Genetic instability in lymphoblastoid cell lines expressing biallelic and monoallelic variants in the human MUTYH gene

The recent possible strategies for breeding ultraviolet-B-resistant crops

The sensitivity of crops to ultraviolet B (UVB, 280-315 nm) radiation varies significantly. Plants' sensitivity to UVB is heavily influenced by the activity of the enzyme cyclobutane pyrimidine dimer (CPD) photolyase, which fixes UVB-induced CPDs. Crops grown in tropical areas with high level of UVB radiation, like O. glaberrima from Africa and O. sativa ssp. indica rice from Bengal, are more sensitive to UVB radiation and could suffer more as a result of rising UVB levels on the earth's surface. Therefore, creating crops that can withstand high UVB is crucial in tropical regions. There is, however, little information on current techniques for breeding UVB-resistant plants. The most recent techniques for producing UVB-resistant crops are presented in this review. The use of DNA methylation, boosting the antioxidant system, regulating the expression of micro-RNA396, and overexpressing CPD photolyase in transgenic plants are some of the methods that are discussed. CPD photolyase overexpression in transgenic plants is the most popular technique for producing UVB-resistant rice. The study also offers several strategies for creating UVB-resistant plants using gene editing techniques. To feed the world's rapidly expanding population, researchers can use the information from this study to improve food production.

Oxidative stress accelerates intestinal tumorigenesis by enhancing 8-oxoguanine-mediated mutagenesis in MUTYH-deficient mice

Oxidative stress-induced DNA damage and its repair systems are related to cancer etiology; however, the molecular basis triggering tumorigenesis is not well understood. Here, we aimed to explore the causal relationship between oxidative stress, somatic mutations in pre-tumor-initiated normal tissues, and tumor incidence in the small intestines of MUTYH-proficient and MUTYH-deficient mice. MUTYH is a base excision repair enzyme associated with human colorectal cancer. Mice were administered different concentrations of potassium bromate (KBrO3; an oxidizing agent)-containing water for 4 wk for mutagenesis studies or 16 wk for tumorigenesis studies. All Mutyh -/- mice treated with >0.1% KBrO3 developed multiple tumors, and the average tumor number increased dose dependently. Somatic mutation analysis of Mutyh -/-/rpsL transgenic mice revealed that G:C  > T:A transversion was the only mutation type correlated positively with KBrO3 dose and tumor incidence. These mutations preferentially occurred at 5'G in GG and GAA sequences in rpsL This characteristic mutation pattern was also observed in the genomic region of Mutyh -/- tumors using whole-exome sequencing. It closely corresponded to signature 18 and SBS36, typically caused by 8-oxo-guanine (8-oxoG). 8-oxoG-induced mutations were sequence context dependent, yielding a biased amino acid change leading to missense and stop-gain mutations. These mutations frequently occurred in critical amino acid codons of known cancer drivers, Apc or Ctnnb1, known for activating Wnt signal pathway. Our results indicate that oxidative stress contributes to increased tumor incidence by elevating the likelihood of gaining driver mutations by increasing 8-oxoG-mediated mutagenesis, particularly under MUTYH-deficient conditions.

Germline MUTYH mutations and high-grade gliomas: novel evidence for a potential association

There is growing body of evidence supporting the role of germline mutations in the pathogenesis of pediatric central nervous system (CNS) tumors, and the widespread use of next‐generation sequencing (NGS) panels facilitates their detection. Variants of the MUTYH gene are increasingly recognized as suspected germline background of various extraintestinal malignancies, besides their well‐characterized role in the polyposis syndrome associated with biallelic mutations. Using a multigene NGS panel (Illumina TruSight Oncology 500), we detected one H3 G34V‐ and one H3 K27M‐mutant pediatric high‐grade diffuse glioma, in association with c.1178G>A (p.G393D) and c.916C>T (p.R306C) MUTYH variants, respectively. Both MUTYH mutations were germline, heterozygous and inherited, according to the subsequent genetic testing of the patients and their first‐degree relatives. In the H3 K27M‐mutant glioma, amplifications affecting the 4q12 region were also detected, in association with KDR‐PDGFRA, KIT‐PDGFRA, and KDR‐CHIC2 fusions, previously unreported in this entity. Among 47 other CNS tumors of various histological types tested with the same NGS panel in our institution, only one adult glioblastoma harbored MUTYH mutation. Together with a single previous report, our data raises the possibility of an association between germline MUTYH mutations and CNS malignancies, particularly in pediatric histone H3‐mutant gliomas.

Multiple neoplasia in a patient with Gitelman syndrome harboring germline monoallelic MUTYH mutation

Gitelman syndrome is a rare, recessively inherited disease characterized by chronic hypokalemia and hypomagnesemia as a result of defective electrolyte co-transport at the level of the distal convoluted tubule of the kidney. Here, we present the first report of a patient with Gitelman syndrome who developed multiple neoplasia including colorectal polyposis, synchronous colorectal cancers, recurrent breast fibroadenomata and a desmoid tumor. Whole-exome sequencing confirmed germline compound heterozygous mutations of c.179C > T and c.1326C > G in SLC12A3, and in addition, identified a monoallelic germline c.934-2A > G splice site mutation in MUTYH. In vitro, magnesium deficiency potentiated oxidative DNA damage in lymphoblastoid cell lines derived from the same patient. We postulate that monoallelic MUTYH mutations may manifest in the presence of cooperative non-genetic mechanisms, in this case possibly magnesium deficiency from Gitelman syndrome.

Repair of UV-Induced DNA Damage Independent of Nucleotide Excision Repair Is Masked by MUTYH

DNA lesions caused by UV damage are thought to be repaired solely by the nucleotide excision repair (NER) pathway in human cells. Patients carrying mutations within genes functioning in this pathway display a range of pathologies, including an increased susceptibility to cancer, premature aging, and neurological defects. There are currently no curative therapies available. Here we performed a high-throughput chemical screen for agents that could alleviate the cellular sensitivity of NER-deficient cells to UV-induced DNA damage. This led to the identification of the clinically approved anti-diabetic drug acetohexamide, which promoted clearance of UV-induced DNA damage without the accumulation of chromosomal aberrations, hence promoting cellular survival. Acetohexamide exerted this protective function by antagonizing expression of the DNA glycosylase, MUTYH. Together, our data reveal the existence of an NER-independent mechanism to remove UV-induced DNA damage and prevent cell death.

The Pig-a Gene Mutation Assay in Mice and Human Cells: A Review

This MiniReview describes the principle of mutation assays based on the endogenous Pig-a gene and summarizes results for two species of toxicological interest, mice and human beings. The work summarized here largely avoids rat-based studies, as are summarized elsewhere. The Pig-a gene mutation assay has emerged as a valuable tool for quantifying in vivo and in vitro mutational events. The Pig-a locus is located at the X-chromosome, giving the advantage that one inactivated allele can give rise to a mutated phenotype, detectable by multicolour flow cytometry. For in vivo studies, only minute blood volumes are required, making it easily incorporated into ongoing studies or experiments with limited biological materials. Low blood volumes also allow individuals to serve as their own controls, providing temporal information of the mutagenic process, and/or outcome of intervention. These characteristics make it a promising exposure marker. To date, the Pig-a gene mutation assay has been most commonly performed in rats, while reports regarding its usefulness in other species are accumulating. Besides its applicability to in vivo studies, it holds promise for genotoxicity testing using cultured cells, as shown in recent studies. In addition to safety assessment roles, it is becoming a valuable tool in basic research to identify mutagenic effects of different interventions or to understand implications of various gene defects by investigating modified mouse models or cell systems. Human blood-based assays are also being developed that may be able to identify genotoxic environmental exposures, treatment- and lifestyle-related factors or endogenous host factors that contribute to mutagenesis.

A Specific Mutational Signature Associated with DNA 8-Oxoguanine Persistence in MUTYH-defective Colorectal Cancer

8-Oxoguanine, a common mutagenic DNA lesion, generates G:C>T:A transversions via mispairing with adenine during DNA replication. When operating normally, the MUTYH DNA glycosylase prevents 8-oxoguanine-related mutagenesis by excising the incorporated adenine. Biallelic MUTYH mutations impair this enzymatic function and are associated with colorectal cancer (CRC) in MUTYH-Associated Polyposis (MAP) syndrome. Here, we perform whole-exome sequencing that reveals a modest mutator phenotype in MAP CRCs compared to sporadic CRC stem cell lines or bulk tumours. The excess G:C>T:A transversion mutations in MAP CRCs exhibits a novel mutational signature, termed Signature 36, with a strong sequence dependence. The MUTYH mutational signature reflecting persistent 8-oxoG:A mismatches occurs frequently in the APC, KRAS, PIK3CA, FAT4, TP53, FAT1, AMER1, KDM6A, SMAD4 and SMAD2 genes that are associated with CRC. The occurrence of Signature 36 in other types of human cancer indicates that DNA 8-oxoguanine-related mutations might contribute to the development of cancer in other organs.

Type and frequency of MUTYH variants in Italian patients with suspected MAP: a retrospective multicenter study

To determine prevalence, spectrum and genotype-phenotype correlations of MUTYH variants in Italian patients with suspected MAP (MUTYH-associated polyposis), a retrospective analysis was conducted to identify patients who had undergone MUTYH genetic testing from September 2002 to February 2014. Results of genetic testing and patient clinical characteristics were collected (gender, number of polyps, age at polyp diagnosis, presence of colorectal cancer (CRC) and/or other cancers, family data). The presence of large rearrangements of the MUTYH gene was evaluated by Multiplex Ligation-dependent Probe Amplification analysis. In all, 299 patients with colorectal neoplasia were evaluated: 61.2% were males, the median age at polyps or cancer diagnosis was 50 years (16-80 years), 65.2% had <100 polyps and 51.8% had CRC. A total of 36 different MUTYH variants were identified: 13 (36.1%) were classified as pathogenetic, whereas 23 (63.9%) were variants of unknown significance (VUS). Two pathogenetic variants were observed in 78 patients (26.1%). A large homozygous deletion of exon 15 was found in one patient (<1.0%). MAP patients were younger than those with negative MUTYH testing at polyps diagnosis (P<0.0001) and at first cancer diagnosis (P=0.007). MAP patients carrying the p.Glu480del variant presented with a younger age at polyp diagnosis as compared to patients carrying p.Gly396Asp and p.Tyr179Cys variants. A high heterogeneity of MUTYH variants and a high rate of VUS were identified in a cohort of Italian patients with suspected MAP. Genotype-phenotype analysis suggests that the p.Glu480del variant is associated with a severe phenotype.

The MUTYH base excision repair gene protects against inflammation-associated colorectal carcinogenesis

MUTYH DNA glycosylase removes mismatched adenine opposite 7, 8-dihydro-8-oxoguanine (8-oxoG), which is the major mutagenic lesion induced by oxidative stress. Biallelic mutations in MUTYH are associated with MUTYH-Associated polyposis (MAP) and increased risk in colorectal cancer (CRC). We investigated cancer susceptibility associated with MUTYH inactivation in a mouse model of inflammation-dependent carcinogenesis induced by azoxymethane (AOM) and dextran sulphate (DSS). Mutyh^−/− mice were more sensitive than wild-type (WT) animals to AOM/DSS toxicity and accumulated DNA 8-oxoG in their gastrointestinal tract. AOM/DSS-induced colonic adenomas were significantly more numerous in Mutyh^−/− than in WT animals, and frequently showed a tubulo-villous feature along with high-grade dysplasia and larger size lesions. This condition resulted in a greater propensity to develop adenocarcinomas. The colon of untreated Mutyh^−/− mice expressed higher basal levels of pro-inflammatory cytokines GM-CSF and IFNγ, and treatment with AOM/DSS induced an early decrease in circulating CD4+ and CD8+ T lymphocytes and an increase in myeloid-derived suppressor cells (MDSCs). Adenomas from Mutyh^−/− mice had a greater infiltrate of Foxp3+ T regulatory cells, granulocytes, macrophages, MDSCs and strong expression of TGF-β-latency-associated peptide and IL6. Our findings indicate that MUTYH loss is associated with an increase in CRC risk, which involves immunosuppression and altered inflammatory response. We propose that the AOM/DSS initiation/promotion protocol in Mutyh^−/− mice provides a good model for MAP.

Adenoma development in familial adenomatous polyposis and MUTYH-associated polyposis: somatic landscape and driver genes

Familial adenomatous polyposis (FAP) and MUTYH‐associated polyposis (MAP) are inherited disorders associated with multiple colorectal adenomas that lead to a very high risk of colorectal cancer. The somatic mutations that drive adenoma development in these conditions have not been investigated comprehensively. In this study we performed analysis of paired colorectal adenoma and normal tissue DNA from individuals with FAP or MAP, sequencing 14 adenoma whole exomes (eight MAP, six FAP), 55 adenoma targeted exomes (33 MAP, 22 FAP) and germline DNA from each patient, and a further 63 adenomas by capillary sequencing (41 FAP, 22 MAP). With these data we examined the profile of mutated genes, the mutational signatures and the somatic mutation rates, observing significant diversity in the constellations of mutated driver genes in different adenomas, and loss‐of‐function mutations in WTX (9%; p < 9.99e‐06), a gene implicated in regulation of the WNT pathway and p53 acetylation. These data extend our understanding of the early events in colorectal tumourigenesis in the polyposis syndromes. © 2015 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

DNA repair capacity is impaired in healthy BRCA1 heterozygous mutation carriers

BRCA1 germline mutations increase the lifetime risk of developing breast and ovarian cancers. However, taking into account the differences in disease manifestation among mutation carriers, it is probable that different BRCA1 mutations have distinct haploinsufficiency effects and lead to the formation of different phenotypes. Using lymphoblastoid cell lines derived from heterozygous BRCA1 mutation carriers and non-carriers, we investigated the haploinsufficiency effects of various mutation types using qPCR, immunofluorescence, and microarray technology. Lymphoblastoid cell lines carrying a truncating mutation showed significantly lower BRCA1 mRNA and protein levels and higher levels of gamma-H2AX than control cells or those harboring a missense mutation, indicating greater spontaneous DNA damage. Cells carrying either BRCA1 mutation type showed impaired RAD51 foci formation, suggesting defective repair in mutated cells. Moreover, compared to controls, cell lines carrying missense mutations displayed a more distinct expression profile than cells with truncating mutations, which is consistent with different mutations giving rise to distinct phenotypes. Alterations in the immune response pathway in cells harboring missense mutations point to possible mechanisms of breast cancer initiation in carriers of these mutations. Our findings offer insight into how various heterozygous mutations in BRCA1 could lead to impairment of BRCA1 function and provide strong evidence of haploinsufficiency in BRCA1 mutation carriers.

MUTYH mediates the toxicity of combined DNA 6-thioguanine and UVA radiation

The therapeutic thiopurines, including the immunosuppressant azathioprine (Aza) cause the accumulation of the UVA photosensitizer 6-thioguanine (6-TG) in the DNA of the patients' cells. DNA 6-TG and UVA are synergistically cytotoxic and their interaction causes oxidative damage. The MUTYH DNA glycosylase participates in the base excision repair of oxidized DNA bases. Using Mutyh-nullmouse fibroblasts (MEFs) we examined whether MUTYH provides protection against the lethal effects of combined DNA 6-TG/UVA. Surprisingly, Mutyh-null MEFs were more resistant than wild-type MEFs, despite accumulating higher levels of DNA 8-oxo-7,8-dihydroguanine (8-oxoG).Their enhanced 6-TG/UVA resistance reflected the absence of the MUTYH protein and MEFs expressing enzymatically-dead human variants were as sensitive as wild-type cells. Consistent with their enhanced resistance, Mutyh-null cells sustained fewer DNA strand breaks and lower levels of chromosomal damage after 6-TG/UVA. Although 6-TG/UVA treatment caused early checkpoint activation irrespective of the MUTYH status, Mutyh-null cells failed to arrest in S-phase at late time points. MUTYH-dependent toxicity was also apparent in vivo. Mutyh-/- mice survived better than wild-type during a 12-month chronicexposure to Aza/UVA treatments that significantly increased levels of skin DNA 8-oxoG. Two squamous cell skin carcinomas arose in Aza/UVA treated Mutyh-/- mice whereas similarly treated wild-type animals remained tumor-free.