Abstract 4074: Pten haploinsufficiency promotes tumor invasion and carcinogenesis in mouse colon epithelium with Apc deficiency

Background and Aim: The loss of PTEN (phosphatase and tensin homologue) expression in human colorectal cancer (CRC) is found in approximately 40% of cases, and its functional contribution is not fully understood. To address this question, the colon tumors from the Pten-deficient mice with a disruption of the Adenomatous polyposis coli (Apc) were compared with those from the mice with Apc deficiency alone. Method: To recapitulate human CRC, mouse models carrying transgenes regulated by a 9.5-kb fragment containing sequences from the human CDX2 promoter (CDX2P9.5), which were previously shown to have tightly restricted transgene expression in the colon epithelium, were prepared. CDX2P9.5-NLSCre;Apcflox/+;Ptenflox/+ (referred to as CPC;Apc+Pten mouse) and CDX2P9.5-NLSCre;Apcflox/+(;Pten+/+) mice (referred to as CPC;Apc mouse) were generated, and the overall survival and tumor phenotypes (number, size and severity) at 4, 6, 9, 12 and 15 weeks of age were compared between the two models. In addition, the loss of heterozygosity status of the Apc and Pten wild-type alleles in tumor tissue and normal mucosa in the two mouse models were compared with multiplex polymerase chain reaction (PCR), and Pten transcripts and protein expression were evaluated by quantitative reverse transcription PCR and immunohistochemistry, respectively. Results: CPC;Apc+Pten mice had a significantly shorter life span than CPC;Apc mice (median survival time of CPC;Apc+Pten mice and CPC;Apc mice: 14.4 weeks [n=20] vs. 21.6 weeks [n=20], respectively). The CPC;Apc+Pten mice had more tumors than CPC;Apc mice at all time points measured, but the tumor size in CPC;Apc+Pten mice was significantly larger than that in CPC;Apc mice only at 9 weeks of age. Invasion into the submucosa was more frequently observed in CPC;Apc+Pten mice than in CPC;Apc mice (CPC;Apc+Pten mice and CPC;Apc mice: 36.3%/58.3% and 4.9%/10% at 12/15 weeks of age, respectively). Regarding the findings on multiplex PCR, while the tumors from both models showed biallelic Apc inactivation, the tumors from CPC;Apc+Pten mice showed no loss of the wild-type allele of Pten. Regarding the Pten transcript levels, the levels in the tumors and mucosa from CPC;Apc+Pten mice were almost half of those in CPC;Apc mice. The Pten protein expression level in tumors from CPC;Apc+Pten mice was lower than that in tumors from CPC;Apc mice but not completely suppressed. Conclusion: Pten promotes tumor invasion and carcinogenesis without two hits on