Upstream eventIncreased, Induced Mutations in Critical Genes
Tumorigenesis, Hepatocellular carcinoma
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Directness||Weight of Evidence||Quantitative Understanding|
|AFB1: Mutagenic Mode-of-Action leading to Hepatocellular Carcinoma (HCC)||indirectly leads to||Moderate||Weak|
Life Stage Applicability
How Does This Key Event Relationship Work
There is no direct evidence addressing AFB1 induced critical gene mutations and the subsequent progression through AHF to HCC. In general it is clear that chemicals that induce the critical cancer gene mutations have a mutagenic MOA for the adverse outcome pathway for cancer. The cells that are mutant for the critical cancer gene undergo a change in phenotype and clonally expand into pre-neoplastic lesions, some of which go on to form hepatocellular carcinoma (HCC).
Weight of Evidence
AFB1 can induce mutations in a wide number of species from bacteria to mammals. The primary mutation induced both in bacteria and mammalian cells is a G:C to T:A transversion, consistent with the pro-mutagenic DNA adducts formed by AFB1 (Foster et al., PNAS 80: 2695-2698 (1983), and also with the mutation observed in the p53 gene from the tumors observed in humans residing in high AFB1 exposure regions.
Empirical Support for Linkage
A mutation in codon 249 in the human p53 gene is observed up to 50% of liver cancers in Qidong, China (Hsu et al., Nature 350: 427-428, 1991) and in Mozambique, areas with high exposure to AFB1. A G:C to T:A transversion in the third base of codon 249 in p53 is observed in up to 75% of HCC in high-incidence areas of China and East Africa (Gouas et al., 2009). This specific mutation is very rare in HCC from no or low aflatoxin areas (Hsu et al., 1991 and Bressac et al. Nature, 350:429-431, 1991). This mutation is also very rare in other types of tumors (Gouas et al., 2009).
Evidence for temporality for mutation and HCC The Dycaico et al. (1996) Big Blue™ rat study and the Chen et al. Big Blue™ neonatal mouse study (2010) show that AFB1 can induce mutation (in a surrogate gene) following a relatively short in vivo exposure time.
The codon 249 mutation is detectable in exposed tissues ahead of cancer development (Gouas et al., Cancer Letters, 286: 29-37, 2009)
In a review of p53 mutation at codon 249 (R249S), Gouas et al., 2009 report that cancer patients have significantly higher levels of DNA fragments circulating in their blood than healthy individuals. Using restriction digestion methods (RFLP-PCR) R249S has been detected in patients from The Gambia, and this was strongly associated with HCC. The presence of this mutation in plasma was linked to the codon 249 p53 mutation in the tumors of individual patients. In addition, using a sensitive and quantitative mass spectrometric method, combined with other molecular techniques, Jackson et al., 2003 were able to detect the R249S mutation in the plasma of patients from The Gambia both before and after clinical diagnosis of HCC (in the same patient) (Jackson et al., 2003). These data provide good evidence of an appropriate temporal sequence of events: induction of the mutation precedes tumor formation.
Uncertainties or Inconsistencies
No data on the relationship between cells with mutations in specific critical genes and the induction of HCC. A substantial body of general information and evidence, is, however available on the etiology of tumors and the involvement of mutations in cancer relevant genes in that etiology.
Quantitative Understanding of the Linkage
Detecting mutation in the critical cancer gene following AFB1 exposure is technically challenging, and the techniques to do this are not widely available. Because there is no quantitative information for the induction of these mutations, there is no data-driven quantitative understanding of the relationship between the AFB1-induced critical cancer gene mutations and HCC.
Evidence Supporting Taxonomic Applicability
While exposure to AFB1 does result in the formation of HCC in various species, experiments investigating codon 249 mutation in the tumors of nonhuman primates, ducks, rats and squirrels do not show a high frequency of this mutation.
Taxonomic Applicability (of this KER)
e.g., Rats, Mice, Woodchucks, Humans, Monkeys, Birds, Trout, Tree shrews This specific KER has not been directly measured; however, there is indirect evidence for rats, humans, and trout.
All references have been cited elsewhere in the AOP.