Mbd4 inactivation increases Cright-arrowT transition mutations and promotes gastrointestinal tumor formation

The mutagenic consequences of defective DNA repair

Multiple separate repair mechanisms safeguard the genome against various types of DNA damage, and their failure can increase the rate of spontaneous mutagenesis. The malfunction of distinct repair mechanisms leads to genomic instability through different mutagenic processes. For example, defective mismatch repair causes high base substitution rates and microsatellite instability, whereas homologous recombination deficiency is characteristically associated with deletions and chromosome instability. This review presents a comprehensive collection of all mutagenic phenotypes associated with the loss of each DNA repair mechanism, drawing on data from a variety of model organisms and mutagenesis assays, and placing greatest emphasis on systematic analyses of human cancer datasets. We describe the latest theories on the mechanism of each mutagenic process, often explained by reliance on an alternative repair pathway or the error-prone replication of unrepaired, damaged DNA. Aided by the concept of mutational signatures, the genomic phenotypes can be used in cancer diagnosis to identify defective DNA repair pathways.

A naturally occurring variant of MBD4 causes maternal germline hypermutation in primates

As part of an ongoing genome sequencing project at the Oregon National Primate Research Center, we identified a rhesus macaque with a rare homozygous frameshift mutation in the gene methyl-CpG binding domain 4, DNA glycosylase (MBD4). MBD4 is responsible for the repair of C > T deamination mutations at CpG dinucleotides and has been linked to somatic hypermutation and cancer predisposition in humans. We show here that MBD4-associated hypermutation also affects the germline: The six offspring of the MBD4-null dam have a fourfold to sixfold increase in de novo mutation burden. This excess burden was predominantly C > T mutations at CpG dinucleotides consistent with MBD4 loss of function in the dam. There was also a significant excess of C > T at CpA sites, indicating an important, unappreciated role for MBD4 to repair deamination in CpA contexts. The MBD4-null dam developed sustained eosinophilia later in life, but we saw no other signs of neoplastic processes associated with MBD4 loss of function in humans nor any obvious disease in the hypermutated offspring. This work provides the first evidence for a genetic factor causing hypermutation in the maternal germline of a mammal and adds to the very small list of naturally occurring variants known to modulate germline mutation rates in mammals.

Methyl-CpG binding domain (MBD)2/3 specifically recognizes and binds to the genomic mCpG site with a β-sheet in the MBD to affect embryonic development in Bombyx mori

Methyl-CpG (mCpG) binding domain (MBD) proteins especially bind with methylated DNA, and are involved in many important biological processes; however, the binding mechanism between insect MBD2/3 and mCpG remains unclear. In this study, we identified 2 isoforms of the MBD2/3 gene in Bombyx mori, MBD2/3-S and MBD2/3-L. Binding analysis of MBD2/3-L, MBD2/3-S, and 7 mutant MBD2/3-L proteins deficient in β1-β6 or α1 in the MBD showed that β2-β3-turns in the β-sheet of the MBD are necessary for the formation of the MBD2/3-mCpG complex; furthermore, other secondary structures, namely, β4-β6 and an α-helix, play a role in stabilizing the β-sheet structure to ensure that the MBD is able to bind mCpG. In addition, sequence alignment and binding analyses of different insect MBD2/3s indicated that insect MBD2/3s have an intact and conserved MBD that binds to the mCpG of target genes. Furthermore, MBD2/3 RNA interference results showed that MBD2/3-L plays a role in regulating B. mori embryonic development, similar to that of DNA methylation; however, MBD2/3-S without β4-β6 and α-helix does not alter embryonic development. These results suggest that MBD2/3-L recognizes and binds to mCpG through the intact β-sheet structure in its MBD, thus ensuring silkworm embryonic development.

MBD4 loss results in global reactivation of promoters and retroelements with low methylated CpG density

BACKGROUND

Inherited defects in the base-excision repair gene MBD4 predispose individuals to adenomatous polyposis and colorectal cancer, which is characterized by an accumulation of C > T transitions resulting from spontaneous deamination of 5'-methylcytosine.

METHODS

Here, we have investigated the potential role of MBD4 in regulating DNA methylation levels using genome-wide transcriptome and methylome analyses. Additionally, we have elucidated its function through a series of in vitro experiments.

RESULTS

Here we show that the protein MBD4 is required for DNA methylation maintenance and G/T mismatch repair. Transcriptome and methylome analyses reveal a genome-wide hypomethylation of promoters, gene bodies and repetitive elements in the absence of MBD4 in vivo. Methylation mark loss is accompanied by a broad transcriptional derepression phenotype affecting promoters and retroelements with low methylated CpG density. MBD4 in vivo forms a complex with the mismatch repair proteins (MMR), which exhibits high bi-functional glycosylase/AP-lyase endonuclease specific activity towards methylated DNA substrates containing a G/T mismatch. Experiments using recombinant proteins reveal that the association of MBD4 with the MMR protein MLH1 is required for this activity.

CONCLUSIONS

Our data identify MBD4 as an enzyme specifically designed to repair deaminated 5-methylcytosines and underscores its critical role in safeguarding against methylation damage. Furthermore, it illustrates how MBD4 functions in normal and pathological conditions.

The Arabidopsis DNA glycosylase MBD4L repairs the nuclear genome in vivo

DNA glycosylases remove mispaired or modified bases from DNA initiating the base excision repair (BER) pathway. The DNA glycosylase MBD4 (methyl-CpG-binding domain protein 4) has been functionally characterized in mammals, but not yet in plants, where it is called MBD4-like (MBD4L). Mammalian MBD4 and Arabidopsis recombinant MBD4L excise U and T mispaired with G, as well as 5-fluorouracil (5-FU) and 5-bromouracil (5-BrU) in vitro. Here, we investigate the ability of Arabidopsis MBD4L to remove some of these substrates from the nuclear genome in vivo in coordination with uracil DNA glycosylase (AtUNG). We found that mbd4l mutants are hypersensitive to 5-FU and 5-BrU, as they displayed smaller size, less root growth, and higher cell death than control plants in both media. Using comet assays, we determined BER-associated DNA fragmentation in isolated nuclei and observed reduced DNA breaks in mbd4l plants under both conditions, but particularly with 5-BrU. The use of ung and ung x mbd4l mutants in these assays indicated that both MBD4L and AtUNG trigger nuclear DNA fragmentation in response to 5-FU. Consistently, we here report the nuclear localization of AtUNG based on the expression of AtUNG-GFP/RFP constructs in transgenic plants. Interestingly, MBD4L and AtUNG are transcriptionally coordinated but display not completely overlapping functions. MBD4L-deficient plants showed reduced expression of BER genes and enhanced expression of DNA damage response (DDR) gene markers. Overall, our findings indicate that Arabidopsis MBD4L is critical for maintaining nuclear genome integrity and preventing cell death under genotoxic stress conditions.

From Chaos to Opportunity: Decoding Cancer Heterogeneity for Enhanced Treatment Strategies

Cancer manifests as a multifaceted disease, characterized by aberrant cellular proliferation, survival, migration, and invasion. Tumors exhibit variances across diverse dimensions, encompassing genetic, epigenetic, and transcriptional realms. This heterogeneity poses significant challenges in prognosis and treatment, affording tumors advantages through an increased propensity to accumulate mutations linked to immune system evasion and drug resistance. In this review, we offer insights into tumor heterogeneity as a crucial characteristic of cancer, exploring the difficulties associated with measuring and quantifying such heterogeneity from clinical and biological perspectives. By emphasizing the critical nature of understanding tumor heterogeneity, this work contributes to raising awareness about the importance of developing effective cancer therapies that target this distinct and elusive trait of cancer.

Targeted DNA Demethylation: Vectors, Effectors and Perspectives

Aberrant DNA hypermethylation at regulatory cis-elements of particular genes is seen in a plethora of pathological conditions including cardiovascular, neurological, immunological, gastrointestinal and renal diseases, as well as in cancer, diabetes and others. Thus, approaches for experimental and therapeutic DNA demethylation have a great potential to demonstrate mechanistic importance, and even causality of epigenetic alterations, and may open novel avenues to epigenetic cures. However, existing methods based on DNA methyltransferase inhibitors that elicit genome-wide demethylation are not suitable for treatment of diseases with specific epimutations and provide a limited experimental value. Therefore, gene-specific epigenetic editing is a critical approach for epigenetic re-activation of silenced genes. Site-specific demethylation can be achieved by utilizing sequence-dependent DNA-binding molecules such as zinc finger protein array (ZFA), transcription activator-like effector (TALE) and clustered regularly interspaced short palindromic repeat-associated dead Cas9 (CRISPR/dCas9). Synthetic proteins, where these DNA-binding domains are fused with the DNA demethylases such as ten-eleven translocation (Tet) and thymine DNA glycosylase (TDG) enzymes, successfully induced or enhanced transcriptional responsiveness at targeted loci. However, a number of challenges, including the dependence on transgenesis for delivery of the fusion constructs, remain issues to be solved. In this review, we detail current and potential approaches to gene-specific DNA demethylation as a novel epigenetic editing-based therapeutic strategy.

Genetic and chemotherapeutic influences on germline hypermutation

Mutations in the germline generates all evolutionary genetic variation and is a cause of genetic disease. Parental age is the primary determinant of the number of new germline mutations in an individual's genome1,2. Here we analysed the genome-wide sequences of 21,879 families with rare genetic diseases and identified 12 individuals with a hypermutated genome with between two and seven times more de novo single-nucleotide variants than expected. In most families (9 out of 12), the excess mutations came from the father. Two families had genetic drivers of germline hypermutation, with fathers carrying damaging genetic variation in DNA-repair genes. For five of the families, paternal exposure to chemotherapeutic agents before conception was probably a key driver of hypermutation. Our results suggest that the germline is well protected from mutagenic effects, hypermutation is rare, the number of excess mutations is relatively modest and most individuals with a hypermutated genome will not have a genetic disease.

Heterozygous loss of Zbtb38 leads to early embryonic lethality via the suppression of Nanog and Sox2 expression

Objectives

Mammalian DNA methyltransferases are essential to re‐establish global DNA methylation patterns during implantation, which is critical for transmitting epigenetic information to the next generation. In contrast, the significance of methyl‐CpG binding proteins (MBPs) that bind methylated CpG remains almost unknown at this stage. We previously demonstrated that Zbtb38 (also known as CIBZ)—a zinc finger type of MBP—is required for mouse embryonic stem (ES) cell proliferation by positively regulating Nanog expression. However, the physiological function of Zbtb38 in vivo remains unclear.

Materials and Methods

This study used the Cre‐loxP system to generate conditional Zbtb38 knockout mice. Cell proliferation and apoptosis were studied by immunofluorescence staining. Quantitative real‐time PCR, immunoblotting and immunofluorescence were performed to investigate the molecular mechanisms.

Results

Germline loss of the Zbtb38 single allele resulted in decreased epiblast cell proliferation and increased apoptosis shortly after implantation, leading to early embryonic lethality. Heterozygous loss of Zbtb38 reduced the expression of Nanog, Sox2, and the genes responsible for epiblast proliferation, differentiation, and cell viability. Although this early lethal phenotype, Zbtb38 is dispensable for ES cell establishment and identity.

Conclusions

These findings indicate that Zbtb38 is essential for early embryonic development via the suppression of Nanog and Sox2 expression.

Role of mismatch repair in aging

A common feature of aging is the accumulation of genetic damage throughout life. DNA damage can lead to genomic instability. Many diseases associated with premature aging are a result of increased accumulation of DNA damage. In order to minimize these damages, organisms have evolved a complex network of DNA repair mechanisms, including mismatch repair (MMR). In this review, we detail the effects of MMR on genomic instability and its role in aging emphasizing on the association between MMR and the other hallmarks of aging, serving to drive or amplify these mechanisms. These hallmarks include telomere attrition, epigenetic alterations, mitochondrial dysfunction, altered nutrient sensing and cell senescence. The close relationship between MMR and these markers may provide prevention and treatment strategies, to reduce the incidence of age-related diseases and promote the healthy aging of human beings.

The complex relationship between MITF and the immune system: a Melanoma ImmunoTherapy (response) Factor?

The clinical benefit of immune checkpoint inhibitory therapy (ICT) in advanced melanomas is limited by primary and acquired resistance. The molecular determinants of the resistance have been extensively studied, but these discoveries have not yet been translated into therapeutic benefits. As such, a paradigm shift in melanoma treatment, to surmount the therapeutic impasses linked to the resistance, is an important ongoing challenge.This review outlines the multifaceted interplay between microphthalmia-associated transcription factor (MITF), a major determinant of the biology of melanoma cells, and the immune system. In melanomas, MITF functions downstream oncogenic pathways and microenvironment stimuli that restrain the immune responses. We highlight how MITF, by controlling differentiation and genome integrity, may regulate melanoma-specific antigen expression by interfering with the endolysosomal pathway, KARS1, and antigen processing and presentation. MITF also modulates the expression of coinhibitory receptors, i.e., PD-L1 and HVEM, and the production of an inflammatory secretome, which directly affects the infiltration and/or activation of the immune cells.Furthermore, MITF is also a key determinant of melanoma cell plasticity and tumor heterogeneity, which are undoubtedly one of the major hurdles for an effective immunotherapy. Finally, we briefly discuss the role of MITF in kidney cancer, where it also plays a key role, and in immune cells, establishing MITF as a central mediator in the regulation of immune responses in melanoma and other cancers.We propose that a better understanding of MITF and immune system intersections could help in the tailoring of current ICT in melanomas and pave the way for clinical benefits and long-lasting responses.

Active DNA demethylation-The epigenetic gatekeeper of development, immunity, and cancer

DNA methylation is a critical process in the regulation of gene expression with dramatic effects in development and continually expanding roles in oncogenesis. 5-Methylcytosine was once considered to be an inherited and stably repressive epigenetic mark, which can be only removed by passive dilution during multiple rounds of DNA replication. However, in the past two decades, physiologically controlled DNA demethylation and deamination processes have been identified, thereby revealing the function of cytosine methylation as a highly regulated and complex state-not simply a static, inherited signature or binary on-off switch. Alongside these fundamental discoveries, clinical studies over the past decade have revealed the dramatic consequences of aberrant DNA demethylation. In this review we discuss DNA demethylation and deamination in the context of 5-methylcytosine as critical processes for physiological and physiopathological transitions within three states-development, immune maturation, and oncogenic transformation; and we describe the expanding role of DNA demethylating drugs as therapeutic agents in cancer.

Multi-Omics Analysis to Characterize Cigarette Smoke Induced Molecular Alterations in Esophageal Cells

Though smoking remains one of the established risk factors of esophageal squamous cell carcinoma, there is limited data on molecular alterations associated with cigarette smoke exposure in esophageal cells. To investigate molecular alterations associated with chronic exposure to cigarette smoke, non-neoplastic human esophageal epithelial cells were treated with cigarette smoke condensate (CSC) for up to 8 months. Chronic treatment with CSC increased cell proliferation and invasive ability of non-neoplastic esophageal cells. Whole exome sequence analysis of CSC treated cells revealed several mutations and copy number variations. This included loss of high mobility group nucleosomal binding domain 2 (HMGN2) and a missense variant in mediator complex subunit 1 (MED1). Both these genes play an important role in DNA repair. Global proteomic and phosphoproteomic profiling of CSC treated cells lead to the identification of 38 differentially expressed and 171 differentially phosphorylated proteins. Bioinformatics analysis of differentially expressed proteins and phosphoproteins revealed that most of these proteins are associated with DNA damage response pathway. Proteomics data revealed decreased expression of HMGN2 and hypophosphorylation of MED1. Exogenous expression of HMGN2 and MED1 lead to decreased proliferative and invasive ability of smoke exposed cells. Immunohistochemical labeling of HMGN2 in primary ESCC tumor tissue sections (from smokers) showed no detectable expression while strong to moderate staining of HMGN2 was observed in normal esophageal tissues. Our data suggests that cigarette smoke perturbs expression of proteins associated with DNA damage response pathways which might play a vital role in development of ESCC.

Base Excision Repair in the Immune System: Small DNA Lesions With Big Consequences

The integrity of the genome is under constant threat of environmental and endogenous agents that cause DNA damage. Endogenous damage is particularly pervasive, occurring at an estimated rate of 10,000–30,000 per cell/per day, and mostly involves chemical DNA base lesions caused by oxidation, depurination, alkylation, and deamination. The base excision repair (BER) pathway is primary responsible for removing and repairing these small base lesions that would otherwise lead to mutations or DNA breaks during replication. Next to preventing DNA mutations and damage, the BER pathway is also involved in mutagenic processes in B cells during immunoglobulin (Ig) class switch recombination (CSR) and somatic hypermutation (SHM), which are instigated by uracil (U) lesions derived from activation-induced cytidine deaminase (AID) activity. BER is required for the processing of AID-induced lesions into DNA double strand breaks (DSB) that are required for CSR, and is of pivotal importance for determining the mutagenic outcome of uracil lesions during SHM. Although uracils are generally efficiently repaired by error-free BER, this process is surprisingly error-prone at the Ig loci in proliferating B cells. Breakdown of this high-fidelity process outside of the Ig loci has been linked to mutations observed in B-cell tumors and DNA breaks and chromosomal translocations in activated B cells. Next to its role in preventing cancer, BER has also been implicated in immune tolerance. Several defects in BER components have been associated with autoimmune diseases, and animal models have shown that BER defects can cause autoimmunity in a B-cell intrinsic and extrinsic fashion. In this review we discuss the contribution of BER to genomic integrity in the context of immune receptor diversification, cancer and autoimmune diseases.

Genetic variants of the peroxisome proliferator-activated receptor (PPAR) signaling pathway genes and risk of pancreatic cancer

Because the peroxisome proliferator-activated receptor (PPAR) signaling pathway is involved in development and progression of pancreatic cancer, we investigated associations between genetic variants of the PPAR pathway genes and pancreatic cancer risk by using three published genome-wide association study datasets including 8477 cases and 6946 controls of European ancestry. Expression quantitative trait loci (eQTL) analysis was also performed for correlations between genotypes of the identified genetic variants and messenger RNA (mRNA) expression levels of their genes by using available databases of the 1000 Genomes, TCGA, and GTEx projects. In the single-locus logistic regression analysis, we identified 1141 out of 17 532 significant single-nucleotide polymorphisms (SNPs) in 112 PPAR pathway genes. Further multivariate logistic regression analysis identified three independent, potentially functional loci (rs12947620 in MED1, rs11079651 in PRKCA, and rs34367566 in PRKCB) for pancreatic cancer risk (odds ratio [OR] = 1.11, 95% confidence interval [CI], [1.06-1.17], P = 5.46 × 10^-5 ; OR = 1.10, 95% CI, [1.04-1.15], P = 1.99 × 10^-4 ; and OR = 1.09, 95% CI, [1.04-1.14], P = 3.16 × 10^-4 , respectively) among 65 SNPs that passed multiple comparison correction by false discovery rate (< 0.2). When risk genotypes of these three SNPs were combined, carriers with 2 to 3 unfavorable genotypes (NUGs) had a higher risk of pancreatic cancer than those with 0 to 1 NUGs. The eQTL analysis showed that rs34367566 A>AG was associated with decreased expression levels of PRKCB mRNA in 373 lymphoblastoid cell lines. Our findings indicate that genetic variants of the PPAR pathway genes, particularly MED1, PRKCA, and PRKCB, may contribute to susceptibility to pancreatic cancer.

The Molecular Basis of MeCP2 Function in the Brain

MeCP2 is a reader of the DNA methylome that occupies a large proportion of the genome due to its high abundance and the frequency of its target sites. It has been the subject of extensive study because of its link with 'MECP2-related disorders', of which Rett syndrome is the most prevalent. This review integrates evidence from patient mutation data with results of experimental studies using mouse models, cell lines and in vitro systems to critically evaluate our understanding of MeCP2 protein function. Recent evidence challenges the idea that MeCP2 is a multifunctional hub that integrates diverse processes to underpin neuronal function, suggesting instead that its primary role is to recruit the NCoR1/2 co-repressor complex to methylated sites in the genome, leading to dampening of gene expression.

Base Excision DNA Repair Deficient Cells: From Disease Models to Genotoxicity Sensors

Base excision DNA repair (BER) is a vitally important pathway that protects the cell genome from many kinds of DNA damage, including oxidation, deamination, and hydrolysis. It involves several tightly coordinated steps, starting from damaged base excision and followed by nicking one DNA strand, incorporating an undamaged nucleotide, and DNA ligation. Deficiencies in BER are often embryonic lethal or cause morbid diseases such as cancer, neurodegeneration, or severe immune pathologies. Starting from the early 1980s, when the first mammalian cell lines lacking BER were produced by spontaneous mutagenesis, such lines have become a treasure trove of valuable information about the mechanisms of BER, often revealing unexpected connections with other cellular processes, such as antibody maturation or epigenetic demethylation. In addition, these cell lines have found an increasing use in genotoxicity testing, where they provide increased sensitivity and representativity to cell-based assay panels. In this review, we outline current knowledge about BER-deficient cell lines and their use.

Additional functions of selected proteins involved in DNA repair

Protein moonlighting is a phenomenon in which a single polypeptide chain can perform a number of different unrelated functions. Here we present our analysis of moonlighting in the case of selected DNA repair proteins which include G:T mismatch-specific thymine DNA glycosylase (TDG), methyl-CpG-binding domain protein 4 (MBD4), apurinic/apyrimidinic endonuclease 1 (APE1), AlkB homologs, poly (ADP-ribose) polymerase 1 (PARP-1) and single-strand selective monofunctional uracil DNA glycosylase 1 (SMUG1). Most of their additional functions are not accidental and clear patterns are emerging. Participation in RNA metabolism is not surprising as bases occurring in RNA are the same or very similar to those in DNA. Other common additional function involves regulation of transcription. This is not unexpected as these proteins bind to specific DNA regions for DNA repair, hence they can also be recruited to regulate transcription. Participation in demethylation and replication of DNA appears logical as well. Some of the multifunctional DNA repair proteins play major roles in many diseases, including cancer. However, their moonlighting might prove a major difficulty in the development of new therapies because it will not be trivial to target a single protein function without affecting its other functions that are not related to the disease.

The genomics of undifferentiated sarcoma of soft tissue: Progress, challenges and opportunities

Undifferentiated sarcoma of soft tissue (USTS) are aggressive sarcomas that remain a diagnosis of exclusion and show extreme genomic complexity. Many advances in diagnostic criteria have resulted in several revisions in the definition of this rare cancer type. Recent sequencing efforts have illuminated the nature of the genome complexity and have revealed extensive copy number heterogeneity and multiple evolutionary patterns of development. This review places these recent advances into their historical and translational context and covers the changes in nomenclature, molecular classification, and the promise of personalised medicine.

Investigating Tumor Heterogeneity in Mouse Models

Cancer arises from a single cell through a series of acquired mutations and epigenetic alterations. Tumors gradually develop into a complex tissue comprised of phenotypically heterogeneous cancer cell populations, as well as noncancer cells that make up the tumor microenvironment. The phenotype, or state, of each cancer and stromal cell is influenced by a plethora of cell-intrinsic and cell-extrinsic factors. The diversity of these cellular states promotes tumor progression, enables metastasis, and poses a challenge for effective cancer treatments. Thus, the identification of strategies for the therapeutic manipulation of tumor heterogeneity would have significant clinical implications. A major barrier in the field is the difficulty in functionally investigating heterogeneity in tumors in cancer patients. Here we review how mouse models of human cancer can be leveraged to interrogate tumor heterogeneity and to help design better therapeutic strategies.

Modification of the base excision repair enzyme MBD4 by the small ubiquitin-like molecule SUMO1

The base excision repair DNA N-glycosylase MBD4 (also known as MED1), an interactor of the DNA mismatch repair protein MLH1, plays a central role in the maintenance of genomic stability of CpG sites by removing thymine and uracil from G:T and G:U mismatches, respectively. MBD4 is also involved in DNA damage response and transcriptional regulation. The interaction with other proteins is likely critical for understanding MBD4 functions. To identify novel proteins that interact with MBD4, we used tandem affinity purification (TAP) from HEK-293 cells. The MBD4-TAP fusion and its co-associated proteins were purified sequentially on IgG and calmodulin affinity columns; the final eluate was shown to contain MLH1 by western blotting, and MBD4-associated proteins were identified by mass spectrometry. Bands with molecular weight higher than that expected for MBD4 (˜66 kD) yielded peptides corresponding to MBD4 itself and the small ubiquitin-like molecule-1 (SUMO1), suggesting that MBD4 is sumoylated in vivo. MBD4 sumoylation was validated by co-immunoprecipitation in HEK-293 and MCF7 cells, and by an in vitrosumoylation assay. Sequence and mutation analysis identified three main sumoylation sites: MBD4 is sumoylated preferentially on K137, with additional sumoylation at K215 and K377. Patterns of MBD4 sumoylation were altered, in a DNA damage-specific way, by the anti-metabolite 5-fluorouracil, the alkylating agent N-Methyl-N-nitrosourea and the crosslinking agent cisplatin. MCF7 extract expressing sumoylated MBD4 displays higher thymine glycosylase activity than the unmodified species. Of the 67 MBD4 missense mutations reported in The Cancer Genome Atlas, 14 (20.9%) map near sumoylation sites. These results indicate that MBD4 is sumoylated in vivo in a DNA damage-specific manner, and suggest that sumoylation serves to regulate its repair activity and could be compromised in cancer. This study expands the role played by sumoylation in fine-tuning DNA damage response and repair.

DNA Methylation Readers and Cancer: Mechanistic and Therapeutic Applications

DNA methylation is a major epigenetic process that regulates chromatin structure which causes transcriptional activation or repression of genes in a context-dependent manner. In general, DNA methylation takes place when methyl groups are added to the appropriate bases on the genome by the action of "writer" molecules known as DNA methyltransferases. How these methylation marks are read and interpreted into different functionalities represents one of the main mechanisms through which the genes are switched "ON" or "OFF" and typically involves different types of "reader" proteins that can recognize and bind to the methylated regions. A tightly balanced regulation exists between the "writers" and "readers" in order to mediate normal cellular functions. However, alterations in normal methylation pattern is a typical hallmark of cancer which alters the way methylation marks are written, read and interpreted in different disease states. This unique characteristic of DNA methylation "readers" has identified them as attractive therapeutic targets. In this review, we describe the current state of knowledge on the different classes of DNA methylation "readers" identified thus far along with their normal biological functions, describe how they are dysregulated in cancer, and discuss the various anti-cancer therapies that are currently being developed and evaluated for targeting these proteins.

Defining the Role of Nucleotide Flipping in Enzyme Specificity Using 19F NMR

A broad range of proteins employ nucleotide flipping to recognize specific sites in nucleic acids, including DNA glycosylases, which remove modified nucleobases to initiate base excision repair. Deamination, a pervasive mode of damage, typically generates lesions that are recognized by glycosylases as being foreign to DNA. However, deamination of 5-methylcytosine (mC) generates thymine, a canonical DNA base, presenting a challenge for damage recognition. Nevertheless, repair of mC deamination is important because the resulting G·T mispairs cause C → T transition mutations, and mC is abundant in all three domains of life. Countering this threat are three types of glycosylases that excise thymine from G·T mispairs, including thymine DNA glycosylase (TDG). These enzymes must minimize excision of thymine that is not generated by mC deamination, in A·T pairs and in polymerase-generated G·T mispairs. TDG preferentially removes thymine from DNA contexts in which cytosine methylation is prevalent, including CG and one non-CG site. This remarkable context specificity could be attained through modulation of nucleotide flipping, a reversible step that precedes base excision. We tested this idea using fluorine NMR and DNA containing 2'-fluoro-substituted nucleotides. We find that dT nucleotide flipping depends on DNA context and is efficient only in contexts known to feature cytosine methylation. We also show that a conserved Ala residue limits thymine excision by hindering nucleotide flipping. A linear free energy correlation reveals that TDG attains context specificity for thymine excision through modulation of nucleotide flipping. Our results provide a framework for characterizing nucleotide flipping in nucleic acids using ¹⁹F NMR.

Genome-wide differences in DNA methylation changes in caprine ovaries between oestrous and dioestrous phases

Background

DNA methylation plays a vital role in reproduction. Entire genome DNA methylation changes during the oestrous phase (ES) and dioestrous phase (DS) in the ovaries of Guanzhong dairy goats were investigated using bisulphite sequencing to understand the molecular biological mechanisms of these goats’ oestrous cycle.

Results

We discovered distinct genome-wide DNA methylation patterns in ES and DS ovaries. A total of 26,910 differentially methylated regions were upregulated and 21,453 differentially methylated regions were downregulated in the ES samples compared with the DS samples (P-values ≤0.05 and fold change of methylation ratios ≥2). Differentially methylated region analysis showed hypomethylation in the gene body regions and hypermethylation in the joining region between upstream regions and gene bodies. The methylation ratios of the STAR, FGF2, FGF12, BMP5 and SMAD6 genes in the ES samples were lower than those of the DS samples (P-values ≤0.05 and fold change of methylation ratios ≥2). Conversely, the methylation ratios of the EGFR, TGFBR2, IGF2BP1 and MMD2 genes increased in the ES samples compared with the DS samples. In addition, 223 differentially methylated genes were found in the GnRH signalling pathway (KO04912), ovarian steroidogenesis pathway (KO04913), oestrogen signalling pathway (KO04915), oxytocin signalling pathway (KO04921), insulin secretion pathway (KO04911) and MAPK signalling pathway (KO04010).

Conclusions

This study is the first large-scale comparison of the high-resolution DNA methylation landscapes of oestrous and dioestrous ovaries from dairy goats. Previous studies and our investigations have shown that the NR5A2, STAR, FGF2 and BMP5 genes might have potential application value in regulating caprine oestrus.

Electronic supplementary material

The online version of this article (10.1186/s40104-018-0301-x) contains supplementary material, which is available to authorized users.

MBD4 guards against methylation damage and germ line deficiency predisposes to clonal hematopoiesis and early-onset AML

The tendency of 5-methylcytosine (5mC) to undergo spontaneous deamination has had a major role in shaping the human genome, and this methylation damage remains the primary source of somatic mutations that accumulate with age. How 5mC deamination contributes to cancer risk in different tissues remains unclear. Genomic profiling of 3 early-onset acute myeloid leukemias (AMLs) identified germ line loss of MBD4 as an initiator of 5mC-dependent hypermutation. MBD4-deficient AMLs display a 33-fold higher mutation burden than AML generally, with >95% being C>T in the context of a CG dinucleotide. This distinctive signature was also observed in sporadic cancers that acquired biallelic mutations in MBD4 and in Mbd4 knockout mice. Sequential sampling of germ line cases demonstrated repeated expansion of blood cell progenitors with pathogenic mutations in DNMT3A, a key driver gene for both clonal hematopoiesis and AML. Our findings reveal genetic and epigenetic factors that shape the mutagenic influence of 5mC. Within blood cells, this links methylation damage to the driver landscape of clonal hematopoiesis and reveals a conserved path to leukemia. Germ line MBD4 deficiency enhances cancer susceptibility and predisposes to AML.

The Mbd4 DNA glycosylase protects mice from inflammation-driven colon cancer and tissue injury

Much of the global cancer burden is associated with longstanding inflammation accompanied by release of DNA-damaging reactive oxygen and nitrogen species. Here, we report that the Mbd4 DNA glycosylase is protective in the azoxymethane/dextran sodium sulfate (AOM/DSS) mouse model of inflammation-driven colon cancer. Mbd4 excises T and U from T:G and U:G mismatches caused by deamination of 5-methylcytosine and cytosine. Since the rate of deamination is higher in inflamed tissues, we investigated the role of Mbd4 in inflammation-driven tumorigenesis. In the AOM/DSS assay, Mbd4-/- mice displayed more severe clinical symptoms, decreased survival, and a greater tumor burden than wild-type (WT) controls. The increased tumor burden in Mbd4-/- mice did not arise from impairment of AOM-induced apoptosis in the intestinal crypt. Histopathological analysis indicated that the colonic epithelium of Mbd4-/- mice is more vulnerable than WT to DSS-induced tissue damage. We investigated the role of the Mbd4-/- immune system in AOM/DSS-mediated carcinogenesis by repeating the assay on WT and Mbd4-/- mice transplanted with WT bone marrow. Mbd4-/- mice with WT bone marrow behaved similarly to Mbd4-/- mice. Together, our results indicate that the colonic epithelium of Mbd4-/- mice is more vulnerable to DSS-induced injury, which exacerbates inflammation-driven tissue injury and cancer.

The interaction between cytosine methylation and processes of DNA replication and repair shape the mutational landscape of cancer genomes

Methylated cytosines (5mCs) are frequently mutated in the genome. However, no studies have yet comprehensively analysed mutation–methylation associations across cancer types. Here we analyse 916 cancer genomes, together with tissue type-specific methylation and replication timing data. We describe a strong mutation–methylation association across colorectal cancer subtypes, most interestingly in samples with microsatellite instability (MSI) or Polymerase epsilon (POLE) exonuclease domain mutations. By analysing genomic regions with differential mismatch repair (MMR) efficiency, we suggest a possible role for MMR in the correction of 5mC deamination events, potentially accounting for the high rate of 5mC mutation accumulation in MSI tumours. Additionally, we propose that mutant POLE asserts a mutator phenotype specifically at 5mCs, and we find coding mutation hotspots in POLE-mutant cancers at highly-methylated CpGs in the tumour-suppressor genes APC and TP53. Finally, using multivariable regression models, we demonstrate that different cancers exhibit distinct mutation–methylation associations, with DNA repair influencing such associations in certain cancer genomes. Taken together, we find differential associations with methylation that are vital for accurately predicting expected mutation loads across cancer types. Our findings reveal links between methylation and common mutation and repair processes, with these mechanisms defining a key part of the mutational landscape of cancer genomes.

Thymine DNA Glycosylase (TDG) is involved in the pathogenesis of intestinal tumors with reduced APC expression

Thymine DNA Glycosylase (TDG) is a base excision repair enzyme that acts as a thymine and uracil DNA N-glycosylase on G:T and G:U mismatches, thus protecting CpG sites in the genome from mutagenesis by deamination. In addition, TDG has an epigenomic function by removing the novel cytosine derivatives 5-formylcytosine and 5-carboxylcytosine (5caC) generated by Ten-Eleven Translocation (TET) enzymes during active DNA demethylation. We and others previously reported that TDG is essential for mammalian development. However, its involvement in tumor formation is unknown. To study the role of TDG in tumorigenesis, we analyzed the effects of its inactivation in a well-characterized model of tumor predisposition, the Apc^Min mouse strain. Mice bearing a conditional Tdg^flox allele were crossed with Fabpl::Cre transgenic mice, in the context of the Apc^Min mutation, in order to inactivate Tdg in the small intestinal and colonic epithelium. We observed an approximately 2-fold increase in the number of small intestinal adenomas in the test Tdg-mutant Apc^Min mice in comparison to control genotypes (p=0.0001). This increase occurred in female mice, and is similar to the known increase in intestinal adenoma formation due to oophorectomy. In the human colorectal cancer (CRC) TCGA database, the subset of patients with TDG and APC expression in the lowest quartile exhibits an excess of female cases. We conclude that TDG inactivation plays a role in intestinal tumorigenesis initiated by mutation/underexpression of APC. Our results also indicate that TDG may be involved in sex-specific protection from CRC.

Uracil Accumulation and Mutagenesis Dominated by Cytosine Deamination in CpG Dinucleotides in Mice Lacking UNG and SMUG1

Both a DNA lesion and an intermediate for antibody maturation, uracil is primarily processed by base excision repair (BER), either initiated by uracil-DNA glycosylase (UNG) or by single-strand selective monofunctional uracil DNA glycosylase (SMUG1). The relative in vivo contributions of each glycosylase remain elusive. To assess the impact of SMUG1 deficiency, we measured uracil and 5-hydroxymethyluracil, another SMUG1 substrate, in Smug1^−/− mice. We found that 5-hydroxymethyluracil accumulated in Smug1^−/− tissues and correlated with 5-hydroxymethylcytosine levels. The highest increase was found in brain, which contained about 26-fold higher genomic 5-hydroxymethyluracil levels than the wild type. Smug1^−/− mice did not accumulate uracil in their genome and Ung^−/− mice showed slightly elevated uracil levels. Contrastingly, Ung^−/−Smug1^−/− mice showed a synergistic increase in uracil levels with up to 25-fold higher uracil levels than wild type. Whole genome sequencing of UNG/SMUG1-deficient tumours revealed that combined UNG and SMUG1 deficiency leads to the accumulation of mutations, primarily C to T transitions within CpG sequences. This unexpected sequence bias suggests that CpG dinucleotides are intrinsically more mutation prone. In conclusion, we showed that SMUG1 efficiently prevent genomic uracil accumulation, even in the presence of UNG, and identified mutational signatures associated with combined UNG and SMUG1 deficiency.

Single nucleotide polymorphisms in DNA glycosylases: From function to disease

Oxidative stress is associated with a growing number of diseases that span from cancer to neurodegeneration. Most oxidatively induced DNA base lesions are repaired by the base excision repair (BER) pathway which involves the action of various DNA glycosylases. There are numerous genome wide studies attempting to associate single-nucleotide polymorphisms (SNPs) with predispositions to various types of disease; often, these common variants do not have significant alterations in their biochemical function and do not exhibit a convincing phenotype. Nevertheless several lines of evidence indicate that SNPs in DNA repair genes may modulate DNA repair capacity and contribute to risk of disease. This overview provides a convincing picture that SNPs of DNA glycosylases that remove oxidatively generated DNA lesions are susceptibility factors for a wide disease spectrum that includes besides cancer (particularly lung, breast and gastrointestinal tract), cochlear/ocular disorders, myocardial infarction and neurodegenerative disorders which can be all grouped under the umbrella of oxidative stress-related pathologies.

Aberrant base excision repair pathway of oxidatively damaged DNA: Implications for degenerative diseases

In cellular organisms composition of DNA is constrained to only four nucleobases A, G, T and C, except for minor DNA base modifications such as methylation which serves for defence against foreign DNA or gene expression regulation. Interestingly, this severe evolutionary constraint among other things demands DNA repair systems to discriminate between regular and modified bases. DNA glycosylases specifically recognize and excise damaged bases among vast majority of regular bases in the base excision repair (BER) pathway. However, the mismatched base pairs in DNA can occur from a spontaneous conversion of 5-methylcytosine to thymine and DNA polymerase errors during replication. To counteract these mutagenic threats to genome stability, cells evolved special DNA repair systems that target the non-damaged DNA strand in a duplex to remove mismatched regular DNA bases. Mismatch-specific adenine- and thymine-DNA glycosylases (MutY/MUTYH and TDG/MBD4, respectively) initiated BER and mismatch repair (MMR) pathways can recognize and remove normal DNA bases in mismatched DNA duplexes. Importantly, in DNA repair deficient cells bacterial MutY, human TDG and mammalian MMR can act in the aberrant manner: MutY and TDG removes adenine and thymine opposite misincorporated 8-oxoguanine and damaged adenine, respectively, whereas MMR removes thymine opposite to O⁶-methylguanine. These unusual activities lead either to mutations or futile DNA repair, thus indicating that the DNA repair pathways which target non-damaged DNA strand can act in aberrant manner and introduce genome instability in the presence of unrepaired DNA lesions. Evidences accumulated showing that in addition to the accumulation of oxidatively damaged DNA in cells, the aberrant DNA repair can also contribute to cancer, brain disorders and premature senescence. For example, the aberrant BER and MMR pathways for oxidized guanine residues can lead to trinucleotide expansion that underlies Huntington's disease, a severe hereditary neurodegenerative syndrome. This review summarises the present knowledge about the aberrant DNA repair pathways for oxidized base modifications and their possible role in age-related diseases.

Tet2 loss leads to hypermutagenicity in haematopoietic stem/progenitor cells

TET2 is a dioxygenase that catalyses multiple steps of 5-methylcytosine oxidation. Although TET2 mutations frequently occur in various types of haematological malignancies, the mechanism by which they increase risk for these cancers remains poorly understood. Here we show that Tet2^-/- mice develop spontaneous myeloid, T- and B-cell malignancies after long latencies. Exome sequencing of Tet2^-/- tumours reveals accumulation of numerous mutations, including Apc, Nf1, Flt3, Cbl, Notch1 and Mll2, which are recurrently deleted/mutated in human haematological malignancies. Single-cell-targeted sequencing of wild-type and premalignant Tet2^-/- Lin^-c-Kit⁺ cells shows higher mutation frequencies in Tet2^-/- cells. We further show that the increased mutational burden is particularly high at genomic sites that gained 5-hydroxymethylcytosine, where TET2 normally binds. Furthermore, TET2-mutated myeloid malignancy patients have significantly more mutational events than patients with wild-type TET2. Thus, Tet2 loss leads to hypermutagenicity in haematopoietic stem/progenitor cells, suggesting a novel TET2 loss-mediated mechanism of haematological malignancy pathogenesis.

Tagging polymorphisms of methyl-CpG binding domain 4 and gastric cardiac adenocarcinoma risk in a Chinese population

Potential effects of genetic factors on carcinogenesis of gastric cardiac adenocarcinoma (GCA) may exist. The present experiment specifically evaluated the genetic influence of single nucleotide in methyl-CpG binding domain 4 (MBD4) on GCA tumorigenesis. A case-control experiment based on hospital recruited 330 GCA patients and 608 non-cancer patients was carried out. We employed ligation detection reaction method to detect the genotypes. The results revealed that MBD4 rs3138373, rs2005618, and rs3138355 mutations had no significant association with the risk of GCA. However, a lower risk of GCA presented in male patients who carried the MBD4 rs3138355 G>A polymorphic loci by the stratified analyses. In general, The MBD4 gene polymorphism could not influence GCA hereditary predisposition. Nevertheless, whether the finding learned from our experiment could apply to other ethnic groups will remain vague until future multicenter studies further test and verify our conclusions.

How does inflammation drive mutagenesis in colorectal cancer?

Colorectal cancer (CRC) is a major health challenge worldwide. Factors thought to be important in CRC etiology include diet, microbiome, exercise, obesity, a history of colon inflammation and family history. Interventions, including the use of non-steroidal anti-Inflammatory drugs (NSAIDs) and anti-inflammatory agents, have been shown to decrease incidence in some settings. However, our current understanding of the mechanistic details that drive CRC are insufficient to sort out the complex and interacting factors responsible for cancer-initiating events. It has been known for some time that the development of CRC involves mutations in key genes such as p53 and APC, and the sequence in which these mutations occur can determine tumor presentation. Observed recurrent mutations are dominated by C to T transitions at CpG sites, implicating the deamination of 5-methylcytosine (5mC) as a key initiating event in cancer-driving mutations. While it has been widely assumed that inflammation-mediated oxidation drives mutations in CRC, oxidative damage to DNA induces primarily G to T transversions, not C to T transitions. In this review, we discuss this unresolved conundrum, and specifically, we elucidate how the known nucleotide excision repair (NER) and base excision repair (BER) pathways, which are partially redundant and potentially competing, might provide a critical link between oxidative DNA damage and C to T mutations. Studies using recently developed next-generation DNA sequencing technologies have revealed the genetic heterogeneity in human tissues including tumors, as well as the presence of DNA damage. The capacity to follow DNA damage, repair and mutagenesis in human tissues using these emerging technologies could provide a mechanistic basis for understanding the role of oxidative damage in CRC tumor initiation. The application of these technologies could identify mechanism-based biomarkers useful in earlier diagnosis and aid in the development of cancer prevention strategies.

Involvement of MBD4 inactivation in mismatch repair-deficient tumorigenesis

The DNA glycosylase gene MBD4 safeguards genomic stability at CpG sites and is frequently mutated at coding poly-A tracks in mismatch repair (MMR)-defective colorectal tumors (CRC). Mbd4 biallelic inactivation in mice provided conflicting results as to its role in tumorigenesis. Thus, it is unclear whether MBD4 alterations are only secondary to MMR defects without functional consequences or can contribute to the mutator phenotype. We investigated MBD4 variants in a large series of hereditary/familial and sporadic CRC cases. Whereas MBD4 frameshifts were only detected in tumors, missense variants were found in both normal and tumor DNA. In CRC with double-MBD4/MMR and single-MBD4 variants, transition mutation frequency was increased, indicating that MBD4 defects may affect the mutational landscape independently of MMR defect. Mbd4-deficient mice showed reduced survival when combined with Mlh1^−/− genotype. Taken together, these data suggest that MBD4 inactivation may contribute to tumorigenesis, acting as a modifier of MMR-deficient cancer phenotype.

Are there specific readers of oxidized 5-methylcytosine bases?

5-methylcytosine (5mC) was long thought to be the only enzymatically created modified DNA base in mammalian cells. The discovery of 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine as reaction products of the TET family 5mC oxidases has prompted extensive searches for proteins that specifically bind to these oxidized bases. However, only a few of such "reader" proteins have been identified and verified so far. In this review, we discuss potential biological functions of oxidized 5mC as well as the role the presumed reader proteins may play in interpreting the genomic signals of 5mC oxidation products.

Production of truncated MBD4 protein by frameshift mutation in DNA mismatch repair-deficient cells enhances 5-fluorouracil sensitivity that is independent of hMLH1 status

Methyl-CpG binding domain protein 4 (MBD4) is a DNA glycosylase that can remove 5-fluorodeoxyuracil from DNA as well as repair T:G or U:G mismatches. MBD4 is a target for frameshift mutation with DNA mismatch repair (MMR) deficiency, creating a truncated MBD4 protein (TruMBD4) that lacks its glycosylase domain. Here we show that TruMBD4 plays an important role for enhancing 5-fluorouracil (5FU) sensitivity in MMR-deficient colorectal cancer cells. We found biochemically that TruMBD4 binds to 5FU incorporated into DNA with higher affinity than MBD4. TruMBD4 reduced the 5FU affinity of the MMR recognition complexes that determined 5FU sensitivity by previous reports, suggesting other mechanisms might be operative to trigger cytotoxicity. To analyze overall 5FU sensitivity with TruMBD4, we established TruMBD4 overexpression in hMLH1-proficient or -deficient colorectal cancer cells followed by treatment with 5FU. 5FU-treated TruMBD4 cells demonstrated diminished growth characteristics compared to controls, independently of hMLH1 status. Flow cytometry revealed more 5FU-treated TruMBD4 cells in S phase than controls. We conclude that patients with MMR-deficient cancers, which show characteristic resistance to 5FU therapy, may be increased for 5FU sensitivity via secondary frameshift mutation of the base excision repair gene MBD4.

Proteins That Read DNA Methylation

Covalent modification of DNA via deposition of a methyl group at the 5' position on cytosine residues alters the chemical groups available for interaction in the major groove of DNA. The information content inherent in this modification alters the affinity and the specificity of DNA binding; some proteins favor interaction with methylated DNA, and others disfavor it. Molecular recognition of cytosine methylation by proteins often initiates sequential regulatory events which impact gene expression and chromatin structure. The known methyl-DNA-binding proteins have unique domains responsible for DNA methylation recognition: (1) the methyl-CpG-binding domain (MBD), (2) the C2H2 zinc finger domain, and (3) the SET- and RING finger-associated (SRA) domain. Structural analyses have revealed that each domain has a characteristic methylated DNA-binding pattern, and this difference in the recognition mechanism renders the DNA methylation mark able to transmit complicated biological information. Recent genetic and genomic studies have revealed novel functions of methyl-DNA-binding proteins. These emerging data have also provided glimpses into how methyl-DNA-binding proteins possess unique features and, presumably, functions. In this review, we summarize structural and biochemical analyses elucidating the mechanism for recognition of DNA methylation and correlate this information with emerging genomic and functional data.

MBD4 interacts with and recruits USP7 to heterochromatic foci

MBD4 is the only methyl‐CpG binding protein that possesses a C‐terminal glycosylase domain. It has been associated with a number of nuclear pathways including DNA repair, DNA damage response, the initiation of apoptosis, transcriptional repression, and DNA demethylation. However, the precise contribution of MBD4 to these processes in development and relevant diseases remains elusive. We identified UHRF1 and USP7 as two new interaction partners for MBD4. Both UHRF1, a E3 ubiquitin ligase, and USP7, a de‐ubiquinating enzyme, regulate the stability of the DNA maintenance methyltransferase, Dnmt1. The ability of MBD4 to directly interact with and recruit USP7 to chromocenters implicates it as an additional factor that can potentially regulate Dnmt1 activity during cell proliferation. J. Cell. Biochem. 116: 476–485, 2015. © 2014 The Authors. Journal of Cellular Biochemistry published by Wiley Periodicals, Inc.

Methyl-CpG-binding domain proteins: readers of the epigenome

How DNA methylation is interpreted and influences genome regulation remains largely unknown. Proteins of the methyl-CpG-binding domain (MBD) family are primary candidates for the readout of DNA methylation as they recruit chromatin remodelers, histone deacetylases and methylases to methylated DNA associated with gene repression. MBD protein binding requires both functional MBD domains and methyl-CpGs; however, some MBD proteins also bind unmethylated DNA and active regulatory regions via alternative regulatory domains or interaction with the nucleosome remodeling deacetylase (NuRD/Mi-2) complex members. Mutations within MBD domains occur in many diseases, including neurological disorders and cancers, leading to loss of MBD binding specificity to methylated sites and gene deregulation. Here, we summarize the current state of knowledge about MBD proteins and their role as readers of the epigenome.

Expression and function of AtMBD4L, the single gene encoding the nuclear DNA glycosylase MBD4L in Arabidopsis

DNA glycosylases recognize and excise damaged or incorrect bases from DNA initiating the base excision repair (BER) pathway. Methyl-binding domain protein 4 (MBD4) is a member of the HhH-GPD DNA glycosylase superfamily, which has been well studied in mammals but not in plants. Our knowledge on the plant enzyme is limited to the activity of the Arabidopsis recombinant protein MBD4L in vitro. To start evaluating MBD4L in its biological context, we here characterized the structure, expression and effects of its gene, AtMBD4L. Phylogenetic analysis indicated that AtMBD4L belongs to one of the seven families of HhH-GPD DNA glycosylase genes existing in plants, and is unique on its family. Two AtMBD4L transcripts coding for active enzymes were detected in leaves and flowers. Transgenic plants expressing the AtMBD4L:GUS gene confined GUS activity to perivascular leaf tissues (usually adjacent to hydathodes), flowers (anthers at particular stages of development), and the apex of immature siliques. MBD4L-GFP fusion proteins showed nuclear localization in planta. Interestingly, overexpression of the full length MBD4L, but not a truncated enzyme lacking the DNA glycosylase domain, induced the BER gene LIG1 and enhanced tolerance to oxidative stress. These results suggest that endogenous MBD4L acts on particular tissues, is capable of activating BER, and may contribute to repair DNA damage caused by oxidative stress.

Role of base excision repair in maintaining the genetic and epigenetic integrity of CpG sites

Cytosine methylation at CpG dinucleotides is a central component of epigenetic regulation in vertebrates, and the base excision repair (BER) pathway is important for maintaining both the genetic stability and the methylation status of CpG sites. This perspective focuses on two enzymes that are of particular importance for the genetic and epigenetic integrity of CpG sites, methyl binding domain 4 (MBD4) and thymine DNA glycosylase (TDG). We discuss their capacity for countering C to T mutations at CpG sites, by initiating base excision repair of G · T mismatches generated by deamination of 5-methylcytosine (5mC). We also consider their role in active DNA demethylation, including pathways that are initiated by oxidation and/or deamination of 5mC.

DNA methylation, its mediators and genome integrity

DNA methylation regulates many cellular processes, including embryonic development, transcription, chromatin structure, X-chromosome inactivation, genomic imprinting and chromosome stability. DNA methyltransferases establish and maintain the presence of 5-methylcytosine (5mC), and ten-eleven translocation cytosine dioxygenases (TETs) oxidise 5mC to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC), which can be removed by base excision repair (BER) proteins. Multiple forms of DNA methylation are recognised by methyl-CpG binding proteins (MeCPs), which play vital roles in chromatin-based transcriptional regulation, DNA repair and replication. Accordingly, defects in DNA methylation and its mediators may cause silencing of tumour suppressor genes and misregulation of multiple cell cycles, DNA repair and chromosome stability genes, and hence contribute to genome instability in various human diseases, including cancer. Thus, understanding functional genetic mutations and aberrant expression of these DNA methylation mediators is critical to deciphering the crosstalk between concurrent genetic and epigenetic alterations in specific cancer types and to the development of new therapeutic strategies.

No evidence for AID/MBD4-coupled DNA demethylation in zebrafish embryos

The mechanisms responsible for active DNA demethylation remain elusive in Metazoa. A previous study that utilized zebrafish embryos provided a potent mechanism for active demethylation in which three proteins, AID, MBD4, and GADD45 are involved. We recently found age-dependent DNA hypomethylation in zebrafish, and it prompted us to examine if AID and MBD4 could be involved in the phenomenon. Unexpectedly, however, we found that most of the findings in the previous study were not reproducible. First, the injection of a methylated DNA fragment into zebrafish eggs did not affect either the methylation of genomic DNA, injected methylated DNA itself, or several loci tested or the expression level of aid, which has been shown to play a role in demethylation. Second, aberrant methylation was not observed at certain CpG islands following the injection of antisense morpholino oligonucleotides against aid and mbd4. Furthermore, we demonstrated that zebrafish MBD4 cDNA lacked a coding region for the methyl-CpG binding domain, which was assumed to be necessary for guidance to target regions. Taken together, we concluded that there is currently no evidence to support the proposed roles of AID and MBD4 in active demethylation in zebrafish embryos.

Mutagenicity of furan in female Big Blue B6C3F1 mice

Furan is an abundant food and environmental contaminant that is a potent liver carcinogen in rodent models. To determine if furan is genotoxic in vivo, female B6C3F1 Big Blue transgenic mice were treated with 15 mg/kg bw furan by gavage 5 days a week for 6 weeks, or once weekly for 3 weeks. Liver cII transgene mutation-frequency and mutation spectra were determined. Furan did not increase the mutation frequency under either treatment condition. In the 6-week treatment regimen, there was a change in the cII transgene mutation-spectrum, with the fraction of GC to AT transitions significantly reduced. The only other significant change was an increase in GC to CG transversions; these represented a minor contribution to the overall mutation spectrum. A much larger furan-dependent shift was observed in the 3-week study. There was a significant increase in transversion mutations, predominantly GC to TA transversions as well as smaller non-significant changes in GC to CG and AT to TA transversions. To determine if these mutations were caused by cis-2-butene-1,4-dial (BDA), a reactive metabolite of furan, the mutagenic activity and the mutation spectrum of BDA was determined in vitro, in Big Blue mouse embryonic fibroblasts. This compound did not increase the cII gene mutation-frequency but caused a substantial increase in AT to CG transversions. This increase, however, lost statistical significance when adjusted for multiple comparisons. Together, these findings suggest that BDA may not be directly responsible for the in-vivo effects of furan on mutational spectra. Histopathological analysis of livers from furan-treated mice revealed that furan induced multifocal, hepatocellular necrosis admixed with reactive leukocytes and pigment-laden Kupffer cells, enhanced oval-cell hyperplasia, and increased hepatocyte mitoses, some of which were atypical. An indirect mechanism of genotoxicity is proposed in which chronic toxicity followed by inflammation and secondary cell proliferation triggers cancer development in furan-exposed rodents.