**Author details**

Another suggested possibility is that the activity of phase II detoxification enzymes (gluta‐ thione S transferase (GST) and epoxide hydrolase(EPHX)) may play a role in the genesis of HCC induced jointly by AFB1 and HBV [94; 73; 56]. A multiplicative interaction in the gene‐ sis of HCC in West African and Chinese patients was demonstrated between HBV infection and mutations of EPHX [56]: patients with chronic HBV infection but with normal EPHX al‐ leles were at a 15-fold increase in risk, and those with both HBV infection and at least one EPHX mutant were at a 77-fold increased risk. In further studies in these patients a positive interaction between HBV and AFB1 seemed to depend on the presence of a polymorphism of the GST M1, GST T1, and EPHX genes that are normally responsible for converting the car‐ cinogenic AFB1 -8,9-epoxide to non-reactive metabolites [56, 8, 94, 73]. But again no consis‐ tent pattern has emerged. In one analysis in Taiwan the risk of HCC formation was greater in HBV carriers who had the GST M1 null genotype compared with the non-null genotype [94], in a second study the risk appeared to depend on the presence of a GST T1 null geno‐ type [73], and in a third the risk was considerably greater in those with null genotypes of

Another possible mechanism for a carcinogenic interaction between AFB1 and HBV is that increased hepatocyte necrosis and proliferation cause by chronic HBV infection increases the likelihood of both AFB1 mutations, including 249ser, and the subsequent clonal expansion of cells containing these mutations [13]. Chronic necroinflammatory hepatic disease, including that resulting from HBV infection, results in the generation of oxygen and nitrogen reactive species [50, 35]. Both of the latter are mutagenic, but, in addition, increased oxidative stress

The HBV x gene is frequently included in sequences of the virus that are integrated into cel‐ lular DNA [43]. AFB1-DNA adducts are normally repaired by the nucleotide excision repair pathway. The HBV X protein interferes with the nuclear excision repair pathway [38; 43] and might, by this means, favour persistence of existing mutations or impaired DNA. DNA repair is also compromised by the rapid cell turnover rate in chronic hepatitis. In the pres‐ ence of dietary exposure to AFB1, the HB X protein may contribute to the uncontrolled cell proliferation in other ways. The transcription of p21 waf1/ cip1, which induces cell cycle arrest at the G1-S checkpoint, is activated by HB X protein in a dose-dependent manner in the pres‐ ence of functional p53. This transcription is, however, repressed by HB X protein when p53 is not functional or is functional at a low level [1]. The expression of HB X protein also corre‐ lates with an increase in the overall frequency of DNA mutations in transgenic mice and a 2 fold increase the incidence of the 249ser mutation in transgenic mice exposed to AFB1 [55]. Altered methylation of genes may play a role in hepatocarcinogenesis [43]. For example, the methylation status of the human *ras* association domain gene (RASSF1A) and the P16 gene has been incriminated in the pathogenesis of HCC [95]. No association was found between methylation status and P53 status {95]. A statistically significant association was, however, found between RASSF1A methylation status and the level of AFB1-DNA adducts in HCC

An understanding of the mechanisms responsible for the heightened risk of malignant transformation in patients chronically infected with HBV and exposed to AFB1 is far from

both GST M1 and GST T1 [8].

230 Aflatoxins - Recent Advances and Future Prospects

tissues [95].

has been shown to induce 249ser mutations [29].

Michael C. Kew1\*

Address all correspondence to: michael.kew@uct.ac.za

1 Department of Medicine, Faculty of Health Sciences, University of Cape Town, Depart‐ ment of Medicine, Faculty of Health Sciences, University of the Witwatersrand, Johannes‐ burg, South Africa
