The azoxymethane (AOM) model has been widely used to investigate the pathology and genetics of colorectal cancer in rodents. However, there has been wide variation in treatment regimes, making it difficult to compare across studies. Consequently, standardizing AOM treatment and identifying sources of experimental variation would allow better comparisons across studies. In order to establish an optimal dosing regime for detecting experiment-dependent differences in tumorigenesis, we performed a dose curve analysis using AKR/J, SWR/J, and A/J mouse strains previously reported to vary widely in susceptibility to AOM. Although intraperitoneal or subcutaneous administration, but not in utero exposure, resulted in similar levels of tumor induction, significant dose- and strain-dependent effects of AOM were observed. No sex-dependent differences were observed. Increasing the number of treatments uncovered a significant strain-dependent effect on tumor promotion, independent of susceptibility to tumor initiation. Similarly, we used C57BL/6J and DBA/2J intercrosses to demonstrate that small diet modifications can significantly alter AOM-induced tumorigenesis in a background-dependent manner. These results provide experimental support for a standardized AOM treatment and for the importance of controlling both genetic and non-genetic factors when using this model.

Growing evidence suggests that flat colorectal cancers (CRC) account for 10% to 20% of all CRCs and that these are frequently associated with more advanced pathologies. However, controversy exists as to the origin and progression of flat CRCs compared with the more common polypoid-type morphology. We report using the azoxymethane mouse model for human CRC that KK/HIJ and I/LNJ mice develop different frequencies of flat and polypoid tumors; 83% of colon tumors in I/LNJ mice are flat compared with only 19% in KK/HIJ mice, indicating a strong genetic predisposition to the development of specific CRC morphologies. Like polypoid tumors, all flat tumors show a significant increase in the level of nuclear B-catenin (CATNNB1), supported by similar frequencies of mutations in the phosphorylation domain-coding region (codons 32-41) of Catnnb1. However, in contrast to previous reports, tumors bearing higher ''oncogenic potential'' do not cluster in codon 41 of Catnnb1. There are no differences between flat and polypoid tumors in the frequency of mutations in codons 12 and 13 of Kras or codon 624 of Braf. Similarly, there are no differences between tumor morphologies in their location along the proximal-to-distal colonic axis or in the relative quantity of intratumor stromal myofibrobasts as marked by the expression of A-smooth muscle actin. Using a combination of serial colonoscopic and histologic analyses, we definitively show that flat CRCs do not develop de novo but progress through a flat adenomatous stage to invasive carcinoma without transit through an intermediary polypoid stage. [Cancer Res 2007;67(24):11594-600]