Mutational Strand Asymmetries in Cancer Genomes Reveal Mechanisms of DNA Damage and Repair.

Mutational processes constantly shape the somatic genome, leading to immunity, aging, cancer, and other diseases. When cancer is the outcome, we are afforded a glimpse into these processes by the clonal expansion of the malignant cell. Here, we characterize a less explored layer of the mutational landscape of cancer: mutational asymmetries between the two DNA strands. Analyzing whole-genome sequences of 590 tumors from 14 different cancer types, we reveal widespread asymmetries across mutagenic processes, with transcriptional ("T-class") asymmetry dominating UV-, smoking-, and liver-cancer-associated mutations and replicative ("R-class") asymmetry dominating POLE-, APOBEC-, and MSI-associated mutations. We report a striking phenomenon of transcription-coupled damage (TCD) on the non-transcribed DNA strand and provide evidence that APOBEC mutagenesis occurs on the lagging-strand template during DNA replication. As more genomes are sequenced, studying and classifying their asymmetries will illuminate the underlying biological mechanisms of DNA damage and repair.

A model for transcription-dependent R-loop formation at double-stranded DNA breaks: Implications for their detection and biological effects.

R-loops are structures containing an RNA-DNA duplex and an unpaired DNA strand. During R-loop formation an RNA strand invades the DNA duplex, displacing the homologous DNA strand and binding the complementary DNA strand. Here we analyze a model for transcription-dependent R-loop formation at double-stranded DNA breaks (DSBs). In this model, R-loop formation is preceded by detachment of the non-template DNA strand from the RNA polymerase (RNAP). Then, strand exchange is initiated between the nascent RNA and the non-template DNA strand. During that strand exchange the length of the R-loop could either increase, or decrease in a biased random-walk fashion, in which the bias would depend upon the DNA sequence. Eventually, the restoration of the DNA duplex would completely displace the RNA. However, as long as the RNAP remains bound to the template DNA strand it prevents that displacement. Thus, according to the model, RNAPs stalled at DSBs can increase the lifespan of R-loops, increasing their detectability in experiments, and perhaps enhancing their biological effects.

Topology and kinetics of R-loop formation.

R-loops are structures containing an RNA-DNA duplex and an unpaired DNA strand. They can be formed upon "invasion" of an RNA strand into a DNA duplex, during which the RNA displaces the homologous DNA strand and binds the complementary strand. R-loops have many significant beneficial or deleterious biological effects, so it is important to understand the mechanisms for their generation and processing. We propose a model for co-transcriptional R-loop formation, in which their generation requires passage of the nascent RNA "tail" through the gap between the separated DNA strands. This passage becomes increasingly difficult with lengthening of the RNA tail. The length of the tail increases upon increasing distance between the transcription start site and the site of R-loop initiation. This causes reduced yields of R-loops with greater distance from the transcription start site. However, alternative pathways for R-loop formation are possible, involving either transient disruption of the transcription complex or the hypothetical formation of a triple-stranded structure, as a "collapsed R-loop." These alternative pathways could account for the fact that in many systems R-loops are observed very far from the transcription start site. Our model is consistent with experimental data and makes general predictions about the kinetics of R-loop formation.

Thymineless Death Lives On: New Insights into a Classic Phenomenon.

The primary mechanisms by which bacteria lose viability when deprived of thymine have been elusive for over half a century. Early research focused on stalled replication forks and the deleterious effects of uracil incorporation into DNA from thymidine-deficient nucleotide pools. The initiation of the replication cycle and origin-proximal DNA degradation during thymine starvation have now been quantified via whole-genome microarrays and other approaches. These advances have fostered innovative models and informative experiments in bacteria since this topic was last reviewed. Given that thymineless death is similar in mammalian cells and that certain antibacterial and chemotherapeutic drugs elicit thymine deficiency, a mechanistic understanding of this phenomenon might have valuable biomedical applications.

Transcription-coupled DNA repair: two decades of progress and surprises.

Expressed genes are scanned by translocating RNA polymerases, which sensitively detect DNA damage and initiate transcription-coupled repair (TCR), a subpathway of nucleotide excision repair that removes lesions from the template DNA strands of actively transcribed genes. Human hereditary diseases that present a deficiency only in TCR are characterized by sunlight sensitivity without enhanced skin cancer. Although multiple gene products are implicated in TCR, we still lack an understanding of the precise signals that can trigger this pathway. Futile cycles of TCR at naturally occurring non-canonical DNA structures might contribute to genomic instability and genetic disease.

Strong transcription blockage mediated by R-loop formation within a G-rich homopurine-homopyrimidine sequence localized in the vicinity of the promoter.

Guanine-rich (G-rich) homopurine-homopyrimidine nucleotide sequences can block transcription with an efficiency that depends upon their orientation, composition and length, as well as the presence of negative supercoiling or breaks in the non-template DNA strand. We report that a G-rich sequence in the non-template strand reduces the yield of T7 RNA polymerase transcription by more than an order of magnitude when positioned close (9 bp) to the promoter, in comparison to that for a distal (∼250 bp) location of the same sequence. This transcription blockage is much less pronounced for a C-rich sequence, and is not significant for an A-rich sequence. Remarkably, the blockage is not pronounced if transcription is performed in the presence of RNase H, which specifically digests the RNA strands within RNA-DNA hybrids. The blockage also becomes less pronounced upon reduced RNA polymerase concentration. Based upon these observations and those from control experiments, we conclude that the blockage is primarily due to the formation of stable RNA-DNA hybrids (R-loops), which inhibit successive rounds of transcription. Our results could be relevant to transcription dynamics in vivo (e.g. transcription 'bursting') and may also have practical implications for the design of expression vectors.

Modulation of Cytotoxicity by Transcription-Coupled Nucleotide Excision Repair Is Independent of the Requirement for Bioactivation of Acylfulvene.

Bioactivation as well as DNA repair affects the susceptibility of cancer cells to the action of DNA-alkylating chemotherapeutic drugs. However, information is limited with regard to the relative contributions of these processes to the biological outcome of metabolically activated DNA alkylating agents. We evaluated the influence of cellular bioactivation capacity and DNA repair on cytotoxicity of the DNA alkylating agent acylfulvene (AF). We compared the cytotoxicity and RNA synthesis inhibition by AF and its synthetic activated analogue iso-M0 in a panel of fibroblast cell lines with deficiencies in transcription-coupled (TC-NER) or global genome nucleotide excision repair (GG-NER). We related these data to the inherent bioactivation capacity of each cell type on the basis of mRNA levels. We demonstrated that specific inactivation of TC-NER by siRNA had the largest positive impact on AF activity in a cancer cell line. These findings establish that transcription-coupled DNA repair reduces cellular sensitivity to AF, independent of the requirement for bioactivation.

Transcription blockage by stable H-DNA analogs in vitro.

DNA sequences that can form unusual secondary structures are implicated in regulating gene expression and causing genomic instability. H-palindromes are an important class of such DNA sequences that can form an intramolecular triplex structure, H-DNA. Within an H-palindrome, the H-DNA and canonical B-DNA are in a dynamic equilibrium that shifts toward H-DNA with increased negative supercoiling. The interplay between H- and B-DNA and the fact that the process of transcription affects supercoiling makes it difficult to elucidate the effects of H-DNA upon transcription. We constructed a stable structural analog of H-DNA that cannot flip into B-DNA, and studied the effects of this structure on transcription by T7 RNA polymerase in vitro. We found multiple transcription blockage sites adjacent to and within sequences engaged in this triplex structure. Triplex-mediated transcription blockage varied significantly with changes in ambient conditions: it was exacerbated in the presence of Mn(2+) or by increased concentrations of K(+) and Li(+). Analysis of the detailed pattern of the blockage suggests that RNA polymerase is sterically hindered by H-DNA and has difficulties in unwinding triplex DNA. The implications of these findings for the biological roles of triple-stranded DNA structures are discussed.

Altered minor-groove hydrogen bonds in DNA block transcription elongation by T7 RNA polymerase.

DNA transcription depends upon the highly efficient and selective function of RNA polymerases (RNAPs). Modifications in the template DNA can impact the progression of RNA synthesis, and a number of DNA adducts, as well as abasic sites, arrest or stall transcription. Nonetheless, data are needed to understand why certain modifications to the structure of DNA bases stall RNA polymerases while others are efficiently bypassed. In this study, we evaluate the impact that alterations in dNTP/rNTP base-pair geometry have on transcription. T7 RNA polymerase was used to study transcription over modified purines and pyrimidines with altered H-bonding capacities. The results suggest that introducing wobble base-pairs into the DNA:RNA heteroduplex interferes with transcriptional elongation and stalls RNA polymerase. However, transcriptional stalling is not observed if mismatched base-pairs do not H-bond. Together, these studies show that RNAP is able to discriminate mismatches resulting in wobble base-pairs, and suggest that, in cases of modifications with minor steric impact, DNA:RNA heteroduplex geometry could serve as a controlling factor for initiating transcription-coupled DNA repair.

PNA binding to the non-template DNA strand interferes with transcription, suggesting a blockage mechanism mediated by R-loop formation.

Peptide Nucleic Acids (PNAs) are artificial DNA mimics with superior nucleic acid binding capabilities. T7 RNA polymerase (T7 RNAP) transcription upon encountering PNA bound to the non-template DNA strand was studied in vitro. A characteristic pattern of blockage signals was observed, extending downstream from the PNA binding site, similar to that produced by G-rich homopurine-homopyrimidine (hPu-hPy) sequences and likely caused by R-loop formation. Since blocked transcription complexes in association with stable R-loops may interfere with replication and in some cases trigger apoptosis, targeted R-loop formation might be employed to inactivate selected cells, such as those in tumors, based upon their unique complement of expressed genes.

Comet-FISH with strand-specific probes reveals transcription-coupled repair of 8-oxoGuanine in human cells.

Oxidized bases in DNA have been implicated in cancer, aging and neurodegenerative disease. We have developed an approach combining single-cell gel electrophoresis (comet) with fluorescence in situ hybridization (FISH) that enables the comparative quantification of low, physiologically relevant levels of DNA lesions in the respective strands of defined nucleotide sequences and in the genome overall. We have synthesized single-stranded probes targeting the termini of DNA segments of interest using a polymerase chain reaction-based method. These probes facilitate detection of damage at the single-molecule level, as the lesions are converted to DNA strand breaks by lesion-specific endonucleases or glycosylases. To validate our method, we have documented transcription-coupled repair of cyclobutane pyrimidine dimers in the ataxia telangiectasia-mutated (ATM) gene in human fibroblasts irradiated with 254 nm ultraviolet at 0.1 J/m2, a dose ∼100-fold lower than those typically used. The high specificity and sensitivity of our approach revealed that 7,8-dihydro-8-oxoguanine (8-oxoG) at an incidence of approximately three lesions per megabase is preferentially repaired in the transcribed strand of the ATM gene. We have also demonstrated that the hOGG1, XPA, CSB and UVSSA proteins, as well as actively elongating RNA polymerase II, are required for this process, suggesting cross-talk between DNA repair pathways.

Transcription blockage by homopurine DNA sequences: role of sequence composition and single-strand breaks.

The ability of DNA to adopt non-canonical structures can affect transcription and has broad implications for genome functioning. We have recently reported that guanine-rich (G-rich) homopurine-homopyrimidine sequences cause significant blockage of transcription in vitro in a strictly orientation-dependent manner: when the G-rich strand serves as the non-template strand [Belotserkovskii et al. (2010) Mechanisms and implications of transcription blockage by guanine-rich DNA sequences., Proc. Natl Acad. Sci. USA, 107, 12816-12821]. We have now systematically studied the effect of the sequence composition and single-stranded breaks on this blockage. Although substitution of guanine by any other base reduced the blockage, cytosine and thymine reduced the blockage more significantly than adenine substitutions, affirming the importance of both G-richness and the homopurine-homopyrimidine character of the sequence for this effect. A single-strand break in the non-template strand adjacent to the G-rich stretch dramatically increased the blockage. Breaks in the non-template strand result in much weaker blockage signals extending downstream from the break even in the absence of the G-rich stretch. Our combined data support the notion that transcription blockage at homopurine-homopyrimidine sequences is caused by R-loop formation.

DNA slip-outs cause RNA polymerase II arrest in vitro: potential implications for genetic instability.

The abnormal number of repeats found in triplet repeat diseases arises from 'repeat instability', in which the repetitive section of DNA is subject to a change in copy number. Recent studies implicate transcription in a mechanism for repeat instability proposed to involve RNA polymerase II (RNAPII) arrest caused by a CTG slip-out, triggering transcription-coupled repair (TCR), futile cycles of which may lead to repeat expansion or contraction. In the present study, we use defined DNA constructs to directly test whether the structures formed by CAG and CTG repeat slip-outs can cause transcription arrest in vitro. We found that a slip-out of (CAG)(20) or (CTG)(20) repeats on either strand causes RNAPII arrest in HeLa cell nuclear extracts. Perfect hairpins and loops on either strand also cause RNAPII arrest. These findings are consistent with a transcription-induced repeat instability model in which transcription arrest in mammalian cells may initiate a 'gratuitous' TCR event leading to a change in repeat copy number. An understanding of the underlying mechanism of repeat instability could lead to intervention to slow down expansion and delay the onset of many neurodegenerative diseases in which triplet repeat expansion is implicated.

Role of RecA and the SOS response in thymineless death in Escherichia coli.

Thymineless death (TLD) is a classic and enigmatic phenomenon, documented in bacterial, yeast, and human cells, whereby cells lose viability rapidly when deprived of thymine. Despite its being the essential mode of action of important chemotherapeutic agents, and despite having been studied extensively for decades, the basic mechanisms of TLD have remained elusive. In Escherichia coli, several proteins involved in homologous recombination (HR) are required for TLD, however, surprisingly, RecA, the central HR protein and activator of the SOS DNA-damage response was reported not to be. We demonstrate that RecA and the SOS response are required for a substantial fraction of TLD. We show that some of the Rec proteins implicated previously promote TLD via facilitating activation of the SOS response and that, of the roughly 40 proteins upregulated by SOS, SulA, an SOS-inducible inhibitor of cell division, accounts for most or all of how SOS causes TLD. The data imply that much of TLD results from an irreversible cell-cycle checkpoint due to blocked cell division. FISH analyses of the DNA in cells undergoing TLD reveal blocked replication and apparent DNA loss with the region near the replication origin underrepresented initially and the region near the terminus lost later. Models implicating formation of single-strand DNA at blocked replication forks, a SulA-blocked cell cycle, and RecQ/RecJ-catalyzed DNA degradation and HR are discussed. The data predict the importance of DNA damage-response and HR networks to TLD and chemotherapy resistance in humans.

Mechanisms and implications of transcription blockage by guanine-rich DNA sequences.

Various DNA sequences that interfere with transcription due to their unusual structural properties have been implicated in the regulation of gene expression and with genomic instability. An important example is sequences containing G-rich homopurine-homopyrimidine stretches, for which unusual transcriptional behavior is implicated in regulation of immunogenesis and in other processes such as genomic translocations and telomere function. To elucidate the mechanism of the effect of these sequences on transcription we have studied T7 RNA polymerase transcription of G-rich sequences in vitro. We have shown that these sequences produce significant transcription blockage in an orientation-, length- and supercoiling-dependent manner. Based upon the effects of various sequence modifications, solution conditions, and ribonucleotide substitutions, we conclude that transcription blockage is due to formation of unusually stable RNA/DNA hybrids, which could be further exacerbated by triplex formation. These structures are likely responsible for transcription-dependent replication blockage by G-rich sequences in vivo.

The awakening of DNA repair at Yale.

As a graduate student with Professor Richard Setlow at Yale in the late 1950s, I studied the effects of ultraviolet and visible light on the syntheses of DNA, RNA, and protein in bacteria. I reflect upon my research in the Yale Biophysics Department, my subsequent postdoctoral experiences, and the eventual analyses in the laboratories of Setlow, Paul Howard-Flanders, and myself that constituted the discovery of the ubiquitous pathway of DNA excision repair in the early 1960s. I then offer a brief perspective on a few more recent developments in the burgeoning DNA repair field and their relationships to human disease.

Unbalanced Growth, the DNA Replication Cycle and Discovery of Repair Replication.

This article recounts my graduate research at Yale University (1954-1958) on unbalanced growth in Eschericia coli during thymine deprivation or following ultraviolet (UV) irradiation, with early evidence for the repair of UV-induced DNA damage. Follow-up studies in Copenhagen (1958-1960) in the laboratory of Ole Maaløe led to my discovery that the DNA replication cycle can be synchronized by inhibiting protein and RNA syntheses and that an RNA synthesis step is essential for initiation of the cycle, but not for its completion. This work set the stage for my subsequent research at Stanford University, where the repair replication of damaged DNA was documented, to provide compelling evidence for an excision-repair pathway. That universal pathway validates the requirement for the redundant information in the complementary strands of duplex DNA to ensure genomic stability.

Evelyn M. Witkin (1921-2023).

Pioneer of cell mutagenesis and DNA repair research.

Transcription blockage by bulky end termini at single-strand breaks in the DNA template: differential effects of 5' and 3' adducts.

RNA polymerases from phage-infected bacteria and mammalian cells have been shown to bypass single-strand breaks (SSBs) with a single-nucleotide gap in the template DNA strand during transcription elongation; however, the SSB bypass efficiency varies significantly depending upon the backbone end chemistries at the break. Using a reconstituted T7 phage transcription system (T7 RNAP) and RNA polymerase II (RNAPII) in HeLa cell nuclear extracts, we observe a slight reduction in the level of transcription arrest at SSBs with no gap as compared to those with a single-nucleotide gap. We have shown that biotin and carbon-chain moieties linked to the 3' side, and in select cases the 5' side, of an SSB in the template strand strongly increase the level of transcription arrest when compared to unmodified SSBs. We also find that a small carbon-chain moiety linked to the upstream side of an SSB aids transcriptional bypass of SSBs for both T7 RNAP and RNAP II. Analysis of transcription across SSBs flanked by bulky 3' adducts reveals the ability of 3' end chemistries to arrest T7 RNAP in a size-dependent manner. T7 RNAP is also completely arrested when 3' adducts or 3'-phosphate groups are placed opposite 5'-phosphate groups at an SSB. We have also observed that a biotinylated thymine in the template strand (without a break) does not pose a strong block to transcription. Taken together, these results emphasize the importance of the size of 3', but usually not 5', end chemistries in arresting transcription at SSBs, substantiating the notion that bulky 3' lesions (e.g., topoisomerase cleavable complexes, 3'-phosphoglycolates, and 3'-unsaturated aldehydes) pose very strong blocks to transcribing RNA polymerases. These findings have implications for the processing of DNA damage through SSB intermediates and the mechanism of SSB bypass by T7 RNAP and mammalian RNAPII.

A UV-sensitive syndrome patient with a specific CSA mutation reveals separable roles for CSA in response to UV and oxidative DNA damage.

UV-sensitive syndrome (UV(S)S) is a recently-identified autosomal recessive disorder characterized by mild cutaneous symptoms and defective transcription-coupled repair (TC-NER), the subpathway of nucleotide excision repair (NER) that rapidly removes damage that can block progression of the transcription machinery in actively-transcribed regions of DNA. Cockayne syndrome (CS) is another genetic disorder with sun sensitivity and defective TC-NER, caused by mutations in the CSA or CSB genes. The clinical hallmarks of CS include neurological/developmental abnormalities and premature aging. UV(S)S is genetically heterogeneous, in that it appears in individuals with mutations in CSB or in a still-unidentified gene. We report the identification of a UV(S)S patient (UV(S)S1VI) with a novel mutation in the CSA gene (p.trp361cys) that confers hypersensitivity to UV light, but not to inducers of oxidative damage that are notably cytotoxic in cells from CS patients. The defect in UV(S)S1VI cells is corrected by expression of the WT CSA gene. Expression of the p.trp361cys-mutated CSA cDNA increases the resistance of cells from a CS-A patient to oxidative stress, but does not correct their UV hypersensitivity. These findings imply that some mutations in the CSA gene may interfere with the TC-NER-dependent removal of UV-induced damage without affecting its role in the oxidative stress response. The differential sensitivity toward oxidative stress might explain the difference between the range and severity of symptoms in CS and the mild manifestations in UV(s)S patients that are limited to skin photosensitivity without precocious aging or neurodegeneration.