Genomic instability in repeated sequences is an early somatic event in colorectal tumorigenesis that persists after transformation.

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.