Adaptive reversions of a lac frameshift mutation in Escherichia coli are -1 deletions in small mononucleotide repeats, whereas growth-dependent reversions are heterogeneous. The adaptive mutations resemble instability of simple repeats, which, in hereditary colon cancer, in yeast, and in E. coli occurs in the absence of mismatch repair. The postulate that mismatch repair is disabled transiently during adaptive mutation in E. coli is supported here by the demonstration that the growth-dependent mutation spectrum can be made indistinguishable from adaptive mutations by disallowing mismatch repair during growth. Physiologically induced mismatch repair deficiency could be an important mutagenic mechanism in cancers and in evolution.Adaptive mutations occur in the apparent absence of cell division, in cells exposed to a nonlethal selection (1-6), and have been detected only in genes whose function was selected (refs. 2, 4, .and 7, but see ref. 6). Thus, these mutations differ from spontaneous mutations in growing cells. In an experimental system that detects reversion of a + 1 frameshift mutation in an Escherichia coli lacI-lacZ fusion gene (2), the adaptive mutations are further distinguished from the growthdependent reversions by their molecular mechanism of formation and by their sequences. The adaptive reversions alone require genes of the E. coli RecBCD recombination system (8), implying that genetic recombination is part of the molecular mechanism by which they form. Also, the adaptive Lac' reversions are almost all single-base deletions in small mononucleotide repeats, whereas the growth-dependent Lac+ reversions include large and small insertions, deletions, and duplications and are not confined to simple repeats (9,10). This paper addresses the origin of the unusual sequence spectrum of the adaptive reversions of the lac +1 frameshift mutation.

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Anderson, C. W., Dunn, J. J., Freimuth, P. I., Galloway, A. M. and Allalunis-Turner, M. J. Frameshift Mutation in PRKDC, the Gene for DNA-PKcs, in the DNA Repair-Defective, Human, Glioma-Derived Cell Line M059J. Radiat. Res. 156, 2-9 (2001). The glioma-derived cell line M059J is hypersensitive to ionizing radiation, lacks DNA-PK activity, and fails to express protein for the catalytic subunit, DNA-PKcs, while a sister cell line, M059K, derived from the same tumor, has normal DNA-PK activity. Both cell lines are near pentaploid and have multiple copies of chromosome 8, the chromosome on which the DNA-PKcs gene, PRKDC, is located. Sequence analysis of PCR-amplified exons revealed the loss in M059J cells of a single "A" nucleotide in exon 32, corresponding to the first nucleotide of codon 1351 (ACC, Thr) of PRKDC. Loss of the "A" nucleotide would terminate the DNA-PKcs reading frame early in exon 33. DNA from M059K cells had only the wild-type sequence. An analysis of sequences surrounding PRKDC exon 32 from 87 unrelated individuals revealed no polymorphic nucleotides except for a triplet repeat near the 3' end of this exon; no individual had a frameshift mutation in exon 32. No other sequence differences in PRKDC between M059J and M059K cells were observed in approximately 15,000 bp of genomic sequence including the sequences of exons 5 through 38 and surrounding intron sequence, suggesting a possible reduction to homozygosity at this locus prior to acquisition of the mutation leading to the M059J cell line.

Variations in the levels of ATM protein are insufficient to explain the differences in cellular radiosensitivity observed in a panel of human malignant glioma cell lines.