Colorectal cancer kills more Americans than any other cancer except lung cancer and is the third most common cancer among men and women (American Cancer Society). Effectively treating this and other cancers depends on detecting them early and on using appropriate animal models to test drug efficacy. To that end, researchers from the University of Missouri, Columbia, led by Dr. Craig Franklin, have improved the utility of a mouse model of human colorectal cancer. They found that fecal levels of three cytokine mRNAs were early predictors of the likelihood that 129-Smad3tm1Par/J mice will develop colon cancer (Ericsson et al. 2011). The findings may allow scientists to detect early signs of, and develop better therapies for, human colorectal cancer.
The Smad3-/- mouse
Among the several models of human colorectal cancer is the 129-Smad3tm1Par/J mouse (Smad3-/-, 003451). Although most Smad3-/- mice develop relatively normally and cancer-free if raised in a specific pathogen-free environment, about two-thirds of them develop mucinous adenocarcinoma (MUC), similar to human colorectal cancer, in the proximal colon six weeks after being inoculated with either Helicobacter bilis or Helicobacter hepaticus.
The MUC developed by the Smad3-/- mouse simulates two key features of human colorectal cancer. First, the Mad homolog 3 (Smad3) gene for which the mouse is deficient is a transcription factor downstream of the anti-inflammatory and proapoptotic cytokine transforming growth factor beta (TGFB). In humans, TGFB deficiency is an indicator of malignant colorectal cancer. Second, the MUC develops in response to an inflammatory state. In humans, one of the risk factors for colorectal cancer is also an inflammatory state – inflammatory bowel disease. Additionally, MUC in the Smad3-/- mouse progresses predictably and orthotopically – in the colon. In contrast, MUC in more commonly used models develops from human cancer cell lines injected in the flanks of immunodeficient mice.
An improved colorectal cancer model
The most commonly used screens for detecting human colorectal cancer are fecal occult blood testing, sigmoidoscopy, colonoscopy, and computed tomographic colonography. None of these screens can satisfactorily detect all the indicators of colorectal cancer or detect them early enough. Using these screens in an animal model as small as the mouse accentuates their shortcomings. Franklin and his team sought to circumvent these difficulties. Their primary goal was to devise a screen that would earlier and more reliably predict which H. bilis-infected Smad3-/- mice actually develop MUC lesions. They experimented with two approaches: magnetic resonance imaging (MRI) and fecal levels of several inflammatory cytokine mRNAs. Not surprisingly, they found MRIs to be unsatisfactory: The mice need to be anesthetized, operating the equipment requires considerable resources and expertise, and interpreting the inconsistent images, even if done by experts, is subjective.
Most importantly, MRIs cannot detect potentially cancerous lesions in Smad3-/- mice before eight weeks post H. bilis injection, and only 58% of the mice in which lesions are detected at that time actually develop histologically confirmed MUC lesions. In contrast, Franklin and his team found that the expression of fecal mRNA for interleukin 1B (IL1B), macrophage inflammatory protein 1A (MIP1A), and chemokine, CC motif, ligand 5 (CCL5, formerly called RANTES) are much better and earlier predictors of which mice actually develop MUC lesions. Fecal IL1B mRNA levels can be used as early as one week post injection; fecal MIP1A and CCL5 mRNA levels can be used three weeks post injection.
In summary, the Franklin team improved the Smad3−/− mouse as a model of human colorectal cancer. By assessing the fecal levels of IL1B, MIP1A, and CCL5 of Smad3−/− mice as early as one week and no later than three weeks post H.bilis inoculation, researchers can reliably identify which mice have a high probability of developing MUC. Eliminating MUC negative Smad3−/− mice from therapeutic treatment studies increases the statistical power of data generated and reduces the number of mice needed for expensive compound efficacy tests at different MUC stages. Additionally, fecal levels of the three cytokines might be useful in detecting early stages of, and appropriate treatments for, colorectal cancer in humans.
Reference
Ericsson AC, Myles M, Davis W, Ma L, Lewis M, Maggio-Price L, Franklin C. 2010. Noninvasive detection of inflammation-associated colon cancer in a mouse model. Neoplasia 12:1054-65.