Multiple mutations are common at mouse Aprt in genotoxin-exposed mismatch repair deficient cells

Deficient mismatch repair is detected in large-to-giant congenital melanocytic naevi: providing new insight into aetiology and diagnosis

BACKGROUND

The aetiologies of large-to-giant congenital melanocytic naevi (LGCMN) remain ambiguous. A previous study discovered signatures associated with deficient mismatch repair (dMMR) in patients with LGCMN. However, a screening diagnostic immunohistochemistry (IHC) panel of dMMR in patients with LGCMN has not been performed to date.

OBJECTIVES

To identify the MMR status and aetiologies of LGCMN.

METHODS

A total of 110 patients with CMN, including 30 giant CMN, 30 large CMN, 30 medium CMN and 20 small CMN, underwent diagnostic IHC (for MSH6, MSH2, PMS2 and MLH1) screening of dMMR. The control group comprised normal skin samples from 20 healthy people. MMR proteins with little effect (MSH3 and PMS1) on the MMR system were stained in all samples. The surgical procedures conducted on each patient were noted because they might alter the behaviour of CMN and confound the results. Binary logistic regression analyses were performed between the phenotypic data and MMR status to identify associations. Whole-exome sequencing was performed on the main naevi, satellite naevi and normal skin tissues of four patients to detect variants. Mutational signature analyses were conducted to explore the aetiologies of LGCMN.

RESULTS

dMMR was detected in 37% (11 of 30) of giant, 23% (7 of 30) of large and 7% (2 of 30) of medium CMNs, but were not identified in small CMNs or normal skin tissues. Moreover, multiple LGCMNs had a much higher dMMR rate than did single LGCMNs. The regression analyses showed that MMR status was significantly associated with CMN size and the presence of satellites, but was not correlated with age, sex, location, satellite diversity or tissue expansion. Notably, the pattern of protein loss in LGCMN mainly consisted of PMS2 loss. Mutational signature analyses detected dMMR-related signatures in patients with LGCMN. Additionally, rare deleterious germline mutations in DNA repair genes were detected in LGCMN, mainly in MSH6, ATM, RAD50, BRCA1 and ERCC8. These germline mutations were single-patient variants with unknown significance.

CONCLUSIONS

dMMR is one of the aetiologies underlying LGCMN, particularly in patients with giant main lesions and multiple satellite lesions. Further studies are necessary to investigate the role of the DNA repair system, particularly MMR, in LGCMN.

The expression of MLH1 and MSH2 genes among inhabitants of high background radiation area of Ramsar, Iran

Previous studies evidenced the critical role of the mismatch repair system in DNA damage recognition, cell cycle arrest, apoptosis and DNA repair. MLH1 and MSH2 genes belong to repairing complexes of mismatch repair system. The side effects of ionizing radiation on the human health were proved, but researches on the inhabitants of high background radiation areas, with extra-ordinary radiation exposure, showed that the prevalence of cancer or radiation-related diseases is not significantly higher than normal background areas. The city of Ramsar, in northern Iran, has the highest level of natural background radiation in the world and in this study, we aimed to evaluate the expression of MLH1 and MSH2 genes among the inhabitants of high background radiation areas of Ramsar compared to normal background radiation areas. In the present study, 60 blood sample from high and normal background inhabitants were collected and we MLH1, and MSH2 genes expressions in residents of high background radiation area compared with normal background radiation area were evaluated by Quantitative Real-Time PCR. Our results showed a significant upregulation of MLH1 in residents of high background radiation area. Also, there is a significant association between MLH1 and MSH2 gene expression in both sexes. Also, the increased expression of MLH1 in HBRA is notable. There is an increased expression of MLH1 in age above 50 and a decreased expression of MSH2 in ages under 50 years (P < 0.0001). These findings are suggesting the triggering of Mismatch Repair system in response to high-level of natural background radiation.

Role of the Msh2 gene in genome maintenance and development in mouse fetuses

In an attempt to evaluate the roles of the mismatch repair gene Msh2 in genome maintenance and in development during the fetal stage, spontaneous mutations and several developmental indices were studied in Msh2-deficient lacZ-transgenic mouse fetuses. Mutation levels in fetuses were elevated at 9.5 dpc (days post coitum) when compared to wild-type mice, and the level of mutations continued to increase until the fetuses reached the newborn stage. The mutation levels in 4 different tissues of newborns showed similar magnitudes to those in the whole body. The levels remained similar after birth until 6 months of age. The molecular nature of the mutations examined in 12.5 dpc fetuses of Msh2(+/+) and Msh2(-/-) revealed unique spectra which reflect errors produced during the DNA replication process, and those corrected by a mismatch repair system. Most base substitutions and simple deletions were reduced by the presence of the Msh2 gene, whereas G:C to A:T changes at CpG sequences were not affected, suggesting that the latter change was not influenced by mismatch repair. On the other hand, analysis of developmental indices revealed that there was very little effect, including the presence of malformations, resulting from Msh2-deficiencies. These results indicate that elevated mutation levels have little effect on the development of the fetus, even if a mutator phenotype appears at the organogenesis stage.

Colorectal cancers choosing sides

In contrast to the majority of sporadic colorectal cancer which predominantly occur in the distal colon, most mismatch repair deficient tumours arise at the proximal side. At present, these regional preferences have not been explained properly. Recently, we have screened colorectal tumours for mutations in Wnt-related genes focusing specifically on colorectal location. Combining this analysis with published data, we propose a mechanism underlying the side-related preferences of colorectal cancers, based on the specific acquired genetic defects in β-catenin signalling.

High frequency induction of CC to TT tandem mutations in DNA repair-proficient mammalian cells

The CC to TT tandem mutation is induced by UV radiation exposure, though at relatively low frequencies when compared with the more commonly induced C to T mutation. Induction of the tandem mutation by UV is enhanced in mammalian cells with certain genetic deficiencies; however, conditions have not been described in which the frequency of this mutation is enhanced in DNA repair-proficient mammalian cells. For this study, an integrated construct that detects C to T and CC to TT mutations at a single codon in mouse Aprt was used to examine UVB mutagenesis under various conditions. Oxidative stress, in the form of intracellular hydrogen peroxide, increased the frequency of UVB-induced CC to TT mutations. Surprisingly, exposure of the cells to two antioxidants (N-acetylcysteine and trolox), either alone or in combination, also enhanced UVB induction of CC to TT tandem mutations. These results demonstrate, for the first time, that the frequency of UVB-induced CC to TT tandem mutations can be enhanced dramatically in DNA repair-proficient mammalian cells, and suggest that the enhancing effect does not require direct damage to DNA.

Reduced host cell reactivation of oxidative DNA damage in human cells deficient in the mismatch repair gene hMSH2

Germ line mutations in the mismatch repair (MMR) genes hMSH2 and hMLH1 account for approximately 98% of hereditary non-polyposis colorectal cancers. In addition, there is increasing evidence for an involvement of MMR gene expression in the response of cells to UV-induced skin cancer. The link between MMR and skin cancer suggests an involvement of MMR gene expression in the response of skin cells to UV-induced DNA damage. In this report, we have used two reporter gene assays to examine the role of hMSH2 and hMLH1 in the repair of oxidative DNA damage induced by UVA light and DNA damage caused by methylene blue plus visible light (MB+VL). UVA and MB+VL produce 8-hydroxyguanines in DNA that are repaired by base excision repair (BER). AdHCMVlacZ is a replication-deficient recombinant adenovirus that expresses the beta-galactosidase (beta-gal) reporter gene under the control of the human cytomegalovirus (CMV) immediate-early promoter. We show a reduced host cell reactivation for beta-gal expression of UVA-treated and MB+VL-treated AdHCMVlacZ in hMSH2-deficient LoVo human colon adenocarcinoma cells compared to their hMSH2-proficient counterpart SW480 cells, but not in hMLH1-deficient HCT116 human colon adenocarcinoma cells compared to hMLH1-proficient HCT116-chr3 cells. We have also reported previously that enhanced expression of the undamaged AdHCMVlacZ reporter gene is induced by the pre-treatment of cells with lower levels of the DNA-damaging agent and to higher expression levels in transcription-coupled repair (TCR)-deficient compared to TCR-proficient cells. Here we show that pre-treatment of cells with UVA or MB+VL enhanced expression of the undamaged reporter gene to a higher level in LoVo compared to SW480 cells but there was little or no difference in HCT116 compared to HCT116-chr3 cells. These results suggest a substantial involvement of hMSH2 but little or no involvement of hMLH1 in the repair of UVA- and MB+VL-induced oxidative DNA damage by BER.

Frequency and types of spontaneous Hprt lymphocyte mutations in Pms2-deficient mice

Deficiencies in DNA mismatch repair (MMR) result in predisposition to neoplasia in both rodents and humans. Pms2 is one of the several proteins involved in the eukaryotic MMR system. In order to determine the effect of Pms2-deficiency on mutation, we measured mutant frequencies in the endogenous Hprt gene of lymphocytes from male Pms2(-/-), Pms2(+/-), and Pms2(+/+) mice. Spleens were removed from mice of various ages and lymphocytes isolated from spleens were cultured to determine the frequency of 6-thioguanine-resistant mutants. Mean mutant frequencies in Pms2(-/-) mice at 6, 10, 18, and 34 weeks of age [42.6 x 10(-6) (n=6), 38.5 x 10(-6) (n=6), 58.2 x 10(-6) (n=9), and 49.1 x 10(-6) (n=5), respectively] were significantly higher than those of comparably aged Pms2(+/+) and Pms2(+/-) mice (all less than 3 x 10(-6)). Mutant clones from the mice were expanded, RNA extracted, and Hprt cDNA amplified by RT-PCR. DNA sequencing analysis of 221 mutant cDNAs from the three different Pms2 genotypes identified 182 clones with independent mutations, including five clones that contained multiple mutations. When compared to the mutational spectrum observed in Pms2(+/+) and Pms2(+/-) mice, the mutational spectrum for Pms2(-/-) mice was significantly different. The Pms2(-/-) mutational analysis indicated that loss of the Pms2 protein causes increases in the frequencies of strand-slippage-type frameshift mutations and of A:T --> G:C transitions in the Hprt gene. The absolute frequencies of A:T --> G:C transitions in MMR-deficient mice suggest increases in this mutation may be a common feature of MMR-deficient mice, not just of Pms2-deficient mice, and may be related to the cancer predisposition that results from loss of MMR function.

High frequency induction of mitotic recombination by ionizing radiation in Mlh1 null mouse cells

Mitotic recombination in somatic cells involves crossover events between homologous autosomal chromosomes. This process can convert a cell with a heterozygous deficiency to one with a homozygous deficiency if a mutant allele is present on one of the two homologous autosomes. Thus mitotic recombination often represents the second mutational step in tumor suppressor gene inactivation. In this study we examined the frequency and spectrum of ionizing radiation (IR)-induced autosomal mutations affecting Aprt expression in a mouse kidney cell line null for the Mlh1 mismatch repair (MMR) gene. The mutant frequency results demonstrated high frequency induction of mutations by IR exposure and the spectral analysis revealed that most of this response was due to the induction of mitotic recombinational events. High frequency induction of mitotic recombination was not observed in a DNA repair-proficient cell line or in a cell line with an MMR-independent mutator phenotype. These results demonstrate that IR exposure can initiate a process leading to mitotic recombinational events and that MMR function suppresses these events from occurring.

Levels of H-ras codon 61 CAA to AAA mutation: response to 4-ABP-treatment and Pms2-deficiency

DNA mismatch repair (MMR) deficiencies result in increased frequencies of spontaneous mutation and tumor formation. In the present study, we tested the hypothesis that a chemically-induced mutational response would be greater in a mouse with an MMR-deficiency than in the MMR-proficient mouse models commonly used to assay for chemical carcinogenicity. To accomplish this, the induction of H-ras codon 61 CAA-->AAA mutation was examined in Pms2 knockout mice (Pms2-/-, C57BL/6 background) and sibling wild-type mice (Pms2+/+). Groups of five or six neonatal male mice were treated with 0.3 micromol 4-aminobiphenyl (4-ABP) or the vehicle control, dimethylsulfoxide. Eight months after treatment, liver DNAs were isolated and analysed for levels of H-ras codon 61 CAA-->AAA mutation using allele-specific competitive blocker-PCR. In Pms2-proficient and Pms2-deficient mice, 4-ABP treatment caused an increase in mutant fraction (MF) from 1.65x10(-5) to 2.91x10(-5) and from 3.40x10(-5) to 4.70x10(-5), respectively. Pooling data from 4-ABP-treated and control mice, the approximately 2-fold increase in MF observed in Pms2-deficient as compared with Pms2-proficient mice was statistically significant (P=0.0207) and consistent with what has been reported previously in terms of induction of G:C-->T:A mutation in a Pms2-deficient background. Pooling data from both genotypes, the increase in H-ras MF in 4-ABP-treated mice, as compared with control mice, did not reach the 95% confidence level of statistical significance (P=0.0606). The 4-ABP treatment caused a 1.76-fold and 1.38-fold increase in average H-ras MF in Pms2-proficient and Pms2-deficient mice, respectively. Furthermore, the levels of induced mutation in Pms2-proficient and Pms2-deficient mice were nearly identical (1.26x10(-5) and 1.30x10(-5), respectively). We conclude that Pms2-deficiency does not result in an amplification of the H-ras codon 61 CAA-->AAA mutational response induced by 4-ABP.

A role for Pms2 in the prevention of tandem CC --> TT substitutions induced by ultraviolet radiation and oxidative stress

DNA mismatch repair (MMR) is important for preventing base-pair substitutions caused by spontaneous or damage-related DNA polymerase errors. We have used a reversion assay based on mouse Aprt to investigate the role of MMR in preventing ultraviolet radiation (UV) and oxidative stress induced tandem CC --> TT base pair substitutions in cultured mammalian cells. The reversion construct used for this assay can detect both C --> T and CC --> TT mutational events. Most spontaneous mutations in Pms2-deficient cells were single C --> T substitutions (88%), with the remainder being tandem CC --> TT substitutions (12%). The percentage of tandem CC --> TT substitutions rose to 64% and 94% for Pms2-deficient cells exposed to UV and a mixture of hydrogen peroxide and metals (Cu/Fe), respectively. Exposure to hydrogen peroxide alone or metals alone did not induce the tandem substitutions, nor did treatment of the cells with the alkylating agent ethylmethane sulfonate, which induces G --> A substitutions on the opposite strand. Tandem CC --> TT substitutions were also induced by UV irradiation and the hydrogen peroxide/metal mixture in Pms2-proficient cells, but at frequencies significantly lower than those observed in the Pms2-deficient cells. We conclude that mismatch repair plays an important role in preventing tandem CC --> TT substitutions induced by certain genotoxin exposures.

Mlh1 mediates tissue-specific regulation of mitotic recombination

Mitotic recombination (MR) between chromosome homologs in somatic cells is a major pathway to the loss of heterozygosity (LOH), which may cause cancer if tumor suppressor genes are involved. MR can be suppressed by DNA sequence heterology (homeology) in hybrid mice from matings between species or between subspecies. We now report that MR is relatively suppressed in F1 hybrids between inbred strains C57BL/6 and 129S2. The frequency of MR in fibroblasts is lower in F1 hybrid mice than in either of the two parental strains. However, MR in T cells is not affected by strain background. Thus, relatively small genetic differences are capable of restricting MR in a tissue-specific manner. Using Mlh1-deficient mice, we tested the role of mismatch repair in MR in two isogenic cell types. In fibroblasts of C57BL/6 x 129S2 F1 mice, the suppression of MR is alleviated in the absence of MLH1. In contrast, MR is not affected by Mlh1 status in T cells. The frequency of point mutations at the reporter gene loci Aprt and Hprt, on the other hand, is significantly increased in both T cells and fibroblasts of Mlh1(-/-) mice. Thus, different cell types respond differently to MLH1 deficiency, and the contribution of MR to tumorigenesis may be tissue-dependent in the absence of mismatch repair.

Oxidative mutagenesis, mismatch repair, and aging

A PubMed search for the term "oxidative stress" yields over 29,000 articles published on the subject over the past 10 years; more than 2000 of these articles also include the term "aging" in their title or abstract. Many theories of aging predict causal roles for oxidative stress in the myriad of pathological changes that occur as a function of age, including an increasing propensity to develop cancer. A possible link between aging and cancer is the induction and accumulation of somatic mutations caused by oxidative stress. This Review focuses on small mutational events that are induced by oxidative stress and the role of mismatch repair (MMR) in preventing their formation. It also discusses a possible inhibitory effect of oxidative stress on MMR. We speculate that a synergistic interaction between oxidative damage to DNA and reduced MMR levels will, in part, account for an accumulation of small mutational events, and hence cancer, with aging.

DNA mismatch repair protein Msh6 is required for optimal levels of ultraviolet-B-induced apoptosis in primary mouse fibroblasts

Recent data support a role for DNA mismatch repair in the cellular response to some forms of exogenous DNA damage beyond that of DNA repair; cells with defective DNA mismatch repair have partial or complete failure to undergo apoptosis and/or G2M arrest following specific types of damage. We propose that the DNA mismatch repair Msh2/Msh6 heterodimer, responsible for the detection of DNA damage, promotes apoptosis in normal cells, thus protecting mammals from ultraviolet-induced malignant transformation. Using primary mouse embryonic fibroblasts derived from Msh6+/+ and Msh6-/- mice, we compare the response of DNA-mismatch repair-proficient and -deficient cells to ultraviolet B radiation. In the wild-type mouse embryonic fibroblasts, ultraviolet-B-induced increases in Msh6 protein levels were not dependent on p53. Msh6-/- mouse embryonic fibroblasts were significantly less sensitive to the cytotoxic effects of ultraviolet B radiation. Further comparison of the Msh6+/+ and Msh6-/- mouse embryonic fibroblasts revealed that Msh6-/- mouse embryonic fibroblasts undergo significantly less apoptosis following ultraviolet B irradiation, thus indicating that ultraviolet-B-induced apoptosis is partially Msh6 dependent. These data support a role for Msh6 in protective cellular responses of primary cells to ultraviolet-B-induced mutagenesis and, hence, the prevention of skin cancer.

DNA mismatch repair proteins: potential guardians against genomic instability and tumorigenesis induced by ultraviolet photoproducts

In addition to their established role in repairing post-replicative DNA errors, DNA mismatch repair proteins contribute to cell cycle arrest and apoptosis in response to a wide range of exogenous DNA damage (e.g., alkylation-induced lesions). The role of DNA mismatch repair in response to ultraviolet-induced DNA damage has been historically controversial. Recent data, however, suggest that DNA mismatch repair proteins probably do not contribute to the removal of ultraviolet-induced DNA damage, but may be important in suppressing mutagenesis, effecting apoptosis, and suppressing tumorigenesis following exposure to ultraviolet radiation.

A:T --> G:C base pair substitutions occur at a higher rate than other substitution events in Pms2 deficient mouse cells

The mismatch repair pathway involves multiple proteins that are required to correct DNA polymerase generated mismatches before they become mutations. It has been shown recently, that the predominant base-pair substitution events leading to loss of endogenous Aprt activity in Pms2 null mouse cells are A:T --> G:C mutations (Oncogene 21 (2002) 1768, Oncogene 21 (2002) 2840). To determine if this observation could be explained by an increased rate of A:T --> G:C mutations relative to other base-pair substitutions, we developed a reversion assay to examine G:C --> A:T, C:G --> A:T, and A:T --> G:C mutations within mouse Aprt in a Pms2 null mouse kidney cell line. The results demonstrated a 6-50-fold increase in the rate of the A:T --> G:C mutations relative to the other base-pair substitutions. Additional work demonstrated that growth of the Pms2 null cells in antioxidant containing medium reduced the rate of the A:T --> G:C mutations. The results are discussed with regards to the role of mismatch repair proteins in preventing base-pair substitutions, including those induced by oxidative stress.

Mice defective in the mismatch repair gene Msh2 show increased predisposition to UVB radiation-induced skin cancer

Mice defective in the mismatch repair (MMR) gene Msh2 manifest an enhanced predisposition to skin cancer associated with exposure to UVB radiation. This predisposition is further heightened if the mice are additionally defective for the nucleotide excision repair gene Xpc. To test the hypothesis that the predisposition of Msh2 mutant mice to skin cancer reflects a mutator phenotype associated with increased proliferation of skin cells following exposure to UV radiation, Msh2 mutant mice were exposed to the tumor promoter TPA. Such mice showed a robust proliferative response in the skin, but did not manifest evidence of dysplasia or neoplasia. We conclude that the predisposition of Msh2 mice to UVB radiation-induced skin cancer reflects an interaction between the processes of mismatch repair and some other excision repair mode, the exact nature of which remains to be established.

A mouse kidney cell line with a G:C --> C:G transversion mutator phenotype

We report the identification of a mouse kidney epithelial cell line (K435) in which G:C-->C:G transversion mutations occur at an elevated rate and are the predominant spontaneous events observed at the selectable Aprt locus. Of three genotoxins tested, ultraviolet radiation (UV), ionizing radiation, and hydrogen peroxide, only UV exposure was able to alter the spectrum of small mutational events. To determine if the G:C-->C:G mutator phenotype was due to a deficiency in the mismatch repair pathway, the K435 cells were tested for resistance to 6-thioguanine, cisplatin, and MNNG. Although the K435 cells were as resistant to 6-thioguanine and cisplatin as Pms2 and Mlh1 null kidney cells, they were hypersensitive to MNNG. Moreover, the K435 cells do not exhibit microsatellite instability, a hallmark of mismatch repair deficiency. These results suggest that a novel mechanism, which does not include a classical deficiency in mismatch repair, accounts for the G:C-->C:G mutator phenotype.

Msh2 deficiency increases the mutation frequency in all parts of the mouse colon

The Msh2 DNA mismatch repair gene is one of five genes implicated in the pathogenesis of hereditary nonpolyposis colorectal cancer (HNPCC). To address the possible mechanisms of the site-specific occurrence of HNPCC, the effect of Msh2 deficiency on mutations in different parts of the colon was investigated using the BC-1(lacI)/Msh2 double transgenic mouse. Compared to the Msh2(+/+) mice, Msh2(-/-) mice had an 8-9-fold increase of mutation frequency (MF) in the lacI gene from the cecum and the proximal and distal colon. The mutational spectra were also significantly different between Msh2(+/+) and Msh2(-/-) mice, with a significant increase in the frequency of -1 frameshifts and G:C-->A:T base substitutions in the repair-deficient mice. However, in spite of the site-specific predisposition of HNPCC in humans, we found no significant difference in the MF or mutation spectrum between the three parts of the colon in Msh2(+/+), Msh2(+/-), or Msh2(-/-) mice. In addition, 11 independent mutants harboring complex mutations within the lacI gene were recovered in the Msh2(-/-) mice. Interestingly, while the Msh2(+/-) mice displayed an overall MF similar to that observed in the wild-type mice, sequencing revealed a significantly different mutational spectrum between Msh2(+/+) and Msh2(+/-) mice, mainly characterized by an increase in -1 frameshifts. Due to the prevalence of frameshift mutations in HNPCC patients, this haploinsufficiency effect of the Msh2 gene in safeguarding genomic integrity may have important implications for human carrier status.