Expression of wnt signaling target genes in colorectal cancer progression

Abstract

Canonical wnt signaling is important in embryonic development and tumorigenesis of various tissues including the colon. Activation of wnt signaling leads to transcription of wnt target genes that mediate processes including cell cycle regulation, apopotosis and cell sorting. Exploring the expression of wnt target genes on tumour progression may facilitate understanding the complexity of wnt signaling and improving treatment strategy. Specifically, some wnt target genes, including the EphB2 and TIAM1 genes, may function to suppress tumour progression in animal models. Using tissue microarrays containing hundreds of colorectal adenomas and cancers, lymph nodes and liver metastases, we systematically characterized the expression of 7 putative wnt target genes (including the Axin2, Rnf43, Gpr49, Ascl2, Sox4, EphB2 and Pla2g2a genes) using either immunohistochemistry or in-situ hybridization and correlated the results with aberrant β-catenin expression. All 7 genes showed a high level of expression at the base of the crypts in normal colon, which coincides with cells displaying aberrant β-catenin expression. Compared with normal colonic epithelium, expressions of these genes were significantly higher in a large proportion of adenomas. The expression of each of these 7 genes was significantly associated with aberrant β-catenin expression, providing additional evidence in support of them being wnt target genes. In primary CRCs, they can be distinguished into two groups, a group with high mean expression of 7 genes (35.4%) and a group with low mean expression (64.6%). CRCs with low mean gene expression were associated with advanced tumour stage (p=0.009), poor tumor differentiation (p=0.01), mucinous or signet ring cell morphology (p=0.009) and DNA mismatch repair deficiency (p=0.04). Furthermore, by multivariate analysis, a high mean gene expression level was an independent factor predictive of prolonged overall (adjusted HR 0.64, p=0.047) and disease free survival (adjusted HR=0.362, p<0.001). Based on Kaplan-meier survival analysis, CRCs with a high mean gene expression were associated with prolonged overall and disease-free survival in both Dukes’ stage B and Dukes’ stage C patients. Furthermore, liver metastasis were more likely to show a low mean gene expression compared with primary CRCs (p<0.001). Our results suggest that activation of wnt signaling and induction of these 7 wnt target genes occurs early in adenoma phase, but there is a trend of progressive downregulation of these genes in invasive cancers that correlates with aggressive tumor behaviour. The strong prognostic predictive power of the combined expression levels of these wnt target genes may enable us to identify subgroups of high risk patients for receiving adjuvant chemotherapy. Finally, our result supports the complexity of function of wnt signaling and calls for caution in therapeutic strategy utilizing wnt inhibitors.