Altered DNA Mutation Spectrum in Aflatoxin B1-Treated Transgenic Mice That Express the Hepatitis B Virus X Protein

Abstract

Humans chronically infected with hepatitis B virus (HBV) are at further risk of liver cancer upon exposure to dietary aflatoxin B1 (AFB1), a carcinogenic product of the mold Aspergillus flavus. For the present study, we utilized double-transgenic mice (ATX mice) that express the HBV X protein (HBx) and possess a bacteriophage lambda transgene to evaluate the in vivo effect of HBx expression on AFB1-induced DNA mutations. The expression of HBx correlated with a 24% increase in mutation frequency overall and an approximately twofold increase in the incidence of G/C-to-T/A transversion mutations following AFB1 exposure. These results are consistent with a model in which expression of HBx during chronic HBV infection may contribute to the development of hepatocellular carcinoma following exposure to environmental carcinogens.Chronic infection with hepatitis B virus (HBV) is a primary risk factor for the development of hepatocellular carcinoma (HCC) (2, 49). Additional risk factors include chemical agents, such as aflatoxin and ethanol, which can result in liver damage upon repeated exposure (7,56,58). Epidemiological data, as well as experimental studies utilizing transgenic animals, have indicated that chronic HBV infection and exposure to hepatocarcinogens act synergistically in the development of HCC (10,14,31,33,46,50,53,57). However, the mechanism by which chronic HBV infection sensitizes an individual to carcinogeninduced liver cancer remains unknown.The 17-kDa HBV X protein (HBx) is a nonstructural protein necessary for the establishment of hepadnaviral infection in woodchucks and presumably in all mammals (8, 59). Depending upon experimental conditions, HBx is reported to inhibit nucleotide excision repair (NER) (3,17,21,32,41), to influence apoptosis (11,23,39,40,52,54,55), and to induce cell cycle progression (4,26,27). By affecting these pathways, HBx could facilitate the mutagenic effects of hepatocarcinogens or promote the survival and proliferation of hepatocytes altered after exposure. Although transgenic mice that express HBx are more sensitive to the carcinogenic effects of the hepatocarcinogen diethylnitrosamine (DEN [10,33,50]), they do not demonstrate significant increases in overall mutation accumulation (32, 33). These results suggest that HBx contributes to DEN-mediated carcinogenesis by a mechanism that does not involve a dramatic inhibition of DNA repair. Recently, a study demonstrated that expression of HBx increased mutagenesis in a human liver cell line treated with aflatoxin B1 (AFB1) (17). The effect of HBx on AFB1-induced damage in vivo has not been examined.Aflatoxins are a family of potent hepatocarcinogens produced by the mold Aspergillus flavus. A common dietary contaminant in many parts of the world, aflatoxins are metabolized in vivo by cytochrome P450 isoenzymes to form mutagenic epoxides (13). The induction of P450 enzyme levels may explain the increased AFB1 sensitivity of mice that overexpress the hepatitis B surface antigen (24). However, the levels of carcinogen-metabolizin...

TP53 mutations and hepatocellular carcinoma: insights into the etiology and pathogenesis of liver cancer

“…HBx itself has been shown to attenuate nucleotide excision repair and alter the AFB 1 -induced mutation spectrum (Becker et al, 1998;Groisman et al, 1999;Jia et al, 1999;Mathonnet et al, 2004). HBx transgenic mice have increased G:C to T:A transversions induced by AFB 1 when compared with AFB 1 -exposed wild-type mice (Madden et al, 2002). These interferences of HBx with the cellular DNA repair system provide another mechanism of how HBV contributes to liver carcinogenesis.…”

A zebrafish model of intrahepatic cholangiocarcinoma by dual expression of hepatitis B virus X and hepatitis C virus core protein in liver

“…38 Other HBx transgenic mice display a phenotype in the presence of a carcinogen. [39][40][41][42] The aforementioned studies indicated an indirect role for HBx in oncogenesis. However, several studies have supported a direct oncogenic role for HBx in liver cancer.…”

Protective effects of Amaranthus hybridus against aflatoxin B1and fumonisin B1-induced genotoxicity in H4IIE-luc cells

“…For example, AFB 1 adduct induces G > T transversion at specific loci within p53. [54,55] The resulting alterations in DNA can induce changes in the DNA sequence at specific places generating different annealing primer-template sites. [56] This is probably the reason why altered bands were always the same in most of the concentrations and the exposure periods in both the qualitative and the quantitative analysis.…”