Even in a human whose DNA repair system works normally, the estimated error rate of DNA in newly made cells is about one mutation in 10 billion bases replicated. Each adult human being has 50 to 100 trillion cells, and since each cell contains 6 billion base pairs, during a normal lifetime there must be a truly astronomical number of DNA errors. Fortunately, most of these mutations do not strike genes that are relevant to cancer and most are eliminated by the sophisticated mechanisms of DNA repair. Working with bacteria, HHMI investigator Paul Modrich at the Duke University Medical Center in Durham, North Carolina, had previously found that mismatch repair depends largely on the activity of two proteins, MutS and MutL, which lead the mismatched pairs to be removed and new DNA bases to be inserted. He then identified homologs of these proteins in human cells.
Most colon cancers are non-hereditary, however.
So a very recent finding by Modrich and Vogelstein may affect a far larger proportion of the 500,000 people, worldwide, who are diagnosed with colon cancer every year. The new findings may also hold promise of new treatments for such cancers.