Cells with pathogenic biallelic mutations in the human MUTYH gene are defective in DNA damage binding and repair

Hereditary variants of unknown significance in African American women with breast cancer

Expanded implementation of genetic sequencing has precipitously increased the discovery of germline and somatic variants. The direct benefit of identifying variants in actionable genes may lead to risk reduction strategies such as increased surveillance, prophylactic surgery, as well as lifestyle modifications to reduce morbidity and mortality. However, patients with African ancestry are more likely to receive inconclusive genetic testing results due to an increased number of variants of unknown significance decreasing the utility and impact on disease management and prevention. This study examines whole exome sequencing results from germline DNA samples in African American women with a family history of cancer including 37 cases that were diagnosed with breast cancer and 51 family members. Self-identified ancestry was validated and compared to the 1000 genomes population. The analysis of sequencing results was limited to 85 genes from three clinically available common genetic screening platforms. This target region had a total of 993 variants of which 6 (<1%) were pathogenic or likely pathogenic, 736 (74.1%) were benign, and 170 (17.1%) were classified as a variant of unknown significance. There was an average of 3.4±1.8 variants with an unknown significance per individual and 85 of 88 individuals (96.6%) harbored at least one of these in the targeted genes. Pathogenic or likely pathogenic variants were only found in 6 individuals for the BRCA1 (p.R1726fs, rs80357867), BRCA2 (p.K589fs, rs397507606 & p.L2805fs, rs397507402), RAD50 (p.E995fs, rs587780154), ATM (p.V2424G, rs28904921), or MUTYH (p.G396D, rs36053993) genes. Strategies to functionally validate the remaining variants of unknown significance, especially in understudied and hereditary cancer populations, are greatly needed to increase the clinical utility and utilization of clinical genetic screening platforms to reduce cancer incidence and mortality.

Inherited MUTYH mutations cause elevated somatic mutation rates and distinctive mutational signatures in normal human cells

Cellular DNA damage caused by reactive oxygen species is repaired by the base excision repair (BER) pathway which includes the DNA glycosylase MUTYH. Inherited biallelic MUTYH mutations cause predisposition to colorectal adenomas and carcinoma. However, the mechanistic progression from germline MUTYH mutations to MUTYH-Associated Polyposis (MAP) is incompletely understood. Here, we sequence normal tissue DNAs from 10 individuals with MAP. Somatic base substitution mutation rates in intestinal epithelial cells were elevated 2 to 4-fold in all individuals, except for one showing a 31-fold increase, and were also increased in other tissues. The increased mutation burdens were of multiple mutational signatures characterised by C > A changes. Different mutation rates and signatures between individuals are likely due to different MUTYH mutations or additional inherited mutations in other BER pathway genes. The elevated base substitution rate in normal cells likely accounts for the predisposition to neoplasia in MAP. Despite ubiquitously elevated mutation rates, individuals with MAP do not display overt evidence of premature ageing. Thus, accumulation of somatic mutations may not be sufficient to cause the global organismal functional decline of ageing.

Monoallelic deleterious MUTYH germline variants as a driver for tumorigenesis

MUTYH encodes a glycosylase involved in the base excision repair of DNA. Biallelic pathogenic germline variants in MUTYH cause an autosomal recessive condition known as MUTYH-associated adenomatous polyposis and consequently increase the risk of colorectal cancer. However, reports of increased cancer risk in individuals carrying only one defective MUTYH allele are controversial and based on studies involving few individuals. Here, we describe a comprehensive investigation of monoallelic pathogenic MUTYH germline variants in 10,389 cancer patients across 33 different tumour types and 117,000 healthy individuals. Our results indicate that monoallelic pathogenic MUTYH germline variants can lead to tumorigenesis through a mechanism of somatic loss of heterozygosity of the functional MUTYH allele in the tumour. We confirmed that the frequency of monoallelic pathogenic MUTYH germline variants is higher in individuals with cancer than in the general population, although this frequency is not homogeneous among tumour types. We also demonstrated that the MUTYH mutational signature is present only in tumours with loss of the functional allele and found that the characteristic MUTYH base substitution (C>A) increases stop-codon generation. We identified key genes that are affected during tumorigenesis. In conclusion, we propose that carriers of the monoallelic pathogenic MUTYH germline variant are at a higher risk of developing tumours, especially those with frequent loss of heterozygosity events, such as adrenal adenocarcinoma, although the overall risk is still low. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Psychometric properties of the Emotional Quotient Inventory: Youth Version-EQ-i:YV in Spanish adolescents with Down syndrome

BACKGROUND

The people with Down syndrome who are now adolescents can face a self-report questionnaire with guarantees of being answered with validity to evaluate areas of development that traditionally have not been able to be evaluated. This is the case of emotional intelligence, measured in this research with the Emotional Quotient Inventory: Youth Version-EQ-i:YV.

AIMS

To validate and analyse the scale's psychometric properties in adolescents with Down syndrome.

METHODS

A two-stage cross-sectional investigation was conducted. The inventory consists of 60 items that measure 5 dimensions. The test was administered to 644 adolescents with Down syndrome. We carried out exploratory and confirmatory factor analyses.

OUTCOMES

The 5-factor structure of the test was confirmed. The internal consistency of four dimensions and the EQ-i:YV's total calculated score yielded high values.

CONCLUSIONS

This new version of the EQ-i:YV represents a valid and reliable tool to assess emotional intelligence in Spanish adolescents with Down syndrome.

Assessment of emotional intelligence in adults with down syndrome: Psychometric properties of the Emotional Quotient Inventory

Introduction

The Emotional Quotient Inventory: Youth version-EQ-i:YV was developed by Bar-On & Parker in 2000 and later translated and adapted for the general Spanish adolescent population by Ferrandiz et al. in 2012. The Spanish scale presents similar psychometric properties to the original version (54 items and five subdimensions). The Emotional Quotient Inventory assesses a set of personal, emotional, and social skills that influence adaptation to and coping with environmental demands and pressures. These factors can influence an adolescent’s success later in life, health, and psychological well-being. Traditionally, research in Down syndrome (DS) has focused on identifying cognitive deficits, relatively little is known about emotional intelligence (EI) and there are no scales that measure EI in people with DS adults.

Objectives

To validate and analyze the psychometric properties of the scale in the clinical population, specifically in Spanish adults with DS (EQ-i: SVDS).

Methods

A cross-sectional investigation was carried out in several stages. Descriptive, exploratory factorial (n = 345), confirmatory (n = 397), and scale reliability analyses were performed with better goodness-of-adjustment indices.

Results

A new scale named Emotional Quotient Inventory: Short Version for DS adults was obtained with a structure of four factors called mood, stress management, interpersonal, and intrapersonal. This new scale was reduced to 25 items. Goodness-of-fit indices were excellent (RMSEA [95% CI] = 02[.01; .03]; CFI = .99; TLI = .98; GFI = .87; AGFI = .89). The internal consistency of the four dimensions and the calculated total score (α = .91, ω = .93 and divided halves = .90) yielded high values in this clinical sample.

Discussion

The results recommend the use of the revised EQ-i: YV, the EQ-i: SVDS, to assess EI in adults with DS. The psychometric properties of this study are satisfactory but have four factors. The findings are discussed in terms of future research and practical implication to gain a more thorough understanding of how this population behaves on both a general and preventive level in order to teach EI properly.

Conclusions

This new version is a valid and reliable tool to evaluate emotional intelligence in people with intellectual disabilities and specifically in Spanish adults with DS.

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.

Single molecule glycosylase studies with engineered 8-oxoguanine DNA damage sites show functional defects of a MUTYH polyposis variant

Proper repair of oxidatively damaged DNA bases is essential to maintain genome stability. 8-Oxoguanine (7,8-dihydro-8-oxoguanine, 8-oxoG) is a dangerous DNA lesion because it can mispair with adenine (A) during replication resulting in guanine to thymine transversion mutations. MUTYH DNA glycosylase is responsible for recognizing and removing the adenine from 8-oxoG:adenine (8-oxoG:A) sites. Biallelic mutations in the MUTYH gene predispose individuals to MUTYH-associated polyposis (MAP), and the most commonly observed mutation in some MAP populations is Y165C. Tyr165 is a ‘wedge’ residue that intercalates into the DNA duplex in the lesion bound state. Here, we utilize single molecule fluorescence microscopy to visualize the real-time search behavior of Escherichia coli and Mus musculus MUTYH WT and wedge variant orthologs on DNA tightropes that contain 8-oxoG:A, 8-oxoG:cytosine, or apurinic product analog sites. We observe that MUTYH WT is able to efficiently find 8-oxoG:A damage and form highly stable bound complexes. In contrast, MUTYH Y150C shows decreased binding lifetimes on undamaged DNA and fails to form a stable lesion recognition complex at damage sites. These findings suggest that MUTYH does not rely upon the wedge residue for damage site recognition, but this residue stabilizes the lesion recognition complex.

DNA Repair Molecular Beacon assay: a platform for real-time functional analysis of cellular DNA repair capacity

Numerous studies have shown that select DNA repair enzyme activities impact response and/or toxicity of genotoxins, suggesting a requirement for enzyme functional analyses to bolster precision medicine or prevention. To address this need, we developed a DNA Repair Molecular Beacon (DRMB) platform that rapidly measures DNA repair enzyme activity in real-time. The DRMB assay is applicable for discovery of DNA repair enzyme inhibitors, for the quantification of enzyme rates and is sufficiently sensitive to differentiate cellular enzymatic activity that stems from variation in expression or effects of amino acid substitutions. We show activity measures of several different base excision repair (BER) enzymes, including proteins with tumor-identified point mutations, revealing lesion-, lesion-context- and cell-type-specific repair dependence; suggesting application for DNA repair capacity analysis of tumors. DRMB measurements using lysates from isogenic control and APE1-deficient human cells suggests the major mechanism of base lesion removal by most DNA glycosylases may be mono-functional base hydrolysis. In addition, development of a microbead-conjugated DRMB assay amenable to flow cytometric analysis further advances its application. Our studies establish an analytical platform capable of evaluating the enzyme activity of select DNA repair proteins in an effort to design and guide inhibitor development and precision cancer therapy options.

Common Genetic Variants of MUTYH are not Associated with Cutaneous Malignant Melanoma: Application of Molecular Screening by Means of High-Resolution Melting Technique in a Pilot Case-Control Study

MUTYH glycosylase recognizes the 8-oxoG:A mismatch and is able to excise the adenine base using proofreading mechanisms. Some papers have reported a strong association between cancer development or aggressiveness and MUTYH gene mutations. The aim of this study was to find a possible association between the most frequent MUTYH mutations and melanoma in the context of a case-control pilot study. One hundred ninety-five melanoma patients and 195 healthy controls were matched for sex and age. Clinical and laboratory data were collected in a specific database and all individuals were analyzed for MUTYH mutations by high-resolution melting and direct sequencing techniques. Men and women had significantly different distributions of tumor sites and phototypes. No significant associations were observed between the Y165C, G382D and V479F MUTYH mutations and risk of melanoma development or aggressiveness. Our preliminary findings therefore do not confirm a role for MUTYH gene mutations in the melanoma risk. Further studies are necessary for the assessment of MUTYH not only in melanoma but also other cancer types with the same embryonic origin, in the context of larger arrays studies of genes involved in DNA stability or integrity.

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.

Understanding the role of the Q338H MUTYH variant in oxidative damage repair

The MUTYH DNA–glycosylase is indirectly engaged in the repair of the miscoding 7,8-dihydro-8-oxo-2′-deoxyguanine (8-oxodG) lesion by removing adenine erroneously incorporated opposite the oxidized purine. Inherited biallelic mutations in the MUTYH gene are responsible for a recessive syndrome, the MUTYH-associated polyposis (MAP), which confers an increased risk of colorectal cancer. In this study, we functionally characterized the Q338H variant using recombinant proteins, as well as cell-based assays. This is a common variant among human colorectal cancer genes, which is generally considered, unrelated to the MAP phenotype but recently indicated as a low-penetrance allele. We demonstrate that the Q338H variant retains a wild-type DNA–glycosylase activity in vitro, but it shows a reduced ability to interact with the replication sensor RAD9:RAD1:HUS1 (9–1–1) complex. In comparison with Mutyh^−/− mouse embryo fibroblasts expressing a wild-type MUTYH cDNA, the expression of Q338H variant was associated with increased levels of DNA 8-oxodG, hypersensitivity to oxidant and accumulation of the population in the S phase of the cell cycle. Thus, an inefficient interaction of MUTYH with the 9–1–1 complex leads to a repair-defective phenotype, indicating that a proper communication between MUTYH enzymatic function and the S phase checkpoint is needed for effective repair of oxidative damage.

Role of MUTYH in human cancer

MUTYH, a human ortholog of MutY, is a post-replicative DNA glycosylase, highly conserved throughout evolution, involved in the correction of mismatches resulting from a faulty replication of the oxidized base 8-hydroxyguanine (8-oxodG). In particular removal of adenine from A:8-oxodG mispairs by MUTYH activity is followed by error-free base excision repair (BER) events, leading to the formation of C:8-oxodG base pairs. These are the substrate of another BER enzyme, the OGG1 DNA glycosylase, which then removes 8-oxodG from DNA. Thus the combined action of OGG1 and MUTYH prevents oxidative damage-induced mutations, i.e. GC->TA transversions. Germline mutations in MUTYH are associated with a recessively heritable colorectal polyposis, now referred to as MUTYH-associated polyposis (MAP). Here we will review the phenotype(s) associated with MUTYH inactivation from bacteria to mammals, the structure of the MUTYH protein, the molecular mechanisms of its enzymatic activity and the functional characterization of MUTYH variants. The relevance of these results will be discussed to define the role of specific human mutations in colorectal cancer risk together with the possible role of MUTYH inactivation in sporadic cancer.

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.

Loss of MUTYH function in human cells leads to accumulation of oxidative damage and genetic instability

The DNA glycosylase MUTYH (mutY homolog (Escherichia coli)) counteracts the mutagenic effects of 8-oxo-7,8-dihydroguanine (8-oxodG) by removing adenine (A) misincorporated opposite the oxidized purine. Biallelic germline mutations in MUTYH cause the autosomal recessive MUTYH-associated adenomatous polyposis (MAP). Here we designed new tools to investigate the biochemical defects and biological consequences associated with different MUTYH mutations in human cells. To identify phenotype(s) associated with MUTYH mutations, lymphoblastoid cell lines (LCLs) were derived from seven MAP patients harboring missense as well as truncating mutations in MUTYH. These included homozygous p.Arg245His, p.Gly264TrpfsX7 or compound heterozygous variants (p.Gly396Asp/Arg245Cys, p.Gly396Asp/Tyr179Cys, p.Gly396Asp/Glu410GlyfsX43, p.Gly264TrpfsX7/Ala385ProfsX23 and p.Gly264TrpfsX7/Glu480del). DNA glycosylase assays of MAP LCL extracts confirmed that all these variants were defective in removing A from an 8-oxoG:A DNA substrate, but retained wild-type OGG1 activity. As a consequence of this defect, MAP LCLs accumulated DNA 8-oxodG in their genome and exhibited a fourfold increase in spontaneous mutagenesis at the PIG-A gene compared with LCLs from healthy donors. They were also hypermutable by KBrO3--a source of DNA 8-oxodG--indicating that the relatively modest spontaneous mutator phenotype associated with MUTYH loss can be significantly enhanced by conditions of oxidative stress. These observations identify accumulation of DNA 8-oxodG and a mutator phenotype as likely contributors to the pathogenesis of MUTYH variants.

MUTYH c.933+3A>C, associated with a severely impaired gene expression, is the first Italian founder mutation in MUTYH-Associated Polyposis

MUTYH variants are differently distributed in geographical areas of the world. In MUTYH-associated polyposis (MAP) patients from North-Eastern Italy, c.933+3A>C (IVS10+3A>C), a transversion causing an aberrant splicing process, accounts for nearly 1/5 of all mutations. The aim of this study was to verify whether its high frequency in North-Eastern Italy is due to a founder effect and to clarify its impact on MUTYH transcripts and protein. Haplotype analysis and age estimate performed on members of eleven Italian MAP families and cancer-free controls provided evidence that c.933+3A>C is a founder mutation originated about 83 generations ago. In addition, the Italian haplotype associated with the c.933+3A>C was also found in German families segregating the same mutation, indicating it had a common origin in Western Europe. Altogether c.933+3A>C and the two common Caucasian mutations p.Tyr179Cys and p.Gly396Asp represent about 60% of MUTYH alterations in MAP patients from North-Eastern Italy, suggesting the opportunity to perform targeted molecular screening for these variants in the diagnostic setting. Expression analyses performed on lymphoblastoid cell lines supported the notion that MUTYH c.933+3A>C alters splicing causing the synthesis of a non functional protein. However, some primary transcripts escape aberrant splicing, producing traces of full-length transcript and wild-type protein in a homozygote; this is in agreement with clinical findings that suggest a relatively mild phenotypic effect for this mutation. Overall, these data, that demonstrate a founder effect and further elucidate the splicing alterations caused by the MUTYH c.933+3A>C mutation, have important implications for genetic counseling and molecular diagnosis of MAP.

DNA damage response pathways and cell cycle checkpoints in colorectal cancer: current concepts and future perspectives for targeted treatment

Although several drugs have been designed in the last few years to target specific key pathways and functions in colorectal cancer (CRC), the backbone of CRC treatment is still made up of compounds which rely on DNA damage to accomplish their role. DNA damage response (DDR) and checkpoint pathways are intertwined signaling networks that arrest cell cycle, recognize and repair genetic mistakes which arise during DNA replication and transcription, as well as through the exposure to chemical and physical agents that interact with nucleic acids. The good but highly variable activity of DNA damaging agents in the treatment of CRC suggests that intrinsic alterations in DDR pathways and cell cycle checkpoints may contribute differentially to the way cancer cells react to DNA damage. In the present review, our aim is to depict the recent advances in understanding the molecular basis of the activity of DNA damaging agents used for the treatment of CRC. We focus on the known and potential drug targets that are part of these complex and intertwined pathways. We describe the potential role of the checkpoints in CRC, and how their pharmacological manipulation could lead to chemopotentiation or synergism with currently used drugs. Novel therapeutic agents playing a role in DDR and checkpoint inhibition are assessed. We discuss the possible rationale for combining PARP inhibition with DNA damaging agents, and we address the link between DDR and EGFR pathways in CRC.

MUTYH-associated polyposis (MAP), the syndrome implicating base excision repair in inherited predisposition to colorectal tumors

In 2002, Al-Tassan and co-workers described for the first time a recessive form of inherited polyposis associated with germline mutations of MUTYH, a gene encoding a base excision repair (BER) protein that counteracts the DNA damage induced by the oxidative stress. MUTYH-associated polyposis (MAP) is now a well-defined cancer susceptibility syndrome, showing peculiar molecular features that characterize disease progression. However, some aspects of MAP, including diagnostic criteria, genotype-phenotype correlations, pathogenicity of variants, as well as relationships between BER and other DNA repair pathways, are still poorly understood. A deeper knowledge of the MUTYH expression pattern is likely to refine our understanding of the protein role and, finally, to improve guidances for identifying and handling MAP patients.

MUTYH Associated Polyposis (MAP)

MUTYH Associated Polyposis (MAP), a Polyposis predisposition caused by biallelic mutations in the Base Excision Repair (BER) gene MUTYH, confers a marked risk of colorectal cancer (CRC). The MAP phenotype is difficult to distinguish from other hereditary CRC syndromes. Especially from Familial Adenomatous Polyposis (FAP) and to a lesser extend Lynch Syndrome, which are caused by germline mutations in the APC and Mismatch Repair (MMR) genes, respectively.

Here we review research findings regarding MUTYH interactions, genotypic and phenotypic characteristics of MAP, as well as surveillance and treatment of the disease. The applied papers, published between 1/1 2002- 1/2 2008, were found through PubMed.

The exact role of MUTYH in CRC tumorgenesis is still uncertain, although MAP tumors show distinct molecular features, including somatic G:C>T:A transversions in the APC gene. Furthermore, cooperation between the BER and the MMR systems exists, as MUTYH interacts with MMR gene-products. Possibly, monoallelic defects in both pathways are of significance to CRC development.

Specific MUTYH variants are found to be characteristic in distinct ethnic populations, which could facilitate future genetic screening. Knowledge concerning functional consequences of many MUTYH germline mutations remains sparse. Most thoroughly investigated are the two most common MUTYH variants, Y179C and G396D, both generating dysfunctional gene products.

Phenotypic features of MAP include: development of 10-100 colorectal adenomas, debuting at 46-47 years, often CRC at time of clinical diagnosis, and in some, development of extracolonic manifestations.

Frequency of the common germline MUTYH mutations p.G396D and p.Y179C in patients diagnosed with colorectal cancer in Southern Brazil

INTRODUCTION

MUTYH-associated polyposis (MAP) is an autosomal recessive cancer predisposition syndrome associated with the development of colorectal tumors and colonic polyps at an early age. MAP syndrome is associated to germline biallelic mutations in the MUTYH gene which lead to deficient DNA repair through the base-excision repair system and accumulation of G:C→T:A transversions. Occurrence of such mutations in oncogenes and tumor suppressor genes drives colorectal carcinogenesis and is associated with the development of colonic polyps. Two common mutations, p.Y179C and p.G396D, are present in approximately 70-80% of MAP in European families with identified MUTYH germline mutations. The aim of this study was to assess the frequency of the germline MUTYH mutations p.Y179C and p.G396D in Brazilian patients with MAP and other hereditary colorectal cancer (CRC) phenotypes, as well as in sporadic CRC cases.

MATERIALS AND METHODS

A total of 75 patients were included. Samples were screened for the MUTYH germline mutations p.Y179C and p.G396D by allelic discrimination assays using allele-specific TaqMan® probes. In all mutation-positive cases, results were confirmed by sequencing.

RESULTS AND CONCLUSIONS

Biallelic germline MUTYH mutations were identified in 4 of 60 (6.6%) patients with a phenotype of hereditary colorectal cancer. Germline MUTYH mutation screening should be considered in the differential diagnosis of hereditary colorectal syndromes, and not only in MAP, but also in familial adenomatous polyposis and Bethesda criteria-positive families. Additional mutation screening studies of the MUTYH gene in a larger number of Brazilian patients will be necessary to confirm these results and determine the validity and applicability of MUTYH mutation screening in our population.

Base excision repair and lesion-dependent subpathways for repair of oxidative DNA damage

Nuclear and mitochondrial genomes are under continuous assault by a combination of environmentally and endogenously derived reactive oxygen species, inducing the formation and accumulation of mutagenic, toxic, and/or genome-destabilizing DNA lesions. Failure to resolve these lesions through one or more DNA-repair processes is associated with genome instability, mitochondrial dysfunction, neurodegeneration, inflammation, aging, and cancer, emphasizing the importance of characterizing the pathways and proteins involved in the repair of oxidative DNA damage. This review focuses on the repair of oxidative damage-induced lesions in nuclear and mitochondrial DNA mediated by the base excision repair (BER) pathway in mammalian cells. We discuss the multiple BER subpathways that are initiated by one of 11 different DNA glycosylases of three subtypes: (a) bifunctional with an associated β-lyase activity; (b) monofunctional; and (c) bifunctional with an associated β,δ-lyase activity. These three subtypes of DNA glycosylases all initiate BER but yield different chemical intermediates and hence different BER complexes to complete repair. Additionally, we briefly summarize alternate repair events mediated by BER proteins and the role of BER in the repair of mitochondrial DNA damage induced by ROS. Finally, we discuss the relation of BER and oxidative DNA damage in the onset of human disease.

A large-scale meta-analysis to refine colorectal cancer risk estimates associated with MUTYH variants

Background:

Defective DNA repair has a causal role in hereditary colorectal cancer (CRC). Defects in the base excision repair gene MUTYH are responsible for MUTYH-associated polyposis and CRC predisposition as an autosomal recessive trait. Numerous reports have suggested MUTYH mono-allelic variants to be low penetrance risk alleles. We report a large collaborative meta-analysis to assess and refine CRC risk estimates associated with bi-allelic and mono-allelic MUTYH variants and investigate age and sex influence on risk.

Methods:

MUTYH genotype data were included from 20 565 cases and 15 524 controls. Three logistic regression models were tested: a crude model; adjusted for age and sex; adjusted for age, sex and study.

Results:

All three models produced very similar results. MUTYH bi-allelic carriers demonstrated a 28-fold increase in risk (95% confidence interval (CI): 6.95–115). Significant bi-allelic effects were also observed for G396D and Y179C/G396D compound heterozygotes and a marginal mono-allelic effect for variant Y179C (odds ratio (OR)=1.34; 95% CI: 1.00–1.80). A pooled meta-analysis of all published and unpublished datasets submitted showed bi-allelic effects for MUTYH, G396D and Y179C (OR=10.8, 95% CI: 5.02–23.2; OR=6.47, 95% CI: 2.33–18.0; OR=3.35, 95% CI: 1.14–9.89) and marginal mono-allelic effect for variants MUTYH (OR=1.16, 95% CI: 1.00–1.34) and Y179C alone (OR=1.34, 95% CI: 1.01–1.77).

Conclusions:

Overall, this large study refines estimates of disease risk associated with mono-allelic and bi-allelic MUTYH carriers.

Ser 524 is a phosphorylation site in MUTYH and Ser 524 mutations alter 8-oxoguanine (OG): a mismatch recognition

MUTYH-associated polyposis (MAP) is a colorectal cancer predisposition syndrome that is caused by inherited biallelic mutations in the base excision repair (BER) gene, MUTYH. MUTYH is a DNA glycosylase that removes adenine (A) misinserted opposite 8-oxo-7,8-dihydro-2'-deoxyguanosine (OG). In this work, wild type (WT) MUTYH overexpressed using a baculovirus-driven insect cell expression system (BEVS) provided significantly higher levels of enzyme compared to bacterial overexpression. The isolated MUTYH enzyme was analyzed for potential post-translational modifications using mass spectrometry. An in vivo phosphorylation site was validated at Serine 524, which is located in the C-terminal OG recognition domain within the proliferating cell nuclear antigen (PCNA) binding region. Characterization of the phosphomimetic (S524D) and phosphoablating (S524A) mutants together with the observation that Ser 524 can be phosphorylated suggest that this residue may play an important regulatory role in vivo by altering stability and OG:A mismatch affinity.

MUTYH-associated polyposis (MAP)

The human mutY homologue (MUTYH) gene is responsible for inheritable polyposis and colorectal cancer. This review discusses the molecular genetic aspects of the MUTYH gene and protein, the clinical impact of mono- and biallelic MUTYH mutations and histological aspects of the MUTYH tumors. Furthermore, the relationship between MUTYH and the mismatch repair genes in colorectal cancer (CRC) families is examined. Finally, the role of other base excision repair genes in polyposis and CRC patients is discussed.

MUTYH mutations associated with familial adenomatous polyposis: functional characterization by a mammalian cell-based assay

MUTYH-associated polyposis (MAP) is a colorectal cancer syndrome, due to biallelic mutations of MUTYH. This Base Excision Repair gene encodes for a DNA glycosylase that specifically mitigates the high mutagenic potential of the 8-hydroxyguanine (8-oxodG) along the DNA. Aim of this study was to characterize the biological effects, in a mammalian cell background, of human MUTYH mutations identified in MAP patients (137insIW [c.411_416dupATGGAT; p.137insIleTrp]; R171W [c.511C>T; p.Arg171Trp]; E466del [c.1395_1397delGGA; p.Glu466del]; Y165C [c.494A>G; p.Tyr165Cys]; and G382D [c.1145G>A; p.Gly382Asp]). We set up a novel assay in which the human proteins were expressed in Mutyh(-/-) mouse defective cells. Several parameters, including accumulation of 8-oxodG in the genome and hypersensitivity to oxidative stress, were then used to evaluate the consequences of MUTYH expression. Human proteins were also obtained from Escherichia coli and their glycosylase activity was tested in vitro. The cell-based analysis demonstrated that all MUTYH variants we investigated were dysfunctional in Base Excision Repair. In vitro data complemented the in vivo observations, with the exception of the G382D mutant, which showed a glycosylase activity very similar to the wild-type protein. Our cell-based assay can provide useful information on the significance of MUTYH variants, improving molecular diagnosis and genetic counseling in families with mutations of uncertain pathogenicity.

Adenine removal activity and bacterial complementation with the human MutY homologue (MUTYH) and Y165C, G382D, P391L and Q324R variants associated with colorectal cancer

MUTYH-associated polyposis (MAP) is the only inherited colorectal cancer syndrome that is associated with inherited biallelic mutations in a base excision repair gene. The MUTYH glycosylase plays an important role in preventing mutations associated with 8-oxoguanine (OG) by removing adenine residues that have been misincorporated opposite OG. MAP-associated mutations are present throughout MUTYH, with a large number coding for missense variations. To date the available information on the functional properties of MUTYH variants is conflicting. In this study, a kinetic analysis of the adenine glycosylase activity of MUTYH and several variants was undertaken using a correction for active fraction to control for differences due to overexpression and purification. Using these methods, the rate constants for steps involved in the adenine removal process were determined for the MAP variants Y165C, G382D, P391L and Q324R MUTYH. Under single-turnover conditions, the rate of adenine removal for these four variants was found to be 30-40% of WT MUTYH. In addition, the ability of MUTYH and the variants to suppress mutations and complement for the absence of MutY in Escherichia coli was assessed using rifampicin resistance assays. The presence of WT and Q324R MUTYH resulted in complete suppression of the mutation frequency, while G382D MUTYH showed reduced ability to suppress the mutation frequency. In contrast, the mutation frequency observed upon expression of P391L and Y165C MUTYH were similar to the controls, suggesting no activity toward preventing DNA mutations. Notably, though all variations studied herein resulted in similar reductions in adenine glycosylase activity, the effects in the bacterial complementation are quite different. This suggests that the consequences of a specific amino acid variation on overall repair in a cellular context may be magnified.

MUTYH-associated polyposis and colorectal cancer

This article reviews the role of defective base excision repair, and MUTYH specifically, in colorectal cancer etiology and discusses the consequences of MUTYH gene defects, with particular emphasis on clinical relevance to colorectal polyposis, colorectal cancer risk, and appraising the risk of extra-colonic malignancy. Evidence guiding clinical practice, in terms of surveillance recommendations and options for surgical and other prophylactic interventions, is reviewed.

Clinical implications of the colorectal cancer risk associated with MUTYH mutation

PURPOSE

Biallelic mutations in the base excision DNA repair gene MUTYH predispose to colorectal cancer (CRC). Evidence that monoallelic mutations also confer an elevated CRC risk is controversial. Precise quantification of the CRC risk and the phenotype associated with MUTYH mutations is relevant to the counseling, surveillance, and clinical management of at-risk individuals.

METHODS

We analyzed a population-based series of 9,268 patients with CRC and 5,064 controls for the Y179C and G396D MUTYH mutations. We related genotypes to phenotype and calculated genotype-specific CRC risks.

RESULTS

Overall, biallelic mutation status conferred a 28-fold increase in CRC risk (95% CI,17.66 to 44.06); this accounted for 0.3% of CRCs in the cohort. Genotype relative risks of CRC were strongly age dependent, but penetrance was incomplete at age 60 years. CRC that developed in the context of biallelic mutations were microsatellite stable. Biallelic mutation carriers were more likely to have proximal CRC (P = 4.0 x 10(-4)) and synchronous polyps (P = 5.7 x 10(-9)) than noncarriers. The performance characteristics of clinicopathologic criteria for the identification of biallelic mutations are poor. Monoallelic mutation was not associated with an increased CRC risk (odds ratio, 1.07; 95% CI, 0.87 to 1.31).

CONCLUSION

The high risk and the propensity for proximal disease associated with biallielic MUTYH mutation justify colonoscopic surveillance. Although mutation screening should be directed to patients with APC-negative polyposis and early-onset proximal MSS CRC in whom detection rates will be highest, the expanded phenotype associated with MUTYH mutation needs to be recognized. There is no evidence than monoallelic mutation status per se is clinically important.

Germline MutY human homologue mutations and colorectal cancer: a multisite case-control study

BACKGROUND & AIMS

The MutY human homologue (MYH) gene is a member of the base-excision repair pathway involved in the repair of oxidative DNA damage. The objective of this study was to determine colorectal cancer (CRC) risk associated with mutations in the MYH gene.

METHODS

A total of 3811 CRC cases and 2802 controls collected from a multisite CRC registry were screened for 9 germline MYH mutations; subjects with any mutation underwent screening of the entire MYH gene. Logistic regression was used to estimate age- and sex-adjusted odds ratios (AOR). Clinicopathologic and epidemiologic data were reviewed to describe the phenotype associated with MYH mutation status and assess for potential confounding and effect modification.

RESULTS

Twenty-seven cases and 1 control subject carried homozygous or compound heterozygous MYH mutations (AOR, 18.1; 95% confidence interval, 2.5-132.7). CRC cases with homozygous/compound heterozygous mutations were younger at diagnosis (P=.01), had a higher proportion of right-sided (P=.01), synchronous cancers (P<.01), and personal history of adenomatous polyps (P=.003). Heterozygous MYH mutations were identified in 87 CRC cases and 43 controls; carriers were at increased risk of CRC (AOR, 1.48; 95% confidence interval, 1.02-2.16). There was a higher prevalence of low-frequency microsatellite instability (MSI) in tumors from heterozygous and homozygous/compound heterozygous MYH mutation carriers (P=.02); MSI status modified the CRC risk associated with heterozygous MYH mutations (P interaction<.001).

CONCLUSIONS

Homozygous/compound heterozygous MYH mutations account for less than 1% of CRC cases. Heterozygous carriers are at increased risk of CRC. Further studies are needed to understand the possible interaction between the base excision repair and low-frequency MSI pathways.

MUTYH-associated polyposis

MUTYH-associated polyposis (MAP) is an autosomal recessive disorder characterised by adenomatous polyps of the colorectum and a very high risk of colorectal cancer. It appears to be at least as prevalent as autosomal dominant familial adenomatous polyposis (that is caused by truncating mutations in the APC gene) with which it shares important gastroenterological features. It was first recognised as recently as 2002 and its full phenotype and natural history are still being characterised. Key extracolonic manifestations include a predisposition to duodenal adenomas and cancer and a modest increase in risk for several extraintestinal tumours. Testing for mutations in the MUTYH gene is indicated in patients who have multiple colorectal adenomas or a family history suggestive of autosomal recessive colorectal cancer and for the siblings and spouses of patients with MAP in order to inform surveillance and treatment for patients and their families.

Analysis of MUTYH genotypes and colorectal phenotypes in patients With MUTYH-associated polyposis

BACKGROUND & AIMS

Biallelic mutations in the base excision DNA repair gene MUTYH lead to MUTYH-associated polyposis (MAP) and predisposition to colorectal cancer (CRC). Functional studies have demonstrated significant differences in base recognition and glycosylase activity between various MUTYH mutations, notably for the 2 mutations most frequently reported in MAP patients: Y179C and G396D (previously annotated as Y165C and G382D). Our goal was to establish correlations between genotypes and colorectal phenotype of patients with MAP.

METHODS

In this multicenter study, we analyzed genotype and phenotype data from 257 MAP patients. Data included age at presentation of MAP, polyp count, and the occurrence, location, and age at presentation of CRC.

RESULTS

Patients with a homozygous G396D mutation or compound heterozygous G396D/Y179C mutations presented later with MAP and had a significantly lower hazard of developing CRC than patients with a homozygous Y179C mutation (P < .001). The mean ages of CRC diagnosis in patients were 58 years (homozygous G396D) and 52 years (compound heterozygous G396D/Y179C) versus 46 years (homozygous Y179C; P = .001, linear regression).

CONCLUSIONS

Our study identified the phenotypic effects of Y179C as relatively severe and of G396D as relatively mild. These clinical data are in accord with findings from in vitro functional assays. Genotypic stratification may become useful in the development of guidelines for counseling, surveillance, and management of families with MAP.

Characterization of mutant MUTYH proteins associated with familial colorectal cancer

BACKGROUND & AIMS

The human mutyh gene encodes a base excision repair protein that prevents G:C to T:A transversions in DNA. Biallelic mutations in this gene are associated with recessively inherited familial colorectal cancer. The aim of this study was to characterize the functional activity of mutant-MUTYH and single-nucleotide polymorphism (SNP)-MUTYH proteins involving familial colorectal cancer.

METHODS

MUTYH variants were cloned and assayed for their glycosylase and DNA binding activities using synthetic double-stranded oligonucleotide substrates by analyzing cleavage products by polyacrylamide gel electrophoresis.

RESULTS

In this study, we have characterized 9 missense/frameshift mutants and 2 SNPs for their DNA binding and repair activity in vitro. Two missense mutants (R260Q and G382D) were found to be partially active in both glycosylase and DNA binding, whereas 3 other missense mutants (Y165C, R231H, and P281L) were severely defective in both activities. All of the frameshift mutants (Y90X, Q377X, E466X, and 1103delC) were completely devoid of both glycosylase and DNA binding activities. One SNP (V22M) showed the same activity as wild-type MUTYH protein, but the other SNP (Q324H) was partially impaired in adenine removal.

CONCLUSIONS

This study of MUTYH mutants suggests that certain SNPs may be as partially dysfunctional in base excision repair as missense-MUTYH mutants and lead to colorectal carcinogenesis.