IARC MONOGRAPHS ON THE EVALUATION OF CARCINOGENIC ...

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In eight subjects (four from Hong Kong and four from Shanghai), single-strand conformation
polymorphism (SSCP) analysis and DNA sequencing of exons 5 to 9 of the TP53
gene revealed the 249 ser mutation. However, the authors also performed comparative
genomic hybridization. In HCC from Shanghai, there were significantly more alterations
per sample in these HBV-related cases than in those from Hong Kong or in the HCCs
from Japan and the USA; approximately double the number of alterations per sample was
observed in Shanghai compared with Hong Kong. The most frequent changes responsible
for this increase were deletions on chromosomes 4q, 8p and 16q and gain of 5p. The
authors suggested that this might reflect broader genetic effects of aflatoxin than simply
the 249 ser mutation in the TP53 gene.
These studies show that, in addition to TP53 mutations, geographical location may
influence other genetic alterations in HCC, but the data are insufficient to ascribe any of
these specifically to aflatoxin exposure.
4.4.2 Experimental systems
(a) General
IARC MONOGRAPHS VOLUME 82
Aflatoxin B 1 induces mutations in Salmonella typhimurium strains TA98 and TA100,
and unscheduled DNA synthesis, chromosomal aberrations, sister chromatid exchange,
micronucleus formation and cell transformation in various in-vivo and in-vitro mammalian
systems (IARC, 1993; for references and details on results published since 1993, see
Table 16).
Aflatoxin B 1 can induce mitotic recombination in addition to point mutations. This
has been demonstrated in both yeast and mammalian cells. In human lymphoblastoid
cells, aflatoxin B 1 treatment led to mitotic recombination and LOH. A reversion assay
demonstrated aflatoxin B 1-induced intrachromosomal recombination in a mutant cell line
derived from V79 cells harbouring an inactivating tandem duplication in the Hprt gene.
Aflatoxin B 1 also induced recombination in minisatellite sequences in yeast
expressing recombinant human CYP1A2. In addition, liver tumours derived from HBVtransgenic
mice treated with aflatoxin B 1 transplacentally contained rearrangements in
minisatellite sequences, but no such alterations were observed in tumours from HBVtransgenic
mice not exposed to aflatoxin B 1 (Kaplanski et al., 1997). This suggests that
aflatoxin can promote genetic instability in addition to point mutations. Mitotic recombination
and genetic instability may therefore be alternative mechanisms by which aflatoxin
contributes to genetic alterations such as LOH in HCC (see Section 4.4.1(c)).
As expected, aflatoxin B 1 is significantly more mutagenic following metabolic activation.
The mutagenicity of aflatoxin B 1 in Salmonella tester strains TA98 and TA100
without S9 was approximately 1000 times lower than in the presence of S9.
Splenic lymphocytes were examined for mutant frequency at the Hprt locus in
Fischer 344 rats exposed to aflatoxin B 1. Hprt mutants (frequency, 19.4–31.0 × 10 –6 )
were induced after a three-week exposure of male Fischer 344 rats to aflatoxin B 1 by
repeated intragastric dosing to a total dose of 1500 μg/kg bw. In the same experiment,