Bladder and liver tumorigenesis induced by 2‐acetylaminofluorene in different F1mouse hybrids: Variation within genotypes and effects of using more than one genotype on risk assessment

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Abstract
Several F1 mouse hybrids were used in a chronic bioassay to determine whether such an experimental design would provide greater statistical power than using only the B6C3F1 hybrid. For this purpose, the dose response of formation of hepatocellular and bladder tumors after 30 mo of feeding 2‐acetylaminofluorene (2‐AAF) in the diet was assessed in 4 F1 mouse hybrids, including the B6C3F1 hybrid. No strain background‐related differences in frequency of bladder neoplasms between any F1 hybrids were detected. Bladder tumors occurred only at the highest 2‐AAF dose in female mice. In males the lowest dose was already sufficient to induce bladder neoplasms with incidences of 25–48% adjusted for different nontumor mortality patterns across doses. No marked strain‐related differences in hepatocellular tumor rates were apparent in either sex. Higher frequencies of hepatocellular neoplasms were observed among the untreated control males of the B6C3, AY, and CVA F1 hybrids than among the comparable females. Among treated mice, the lowest 2‐AAF dose increased liver tumor incidence, more so among the females than among the males. The different background genomes resulted in somewhat different risk assessments for liver tumor formation in males due to differences in the time‐to‐tumor curves. Except for the much higher background liver tumor rate in the CVY mice, the adjusted liver tumor incidences were similar across the four hybrids. Hence, the levels of statistical significance obtained for dose‐response trends and comparisons of treated and control groups were similar using 48 animals per dose group with B6C3 mice, or combinations of 24 animals per dose from 2 genotypes, or 12 animals per dose from the 4 hybrid genotypes. Estimates of carcinogenic potency for bladder tumors were similar, within a factor of two, across the four hybrids. However, estimates of liver tumor potency across genotypes varied by a factor of two and six for females and males, respectively. Thus, the mean of cancer potency estimates across these genotypes would be more representative for mice than results from any single genotype. As in chronic carcinogenesis studies with other test agents, neoplasms developed in only a certain proportion, rather than in all, of the genetically identical animals exposed to a given dose of the toxicant for the same length of time under the same controlled environmental conditions. This phenotypic variability in toxic responses may reflect differential regulation of gene expression among the genetically identical test animals.