The azoxymethane (AOM)-induced mouse colon tumor model recapitulates many of the features associated with the multistage progression of human sporadic colorectal cancers (CRCs). To better define the genetic events associated with tumorigenesis in this murine model, we analyzed tumors from A/J mice for chromosomal (CIN) and microsatellite (MSI) instabilities, two important pathways of genomic instability that play a critical role in the pathogenesis of human CRCs. Six week-old male A/J mice were injected with either AOM (n=5) (10mg/kg b.w., ip) or vehicle (n=5) (0.9% NaCl solution) once a week for six weeks. Thirty-two weeks after the last dose, comparative genomic hybridization (CGH) was performed on 16 tumors harvested from five animals. While 25% of the tumors displayed either a gain of chromosome 2 or loss of Y, the majority (75%) showed no genomic imbalances. However, 81% of the tumors revealed low-level microsatellite instability (MSI-L) when analyzed using mono- and di-nucleotide repeat markers. Further analysis of chromosomal aberrations and MSI was performed in our recently established A/J colon tumor cell line (SP2u). While data from CGH showed loss of chromosomes 4, 14 and Y, spectral karyotyping (SKY) analysis revealed pseudo-tetraploidy with a loss of the Y chromosome. More importantly, high-level instability (MSI-H) was evident in SP2u cells, suggesting a potential role for the MSI pathway in this tumor model. In addition, each sample was analyzed for mutational changes in the K-ras and Apc genes, two key players in colon tumor progression. A very low frequency (6%) of K-ras mutations, together with the absence of truncation mutations in Apc, was observed. However, a nuclear localization of β-catenin observed in these tumors may indicate a dysregulated Wnt pathway in these mice. In conclusion, our findings suggest a minimal role for CIN and K-ras and Apc-mutations in AOM-induced tumors in mice. However, there remains the possibility that the MSI-L, or mild mutator pathway, may have a potential role in promoting tumor development in this model.
[Proc Amer Assoc Cancer Res, Volume 45, 2004]