Detection of mutations in the DNA polymerase delta gene of human sporadic colorectal cancers and colon cancer cell lines

To test the hypothesis whether DNA polymerases acquire mutator properties d uring tumor development (mutator hypothesis), we examined DNA polymerase de lta mRNA in 6 colon cancer cell lines (DLD-1, HCT116, SW48, HT29, SW480 and SW620) and 7 sporadic human colorectal cancers. For analysis we used ampli fication of cDNA by polymerase chain reaction, single-strand conformation p olymorphism and sequencing techniques. In 5 of the cell lines, 9 mutations leading to changes of the amino acid sequence of DNA polymerase delta were detected. Most mutations were found in the cell lines DLD-1, HCT116 and SW4 8 for which defects in mismatch repair genes had been identified previously . In the majority of cases, wild type and mutated sequences were present. I n 2 cell lines (HCT116 and SW48), a single-nucleotide deletion occurred at the same position. This resulted in a premature termination codon by which the DNA interaction domain of the enzyme was eliminated. Furthermore, seque nce deviations were found in the tumor tissues of 4 colon cancer patients. Wild-type and altered sequences were present simultaneously. The deviations included missense mutations (2 cases) and silent mutations (2 cases). The missense mutations and one of the silent mutations were found in normal muc osa as well, in addition, the mutation clustered region of a tumor suppress or gene, often found to be defective in colon cancer, the adenomatous polyp osis coli (APC) gene, was investigated in surgical specimens and cell lines . One carcinoma and 2 cell lines exhibited amino acid changes in both the D NA polymerase delta gene and in the mutation clustered region of the APC ge ne. Since most of the mutations detected in the DNA polymerase delta mRNA a re likely to alter the structure of the protein, the enzyme is expected to be functionally impaired. In particular, copying fidelity might be decrease d, thus contributing to the high mutation rate observed in colorectal cance r. Int. J. Cancer, 80:919-929, 1999. (C) 1999 Wiley-Liss, Inc.