Differential subcellular localization of human MutY homolog (hMYH) and the functional activity of adenine:8-oxoguanine DNA glycosylase

A novel AluYb8 insertion-associated non-coding RNA, lncMUTYH, impairs mitochondrial function and dampens the M2-like polarization of macrophages

An inverted AluYb8 insertion in the MUTYH intron 15 (AluYb8MUTYH variant) has been reported to be associated with reduced MUTYH1 expression and mitochondrial dysfunction with age. However, the underlying mechanism remains unknown. In this study, we identified a novel transcript associated with the AluYb8MUTYH variant, which revealed that this transcript is about 780 nucleotides in length with a poly-A tail, lacks protein-coding potential, referred to as lncMUTYH. The results from the reporter gene system confirmed that the lncMUTYH down-regulates MUTYH1 expression at the translational level. Site-directed mutagenesis on the 5'-terminal exon sequences of α-MUTYH and lncMUTYH constructs revealed that lncMUTYH can act as a trans-regulator that depends on the partial base pairing between its exonized AluYb8 sequence and the 5'UTR of α-MUTYH to impede MUTYH 1 expression. Furthermore, we have demonstrated a correlation between decreased mitochondrion-localized MUTYH1 caused by lncMUTYH and lowered levels of mitochondrial biological function indicators, such as mtDNA content, mitochondrial regulatory gene expression, oxygen consumption rate, ATP product, and mitochondrial respiratory capacity. Notably, we found that lncMUTYH inhibited the M2-like polarization of macrophages, and CD68/CD206-positive cell fractions were significantly lower in lncMUTYH ectopically expressing cells. The results confirmed that the AluYb8MUTYH-associated lncMUTYH, derived from an AluYb8 insertion mutation, acts as a trans-regulatory factor that inhibits the MUTYH1 protein expression, leading to a progressive mitochondrial dysfunction that may disrupt macrophage differentiation. In summary, lncMUTYH can contribute to AluYb8MUTYH-associated mitochondrial dysfunction with age and hamper the macrophage polarization process, potentially increasing the risk of developing age-related diseases.

Dynamic features of human mitochondrial DNA maintenance and transcription

Mitochondria are the primary sites for cellular energy production and are required for many essential cellular processes. Mitochondrial DNA (mtDNA) is a 16.6 kb circular DNA molecule that encodes only 13 gene products of the approximately 90 different proteins of the respiratory chain complexes and an estimated 1,200 mitochondrial proteins. MtDNA is, however, crucial for organismal development, normal function, and survival. MtDNA maintenance requires mitochondrially targeted nuclear DNA repair enzymes, a mtDNA replisome that is unique to mitochondria, and systems that control mitochondrial morphology and quality control. Here, we provide an overview of the current literature on mtDNA repair and transcription machineries and discuss how dynamic functional interactions between the components of these systems regulate mtDNA maintenance and transcription. A profound understanding of the molecular mechanisms that control mtDNA maintenance and transcription is important as loss of mtDNA integrity is implicated in normal process of aging, inflammation, and the etiology and pathogenesis of a number of diseases.

Impact of Oxidative DNA Damage and the Role of DNA Glycosylases in Neurological Dysfunction

The human brain requires a high rate of oxygen consumption to perform intense metabolic activities, accounting for 20% of total body oxygen consumption. This high oxygen uptake results in the generation of free radicals, including reactive oxygen species (ROS), which, at physiological levels, are beneficial to the proper functioning of fundamental cellular processes. At supraphysiological levels, however, ROS and associated lesions cause detrimental effects in brain cells, commonly observed in several neurodegenerative disorders. In this review, we focus on the impact of oxidative DNA base lesions and the role of DNA glycosylase enzymes repairing these lesions on brain function and disease. Furthermore, we discuss the role of DNA base oxidation as an epigenetic mechanism involved in brain diseases, as well as potential roles of DNA glycosylases in different epigenetic contexts. We provide a detailed overview of the impact of DNA glycosylases on brain metabolism, cognition, inflammation, tissue loss and regeneration, and age-related neurodegenerative diseases based on evidence collected from animal and human models lacking these enzymes, as well as post-mortem studies on patients with neurological disorders.

Importance of Angomonas deanei KAP4 for kDNA arrangement, cell division and maintenance of the host-bacterium relationship

Angomonas deanei coevolves in a mutualistic relationship with a symbiotic bacterium that divides in synchronicity with other host cell structures. Trypanosomatid mitochondrial DNA is contained in the kinetoplast and is composed of thousands of interlocked DNA circles (kDNA). The arrangement of kDNA is related to the presence of histone-like proteins, known as KAPs (kinetoplast-associated proteins), that neutralize the negatively charged kDNA, thereby affecting the activity of mitochondrial enzymes involved in replication, transcription and repair. In this study, CRISPR-Cas9 was used to delete both alleles of the A. deanei KAP4 gene. Gene-deficient mutants exhibited high compaction of the kDNA network and displayed atypical phenotypes, such as the appearance of a filamentous symbionts, cells containing two nuclei and one kinetoplast, and division blocks. Treatment with cisplatin and UV showed that Δkap4 null mutants were not more sensitive to DNA damage and repair than wild-type cells. Notably, lesions caused by these genotoxic agents in the mitochondrial DNA could be repaired, suggesting that the kDNA in the kinetoplast of trypanosomatids has unique repair mechanisms. Taken together, our data indicate that although KAP4 is not an essential protein, it plays important roles in kDNA arrangement and replication, as well as in the maintenance of symbiosis.

Role of Mitochondrial DNA in Inflammatory Airway Diseases

The mitochondrial genome is a small, circular, and highly conserved piece of DNA which encodes only 13 protein subunits yet is vital for electron transport in the mitochondrion and, therefore, vital for the existence of multicellular life on Earth. Despite this importance, mitochondrial DNA (mtDNA) is located in one of the least-protected areas of the cell, exposing it to high concentrations of intracellular reactive oxygen species (ROS) and threat from exogenous substances and pathogens. Until recently, the quality control mechanisms which ensured the stability of the nuclear genome were thought to be minimal or nonexistent in the mitochondria, and the thousands of redundant copies of mtDNA in each cell were believed to be the primary mechanism of protecting these genes. However, a vast network of mechanisms has been discovered that repair mtDNA lesions, replace and recycle mitochondrial chromosomes, and conduct alternate RNA processing for previously undescribed mitochondrial proteins. New mtDNA/RNA-dependent signaling pathways reveal a mostly undiscovered biochemical landscape in which the mitochondria interface with their host cells/organisms. As the myriad ways in which the function of the mitochondrial genome can affect human health have become increasingly apparent, the use of mitogenomic biomarkers (such as copy number and heteroplasmy) as toxicological endpoints has become more widely accepted. In this article, we examine several pathologies of human airway epithelium, including particle exposures, inflammatory diseases, and hyperoxia, and discuss the role of mitochondrial genotoxicity in the pathogenesis and/or exacerbation of these conditions. © 2021 American Physiological Society. Compr Physiol 11:1485-1499, 2021.

New perspectives in cancer biology from a study of canonical and non-canonical functions of base excision repair proteins with a focus on early steps

Alterations of DNA repair enzymes and consequential triggering of aberrant DNA damage response (DDR) pathways are thought to play a pivotal role in genomic instabilities associated with cancer development, and are further thought to be important predictive biomarkers for therapy using the synthetic lethality paradigm. However, novel unpredicted perspectives are emerging from the identification of several non-canonical roles of DNA repair enzymes, particularly in gene expression regulation, by different molecular mechanisms, such as (i) non-coding RNA regulation of tumour suppressors, (ii) epigenetic and transcriptional regulation of genes involved in genotoxic responses and (iii) paracrine effects of secreted DNA repair enzymes triggering the cell senescence phenotype. The base excision repair (BER) pathway, canonically involved in the repair of non-distorting DNA lesions generated by oxidative stress, ionising radiation, alkylation damage and spontaneous or enzymatic deamination of nucleotide bases, represents a paradigm for the multifaceted roles of complex DDR in human cells. This review will focus on what is known about the canonical and non-canonical functions of BER enzymes related to cancer development, highlighting novel opportunities to understand the biology of cancer and representing future perspectives for designing new anticancer strategies. We will specifically focus on APE1 as an example of a pleiotropic and multifunctional BER protein.

AluYb8 insertion polymorphism in the MUTYH gene impairs mitochondrial DNA maintenance and affects the age of onset of IPF

Background: Idiopathic pulmonary fibrosis (IPF) is an age-related fatal disease with an unknown etiology. Increased oxidative stress and mitochondrial dysfunction are thought to be involved in its pathogenesis. However, the effect of the AluYb8MUTYH polymorphism on IPF is not known.

Results: The mean age of onset for IPF in patients homozygous for the AluYb8MUTYH variant (P/P) was 66.5 years old, which was significantly earlier than that in patients with the wild-type (A/A, 70.45 years old). For the 97 male IPF patients with lung function data, the FVC% of the P/P patients was lower than that of the wild-type (A/A) or heterozygous (A/P) patients. The laboratory analysis indicated that an increased mtDNA content and impaired mitochondrial quality control were associated with the P/P genotype. We also confirmed that AluYb8 insertion into MUTYH caused decreased MUTYH1 expression in lung tissues.

Methods: We compared the lung function of IPF patients and observed the mtDNA content, mtDNA integrity and molecular expression of mitochondrial quality control among subjects with different AluYb8MUTYH genotypes. Additionally, immunoblotting and a reporter gene system were used to test whether altered mitochondrial MUTYH1 expression was linked to AluYb8MUTYH.

Conclusions: The AluYb8 insertion polymorphism in MUTYH impairs mtDNA stability and affects the age of onset of IPF.

Base-excision repair deficiency alone or combined with increased oxidative stress does not increase mtDNA point mutations in mice

Mitochondrial DNA (mtDNA) mutations become more prevalent with age and are postulated to contribute to the ageing process. Point mutations of mtDNA have been suggested to originate from two main sources, i.e. replicative errors and oxidative damage, but the contribution of each of these processes is much discussed. To elucidate the origin of mtDNA mutations, we measured point mutation load in mice with deficient mitochondrial base-excision repair (BER) caused by knockout alleles preventing mitochondrial import of the DNA repair glycosylases OGG1 and MUTYH (Ogg1 dMTS, Mutyh dMTS). Surprisingly, we detected no increase in the mtDNA mutation load in old Ogg1 dMTS mice. As DNA repair is especially important in the germ line, we bred the BER deficient mice for five consecutive generations but found no increase in the mtDNA mutation load in these maternal lineages. To increase reactive oxygen species (ROS) levels and oxidative damage, we bred the Ogg1 dMTS mice with tissue specific Sod2 knockout mice. Although increased superoxide levels caused a plethora of changes in mitochondrial function, we did not detect any changes in the mutation load of mtDNA or mtRNA. Our results show that the importance of oxidative damage as a contributor of mtDNA mutations should be re-evaluated.

Diverse functions of DNA glycosylases processing oxidative base lesions in brain

Endogenous and exogenous oxidative agents continuously damage genomic DNA, with the brain being particularly vulnerable. Thus, preserving genomic integrity is key for brain health and neuronal function. Accumulation of DNA damage is one of the causative factors of ageing and increases the risk of a wide range of neurological disorders. Base excision repair is the major pathway for removal of oxidized bases in the genome and initiated by DNA glycosylases. Emerging evidence suggest that DNA glycosylases have non-canonical functions important for genome regulation. Understanding canonical and non-canonical functions of DNA glycosylases processing oxidative base lesions modulating brain function will be crucial for the development of novel therapeutic strategies.

Isoforms of Base Excision Repair Enzymes Produced by Alternative Splicing

Transcripts of many enzymes involved in base excision repair (BER) undergo extensive alternative splicing, but functions of the corresponding alternative splice variants remain largely unexplored. In this review, we cover the studies describing the common alternatively spliced isoforms and disease-associated variants of DNA glycosylases, AP-endonuclease 1, and DNA polymerase beta. We also discuss the roles of alternative splicing in the regulation of their expression, catalytic activities, and intracellular transport.

Analysis of MUTYH alternative transcript expression, promoter function, and the effect of human genetic variants

The human DNA repair gene MUTYH, whose mutational loss causes a colorectal polyposis and cancer predisposition, contains three alternative first exons. In order to analyze alternative transcription and the effect of genetic alterations found in humans, we established a cell-based minigene experimental model supporting transcription and splicing and thoroughly verified its functionality. We identified highly conserved promoter areas and inactivated them in the minigene, and also introduced six human variants. Moreover, the potential contribution of CpG island methylation and specific transcription factors on MUTYH transcription was addressed. The findings allowed to attribute regulatory roles to three conserved motifs in the promoter: an M4 motif, a transcription factor IIB recognition element, and a GC box. Moreover, the data showed that three patient variants compromised MUTYH expression and therefore have the potential to cause pathogenic effects. We did not find evidence for a biologically relevant contribution of CpG island methylation or a direct transcriptional activation by DNA damage. Besides insight into the regulation of MUTYH transcription, the work therefore provides a functional MUTYH minigene experimental system suitable as a diagnostic tool for analyzing patient variants, and a functional map of the promotor that also can facilitate pathogenicity classifications of human variants.

Mechanisms of Mitochondrial DNA Repair in Mammals

Accumulation of mutations in mitochondrial DNA leads to the development of severe, currently untreatable diseases. The contribution of these mutations to aging and progress of neurodegenerative diseases is actively studied. Elucidation of DNA repair mechanisms in mitochondria is necessary for both developing approaches to the therapy of diseases caused by mitochondrial mutations and understanding specific features of mitochondrial genome functioning. Mitochondrial DNA repair systems have become a subject of extensive studies only in the last decade due to development of molecular biology methods. DNA repair systems of mammalian mitochondria appear to be more diverse and effective than it had been thought earlier. Even now, one may speak about the existence of mitochondrial mechanisms for the repair of single- and double-stranded DNA lesions. Homologous recombination also takes place in mammalian mitochondria, although its functional significance and molecular mechanisms remain obscure. In this review, I describe DNA repair systems in mammalian mitochondria, such as base excision repair (BER) and microhomology-mediated end joining (MMEJ) and discuss a possibility of existence of mitochondrial DNA repair mechanisms otherwise typical for the nuclear DNA, e.g., nucleotide excision repair (NER), mismatch repair (MMR), homologous recombination, and classical non-homologous end joining (NHEJ). I also present data on the mechanisms for coordination of the nuclear and mitochondrial DNA repair systems that have been actively studied recently.

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.

Removal of oxidatively generated DNA damage by overlapping repair pathways

It is generally believed that the mammalian nucleotide excision repair pathway removes DNA helix-distorting bulky DNA lesions, while small non-bulky lesions are repaired by base excision repair (BER). However, recent work demonstrates that the oxidativly generated guanine oxidation products, spiroimininodihydantoin (Sp), 5-guanidinohydantoin (Gh), and certain intrastrand cross-linked lesions, are good substrates of NER and BER pathways that compete with one another in human cell extracts. The oxidation of guanine by peroxynitrite is known to generate 5-guanidino-4-nitroimidazole (NIm) which is structurally similar to Gh, except that the 4-nitro group in NIm is replaced by a keto group in Gh. However, unlike Gh, NIm is an excellent substrate of BER, but not of NER. These and other related results are reviewed and discussed in this article.

Risky repair: DNA-protein crosslinks formed by mitochondrial base excision DNA repair enzymes acting on free radical lesions

Oxygen is both necessary and dangerous for aerobic cell function. ATP is most efficiently made by the electron transport chain, which requires oxygen as an electron acceptor. However, the presence of oxygen, and to some extent the respiratory chain itself, poses a danger to cellular components. Mitochondria, the sites of oxidative phosphorylation, have defense and repair pathways to cope with oxidative damage. For mitochondrial DNA, an essential pathway is base excision repair, which acts on a variety of small lesions. There are instances, however, in which attempted DNA repair results in more damage, such as the formation of a DNA-protein crosslink trapping the repair enzyme on the DNA. That is the case for mitochondrial DNA polymerase γ acting on abasic sites oxidized at the 1-carbon of 2-deoxyribose. Such DNA-protein crosslinks presumably must be removed in order to restore function. In nuclear DNA, ubiquitylation of the crosslinked protein and digestion by the proteasome are essential first processing steps. How and whether such mechanisms operate on DNA-protein crosslinks in mitochondria remains to be seen.

Polymorphism and protein expression of MUTYH gene for risk of rheumatoid arthritis

Background

We have previously described the association between rheumatoid arthritis (RA) prevalence and the two mutY Homolog (E. coli) (MUTYH) SNPs (rs3219463 and rs3219476) among the Taiwanese population. This present study will aim to elucidate whether the SNPs can alter the expression of EGFR in the progression of RA.

Methods

The cohort study included 368 Taiwan’s Han Chinese RA patients and 364 healthy controls. Blood samples collected from the participants were analyzed to determine their serum MUTYH levels and to identify rs3219463 SNP of MUTYH from their genomic DNA.

Results

Our data resulted in a statistically significant difference in genotype frequency distributions at rs3219463 for RA patients and controls (p < 0.0002). Also, the patients with G carrier at rs3219463 were less likely to suffer from painful joints (p < 0.006) and DAS28 scores (p < 0.003). Furthermore, the increase in serum level of MUTYH was also observed in RA patients (p < 0.005).

Conclusions

Our study showed that RA is associated with rs3219463 SNP in EGFR gene and an increased serum level of the MUTYH protein. These findings suggest MUTYH is worthy of further investigation as a therapeutic target for RA.

Mammalian Mitochondria and Aging: An Update

Mitochondria were first postulated to contribute to aging more than 40 years ago. During the following decades, multiple lines of evidence in model organisms and humans showed that impaired mitochondrial function can contribute to age-associated disease phenotypes and aging. However, in contrast to the original theory favoring oxidative damage as a cause for mtDNA mutations, there are now strong data arguing that most mammalian mtDNA mutations originate as replication errors made by the mtDNA polymerase. Currently, a substantial amount of mitochondrial research is focused on finding ways to either remove or counteract the effects of mtDNA mutations with the hope of extending the human health- and lifespan. This review summarizes the current knowledge regarding the formation of mtDNA mutations and their impact on mitochondrial function. We also critically discuss proposed pathways interlinked with mammalian mtDNA mutations and suggest future research strategies to elucidate the role of mtDNA mutations in aging.

Translesion Synthesis of 2'-Deoxyguanosine Lesions by Eukaryotic DNA Polymerases

With the discovery of translesion synthesis DNA polymerases, great strides have been made in the last two decades in understanding the mode of replication of various DNA lesions in prokaryotes and eukaryotes. A database search indicated that approximately 2000 articles on this topic have been published in this period. This includes research involving genetic and structural studies as well as in vitro experiments using purified DNA polymerases and accessory proteins. It is a daunting task to comprehend this exciting and rapidly emerging area of research. Even so, as the majority of DNA damage occurs at 2′-deoxyguanosine residues, this perspective attempts to summarize a subset of this field, focusing on the most relevant eukaryotic DNA polymerases responsible for their bypass.

Base excision DNA repair in the embryonic development of the sea urchin, Strongylocentrotus intermedius

In actively proliferating cells, such as the cells of the developing embryo, DNA repair is crucial for preventing the accumulation of mutations and synchronizing cell division. Sea urchin embryo growth was analyzed and extracts were prepared. The relative activity of DNA polymerase, apurinic/apyrimidinic (AP) endonuclease, uracil-DNA glycosylase, 8-oxoguanine-DNA glycosylase, and other glycosylases was analyzed using specific oligonucleotide substrates of these enzymes; the reaction products were resolved by denaturing 20% polyacrylamide gel electrophoresis. We have characterized the profile of several key base excision repair activities in the developing embryos (2 blastomers to mid-pluteus) of the grey sea urchin, Strongylocentrotus intermedius. The uracil-DNA glycosylase specific activity sharply increased after blastula hatching, whereas the specific activity of 8-oxoguanine-DNA glycosylase steadily decreased over the course of the development. The AP-endonuclease activity gradually increased but dropped at the last sampled stage (mid-pluteus 2). The DNA polymerase activity was high at the first cleavage division and then quickly decreased, showing a transient peak at blastula hatching. It seems that the developing sea urchin embryo encounters different DNA-damaging factors early in development within the protective envelope and later as a free-floating larva, with hatching necessitating adaptation to the shift in genotoxic stress conditions. No correlation was observed between the dynamics of the enzyme activities and published gene expression data from developing congeneric species, S. purpuratus. The results suggest that base excision repair enzymes may be regulated in the sea urchin embryos at the level of covalent modification or protein stability.

Functional Complementation Assay for 47 MUTYH Variants in a MutY-Disrupted Escherichia coli Strain

MUTYH-associated polyposis (MAP) is an adenomatous polyposis transmitted in an autosomal-recessive pattern, involving biallelic inactivation of the MUTYH gene. Loss of a functional MUTYH protein will result in the accumulation of G:T mismatched DNA caused by oxidative damage. Although p.Y179C and p.G396D are the two most prevalent MUTYH variants, more than 200 missense variants have been detected. It is difficult to determine whether these variants are disease-causing mutations or single-nucleotide polymorphisms. To understand the functional consequences of these variants, we generated 47 MUTYH gene variants via site-directed mutagenesis, expressed the encoded proteins in MutY-disrupted Escherichia coli, and assessed their abilities to complement the functional deficiency in the E. coli by monitoring spontaneous mutation rates. Although the majority of variants exhibited intermediate complementation relative to the wild type, some variants severely interfered with this complementation. However, some variants retained functioning similar to the wild type. In silico predictions of functional effects demonstrated a good correlation. Structural prediction of MUTYH based on the MutY protein structure allowed us to interpret effects on the protein stability or catalytic activity. These data will be useful for evaluating the functional consequences of missense MUTYH variants detected in patients with suspected MAP.

Functional Evaluation of Nine Missense-Type Variants of the Human DNA Glycosylase Enzyme MUTYH in the Japanese Population

Biallelic germline mutations of MUTYH, the gene encoding DNA glycosylase, cause MUTYH-associated polyposis (MAP), characterized by multiple colorectal adenomas and carcinoma(s). However, a considerable number of MUTYH variants are still functionally uncharacterized. Herein, we report the results of functional evaluation of nine missense-type MUTYH variant proteins in the Japanese population. The DNA glycosylase activity and ability to suppress mutations caused by 8-hydroxyguanine, an oxidized form of guanine, were examined for the nine variants of type 2 MUTYH, a nuclear form of the enzyme, by DNA cleavage activity assay and supF forward mutation assay, respectively. Both activities were severely defective in the p.N210S MUTYH type 2 variant corresponding to p.N238S in the reference MUTYH form and partially defective in p.R219G variant corresponding to p.R247G, but nearly fully retained in seven other variants examined. Our results suggest that p.N238S and p.R247G are likely to be pathogenic alleles for MAP.

Induction of action-at-a-distance mutagenesis by 8-oxo-7,8-dihydroguanine in DNA pol λ-knockdown cells

Introduction

In DNA, 8-oxo-7,8-dihydroguanine (G^O, 8-hydroxyguanine) is one of the most pivotal oxidatively damaged bases and induces G:C → T:A transversion mutations. DNA polymerase λ preferentially inserts dCTP opposite G^Oin vitro, and this error-free bypass function is considered to be important after A base removal from G^O:A pairs by the MUTYH DNA glycosylase. To examine the effects of reduced levels of DNA polymerase λ on the G^O-induced mutations, the polymerase was knocked-down in human U2OS cells, and a shuttle plasmid DNA containing a G^O:C pair at position 122 in the supF gene was transfected into the cells. The plasmid DNA replicated in the cells was introduced into an Escherichia coli indicator strain, to measure the supF mutant frequency.

Results

The knockdown of DNA polymerase λ significantly enhanced the mutant frequency of the G^O plasmid DNA. Contrary to our expectations, the knockdown did not promote the targeted G:C → T:A transversion. Instead, substitution mutations at G:C sites other than position 122 were enhanced in the cells.

Conclusions

These results suggested that the knockdown of DNA polymerase λ induced action-at-a-distance mutagenesis in human cells when the G^O:C pair was present in the DNA.

Association of AluYb8 insertion/deletion polymorphism in the MUTYH gene with mtDNA maintain in the type 2 diabetes mellitus patients

A common AluYb8-element insertion/deletion polymorphism of the MUTYH gene (AluYb8MUTYH) is a novel genetic risk factor for type 2 diabetes mellitus (T2DM). In the present study, mtDNA sequencing analysis indicated that the mtDNA sequence heteroplasmy was not associated with AluYb8MUTYH polymorphism. To better understand the genetic risk for T2DM, we investigated the association of this polymorphism with mtDNA content, mtDNA breakage and mtDNA transcription in the leukocytes of T2DM patients. The mtDNA content and unbroken mtDNA were significantly increased in the mutant patients than in the wild-type patients (P <0.05, respectively). However, no association between mtDNA transcription and AluYb8MUTYH variant was observed. The results suggested that the AluYb8MUTYH variant was associated with an altered mtDNA maintain in T2DM patients. The high level of mtDNA content observed in the mutant patients may have resulted from inefficient base excision repair of mitochondrial MUTYH and a compensatory mechanism that is triggered by elevated oxidative stress.

Mitochondrial DNA: Radically free of free-radical driven mutations

Mitochondrial DNA has long been posited as a likely target of oxidative damage induced mutation during the ageing process. Research over the past decades has uncovered the accumulation of mitochondrial DNA mutations in association with a mosaic pattern of cells displaying mitochondrial dysfunction in ageing individuals. Unfortunately, the underlying mechanisms are far less straightforward than originally anticipated. Recent research on mitochondria reveals that these genomes are far less helpless than originally envisioned. Additionally, new technologies have allowed us to analyze the mutational signatures of many more somatic mitochondrial DNA mutations, revealing surprising patterns that are inconsistent with a DNA-oxidative damage based hypothesis. In this review, we will discuss these recent observations and new insights into the eccentricities of mitochondrial genetics, and their impact on our understanding of mitochondrial mutations and their role in the ageing process. This article is part of a Special Issue entitled: Mitochondrial Dysfunction in Aging.

The genetic basis of familial adenomatous polyposis and its implications for clinical practice and risk management

Familial adenomatous polyposis (FAP) is an inherited disorder that represents the most common gastrointestinal polyposis syndrome. Germline mutations in the APC gene were initially identified as responsible for FAP, and later, several studies have also implicated the MUTYH gene as responsible for this disease, usually referred to as MUTYH-associated polyposis (MAP). FAP and MAP are characterized by the early onset of multiple adenomatous colorectal polyps, a high lifetime risk of colorectal cancer (CRC), and in some patients the development of extracolonic manifestations. The goal of colorectal management in these patients is to prevent CRC mortality through endoscopic and surgical approaches. Individuals with FAP and their relatives should receive appropriate genetic counseling and join surveillance programs when indicated. This review is focused on the description of the main clinical and genetic aspects of FAP associated with germline APC mutations and MAP.

Germline MUTYH gene mutations are not frequently found in unselected patients with papillary breast carcinoma

MUTYH-associated polyposis (MAP) is an autosomal recessive disease, which predisposes to polyposis and colorectal cancer. There is a trend towards an increased risk of breast cancer in MAP patients, with a remarkable proportion of papillary breast cancers. To determine whether MUTYH mutations are associated with this specific and rare type of breast cancer, 53 unselected patients with papillary breast cancer were analyzed for founder mutations in the MUTYH gene. No germline mutations were identified, indicating that biallelic MUTYH mutations are not a frequent underlying cause for the development of papillary carcinomas of the breast.

Oxidatively induced DNA damage and its repair in cancer

Oxidatively induced DNA damage is caused in living organisms by endogenous and exogenous reactive species. DNA lesions resulting from this type of damage are mutagenic and cytotoxic and, if not repaired, can cause genetic instability that may lead to disease processes including carcinogenesis. Living organisms possess DNA repair mechanisms that include a variety of pathways to repair multiple DNA lesions. Mutations and polymorphisms also occur in DNA repair genes adversely affecting DNA repair systems. Cancer tissues overexpress DNA repair proteins and thus develop greater DNA repair capacity than normal tissues. Increased DNA repair in tumors that removes DNA lesions before they become toxic is a major mechanism for development of resistance to therapy, affecting patient survival. Accumulated evidence suggests that DNA repair capacity may be a predictive biomarker for patient response to therapy. Thus, knowledge of DNA protein expressions in normal and cancerous tissues may help predict and guide development of treatments and yield the best therapeutic response. DNA repair proteins constitute targets for inhibitors to overcome the resistance of tumors to therapy. Inhibitors of DNA repair for combination therapy or as single agents for monotherapy may help selectively kill tumors, potentially leading to personalized therapy. Numerous inhibitors have been developed and are being tested in clinical trials. The efficacy of some inhibitors in therapy has been demonstrated in patients. Further development of inhibitors of DNA repair proteins is globally underway to help eradicate cancer.

MUTYH, an adenine DNA glycosylase, mediates p53 tumor suppression via PARP-dependent cell death

p53-regulated caspase-independent cell death has been implicated in suppression of tumorigenesis, however, the regulating mechanisms are poorly understood. We previously reported that 8-oxoguanine (8-oxoG) accumulation in nuclear DNA (nDNA) and mitochondrial DNA triggers two distinct caspase-independent cell death through buildup of single-strand DNA breaks by MutY homolog (MUTYH), an adenine DNA glycosylase. One pathway depends on poly-ADP-ribose polymerase (PARP) and the other depends on calpains. Deficiency of MUTYH causes MUTYH-associated familial adenomatous polyposis. MUTYH thereby suppresses tumorigenesis not only by avoiding mutagenesis, but also by inducing cell death. Here, we identified the functional p53-binding site in the human MUTYH gene and demonstrated that MUTYH is transcriptionally regulated by p53, especially in the p53/DNA mismatch repair enzyme, MLH1-proficient colorectal cancer-derived HCT116+Chr3 cells. MUTYH-small interfering RNA, an inhibitor for p53 or PARP suppressed cell death without an additive effect, thus revealing that MUTYH is a potential mediator of p53 tumor suppression, which is known to be upregulated by MLH1. Moreover, we found that the p53-proficient, mismatch repair protein, MLH1-proficient colorectal cancer cell line express substantial levels of MUTYH in nuclei but not in mitochondria, suggesting that 8-oxoG accumulation in nDNA triggers MLH1/PARP-dependent cell death. These results provide new insights on the molecular mechanism of tumorigenesis and potential new strategies for cancer therapies.

Visualization of the physical and functional interaction between hMYH and hRad9 by Dronpa bimolecular fluorescence complementation

BACKGROUND

Human MutY glycosylase homolog (hMYH), a component of the base excision repair pathway, is responsible for the generation of apurinic/apyrimidinic sites. Rad9-Rad1-Hus1 (9-1-1) is a heterotrimeric protein complex that plays a role in cell cycle checkpoint control and DNA repair. In humans, hMYH and 9-1-1 interact through Hus1 and to a lesser degree with Rad1 in the presence of DNA damage. In Saccharomyces pombe, each component of the 9-1-1 complex interacts directly with SpMYH. The glycosylase activity of hMYH is stimulated by Hus1 and the 9-1-1 complex and enhanced by DNA damage treatment. Cells respond to different stress conditions in different manners. Therefore, we investigated whether Rad9 interacted with hMYH under different stresses. Here, we identified and visualized the interaction between hRad9 and hMYH and investigated the functional consequences of this interaction.

RESULTS

Co-IP and BiFC indicates that hMYH interacts with hRad9. As shown by GST-pull down assay, this interaction is direct. Furthermore, BiFC with deletion mutants of hMYH showed that hRad9 interacts with N-terminal region of hMYH. The interaction was enhanced by hydroxyurea (HU) treatment. mRNA and protein levels of hMYH and hRad9 were increased following HU treatment. A marked increase in p-Chk1 (S345) and p-Cdk2 (T14, Y15) was observed. But this phosphorylation decreased in siMYH- or siRad9-transfected cells, and more pronounced decrease observed in co-transfected cells.

CONCLUSIONS

Our data reveal that hRad9 interacts directly with N-terminal region of hMYH. This interaction is enhanced by HU treatment. Knockdown of one or both protein result in decreasing Chk1 and Cdk2 phosphorylation. Since both protein functions in the early detection of DNA damage, we suggest that this interaction occurs early in DNA damage pathway.

Impaired 8-hydroxyguanine repair activity of MUTYH variant p.Arg109Trp found in a Japanese patient with early-onset colorectal cancer

PURPOSE

The biallelic inactivation of the 8-hydroxyguanine repair gene MUTYH leads to MUTYH-associated polyposis (MAP), which is characterized by colorectal multiple polyps and carcinoma(s). However, only limited information regarding MAP in the Japanese population is presently available. Since early-onset colorectal cancer (CRC) is a characteristic of MAP and might be caused by the inactivation of another 8-hydroxyguanine repair gene, OGG1, we investigated whether germline MUTYH and OGG1 mutations are involved in early-onset CRC in Japanese patients.

METHODS

Thirty-four Japanese patients with early-onset CRC were examined for germline MUTYH and OGG1 mutations using sequencing.

RESULTS

Biallelic pathogenic mutations were not found in any of the patients; however, a heterozygous p.Arg19∗  MUTYH variant and a heterozygous p.Arg109Trp MUTYH variant were detected in one patient each. The p.Arg19∗ and p.Arg109Trp corresponded to p.Arg5∗ and p.Arg81Trp, respectively, in the type 2 nuclear-form protein. The defective DNA repair activity of p.Arg5∗ is apparent, while that of p.Arg81Trp has been demonstrated using DNA cleavage and supF forward mutation assays.

CONCLUSION

These results suggest that biallelic MUTYH or OGG1 pathogenic mutations are rare in Japanese patients with early-onset CRC; however, the p.Arg19∗ and p.Arg109Trp MUTYH variants are associated with functional impairments.

Regulation of human MutYH DNA glycosylase by the E3 ubiquitin ligase mule

Oxidation of DNA is a frequent and constantly occurring event. One of the best characterized oxidative DNA lesions is 7,8-dihydro-8-oxoguanine (8-oxo-G). It instructs most DNA polymerases to preferentially insert an adenine (A) opposite 8-oxo-G instead of the appropriate cytosine (C) thus showing miscoding potential. The MutY DNA glycosylase homologue (MutYH) recognizes A:8-oxo-G mispairs and removes the mispaired A giving way to the canonical base excision repair that ultimately restores undamaged guanine (G). Here we characterize for the first time in detail a posttranslational modification of the human MutYH DNA glycosylase. We show that MutYH is ubiquitinated in vitro and in vivo by the E3 ligase Mule between amino acids 475 and 535. Mutation of five lysine residues in this region significantly stabilizes MutYH, suggesting that these are the target sites for ubiquitination. The endogenous MutYH protein levels depend on the amount of expressed Mule. Furthermore, MutYH and Mule physically interact. We found that a ubiquitination-deficient MutYH mutant shows enhanced binding to chromatin. The mutation frequency of the ovarian cancer cell line A2780, analyzed at the HPRT locus can be increased upon oxidative stress and depends on the MutYH levels that are regulated by Mule. This reflects the importance of tightly regulated MutYH levels in the cell. In summary our data show that ubiquitination is an important regulatory mechanism for the essential MutYH DNA glycosylase in human cells.

Mismatch repair deficient mice show susceptibility to oxidative stress-induced intestinal carcinogenesis

We have previously established an experimental system for oxidative DNA damage-induced tumorigenesis in the small intestine of mice. To elucidate the roles of mismatch repair genes in the tumor suppression, we performed oxidative DNA damage-induced tumorigenesis experiments using Msh2-deficient mice. Oral administration of 0.2% Potassium Bromate, KBrO₃, effectively induced epithelial tumors in the small intestines of Msh2-deficient mice. We observed a 22.5-fold increase in tumor formation in the small intestines of Msh2-deficient mice compared with the wild type mice. These results indicate that mismatch repair is involved in the suppression of oxidative stress-induced intestinal tumorigenesis in mice. A mutation analysis of the Ctnnb1 gene of the tumors revealed predominant occurrences of G:C to A:T transitions. The TUNEL analysis showed a decreased number of TUNEL-positive cells in the crypts of small intestines from the Msh2-deficient mice compared with the wild type mice after treatment of KBrO₃. These results suggest that the mismatch repair system may simultaneously function in both avoiding mutagenesis and inducing cell death to suppress the tumorigenesis induced by oxidative stress in the small intestine of mice.

Mammalian MutY homolog (MYH or MUTYH) protects cells from oxidative DNA damage

MutY DNA glycosylase homologs (MYH or MUTYH) reduce G:C to T:A mutations by removing misincorporated adenines or 2-hydroxyadenines paired with guanine or 8-oxo-7,8-dihydroguanine (8-oxo-G). Mutations in the human MYH (hMYH) gene are associated with the colorectal cancer predisposition syndrome MYH-associated polyposis. To examine the function of MYH in human cells, we regulated MYH gene expression by knockdown or overproduction. MYH knockdown human HeLa cells are more sensitive to the killing effects of H2O2 than the control cells. In addition, hMYH knockdown cells have altered cell morphology, display enhanced susceptibility to apoptosis, and have altered DNA signaling activation in response to oxidative stress. The cell cycle progression of hMYH knockdown cells is also different from that of the control cells following oxidative stress. Moreover, hMYH knockdown cells contain higher levels of 8-oxo-G lesions than the control cells following H2O2 treatment. Although MYH does not directly remove 8-oxo-G, MYH may generate favorable substrates for other repair enzymes. Overexpression of mouse Myh (mMyh) in human mismatch repair defective HCT15 cells makes the cells more resistant to killing and refractory to apoptosis by oxidative stress than the cells transfected with vector. In conclusion, MYH is a vital DNA repair enzyme that protects cells from oxidative DNA damage and is critical for a proper cellular response to DNA damage.

The polymorphic AluYb8 insertion in the MUTYH gene is associated with reduced type 1 protein expression and reduced mitochondrial DNA content

The human mutY homolog (MUTYH) participates in base excision repair (BER), which is critical for repairing oxidized DNA bases and maintaining DNA replication fidelity. The polymorphic AluYb8 insertion in the 15^th intron of the MUTYH gene (AluYb8MUTYH) has been shown to associate with an aggregated 8-hydroxy-2′-deoxyguanosine (8-OH-dG) lesion in genomic DNA and to serve as a risk factor for age-related diseases. In this work, we demonstrate that this variant is associated with a significant reduction of the type 1 MUTYH protein that localizes to mitochondria. Notably, this variant affects mitochondrial DNA (mtDNA) maintenance and functional mitochondrial mass in individuals homozygous for the AluYb8MUTYH variant. These findings provide evidence for an association between the AluYb8MUTYH variant and decreased mitochondrial homeostasis and, consequently, contribute to elucidating the roles of the AluYb8MUTYH variant in impairing the mitochondrial base excision repair (mtBER) system and increasing the risk of acquiring an age-related disease.

MUTYH-associated colorectal cancer and adenomatous polyposis

MUTYH-associated polyposis (MAP) was first described in 2002. MUTYH is a component of a base excision repair system that protects the genomic information from oxidative damage. When the MUTYH gene product is impaired by bi-allelic germline mutation, it leads to the mutation of cancer-related genes, such as the APC and/or the KRAS genes, via G to T transversion. MAP is a hereditary colorectal cancer syndrome inherited in an autosomal-recessive fashion. The clinical features of MAP include the presence of 10-100 adenomatous polyps in the colon, and early onset of colorectal cancer. Ethnic and geographical differences in the pattern of the MUTYH gene mutations have been suggested. In Caucasian patients, c.536A>G (Y179C) and c.1187G>A (G396D) mutations are frequently detected. In the Asian population, Y179C and G396D are uncommon, whereas other variants are suggested to be the major causes of MAP. We herein review the literature on MUTYH-associated colorectal cancer and adenomatous polyposis.

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.

Alkyladenine DNA glycosylase (AAG) localizes to mitochondria and interacts with mitochondrial single-stranded binding protein (mtSSB)

Due to a harsh environment mitochondrial genomes accumulate high levels of DNA damage, in particular oxidation, hydrolytic deamination, and alkylation adducts. While repair of alkylated bases in nuclear DNA has been explored in detail, much less is known about the repair of DNA alkylation damage in mitochondria. Alkyladenine DNA glycosylase (AAG) recognizes and removes numerous alkylated bases, but to date AAG has only been detected in the nucleus, even though mammalian mitochondria are known to repair DNA lesions that are specific substrates of AAG. Here we use immunofluorescence to show that AAG localizes to mitochondria, and we find that native AAG is present in purified human mitochondrial extracts, as well as that exposure to alkylating agent promotes AAG accumulation in the mitochondria. We identify mitochondrial single-stranded binding protein (mtSSB) as a novel interacting partner of AAG; interaction between mtSSB and AAG is direct and increases upon methyl methanesulfonate (MMS) treatment. The consequence of this interaction is specific inhibition of AAG glycosylase activity in the context of a single-stranded DNA (ssDNA), but not a double-stranded DNA (dsDNA) substrate. By inhibiting AAG-initiated processing of damaged bases, mtSSB potentially prevents formation of DNA breaks in ssDNA, ensuring that base removal primarily occurs in dsDNA. In summary, our findings suggest the existence of AAG-initiated BER in mitochondria and further support a role for mtSSB in DNA repair.

Impaired suppressive activities of human MUTYH variant proteins against oxidative mutagenesis

AIM: To investigate the suppressive activity of MUTYH variant proteins against mutations caused by oxidative lesion, 8-hydroxyguanine (8OHG), in human cells.

METHODS: p.R154H, p.M255V, p.L360P, and p.P377L MUTYH variants, which were previously found in patients with colorectal polyposis and cancer, were selected for use in this study. Human H1299 cancer cell lines inducibly expressing wild-type (WT) MUTYH (type 2) or one of the 4 above-mentioned MUTYH variants were established using the piggyBac transposon vector system, enabling the genomic integration of the transposon sequence for MUTYH expression. MUTYH expression was examined after cumate induction using Western blotting analysis and immunofluorescence analysis. The intracellular localization of MUTYH variants tagged with FLAG was also immunofluorescently examined. Next, the mutation frequency in the supF of the shuttle plasmid pMY189 containing a single 8OHG residue at position 159 of the supF was compared between empty vector cells and cells expressing WT MUTYH or one of the 4 MUTYH variants using a supF forward mutation assay.

RESULTS: The successful establishment of human cell lines inducibly expressing WT MUTYH or one of the 4 MUTYH variants was concluded based on the detection of MUTYH expression in these cell lines after treatment with cumate. All of the MUTYH variants and WT MUTYH were localized in the nucleus, and nuclear localization was also observed for FLAG-tagged MUTYH. The mutation frequency of supF was 2.2 × 10^-2 in the 8OHG-containing pMY189 plasmid and 2.5 × 10^-4 in WT pMY189 in empty vector cells, which was an 86-fold increase with the introduction of 8OHG. The mutation frequency (4.7 × 10^-3) of supF in the 8OHG-containing pMY189 plasmid in cells overexpressing WT MUTYH was significantly lower than in the empty vector cells (P < 0.01). However, the mutation frequencies of the supF in the 8OHG-containing pMY189 plasmid in cells overexpressing the p.R154H, p.M255V, p.L360P, or p.P377L MUTYH variant were 1.84 × 10^-2, 1.55 × 10^-2, 1.91 × 10^-2, and 1.96 × 10^-2, respectively, meaning that no significant difference was observed in the mutation frequency between the empty vector cells and cells overexpressing MUTYH mutants.

CONCLUSION: The suppressive activities of p.R154H, p.M255V, p.L360P, and p.P377L MUTYH variants against mutations caused by 8OHG are thought to be severely impaired in human cells.

Genetic variability in DNA repair proteins in age-related macular degeneration

The pathogenesis of age-related macular degeneration (AMD) is complex and involves interactions between environmental and genetic factors, with oxidative stress playing an important role inducing damage in biomolecules, including DNA. Therefore, genetic variability in the components of DNA repair systems may influence the ability of the cell to cope with oxidative stress and in this way contribute to the pathogenesis of AMD. However, few reports have been published on this subject so far. We demonstrated that the c.977C>G polymorphism (rs1052133) in the hOGG1 gene and the c.972G>C polymorphism (rs3219489) in the MUTYH gene, the products of which play important roles in the repair of oxidatively damaged DNA, might be associated with the risk of AMD. Oxidative stress may promote misincorporation of uracil into DNA, where it is targeted by several DNA glycosylases. We observed that the g.4235T>C (rs2337395) and c.–32A>G (rs3087404) polymorphisms in two genes encoding such glycosylases, UNG and SMUG1, respectively, could be associated with the occurrence of AMD. Polymorphisms in some other DNA repair genes, including XPD (ERCC2), XRCC1 and ERCC6 (CSB) have also been reported to be associated with AMD. These data confirm the importance of the cellular reaction to DNA damage, and this may be influenced by variability in DNA repair genes, in AMD pathogenesis.

Involvement of Werner syndrome protein in MUTYH-mediated repair of oxidative DNA damage

Reactive oxygen species constantly generated as by-products of cellular metabolism readily attack genomic DNA creating mutagenic lesions such as 7,8-dihydro-8-oxo-guanine (8-oxo-G) that promote aging. 8-oxo-G:A mispairs arising during DNA replication are eliminated by base excision repair initiated by the MutY DNA glycosylase homologue (MUTYH). Here, by using formaldehyde crosslinking in mammalian cell extracts, we demonstrate that the WRN helicase/exonuclease defective in the premature aging disorder Werner syndrome (WS) is recruited to DNA duplex containing an 8-oxo-G:A mispair in a manner dependent on DNA polymerase λ (Polλ) that catalyzes accurate DNA synthesis over 8-oxo-G. Similarly, by immunofluorescence, we show that Polλ is required for accumulation of WRN at sites of 8-oxo-G lesions in human cells. Moreover, we show that nuclear focus formation of WRN and Polλ induced by oxidative stress is dependent on ongoing DNA replication and on the presence of MUTYH. Cell viability assays reveal that depletion of MUTYH suppresses the hypersensitivity of cells lacking WRN and/or Polλ to oxidative stress. Biochemical studies demonstrate that WRN binds to the catalytic domain of Polλ and specifically stimulates DNA gap filling by Polλ over 8-oxo-G followed by strand displacement synthesis. Our results suggest that WRN promotes long-patch DNA repair synthesis by Polλ during MUTYH-initiated repair of 8-oxo-G:A mispairs.

Mitochondrial DNA damage and its consequences for mitochondrial gene expression

How mitochondria process DNA damage and whether a change in the steady-state level of mitochondrial DNA damage (mtDNA) contributes to mitochondrial dysfunction are questions that fuel burgeoning areas of research into aging and disease pathogenesis. Over the past decade, researchers have identified and measured various forms of endogenous and environmental mtDNA damage and have elucidated mtDNA repair pathways. Interestingly, mitochondria do not appear to contain the full range of DNA repair mechanisms that operate in the nucleus, although mtDNA contains types of damage that are targets of each nuclear DNA repair pathway. The reduced repair capacity may, in part, explain the high mutation frequency of the mitochondrial chromosome. Since mtDNA replication is dependent on transcription, mtDNA damage may alter mitochondrial gene expression at three levels: by causing DNA polymerase γ nucleotide incorporation errors leading to mutations, by interfering with the priming of mtDNA replication by the mitochondrial RNA polymerase, or by inducing transcriptional mutagenesis or premature transcript termination. This review summarizes our current knowledge of mtDNA damage, its repair, and its effects on mtDNA integrity and gene expression. This article is part of a special issue entitled: Mitochondrial Gene Expression.

A DNA oligomer containing 2,2,4-triamino-5(2H)-oxazolone is incised by human NEIL1 and NTH1

The nucleobase derivative, 2,2,4-triamino-5(2H)-oxazolone (Oz), is an oxidation product of guanine or of 8-oxo-7,8-dihydroguanine that causes G-to-C transversions in DNA. Human NEIL1 (hNEIL1) and NTH1 (hNTH1) are homologues of two prokaryotic base excision repair enzymes, FPG/NEI and NTH, respectively. Here, we demonstrated that hNEIL1 and hNTH1 cleave Oz sites as efficiently as 5-hydroxyuracil sites. Thus, hNEIL1 and hNTH1 can repair Oz lesions. Furthermore, the nicking activities of these enzymes are largely independent of nucleobases opposite Oz; this finding indicates that removing Oz from Oz:G and Oz:A base pairs might cause an increase in the rate of point mutations in human cells.