Primary Mutation of Exon 3 of β-catenin (CTNNB1) Gene in Colorectal Tumors: Correlation with Mutation of Adenomatous Polyposis Coli Gene, Microsatellite Instability, and Prognostic Significance

Abstract

Objective: We explore the mutational patterns of the exon 3 of the β-catenin (CTNNB1) gene in colorectal tumors and assess whether primary mutation in this region is associated with mutation of the adenomatous polyposis coli (APC) gene, highfrequency microsatellite instability (MSI-H), and prognostic significance. Methods: The mutational patterns of the exon 3 of the β-catenin gene in 464 colorectal tumors were assessed by direct DNA sequencing of polymerase chain reaction-amplified products. Mutation of APC gene was first analyzed by topographic Denaturing High Performance Liquid Chromatography, followed by direct DNA sequencing and protein truncation test. The subcellular localization of β-catenin was done by immunocytochemistry. MSI-H was defined as the appearance of MSI in at least 2 of the 5 examined chromosomal loci (BAT-25, BAT-26, D5S346, D2S123, D17S250). Results: We found that primary mutation of the β-catenin gene in colorectal tumors was very low (n=12, 2.59%) in incidence. Besides previously reported conserved mutational sites including codon 34,37,41 and 45, two novel mutations in codon 59 (threonine→alanine) and 60 (serine→alanine)were detected. The colorectal tumors with mutation in codon 59 or 60 seem to be similar to those with mutation in codon 34, 37, 41, or 45 in terms of nuclear translocation of β-catenin, mutational status of the APC gene, and other clinicopathologic features. Compared to the control group (n=48), randomly selected from the 452 tumors with wild-type β-catenin, tumors with primary β-catenin mutation tended to present with nuclear accumulation of β-catenin protein (p=0.015). Primary β-catenin mutation was mutually exclusive with the mutation of APC gene in advanced colorectal cancer (p＜0.001). Moreover, the mutational rate of β-catenin was significantly higher in tumors with MSI-H (p＜0.001). Multivariate analysis indicated that MSI-H was a significantly (p=0.045) independent favorable prognostic factor with a relative risk of 0.112 (95% Confidence Interval: 0.013-0.954). However, primary mutation in exon 3 of the β-catenin gene, nuclear β-catenin expression, and APC mutation were without prognostic significance. Conclusions: DNA sequences encoding the regulatory domain of β-catenin protein may extend from codon 29 to codon 60 in the exon 3 of this gene. Primary mutation of exon 3 of the β-catenin gene was very rare in colorectal tumors, mutually exclusive with APC mutation only in advanced disease, and associated with MSI-H.