8313

AXIN2

AXIL|ODCRCS;axin 2;AXIN2;axin 2

axin-2|axin-like protein|axis inhibition protein 2|conductin

17q24.1

protein-coding

Medulloblastomas (MBs) represent the most common malignant brain tumors in children. Most MBs develop sporadically in the cerebellum, but their incidence is highly elevated in patients with familial adenomatous polyposis coli. These patients carry germline mutations in the APC tumor suppressor gene. APC is part of a multiprotein complex involved in the Wnt signaling pathway that controls the stability of beta-catenin, the central effector in this cascade. Previous genetic studies in MBs have identified mutations in genes coding for beta-catenin and its partners, APC and AXIN1, which cause activation of Wnt signaling. The pathway is negatively controlled by the tumor suppressor AXIN2 (Conductin), a scaffold protein of this signaling complex. To investigate whether alterations in AXIN2 may also be involved in the pathogenesis of sporadic MBs, we performed a mutational screening of the AXIN2 gene in 116 MB biopsy samples and 11 MB cell lines using single-strand conformation polymorphism and sequencing analysis. One MB displayed a somatic, tumor-specific 2 bp insertion in exon 5, leading to carboxy-terminal truncation of the AXIN2 protein. This tumor biopsy showed nuclear accumulation of beta-catenin protein, indicating an activation of Wnt signaling. In 2 further MB biopsies, mutations were identified in exon 5 (Glu408Lys) and exon 8 (Ser738Phe) of the AXIN2 gene, which are due to predicted germline mutations and rare polymorphisms. mRNA expression analysis in 22 MBs revealed reduced expression of AXIN2 mRNA compared to 8 fetal cerebellar tissues. Promoter hypermethylation could be ruled out as a major cause for transcriptional silencing by bisulfite sequencing. To study the functional role of AXIN2 in MBs, wild-type AXIN2 was overexpressed in MB cell lines in which the Wnt signaling pathway was activated by Wnt-3a. In this assay, AXIN2 inhibited Wnt signaling demonstrated in luciferase reporter assays. In contrast, overexpression of mutated AXIN2 with a deleted C-terminal DIX-domain resulted in an activation of the Wnt signaling pathway. These findings indicate that mutations of AXIN2 can lead to an oncogenic activation of the Wnt pathway in MBs.

Aberrant Wnt/beta-catenin signaling leading to nuclear accumulation of the oncogene product beta-catenin is observed in a wide spectrum of human malignancies. The destruction complex in the Wnt/beta-catenin pathway is critical for regulating the level of beta-catenin in the cytoplasm and in the nucleus. Here, we report a comprehensive study of the contribution of genetic variation in six genes encoding the beta-catenin destruction complex (APC, AXIN1, AXIN2, CSNK1D, CSNK1E, and GSK3B) to breast cancer using a Mayo Clinic Breast cancer Case-Control Study. A total of 79 candidate functional and tagging single nucleotide polymorphisms (SNP) were genotyped in 798 invasive cases and 843 unaffected controls. Of these, rs454886 in the APC tumor suppressor gene was associated with increased breast cancer risk (per allele odds ratio, 1.23; 95% confidence intervals, 1.05-1.43; P(trend) = 0.01). In addition, five SNPs in AXIN2 were associated with increased risk of breast cancer (P(trend) < 0.05). Haplotype-based tests identified significant associations between specific haplotypes in APC and AXIN2 (P < or = 0.03) and breast cancer risk. Further characterization of the APC and AXIN2 variants suggested that AXIN2 rs4791171 was significantly associated with risk in premenopausal (P(trend) = 0.0002) but not in postmenopausal women. The combination of our findings and numerous genetic and functional studies showing that APC and AXIN2 perform crucial tumor suppressor functions suggest that further investigation of the contribution of AXIN2 and APC SNPs to breast cancer risk are needed.

Frameshift mutations of genes with mononucleotide repeats are features of colorectal and gastric cancers with microsatellite instability (MSI). Deregulation of Wnt pathway is involved in the mechanisms of cancer development, and mutations of the Wnt-pathway genes have frequently been detected in cancers, indicating somatic mutations are important deregulation mechanisms of the Wnt signaling in cancer development. Both AXIN2 and TCF7L2 genes in the Wnt pathway possess mononucleotide repeats in their coding sequences and are considered as candidate tumor suppressor genes. The aim of this study was to see whether AXIN2 and TCF7L2 are altered by frameshift mutations in gastric carcinomas with MSI. For this, we analyzed human AXIN2 exon 8 and TCF7L2 exon 14 in 32 gastric carcinomas with high MSI, 13 gastric carcinomas with low MSI, and 47 gastric carcinomas without MSI by a single-strand conformation polymorphism analysis. Overall, we detected 9 AXIN2 and 6 TCF7L2 frameshift mutations in the mononucleotide repeats in the cancers with MSH-H, and all of them were found in MSH-H cancers (AXIN2, 28.1%; TCF7L2, 18.8%). Of the 32 high MSI cancers, 13 cancers (40.6%) harbored at least one of AXIN2 and TCF7L2 mutation, whereas 19 cancers (59.4%) harbored neither. The present data indicate that frameshift mutations in both AXIN2 and TCF7L2 genes are common in gastric carcinomas with high MSI and suggest that these mutations may contribute to development of gastric cancers with high MSI by deregulating the Wnt signaling in the affected cancer cells.

Accumulated evidence suggests that alterations due to mutations or genetic polymorphisms in the AXIN2 tumor suppressor gene, a component of the Wnt signaling pathway, contributes to carcinogenesis. The effect of the AXIN2 exon 1 148 Cright curved arrow T polymorphism was recently investigated in a Japanese population, but has not been investigated in other populations. Additionally, other common polymorphisms of this gene have not been studied. In the present study, 8 polymorphisms of the AXIN2 gene, including exon 1 148 Cright curved arrow T, were investigated in a Turkish population of 100 lung cancer patients using PCR-RFLP methods. For the exon 1 432 Cright curved arrow T, exon 5 1365 Gright curved arrow A, exon 5 1386 Cright curved arrow T, intron 5 1712+19 Gright curved arrow T, exon 7 2062 Cright curved arrow T and intron 7 2141+73 Gright curved arrow A single nucleotide polymorphisms of AXIN2, no significant association was found between the controls and the lung cancer patients. For exon 1 148 Cright curved arrow T, a statistically significant association between the controls and lung cancer patients was found. For this region, lung cancer patients with the TT genotype showed a decreased risk [odds ratio (ORTT) 0.33, 95% confidence interval (CI) 0.12-0.89; p=0.032 (adjusted for age, gender and smoking status)] as compared with the controls with the CC genotype. Concerning histological tumor type, it has been found that exon 1 148 Cright curved arrow T SNP is associated with a significant decreased risk in squamous cell carcinoma patients (ORTT 0.16; 95% CI 0.03-0.79; p=0.014). Male (ORTT 0.19; 95% CI 0.04-0.77; p=0.015) and smoker (ORTT 0.11; 95% CI 0.01-0.71; p=0.019) lung cancer patients with the TT genotype showed a decreased risk for the same region. Our results suggest that the risk of lung cancer in a Turkish population is related to polymorphisms of the AXIN2 gene.