Potassium bromate treatment predominantly causes large deletions, but not GC > TA transversion in human cells

Potassium bromate (KBrO3) is strongly carcinogenic in rodents and mutagenic in bacteria and mammalian cells in vitro. The proposed genotoxic mechanism for KBrO3 is oxidative DNA damage. KBrO3 can generate high yields of 8-hydroxydeoxyguanosine (8OHdG) DNA adducts, which cause GC > TA transversions in cell-free systems. In this study, we investigated the in vitro genotoxicity of KBrO3 in human lymphoblastoid TK6 cells using the comet (COM) assay, the micronucleus (MN) test, and the thymidine kinase (TK) gene mutation assay. After a 4 h treatment, the alkaline and neutral COM assay demonstrated that KBrO3 directly yielded DNA damages including DNA double strand breaks (DSBs). KBrO3 also induced MN and TK mutations concentration-dependently. At the highest concentration (5 mM), KBrO3 induced MN and TK mutation frequencies that were over 30 times the background level. Molecular analysis revealed that 90% of the induced mutations were large deletions that involved loss of heterozygosity (LOH) at the TK locus. Ionizing-irradiation exhibited similar mutational spectrum in our system. These results indicate that the major genotoxicity of KBrO3 may be due to DSBs that lead to large deletions rather than to 8OHdG adducts that lead to GC > TA transversions, as is commonly believed. To better understand the genotoxic mechanism of KBrO3, we analyzed gene expression profiles of TK6 cells using Affymetrix Genechip. Some genes involved in stress, apoptosis, and DNA repair were up-regulated by the treatment of KBrO3. However, we could not observe the similarity of gene expression profile in the treatment of KBrO3 to ionizing-irradiation as well as oxidative damage inducers.