Why are there more mutations in males?
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Animals↗
Biomedical subjects
D G MacPhee
Publications and source records attributed to D G MacPhee.
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.
Anticarcinogenic Agents↗
Epigenetics and epimutagens: some new perspectives on cancer, germ line effects and endocrine disrupters.
It is known that a variety of chemicals, including certain base analogues and reactive oxygen species, can alter the phenotypes of mammalian cells epigenetically, i.e., without changing their DNA sequence information in any way. The implications of such findings are not trivial, but do not seem to have been the focus of a great deal of attention amongst mutation researchers to date. In part this may be a reflection of the confused state of terminology in the chemical carcinogenesis research area and in part may signal a reluctance on the part of many of us to come to terms with the idea of heritable non-sequence changes to DNA molecules. In this review, some of the most obvious outcomes of spontaneous and induced epimutagenic change for human carcinogenesis and germ line inheritance are discussed, and an attempt is made to place the so-called endocrine disrupters in a context in which their modes of action may be more readily analysed and integrated into the broader chemical hazard framework.
Animals↗
Catabolite repressors are potent antimutagens in Escherichia coli plate incorporation assays: experiments with glucose, glucose-6-phosphate and methyl-alpha-D-glucopyranoside.
Having previously found that the yields of spontaneous valine-resistant (Val(r)) Escherichia coli mutants which appeared on plates containing 40 microg/ml of valine were always much lower when glucose was present in the glycerol-containing defined medium normally used to select them, we now sought to determine whether or not the global regulatory mechanism known as catabolite repression (formerly also called glucose repression) might be involved. We therefore tested glucose (the archetypal catabolite repressor), glycerol (a non catabolite-repressing substrate), glucose-6-phosphate (G6P, an exceptionally powerful catabolite repressor) and methyl-alpha-D-glucopyranoside (alphaMG, a strongly catabolite-repressing but non-utilisable glucose analogue), as potential inhibitors of spontaneous mutagenesis in plate incorporation assays, using three distinct mutation detection systems. We found that the numbers of spontaneous Val(r) and Lac+ mutations appearing on the selective plates tended to be highest when the medium contained only a non-repressing primary carbon source (glycerol in the Val(s) --> Val(r) system, lactose in the Lac- --> Lac+ system) and lowest when it had been supplemented with a strongly catabolite-repressing compound such as alphaMG, G6P or glucose. These results would seem to establish that catabolite repression is an important factor in determining the outcome of the spontaneous mutation generation process in E. coli and hence that the numbers of spontaneous mutations which can be expected to arise in any given set of mutation assay conditions may often be dependent upon the levels of catabolite repression which prevail during the course of the assay. The implications of these results for conventional plate-incorporation mutation assays are discussed.
Antimutagenic Agents↗
Frameshift mutagenesis by 9-aminoacridine: antimutagenic effects of adenosine compounds.
It has been shown that frameshift mutagenesis by 9-aminoacridine (9AA) in Salmonella typhimurium is significantly inhibited if glucose is present while cells are being treated in liquid defined medium. We suggested that this effect might be a result of glucose-provoked alterations of cAMP levels within the cell. We therefore sought to investigate the effects of exogenous cAMP on mutagenesis by 9-aminoacridine in both Salmonella typhimurium and Escherichia coli. Contrary to expectation, we found that frameshift mutagenesis was significantly depressed when high concentrations of cAMP were added to the defined medium during liquid treatment with 9AA. Other adenosine 5'-phosphates such as adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP) and adenosine 5'-monophosphate (AMP) added to the liquid medium during 9AA treatment also substantially decreased the reversion rate to prototrophy in both S. typhimurium and E. coli, as did adenosine itself. Further experiments showed that neither influx nor efflux of 9-aminoacridine molecules were greatly affected by adenosine compounds, and that although cAMP and adenosine exerted similar antimutagenic effects on 9AA-treated stationary phase cells, their effects on log phase cells were quite different. The antimutagenic effect of a representative adenosine compound (ATP) was found to persist for some time after stationary phase cells had been washed, with maximal mutability being regained only after about 3 h.
Adenine Nucleotides↗
Mismatch repair as an important source of new mutations in non-dividing cells.
This paper describes a mechanism which permits somatic cells to generate random mutations in the complete absence of cell proliferation. Knowledge of the existence of this mechanism should provide us with the basis for a better understanding of a number of important biological phenomena, and in particular may help to explain the origins of many human cancers.
Cell Division↗
Mismatch repair as a source of mutations in non-dividing cells.
This paper describes a mechanism which permits somatic cells to generate random mutations in the complete absence of cell proliferation. The mechanism itself is remarkably simple, involving a well-known cellular process (mismatch repair or MMR) which is primarily associated with mutation avoidance, but which is also capable of generating mutations when circumstances are not ideal for avoidance. When MMR operates in its so-called 'methylation-instructed' mode to remove mismatches from newly-replicated portions of genomic DNA, it does so in a way which serves to minimize mutation yields. By contrast, when MMR operates in a non-instructed or 'randomly-templated' way to remove mismatches from DNA molecules, it does so without distinguishing between the two strands of DNA that contain the mismatched bases. Randomly-templated mismatch repair (RT-MMR) therefore generates new and complete mutations whenever it removes the correct bases from either base-pair mismatches or frameshift mispairs and replaces them without incorrect bases or sequences. Wider recognition of the existence of this mechanism--and especially of its proclivity for mutation generation when it is operating in non-dividing cells--should help us to develop a better understanding of a number of important biological phenomena, and may be of particular value in our attempts to explain the origins of many human cancers.
Animals↗
Spontaneous mutations in bacteria: chance or necessity?
Several investigators have recently reported that significant numbers of appropriately adapted mutants can be induced in bacterial and yeast strains by exposing stationary phase cells to specific environmental challenges. The resulting mutants are said to be both selection-induced and demonstrably non-random in origin; if this interpretation is correct, it is in direct conflict with the conventional neo-Darwinian view, which is that spontaneous mutants are truly random in origin and arise without the intervention of any overtly adaptive forces. We believe that there are alternative ways of accounting for the appearance of many (and probably all) of the additional mutants which proponents of the adaptive mutation theory claim are observed only after they applied the appropriate selective pressure. Having reviewed the available evidence, we consider that most (if not all) of the sorts of mutants which are said to have been induced following exposure of stationary-phase cells to intense selective pressure are equally likely to have been generated during the operation of certain well-known, conventional (and essentially random) cellular DNA repair processes. Evidence in support of our view can be found in the mainstream literature on the origins of spontaneous mutations. We also note that some of the molecular models which have recently been proposed to explain the production of selection-induced mutations preferentially (or even only) in genes of adaptive significance may turn out to be of considerable interest in their own right, even although the mutants whose origins they were intended to explain may turn out to have arisen in a manner which is totally independent of the conditions used for their selection.
Adaptation, Physiological↗
Mismatch repair, somatic mutations, and the origins of cancer.
This paper outlines the basic properties of a newly recognized pathway that should enable somatic cells to generate double-stranded mutations in the complete absence of cell proliferation. Recognition of the existence of this pathway provides us with the basis for a better understanding of a number of important biological phenomena and, in particular, may help us to understand the origins of cancers in unselected human populations.
Cell Division↗
Regulatory processes and the origins of spontaneous mutations.
A review of information currently available about the origins of spontaneous mutational events suggests that there may be a role for known cellular control mechanisms in determining the frequencies with which such events can occur. Attention is also directed to recent findings with antimutator (dnaE) mutants of Escherichia coli which indicate that the final step involved in generating a spontaneous mutational event may be different from that involved in generating an SOS-dependent mutational event. Finally, the possible involvement of various sorts of treatments collectively referred to as stress responses (heat shock, cold shock or oxidative damage, etc.) in generating random mutations is discussed; if there is such an involvement, this may represent one way in which organisms are programmed to adapt to a wide variety of environmental challenges.
Escherichia coli↗
Mutagenesis by 9-aminoacridine in Salmonella typhimurium: inhibition by glucose and other PTS class A carbon sources.
Reversion of the hisC3076 frameshift marker of Salmonella typhimurium has been measured following treatment of cells in growth and non-growth media with 9-aminoacridine (9AA). By varying the carbon source present in a defined medium, it has been shown that mutagenesis is reduced close to the spontaneous level in the presence of glucose whilst significant reductions are also observed with glucosamine, mannose, mannitol, fructose or glucose 6-phosphate. Intermediate mutant yields are observed when lactic acid or glycerol are present, whereas any one of a further group of carbon sources (gluconate, arabinose, ribose, succinate or casein hydrolysate) permit relatively large numbers of mutants to be recovered. Interestingly, when any one of these "high yield" carbon sources is supplemented with glucose the strong inhibitory effect characteristic of glucose is again observed. On the basis of these results, it can be concluded that inhibition of 9AA-induced reversion by a carbon source is not an exclusive property of glucose, although when more than one carbon source is present the inhibitory effect of glucose predominates. Possible explanations for these findings include the active exclusion of 9AA from cells as a direct consequence of glucose transport across the cell membrane. To address this possibility, cells were pre-grown in verapamil, a calcium channel antagonist which is known to increase the mutagenicity of various 9-anilinoacridine derivatives in S. typhimurium. We found that glucose inhibition of 9AA-induced mutagenesis was not relaxed to any significant extent following treatment with verapamil. In a further experiment, two glucose analogues (2'-deoxyglucose and methyl-D-glucoside) known to be actively transported into the cell but not metabolised past the first phosphorylation step were used. These analogues inhibit the transport into the cell of several types of molecules, but since they do not significantly depress 9AA mutagenesis it seems unlikely that blockage of 9AA transport across the cell membrane can be invoked to explain the inhibitory effect of glucose on 9AA mutagenesis. An alternative explanation based on glucose-mediated repression of an error-prone, mutation-generating, DNA-repair process is presented.
Aminacrine↗
Glucose inhibition of mutagenesis by 9-aminoacridine in Salmonella typhimurium.
Back mutation to prototrophy of the hisC3076 marker of Salmonella typhimurium has been measured following treatment with 9-aminoacridine (9AA) under conditions in which growth is either permitted or not permitted. Cells treated with 9AA in buffer (i.e. under conditions in which little or no replication was possible) were found to respond well to 9AA-induced mutagenesis. This remained true whether or not the plating medium contained trace amounts of histidine to allow residual replication of His- cells (as for example in the Ames test); yields of His+ mutants were also about the same regardless of whether the plating medium contained glucose or glycerol as the sole carbon source. By contrast, 9AA-induced mutagenesis was essentially abolished when cells were treated in buffer containing 1% glucose (i.e. under conditions which strongly favoured replication). Glucose inhibition of 9AA-induced mutagenesis began to be apparent at glucose concentrations of about 0.02%, whilst total abolition was observed at about 0.2%. In addition, glucose inhibition was found to be dependent on the concentration of 9AA, the induced mutation rate increasing at levels of up to 100-150 micrograms/ml of 9AA and then declining steeply at higher levels. Further kinetic studies indicated that glucose could depress the 9AA-induced mutation rate significantly even in cells exposed to the mutagen for up to 18 min prior to the addition of glucose, whilst inhibition of 9AA mutagenesis by glucose was both temporary and reversible.
Aminacrine↗
Directed mutation: paradigm postponed.
Claims have been made in recent years that spontaneous mutational events are "directed" by (i) the presence in a selective medium of a single carbon source which the cells are unable to utilize (e.g. lactose), or (ii) the absence from a selective medium of an amino acid which the cells are unable to make for themselves (e.g. tryptophan). In resurrecting the previously rejected hypothesis of a "directed" origin for spontaneous mutants, it was noted that selecting for valine-resistant mutants did not allow comparable increases in colony numbers to be observed when overlays containing valine and glucose were added to plates which had accumulated (or were still accumulating) large numbers of "directed" Lac+ or Trp+ mutants. The present paper shows that Valr mutants do occur in much greater numbers if a carbon source other than glucose is added to the plates along with the valine-containing overlays; the evidence is that even small amounts (< 0.02%) of glucose in the overlays prevent the recovery of Valr mutants. From these results, it is argued that the apparent "directed" nature of the spontaneous mutation process is actually a manifestation of the long-known phenomenon of glucose repression.
Animals↗
Is there evidence for directed mutation in bacteria?
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Bacteria↗
Origins of spontaneous mutants: cellular strategies for controlling their formation in response to changing environments.
Spontaneous and induced mutational events may not be as readily distinguishable as has been generally assumed. It is suggested that the later stages of the processes leading to the establishment of fixation of mutations may consist of a 'mutational pathway', with many features in common with the better-known metabolic pathways of organisms like Escherichia coli. If so, the mutational pathway may be controlled by catabolite repression, a cellular mechanism which switches metabolic pathways on and off (or more probably up and down) in response to intracellular levels of a molecular messenger known as cyclic adenosine monophosphate (cAMP). Mutation rates could then be regulated by an intracellular messenger in precisely the way which we would predict they ought to be regulated, if they are to cope with evolutionary pressures for increased variability.
Biological Evolution↗
Substitution of animals in toxicology testing--a rush to judgement?
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Animal Testing Alternatives↗
Precise excision of Tn10 in Salmonella typhimurium: effects of mutations in the polA, dam, mutH and mutB genes and of methionine or ethionine in the plating medium.
Precise excision of Tn10 occurs at significantly elevated frequencies in cultures of the polA7 mutant strain of Salmonella typhimurium, is further increased in polA7 dam-1 and polA7 mutB strains and decreased in a polA7 mutH background. The numbers of precise excision events occurring in polA7 strains are also significantly increased when methionine (20 micrograms/ml or less) is present in the medium but decreased when ethionine (again, 20 micrograms/ml or less) is present. When both amino present, the outcome is about a 2-fold increase in precise excision events. The involvement of mismatch repair and methylation patterns in precise excision events is discussed.
DNA Repair↗
Fluctuation tests are more sensitive than plate tests in detection of chemicals which cause enhanced excision of transposon Tn10 in Salmonella typhimurium.
Precise excision of transposon Tn10, as judged by reversion of Salmonella typhimurium strain LT2 trp1014::Tn10 to Trp+, was not detectably enhanced following exposure to 9-aminoacridine, 5-azacytidine or mitomycin C in conventional treat-and-plate assays. By contrast, 7/13 chemicals, including 5-azacytidine and mitomycin C, were found to be capable of enhancing precise excision of Tn10 when tested in modified fluctuation assays. Despite earlier reports, precise excision is one activity of transposons which is not therefore refractory to enhancement by chemical mutagens.
DNA Transposable Elements↗