Acridine-induced mitotic gene conversion (paramutation) in Saccharomyces cerevisiae. The effect of two different modes of binding to DNA

Enhancement of the recombinagenic and mutagenic activities of bleomycin in yeast by intercalation of acridine compounds into DNA

Strain D7 of Saccharomyces cerevisiae was used to measure the induction by bleomycin (BLM) of mitotic recombination at the trp5 locus and point mutations at ilv1 in the presence and absence of acridine compounds. BLM is a potent mutagen and recombinagen in the D7 assay. The acridines vary, some being mutagenic or recombinagenic and others not. Combined treatments were used to distinguish whether a genetically inactive acridine has no effect on the genetic activity of BLM or modulates its action. When an acridine is itself genetically active, combined treatments were used to determine whether its effects are additive with those of BLM or whether there is interaction between the two compounds. Acridine compounds that share the ability to intercalate between the base pairs of DNA but differ in their mutagenic specificity owing to the presence of different substituent groups were analysed. Clear potentiation and synergistic interactions were detected in combined treatments with BLM and aminoacridines, nitroacridines or an acridine mustard. Potentiation and synergy were also observed in sequential exposures in which the yeast were grown in the presence of acridine compounds and then treated with BLM in the absence of free acridine. The results are consistent with an increase in BLM susceptibility conferred by acridine intercalation. It is likely that the intercalating agents increase the access of BLM to the minor groove of DNA, where it abstracts a hydrogen from the 4' position of deoxyribose, creating a free radical that is processed into strand breaks.

The genetic toxicology of acridines

Acridine and its derivatives are planar polycyclic aromatic molecules which bind tightly but reversibly to DNA by intercalation, but do not usually covalently interact with it. Acridines have a broad spectrum of biological activities, and a number of derivatives are widely used as antibacterial, antiprotozoal and anticancer drugs. Simple acridines show activity as frameshift mutagens, especially in bacteriophage and bacterial assays, by virtue of their intercalative DNA-binding ability. Acridines bearing additional fused aromatic rings (benzacridines) show little activity as frameshift mutagens, but interact covalently with DNA following metabolic activation (forming predominantly base-pair substitution mutations). Compounds where the acridine acts as a carrier to target alkylating agents to DNA (e.g. the ICR compounds) cause predominantly frameshift as well as base-pair substitution mutations in both bacterial and mammalian cells. Nitroacridines may act as simple acridines or (following nitro group reduction) as alkylating agents, depending upon the position of the nitro group. Acridine-based topoisomerase II inhibitors, although frameshift mutagens in bacteria and bacteriophage systems, are primarily chromosomal mutagens in mammalian cells. These mutagenic activities are important, since the compounds have considerable potential as clinical antitumour drugs. Although evidence suggests that simple acridines are not animal or human carcinogens, a number of the derived compounds are highly active in this capacity.

Testing of chemicals for genetic activity with Saccharomyces cerevisiae: a report of the U.S. Environmental Protection Agency Gene-Tox Program

The yeast Saccharomyces cerevisiae is a unicellular fungus that can be cultured as a stable haploid or a stable diploid . Diploid cultures can be induced to undergo meiosis in a synchronous fashion under well-defined conditions. Consequently, yeasts can be used to study genetic effects both in mitotic and in meiotic cells. Haploid strains have been used to study the induction of point mutations. In addition to point mutation induction, diploid strains have been used for studying mitotic recombination, which is the expression of the cellular repair activities induced by inflicted damage. Chromosomal malsegregation in mitotic and meiotic cells can also be studied in appropriately marked strains. Yeast has a considerable potential for endogenous activation, provided the tests are performed with appropriate cells. Exogenous activation has been achieved with S9 rodent liver in test tubes as well as in the host-mediated assay, where cells are injected into rodents. Yeast cells can be recovered from various organs and tested for induced genetic effects. The most commonly used genetic end point has been mitotic recombination either as mitotic crossing-over or mitotic gene conversion. A number of different strains are used by different authors. This also applies to haploid strains used for monitoring induction of point mutations. Mitotic chromosome malsegregation has been studied mainly with strain D6 and meiotic malsegregation with strain DIS13 . Data were available on tests with 492 chemicals, of which 249 were positive, as reported in 173 articles or reports. The genetic test/carcinogenicity accuracy was 0.74, based on the carcinogen listing established in the Gene-Tox Program. The yeast tests supplement the bacterial tests for detecting agents that act via radical formation, antibacterial drugs, and other chemicals interfering with chromosome segregation and recombination processes.

Genetic effects of acridine compounds

Acridines and a very large number of acridine derivatives are used in enormous quantities both in medicine and industry. The mutagenic action of these compounds has been demonstrated in a wide variety of organisms and is known to occur both in the dark as well as in the presence of light (photodynamic action). At the molecular level, acridines have been shown to cause frameshift mutations of both the addition and deletion types, a characteristic which has been of tremendous help in elucidating the nature of the genetic code. These and various other biological effects of acridines, such as inhibition of DNA repair, curing of plasmids and cell-growth inhibition, are examined in this review.

Induction of gene conversion in diploid yeast by chemicals: correlation with mutagenic action and its relevance in genotoxicity screening

More than 200 chemicals, comprising drugs, pesticides, herbicides, industrial chemicals, food additives, etc. tested by various workers for their ability to induce gene conversion in diploid yeast, have been reviewed. This review shows that a strong correlation exists between the convertogenic and mutagenic abilities of these chemicals. This conclusion confirms the earlier one by Zimmermann which was based on fewer mutagenic chemicals. Published data have further been analysed to confirm quantitatively Zimmermann's other important conclusion that gene conversion is not mutagen-specific. This is a positive advantage in using gene conversion in preliminary screening of chemicals because it reduces the chances of false negatives.

The effect of dose and time on the induction of genetic alterations in Saccharomyces cerevisiae by aminoacridines in the presence and absence of visible light irradiation in comparison with the dose-effect-curves of mutagens with other type of action

Aminoacridines induce frameshift mutations and are photodynamically active, depending on whether visible light is absent or present. Therefore, a test system which allows to compare quantitatively the genetic effects of aminoacridines irradiated or unirradiated by visible light ought to be susceptible to the different DNA alterations which can be induced by these substances. For this reason in most experiments mitotic gene conversion and only in some selected experiments reverse mutation was chosen as the indicator of genetic activity. In contrast to mutation systems mitotic gene conversion has never shown a response specific to only some types of mutagens. The three aminoacridine derivatives used-acridine orange (AO), proflavine (PF), and acridine yellow (AY)-were successful in the induction of convertants at two different loci. No locus-specificity could be observed. The time-dependent induction of convertants proceeds quickly but soon reaches-especially after treatment without light-a saturation point. The dose/effect-curve after treatment in the dark has a slope increasing with increasing concentration. Irradiation with visible light results in a dose/effect-curve consisting of three parts. Af first the increase of convertants is nearly linear extending one (AY) to three (AO) orders of magnitude. After a saturation effect begins at the point at which an effectiveness of the acridines in the dark is apparent. At high concentrations an induction of convertants can again be observed which is nearly the same as that after treatment in the dark. To determine whether the dose/effect-curves obtained for gene conversion refer to similar curves for gene mutations after treatment with AO at the same locus not only gene conversions but also reverse mutations were scored for. AO-treatment in the dark is ineffective in inducing reverse mutations. Irradiation with visible light results in a dose/effect-curve beeing parallel only in its first part to the dose/effect-curve obtained for gene conversion, while in its second part a mutation frequency decline can be observed. Comparing the dose/effect-curves of AO resulting from the induction of gene conversion and gene mutation, and taking into account that no mutants can be induced by AO-treatment in the dark, the increase in convertants at high acridine-concentrations can be explained as an addition of light-dependent and light-independent effects. That means, in mutation systems at low concentrations of aminoacridines irradiation with visible light should cause transitions, transversions and microlesions, at intermediate concentrations frameshift lesions should begin to appear, and at very high concentrations nearly exclusively frameshift lesions should occur. The dose/effect-curves of aminoacridines compared with those of other mutagens are very complex. The dose/effect-curves of the mutagens of other type of action tested are linear in a double logarithmic scale, and parallel for induced gene conversion and induced gene mutation...

Host-mediated assay with yeast and rats using probenecid (Benemid) to block the renal tubular excretion of cyclophosphamide metabolites

The genetic activity of cyclophosphamide (Cy) was tested in the host-mediated assay (injection of yeasts into the peritoneal cavity of rats) modified by the use of probenecid (Pro) (Benemid) to block the renal tubular excretion of the genetically active metabolites. The genetic test system used was the induction of mitotic gene conversion in two unliked loci of a diploid strain of Saccharomyces cerevisiae. By this method the genetic effect of By was doubled in comparison with the case of administering Cy alone. Compared with the animals which received only Pro, increases of conversion frequencies of 20 times in the ade2 locus and of 15 times in the trp5 locus were found.