Dietary exposure to aflatoxin B(1) (AFB(1)) is associated with an increased incidence of hepatocellular carcinoma (HCC), especially in populations in which exposure to hepatitis B virus (HBV) is a common occurrence. Most HCC samples from people living where HBV is prevalent have one striking mutational hotspot: a GC-->TA transversion at the third position of codon 249 of the p53 gene. In this review, the chemical reaction of an electrophilic derivative of aflatoxin with specific DNA sequences is examined, along with the types of mutations caused by AFB(1) and the sequence context dependence of those mutations. An attempt is made to assign the source of these mutations to specific chemical forms of AFB(1)-DNA damage. In addition, epidemiological and experimental data are examined regarding the synergistic effects of AFB(1) and HBV on HCC formation and the predominance of one hotspot GC-->TA transversion in the p53 gene of affected individuals.

A G to T mutation has been observed at the third position of codon 249 of the p53 tumor-suppressor gene in over 50% of the hepatocellular carcinoma cases associated with high exposure to aflatoxin B1 (AFB1). Hypotheses have been put forth that AFB1, in concert with hepatitis B virus (HBV), may play a role in the formation of, and͞or the selection for, this mutation. The primary DNA adduct of AFB1 is 8,9-dihydro-8-(N 7 -guanyl)-9-hydroxyaflatoxin B1 (AFB1-N7-Gua), which is converted naturally to two secondary lesions, an apurinic site and an AFB1-formamidopyrimidine (AFB1-FAPY) adduct. AFB1-FAPY is detected at near maximal levels in rat DNA days to weeks after AFB1 exposure, underscoring its high persistence in vivo. The present study reveals two striking properties of this DNA adduct: (i) AFB1-FAPY was found to cause a G to T mutation frequency in Escherichia coli approximately 6 times higher than that of AFB1-N7-Gua, and (ii) one proposed rotamer of AFB1-FAPY is a block to replication, even when the efficient bypass polymerase MucAB is used by the cell. Taken together, these characteristics make the FAPY adduct the prime candidate for both the genotoxicity of aflatoxin, because mammalian cells also have similar bypass mechanisms for combating DNA damage, and the mutagenicity that ultimately may lead to liver cancer. Aflatoxin B 1 (AFB 1 ), one of the most potent known liver carcinogens, is produced by the common soil fungus Aspergillus flavus. Exposure to this toxin is high in regions of the world where certain foods are improperly stored (1). Hepatitis B virus (HBV) is also common in these regions, and epidemiological evidence indicates that there is a synergistic interaction between AFB 1 exposure and HBV infection on the induction of hepatocellular carcinoma (HCC). In over 50% of HCC cases studied in these areas, a characteristic G to T mutation is observed at the third position of codon 249 of the p53 tumorsuppressor gene (2, 3). Whether this specific sequence is an exceptional target for mutations caused by AFB 1 or whether the mutation is selected for once it occurs remains to be determined. However, each of these scenarios shares the fundamental early step involving generation of a G to T mutation.There is substantial evidence that AFB 1 -induced G to T mutations in cellular ras genes may also be a step in transformation of normal cells to malignant cells (4-6). In humans these mutations occur at the first and second positions of codon 12 in the Ha-ras protooncogene (7) and they are in sequence contexts similar, but not identical, to that of codon 249 in p53.Many studies have defined the mutational spectrum produced after exposure of cells to either the epoxide, which is the toxicologically relevant natural metabolite of AFB 1 (8), or to other electrophilic derivatives that serve as models for the epoxide (9). The G to T mutation is predominantly observed (2,3,(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Studies of mutational landscapes, by their nature, do not elucidate which specific chemical form o...