Abstract 4417: Induction of colonic cre-recombinase expression in carbonic anhydrase1-cre transgenic mice by dextran sulfate sodium
Authors: Johnson Robert L. , Fleet James , Snyder Paul 
Published: August 2012

Abstract
We previously reported the development of C57BL/6-Tg(Car1-cre)5Flt transgenic mice (CAC) with carbonic anhydrase-1 promoter/enhancer driven expression of cre-recombinase (Cre) in approximately 10 percent of colon epithelial cells. Here we report a novel observation in these mice that expands their utility for colorectal cancer research. Our original goal was to use CAC-ApcΔ580/+ mice, with conditional deletion of one Apc allele from colonic epithelial cells, to determine the mechanism of the interaction between Apc allele deletion and dextran sulfate sodium (DSS)-induced colitis on carcinogenesis. To detect cells with recombined floxed alleles, CAC-ApcΔ580/+ mice were crossed with B6.129S4-Gt(ROSA)26Sortm1Sor/J mice (Rosa26R), which have a beta-galactosidase (Bgal) transgene preceded by a floxed stop codon. We administered 0%, 0.65%, 1.35%, or 2% DSS in drinking water for 5 days to 8 wk old mice. Mice were killed and tissues harvested at 2 or 10 days after DSS was withdrawn. Histologic sections of colon were scored for ulceration and regeneration. Immunohistochemical labeling for Bgal was used as a reporter of CAC expression and recombination of floxed alleles. All levels of DSS caused ulcerative colitis, with severity increasing in a dose dependent manner. With time, ulceration was repaired by regeneration and hyperplasia of remaining epithelial cells. Surprisingly, while only 10 percent of colon crypts express Cre in control mice, nearly 100% of regenerative foci and many adjacent hyperplastic crypts were positive for Bgal after DSS exposure. After 2-3 epithelial replacement cycles (10 d post-DSS), all 3 doses significantly increased Bgal labeling (0.65% DSS-18.4%±.06, 1.35% DSS-17.7%± .04, 2% DSS-18.2%±.06) relative to controls (10.2%±.04). Similar results were seen in CAC-Rosa26R mice with normal Apc alleles, suggesting that the induction of the CAC transgene is directly attributable to DSS. Despite similar allelic recombination throughout the proximal and distal colon after DSS treatment, 10 wk old CAC-ApcΔ580/+ mice treated with 2% DSS for 5 days, developed more tumors at 15 wks of age in the distal colon (74%) than in the proximal colon (26%). This suggests that Apc allele deletion differentially influences cancer risk in proximal versus distal colon, which is consistent with observations of higher Apc mutation rates in human distal colorectal tumors. In conclusion, we report that administration of DSS at doses as low as 0.65% for 5 days to CAC mice causes mild ulcerative colitis that induces transgene expression and recombination of floxed alleles throughout the colon. This model provides a novel tool to differentiate the impact of gene by environment interaction in colon epithelial cells following colitis and offers an innovative approach to discover the mechanisms responsible for disparate pathways to colorectal carcinogenesis in the proximal versus distal colon.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4417. doi:1538-7445.AM2012-4417