Functional analysis of chemically-induced mutations at the flounder TP53 locus, the FACIM assay

Yeast-based genotoxicity tests for assessing DNA alterations and DNA stress responses: a 40-year overview

By damaging DNA molecules, genotoxicants cause genetic mutations and also increase human susceptibility to cancers and genetic diseases. Over the past four decades, several assays have been developed in the budding yeast Saccharomyces cerevisiae to screen potential genotoxic substances and provide alternatives to animal-based genotoxicity tests. These yeast-based genotoxicity tests are either DNA alteration-based or DNA stress-response reporter-based. The former, which came first, were developed from the genetic studies conducted on various types of DNA alterations in yeast cells. Despite their limited throughput capabilities, some of these tests have been used as short-term genotoxicity tests in addition to bacteria- or mammalian cell-based tests. In contrast, the latter tests are based on the emergent transcriptional induction of DNA repair-related genes via activation of the DNA damage checkpoint kinase cascade triggered by DNA damage. Some of these reporter assays have been linked to DNA damage-responsive promoters to assess chemical carcinogenicity and ecotoxicity in environmental samples. Yeast-mediated genotoxicity tests are being continuously improved by increasing the permeability of yeast cell walls, by the ectopic expression of mammalian cytochrome P450 systems, by the use of DNA repair-deficient host strains, and by integrating them into high-throughput formats or microfluidic devices. Notably, yeast-based reporter assays linked with the newer toxicogenomic approaches are becoming powerful short-term genotoxicity tests for large numbers of compounds. These tests can also be used to detect polluted environmental samples, and as effective screening tools during anticancer drug development.

Transcriptional responses of cancer-related genes in turbot Scophthalmus maximus and mussels Mytilus edulis exposed to heavy fuel oil no. 6 and styrene

Recent spills in European waters have released polycyclic aromatic hydrocarbons, important components of heavy fuel oil, and the hydrocarbon styrene. Heavy fuel oil and styrene are classified as potentially genotoxic and carcinogenic. Here we investigate transcription of genes involved in cancer development in the liver of juvenile turbots and in the digestive gland of mussels exposed to heavy fuel oil and to styrene and after a recovery period. In turbot, oil produced a significant up-regulation of p53 and gadd45α after 14 days exposure. cyclin G1 was up-regulated after 7 days treatment with styrene. In mussels, ras was down-regulated in both treatments after the recovery periods. No mutations in ras hotspots were detected in exposed mussels. gadd45α was up-regulated after the recovery period of the styrene experiment. Overall, transcriptional responses differed in mussels compared to turbot. Turbot responded to hydrocarbon exposure by triggering cell cycle arrest (p53) and DNA repair (gadd45α).

8-Oxoguanine incision activity is impaired in lung tissues of NSCLC patients with the polymorphism of OGG1 and XRCC1 genes

Decreased repair of oxidative DNA damage is a risk factor for developing certain human malignancies. We have previously found that the capacity of 8-oxo-7,8-dihydroguanine repair was lower in leukocytes of NSCLC patients than in controls. To explain these observations, we searched for mutations and polymorphisms in the OGG1 gene among 88 NSCLC patients and 79 controls. One patient exhibited a heterozygous mutation in exon 1, which resulted in Arg46Gln substitution. Normal lung and tumor tissue carrying this mutation showed markedly lower 8-oxoG incision activity than the mean for all patients. The predominant polymorphism of OGG1 was Ser326Cys. A significant difference was observed in the frequencies of the OGG1 variants between populations of NSCLC patients and controls. The frequency of the Cys326 allele and the number of Cys326Cys homozygotes was higher among patients than controls. In individuals with either Ser326Cys or Cys326Cys genotype 8-oxoG incision rate was lower than in those with both Ser326 alleles, either in lung or leukocytes. Moreover, 8-oxodG level was higher in lung tissue and leukocytes of patients carrying two Cys326 alleles and in leukocytes of patients with the Ser326Cys genotype. We also screened for polymorphisms of the XRCC1 gene. Only heterozygotes of the XRCC1 variants Arg194Trp, Arg280His and Arg399Gln were found among patients and controls, with the frequency of Arg280His being significantly higher among patients. NSCLC patients with Arg280His or Arg399Gln polymorphism revealed lower 8-oxoG incision activity in their lung tissues, but not in leukocytes. We can conclude that the OGG1 Ser326Cys polymorphisms may have an impact on the efficiency of 8-oxoG incision in humans and the XRCC1 His280 and Gln399 may influence the OGG1 activity in tissues exposed to chronic oxidative/inflammatory stress. Higher frequency of the OGG1 Cys326 allele among NSCLC patients may partially explain the impairment of the 8-oxoG repair observed in their leukocytes.

Detection of environmental mutagens using the FACIM assay

A genetically engineered diploid yeast strain named yJC2, was specifically developed for environmental mutagen detection and characterization of induced mutations. This strain contains one copy of the human TP53 tumour suppressor gene coding sequence which is used as a molecular target for mutagens and two copies of the ADE2 reporter gene allowing accurate measurement of the TP53 transcriptional activity. The strain sensitivity to mutagens was evaluated by exposing cells to UVC, 4-nitroquinoline (NQO) or to an organic extract of sediment from the Seine estuary. For all studied mutagens, a significant and dose-dependent increase of mutant frequency was observed. The present assay named FACIM II (Functional Analysis of Chemical-Induced TP53 Mutations) is more convenient than the FACIM I and more inducible than the SOS Chromotest to detect direct-acting mutagens in the environment.

Predicting the oncogenicity of missense mutations reported in the International Agency for Cancer Research (IARC) mutation database on p53

Many mutation databases, comprising thousands of reported mutations, are available. Often the clinical significance of the reported mutations is unknown. In this study we developed an algorithm that allows prediction of the clinical significance of missense mutations reported in a mutation database. Nonsense mutations are used as a referent group for this assessment. We used the International Association for Research on Cancer (IARC) mutation database on TP53 to implement the algorithm. First, on the basis of published data [Nachman MW, Crowell SL. 2000. Genetics 156:297-304], we ascribed mutation rates to every single nucleotide substitution (SNS) in the core domain of the TP53 gene. Second, for every possible SNS we computed the expected number of missense mutations, under the assumption that missense mutations are as oncogenic as nonsense ones. The natural logarithm of the ratio of the observed to the expected number of missense mutations (LR) was used as a quantitative measure of oncogenicity (i.e., the ability of a mutation to produce cancer). We estimated the relative oncogenicity of all missense mutations reported in the IARC p53 mutation database, and constructed a profile of oncogenicity of the missense mutations along the DNA-binding region of p53.

Genotoxicological studies in aquatic organisms: an overview

Substantial progress has been made in the lat two decades to evaluate the impact of physical and chemical genotoxins in aquatic organisms. This overview (a) summarises the major high lights in this stimulating area of research, (b) compares the developments in this field with the developments in mammalian genotoxicological studies, where appropriate, (c) introduces 18 different articles presented in this special issue of Mutation Research in the backdrop of main advances and , (d) hypothesises on future directions of research in this exciting field.