Characterizing Requirements for Small Ubiquitin-like Modifier (SUMO) Modification and Binding on Base Excision Repair Activity of Thymine-DNA Glycosylase in Vivo

Kinetic Analysis of the Effect of N-Terminal Acetylation on Thymine DNA Glycosylase

Thymine DNA glycosylase (TDG) is tasked with initiating DNA base excision repair by recognizing and removing T, U, the chemotherapeutic 5-fluorouracil (5-FU), and many other oxidized and halogenated pyrimidine bases. TDG contains a long, unstructured N-terminus that contains four known sites of acetylation: lysine (K) residues 59, 83, 84, and 87. Here, K to glutamine (Q) mutants are used as acetyl-lysine (AcK) analogues to probe the effect of N-terminal acetylation on the kinetics of TDG. We find that mimicking acetylation affects neither the maximal single-turnover rate k_max nor the turnover rate k_TO, indicating that the steps after initial binding, through chemistry and product release, are not affected. Under subsaturating conditions, however, acetylation changes the processing of U substrates. Subtle differences among AcK analogues are revealed with 5-FU in single-stranded DNA. We propose that the subtleties observed among the AcK analogues may be amplified on the genomic scale, leading to regulation of TDG activity. N-terminal acetylation, though, may also play a structural, rather than kinetic role in vivo.

Global non-covalent SUMO interaction networks reveal SUMO-dependent stabilization of the non-homologous end joining complex

In contrast to our extensive knowledge on covalent small ubiquitin-like modifier (SUMO) target proteins, we are limited in our understanding of non-covalent SUMO-binding proteins. We identify interactors of different SUMO isoforms-monomeric SUMO1, monomeric SUMO2, or linear trimeric SUMO2 chains-using a mass spectrometry-based proteomics approach. We identify 379 proteins that bind to different SUMO isoforms, mainly in a preferential manner. Interestingly, XRCC4 is the only DNA repair protein in our screen with a preference for SUMO2 trimers over mono-SUMO2, as well as the only protein in our screen that belongs to the non-homologous end joining (NHEJ) DNA double-strand break repair pathway. A SUMO interaction motif (SIM) in XRCC4 regulates its recruitment to sites of DNA damage and phosphorylation of S320 by DNA-PKcs. Our data highlight the importance of non-covalent and covalent sumoylation for DNA double-strand break repair via the NHEJ pathway and provide a resource of SUMO isoform interactors.

SIZ1-Mediated SUMOylation of ROS1 Enhances Its Stability and Positively Regulates Active DNA Demethylation in Arabidopsis

The 5-methylcytosine DNA glycosylase/lyase REPRESSOR OF SILENCING 1 (ROS1)-mediated active DNA demethylation is critical for shaping the genomic DNA methylation landscape in Arabidopsis. Whether and how the stability of ROS1 may be regulated by post-translational modifications is unknown. Using a methylation-sensitive PCR (CHOP-PCR)-based forward genetic screen for Arabidopsis DNA hyper-methylation mutants, we identified the SUMO E3 ligase SIZ1 as a critical regulator of active DNA demethylation. Dysfunction of SIZ1 leads to hyper-methylation at approximately 1000 genomic regions. SIZ1 physically interacts with ROS1 and mediates the SUMOylation of ROS1. The SUMOylation of ROS1 is reduced in siz1 mutant plants. Compared with that in wild-type plants, the protein level of ROS1 is significantly decreased, whereas there is an increased level of ROS1 transcripts in siz1 mutant plants. Our results suggest that SIZ1-mediated SUMOylation of ROS1 promotes its stability and positively regulates active DNA demethylation.

Defining the impact of sumoylation on substrate binding and catalysis by thymine DNA glycosylase

Thymine DNA glycosylase (TDG) excises thymine from mutagenic G·T mispairs generated by deamination of 5-methylcytosine (mC) and it removes two mC derivatives, 5−formylcytosine (fC) and 5−carboxylcytosine (caC), in a multistep pathway for DNA demethylation. TDG is modified by small ubiquitin-like modifier (SUMO) proteins, but the impact of sumoylation on TDG activity is poorly defined and the functions of TDG sumoylation remain unclear. We determined the effect of TDG sumoylation, by SUMO-1 or SUMO-2, on substrate binding and catalytic parameters. Single turnover experiments reveal that sumoylation dramatically impairs TDG base-excision activity, such that G·T activity is reduced by ≥45-fold and fC and caC are excised slowly, with a reaction half-life of ≥9 min (37°C). Fluorescence anisotropy studies reveal that unmodified TDG binds tightly to G·fC and G·caC substrates, with dissociation constants in the low nanomolar range. While sumoylation of TDG weakens substrate binding, the residual affinity is substantial and is comparable to that of biochemically-characterized readers of fC and caC. Our findings raise the possibility that sumoylation enables TDG to function, at least transiently, as reader of fC and caC. Notably, sumoylation could potentially facilitate TDG recruitment of other proteins, including transcription factors or epigenetic regulators, to these sites in DNA.

Origin of mutations in genes associated with human glioblastoma multiform cancer: random polymerase errors versus deamination

The etiology of glioblastoma multiforme (GBM), the most serious form of brain cancer, remains obscure, although it has been proposed that cancer risk is a function of random polymerase errors that occur during stem cell division and the resulting mutations in oncogenes and tumor suppressor genes. Analysis of the 8 genes (PTEN, TP53, EGFR, PIK3R1, PIK3CA, NF1, RB1, IDH1) that are mutated in at least 5% of GBM tumors indicates a non-random mutation pattern that reflects a significant role for hydrolytic deamination at CpG sites. The formation of activating mutations in some genes, e.g., IDH1, where a very limited set of mutations are oncogenic, statistically cannot involve random mutagenesis due to polymerase errors that occur during each stem cell replication. Comparison of the in vitro misincorporation tendencies of three replicative polymerases and the “random” mutation pattern in a subset of genes indicates non-polymerase based pathways are involved. Analysis of the mutation patterns shows that chemical deamination that occurs at a slow rate at each CpG is favored over random polymerase errors by a factor of more than 10 million. Therefore, if a truncating nonsense mutation in a tumor suppressor, or an activating missense mutation in an oncogene, can occur due to a C > T base substitution at a CpG sequence, it is highly favored over other mutation pathways.

SUMOylation coordinates BERosome assembly in active DNA demethylation during cell differentiation

During active DNA demethylation, 5-methylcytosine (5mC) is oxidized by TET proteins to 5-formyl-/5-carboxylcytosine (5fC/5caC) for replacement by unmethylated C by TDG-initiated DNA base excision repair (BER). Base excision generates fragile abasic sites (AP-sites) in DNA and has to be coordinated with subsequent repair steps to limit accumulation of genome destabilizing secondary DNA lesions. Here, we show that 5fC/5caC is generated at a high rate in genomes of differentiating mouse embryonic stem cells and that SUMOylation and the BER protein XRCC1 play critical roles in orchestrating TDG-initiated BER of these lesions. SUMOylation of XRCC1 facilitates physical interaction with TDG and promotes the assembly of a TDG-BER core complex. Within this TDG-BERosome, SUMO is transferred from XRCC1 and coupled to the SUMO acceptor lysine in TDG, promoting its dissociation while assuring the engagement of the BER machinery to complete demethylation. Although well-studied, the biological importance of TDG SUMOylation has remained obscure. Here, we demonstrate that SUMOylation of TDG suppresses DNA strand-break accumulation and toxicity to PARP inhibition in differentiating mESCs and is essential for neural lineage commitment.

Nucleosomes and the three glycosylases: High, medium, and low levels of excision by the uracil DNA glycosylase superfamily

Human cells express the UDG superfamily of glycosylases, which excise uracil (U) from the genome. The three members of this structural superfamily are uracil DNA glycosylase (UNG/UDG), single-strand selective monofunctional uracil DNA glycosylase (SMUG1), and thymine DNA glycosylase (TDG). We previously reported that UDG is efficient at removing U from DNA packaged into nucleosome core particles (NCP) and is minimally affected by the histone proteins when acting on an outward-facing U in the dyad region. In an effort to determine whether this high activity is a general property of the UDG superfamily of glycosylases, we compare the activity of UDG, SMUG1, and TDG on a U:G wobble base pair using NCP assembled from Xenopus laevis histones and the Widom 601 positioning sequence. We found that while UDG is highly active, SMUG1 is severely inhibited on NCP and this inhibition is independent of sequence context. Here we also provide the first report of TDG activity on an NCP, and found that TDG has an intermediate level of activity in excision of U and is severely inhibited in its excision of T. These results are discussed in the context of cellular roles for each of these enzymes.

Functions of SUMO in the Maintenance of Genome Stability

Like in most other areas of cellular metabolism, the functions of the ubiquitin-like modifier SUMO in the maintenance of genome stability are manifold and varied. Perturbations of global sumoylation causes a wide spectrum of phenotypes associated with defects in DNA maintenance, such as hypersensitivity to DNA-damaging agents, gross chromosomal rearrangements and loss of entire chromosomes. Consistent with these observations, many key factors involved in various DNA repair pathways have been identified as SUMO substrates. However, establishing a functional connection between a given SUMO target, the cognate SUMO ligase and a relevant phenotype has remained a challenge, mainly because of the difficulties involved in identifying important modification sites and downstream effectors that specifically recognize the target in its sumoylated state. This review will give an overview over the major pathways of DNA repair and genome maintenance influenced by the SUMO system and discuss selected examples of SUMO's actions in these pathways where the biological consequences of the modification have been elucidated.

BERing the burden of damage: Pathway crosstalk and posttranslational modification of base excision repair proteins regulate DNA damage management

DNA base damage and non-coding apurinic/apyrimidinic (AP) sites are ubiquitous types of damage that must be efficiently repaired to prevent mutations. These damages can occur in both the nuclear and mitochondrial genomes. Base excision repair (BER) is the frontline pathway for identifying and excising damaged DNA bases in both of these cellular compartments. Recent advances demonstrate that BER does not operate as an isolated pathway but rather dynamically interacts with components of other DNA repair pathways to modulate and coordinate BER functions. We define the coordination and interaction between DNA repair pathways as pathway crosstalk. Numerous BER proteins are modified and regulated by post-translational modifications (PTMs), and PTMs could influence pathway crosstalk. Here, we present recent advances on BER/DNA repair pathway crosstalk describing specific examples and also highlight regulation of BER components through PTMs. We have organized and reported functional interactions and documented PTMs for BER proteins into a consolidated summary table. We further propose the concept of DNA repair hubs that coordinate DNA repair pathway crosstalk to identify central protein targets that could play a role in designing future drug targets.

Kinetic Methods for Studying DNA Glycosylases Functioning in Base Excision Repair

Base excision repair (BER) is a conserved and ubiquitous pathway that is initiated by DNA glycosylases, which recognize and remove damaged or mismatched nucleobases, setting the stage for restoration of the correct DNA sequence by follow-on BER enzymes. DNA glycosylases employ a nucleotide-flipping step prior to cleavage of the N-glycosyl bond, and most exhibit slow release of the abasic DNA product and/or strong product inhibition. As such, studying the catalytic mechanism of these enzymes requires care in the design, execution, and interpretation of single- and multiple-turnover kinetics experiments, which is the topic of this chapter.

DNA repair and erasure of 5-methylcytosine in vertebrates

DNA methylation plays important roles in development and disease. Yet, only recently has the dynamic nature of this epigenetic mark via oxidation and DNA repair-mediated demethylation been recognized. A major conceptual challenge to the model that DNA methylation is reversible is the risk of genomic instability, which may come with widespread DNA repair activity. Here, we focus on recent advances in mechanisms of TET-TDG mediated demethylation and cellular strategies that avoid genomic instability. We highlight the recently discovered involvement of NEIL DNA glycosylases, which cooperate with TDG in oxidative demethylation to accelerate substrate turnover and promote the organized handover of harmful repair intermediates to maintain genome stability.

Structural basis of damage recognition by thymine DNA glycosylase: Key roles for N-terminal residues

Thymine DNA Glycosylase (TDG) is a base excision repair enzyme functioning in DNA repair and epigenetic regulation. TDG removes thymine from mutagenic G·T mispairs arising from deamination of 5-methylcytosine (mC), and it processes other deamination-derived lesions including uracil (U). Essential for DNA demethylation, TDG excises 5-formylcytosine and 5-carboxylcytosine, derivatives of mC generated by Tet (ten-eleven translocation) enzymes. Here, we report structural and functional studies of TDG^82-308, a new construct containing 29 more N-terminal residues than TDG^111-308, the construct used for previous structures of DNA-bound TDG. Crystal structures and NMR experiments demonstrate that most of these N-terminal residues are disordered, for substrate- or product-bound TDG^82-308 Nevertheless, G·T substrate affinity and glycosylase activity of TDG^82-308 greatly exceeds that of TDG^111-308 and is equivalent to full-length TDG. We report the first high-resolution structures of TDG in an enzyme-substrate complex, for G·U bound to TDG^82-308 (1.54 Å) and TDG^111-308 (1.71 Å), revealing new enzyme-substrate contacts, direct and water-mediated. We also report a structure of the TDG^82-308 product complex (1.70 Å). TDG^82-308 forms unique enzyme-DNA interactions, supporting its value for structure-function studies. The results advance understanding of how TDG recognizes and removes modified bases from DNA, particularly those resulting from deamination.

Role of Base Excision "Repair" Enzymes in Erasing Epigenetic Marks from DNA

Base excision repair (BER) is one of several DNA repair pathways found in all three domains of life. BER counters the mutagenic and cytotoxic effects of damage that occurs continuously to the nitrogenous bases in DNA, and its critical role in maintaining genomic integrity is well established. However, BER also performs essential functions in processes other than DNA repair, where it acts on naturally modified bases in DNA. A prominent example is the central role of BER in mediating active DNA demethylation, a multistep process that erases the epigenetic mark 5-methylcytosine (5mC), and derivatives thereof, converting them back to cytosine. Herein, we review recent advances in the understanding of how BER mediates this critical component of epigenetic regulation in plants and animals.