efsa-opinion-chromium-food-drinking-water

Chromium in food and drinking water
arrests of DNA replication on DNA templates exposed to trivalent or hexavalent chromium in the
presence of ascorbate (Bridgewater et al., 1994, 1998). Cr(VI) ascorbate-generated DNA adducts were
later shown to be mutagenic and replication blocking by using adduct-carrying shuttle vectors
transfected into human cells (Quievryn et al., 2003). Replication of plasmids containing either Cr(III)-
DNA or Asc-Cr(III)-DNA adducts revealed that the ternary adducts have a much greater mutagenic
potential than the binary adducts. It was estimated that Asc-Cr(III)-DNA adducts accounted for
> 90 % mutagenicity induced by ascorbate-dependent reduction of Cr(VI) under these experimental
conditions. An approximately equal number of deletions and G/C targeted point mutations
characterized the Cr(VI) induced mutational spectrum in human cells. The occurrence of deletion is
consistent with the strong replication-blocking potential of these adducts. Voitkun et al. (1998) in their
analysis of ternary DNA adducts [Cr(III)-mediated crosslinks of DNA with cysteine, histidine, or
glutathione (GSH)] found that these adducts were also mutagenic after replication of adducted
plasmids in human fibroblasts. The GSH-Cr(III)-DNA adducts was the most potent pro-mutagenic
lesions while binary adducts were only weakly mutagenic. Single base substitutions at the G:C base
pairs were the predominant type of mutations for all Cr(III) adducts. Cr(III), Cr(III)-Cys and
Cr(III)-His adducts induced G:C--> A:T transitions and G:C--> T:A transversions with almost equal
frequency, whereas the Cr(III)-GSH mutational spectrum was dominated by G:C--> T:A
transversions. Sequence-specificity for adduct-induced mutations was also reported with mutations
occurring preferentially at G:C pairs where a 3’ purine was adjacent to the mutated guanine. The
formation of mutagenic adducts was confirmed by Zhitkovich et al. (2002) using a similar approach.
In this study they also showed that the cysteine-dependent metabolism of Cr (VI) caused the formation
of mutagenic and replication-blocking DNA lesions. These adducts, which are mutagenic in human
fibroblasts, are formed in the absence of oxidative damage to DNA (Zhitkovich, 2000). The
Asc-Cr(III)-DNA adducts appears to be more mutagenic and replication-blocking than His/Cys
adducts and possibly even the GSH adducts (Quievryn et al., 2003). Intracellular replication of
Cr-modified plasmids demonstrated increased mutagenicity of binary Cr(III)-DNA and ternary
cysteine-Cr(III)-DNA adducts in cells with inactive nucleotide excision repair (Reynolds et al., 2004).
Figure 16: Direct coordination of Cr(III) to 5’-phosphate and hydrogen bonding to N-7 of dG. This
binding mode can occur for both binary and ternary Cr(III)-DNA adducts. It has been proposed to
explain the G selective mutagenesis.
To gain insights into the mutagenic properties of chromium induced DNA lesions mutational spectra
have been also analysed in mammalian cells exposed to chromate (Yang et al., 1992; Chen and Thilly,
1994). In the first report where hprt induced mutational spectrum was analysed in CHO cells (Yang et
al, 1992) mutations occurred predominantly at A:T base pairs whereas in the second study in human
lymphoblastoid cells (Chen and Thilly, 1994) G:C base pairs were the mostly frequently mutated with
both GC > AT and GC > TA changes. This last mutational spectrum is consistent with the
mutagenicity of Cr(III)-derived DNA adducts as detected in single-lesion plasmids replicated in
human cells (see above) and differs significantly from the spectra induced by known oxygen radicalproducing
agents (H 2 O 2 , Fe 2+ and X-ray) analysed in the same study (Chen and Thilly, 1994).
EFSA Journal 2014;12(3):3595 109