The Tyr-265-to-Cys mutator mutant of DNA polymerase beta induces a mutatorphenotype in mouse LN12 cells

DNA polymerase beta functions in both base excision repair and meiosis, Err ors committed by polymerase beta during these processes could result in mut ations. Using a complementation system, in which rat DNA polymerase beta su bstitutes for DNA polymerase I of Escherichia coil, we previously isolated a DNA polymerase beta mutant in which Tyr-265 was altered to Cys (Y265C). T he Y265C mutant is dominant to wild-type DNA polymerase beta and possesses an intrinsic mutator activity. We now hale expressed the wild-type DNA poly merase and the Y265C mutator mutant in mouse LN12 cells, which have endogen ous DNA polymerase beta activity. We demonstrate that expression of the Y26 5C mutator mutant in the LN12 cells results in an 8-fold increase in the sp ontaneous mutation frequency of lambda cII mutants compared with expression of the mild-type protein. Expression of Y265C results in at least a 40-fol d increase in the frequency of deletions of three bases or more and a 7-fol d increase in point mutations. Our results suggest that the mutations we ob serve in vivo result directly from the action of the mutator polymerase, To our knowledge, this is the first demonstration of a mutator phenotype resu lting from expression of a DNA polymerase mutator mutant in mammalian cells . This work raises the possibility that variant polymerases may act in a do minant fashion in human cells, leading to genetic instability and carcinoge nesis.