Spontaneous errors in DNA replication have been suggested to play a significant role in neoplastic transformation and to explain the chromosomal alterations seen in cancer cells. A defective replication factor could increase the mutation rate in clonal variants arising during tumour progression, but despite intensive efforts, increases in tumour cell mutation rates have not been unambiguously shown. Here we use an unbiased genomic fingerprinting technique to show that 12 per cent of colorectal carcinomas carry somatic deletions in poly(dA.dT) sequences and other simple repeats. We estimate that cells from these tumours can carry more than 100,000 such mutations. Only tumours with affected poly(dA.dT) sequences carry mutations in the other simple repeats examined, and such mutations can be found in all neoplastic regions of multiple tumours from the same patient, including adenomas. Tumours with these mutations show distinctive genotypic and phenotypic features. We conclude that these mutations reflect a previously undescribed form of carcinogenesis in the colon (predisposition to which may be inherited) mediated by a mutation in a DNA replication factor resulting in reduced fidelity for replication or repair (a 'mutator mutation').

Cancers of the microsatellite mutator phenotype (MMP) show exaggerated genomic instability at simple repeat sequences. More than 50 percent (21 out of 41) of human MMP+ colon adenocarcinomas examined were found to have frameshift mutations in a tract of eight deoxyguanosines [(G)8] within BAX, a gene that promotes apoptosis. These mutations were absent in MMP- tumors and were significantly less frequent in (G)8 repeats from other genes. Frameshift mutations were present in both BAX alleles in some MMP+ colon tumor cell lines and in primary tumors. These results suggest that inactivating BAX mutations are selected for during the progression of colorectal MMP+ tumors and that the wild-type BAX gene plays a suppressor role in a p53-independent pathway for colorectal carcinogenesis.

Genomic instability at simple repeated sequences (SRS) is a landmark for some sporadic and hereditary cancers of the colon. We have identified several human tumour cell lines with up to 1,000-fold increases in mutation rates for endogenous microsatellite sequences, relative to normal cells or tumour cells without the mutator phenotype and show that they are very early events in tumorigenesis. Our in vivo and in vitro results show that the genomic instability persists after transformation and that microsatellite mutations accumulate as consecutive somatic slippage events of a single or a few repeated units. This mechanism may account for the repeat expansions in triplet hereditary diseases and the same defect in replication fidelity in non-polyposis colon cancer could also contribute to the non-mendelian anticipation in these diseases.

Summary Reactive oxygen species (ROS) activate NF-E2-related transcription factor 2 (Nrf2), a key transcriptional regulator driving antioxidant gene expression and protection from oxidant injury. Here we report that in response to elevation of intracellular ROS above a critical threshold, Nrf2 stimulates expression of transcription Kruppel-like factor 9 (Klf9), resulting in further Klf9-dependent increases in ROS and subsequent cell death. We demonstrated that Klf9 independently causes increased ROS levels in various types of cultured cells and in mouse tissues and is required for pathogenesis of bleomycin-induced pulmonary fibrosis in mice. Mechanistically, Klf9 binds to the promoters and alters the expression of several genes involved in the metabolism of ROS, including suppression of thioredoxin reductase 2, an enzyme participating in ROS clearance. Our data reveal an Nrf2-dependent feed-forward regulation of ROS and identify Klf9 as a novel ubiquitous regulator of oxidative stress and lung injury.