Involvement of DNA Methylation and CpG Endonuclease Activity in Environmental Carcinogenesis and Cancer Chemoprevention
DNA hypomethylation has been found in cancer cells and in carcinogen treated normal appearance tissue. It causes chromosome instability and loss of imprinting (LOI), which are found in human cancers. Induction of DNA hypomethylation may be involved in chemical carcinogenesis. Female B6C3F1 mice were administered sodium arsenite,dimethylarsinic acid (DMA), bromodichloromethane (BDCM), dibromoacetic acid(DBA) and dichloroacetic acid (DCA)in their drinking water for up to 28 d. The methylation of liver DNA from the treated animals decreased dramatically and LOI of IGF-II was found. Methionine prevented the LOI induced by arsenic. The mechanism of DNA hypomethylation has been studied. A sensitive CpG nuclease activity assay was developed. Using this assay we determined CpG nuclease was the enzyme causing DNA hypomethylation in mouse lung tumors induced by vinyl carbamate. CpG nuclease preferred double stranded DNA as the substrate and its activity is inhibited by EDTA and 2-mercaptoethanol. In vinyl carbamate-induced mouse lung tumors, the enzyme activity was increased; budesonide prevented the increased of CpG nuclease activity. We also found that there was an inverse relationship between CpG nuclease activity and DNA methylation, which suggests that CpG nuclease is the mechanism to cause DNA hypomethylation in vinyl carbamate-induced mouse lung tumor. Hypermethylation of tumor suppressor genes has been found in many human cancers. In this thesis we found the hypermethylation of estrogen receptor-alpha gene in DCA and TCA-induced mouse liver tumors, but not in non-involved liver. Alteration of mRNA expression of cancer-associated genes in non-involved liver and liver tumor induced by DCA also was measured. We found that survivin was specifically over-expressed in liver tumor; GST-ð and PPAR-ã were over expressed in both non-involved liver and in liver tumor. In summary, DNA hypomethylation, LOI of IGF-II, CpG nuclease activity and alteration of mRNA expression of cancer-associated genes can be used as biomarkers to study the carcinogenic mechanism of nongenotoxic carcinogens. DNA hypomethylation, LOI of IGF-II can be used as surrogate end-point biomarkers to evaluate the efficacy of the cancer chemopreventive agents.
School:University of Toledo Health Science Campus
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:dna methylation demethylation carcinogenesis loss of imprinting cancer chemoprevention environmental carcinogens
Date of Publication:01/01/2006