Colon-specific tumorigenesis in mice driven by Cre-mediated inactivation of Apc and activation of mutant Kras

Abstract

Several genetically engineered mouse (GEM) models of colorectal cancer have been developed and are a mainstay in our efforts to identify means of preventing and treating this disease. Many of these models involve a germline disruption of the adenomatous polyposis coli (Apc) tumor suppressor gene and share the limitation that the great preponderance of tumors appear in the small rather than large intestine. In recent years efforts have been made to increase the similarity of these models to human sporadic colorectal cancer by disrupting Apc in a tissue-specific fashion using the Cre-Lox system so that the genetic aberrations are confined to the colonic epithelium. These models have shown great promise but reproducible and high penetrance colon-specific tumorigenesis has not yet been achieved without invasive techniques to introduce the Cre enzyme. We therefore sought to create a new model with high penetrance colon-specific tumorigenesis but without the need for exogenous Cre administration. We utilized existing mice possessing a conditional knock out for the Apc gene and a latent activated Kras allele and crossed them with mice expressing Cre recombinase solely in the large intestine. Using this approach we generated mice that developed 1–9 colonic adenomas per mouse (average 4.3) but without any tumors in the small intestine or cecum. No invasive tumors were observed. Despite the apparent lack of invasion, the geographical correctness, complete penetrance and intermediate tumor burden make this model a promising addition to our toolkit for the study of colorectal cancer treatment and prevention.

Colorectal cancer models for novel drug discovery

“…Other cre mouse lines that are used to model CRC (with varying polyp burden in small intestine vs. colon) include Fabp-Cre [59], Ah-Cre (Cyp1A promoter) [78], CDX2P-NLS-Cre [79], and CAC-Cre (carbonic anhydrase I gene promoter, expressed only in the large intestine) [80]. The addition of LSL-Kras G12D/+ to Apc loxp14/+ CAC-Cre results in an average of 4.3 colon adenomas (but not carcinomas) per mouse [81]. …”

Mouse model of proximal colon-specific tumorigenesis driven by microsatellite instability-induced Cre-mediated inactivation of Apc and activation of Kras

“…Previously, as Apc knockout mouse strains, Apc 580S/? , [35][36][37] Apc Min , [38] and Apc ?/1638N [39] with Apc protein truncations at codons 580, 850, and 1638, respectively, were crossed with strains expressing oncogenic Kras transgenes. However, tumors from mice crossed with Apc Min mice and Apc ?/1638N mice with heterozygous germline mutation followed by the LOH are predominantly located in the small intestine [38,39].…”

“…Currently, wellestablished conditional knockout technologies with CreloxP-mediated recombination provide more sophisticated mouse models. By flanking exon 14 of Apc gene with loxP sites, Apc floxed mice provided a variety of different phenotypes when Cre-recombinase was introduced by surgical introduction of adeno-Cre [36] or by mating with carbonic anhydrase 1 promoter-Cre [37] or Ah-Cre [35,38] mice. Those mice were also crossed with strains expressing oncogenic Kras transgenes, and the synergistic effect of oncogenic Kras and Apc deficiency demonstrated more aggressive phenotypes than that of our mouse model [39,40].…”

Fascin-1 as a biomarker and prospective therapeutic target in colorectal cancer

“…This goal must be placed in the context that widely used mouse models for intestinal tumorigenesis, such as Apc/min, are limited in their recapitulation of the human disease because most tumors develop within the small intestine [61]. New, genetically modified mouse models are being developed that circumvent this issue and which also address the challenge of achieving robust models to recapitulate colorectal tumor progression to metastatic disease [62,63]. …”