Thompson CM, Suh M, Proctor DM, Rager JE, Haws LC, and Harris MA. 2016. Assessment of the in vivo genotoxicity of CrVI in target organs identified in a two-year cancer bioassay. Presented at the Society of Toxicology’s 55th Annual Meeting, March 13-17, New Orleans, LA.
Studies indicate there is strong correlation between in vivo genotoxic potency and in vivo carcinogenic potency. This correlation exists despite the fact that genotoxicity is often measured in non-target tissue (i.e. those that do not develop tumorigenic responses). Here, we review the in vivo genotoxicity of hexavalent chromium (CrVI) in drinking water in target and non-target tissues. In a 2-yr study, CrVI in drinking water increased duodenal adenomas and carcinomas in B6C3F1 mice at ≥20 ppm, and oral cavity squamous cell carcinomas in F344 rats at ~180 ppm. Results for peripheral blood micronucleus (MN) assays in the literature were generally negative. In the gastrointestinal tract, we measured mutant frequency (MF) in the oral mucosa of Big Blue® TgF344 rats. Relative to controls, a statistically significant (20- to 40-fold) increase in cII MF was observed in rats treated with 10 ppm 4NQO, but no change occurred in rats treated with 180 ppm CrVI. In the duodenum of B6C3F1 mice, we measured a high background MF of k-ras codon 12 GAT mutations in control mice (~1E-3); however, no differences in MF were observed in mice treated with 180 ppm CrVI. Several analyses of duodenal crypts failed to indicate an increase in crypt MN in B6C3F1 mice exposed to 180 ppm CrVI. Phosphorylated H2AX immunostaining corresponded to aberrant crypt enterocytes in cyclophosphamide-treated mice, but not CrVI-treated mice. Consistent with these negative findings, X-ray fluorescence microscopy demonstrated Cr fluorescence in duodenal villi but little or no Cr fluorescence in duodenal crypts. Transcriptomic analyses did not indicate activation of signaling commonly involved in intestinal carcinogenesis (e.g. Wnt/β-catenin signaling). In summary, the low systemic genotoxic potency of CrVI was consistent with the lack of systemic carcinogenic potency; and genotoxic responses were negative in site of contact tumor tissues. The absence of genotoxic responses in target tissues suggest that non-genotoxic mechanisms are likely involved in the tumorigenic responses.