Background: The expression of carcino-embryonic antigen by colorectal cancer is an example of oncogenic activation of embryonic gene expression. Hypothesizing that oncogenesis-recapitulatingontogenesis may represent a broad programmatic commitment, we compared gene expression patterns of human colorectal cancers (CRCs) and mouse colon tumor models to those of mouse colon development embryonic days 13.5-18.5.

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.

The mammalian gastrointestinal (GI) tract is inhabited by over a hundred species of symbiotic bacteria. Differences among individuals in the composition of the GI flora may contribute to variation in in vivo experimental analyses and disease susceptibility. To investigate potential interindividual differences in GI flora composition, we developed real-time quantitative PCR-based assays for the detection of the eight members of the Altered Schaedler Flora (ASF) as representative members of different bacterial niches within the mammalian GI tract. Quantitative and reproducible strain-specific variations in the numbers of the ASF members were observed across 23 different barrier-housed inbred mouse strains, suggesting that the ASF assays can be used as sentinels for changes in GI flora composition. A significant cage effect was also detected. Isogenic mice that cohabited at weaning, whether from the same or different litters, showed little variation in ASF profiles. Conversely, litters split among different cages at weaning showed divergence in ASF profiles after three weeks. Individual ASF profiles, once established, were highly stable over time in the absence of environmental perturbation. Furthermore, cohabitation of different inbred strains maintained most of the interstrain variation in the GI flora, supporting a role of host genetics in determining GI flora composition.

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]

The azoxymethane model of colorectal cancer (CRC) was used to gain insights into the genetic heterogeneity of nonfamilial CRC. We observed significant differences in susceptibility parameters across 40 mouse inbred strains, with 6 new and 18 of 24 previously identified mouse CRC modifier alleles detected using genome-wide association analysis. Tumor incidence varied in F1 as well as intercrosses and backcrosses between resistant and susceptible strains. Analysis of inheritance patterns indicates that resistance to CRC development is inherited as a dominant characteristic genome-wide, and that susceptibility appears to occur in individuals lacking a large-effect, or sufficient numbers of small-effect, polygenic resistance alleles. Our results suggest a new polygenic model for inheritance of nonfamilial CRC, and that genetic studies in humans aimed at identifying individuals with elevated susceptibility should be pursued through the lens of absence of dominant resistance alleles rather than for the presence of susceptibility alleles.