Quantitative measurement of translesion DNA synthesis in mammalian cells

Mass spectrometry-based assays for assessing replicative bypass and repair of DNA alkylation in cells

Endogenous metabolism and environmental exposure can give rise to DNA alkylation, which can elicit deleterious biological consequences. In the search for reliable and quantitative analytical methods to elucidate the impact of DNA alkylation on the flow of genetic information, mass spectrometry (MS) has attracted increasing attention, owing to its unambiguous determination of molecular mass. The MS-based assays obviate conventional colony-picking methods and Sanger sequencing procedures, and retained the high sensitivity of postlabeling methods. With the help of the CRISPR/Cas9 gene editing method, MS-based assays showed high potential in studying individual functions of repair proteins and translesion synthesis (TLS) polymerases in DNA replication. In this mini-review, we have summarized the development of MS-based competitive and replicative adduct bypass (CRAB) assays and their recent applications in assessing the impact of alkylation on DNA replication. With further development of MS instruments for high resolving power and high throughput, these assays should be generally applicable and efficient in quantitative measurement of the biological consequences and repair of other DNA lesions.

TENT4A Non-Canonical Poly(A) Polymerase Regulates DNA-Damage Tolerance via Multiple Pathways That Are Mutated in Endometrial Cancer

TENT4A (PAPD7) is a non-canonical poly(A) polymerase, of which little is known. Here, we show that TENT4A regulates multiple biological pathways and focuses on its multilayer regulation of translesion DNA synthesis (TLS), in which error-prone DNA polymerases bypass unrepaired DNA lesions. We show that TENT4A regulates mRNA stability and/or translation of DNA polymerase η and RAD18 E3 ligase, which guides the polymerase to replication stalling sites and monoubiquitinates PCNA, thereby enabling recruitment of error-prone DNA polymerases to damaged DNA sites. Remarkably, in addition to the effect on RAD18 mRNA stability via controlling its poly(A) tail, TENT4A indirectly regulates RAD18 via the tumor suppressor CYLD and via the long non-coding antisense RNA PAXIP1-AS2, which had no known function. Knocking down the expression of TENT4A or CYLD, or overexpression of PAXIP1-AS2 led each to reduced amounts of the RAD18 protein and DNA polymerase η, leading to reduced TLS, highlighting PAXIP1-AS2 as a new TLS regulator. Bioinformatics analysis revealed that TLS error-prone DNA polymerase genes and their TENT4A-related regulators are frequently mutated in endometrial cancer genomes, suggesting that TLS is dysregulated in this cancer.

Biological Evaluation of DNA Biomarkers in a Chemically Defined and Site-Specific Manner

As described elsewhere in this Special Issue on biomarkers, much progress has been made in the detection of modified DNA within organisms at endogenous and exogenous levels of exposure to chemical species, including putative carcinogens and chemotherapeutic agents. Advances in the detection of damaged or unnatural bases have been able to provide correlations to support or refute hypotheses between the level of exposure to oxidative, alkylative, and other stresses, and the resulting DNA damage (lesion formation). However, such stresses can form a plethora of modified nucleobases, and it is therefore difficult to determine the individual contribution of a particular modification to alter a cell's genetic fate, as measured in the form of toxicity by stalled replication past the damage, by subsequent mutation, and by lesion repair. Chemical incorporation of a modification at a specific site within a vector (site-specific mutagenesis) has been a useful tool to deconvolute what types of damage quantified in biologically relevant systems may lead to toxicity and/or mutagenicity, thereby allowing researchers to focus on the most relevant biomarkers that may impact human health. Here, we will review a sampling of the DNA modifications that have been studied by shuttle vector techniques.

A Small Molecule Targeting Mutagenic Translesion Synthesis Improves Chemotherapy

Intrinsic and acquired drug resistance and induction of secondary malignancies limit successful chemotherapy. Because mutagenic translesion synthesis (TLS) contributes to chemoresistance as well as treatment-induced mutations, targeting TLS is an attractive avenue for improving chemotherapeutics. However, development of small molecules with high specificity and in vivo efficacy for mutagenic TLS has been challenging. Here, we report the discovery of a small-molecule inhibitor, JH-RE-06, that disrupts mutagenic TLS by preventing recruitment of mutagenic POL ζ. Remarkably, JH-RE-06 targets a nearly featureless surface of REV1 that interacts with the REV7 subunit of POL ζ. Binding of JH-RE-06 induces REV1 dimerization, which blocks the REV1-REV7 interaction and POL ζ recruitment. JH-RE-06 inhibits mutagenic TLS and enhances cisplatin-induced toxicity in cultured human and mouse cell lines. Co-administration of JH-RE-06 with cisplatin suppresses the growth of xenograft human melanomas in mice, establishing a framework for developing TLS inhibitors as a novel class of chemotherapy adjuvants.

Potential Strategies to Target Protein-Protein Interactions in the DNA Damage Response and Repair Pathways

This review article discusses some insights about generating novel mechanistic inhibitors of the DNA damage response and repair (DDR) pathways by focusing on protein-protein interactions (PPIs) of the key DDR components. General requirements for PPI strategies, such as selecting the target PPI site on the basis of its functionality, are discussed first. Next, on the basis of functional rationale and biochemical feasibility to identify a PPI inhibitor, 26 PPIs in DDR pathways (BER, MMR, NER, NHEJ, HR, TLS, and ICL repair) are specifically discussed for inhibitor discovery to benefit cancer therapies using a DNA-damaging agent.

Mass Spectrometry-Based Quantitative Strategies for Assessing the Biological Consequences and Repair of DNA Adducts

The genetic integrity of living organisms is constantly threatened by environmental and endogenous sources of DNA damaging agents that can induce a plethora of chemically modified DNA lesions. Unrepaired DNA lesions may elicit cytotoxic and mutagenic effects and contribute to the development of human diseases including cancer and neurodegeneration. Understanding the deleterious outcomes of DNA damage necessitates the investigation about the effects of DNA adducts on the efficiency and fidelity of DNA replication and transcription. Conventional methods for measuring lesion-induced replicative or transcriptional alterations often require time-consuming colony screening and DNA sequencing procedures. Recently, a series of mass spectrometry (MS)-based strategies have been developed in our laboratory as an efficient platform for qualitative and quantitative analyses of the changes in genetic information induced by DNA adducts during DNA replication and transcription. During the past few years, we have successfully used these MS-based methods for assessing the replicative or transcriptional blocking and miscoding properties of more than 30 distinct DNA adducts. When combined with genetic manipulation, these methods have also been successfully employed for revealing the roles of various DNA repair proteins or translesion synthesis DNA polymerases (Pols) in modulating the adverse effects of DNA lesions on transcription or replication in mammalian and bacterial cells. For instance, we found that Escherichia coli Pol IV and its mammalian ortholog (i.e., Pol κ) are required for error-free bypass of N(2)-(1-carboxyethyl)-2'-deoxyguanosine (N(2)-CEdG) in cells. We also found that the N(2)-CEdG lesions strongly inhibit DNA transcription and they are repaired by transcription-coupled nucleotide excision repair in mammalian cells. In this Account, we focus on the development of MS-based approaches for determining the effects of DNA adducts on DNA replication and transcription, where liquid chromatography-tandem mass spectrometry is employed for the identification, and sometimes quantification, of the progeny products arising from the replication or transcription of lesion-bearing DNA substrates in vitro and in mammalian cells. We also highlight their applications to lesion bypass, mutagenesis, and repair studies of three representative types of DNA lesions, that is, the methylglyoxal-induced N(2)-CEdG, oxidatively induced 8,5'-cyclopurine-2'-deoxynucleosides, and regioisomeric alkylated thymidine lesions. Specially, we discuss the similar and distinct effects of the minor-groove DNA lesions including N(2)-CEdG and O(2)-alkylated thymidine lesions, as well as the major-groove O(4)-alkylated thymidine lesions on DNA replication and transcription machinery. For example, we found that the addition of an alkyl group to the O(4) position of thymine may facilitate its preferential pairing with guanine and thus induce exclusively the misincorporation of guanine nucleotide opposite the lesion, whereas alkylation of thymine at the O(2) position may render the nucleobase unfavorable in pairing with any of the canonical nucleobases and thus exhibit promiscuous miscoding properties during DNA replication and transcription. The MS-based strategies described herein should be generally applicable for quantitative measurement of the biological consequences and repair of other DNA lesions in vitro and in cells.

DNA sequence context greatly affects the accuracy of bypass across an ultraviolet light 6-4 photoproduct in mammalian cells

Translesion DNA synthesis (TLS) is a DNA damage tolerance mechanism carried out by low-fidelity DNA polymerases that bypass DNA lesions, which overcomes replication stalling. Despite the miscoding nature of most common DNA lesions, several of them are bypassed in mammalian cells in a relatively accurate manner, which plays a key role maintaining a low mutation load. Whereas it is generally agreed that TLS across the major UV and sunlight induced DNA lesion, the cyclobutane pyrimidine dimer (CPD), is accurate, there were conflicting reports on whether the same is true for the thymine-thymine pyrimidine-pyrimidone(6-4) ultraviolet light photoproduct (TT6-4PP), which represents the second most common class of UV lesions. Using a TLS assay system based on gapped plasmids carrying site-specific TT6-4PP lesions in defined sequence contexts we show that the DNA sequence context markedly affected both the extent and accuracy of TLS. The sequence exhibiting higher TLS exhibited also higher error-frequency, caused primarily by semi-targeted mutations, at the nearest nucleotides flanking the lesion. Our results resolve the discrepancy reported on TLS across TT6-4PP, and suggest that TLS is more accurate in human cells than in mouse cells.

Identification of novel DNA-damage tolerance genes reveals regulation of translesion DNA synthesis by nucleophosmin

Cells cope with replication-blocking lesions via translesion DNA synthesis (TLS). TLS is carried out by low-fidelity DNA polymerases that replicate across lesions, thereby preventing genome instability at the cost of increased point mutations. Here we perform a two-stage siRNA-based functional screen for mammalian TLS genes and identify 17 validated TLS genes. One of the genes, NPM1, is frequently mutated in acute myeloid leukaemia (AML). We show that NPM1 (nucleophosmin) regulates TLS via interaction with the catalytic core of DNA polymerase-η (polη), and that NPM1 deficiency causes a TLS defect due to proteasomal degradation of polη. Moreover, the prevalent NPM1c+ mutation that causes NPM1 mislocalization in ~30% of AML patients results in excessive degradation of polη. These results establish the role of NPM1 as a key TLS regulator, and suggest a mechanism for the better prognosis of AML patients carrying mutations in NPM1.

Cells cope with replication-blocking DNA lesions by translesion DNA synthesis (TLS) polymerases, including polη. Here, the authors show that NPM1, a gene frequently mutated in acute myeloid leukaemia, protects polη from proteasomal degradation, and that NPM1 deficiency causes a TLS defect.