Proliferation of mutators in A cell population

Abstract

A Lac؊ strain of Escherichia coli that reverts by the addition of a G to a G-G-G-G-G-G sequence was used to study the proliferation of mutators in a bacterial culture. Selection for the Lac ؉ phenotype, which is greatly stimulated in mismatch repair-deficient strains, results in an increase in the percentage of mutators in the selected population from less than 1 per 100,000 cells to 1 per 200 cells. All the mutators detected were deficient in the mismatch repair system. Mutagenesis results in a similar increase in the percentage of mutators. Mutagenesis combined with a single selection can result in a population of more than 50% mutators when a sample of several thousand cells is grown out and selected. Mutagenesis combined with two or more successive selections can generate a population that is 100% mutator. These experiments are discussed in relation to ideas that an early step in carcinogenesis is the creation of a mutator phenotype.Although cells with higher spontaneous mutation rates, or mutators, have been studied for many years, the recent finding that the inherited form of human nonpolyposis colon cancer results from a defect in the human counterpart to the bacterial mismatch repair system and that this defect creates a mutator phenotype (1,7,18,33) has generated new excitement in the field of repair systems in general and focused renewed attention on mutators in particular. Loeb has suggested (21, 22) that tumor progression may require the creation of a mutator cell, since the multistage process of carcinogenesis (32), which probably occurs by genetic instability and clonal selection (8), depends on a series of mutations. The spontaneous mutation rate cannot account for the multiple mutations observed in human tumors, but if creation of a mutator occurs early in tumor progression, then the multiple mutations could be generated (21,22). Can mutators arise and proliferate rapidly enough in a cell population to play a role in carcinogenesis? The first mutators discovered in bacteria were found in existing laboratory strains (27,37). Early experiments on detecting mutators dealt with screening isolates in the wild (12, 16) and also involved mutagenized populations of bacteria, sometimes coupled to different selections (6,13,14,19). For example, Jyssum (16) found that 4 of 110 natural isolates of Escherichia coli from patients were mutators, while Gross and Siegel (12) detected mutator activity in 1 of 408 natural E. coli isolates. Such findings suggested that natural populations might be undergoing constant selection, since competition experiments in chemostats showed that under certain conditions mutators have increased fitness in a population (2,4,11,29). Siegel and Bryson (35,36) discovered the mutS gene after observing the mutator phenotype of a strain selected to be azaserine resistant. Also, Helling (13) mutagenized cells and selected for streptomycin-resistant cells and then screened for mutator activity and detected examples of mutators carrying mutT defects, and Hoess and Herman utilized a multip...

A role for DNA mismatch repair in sensing and responding to fluoropyrimidine damage

“…In these circumstances, treatment may result in a cytotoxic response in normal cells with increased resistance to drug treatment due to damage tolerance in tumors that lack MMR activity. Furthermore, analogous to studies observed in bacteria (LeClerc et al, 1996;Mao et al, 1997) MMR-deficient cells could, in fact, be selected for by such DNA-directed FP treatments. In addition, treatment of MMR-deficient tumors may harbor an elevated level of mutation due to FP-induced DNA lesions.…”

Clusters of mutations from transient hypermutability

“…32. Mutator mutations are deleterious in most conditions (33)(34)(35)(36) and are found at correspondingly low frequencies in laboratory populations of, for instance, E. coli and Salmonella typhimurium (37,38). A general argument suggests that excesses of multiples usually occur The reporter gene is natural unless noted to be a transgene.…”

Optimization of DNA polymerase mutation rates during bacterial evolution

“…During long-term adaptation or passage through selective bottlenecks, mutators can arise from a population and outgrow the wild-type strain (15-17). The advantage conferred by mutator genes is indirect because they increase fitness by introducing mutations at other loci that offer selectable growth benefits (14,15).Experiments involving the competition of bacteria with differing mutation rates have generally used only one or a few mutators, have not included antimutators, and have typically entailed pairwise competition. Although mutators often outcompete wild-type strains in serial transfer experiments, theoretical models predict that, as mutation rates increase, a threshold is crossed where hypermutability becomes more deleterious than beneficial (18,19).…”