Analysis of a case of mutagen specificity in Neurospora crassa. I. Dose-response curves

1,3-Butadiene: II. Genotoxicity profile

1,3-Butadiene’s (BD’s) major electrophilic metabolites 1,2-epoxy-3-butene (EB), 1,2-dihydroxy-3,4-epoxybutane (EBD), and 1,2,3,4-diepoxybutane (DEB) are responsible for both its mutagenicity and carcinogenicity. EB, EBD, and DEB are DNA reactive, forming a variety of adducts. All three metabolites are genotoxic in vitro and in vivo, with relative mutagenic potencies of DEB >> EB > EBD. DEB also effectively produces gene deletions and chromosome aberrations. BD’s greater mutagenicity and carcinogenicity in mice over rats as well as its failure to induce chromosome-level mutations in vivo in rats appear to be due to greater production of DEB in mice. Concentrations of EB and DEB in vivo in humans are even lower than in rats. Although most studies of BD-exposed humans have failed to find increases in gene mutations, one group has reported positive findings. Reasons for these discordant results are examined. BD-related chromosome aberrations have never been demonstrated in humans except for the possible production of micronuclei in lymphocytes of workers exposed to extremely high levels of BD in the workplace. The relative potencies of the BD metabolites, their relative abundance in the different species, and the kinds of mutations they can induce are major considerations in BD’s overall genotoxicity profile.

Mutation tests in Neurospora crassa. A report of the U.S. Environmental Protection Agency Gene-Tox Program

Many mutation tests have been developed in Neurospora crassa during the almost 40 years of its use in mutation research. These tests detect two major classes of mutation: gene mutation and meiotic nondisjunction. Within the first class, forward- and reverse-mutation tests have been used. The forward-mutation tests include those that detect mutations at many loci and at specific loci. Both kinds of forward-mutation tests have been done in homokaryons (n) and heterokaryons (n + n'). From the publications that were not rejected by our pre-established criteria, data were extracted for 166 chemicals that had been tested for mutagenicity. Only 6 of the 166 chemicals have been tested in one or more gene mutation test and the meiotic nondisjunction test; these 6 chemicals were positive in the first and negative in the second. Of the 102 chemicals tested in one or more gene mutation tests, 94 were positive and 8 were negative. Of the 70 chemicals tested in the meiotic nondisjunction test, 7 were positive and 63 were negative. Two tests, the ad-3 forward-mutation test and the meiotic nondisjunction test, have been used most frequently. These two tests are especially important for hazard evaluation, because each detects a class of mutations that is likely to be deleterious or lethal in the F1 - disomics by the meiotic nondisjunction test and multilocus deletions by the ad-3 forward-mutation test in heterokaryons. Generally, direct-acting chemicals are mutagenic in the gene mutation tests, but few chemicals that required metabolic activation have been tested. Only 31 of the 166 chemicals tested in N. crassa have been tested for carcinogenicity. Among these chemicals, there is a good association between mutagenicity in gene mutation tests and carcinogenicity but a poorer association between meiotic nondisjunction and carcinogenicity; however, only a small number of chemicals has been tested in the meiotic nondisjunction test. Further use and development of certain mutation tests in N. crassa are desirable.

Assay of cytotoxicity and mutagenicity of alkylating agents by using Neurospora spheroplasts

A system relying on the use of Neurospora crassa spheroplasts has been developed for the assay of cytotoxicity and mutagenicity of chemical compounds. Mutagenicity was assayed by using reversion of alleles in the am gene selected to recognize certain specified transitions and also undefined point mutations. Cytotoxicity was quantified by measuring a 'cytotoxicity parameter', m, which appears in the exponential function that fits the survival/dose curve for each compound (under standard incubation conditions). Of the compounds tested, nitrogen mustard (Cl(CH2)2 NMe(CH2)2Cl) was cytotoxic and non-mutagenic, and ethyl nitrosourea was highly mutagenic but not cytotoxic. Of the remaining compounds tested, methyl nitrosourea, butadiene diepoxide, and cis platin (cis diammonia platinum II chloride) all showed comparable mutagenicity per survivor, although the values of m covered a wide range. Differences were found between the different compounds in the effects of the uvs-2 allele on survival and on the preponderance of G to A transitions.

Genetic effects of dimethyl sulfate, diethyl sulfate, and related compounds

DMS and DES are monofunctional alkylating agents that have been shown to induce mutations, chromosomal aberrations, and other genetic alterations in a diversity of organisms. They have also been shown to be carcinogenic in animals. As an alkylating agent, DMS is a typical SN2 agent, attacking predominantly nitrogen sites in nucleic acids. DES is capable of SN1 alkylations as well as SN2 and thereby causes some alkylation on oxygen sites including the O6-position of guanine which is thought to be significant in mutagenesis by direct mispairing. The mutagenicity of DMS is better explained in terms of indirect, repair-dependent processes. With respect to both alkylating activity and genetic effects, striking similarities are found between DMS and MMS and between DES and EMS. In most systems where they have been tested, both DMS and DES are mutagenic. Results of many of the mutagenesis studies involving these compounds and other alkylating sulfuric acid esters are summarized in Tables 6, 7, 8, 9 and 10 of this review. Most data are consistent with these agents acting primarily as base-pair substitution mutagens. In the case of DES, strong specificity for G.C to A.T transitions has been reported in some systems but has not been clearly supported in some others. Low levels of frameshift mutations of the deletion type are also likely. In addition to the induction of mutations, recombinogenic and clastogenic effects have been described.

The induction of mutation in yeast by hydrogen peroxide

The inactivation and mutation to respiratory deficiency of yeast cells by H2O2 are shown to vary progressively with the phase of cell growth, with a sharp transition occurring as the cells complete logarithmic growth. Respiratory deficient mutants isolated from the wild-type population are of two types, one of which is much more sensitive to H2O2 but forms only a small fraction of the mutant sub-population. Based upon the response of the more resistant type, mutation frequency increases appear to result from selection of pre-existing mutants in log phase populations, while induction occurs in stationary phase cells. The induced mutation frequency fits a (dose)2 relationship, but the frequency is depressed when the dose is high (or number of cells treated is low). All the induced mutants are extranuclear and of the resistant type, and show a wide range of suppressiveness in crosses to respiratory competent cells. This may indicate mitochondrial DNA is altered to different extents by H2O2; by the same criterion, the spontaneously occurring H2O2 -sensitive mutants retain a large amount of mitochondrial DNA information, in agreement with their colonial morphology. A small increase in forward mutation of nuclear genes was also found after H2O2 treatment. Parallels are drawn between the response of yeast cells to ionising radiation and to H2O2, and it is suggested that radical action may be involved in inactivation and mitochondrial genome mutation induced by both agents.

Charlotte Auerbach and chemical mutagenesis

A review is presented of the many conceptual contributions of Charlotte Auerbach to the development of the field of chemical mutagenesis during the past thirty years. The following aspects are discussed: (1) differences between the genetic effects of chemicals and X-rays; (2) mosaicism, delayed mutation and replicating instabilities; (3) mutation as a cellular process; (4) specificity; (5) dose-effect curves, and (6) correlation between different kinds of damage.

The modifying effects of strain and age on the mutagenic specificity of ultraviolet light in Neurospora crassa

Two derivatives of K3/17 ad-3A 38701; inos 37401 of Neurospora crassa are described which show opposite specific reversional responses to UV. Both derivatives carry the same two auxotrophic alleles and appear to differ only in a single gene which influences the pattern of mutagen specificity. The differences between the derivatives only develop after the cultures have been aged for two to four weeks. Various possible explanations are considered.

The mutagenicity of amino acid analogues in Coprinus lagopus

The amino acid analoguesp-fluorophenylalanine (PFP) and ethionine (ETH) are strongly mutagenic inCoprinus lagopus. The most pronounced effect was found with suppressor mutations of themet-1locus. PFP, at a concentration of 2·4 × 10−4M, increased the mutation frequency 500 fold and ETH, at a concentration of 2·4 × 10−3M, 30 fold over the spontaneous mutation frequency. From the spectrum of suppressors of themet-1locus and the dominant revertants of thead-82locus, induced by analogue treatments, it was concluded that both analogues induce single base-change mutations. The dose response curves follow a sigmoid plot, revealing that within a certain range of analogue concentrations, muta-genesis is strongly dose dependent.

Using analogue resistant mutants, it has been shown that PFP mutagenesis is a function of its incorporation into protein. However, ETH mutagenesis is independent of protein incorporation but can be correlated with the degree of ethylation of nucleic acids. The synergistic effect PFP and ETH supports the evidence of the different mutagenic actions of the two analogues.