Catalytic contributions of key residues in the adenine glycosylase MutY revealed by pH-dependent kinetics and cellular repair assays

FSHing for DNA Damage: Key Features of MutY Detection of 8-Oxoguanine:Adenine Mismatches

Base excision repair (BER) enzymes are genomic superheroes that stealthily and accurately identify and remove chemically modified DNA bases. DNA base modifications erode the informational content of DNA and underlie many disease phenotypes, most conspicuously, cancer. The "OG" of oxidative base damage, 8-oxo-7,8-dihydroguanine (OG), is particularly insidious due to its miscoding ability that leads to the formation of rare, pro-mutagenic OG:A mismatches. Thwarting mutagenesis relies on the capture of OG:A mismatches prior to DNA replication and removal of the mis-inserted adenine by MutY glycosylases to initiate BER. The threat of OG and the importance of its repair are underscored by the association between inherited dysfunctional variants of the MutY human homologue (MUTYH) and colorectal cancer, known as MUTYH-associated polyposis (MAP). Our functional studies of the two founder MUTYH variants revealed that both have compromised activity and a reduced affinity for OG:A mismatches. Indeed, these studies underscored the challenge of the recognition of OG:A mismatches that are only subtly structurally different than T:A base pairs. Since the original discovery of MAP, many MUTYH variants have been reported, with most considered to be "variants of uncertain significance." To reveal features associated with damage recognition and adenine excision by MutY and MUTYH, we have developed a multipronged chemical biology approach combining enzyme kinetics, X-ray crystallography, single-molecule visualization, and cellular repair assays. In this review, we highlight recent work in our laboratory where we defined MutY structure-activity relationship (SAR) studies using synthetic analogs of OG and A in cellular and in vitro assays. Our studies revealed the 2-amino group of OG as the key distinguishing feature of OG:A mismatches. Indeed, the unique position of the 2-amino group in the major groove of OGsyn:Aanti mismatches provides a means for its rapid detection among a large excess of highly abundant and structurally similar canonical base pairs. Furthermore, site-directed mutagenesis and structural analysis showed that a conserved C-terminal domain β-hairpin "FSH'' loop is critical for OG recognition with the "His" serving as the lesion detector. Notably, MUTYH variants located within and near the FSH loop have been associated with different forms of cancer. Uncovering the role(s) of this loop in lesion recognition provided a detailed understanding of the search and repair process of MutY. Such insights are also useful to identify mutational hotspots and pathogenic variants, which may improve the ability of physicians to diagnose the likelihood of disease onset and prognosis. The critical importance of the "FSH" loop in lesion detection suggests that it may serve as a unique locus for targeting probes or inhibitors of MutY/MUTYH to provide new chemical biology tools and avenues for therapeutic development.

Cellular Repair of Synthetic Analogs of Oxidative DNA Damage Reveals a Key Structure-Activity Relationship of the Cancer-Associated MUTYH DNA Repair Glycosylase

The base excision repair glycosylase MUTYH prevents mutations associated with the oxidatively damaged base, 8-oxo-7,8-dihydroguanine (OG), by removing undamaged misincorporated adenines from OG:A mispairs. Defects in OG:A repair in individuals with inherited MUTYH variants are correlated with the colorectal cancer predisposition syndrome known as MUTYH-associated polyposis (MAP). Herein, we reveal key structural features of OG required for efficient repair by human MUTYH using structure-activity relationships (SAR). We developed a GFP-based plasmid reporter assay to define SAR with synthetically generated OG analogs in human cell lines. Cellular repair results were compared with kinetic parameters measured by adenine glycosylase assays in vitro. Our results show substrates lacking the 2-amino group of OG, such as 8OI:A (8OI = 8-oxoinosine), are not repaired in cells, despite being excellent substrates in in vitro adenine glycosylase assays, new evidence that the search and detection steps are critical factors in cellular MUTYH repair functionality. Surprisingly, modification of the O8/N7H of OG, which is the distinguishing feature of OG relative to G, was tolerated in both MUTYH-mediated cellular repair and in vitro adenine glycosylase activity. The lack of sensitivity to alterations at the O8/N7H in the SAR of MUTYH substrates is distinct from previous work with bacterial MutY, indicating that the human enzyme is much less stringent in its lesion verification. Our results imply that the human protein relies almost exclusively on detection of the unique major groove position of the 2-amino group of OG within OGsyn:Aanti mispairs to select contextually incorrect adenines for excision and thereby thwart mutagenesis. These results predict that MUTYH variants that exhibit deficiencies in OG:A detection will be severely compromised in a cellular setting. Moreover, the reliance of MUTYH on the interaction with the OG 2-amino group suggests that disrupting this interaction with small molecules may provide a strategy to develop potent and selective MUTYH inhibitors.

Preorganized Internal Electric Field Promotes a Double-Displacement Mechanism for the Adenine Excision Reaction by Adenine DNA Glycosylase

Adenine DNA glycosylase (MutY) is a monofunctional glycosylase, removing adenines (A) misinserted opposite 8-oxo-7,8-dihydroguanine (OG), a common product of oxidative damage to DNA. Through multiscale calculations, we decipher a detailed adenine excision mechanism of MutY that is consistent with all available experimental data, involving an initial protonation step and two nucleophilic displacement steps. During the first displacement step, N-glycosidic bond cleavage is accompanied by the attack of the carboxylate group of residue Asp144 at the anomeric carbon (C1'), forming a covalent glycosyl-enzyme intermediate to stabilize the fleeting oxocarbenium ion. After departure of the excised base, water nucleophiles can be recruited to displace Asp144, completing the catalytic cycle with retention of stereochemistry at the C1' position. The two displacement reactions are found to mostly involve the movement of the oxocarbenium ion, occurring with large charge reorganization and thus sensitive to the internal electric field (IEF) exerted by the polar protein environment. Intriguingly, we find that the negatively charged carboxylate group is a good nucleophile for the oxocarbenium ion, yet an unactivated water molecule is not, and that the electric field catalysis strategy is used by the enzyme to enable its unique double-displacement reaction mechanism. A strong IEF, pointing toward 5' direction of the substrate sugar ring, greatly facilitates the second displacement reaction at the expense of elevating the barrier of the first one, thereby allowing both reactions to occur. These findings not only increase our understanding of the strategies used by DNA glycosylases to repair DNA lesions, but also have important implications for how internal/external electric field can be applied to modulate chemical reactions.

Distinctive Formation of a DNA-Protein Cross-Link during the Repair of DNA Oxidative Damage: Insights into Human Disease from MD Simulations and QM/MM Calculations

Reactive oxygen species damage DNA and result in health issues. The major damage product, 8-oxo-7,8-dihydroguanine (8oG), is repaired by human adenine DNA glycosylase homologue (MUTYH). Although MUTYH misfunction is associated with a genetic disorder called MUTYH-associated polyposis (MAP) and MUTYH is a potential target for cancer drugs, the catalytic mechanism required to develop disease treatments is debated in the literature. This study uses molecular dynamics simulations and quantum mechanics/molecular mechanics techniques initiated from DNA-protein complexes that represent different stages of the repair pathway to map the catalytic mechanism of the wild-type MUTYH bacterial homologue (MutY). This multipronged computational approach characterizes a DNA-protein cross-linking mechanism that is consistent with all previous experimental data and is a distinct pathway across the broad class of monofunctional glycosylase repair enzymes. In addition to clarifying how the cross-link is formed, accommodated by the enzyme, and hydrolyzed for product release, our calculations rationalize why cross-link formation is favored over immediate glycosidic bond hydrolysis, the accepted mechanism for all other monofunctional DNA glycosylases to date. Calculations on the Y126F mutant MutY highlight critical roles for active site residues throughout the reaction, while investigation of the N146S mutant rationalizes the connection between the analogous N224S MUTYH mutation and MAP. In addition to furthering our knowledge of the chemistry associated with a devastating disorder, the structural information gained about the distinctive MutY mechanism compared to other repair enzymes represents an important step for the development of specific and potent small-molecule inhibitors as cancer therapeutics.

Structural snapshots of base excision by the cancer-associated variant MutY N146S reveal a retaining mechanism

DNA glycosylase MutY plays a critical role in suppression of mutations resulted from oxidative damage, as highlighted by cancer-association of the human enzyme. MutY requires a highly conserved catalytic Asp residue for excision of adenines misinserted opposite 8-oxo-7,8-dihydroguanine (OG). A nearby Asn residue hydrogen bonds to the catalytic Asp in structures of MutY and its mutation to Ser is an inherited variant in human MUTYH associated with colorectal cancer. We captured structural snapshots of N146S Geobacillus stearothermophilus MutY bound to DNA containing a substrate, a transition state analog and enzyme-catalyzed abasic site products to provide insight into the base excision mechanism of MutY and the role of Asn. Surprisingly, despite the ability of N146S to excise adenine and purine (P) in vitro, albeit at slow rates, N146S-OG:P complex showed a calcium coordinated to the purine base altering its conformation to inhibit hydrolysis. We obtained crystal structures of N146S Gs MutY bound to its abasic site product by removing the calcium from crystals of N146S-OG:P complex to initiate catalysis in crystallo or by crystallization in the absence of calcium. The product structures of N146S feature enzyme-generated β-anomer abasic sites that support a retaining mechanism for MutY-catalyzed base excision.

Structure, function and evolution of the Helix-hairpin-Helix DNA glycosylase superfamily: Piecing together the evolutionary puzzle of DNA base damage repair mechanisms

The Base Excision Repair (BER) pathway is a highly conserved DNA repair system targeting chemical base modifications that arise from oxidation, deamination and alkylation reactions. BER features lesion-specific DNA glycosylases (DGs) which recognize and excise modified or inappropriate DNA bases to produce apurinic/apyrimidinic (AP) sites and coordinate AP-site hand-off to subsequent BER pathway enzymes. The DG superfamilies identified have evolved independently to cope with a wide variety of nucleobase chemical modifications. Most DG superfamilies recognize a distinct set of structurally related lesions. In contrast, the Helix-hairpin-Helix (HhH) DG superfamily has the remarkable ability to act upon structurally diverse sets of base modifications. The versatility in substrate recognition of the HhH-DG superfamily has been shaped by motif and domain acquisitions during evolution. In this paper, we review the structural features and catalytic mechanisms of the HhH-DG superfamily and draw a hypothetical reconstruction of the evolutionary path where these DGs developed diverse and unique enzymatic features.

Unique Hydrogen Bonding of Adenine with the Oxidatively Damaged Base 8-Oxoguanine Enables Specific Recognition and Repair by DNA Glycosylase MutY

The DNA glycosylase MutY prevents deleterious mutations resulting from guanine oxidation by recognition and removal of adenine (A) misincorporated opposite 8-oxo-7,8-dihydroguanine (OG). Correct identification of OG:A is crucial to prevent improper and detrimental MutY-mediatedadenine excision from G:A or T:A base pairs. Here we present a structure-activity relationship (SAR) study using analogues of A to probe the basis for OG:A specificity of MutY. We correlate observed in vitro MutY activity on A analogue substrates with their experimental and calculated acidities to provide mechanistic insight into the factors influencing MutY base excision efficiency. These data show that H-bonding and electrostatic interactions of the base within the MutY active site modulate the lability of the N-glycosidic bond. A analogues that were not excised from duplex DNA as efficiently as predicted by calculations provided insight into other required structural features, such as steric fit and H-bonding within the active site for proper alignment with MutY catalytic residues. We also determined MutY-mediated repair of A analogues paired with OG within the context of a DNA plasmid in bacteria. Remarkably, the magnitudes of decreased in vitro MutY excision rates with different A analogue duplexes do not correlate with the impact on overall MutY-mediated repair. The feature that most strongly correlated with facile cellular repair was the ability of the A analogues to H-bond with the Hoogsteen face of OG. Notably, base pairing of A with OG uniquely positions the 2-amino group of OG in the major groove and provides a means to indirectly select only these inappropriately placed adenines for excision. This highlights the importance of OG lesion detection for efficient MutY-mediated cellular repair. The A analogue SARs also highlight the types of modifications tolerated by MutY and will guide the development of specific probes and inhibitors of MutY.

When you're strange: Unusual features of the MUTYH glycosylase and implications in cancer

MUTYH is a base-excision repair glycosylase that removes adenine opposite 8-oxoguanine (OG). Variants of MUTYH defective in functional activity lead to MUTYH-associated polyposis (MAP), which progresses to cancer with very high penetrance. Whole genome and whole exome sequencing studies have found MUTYH deficiencies in an increasing number of cancer types. While the canonical OG:A repair activity of MUTYH is well characterized and similar to bacterial MutY, here we review more recent evidence that MUTYH has activities independent of OG:A repair and appear centered on the interdomain connector (IDC) region of MUTYH. We summarize evidence that MUTYH is involved in rapid DNA damage response (DDR) signaling, including PARP activation, 9-1-1 and ATR signaling, and SIRT6 activity. MUTYH alters survival and DDR to a wide variety of DNA damaging agents in a time course that is not consistent with the formation of OG:A mispairs. Studies that suggest MUTYH inhibits the repair of alkyl-DNA damage and cyclopyrimidine dimers (CPDs) is reviewed, and evidence of a synthetic lethal interaction with mismatch repair (MMR) is summarized. Based on these studies we suggest that MUTYH has evolved from an OG:A mispair glycosylase to a multifunctional scaffold for DNA damage response signaling.

Mechanisms of MTH1 inhibition-induced DNA strand breaks: The slippery slope from the oxidized nucleotide pool to genotoxic damage

Unlike normal tissues, tumor cells possess a propensity for genomic instability, resulting from elevated oxidant levels produced by oncogenic signaling and aberrant cellular metabolism. Thus, targeting mechanisms that protect cancer cells from the tumor-inhibitory consequences of their redox imbalance and spontaneous DNA-damaging events is expected to have broad-spectrum efficacy and a high therapeutic index. One critical mechanism for tumor cell protection from oxidant stress is the hydrolysis of oxidized nucleotides. Human MutT homolog 1 (MTH1), the mammalian nudix (nucleoside diphosphate X) pyrophosphatase (NUDT1), protects tumor cells from oxidative stress-induced genomic DNA damage by cleansing the nucleotide pool of oxidized purine nucleotides. Depletion or pharmacologic inhibition of MTH1 results in genomic DNA strand breaks in many cancer cells. However, the mechanisms underlying how oxidized nucleotides, thought mainly to be mutagenic rather than genotoxic, induce DNA strand breaks are largely unknown. Given the recent therapeutic interest in targeting MTH1, a better understanding of such mechanisms is crucial to its successful translation into the clinic and in identifying the molecular contexts under which its inhibition is likely to be beneficial. Here we provide a comprehensive perspective on MTH1 function and its importance in protecting genome integrity, in the context of tumor-associated oxidative stress and the mechanisms that likely lead to irreparable DNA strand breaks as a result of MTH1 inhibition.

DNA scanning by base excision repair enzymes and implications for pathway coordination

Site-specific DNA binding proteins must search the genome to locate their target sites, and many DNA modifying enzymes have the ability to scan along DNA in search of their substrates. This process is termed processive searching, and it serves to decrease the search time by effectively increasing the DNA binding footprint of a protein. The repertoire of proteins capable of processive searching is expanding, highlighting the need to understand the governing principles behind this fundamental process. Many of the enzymes in the base excision DNA repair pathway are capable of processive searching. Here, we briefly summarize methodology for determining if a protein can scan DNA and highlight the discovery that the base excision repair DNA polymerase β performs a processive search. Elucidation of physical models for DNA searching has also provided a plausible mechanism for pathway coordination during repair. The ability of BER enzymes to transiently sample adjacent DNA sites while bound to their product confers accessibility to downstream enzymes and does not require protein-protein interactions for coordination.

Cellular Assays for Studying the Fe-S Cluster Containing Base Excision Repair Glycosylase MUTYH and Homologs

Many DNA repair enzymes, including the human adenine glycosylase MUTYH, require iron-sulfur (Fe-S) cluster cofactors for DNA damage recognition and subsequent repair. MUTYH prokaryotic and eukaryotic homologs are a family of adenine (A) glycosylases that cleave A when mispaired with the oxidatively damaged guanine lesion, 8-oxo-7,8-dihydroguanine (OG). Faulty OG:A repair has been linked to the inheritance of missense mutations in the MUTYH gene. These inherited mutations can result in the onset of a familial colorectal cancer disorder known as MUTYH-associated polyposis (MAP). While in vitro studies can be exceptional at unraveling how MutY interacts with its OG:A substrate, cell-based assays are needed to provide a cellular context to these studies. In addition, strategic comparison of in vitro and in vivo studies can provide exquisite insight into the search, selection, excision process, and the coordination with protein partners, required to mediate full repair of the lesion. A commonly used assay is the rifampicin resistance assay that provides an indirect evaluation of the intrinsic mutation rate in Escherichia coli (E. coli or Ec), read out as antibiotic-resistant cell growth. Our laboratory has also developed a bacterial plasmid-based assay that allows for direct evaluation of repair of a defined OG:A mispair. This assay provides a means to assess the impact of catalytic defects in affinity and excision on overall repair. Finally, a mammalian GFP-based reporter assay has been developed that more accurately models features of mammalian cells. Taken together, these assays provide a cellular context to the repair activity of MUTYH and its homologs that illuminates the role these enzymes play in preventing mutations and disease.

Characterization of Trypanosoma cruzi MutY DNA glycosylase ortholog and its role in oxidative stress response

Trypanosoma cruzi is a protozoan parasite and the causative agent of Chagas disease. Like most living organisms, it is susceptible to oxidative stress, and must adapt to distinct environments. Hence, DNA repair is essential for its survival and the persistence of infection. Therefore, we studied whether T. cruzi has a homolog counterpart of the MutY enzyme (TcMYH), important in the DNA Base Excision Repair (BER) mechanism. Analysis of T. cruzi genome database showed that this parasite has a putative MutY DNA glycosylase sequence. We performed heterologous complementation assays using this genomic sequence. TcMYH complemented the Escherichia coli MutY- strain, reducing the mutation rate to a level similar to wild type. In in vitro assays, TcMYH was able to remove an adenine that was opposite to 8-oxoguanine. We have also constructed a T. cruzi lineage that overexpresses MYH. Although in standard conditions this lineage has similar growth to control cells, the overexpressor is more sensitive to hydrogen peroxide and glucose oxidase than the control, probably due to accumulation of AP sites in its DNA. Localization experiments with GFP-fused TcMYH showed this enzyme is present in both nucleus and mitochondrion. QPCR and MtOX results reinforce the presence and function of TcMYH in these two organelles. Our data suggest T. cruzi has a functional MYH DNA glycosylase, which participates in nuclear and mitochondrial DNA Base Excision Repair.

Structure-Activity Relationships Reveal Key Features of 8-Oxoguanine: A Mismatch Detection by the MutY Glycosylase

Base excision repair glycosylases locate and remove damaged bases in DNA with remarkable specificity. The MutY glycosylases, unusual for their excision of undamaged adenines mispaired to the oxidized base 8-oxoguanine (OG), must recognize both bases of the mispair in order to prevent promutagenic activity. Moreover, MutY must effectively find OG:A mismatches within the context of highly abundant and structurally similar T:A base pairs. Very little is known about the factors that initiate MutY’s interaction with the substrate when it first encounters an intrahelical OG:A mispair, or about the order of recognition checkpoints. Here, we used structure–activity relationships (SAR) to investigate the features that influence the in vitro measured parameters of mismatch affinity and adenine base excision efficiency by E. coli MutY. We also evaluated the impacts of the same substrate alterations on MutY-mediated repair in a cellular context. Our results show that MutY relies strongly on the presence of the OG base and recognizes multiple structural features at different stages of recognition and catalysis to ensure that only inappropriately mispaired adenines are excised. Notably, some OG modifications resulted in more dramatic reductions in cellular repair than in the in vitro kinetic parameters, indicating their importance for initial recognition events needed to locate the mismatch within DNA. Indeed, the initial encounter of MutY with its target base pair may rely on specific interactions with the 2-amino group of OG in the major groove, a feature that distinguishes OG:A from T:A base pairs. These results furthermore suggest that inefficient substrate location in human MutY homologue variants may prove predictive for the early onset colorectal cancer phenotype known as MUTYH-Associated Polyposis, or MAP.

Repair of 8-oxoG:A mismatches by the MUTYH glycosylase: Mechanism, metals and medicine

Reactive oxygen and nitrogen species (RONS) may infringe on the passing of pristine genetic information by inducing DNA inter- and intra-strand crosslinks, protein-DNA crosslinks, and chemical alterations to the sugar or base moieties of DNA. 8-Oxo-7,8-dihydroguanine (8-oxoG) is one of the most prevalent DNA lesions formed by RONS and is repaired through the base excision repair (BER) pathway involving the DNA repair glycosylases OGG1 and MUTYH in eukaryotes. MUTYH removes adenine (A) from 8-oxoG:A mispairs, thus mitigating the potential of G:C to T:A transversion mutations from occurring in the genome. The paramount role of MUTYH in guarding the genome is well established in the etiology of a colorectal cancer predisposition syndrome involving variants of MUTYH, referred to as MUTYH-associated polyposis (MAP). In this review, we highlight recent advances in understanding how MUTYH structure and related function participate in the manifestation of human disease such as MAP. Here we focus on the importance of MUTYH's metal cofactor sites, including a recently discovered "Zinc linchpin" motif, as well as updates to the catalytic mechanism. Finally, we touch on the insight gleaned from studies with MAP-associated MUTYH variants and recent advances in understanding the multifaceted roles of MUTYH in the cell, both in the prevention of mutagenesis and tumorigenesis.

Dispensability of the [4Fe-4S] cluster in novel homologues of adenine glycosylase MutY

7,8-Dihydro-8-deoxyguanine (8oG) is one of the most common oxidative lesions in DNA. DNA polymerases misincorporate an adenine across from this lesion. Thus, 8oG is a highly mutagenic lesion responsible for G:C→T:A transversions. MutY is an adenine glycosylase, part of the base excision repair pathway that removes adenines, when mispaired with 8oG or guanine. Its catalytic domain includes a [4Fe-4S] cluster motif coordinated by cysteinyl ligands. When this cluster is absent, MutY activity is depleted and several studies concluded that the [4Fe-4S] cluster motif is an indispensable component for DNA binding, substrate recognition and enzymatic activity. In the present study, we identified 46 MutY homologues that lack the canonical cysteinyl ligands, suggesting an absence of the [4Fe-4S] cluster. A phylogenetic analysis groups these novel MutYs into two different clades. One clade is exclusive of the order Lactobacillales and another clade has a mixed composition of anaerobic and microaerophilic bacteria and species from the protozoan genus Entamoeba. Structural modeling and sequence analysis suggests that the loss of the [4Fe-4S] cluster is compensated by a convergent solution in which bulky amino acids substitute the [4Fe-4S] cluster. We functionally characterized MutYs from Lactobacillus brevis and Entamoeba histolytica as representative members from each clade and found that both enzymes are active adenine glycosylases. Furthermore, chimeric glycosylases, in which the [4Fe-4S] cluster of Escherichia coli MutY is replaced by the corresponding amino acids of LbY and EhY, are also active. Our data indicates that the [4Fe-4S] cluster plays a structural role in MutYs and evidences the existence of alternative functional solutions in nature.

Structure and stereochemistry of the base excision repair glycosylase MutY reveal a mechanism similar to retaining glycosidases

MutY adenine glycosylases prevent DNA mutations by excising adenine from promutagenic 8-oxo-7,8-dihydroguanine (OG):A mismatches. Here, we describe structural features of the MutY active site bound to an azaribose transition state analog which indicate a catalytic role for Tyr126 and approach of the water nucleophile on the same side as the departing adenine base. The idea that Tyr126 participates in catalysis, recently predicted by modeling calculations, is strongly supported by mutagenesis and by seeing close contact between the hydroxyl group of this residue and the azaribose moiety of the transition state analog. NMR analysis of MutY methanolysis products corroborates a mechanism for adenine removal with retention of stereochemistry. Based on these results, we propose a revised mechanism for MutY that involves two nucleophilic displacement steps akin to the mechanisms accepted for 'retaining' O-glycosidases. This new-for-MutY yet familiar mechanism may also be operative in related base excision repair glycosylases and provides a critical framework for analysis of human MutY (MUTYH) variants associated with inherited colorectal cancer.

Repair of Alkylation Damage in Eukaryotic Chromatin Depends on Searching Ability of Alkyladenine DNA Glycosylase

Human alkyladenine DNA glycosylase (AAG) initiates the base excision repair pathway by excising alkylated and deaminated purine lesions. In vitro biochemical experiments demonstrate that AAG uses facilitated diffusion to efficiently search DNA to find rare sites of damage and suggest that electrostatic interactions are critical to the searching process. However, it remains an open question whether DNA searching limits the rate of DNA repair in vivo. We constructed AAG mutants with altered searching ability and measured their ability to protect yeast from alkylation damage in order to address this question. Each of the conserved arginine and lysine residues that are near the DNA binding interface were mutated, and the functional impacts were evaluated using kinetic and thermodynamic analysis. These mutations do not perturb catalysis of N-glycosidic bond cleavage, but they decrease the ability to capture rare lesion sites. Nonspecific and specific DNA binding properties are closely correlated, suggesting that the electrostatic interactions observed in the specific recognition complex are similarly important for DNA searching complexes. The ability of the mutant proteins to complement repair-deficient yeast cells is positively correlated with the ability of the proteins to search DNA in vitro, suggesting that cellular resistance to DNA alkylation is governed by the ability to find and efficiently capture cytotoxic lesions. It appears that chromosomal access is not restricted and toxic sites of alkylation damage are readily accessible to a searching protein.

Mechanisms for enzymatic cleavage of the N-glycosidic bond in DNA

DNA glycosylases remove damaged or enzymatically modified nucleobases from DNA, thereby initiating the base excision repair (BER) pathway, which is found in all forms of life. These ubiquitous enzymes promote genomic integrity by initiating repair of mutagenic and/or cytotoxic lesions that arise continuously due to alkylation, deamination, or oxidation of the normal bases in DNA. Glycosylases also perform essential roles in epigenetic regulation of gene expression, by targeting enzymatically-modified forms of the canonical DNA bases. Monofunctional DNA glycosylases hydrolyze the N-glycosidic bond to liberate the target base, while bifunctional glycosylases mediate glycosyl transfer using an amine group of the enzyme, generating a Schiff base intermediate that facilitates their second activity, cleavage of the DNA backbone. Here we review recent advances in understanding the chemical mechanism of monofunctional DNA glycosylases, with an emphasis on how the reactions are influenced by the properties of the nucleobase leaving-group, the moiety that varies across the vast range of substrates targeted by these enzymes.

A zinc linchpin motif in the MUTYH glycosylase interdomain connector is required for efficient repair of DNA damage

Mammalian MutY glycosylases have a unique architecture that features an interdomain connector (IDC) that joins the catalytic N-terminal domain and 8-oxoguanine (OG) recognition C-terminal domain. The IDC has been shown to be a hub for interactions with protein partners involved in coordinating downstream repair events and signaling apoptosis. Herein, a previously unidentified zinc ion and its coordination by three Cys residues of the IDC region of eukaryotic MutY organisms were characterized by mutagenesis, ICP-MS, and EXAFS. In vitro kinetics and cellular assays on WT and Cys to Ser mutants have revealed an important function for zinc coordination on overall protein stability, iron-sulfur cluster insertion, and ability to mediate DNA damage repair. We propose that this "zinc linchpin" motif serves to structurally organize the IDC and coordinate the damage recognition and base excision functions of the C- and N-terminal domains.

Specificity and catalytic mechanism in family 5 uracil DNA glycosylase

UDGb belongs to family 5 of the uracil DNA glycosylase (UDG) superfamily. Here, we report that family 5 UDGb from Thermus thermophilus HB8 is not only a uracil DNA glycosyase acting on G/U, T/U, C/U, and A/U base pairs, but also a hypoxanthine DNA glycosylase acting on G/I, T/I, and A/I base pairs and a xanthine DNA glycosylase acting on all double-stranded and single-stranded xanthine-containing DNA. Analysis of potentials of mean force indicates that the tendency of hypoxanthine base flipping follows the order of G/I > T/I, A/I > C/I, matching the trend of hypoxanthine DNA glycosylase activity observed in vitro. Genetic analysis indicates that family 5 UDGb can also act as an enzyme to remove uracil incorporated into DNA through the existence of dUTP in the nucleotide pool. Mutational analysis coupled with molecular modeling and molecular dynamics analysis reveals that although hydrogen bonding to O2 of uracil underlies the UDG activity in a dissociative fashion, Tth UDGb relies on multiple catalytic residues to facilitate its excision of hypoxanthine and xanthine. This study underscores the structural and functional diversity in the UDG superfamily.

Standard role for a conserved aspartate or more direct involvement in deglycosylation? An ONIOM and MD investigation of adenine-DNA glycosylase

8-Oxoguanine (OG) is one of the most frequently occurring forms of DNA damage and is particularly deleterious since it forms a stable Hoogsteen base pair with adenine (A). The repair of an OG:A mispair is initiated by adenine-DNA glycosylase (MutY), which hydrolyzes the sugar-nucleobase bond of the adenine residue before the lesion is processed by other proteins. MutY has been proposed to use a two-part chemical step involving protonation of the adenine nucleobase, followed by SN1 hydrolysis of the glycosidic bond. However, differences between a recent (fluorine recognition complex, denoted as the FLRC) crystal structure and the structure on which most mechanistic conclusions have been based to date (namely, the lesion recognition complex or LRC) raise questions regarding the mechanism used by MutY and the discrete role of various active-site residues. The present work uses both molecular dynamics (MD) and quantum mechanical (ONIOM) models to compare the active-site conformational dynamics in the two crystal structures, which suggests that only the understudied FLRC leads to a catalytically competent reactant. Indeed, all previous computational studies on MutY have been initiated from the LRC structure. Subsequently, for the first time, various mechanisms are examined with detailed ONIOM(M06-2X:PM6) reaction potential energy surfaces (PES) based on the FLRC structure, which significantly extends the mechanistic picture. Specifically, our work reveals that the reaction proceeds through a different route than the commonly accepted mechanism and the catalytic function of various active-site residues (Geobacillus stearothermophilus numbering). Specifically, contrary to proposals based on the LRC, E43 is determined to solely be involved in the initial adenine protonation step and not the deglycosylation reaction as the general base. Additionally, a novel catalytic role is proposed for Y126, whereby this residue plays a significant role in stabilizing the highly charged active site, primarily through interactions with E43. More importantly, D144 is found to explicitly catalyze the nucleobase dissociation step through partial nucleophilic attack. Although this is a more direct role than previously proposed for any other DNA glycosylase, comparison to previous work on other glycosylases justifies the larger contribution in the case of MutY and allows us to propose a unified role for the conserved Asp/Glu in the DNA glycosylases, as well as other enzymes that catalyze nucleotide deglycosylation reactions.

Depurination of N7-methylguanine by DNA glycosylase AlkD is dependent on the DNA backbone

DNA glycosylase AlkD excises N7-methylguanine (7mG) by a unique but unknown mechanism, in which the damaged nucleotide is positioned away from the protein and the phosphate backbone is distorted. Here, we show by methylphosphonate substitution that a phosphate proximal to the lesion has a significant effect on the rate enhancement of 7mG depurination by the enzyme. Thus, instead of a conventional mechanism whereby protein side chains participate in N-glycosidic bond cleavage, AlkD remodels the DNA into an active site composed exclusively of DNA functional groups that provide the necessary chemistry to catalyze depurination.

MUTYH DNA glycosylase: the rationale for removing undamaged bases from the DNA

Maintenance of genetic stability is crucial for all organisms in order to avoid the onset of deleterious diseases such as cancer. One of the many proveniences of DNA base damage in mammalian cells is oxidative stress, arising from a variety of endogenous and exogenous sources, generating highly mutagenic oxidative DNA lesions. One of the best characterized oxidative DNA lesion is 7,8-dihydro-8-oxoguanine (8-oxo-G), which can give rise to base substitution mutations (also known as point mutations). This mutagenicity is due to the miscoding potential of 8-oxo-G that instructs most DNA polymerases (pols) to preferentially insert an Adenine (A) opposite 8-oxo-G instead of the appropriate Cytosine (C). If left unrepaired, such A:8-oxo-G mispairs can give rise to CG→AT transversion mutations. A:8-oxo-G mispairs are proficiently recognized by the MutY glycosylase homologue (MUTYH). MUTYH can remove the mispaired A from an A:8-oxo-G, giving way to the canonical base-excision repair (BER) that ultimately restores undamaged Guanine (G). The importance of this MUTYH-initiated pathway is illustrated by the fact that biallelic mutations in the MUTYH gene are associated with a hereditary colorectal cancer syndrome termed MUTYH-associated polyposis (MAP). In this review, we will focus on MUTYH, from its discovery to the most recent data regarding its cellular roles and interaction partners. We discuss the involvement of the MUTYH protein in the A:8-oxo-G BER pathway acting together with pol λ, the pol that can faithfully incorporate C opposite 8-oxo-G and thus bypass this lesion in a correct manner. We also outline the current knowledge about the regulation of MUTYH itself and the A:8-oxo-G repair pathway by posttranslational modifications (PTM). Finally, to achieve a clearer overview of the literature, we will briefly touch on the rather confusing MUTYH nomenclature. In short, MUTYH is a unique DNA glycosylase that catalyzes the excision of an undamaged base from DNA.

Glycosidic bond cleavage in deoxynucleotides: effects of solvent and the DNA phosphate backbone in the computational model

Density functional theory (B3LYP) was employed to examine the hydrolysis of the canonical 2'-deoxynucleotides in varied environments (gas phase or water) using different computational models for the sugar residue (methyl or phosphate group at C5') and nucleophile (water activated through full or partial proton abstraction). Regardless of the degree of nucleophile activation, our results show that key geometrical parameters along the reaction pathway are notably altered upon direct inclusion of solvent effects in the optimization routine, which leads to significant changes in the reaction energetics and better agreement with experiment. Therefore, despite the wide use of gas-phase calculations in the literature, small model computational work, as well as large-scale enzyme models, that strive to understand nucleotide deglycosylation must adequately describe the environment. Alternatively, although inclusion of the phosphate group at C5' also affects the geometries of important stationary points, the effects cancel to yield unchanged deglycosylation barriers, and therefore smaller computational models can be used to estimate the energy associated with nucleotide deglycosylation, with the 5' phosphate group included if full (geometric) details of the reaction are desired. Hydrogen-bonding interactions with the nucleobase can significantly reduce the barrier to deglycosylation, which supports suggestions that discrete hydrogen-bonding interactions with active-site amino acid residues can play a significant role in enzyme-catalyzed nucleobase excision. Taken together with previous studies, the present work provides vital clues about the components that must be included in future studies of the deglycosylation of isolated noncanonical nucleotides, as well as the corresponding enzyme-catalyzed reactions.

Recent advances in the structural mechanisms of DNA glycosylases

DNA glycosylases safeguard the genome by locating and excising a diverse array of aberrant nucleobases created from oxidation, alkylation, and deamination of DNA. Since the discovery 28years ago that these enzymes employ a base flipping mechanism to trap their substrates, six different protein architectures have been identified to perform the same basic task. Work over the past several years has unraveled details for how the various DNA glycosylases survey DNA, detect damage within the duplex, select for the correct modification, and catalyze base excision. Here, we provide a broad overview of these latest advances in glycosylase mechanisms gleaned from structural enzymology, highlighting features common to all glycosylases as well as key differences that define their particular substrate specificities.

Alkylation sensitivity screens reveal a conserved cross-species functionome

To identify genes that contribute to chemotherapy resistance in glioblastoma, we conducted a synthetic lethal screen in a chemotherapy-resistant glioblastoma-derived cell line with the clinical alkylator temozolomide (TMZ) and an siRNA library tailored toward "druggable" targets. Select DNA repair genes in the screen were validated independently, confirming the DNA glycosylases uracil-DNA glycosylase (UNG) and A/G-specific adenine DNA glycosylase (MYH) as well as methylpurine-DNA glycosylase (MPG) to be involved in the response to high dose TMZ. The involvement of UNG and MYH is likely the result of a TMZ-induced burst of reactive oxygen species. We then compared the human TMZ sensitizing genes identified in our screen with those previously identified from alkylator screens conducted in Escherichia coli and Saccharomyces cerevisiae. The conserved biologic processes across all three species compose an alkylation functionome that includes many novel proteins not previously thought to impact alkylator resistance. This high-throughput screen, validation and cross-species analysis was then followed by a mechanistic analysis of two essential nodes: base excision repair (BER) DNA glycosylases (UNG, human and mag1, S. cerevisiae) and protein modification systems, including UBE3B and ICMT in human cells or pby1, lip22, stp22 and aim22 in S. cerevisiae. The conserved processes of BER and protein modification were dual targeted and yielded additive sensitization to alkylators in S. cerevisiae. In contrast, dual targeting of BER and protein modification genes in human cells did not increase sensitivity, suggesting an epistatic relationship. Importantly, these studies provide potential new targets to overcome alkylating agent resistance.

Mechanistic and conformational flexibility of the covalent linkage formed during β-lyase activity on an AP-site: application to hOgg1

The β/δ-lyase activity of bifunctional glycosylases on damaged nucleotides in DNA involves the formation of a covalent linkage between the protein (lysine or N-terminal proline) and DNA (C1' of the damaged nucleotide). In the present study, the conformational and mechanistic flexibility of the cross-link is examined. Repair of 8-oxoguanine damage by hOgg1 is considered as a representative system, and the glycosylase through β-lyase steps are investigated using density functional theory. (PCM/SMD)-M06-2X/6-311+G(2df,2p)//PCM-B3LYP/6-31G(d) energetics were determined for eight unique mechanisms differing in the conformation of the imine linkage (E/Z), the proton (pro-S/R) abstracted during elimination, and whether the ring-opening step is base catalyzed. This initial study used a model system limited to the damaged nucleoside 3'-monophosphate and a model nucleophile to investigate this series of complex reaction steps. The great flexibility exhibited by the linkage and clustered β-elimination energetics indicate sterics will play a large role in predicting the preferred lyase mechanism for a given enzyme. The stationary points identified herein can be overlaid into a protein structure to assist in generating initial guesses for large model systems. By comparing the characterized geometries and enzyme active sites, methods for catalysis of the various chemical steps can be identified, and these possibilities are discussed in detail for hOgg1. Interestingly, the most stable structure on the potential energy surface occurs before elimination of the 3'-phosphate. Hydrolysis of the protein-DNA cross-link at this point would yield an AP-site, which provides support for the recently observed monofunctional activity of hOgg1.

For MutY, it's all about the OG

MutY and its human ortholog, MUTYH, repair a specific form of DNA damage: adenine mis-paired with the oxidatively modified form of deoxyguanosine, 8-oxo-7,8-dihydro-2'-deoxyguanosine. In a recent issue of Chemistry & Biology, Brinkmeyer et al. utilized mutant forms of MutY to reveal the critical residues in MutY that are required for this selectivity and specificity.

Excision of thymine and 5-hydroxymethyluracil by the MBD4 DNA glycosylase domain: structural basis and implications for active DNA demethylation

The mammalian DNA glycosylase—methyl-CpG binding domain protein 4 (MBD4)—is involved in active DNA demethylation via the base excision repair pathway. MBD4 contains an N-terminal MBD and a C-terminal DNA glycosylase domain. MBD4 can excise the mismatched base paired with a guanine (G:X), where X is uracil, thymine or 5-hydroxymethyluracil (5hmU). These are, respectively, the deamination products of cytosine, 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC). Here, we present three structures of the MBD4 C-terminal glycosylase domain (wild-type and its catalytic mutant D534N), in complex with DNA containing a G:T or G:5hmU mismatch. MBD4 flips the target nucleotide from the double-stranded DNA. The catalytic mutant D534N captures the intact target nucleotide in the active site binding pocket. MBD4 specifically recognizes the Watson–Crick polar edge of thymine or 5hmU via the O₂, N₃ and O₄ atoms, thus restricting its activity to thymine/uracil-based modifications while excluding cytosine and its derivatives. The wild-type enzyme cleaves the N-glycosidic bond, leaving the ribose ring in the flipped state, while the cleaved base is released. Unexpectedly, the C₁′ of the sugar has yet to be hydrolyzed and appears to form a stable intermediate with one of the side chain carboxyl oxygen atoms of D534, via either electrostatic or covalent interaction, suggesting a different catalytic mechanism from those of other DNA glycosylases.