: DNA replication challenges at DNA repeat sequences

Common fragile sites (CFS) are specific genomic regions prone to chromosomal instability under conditions of DNA replication stress. CFSs manifest as breaks, gaps, and constrictions on metaphase chromosomes under mild replication stress. These replication-sensitive CFS regions are preferentially unstable during cancer development, as reflected by their association with copy number variants (CNVs) frequently arise in most tumor types. Over the years, it became clear that a combination of different characteristics underlies the enhanced sensitivity of CFSs to replication stress. As of today, there is a strong evidence that the core fragility regions along CFSs overlap with actively transcribed large genes with delayed replication timing upon replication stress. Recently, the mechanistic basis for CFS instability was further extended to regions which span topologically associated domain (TAD) boundaries, generating a fragility signature composed of replication, transcription and genome organization. The presence of difficult-to-replicate AT-rich repeats was one of the early features suggested to characterize a subgroup of CFSs. These long stretches of AT-dinucleotide have the potential to fold into stable secondary structures which may impede replication fork progression, leaving the region under-replicated. Here, we focus on the molecular mechanisms underlying repeat instability at CFSs and on the proteins involved in the resolution of secondary structure impediments arising along repetitive sequence elements which are essential for the maintenance of genome stability.

Revisiting recent unusual drug-DNA complex structures: Implications for cancer and neurological disease diagnostics and therapeutics

DNA plays a crucial role in various biological processes such as protein production, replication, recombination etc. by adopting different conformations. Targeting these conformations by small molecules is not only important for disease therapy, but also improves our understanding of the mechanisms of disease development. In this review, we provide an overview of some of the most recent ligand-DNA complexes that have diagnostic and therapeutic applications in neurological diseases caused by abnormal repeat expansions and in cancer associated with mismatches. In addition, we have discussed important implications of ligands targeting higher-order structures, such as four-way junctions, G-quadruplexes and triplexes for drug discovery and DNA nanotechnology. We provide an overview of the results and perspectives of such structural studies on ligand-DNA interactions.

Fragile sites, chromosomal lesions, tandem repeats, and disease

Expanded tandem repeat DNAs are associated with various unusual chromosomal lesions, despiralizations, multi-branched inter-chromosomal associations, and fragile sites. Fragile sites cytogenetically manifest as localized gaps or discontinuities in chromosome structure and are an important genetic, biological, and health-related phenomena. Common fragile sites (∼230), present in most individuals, are induced by aphidicolin and can be associated with cancer; of the 27 molecularly-mapped common sites, none are associated with a particular DNA sequence motif. Rare fragile sites ( ≳ 40 known), ≤ 5% of the population (may be as few as a single individual), can be associated with neurodevelopmental disease. All 10 molecularly-mapped folate-sensitive fragile sites, the largest category of rare fragile sites, are caused by gene-specific CGG/CCG tandem repeat expansions that are aberrantly CpG methylated and include FRAXA, FRAXE, FRAXF, FRA2A, FRA7A, FRA10A, FRA11A, FRA11B, FRA12A, and FRA16A. The minisatellite-associated rare fragile sites, FRA10B, FRA16B, can be induced by AT-rich DNA-ligands or nucleotide analogs. Despiralized lesions and multi-branched inter-chromosomal associations at the heterochromatic satellite repeats of chromosomes 1, 9, 16 are inducible by de-methylating agents like 5-azadeoxycytidine and can spontaneously arise in patients with ICF syndrome (Immunodeficiency Centromeric instability and Facial anomalies) with mutations in genes regulating DNA methylation. ICF individuals have hypomethylated satellites I-III, alpha-satellites, and subtelomeric repeats. Ribosomal repeats and subtelomeric D4Z4 megasatellites/macrosatellites, are associated with chromosome location, fragility, and disease. Telomere repeats can also assume fragile sites. Dietary deficiencies of folate or vitamin B12, or drug insults are associated with megaloblastic and/or pernicious anemia, that display chromosomes with fragile sites. The recent discovery of many new tandem repeat expansion loci, with varied repeat motifs, where motif lengths can range from mono-nucleotides to megabase units, could be the molecular cause of new fragile sites, or other chromosomal lesions. This review focuses on repeat-associated fragility, covering their induction, cytogenetics, epigenetics, cell type specificity, genetic instability (repeat instability, micronuclei, deletions/rearrangements, and sister chromatid exchange), unusual heritability, disease association, and penetrance. Understanding tandem repeat-associated chromosomal fragile sites provides insight to chromosome structure, genome packaging, genetic instability, and disease.

OUP accepted manuscript

Conflicts between transcription and replication machinery are a potent source of replication stress and genome instability; however, no technique currently exists to identify endogenous genomic locations prone to transcription–replication interactions. Here, we report a novel method to identify genomic loci prone to transcription–replication interactions termed transcription–replication immunoprecipitation on nascent DNA sequencing, TRIPn-Seq. TRIPn-Seq employs the sequential immunoprecipitation of RNA polymerase 2 phosphorylated at serine 5 (RNAP2s5) followed by enrichment of nascent DNA previously labeled with bromodeoxyuridine. Using TRIPn-Seq, we mapped 1009 unique transcription–replication interactions (TRIs) in mouse primary B cells characterized by a bimodal pattern of RNAP2s5, bidirectional transcription, an enrichment of RNA:DNA hybrids, and a high probability of forming G-quadruplexes. TRIs are highly enriched at transcription start sites and map to early replicating regions. TRIs exhibit enhanced Replication Protein A association and TRI-associated genes exhibit higher replication fork termination than control transcription start sites, two marks of replication stress. TRIs colocalize with double-strand DNA breaks, are enriched for deletions, and accumulate mutations in tumors. We propose that replication stress at TRIs induces mutations potentially contributing to age-related disease, as well as tumor formation and development.

Broken, silent, and in hiding: tamed endogenous pararetroviruses escape elimination from the genome of sugar beet (Beta vulgaris)

BACKGROUND AND AIMS

Endogenous pararetroviruses (EPRVs) are widespread components of plant genomes that originated from episomal DNA viruses of the Caulimoviridae family. Due to fragmentation and rearrangements, most EPRVs have lost their ability to replicate through reverse transcription and to initiate viral infection. Similar to the closely related retrotransposons, extant EPRVs were retained and often amplified in plant genomes for several million years. Here, we characterize the complete genomic EPRV fraction of the crop sugar beet (Beta vulgaris, Amaranthaceae) to understand how they shaped the beet genome and to suggest explanations for their absent virulence.

METHODS

Using next- and third-generation sequencing data and genome assembly, we reconstructed full-length in silico representatives for the three host-specific EPRVs (beetEPRVs) in the B. vulgaris genome. Focusing on the endogenous caulimovirid beetEPRV3, we investigated its chromosomal localization, abundance and distribution by fluorescent in situ and Southern hybridization.

KEY RESULTS

Full-length beetEPRVs range between 7.5 and 10.7 kb in size, are heterogeneous in structure and sequence, and occupy about 0.3 % of the beet genome. Although all three beetEPRVs were assigned to the florendoviruses, they showed variably arranged protein-coding domains, different fragmentation, and preferences for diverse sequence contexts. We observed small RNAs that specifically target the individual beetEPRVs, indicating stringent epigenetic suppression. BeetEPRV3 sequences occur along all sugar beet chromosomes, preferentially in the vicinity of each other and are associated with heterochromatic, centromeric and intercalary satellite DNAs. BeetEPRV3 members also exist in genomes of related wild species, indicating an initial beetEPRV3 integration 13.4-7.2 million years ago.

CONCLUSIONS

Our study in beet illustrates the variability of EPRV structure and sequence in a single host genome. Evidence of sequence fragmentation and epigenetic silencing implies possible plant strategies to cope with long-term persistence of EPRVs, including amplification, fixation in the heterochromatin, and containment of EPRV virulence.

Transcription-Replication Collisions-A Series of Unfortunate Events

Transcription-replication interactions occur when DNA replication encounters genomic regions undergoing transcription. Both replication and transcription are essential for life and use the same DNA template making conflicts unavoidable. R-loops, DNA supercoiling, DNA secondary structure, and chromatin-binding proteins are all potential obstacles for processive replication or transcription and pose an even more potent threat to genome integrity when these processes co-occur. It is critical to maintaining high fidelity and processivity of transcription and replication while navigating through a complex chromatin environment, highlighting the importance of defining cellular pathways regulating transcription-replication interaction formation, evasion, and resolution. Here we discuss how transcription influences replication fork stability, and the safeguards that have evolved to navigate transcription-replication interactions and maintain genome integrity in mammalian cells.

Participation of Multifunctional RNA in Replication, Recombination and Regulation of Endogenous Plant Pararetroviruses (EPRVs)

Pararetroviruses, taxon Caulimoviridae, are typical of retroelements with reverse transcriptase and share a common origin with retroviruses and LTR retrotransposons, presumably dating back 1.6 billion years and illustrating the transition from an RNA to a DNA world. After transcription of the viral genome in the host nucleus, viral DNA synthesis occurs in the cytoplasm on the generated terminally redundant RNA including inter- and intra-molecule recombination steps rather than relying on nuclear DNA replication. RNA recombination events between an ancestral genomic retroelement with exogenous RNA viruses were seminal in pararetrovirus evolution resulting in horizontal transmission and episomal replication. Instead of active integration, pararetroviruses use the host DNA repair machinery to prevail in genomes of angiosperms, gymnosperms and ferns. Pararetrovirus integration - leading to Endogenous ParaRetroViruses, EPRVs - by illegitimate recombination can happen if their sequences instead of homologous host genomic sequences on the sister chromatid (during mitosis) or homologous chromosome (during meiosis) are used as template. Multiple layers of RNA interference exist regulating episomal and chromosomal forms of the pararetrovirus. Pararetroviruses have evolved suppressors against this plant defense in the arms race during co-evolution which can result in deregulation of plant genes. Small RNAs serve as signaling molecules for Transcriptional and Post-Transcriptional Gene Silencing (TGS, PTGS) pathways. Different populations of small RNAs comprising 21-24 nt and 18-30 nt in length have been reported for Citrus, Fritillaria, Musa, Petunia, Solanum and Beta. Recombination and RNA interference are driving forces for evolution and regulation of EPRVs.

Fragile site instability: measuring more than breaks

Genome instability is not only a hallmark of cancer, it is necessary for its initiation and evolution, and naturally accumulates as cells age. Replication stress is a potent source of genome instability found in many tumor types [1]. Chromosomal fragile sites are genomic loci highly prone to DNA damage specifically from replication stress and are frequently mutated in cancer [2-4]2-4]. While tracking the origin of individual mutations has proved challenging, measuring DNA damage and repair at endogenous sites can offer key insights into understanding the etiology of cancer. In the past 15 years, the causal link between replication stress, oncogene activation, and tumor initiation and evolution has become increasingly clear [1, 5-9]. Replication-associated damage accumulates at early stages of tumorigenesis and may promote further transformation. Studying the causes and consequences of fragile site instability can offer a window into the earliest stages of carcinogenesis [10-13]. In particular, fragile site studies will help us understand the molecular underpinnings influencing the frequency of DNA breakage, successful repair processes suppressing genome instability, and unsuccessful repair leading to mutations and chromosome rearrangements. Of these, measuring successful repair is the most challenging as it leaves little evidence behind.

Topoisomerase II contributes to DNA secondary structure-mediated double-stranded breaks

DNA double-stranded breaks (DSBs) trigger human genome instability, therefore identifying what factors contribute to DSB induction is critical for our understanding of human disease etiology. Using an unbiased, genome-wide approach, we found that genomic regions with the ability to form highly stable DNA secondary structures are enriched for endogenous DSBs in human cells. Human genomic regions predicted to form non-B-form DNA induced gross chromosomal rearrangements in yeast and displayed high indel frequency in human genomes. The extent of instability in both analyses is in concordance with the structure forming ability of these regions. We also observed an enrichment of DNA secondary structure-prone sites overlapping transcription start sites (TSSs) and CCCTC-binding factor (CTCF) binding sites, and uncovered an increase in DSBs at highly stable DNA secondary structure regions, in response to etoposide, an inhibitor of topoisomerase II (TOP2) re-ligation activity. Importantly, we found that TOP2 deficiency in both yeast and human leads to a significant reduction in DSBs at structure-prone loci, and that sites of TOP2 cleavage have a greater ability to form highly stable DNA secondary structures. This study reveals a direct role for TOP2 in generating secondary structure-mediated DNA fragility, advancing our understanding of mechanisms underlying human genome instability.

AT-dinucleotide rich sequences drive fragile site formation

Common fragile sites (CFSs) are genomic regions prone to breakage under replication stress conditions recurrently rearranged in cancer. Many CFSs are enriched with AT-dinucleotide rich sequences (AT-DRSs) which have the potential to form stable secondary structures upon unwinding the double helix during DNA replication. These stable structures can potentially perturb DNA replication progression, leading to genomic instability. Using site-specific targeting system, we show that targeted integration of a 3.4 kb AT-DRS derived from the human CFS FRA16C into a chromosomally stable region within the human genome is able to drive fragile site formation under conditions of replication stress. Analysis of >1300 X chromosomes integrated with the 3.4 kb AT-DRS revealed recurrent gaps and breaks at the integration site. DNA sequences derived from the integrated AT-DRS showed in vitro a significantly increased tendency to fold into branched secondary structures, supporting the predicted mechanism of instability. Our findings clearly indicate that intrinsic DNA features, such as complexed repeated sequence motifs, predispose the human genome to chromosomal instability.

Visualizing locus-specific sister chromatid exchange reveals differential patterns of replication stress-induced fragile site breakage

Chromosomal fragile sites are genomic loci sensitive to replication stress which accumulate high levels of DNA damage, and are frequently mutated in cancers. Fragile site damage is thought to arise from the aberrant repair of spontaneous replication stress, however successful fragile site repair cannot be calculated using existing techniques. Here, we report a new assay measuring recombination-mediated repair at endogenous genomic loci by combining a sister chromatid exchange (SCE) assay with fluorescent in situ hybridization (SCE-FISH). Using SCE-FISH, we find that endogenous and exogenous replication stress generated unrepaired breaks and SCEs at fragile sites. We also find that distinct sources of replication stress induce distinct patterns of breakage: ATR inhibition induces more breaks at early replicating fragile sites (ERFS), while ERFS and late-replicating common fragile sites (CFS) are equally fragile in response to aphidicolin. Furthermore, SCEs were suppressed at fragile sites near centromeres in response to replication stress, suggesting that genomic location influences DNA repair pathway choice. SCE-FISH also measured successful recombination in human primary lymphocytes, and identificed the proto-oncogene BCL2 as a replication stress-induced fragile site. These findings demonstrate that SCE-FISH frequency at fragile sites is a sensitive indicator of replication stress, and that large-scale genome organization influences DNA repair pathway choice.

Broken by the Cut: A Journey into the Role of Topoisomerase II in DNA Fragility

DNA topoisomerase II (TOP2) plays a critical role in many processes such as replication and transcription, where it resolves DNA structures and relieves torsional stress. Recent evidence demonstrated the association of TOP2 with topologically associated domains (TAD) boundaries and CCCTC-binding factor (CTCF) binding sites. At these sites, TOP2 promotes interactions between enhancers and gene promoters, and relieves torsional stress that accumulates at these physical barriers. Interestingly, in executing its enzymatic function, TOP2 contributes to DNA fragility through re-ligation failure, which results in persistent DNA breaks when unrepaired or illegitimately repaired. Here, we discuss the biological processes for which TOP2 is required and the steps at which it can introduce DNA breaks. We describe the repair processes that follow removal of TOP2 adducts and the resultant broken DNA ends, and present how these processes can contribute to disease-associated mutations. Furthermore, we examine the involvement of TOP2-induced breaks in the formation of oncogenic translocations of leukemia and papillary thyroid cancer, as well as the role of TOP2 and proteins which repair TOP2 adducts in other diseases. The participation of TOP2 in generating persistent DNA breaks and leading to diseases such as cancer, could have an impact on disease treatment and prevention.

Complex DNA structures trigger copy number variation across the Plasmodium falciparum genome

Antimalarial resistance is a major obstacle in the eradication of the human malaria parasite, Plasmodium falciparum. Genome amplifications, a type of DNA copy number variation (CNV), facilitate overexpression of drug targets and contribute to parasite survival. Long monomeric A/T tracks are found at the breakpoints of many Plasmodium resistance-conferring CNVs. We hypothesize that other proximal sequence features, such as DNA hairpins, act with A/T tracks to trigger CNV formation. By adapting a sequence analysis pipeline to investigate previously reported CNVs, we identified breakpoints in 35 parasite clones with near single base-pair resolution. Using parental genome sequence, we predicted the formation of stable hairpins within close proximity to all future breakpoint locations. Especially stable hairpins were predicted to form near five shared breakpoints, establishing that the initiating event could have occurred at these sites. Further in-depth analyses defined characteristics of these 'trigger sites' across the genome and detected signatures of error-prone repair pathways at the breakpoints. We propose that these two genomic signals form the initial lesion (hairpins) and facilitate microhomology-mediated repair (A/T tracks) that lead to CNV formation across this highly repetitive genome. Targeting these repair pathways in P. falciparum may be used to block adaptation to antimalarial drugs.

The role of fork stalling and DNA structures in causing chromosome fragility

Alternative non-B form DNA structures, also called secondary structures, can form in certain DNA sequences under conditions that produce single-stranded DNA, such as during replication, transcription, and repair. Direct links between secondary structure formation, replication fork stalling, and genomic instability have been found for many repeated DNA sequences that cause disease when they expand. Common fragile sites (CFSs) are known to be AT-rich and break under replication stress, yet the molecular basis for their fragility is still being investigated. Over the past several years, new evidence has linked both the formation of secondary structures and transcription to fork stalling and fragility of CFSs. How these two events may synergize to cause fragility and the role of nuclease cleavage at secondary structures in rare and CFSs are discussed here. We also highlight evidence for a new hypothesis that secondary structures at CFSs not only initiate fragility but also inhibit healing, resulting in their characteristic appearance.

Genomic instability in fragile sites-still adding the pieces

Common fragile sites (CFSs) are specific genomic regions in normal chromosomes that exhibit genomic instability under DNA replication stress. As replication stress is an early feature of cancer development, CFSs are involved in the signature of genomic instability found in malignant tumors. The landscape of CFSs is tissue-specific and differs under different replication stress inducers. Nevertheless, the features underlying CFS sensitivity to replication stress are shared. Here, we review the events generating replication stress and discuss the unique characteristics of CFS regions and the cellular responses aimed to stabilizing these regions.

The Protective Role of Dormant Origins in Response to Replicative Stress

Genome stability requires tight regulation of DNA replication to ensure that the entire genome of the cell is duplicated once and only once per cell cycle. In mammalian cells, origin activation is controlled in space and time by a cell-specific and robust program called replication timing. About 100,000 potential replication origins form on the chromatin in the gap 1 (G1) phase but only 20⁻30% of them are active during the DNA replication of a given cell in the synthesis (S) phase. When the progress of replication forks is slowed by exogenous or endogenous impediments, the cell must activate some of the inactive or "dormant" origins to complete replication on time. Thus, the many origins that may be activated are probably key to protect the genome against replication stress. This review aims to discuss the role of these dormant origins as safeguards of the human genome during replicative stress.

Alternative DNA secondary structure formation affects RNA polymerase II promoter-proximal pausing in human

BACKGROUND

Alternative DNA secondary structures can arise from single-stranded DNA when duplex DNA is unwound during DNA processes such as transcription, resulting in the regulation or perturbation of these processes. We identify sites of high propensity to form stable DNA secondary structure across the human genome using Mfold and ViennaRNA programs with parameters for analyzing DNA.

RESULTS

The promoter-proximal regions of genes with paused transcription are significantly and energetically more favorable to form DNA secondary structure than non-paused genes or genes without RNA polymerase II (Pol II) binding. Using Pol II ChIP-seq, GRO-seq, NET-seq, and mNET-seq data, we arrive at a robust set of criteria for Pol II pausing, independent of annotation, and find that a highly stable secondary structure is likely to form about 10-50 nucleotides upstream of a Pol II pausing site. Structure probing data confirm the existence of DNA secondary structures enriched at the promoter-proximal regions of paused genes in human cells. Using an in vitro transcription assay, we demonstrate that Pol II pausing at HSPA1B, a human heat shock gene, is affected by manipulating DNA secondary structure upstream of the pausing site.

CONCLUSIONS

Our results indicate alternative DNA secondary structure formation as a mechanism for how GC-rich sequences regulate RNA Pol II promoter-proximal pausing genome-wide.

Role of specialized DNA polymerases in the limitation of replicative stress and DNA damage transmission

Replication stress is a strong and early driving force for genomic instability and tumor development. Beside replicative DNA polymerases, an emerging group of specialized DNA polymerases is involved in the technical assistance of the replication machinery in order to prevent replicative stress and its deleterious consequences. During S-phase, altered progression of the replication fork by endogenous or exogenous impediments induces replicative stress, causing cells to reach mitosis with genomic regions not fully duplicated. Recently, specific mechanisms to resolve replication intermediates during mitosis with the aim of limiting DNA damage transmission to daughter cells have been identified. In this review, we detail the two major actions of specialized DNA polymerases that limit DNA damage transmission: the prevention of replicative stress by non-B DNA replication and the recovery of stalled replication forks.

DNA polymerases eta and kappa exchange with the polymerase delta holoenzyme to complete common fragile site synthesis

Common fragile sites (CFSs) are inherently unstable genomic loci that are recurrently altered in human tumor cells. Despite their instability, CFS are ubiquitous throughout the human genome and associated with large tumor suppressor genes or oncogenes. CFSs are enriched with repetitive DNA sequences, one feature postulated to explain why these loci are inherently difficult to replicate, and sensitive to replication stress. We have shown that specialized DNA polymerases (Pols) η and κ replicate CFS-derived sequences more efficiently than the replicative Pol δ. However, we lacked an understanding of how these enzymes cooperate to ensure efficient CFS replication. Here, we designed a model of lagging strand replication with RFC loaded PCNA that allows for maximal activity of the four-subunit human Pol δ holoenzyme, Pol η, and Pol κ in polymerase mixing assays. We discovered that Pol η and κ are both able to exchange with Pol δ stalled at repetitive CFS sequences, enhancing Normalized Replication Efficiency. We used this model to test the impact of PCNA mono-ubiquitination on polymerase exchange, and found no change in polymerase cooperativity in CFS replication compared with unmodified PCNA. Finally, we modeled replication stress in vitro using aphidicolin and found that Pol δ holoenzyme synthesis was significantly inhibited in a dose-dependent manner, preventing any replication past the CFS. Importantly, Pol η and κ were still proficient in rescuing this stalled Pol δ synthesis, which may explain, in part, the CFS instability phenotype of aphidicolin-treated Pol η and Pol κ-deficient cells. In total, our data support a model wherein Pol δ stalling at CFSs allows for free exchange with a specialized polymerase that is not driven by PCNA.

Fragility Extraordinaire: Unsolved Mysteries of Chromosome Fragile Sites

Chromosome fragile sites are a fascinating cytogenetic phenomenon now widely implicated in a slew of human diseases ranging from neurological disorders to cancer. Yet, the paths leading to these revelations were far from direct, and the number of fragile sites that have been molecularly cloned with known disease-associated genes remains modest. Moreover, as more fragile sites were being discovered, research interests in some of the earliest discovered fragile sites ebbed away, leaving a number of unsolved mysteries in chromosome biology. In this review we attempt to recount some of the early discoveries of fragile sites and highlight those phenomena that have eluded intense scrutiny but remain extremely relevant in our understanding of the mechanisms of chromosome fragility. We then survey the literature for disease association for a comprehensive list of fragile sites. We also review recent studies addressing the underlying cause of chromosome fragility while highlighting some ongoing debates. We report an observed enrichment for R-loop forming sequences in fragile site-associated genes than genomic average. Finally, we will leave the reader with some lingering questions to provoke discussion and inspire further scientific inquiries.

Genomic and Cytogenetic Localization of the Carotenoid Genes in the Aphid Genome

Data published in the scientific literature suggests a possible link between chromosomal rearrangements involving autosomes 1 and 3 and the presence of red morphs in the peach-potato aphid Myzus persicae (Sulzer). In order to begin a study of this relationship, we analysed the genomic and chromosomal location of genes involved in carotenoid biosynthesis in M. persicae and the pea aphid, Acyrthosiphon pisum (Harris), since carotenoids are the basis of the colour in many aphid species. Genomic analysis identified a DNA sequence containing carotenoid genes in synteny between the 2 species. According to the results obtained using in situ PCR, carotenoid genes were located in a subterminal portion of autosome 1 in both species. The same localization has also been observed in the onion aphid Neotoxoptera formosana Takahashi that, as M. persicae and A. pisum, belongs to the tribe Macrosiphini, thereby suggesting a synteny of this chromosomal region in aphids. In situ PCR experiments performed on 2 M. persicae asexual lineages bearing heterozygous translocations involving autosomes 1 and 3 revealed that carotenoid genes were located within chromosomal portions involved in recurrent rearrangements. We also verified by bioinformatics analyses the presence of fragile sites that could explain these recurrent rearrangements in M. persicae.

Translocation and deletion breakpoints in cancer genomes are associated with potential non-B DNA-forming sequences

Gross chromosomal rearrangements (including translocations, deletions, insertions and duplications) are a hallmark of cancer genomes and often create oncogenic fusion genes. An obligate step in the generation of such gross rearrangements is the formation of DNA double-strand breaks (DSBs). Since the genomic distribution of rearrangement breakpoints is non-random, intrinsic cellular factors may predispose certain genomic regions to breakage. Notably, certain DNA sequences with the potential to fold into secondary structures [potential non-B DNA structures (PONDS); e.g. triplexes, quadruplexes, hairpin/cruciforms, Z-DNA and single-stranded looped-out structures with implications in DNA replication and transcription] can stimulate the formation of DNA DSBs. Here, we tested the postulate that these DNA sequences might be found at, or in close proximity to, rearrangement breakpoints. By analyzing the distribution of PONDS-forming sequences within ±500 bases of 19 947 translocation and 46 365 sequence-characterized deletion breakpoints in cancer genomes, we find significant association between PONDS-forming repeats and cancer breakpoints. Specifically, (AT)_n, (GAA)_n and (GAAA)_n constitute the most frequent repeats at translocation breakpoints, whereas A-tracts occur preferentially at deletion breakpoints. Translocation breakpoints near PONDS-forming repeats also recur in different individuals and patient tumor samples. Hence, PONDS-forming sequences represent an intrinsic risk factor for genomic rearrangements in cancer genomes.

Environmental and chemotherapeutic agents induce breakage at genes involved in leukemia-causing gene rearrangements in human hematopoietic stem/progenitor cells

Hematopoietic stem and progenitor cells (HSPCs) give rise to all of the cells that make up the hematopoietic system in the human body, making their stability and resilience especially important. Damage to these cells can severely impact cell development and has the potential to cause diseases, such as leukemia. Leukemia-causing chromosomal rearrangements have largely been studied in the context of radiation exposure and are formed by a multi-step process, including an initial DNA breakage and fusion of the free DNA ends. However, the mechanism for DNA breakage in patients without previous radiation exposure is unclear. Here, we investigate the role of non-cytotoxic levels of environmental factors, benzene, and diethylnitrosamine (DEN), and chemotherapeutic agents, etoposide, and doxorubicin, in generating DNA breakage at the patient breakpoint hotspots of the MLL and CBFB genes in human HSPCs. These conditions represent exposure to chemicals encountered daily or residual doses from chemotherapeutic drugs. Exposure of HSPCs to non-cytotoxic levels of environmental chemicals or chemotherapeutic agents causes DNA breakage at preferential sites in the human genome, including the leukemia-related genes MLL and CBFB. Though benzene, etoposide, and doxorubicin have previously been linked to leukemia formation, this is the first study to demonstrate a role for DEN in the generation of DNA breakage at leukemia-specific sites. These chemical-induced DNA breakpoints coincide with sites of predicted topoisomerase II cleavage. The distribution of breakpoints by exposure to non-cytotoxic levels of chemicals showed a similar pattern to fusion breakpoints in leukemia patients. Our findings demonstrate that HSPCs exposed to non-cytotoxic levels of environmental chemicals and chemotherapeutic agents are prone to topoisomerase II-mediated DNA damage at the leukemia-associated genes MLL and CBFB. These data suggest a role for long-term environmental chemical or residual chemotherapeutic drug exposure in generation of DNA breakage at sites with a propensity to form leukemia-causing gene rearrangements.

Replication stress in Mammalian cells and its consequences for mitosis

The faithful transmission of genetic information to daughter cells is central to maintaining genomic stability and relies on the accurate and complete duplication of genetic material during each cell cycle. However, the genome is routinely exposed to endogenous and exogenous stresses that can impede the progression of replication. Such replication stress can be an early cause of cancer or initiate senescence. Replication stress, which primarily occurs during S phase, results in consequences during mitosis, jeopardizing chromosome segregation and, in turn, genomic stability. The traces of replication stress can be detected in the daughter cells during G1 phase. Alterations in mitosis occur in two types: 1) local alterations that correspond to breaks, rearrangements, intertwined DNA molecules or non-separated sister chromatids that are confined to the region of the replication dysfunction; 2) genome-wide chromosome segregation resulting from centrosome amplification (although centrosomes do not contain DNA), which amplifies the local replication stress to the entire genome. Here, we discuss the endogenous causes of replication perturbations, the mechanisms of replication fork restart and the consequences for mitosis, chromosome segregation and genomic stability.

DNA secondary structure at chromosomal fragile sites in human disease

DNA has the ability to form a variety of secondary structures that can interfere with normal cellular processes, and many of these structures have been associated with neurological diseases and cancer. Secondary structure-forming sequences are often found at chromosomal fragile sites, which are hotspots for sister chromatid exchange, chromosomal translocations, and deletions. Structures formed at fragile sites can lead to instability by disrupting normal cellular processes such as DNA replication and transcription. The instability caused by disruption of replication and transcription can lead to DNA breakage, resulting in gene rearrangements and deletions that cause disease. In this review, we discuss the role of DNA secondary structure at fragile sites in human disease.

Massive parallel sequencing uncovers actionable FGFR2-PPHLN1 fusion and ARAF mutations in intrahepatic cholangiocarcinoma

Intrahepatic cholangiocarcinoma (iCCA) is a fatal bile duct cancer with dismal prognosis and limited therapeutic options. By performing RNA- and exome-sequencing analyses, we report a novel fusion event, FGFR2-PPHLN1 (16%), and damaging mutations in the ARAF oncogene (11%). Here we demonstrate that the chromosomal translocation t(10;12)(q26;q12) leading to FGFR2-PPHLN1 fusion possesses transforming and oncogenic activity, which is successfully inhibited by a selective FGFR2 inhibitor in vitro. Among the ARAF mutations, N217I and G322S lead to activation of the pathway and N217I shows oncogenic potential in vitro. Screening of a cohort of 107 iCCA patients reveals that FGFR2 fusions represent the most recurrent targetable alteration (45%, 17/107), while they are rarely present in other primary liver tumours (0/100 of hepatocellular carcinoma (HCC); 1/21 of mixed iCCA-HCC). Taken together, around 70% of iCCA patients harbour at least one actionable molecular alteration (FGFR2 fusions, IDH1/2, ARAF, KRAS, BRAF and FGF19) that is amenable for therapeutic targeting.

Endogenous florendoviruses are major components of plant genomes and hallmarks of virus evolution

The extent and importance of endogenous viral elements have been extensively described in animals but are much less well understood in plants. Here we describe a new genus of Caulimoviridae called ‘Florendovirus’, members of which have colonized the genomes of a large diversity of flowering plants, sometimes at very high copy numbers (>0.5% total genome content). The genome invasion of Oryza is dated to over 1.8 million years ago (MYA) but phylogeographic evidence points to an even older age of 20–34 MYA for this virus group. Some appear to have had a bipartite genome organization, a unique characteristic among viral retroelements. In Vitis vinifera, 9% of the endogenous florendovirus loci are located within introns and therefore may influence host gene expression. The frequent colocation of endogenous florendovirus loci with TA simple sequence repeats, which are associated with chromosome fragility, suggests sequence capture during repair of double-stranded DNA breaks.

Endogenous viral elements have been extensively described in animals but their significance in plants is less well understood. Here, Geering et al. describe a new group of endogenous pararetroviruses, called florendoviruses, which have colonized the genomes of many important crop species.

Updating the mechanisms of common fragile site instability: how to reconcile the different views?

Common fragile sites (CFSs) are large chromosomal regions long identified by conventional cytogenetics as sequences prone to breakage in cells subjected to replication stress. The interest in CFSs came from their key role in the formation of DNA damage, resulting in chromosomal rearrangements. The instability of CFSs was notably correlated with the appearance of genome instability in precancerous lesions and during tumor progression. Identification of the molecular mechanisms responsible for their instability therefore represents a major challenge. A number of data show that breaks result from mitotic entry before replication completion but the mechanisms responsible for such delayed replication of CFSs and relaxed checkpoint surveillance are still debated. In addition, clues to the molecular events leading to breakage just start to emerge. We present here the results of recent reports addressing these questions.

Replication initiation and genome instability: a crossroads for DNA and RNA synthesis

Nuclear DNA replication requires the concerted action of hundreds of proteins to efficiently unwind and duplicate the entire genome while also retaining epigenetic regulatory information. Initiation of DNA replication is tightly regulated, rapidly firing thousands of origins once the conditions to promote rapid and faithful replication are in place, and defects in replication initiation lead to proliferation defects, genome instability, and a range of developmental abnormalities. Interestingly, DNA replication in metazoans initiates in actively transcribed DNA, meaning that replication initiation occurs in DNA that is co-occupied with tens of thousands of poised and active RNA polymerase complexes. Active transcription can induce genome instability, particularly during DNA replication, as RNA polymerases can induce torsional stress, formation of secondary structures, and act as a physical barrier to other enzymes involved in DNA metabolism. Here we discuss the challenges facing mammalian DNA replication, their impact on genome instability, and the development of cancer.

A CTNNA3 compound heterozygous deletion implicates a role for αT-catenin in susceptibility to autism spectrum disorder

Background

Autism spectrum disorder (ASD) is a highly heritable, neurodevelopmental condition showing extreme genetic heterogeneity. While it is well established that rare genetic variation, both de novo and inherited, plays an important role in ASD risk, recent studies also support a rare recessive contribution.

Methods

We identified a compound heterozygous deletion intersecting the CTNNA3 gene, encoding αT-catenin, in a proband with ASD and moderate intellectual disability. The deletion breakpoints were mapped at base-pair resolution, and segregation analysis was performed. We compared the frequency of CTNNA3 exonic deletions in 2,147 ASD cases from the Autism Genome Project (AGP) study versus the frequency in 6,639 controls. Western blot analysis was performed to get a quantitative characterisation of Ctnna3 expression during early brain development in mouse.

Results

The CTNNA3 compound heterozygous deletion includes a coding exon, leading to a putative frameshift and premature stop codon. Segregation analysis in the family showed that the unaffected sister is heterozygote for the deletion, having only inherited the paternal deletion. While the frequency of CTNNA3 exonic deletions is not significantly different between ASD cases and controls, no homozygous or compound heterozygous exonic deletions were found in a sample of over 6,000 controls. Expression analysis of Ctnna3 in the mouse cortex and hippocampus (P0-P90) provided support for its role in the early stage of brain development.

Conclusion

The finding of a rare compound heterozygous CTNNA3 exonic deletion segregating with ASD, the absence of CTNNA3 homozygous exonic deletions in controls and the high expression of Ctnna3 in both brain areas analysed implicate CTNNA3 in ASD susceptibility.

Phenotypic diversity of breast cancer-related mutations in metalloproteinase-disintegrin ADAM12

Six different somatic missense mutations in the human ADAM12 gene have been identified so far in breast cancer. Five of these mutations involve highly conserved residues in the extracellular domain of the transmembrane ADAM12-L protein. Two of these extracellular mutations, D301H and G479E, have been previously characterized in the context of mouse ADAM12. Three other mutations, T596A, R612Q, and G668A, have been reported more recently, and their effects on ADAM12-L protein structure/function are not known. Here, we show that ADAM12-L bearing the G668A mutation is largely retained in the endoplasmic reticulum in its nascent, full-length form, with an intact N-terminal pro-domain. The T596A and R612Q mutants are efficiently trafficked to the cell surface and proteolytically processed to remove their pro-domains. However, the T596A mutant shows decreased catalytic activity at the cell surface, while the R612Q mutant is fully active and comparable to the wild-type ADAM12-L. The D301H and G479E mutants, consistent with the corresponding D299H and G477E mutants of mouse ADAM12 described earlier, are not proteolytically processed and do not exhibit catalytic activity at the cell surface. Among all six breast cancer-associated mutations in ADAM12-L, mutations that preserve the activity - R612Q and L792F - occur in triple-negative breast cancers, while loss-of-function mutations - D301H, G479E, T596A, and G668A - are found in non-triple negative cancers. This apparent association between the catalytic activity of the mutants and the type of breast cancer supports a previously postulated role of an active ADAM12-L in the triple negative breast cancer disease.

DNA topoisomerases participate in fragility of the oncogene RET

Fragile site breakage was previously shown to result in rearrangement of the RET oncogene, resembling the rearrangements found in thyroid cancer. Common fragile sites are specific regions of the genome with a high susceptibility to DNA breakage under conditions that partially inhibit DNA replication, and often coincide with genes deleted, amplified, or rearranged in cancer. While a substantial amount of work has been performed investigating DNA repair and cell cycle checkpoint proteins vital for maintaining stability at fragile sites, little is known about the initial events leading to DNA breakage at these sites. The purpose of this study was to investigate these initial events through the detection of aphidicolin (APH)-induced DNA breakage within the RET oncogene, in which 144 APH-induced DNA breakpoints were mapped on the nucleotide level in human thyroid cells within intron 11 of RET, the breakpoint cluster region found in patients. These breakpoints were located at or near DNA topoisomerase I and/or II predicted cleavage sites, as well as at DNA secondary structural features recognized and preferentially cleaved by DNA topoisomerases I and II. Co-treatment of thyroid cells with APH and the topoisomerase catalytic inhibitors, betulinic acid and merbarone, significantly decreased APH-induced fragile site breakage within RET intron 11 and within the common fragile site FRA3B. These data demonstrate that DNA topoisomerases I and II are involved in initiating APH-induced common fragile site breakage at RET, and may engage the recognition of DNA secondary structures formed during perturbed DNA replication.