Mismatch repair as an important source of new mutations in non-dividing cells

Mutagenesis and the three R's in yeast

Mutagenesis is a prerequisite for evolution and also is an important contributor to human diseases. Most mutations in actively dividing cells originate during DNA replication as errors introduced when copying an undamaged DNA template or during the bypass of DNA lesions. In addition, mutations can be introduced during the repair of DNA double-strand breaks by either homologous recombination or non-homologous end-joining pathways. Finally, although generally considered to be a very high-fidelity process, the excision repair of DNA damage may be an important contributor to mutagenesis in non-dividing cells. In this review, we will discuss the well-known contributions of DNA replication to mutagenesis in Saccharomyces cerevisiae, as well as the less-appreciated contributions of recombination and repair to mutagenesis in this organism.

ENU induces mutations in the heart of lacZ transgenic mice

The use of transgenic mouse models as somatic mutation assays allows determination of mutation in all tissues of the mouse, including non-dividing tissues. In this regard, these models can be used to study the possibility that mutations can be induced in mitotically quiescent organs such as the heart. Mutations are generally thought to be associated with mitotic processes of DNA replication. Mutations, however, are also postulated to occur in the absence of mitosis as the result of DNA repair. In order to determine whether or not mutations could be induced in the heart, we analyzed the mutant frequency in the hearts of F(1) (Muta Mouse X SWR) mice that had been treated acutely with 250 mg/kg ENU and sampled at days 10, 35, and 70 post-treatment. A significant increase in mutant frequency at day 70 shows that mutations can be induced in the heart. Since the heart contains small numbers of non-muscle cells, additional mechanisms that could explain these results were also considered. The effect of ENU-induced cell proliferation or a sub-population of rapidly dividing cells is ruled out by C(14)-thymidine uptake studies which showed minimal proliferation. By the same token, the influence of ex vivo mutations (i.e., DNA adducts fixed as mutations during replication in the bacteria) is ruled out by the observed time course of mutations, as well as experimental evidence showing that such mutations are not detected in the lacZ assay.

Time-dependent mutagenesis and cancer: a new role for antimutagenesis in cancer prevention?

Most attempts to identify potential antimutagens (and/or presumptive anticarcinogens) involve testing individual compounds or mixtures in tandem combinations with specific physical or chemical mutagens and measuring the sought-after reductions in mutation numbers in one or more experimental organisms. Relatively few investigators appear to have set out to identify antimutagens which are efficacious in reducing spontaneous mutation yields (possibly because of the poor mutation yields which tend to be available for downward manipulation in spontaneous mutation assays). The net effect is that we currently know very little about what may well prove to be one of the most interesting and exciting areas of antimutagenesis and anticarcinogenesis research in the future. This paper is primarily concerned with the main features of several interrelated (and often overlapping) pathways which are likely to be involved in the generation of newly-mutant sequences in cellular organisms in the absence of a deliberately-added mutagen. Attempts will be made to highlight some of the cellular processes which may have to be blocked in subtle (or perhaps even unsubtle) ways if we are to achieve our somewhat ambitious goal of discovering antimutagenic anticarcinogens which are both usable and useful in delaying the onset of primarily age-dependent mammalian cancers whose origins may well owe a great deal more to spontaneous mutations than they do to environmentally-provoked ones.

The rate of adaptive mutagenesis in Escherichia coli is enhanced by oxygen (superoxide)

Adaptive mutagenesis is that which occurs in non-dividing cells and which allows growth under the selective conditions imposed. We now report that reversion of amino acid auxotrophies in E. coli fits that definition and is enhanced under conditions conducive to oxidative damage to DNA. Thus adaptive mutagenesis was approximately 4-fold more frequent in a sodA sodB strain than in the superoxide dismutase-replete parental strain and this mutagenesis was suppressed under anaerobic conditions. Moreover, a cell permeant manganic porphyrin, capable of catalyzing the dismutation of O2-, diminished the rate of occurrence of these mutations. Repair of oxidative damage to DNA, in the non-dividing cells, appears to provide the opportunity for adaptive mutagenesis.