Inactivation of the DNA mismatch-repair machinery can drive tumorigenesis, as mutations in genes involved in cell growth and survival are not repaired. But does this mutator phenotype also mutate other repair genes to accelerate genomic instability and tumorigenesis? Sergei Malkhosyan and co-workers, reporting in the 18 December issue of Proceedings of the National Academy of Sciences, show that mutation of two mismatch-repair genes, rather than one, not only increases the mutation rate, but also changes the predominant type of mutation.

The SW48 human colon adenocarcinoma cell line lacks expression of the mismatch-repair gene MLH1 , owing to promoter methylation, and contains a heterozygous frameshift mutation in another, MSH6 . The authors analysed clones of this unstable cell line to isolate cells in which the wild-type allele of MSH6 was also mutated — to generate MLH1−/− MSH6−/− cells — and cells in which the mutated MSH6 allele had reverted to wild type — MLH1−/− MSH6+/+ cells. The mutation rate and type were then determined in the two cell types, using the endogenous HPRT gene as a reporter.

The mutation rate was significantly higher in the double mutant cells: 2.3 × 10−5 per allele per replication, compared with 0.9 × 10−5. Interestingly, the mutation spectrum was also different, depending on whether MSH6 was present. The mutations were all single point mutations, consistent with the mismatch-repair deficiency, but MSH6+/+ cells predominantly had single base-pair insertions or deletions, whereas MSH6−/− cells predominantly had base substitutions and transitions, which resulted in missense or nonsense mutations.

So, mismatch repair — and the mutator phenotype that ensues when the process is deficient — is not quite as simple as we thought. The hypothesis that inactivation of MLH1 would prevent all mismatch repair must be re-evaluated in light of these findings, and the idea that the mutator phenotype accelerates through a mutator 'cascade' gains further ground. The consequences of mutation in more than one mismatch-repair gene are now known, but the extent of the impact that this might have on tumorigenesis remains to be determined.