A DNA polymerase β mutant from colon cancer cells induces mutations

Previous investigations have shown that ≈35% of the 90 tumors analyzed to date contain mutations within the DNA polymerase β (pol β) gene. The existence of pol β mutations in a substantial fraction of human tumors studied suggests a link between DNA pol β and cancer. A DNA pol β variant, in which Lys-289 has been altered to Met, was identified previously in a colorectal carcinoma. The K289M protein was expressed in mouse L cells containing the λ cII mutational target. The λ DNA was packaged and used to infect bacterial cells to obtain the spontaneous mutation frequency. We found that expression of K289M in the mouse cells resulted in a 2.5-fold increase in the mutation frequency. What was most interesting was that expression of K289M in these cells resulted in a 16-fold increase in the frequency of C to G or G to C base substitutions at a specific site within the cII target. By using this cII target sequence, kinetic analysis of the purified K289M protein revealed that it was able to misincorporate dCTP opposite template C and dGTP opposite template G with significantly higher efficiency than the wild-type pol β protein. We provide evidence that misincorporation of nucleotides by K289M results from altered positioning of the DNA within the active site of the enzyme. Our data are consistent with the interpretation that misincorporation of nucleotides resulting from altered DNA positioning by the K289M protein has the potential to result in tumorigenesis or neoplastic progression.