Quantitative assessment of azoxymethane-induced aberrant crypt foci in inbred mice

Heritable differences in tumor susceptibility are observed in mice after repetitive exposures to the organotropic colon carcinogen azoxymethane (AOM). The following study was undertaken to determine whether early morphological alterations within the colonic epithelium correlate with subsequent cancer risk. A/J and SWR/J (susceptible) and AKR/J (resistant) mice were injected once a week with AOM at a dose of 10 mg/kg, i.p., for a total of 6 weeks. Four weeks after the last injection, methylene blue-stained whole-mount colons were examined for the presence of colonic epithelial lesions referred to as aberrant crypt foci (ACF). Putative lesions identified under low magnification were further characterized by H&E staining of corresponding sections. AOM produced a treatment-related increase in ACFs in each of the mouse lines examined. The tumor-susceptible SWR/J and A/J mice developed on average between three- and sixfold more ACFs in the distal colon (32 and 15/cm of colon, respectively) than the resistant AKR/J mice (5/cm colon). The size distribution of ACFs was further analyzed in each of the strains. In SWR/J and A/J, 20-35% of lesions were classified as large ACFs, consisting of 5 or more aberrant crypts per focus. This is in striking contrast to the size distribution of lesions identified in the AKR/J colons, where fewer than 5% of grossly identified lesions were classified as large. In fact, the majority (> 80%) of ACFs in AKR/J mice consisted of only 1-2 aberrant crypts@focus. In addition, there was no evidence of dysplasia in any of the AKR/J lesions examined, whereas the lesions in susceptible mice were dysplastic (adenomas). Our data indicate that tumorigenic response is associated with the extent and multiplicity of ACFs that form within the colonic epithelium at an early time point after carcinogen exposure. These studies further support the use of this morphological biomarker as a short-term endpoint of colon tumorigenesis.