A role for certain mouse Aprt sequences in resistance to toxic adenine analogs

A role for Pms2 in the prevention of tandem CC --> TT substitutions induced by ultraviolet radiation and oxidative stress

DNA mismatch repair (MMR) is important for preventing base-pair substitutions caused by spontaneous or damage-related DNA polymerase errors. We have used a reversion assay based on mouse Aprt to investigate the role of MMR in preventing ultraviolet radiation (UV) and oxidative stress induced tandem CC --> TT base pair substitutions in cultured mammalian cells. The reversion construct used for this assay can detect both C --> T and CC --> TT mutational events. Most spontaneous mutations in Pms2-deficient cells were single C --> T substitutions (88%), with the remainder being tandem CC --> TT substitutions (12%). The percentage of tandem CC --> TT substitutions rose to 64% and 94% for Pms2-deficient cells exposed to UV and a mixture of hydrogen peroxide and metals (Cu/Fe), respectively. Exposure to hydrogen peroxide alone or metals alone did not induce the tandem substitutions, nor did treatment of the cells with the alkylating agent ethylmethane sulfonate, which induces G --> A substitutions on the opposite strand. Tandem CC --> TT substitutions were also induced by UV irradiation and the hydrogen peroxide/metal mixture in Pms2-proficient cells, but at frequencies significantly lower than those observed in the Pms2-deficient cells. We conclude that mismatch repair plays an important role in preventing tandem CC --> TT substitutions induced by certain genotoxin exposures.

Oxidative mutagenesis, mismatch repair, and aging

A PubMed search for the term "oxidative stress" yields over 29,000 articles published on the subject over the past 10 years; more than 2000 of these articles also include the term "aging" in their title or abstract. Many theories of aging predict causal roles for oxidative stress in the myriad of pathological changes that occur as a function of age, including an increasing propensity to develop cancer. A possible link between aging and cancer is the induction and accumulation of somatic mutations caused by oxidative stress. This Review focuses on small mutational events that are induced by oxidative stress and the role of mismatch repair (MMR) in preventing their formation. It also discusses a possible inhibitory effect of oxidative stress on MMR. We speculate that a synergistic interaction between oxidative damage to DNA and reduced MMR levels will, in part, account for an accumulation of small mutational events, and hence cancer, with aging.

A mouse kidney cell line with a G:C --> C:G transversion mutator phenotype

We report the identification of a mouse kidney epithelial cell line (K435) in which G:C-->C:G transversion mutations occur at an elevated rate and are the predominant spontaneous events observed at the selectable Aprt locus. Of three genotoxins tested, ultraviolet radiation (UV), ionizing radiation, and hydrogen peroxide, only UV exposure was able to alter the spectrum of small mutational events. To determine if the G:C-->C:G mutator phenotype was due to a deficiency in the mismatch repair pathway, the K435 cells were tested for resistance to 6-thioguanine, cisplatin, and MNNG. Although the K435 cells were as resistant to 6-thioguanine and cisplatin as Pms2 and Mlh1 null kidney cells, they were hypersensitive to MNNG. Moreover, the K435 cells do not exhibit microsatellite instability, a hallmark of mismatch repair deficiency. These results suggest that a novel mechanism, which does not include a classical deficiency in mismatch repair, accounts for the G:C-->C:G mutator phenotype.

Multiple mutations are common at mouse Aprt in genotoxin-exposed mismatch repair deficient cells

Mismatch repair deficiency is known to contribute to elevated rates of mutations, particularly at mono- and dinucleotide repeat sequences. However, such repeats are often missing from the coding regions of endogenous genes. To determine the types of mutations that can occur within an endogenous gene lacking highly susceptible repeat sequences, we examined mutagenic events at the 2.3 kb mouse Aprt gene in kidney cell lines derived from mice deficient for the PMS2 and MLH1 mismatch repair proteins. The Aprt mutation rate was increased 33-fold and 3.6-20-fold for Mlh1 and Pms2 null cell lines, respectively, when compared with a wild-type kidney cell line. For the Pms2 null cells this increase resulted from both intragenic events, which were predominantly base-pairs substitutions, and loss of heterozygosity events. Almost all mutations in the Mlh1 null cells were due to base-pair substitutions. A:T-->G:C transitions (54% of small events) were predominant in the Pms2 null cells whereas G:C-->A:T transitions (36%) were the most common base-pair change in the Mlh1 null cells. Interestingly, 4-9% of the spontaneous mutant alleles in the mismatch repair deficient cells exhibited two well-separated base-pair substitution events. The percentage of mutant alleles with two and occasionally three base-pair substitutions increased when the Pms2 and Mlh1 null cells were treated with ultraviolet radiation (15-21%) and when the Mlh1 null cells were treated with hydrogen peroxide (35%). In most cases the distance separating the multiple base-pair substitutions on a given allele was in excess of 100 base-pairs, suggesting that the two mutational events were not linked directly to a single DNA lesion. The significance of these results is discussed with regards to the roles for the PMS2 and MLH1 proteins in preventing spontaneous and genotoxin-related mutations.

Spontaneously immortalized cell lines obtained from adult Atm null mice retain sensitivity to ionizing radiation and exhibit a mutational pattern suggestive of oxidative stress

The study of Ataxia-telangiectasia (A-T) has benefited significantly from mouse models with knockout mutations for the Atm (A-T mutation) locus. While these models have proven useful for in vivo studies, cell cultures from Atm null embryos have been reported to grow poorly and then senesce. In this study, we initiated primary cultures from adult ears and kidneys of Atm homozygous mice and found that these cultures immortalized readily without loss of sensitivity to ionizing radiation and other Atm related cell cycle defects. A mutational analysis for loss of expression of an autosomal locus showed that ionizing radiation had a mutagenic effect. Interestingly, some spontaneous mutants exhibited a mutational pattern that is characteristic of oxidative mutagenesis. This result is consistent with chronic oxidative stress in Atm null cells. In total, the results demonstrate that permanent cell lines can be established from the tissues of adult mice homozygous for Atm and that these cell lines will exhibit expected and novel consequences of this deficiency.

The primary function of a redundant Sp1 binding site in the mouse aprt gene promoter is to block epigenetic gene inactivation

The promoter region of the mouse adenine phosphoribosyltransferase (aprt) gene contains one non-consensus Sp1 binding site at its 5' end followed by three consensus Sp1 binding sites. The two 3'-most binding sites are sufficient for maximal expression of aprt , suggesting that the non-consensus and consensus binding sites at the 5' end are redundant. However, the two 3' sites are not sufficient to block epigenetic inactivation, which led to the hypothesis that the redundant consensus and/or non-consensus 5' Sp1 binding sites are required to block inactivation events. To test this hypothesis, promoter region constructs were made in which the two 5' Sp1 binding sites were mutated alone or in tandem, and then each construct was tested for its ability to withstand epigenetic inactivation. A cis -acting methylation center that is normally located 1.2 kb upstream of the promoter was used to induce inactivation. The results demonstrate that the presence of the redundant consensus Sp1 binding site is required to block methylation-associated gene inactivation. Therefore, the Sp1 binding sites comprising the mouse aprt promoter have evolved two distinct functions, one to promote transcription and the other to block epigenetic inactivation.