Similar frequency and signature of untargeted substitutions induced by abasic site analog under reduced human APE1 conditions.

Abasic sites are formed in cells by various factors including environmental mutagens and considered to be involved in cancer initiation, promotion, and progression. A chemically stable abasic site analog (tetrahydrofuran-type analog, THF) induces untargeted base substitutions as well as targeted substitution and large deletion mutations in human cells. The untargeted substitutions may be initiated by the cleavage of the DNA strand bearing THF by the human apurinic/apyrimidinic endonuclease 1 (APE1) protein, the major repair enzyme for THF and abasic sites. To examine the effects of lower APE1 levels, the protein was knocked down by siRNA in human U2OS cells. A plasmid containing a single THF modification outside the supF gene was introduced into the knockdown cells, and the untargeted substitution mutations in the reporter gene were analyzed. Unexpectedly, the knockdown had no evident impact on their frequency and spectrum. The G bases of 5'-GpA-3' dinucleotides on the modified strand were quite frequently substituted, with and without the APE1 knockdown. These results suggested that the DNA strand cleavage by APE1 is not essential for the THF-induced untargeted base substitutions.