Inverse correlation between RER+ status and p53 mutation in colorectal cancer cell lines

The high point mutation rate of replication error-prone (RER+) cells could theoretically lead to inactivation of the p53 gene by polyclonal mutations, which might explain the conflicting results that have been published on the p53 status of RER+ colon cancers. To address this issue, we tested the p53 status of 21 human colorectal cancer cell lines, including four showing microsatellite instability (RER+ phenotype). Denaturing gradient gel electrophoresis (DGGE) followed by sequencing showed that all four RER+ cell lines were wild type for p53 while 15 of the 17 RER- cell lines contained p53 mutations (P=0.001). Eight cell lines (four RER+ and four RER-) were analysed using three complementary methods to test more rigorously the polyclonal mutation hypothesis. (i) Of 87 single-cell clones (seven to 14 per cell line) examined by DGGE, only those derived from known p53 mutant cell lines showed altered profiles. (ii) Antibody DO-7 stained more than 80% of nuclei from the p53 mutant cell lines, but only 15% of nuclei from the RER+ cell lines. (iii) A yeast functional assay which can simultaneously detect polyclonal mutations at over 500 different sites in the p53 cDNA scored all four RER+ cell lines as containing only transcriptionally active p53. These data thus do not support the polyclonal mutation hypothesis and instead suggest that mismatch repair deficiency provides a p53-independent pathway for development of colorectal cancers.