Mutations in AXIN2 cause colorectal cancer with defective mismatch repair by activating beta-catenin/TCF signalling

Abnormalities of the APC/beta-catenin pathway in endometrial cancer

The activation of the APC/beta-catenin signalling pathway due to beta-catenin mutations has been implicated in the development of a subset of endometrial carcinomas (ECs). However, up to 25% of ECs have beta-catenin nuclear accumulation without evidence of beta-catenin mutations, suggesting alterations of other molecules that can modulate the Wnt pathway, such as APC, gamma-catenin, AXIN1 and AXIN2. We investigated the expression pattern of beta- and gamma-catenin in a group of 128 endometrial carcinomas, including 95 endometrioid endometrial carcinomas (EECs) and 33 non-endometrioid endometrial carcinomas (NEECs). In addition, we evaluated the presence of loss of heterozygosity and promoter hypermethylation of the APC gene and mutations in the APC, beta- and gamma-catenin, AXIN1, AXIN2, and RAS genes, and phospho-Akt expression. No APC mutations were detected but LOH at the APC locus was found in 24.3% of informative cases. APC promoter 1A hypermethylation was observed in 46.6% of ECs, and was associated with the endometrioid phenotype (P=0.034) and microsatellite instability (P=0.008). Neither LOH nor promoter hypermethylation of APC was associated with nuclear catenin expression. Nuclear beta-catenin expression was found in 31.2% of EECs and 3% of NEECs (P=0.002), and was significantly associated with beta-catenin gene exon 3 mutations (P<0.0001). beta-catenin gene exon 3 mutations were associated with the endometrioid phenotype, and were detected in 14 (14.9%) EECs, but in none of the NEECs (P=0.02). gamma-catenin nuclear expression was found in 10 ECs; it was not associated with the histological type but was associated with more advanced stages (P=0.042). No mutations in gamma-catenin, AXIN1 and 2 genes were detected in this series. Neither RAS mutations nor phospho-Akt expression, which were found in 16 and 27.6% of the cases, respectively, were associated with beta-catenin nuclear expression. Our results demonstrated a high prevalence of alterations in molecules of the APC/beta-catenin pathway, but only mutations in beta-catenin gene are associated with aberrant nuclear localization of beta-catenin.

Analysis of the beta-catenin/T cell factor signaling pathway in 36 gastrointestinal and liver cancer cells

We investigated the frequency and mechanism of beta-catenin/T cell factor (Tcf) signaling activation in a panel of 36 human gastrointestinal and liver cancer cell lines. Reporter assay and electrophoretic mobility shift assay revealed that the beta-catenin/Tcf signaling was upregulated in 12 of 12 (100%) colorectal, 5 of 8 (68%) gastric, 2 of 7 (29%) hepatic, and none of 9 pancreatic cancer cell lines. The activation of the pathway was mainly due to the mutation of adenomatous polyposis coli (APC) or beta-catenin, and Tcf-4 was highly expressed in these cell lines with upregulated signaling. Nuclear beta-catenin was observed not only in the signaling-activated cell lines, but also in 14 of 25 (56%) primary gastric cancers, 15 of 20 (75%) colon cancers, 5 of 19 (26%) hepatocellular carcinomas, and none of 13 pancreatic cancers. The presence of signaling-upregulated gastric cancer cell lines with intact APC and beta-catenin suggests the involvement of other mechanisms than mutations of APC or beta-catenin.

The beta-catenin/TCF-4 complex imposes a crypt progenitor phenotype on colorectal cancer cells

The transactivation of TCF target genes induced by Wnt pathway mutations constitutes the primary transforming event in colorectal cancer (CRC). We show that disruption of beta-catenin/TCF-4 activity in CRC cells induces a rapid G1 arrest and blocks a genetic program that is physiologically active in the proliferative compartment of colon crypts. Coincidently, an intestinal differentiation program is induced. The TCF-4 target gene c-MYC plays a central role in this switch by direct repression of the p21(CIP1/WAF1) promoter. Following disruption of beta-catenin/TCF-4 activity, the decreased expression of c-MYC releases p21(CIP1/WAF1) transcription, which in turn mediates G1 arrest and differentiation. Thus, the beta-catenin/TCF-4 complex constitutes the master switch that controls proliferation versus differentiation in healthy and malignant intestinal epithelial cells.

Regulation of epithelial cell migration and tumor formation by beta-catenin signaling

Cell migration requires precise control, which is altered or lost when tumor cells become invasive and metastatic. beta-catenin plays a dual role in this process: as a member of adherens junctions it is essential to link cadherins to the cytoskeleton thereby allowing tight intercellular adhesion, and as a member of the Wnt-signaling pathway, beta-catenin is translocated into the nucleus and serves together with the LEF1/TCF-transcription factors to drive gene expression necessary for the epithelial-to-mesenchymal transition (EMT). Activated beta-catenin signaling has been implicated in the genesis of a variety of tumors. Here we demonstrate a pivotal function for beta-catenin signaling in epithelial cell migration and tumorigenesis. Hepatocyte growth factor (HGF) and epidermal growth factor (EGF) induce beta-catenin signaling under conditions where they stimulate cell motility. Ectopic expression of either stabilized beta-catenin or a regulatable form of activated beta-catenin induces cell migration in different cell types and cooperates with EGF and HGF in this process. Activation of beta-catenin signaling induces expression of the new target gene osteopontin during migration. Cells expressing stabilized beta-catenin also exhibit significantly increased capability to form tumors in a nude mouse xenograft model. The data suggest that a critical threshold of beta-catenin signaling, activated by cooperative mechanisms, may be important during the EMT and tumorigenesis.

Oncogenic transformation by beta-catenin: deletion analysis and characterization of selected target genes

Genetic analysis of beta-catenin-induced oncogenic transformation in chicken embryo fibroblasts (CEF) revealed the following prerequisites for oncogenicity: (1) removal of the N terminal phosphorylation sites targeted by glycogen synthase kinase 3beta (GSK3beta), (2) retention of the N terminal transactivation domain, and (3) retention of the armadillo repeats. The C terminal transactivation domain played an ancillary role in the transformation of CEF. There was a rough correlation between the transforming activity of various beta-catenin constructs and their transactivation of the TOPFLASH reporter. Expression levels of the candidate target genes of beta-catenin-LEF, cyclin D1 and myc were not correlated with each other or with the transforming activity of beta-catenin constructs. A new target gene, coding for inositol hexakisphosphate kinase 2 (IP6K2) was identified. Its expression showed concordance with the transforming activity of beta-catenin constructs.

Emerging concepts in colorectal neoplasia

An understanding of the mechanisms that explain the initiation and early evolution of colorectal cancer should facilitate the development of new approaches to effective prevention and intervention. This review highlights deficiencies in the current model for colorectal neoplasia in which APC mutation is placed at the point of initiation. Other genes implicated in the regulation of apoptosis and DNA repair may underlie the early development of colorectal cancer. Inactivation of these genes may occur not by mutation or loss but through silencing mediated by methylation of the gene's promoter region. hMLH1 and MGMT are examples of DNA repair genes that are silenced by methylation. Loss of expression of hMLH1 and MGMT protein has been demonstrated immunohistochemically in serrated polyps. Multiple lines of evidence point to a "serrated" pathway of neoplasia that is driven by inhibition of apoptosis and the subsequent inactivation of DNA repair genes by promoter methylation. The earliest lesions in this pathway are aberrant crypt foci (ACF). These may develop into hyperplastic polyps or transform while still of microscopic size into admixed polyps, serrated adenomas, or traditional adenomas. Cancers developing from these lesions may show high- or low-level microsatellite instability (MSI-H and MSI-L, respectively) or may be microsatellite stable (MSS). The suggested clinical model for this alternative pathway is the condition hyperplastic polyposis. If colorectal cancer is a heterogeneous disease comprising discrete subsets that evolve through different pathways, it is evident that these subsets will need to be studied individually in the future.

Target disruption of the mutant beta-catenin gene in colon cancer cell line HCT116: preservation of its malignant phenotype

Most colorectal carcinomas harbor genetic alterations that result in stabilization of beta-catenin. A colorectal carcinoma cell line, HCT116, which has both mutated and wild-type beta-catenin genes, was engineered by homologous recombination to investigate the significance of beta-catenin gene mutation. As expected, the mutant allele-targeted clones showed decreased beta-catenin expression and downregulation of T-cell factor (TCF)/lymphoid enhancer factor (LEF)-dependent transcription. Morphologically, targeted clones were only minimally altered under usual culture conditions, but under low serum conditions, mutant allele-targeted clones still grew in plane, in contrast to parental cell line and wild allele-targeted clones, which formed spheroids. The mutant allele-targeted clones showed no significant changes in growth rate and anchorage-independent growth in vitro, and displayed rather increased growth in vivo. Although beta-catenin stabilization affects some biological characteristics including adhesive properties, it may not have growth-promoting effects at least in some colorectal carcinomas.

Nr-CAM is a target gene of the beta-catenin/LEF-1 pathway in melanoma and colon cancer and its expression enhances motility and confers tumorigenesis

beta-catenin and plakoglobin (gamma-catenin) are homologous molecules involved in cell adhesion, linking cadherin receptors to the cytoskeleton. beta-catenin is also a key component of the Wnt pathway by being a coactivator of LEF/TCF transcription factors. To identify novel target genes induced by beta-catenin and/or plakoglobin, DNA microarray analysis was carried out with RNA from cells overexpressing either protein. This analysis revealed that Nr-CAM is the gene most extensively induced by both catenins. Overexpression of either beta-catenin or plakoglobin induced Nr-CAM in a variety of cell types and the LEF/TCF binding sites in the Nr-CAM promoter were required for its activation by catenins. Retroviral transduction of Nr-CAM into NIH3T3 cells stimulated cell growth, enhanced motility, induced transformation, and produced rapidly growing tumors in nude mice. Nr-CAM and LEF-1 expression was elevated in human colon cancer tissue and cell lines and in human malignant melanoma cell lines but not in melanocytes or normal colon tissue. Dominant negative LEF-1 decreased Nr-CAM expression and antibodies to Nr-CAM inhibited the motility of B16 melanoma cells. The results indicate that induction of Nr-CAM transcription by beta-catenin or plakoglobin plays a role in melanoma and colon cancer tumorigenesis, probably by promoting cell growth and motility.

Mutational spectrum of beta-catenin, AXIN1, and AXIN2 in hepatocellular carcinomas and hepatoblastomas

Activation of Wnt signaling through beta-catenin mutations contributes to the development of hepatocellular carcinoma (HCC) and hepatoblastoma (HB). To explore the contribution of additional Wnt pathway molecules to hepatocarcinogenesis, we examined beta-catenin, AXIN1 and AXIN2 mutations in 73 HCCs and 27 HBs. beta-catenin mutations were detected in 19.2% (14 out of 73) HCCs and 70.4% (19 out of 27) HBs. beta-catenin mutations in HCCs were primarily point mutations, whereas more than half of the HBs had deletions. AXIN1 mutations occurred in seven (9.6%) HCCs and two (7.4%) HBs. The AXIN1 mutations included seven missense mutations, a 1 bp deletion, and a 12 bp insertion. The predominance of missense mutations found in the AXIN1 gene is different from the small deletions or nonsense mutations described previously. Loss of heterozygosity at the AXIN1 locus was present in four of five informative HCCs with AXIN1 mutations, suggesting a tumor suppressor function of this gene. AXIN2 mutations were found in two (2.7%) HCCs but not in HBs. Two HCCs had both AXIN1 and beta-catenin mutations, and one HCC had both AXIN2 and beta-catenin mutations. About half the HCCs with AXIN1 or AXIN2 mutations showed beta-catenin accumulation in the nucleus, cytoplasm or membrane. Overall, these data indicate that besides the approximately 20% of HCCs and 80% of HBs with beta-catenin mutations contributing to hepatocarcinogenesis, AXIN1 and AXIN2 mutations appear to be important in an additional 10% of HCCs and HBs.

Cadherin and catenin alterations in human cancer

Among the hallmarks of cancer are defective cell-cell and cell-matrix adhesion. Alterations in cadherin-catenin complexes likely have a major contributing role in cell-adhesion defects in carcinomas arising in many different tissues. E-cadherin, the prototypic member of the cadherin transmembrane protein family, regulates cell adhesion by interacting with E-cadherin molecules on opposing cell surfaces. E-cadherin's function in cell adhesion is also critically dependent on its ability to interact through its cytoplasmic domain with catenin proteins. A diverse collection of defects alter cadherin-catenin function in cancer cells, including loss-of-function mutations and defects in the expression of E-cadherin and certain catenins, such as alpha-catenin. Although there is much evidence that beta-catenin is deregulated in cancer as a result of inactivating mutations in the APC and AXIN tumor-suppressor proteins and gain-of-function mutations in beta-catenin itself, the principal consequences of beta-catenin deregulation in cancer appear to be largely distinct from the effects attributable to inactivation of E-cadherin or alpha-catenin. In this review, we highlight some of the specific genetic and epigenetic defects responsible for altered cadherin and catenin function in cancer, as well as potential contributions of cadherin-catenin alterations to the cancer process.

Subcellular distribution of Wnt pathway proteins in normal and neoplastic colon

Mutations in the APC tumor suppressor gene are present in approximately 85% of colorectal tumors and are thought to contribute early in the process of tumorigenesis. The truncated protein resulting from most APC mutations can lead to elevated beta-catenin levels in colon tumor cells. APC and associated proteins thus form a beta-catenin regulatory complex, with axin playing a key role. Although cell culture studies have revealed intriguing aspects of this complex, little characterization has been done in human colonocytes, the target tissue of colon carcinogenesis. The present study of intact human colon crypts, adenomatous polyps, and adenocarcinomas focuses on subcellular localization of some key elements of the complex: beta-catenin, APC, axin, and axin2. We examined endogenous protein localization within the framework of three-dimensional tissue architecture by using laser scanning confocal microscopy, and immunofluorescence staining of whole-mount fixed tissue from more than 50 patients. Expression patterns suggest that APC and axin colocalize in the nucleus and at lateral cell borders, and show that axin2 is limited to the nucleus. Altered nuclear expression of axin seen in colon polyps and carcinomas may be a consequence of the loss of full-length APC and the advent of nuclear beta-catenin. The observation of nuclear beta-catenin in fewer than half of carcinoma images and only rarely in polyps indicates that nuclear translocation of beta-catenin may not be an immediate consequence of the loss of APC.

Activation of AXIN2 expression by beta-catenin-T cell factor. A feedback repressor pathway regulating Wnt signaling

The Wnt pathway regulates cell fate, proliferation, and apoptosis, and defects in the pathway play a key role in many cancers. Although Wnts act to stabilize beta-catenin levels in the cytosol and nucleus, a multiprotein complex containing adenomatous polyposis coli, glycogen synthase kinase 3beta, and Axin1 or its homolog Axin2/Axil/conductin promotes beta-catenin phosphorylation and subsequent proteasomal degradation. We found that the rat Axil gene was strongly induced upon neoplastic transformation of RK3E cells by mutant beta-catenin or gamma-catenin or after ligand-induced activation of a beta-catenin-estrogen receptor fusion protein. Expression of Wnt1 in murine breast epithelial cells activated the conductin gene, and human cancers with defective beta-catenin regulation had elevated AXIN2 gene and protein expression. Expression of AXIN2/Axil was strongly repressed in cancer cells by restoration of wild type adenomatous polyposis coli function or expression of a dominant negative form of T cell factor (TCF)-4. TCF binding sites in the AXIN2 promoter played a key role in the ability of beta-catenin to activate AXIN2 transcription. In contrast to AXIN2/Axil, expression of human or rat Axin1 homologs was nominally affected by beta-catenin-TCF. Because Axin2 can inhibit beta-catenin abundance and function, the data implicate AXIN2 in a negative feedback pathway regulating Wnt signaling. Additionally, although Axin1 and Axin2 have been thought to have comparable functions, the observation that Wnt pathway activation elevates AXIN2 but not AXIN1 expression suggests that there may be potentially significant functional differences between the two proteins.

Molecular causes of colon cancer

Cells in a developing embryo communicate with each other through a limited number of intercellular signalling pathways, of which the Wnt signalling pathway is one. Little is known about the function of Wnt signalling beyond that in embryogenesis. However, recent insights into the molecular etiology of colon cancer have implied a central role for the Wnt signalling pathway. The malignant transformation of colorectal epithelium is well defined, leading to adenoma and sequentially carcinoma formation. Several genes that regulate the Wnt pathway are mutated in cancer of the human colon and other organs. All of these mutations lead to the inappropriate activation of the pathway, which instructs the cell to divide unrestrictedly. These insights now allow the Wnt pathway to be exploited as a new target for drug development in colon cancer.

The APC gene in colorectal cancer

Mutations in the adenomatous polyposis coli (APC) gene are not only responsible for familial adenomatous polyposis (FAP), but also play a rate-limiting role in the majority of sporadic colorectal cancers. Colorectal tumours are known to arise through a gradual series of histological changes, the so-called 'adenoma-carcinoma' sequence, each accompanied by a genetic alteration in a specific oncogene or tumour suppressor gene. Loss of APC function triggers this chain of molecular and histological changes. In general, an intestinal cell needs to comply with two essential requirements to develop into a cancer: it must acquire selective advantage to allow for the initial clonal expansion, and genetic instability to allow for multiple hits at other genes responsible for tumour progression and malignant transformation. Inactivation of APC seems to fulfill both requirements. In this short review, I will discuss the role played by APC in providing, when mutated, selective advantage, through constitutional activation of the Wnt signal transduction pathway, and chromosomal instability to the nascent intestinal tumor cell.

Distinction between familial and sporadic forms of colorectal cancer showing DNA microsatellite instability

Attempts to classify colorectal cancer into subtypes based upon molecular characterisation are overshadowed by the classical stepwise model in which the adenoma-carcinoma sequence serves as the morphological counterpart. Clarity is achieved when cancers showing DNA microsatellite instability (MSI) are distinguished as sporadic MSI-low (MSI-L), sporadic MSI-high (MSI-H) and hereditary non-polyposis colorectal cancer (HNPCC). Divergence of the 'methylator' pathway into MSI-L and MSI-H is at least partly determined by the respective silencing of MGMT and hMLH1. Multiple differences can be demonstrated between sporadic and familial (HNPCC) MSI-H colorectal cancer with respect to early mechanisms, evolution, molecular characterisation, demographics and morphology. By acknowledging the existence of multiple pathways, rapid advances in the fields of basic and translational research will occur and this will lead to improved strategies for the prevention, early detection and treatment of colorectal cancer.

Role of beta-catenin/T-cell factor-regulated genes in ovarian endometrioid adenocarcinomas

In various cancers, inactivating mutations in the adenomatous polyposis coli or Axin tumor suppressor proteins or activating mutations in beta-catenin's amino-terminal domain elevate beta-catenin levels, resulting in marked effects on T-cell factor (TCF)-regulated transcription. Several candidate beta-catenin/TCF-regulated genes in cancer have been proposed. Expression of a few of these genes has been studied in primary human cancers, but most studies have focused on colon cancers and not on other cancer types that harbor mutational defects in adenomatous polyposis coli, AXIN, or beta-catenin. Mutations leading to beta-catenin deregulation are found in nearly half of ovarian endometrioid adenocarcinomas (OEAs). We report here on the expression of 6 candidate beta-catenin/TCF-regulated genes in a panel of 44 primary OEAs, more than a third of which carry demonstrable defects in beta-catenin regulation. Using quantitative assays of gene expression, we found significantly elevated expression of the MMP-7, CCND1 (Cyclin D1), CX43 (Connexin 43), PPAR-delta, and ITF2 genes in OEAs with deregulated beta-catenin. This correlation was not observed for c-myc, another putative beta-catenin/TCF-regulated gene. Immunohistochemical studies confirmed that overexpression of cyclin D1 and MMP-7 was highly associated with nuclear accumulation of beta-catenin and mutational defects of the Wnt/beta-catenin/TCF-signaling pathway. Our findings indicate cyclin D1, MMP-7, connexin 43, PPAR-delta, and ITF-2, likely play important roles in the pathogenesis of those OEAs that manifest defects in beta-catenin regulation.

The Wnt pathway, epithelial-stromal interactions, and malignant progression in phyllodes tumours

In a previous study of phyllodes tumours, it has been shown that both the stroma and the epithelium can exhibit distinct molecular changes, suggesting that both are part of the neoplastic process. In view of this finding, it was decided to study stromal-epithelial interactions in these tumours by examining the Wnt-APC-beta-catenin pathway. Beta-catenin and cyclin D1 immunohistochemistry was performed on 119 phyllodes tumours. Eighty-six (72%) showed stromal nuclear beta-catenin localization and in 57% the staining was moderate or strong; however, of the eight malignant tumours in the series, seven showed absent or weak nuclear staining (p<0.025). In no tumour was nuclear beta-catenin staining seen in the epithelial component. Moderate or strong stromal cyclin D1 staining correlated with nuclear stromal beta-catenin staining (p<0.05). Forty-five of the tumours, including two malignant lesions, were screened for beta-catenin exon 3 mutations using SSCP and sequencing, but none was found. Loss of heterozygosity (LOH) of the marker D5S346 was used to infer APC mutation, but only one (benign) tumour showed LOH. Wnt2 and Wnt5a mRNA was localized by in situ hybridization in 13 cases (three malignant) chosen to reflect the different beta-catenin staining patterns. There was an association between strong nuclear beta-catenin staining of stromal cells and epithelial Wnt5a expression (p<0.0015). These data suggest that stromal proliferation in benign phyllodes tumours relies on abnormalities in the Wnt pathway which result not from mutation, but from Wnt5a expression in the epithelium. In the progression to malignancy, the stromal proliferation appears to become independent of the Wnt pathway and, presumably, of the epithelial component of these tumours.

Genetic instability in human mismatch repair deficient cancers

Cancers showing microsatellite instability (MSI-H) are frequent tumors characterized by inactivating alterations of mismatch repair (MMR) genes that lead to an incapacity to recognize and repair errors that occur during DNA replication. These cancers can be inherited as in the human non-polyposis colorectal cancer syndrome, or can occur sporadically in 10-15% of colorectal, gastric and endometrial cancers. MSI-H tumors have different clinicopathological features compared to cancers without this phenotype, termed MSS, and the repertoire of genetic events involved in tumoral progression of both phenotypes is thought to be different. In MSI-H tumors, most of the genetic changes occur at both non-coding and coding microsatellites that are particularly prone to errors during replication due to their repetitive sequence. This mechanism appears to be the main "genetic pathway" by which functional changes with putative oncogenic effects are accumulated in these tumors.

ITF-2, a downstream target of the Wnt/TCF pathway, is activated in human cancers with beta-catenin defects and promotes neoplastic transformation

In many cancers, inactivation of the adenomatous polyposis coli (APC) or Axin tumor suppressor proteins or activating mutations in beta-catenin lead to elevated beta-catenin levels, enhanced binding of beta-catenin to T cell factor (TCF) proteins, and increased expression of TCF-regulated genes. We found that the gene for the basic helix-loop-helix transcription factor ITF-2 (immunoglobulin transcription factor-2) was activated in rat E1A-immortalized RK3E cells following neoplastic transformation by beta-catenin or ligand-induced activation of a beta-catenin-estrogen receptor fusion protein. Human cancers with beta-catenin regulatory defects had elevated ITF-2 expression, and ITF-2 was repressed by restoring wild-type APC function or inhibiting TCF activity. Of note, ITF-2 promoted neoplastic transformation of RK3E cells. We propose that ITF-2 is a TCF-regulated gene, which functions in concert with other TCF target genes to promote growth and/or survival of cancer cells with defects in beta-catenin regulation.

Activation of the Wnt pathway interferes with serum response element-driven transcription of immediate early genes

Mutational activation of the Wnt signaling pathway is a common early event in colorectal tumorigenesis, and the identification of target genes regulated by this pathway will provide a better understanding of tumor progression. Gene expression profiling on oligonucleotide microarrays revealed reduced expression of the immediate early genes fos and fosB following stimulation of cells by Wnt-1. Further analysis demonstrated that serum or 12-O-tetradecanoylphorbol-13-acetate activation of several immediate early genes including fos, fosB, junB, and egr1 was inhibited by Wnt signaling. Wnt signaling inhibited transcriptional activation driven by the serum response element without altering the activation of the extracellular signal-regulated kinase cascade or ternary complex formation at the fos serum response element promoter. The Wnt-mediated repression of c-Fos, FosB, and JunB expression was consistent with a decrease in their binding to an AP-1 promoter element and decreased target gene transcription. The expression of fos, fosB, junB, and egr1 was also repressed in human colon tumors relative to patient matched normal tissue. By contrast, the fos family member fra-1 was up-regulated in the human colon tumors, suggesting a compensatory mechanism for the reduction in fos and fosB expression. The results indicate that Wnt signaling can repress the expression of certain immediate early genes, and that this effect is consistent with changes in gene expression observed in human colorectal tumors.

Negative feedback loop of Wnt signaling through upregulation of conductin/axin2 in colorectal and liver tumors

Activation of Wnt signaling through beta-catenin/TCF complexes is a key event in the development of various tumors, in particular colorectal and liver tumors. Wnt signaling is controlled by the negative regulator conductin/axin2/axil, which induces degradation of beta-catenin by functional interaction with the tumor suppressor APC and the serine/threonine kinase GSK3beta. Here we show that conductin is upregulated in human tumors that are induced by beta-catenin/Wnt signaling, i.e., high levels of conductin protein and mRNA were found in colorectal and liver tumors but not in the corresponding normal tissues. In various other tumor types, conductin levels did not differ between tumor and normal tissue. Upregulation of conductin was also observed in the APC-deficient intestinal tumors of Min mice. Inhibition of Wnt signaling by a dominant-negative mutant of TCF downregulated conductin but not the related protein, axin, in DLD1 colorectal tumor cells. Conversely, activation of Wnt signaling by Wnt-1 or dishevelled increased conductin levels in MDA MB 231 and Neuro2A cells, respectively. In time course experiments, stabilization of beta-catenin preceded the upregulation of conductin by Wnt-1. These results demonstrate that conductin is a target of the Wnt signaling pathway. Upregulation of conductin may constitute a negative feedback loop that controls Wnt signaling activity.

Wnt/beta-catenin/Tcf signaling induces the transcription of Axin2, a negative regulator of the signaling pathway

Axin2/Conductin/Axil and its ortholog Axin are negative regulators of the Wnt signaling pathway, which promote the phosphorylation and degradation of beta-catenin. While Axin is expressed ubiquitously, Axin2 mRNA was seen in a restricted pattern during mouse embryogenesis and organogenesis. Because many sites of Axin2 expression overlapped with those of several Wnt genes, we tested whether Axin2 was induced by Wnt signaling. Endogenous Axin2 mRNA and protein expression could be rapidly induced by activation of the Wnt pathway, and Axin2 reporter constructs, containing a 5.6-kb DNA fragment including the promoter and first intron, were also induced. This genomic region contains eight Tcf/LEF consensus binding sites, five of which are located within longer, highly conserved noncoding sequences. The mutation or deletion of these Tcf/LEF sites greatly diminished induction by beta-catenin, and mutation of the Tcf/LEF site T2 abolished protein binding in an electrophoretic mobility shift assay. These results strongly suggest that Axin2 is a direct target of the Wnt pathway, mediated through Tcf/LEF factors. The 5.6-kb genomic sequence was sufficient to direct the tissue-specific expression of d2EGFP in transgenic embryos, consistent with a role for the Tcf/LEF sites and surrounding conserved sequences in the in vivo expression pattern of Axin2. Our results suggest that Axin2 participates in a negative feedback loop, which could serve to limit the duration or intensity of a Wnt-initiated signal.

Beta-catenin--a linchpin in colorectal carcinogenesis?

An important role for beta-catenin pathways in colorectal carcinogenesis was first suggested by the protein's association with adenomatous polyposis coli (APC) protein, and by evidence of dysregulation of beta-catenin protein expression at all stages of the adenoma-carcinoma sequence. Recent studies have, however, shown that yet more components of colorectal carcinogenesis are linked to beta-catenin pathways. Pro-oncogenic factors that also release beta-catenin from the adherens complex and/or encourage translocation to the nucleus include ras, epidermal growth factor (EGF), c-erbB-2, PKC-betaII, MUC1, and PPAR-gamma, whereas anti-oncogenic factors that also inhibit nuclear beta-catenin signaling include transforming growth factor (TGF)-beta, retinoic acid, and vitamin D. Association of nuclear beta-catenin with the T cell factor (TCF)/lymphoid enhancer factor (LEF) family of transcription factors promotes the expression of several compounds that have important roles in the development and progression of colorectal carcinoma, namely: c-myc, cyclin D1, gastrin, cyclooxygenase (COX)-2, matrix metalloproteinase (MMP)-7, urokinase-type plasminogen activator receptor (aPAR), CD44 proteins, and P-glycoprotein. Finally, genetic aberrations of several components of the beta-catenin pathways, eg, Frizzled (Frz), AXIN, and TCF-4, may potentially contribute to colorectal carcinogenesis. In discussing the above interactions, this review demonstrates that beta-catenin represents a key molecule in the development of colorectal carcinoma.

Genetics--cellular basis

Colorectal carcinogenesis is a multistep process during which the specialised epithelial cells of intestinal mucosa surface (e.g. colonocytes) accumulate a series of genetic and epigenetic events which lead to a perturbation of their normal cellular functions and turnover. This review will address the mechanisms and biological effects of these abnormalities on the growth control, differentiation, adhesion and survival of the colonocytes.

Frequent alterations in the Wnt signaling pathway in colorectal cancer with microsatellite instability

It is generally accepted that both dysfunction of the Wnt signaling pathway, including mutations in the adenomatous polyposis coli (APC) and beta-catenin genes, and genetic instability play important roles in colorectal carcinogenesis. However, alteration of the components in the Wnt signaling pathway in colorectal cancer (CRC) with microsatellite instability (MSI) has not been elucidated. In order to assess the status of the Wnt signaling components in CRC with MSI, mutational analyses of the beta-catenin, APC, Axin 1, and T cell factor 4 (TCF4) genes were performed. Three of 33 samples had mutations in exon 3 of the beta-catenin gene and two in the APC gene. Eight mutations in seven samples were detected by single-strand conformation polymorphism and subsequent direct sequence analysis of the entire coding region of the Axin 1 gene. Furthermore, TCF4, which is one of the transcriptional factors in the Wnt signaling pathway and has a mononucleotide repeat sequence (a nine- adenine repeat, (A)9) in its C-terminal region, was mutated in 13 of the 33 samples. Thus, alteration in the Wnt signaling pathway is frequently observed in CRC with MSI, including hereditary nonpolyposis colorectal cancer, as well as in familial adenomatous polyposis and sporadic CRC without MSI.

Elevated expression of axin2 and hnkd mRNA provides evidence that Wnt/beta -catenin signaling is activated in human colon tumors

Genetic studies have identified mutations in key regulators of the Wnt/beta-catenin pathway in a variety of cancers, most frequently in colon cancers. However, whether the pathway is activated in clinical cancer samples is not easily determined, and therefore it is useful to find markers that could be surrogates to show activation of the Wnt/beta-catenin pathway. Gene expression profiles were analyzed in SW620, a colon cancer cell line in which beta-catenin levels are stabilized as a consequence of truncated adenomatous polyposis coli and were compared with profiles of the same cells transfected with antisense oligodeoxynucleotides. Treatment of cells with beta-catenin antisense oligodeoxynucleotides resulted in a decrease in the levels of axin2 and human naked cuticle (hnkd) mRNAs. Interestingly, the proteins encoded by both of these mRNAs are known inhibitors of the beta-catenin pathway. In 30 human cell lines derived from different origins, axin2 and hnkd were expressed only in human colon cancer cell lines that are known to have activating mutations in the Wnt/beta-catenin pathway. Further, levels of both axin2 and hnkd mRNA were also found to be elevated in about 65% of laser microdissected cells from human colon tumors compared with laser microdissected cells of normal morphology from the same patient samples. The increased expression of axin2 and hnkd correlated with truncations in adenomatous polyposis coli in the same patient samples. These results reveal that it is possible to detect activation of a carcinogenic pathway in human cancer samples with specific markers.

Loss of CDX2 expression and microsatellite instability are prominent features of large cell minimally differentiated carcinomas of the colon

Most large bowel cancers are moderately to well-differentiated adenocarcinomas comprised chiefly or entirely of glands lined by tall columnar cells. We have identified a subset of poorly differentiated colon carcinomas with a distinctive histopathological appearance that we term large cell minimally differentiated carcinomas (LCMDCs). These tumors likely include a group of poorly differentiated carcinomas previously described by others as medullary adenocarcinomas. To better understand the pathogenesis of these uncommon neoplasms, we compared molecular features of 15 LCMDCs to those present in 25 differentiated adenocarcinomas (DACs) of the colon. Tumors were examined for alterations commonly seen in typical colorectal carcinomas, including increased p53 and beta-catenin immunoreactivity, K-ras gene mutations, microsatellite instability, and loss of heterozygosity of markers on chromosomes 5q, 17p, and 18q. In addition, tumors were evaluated by immunohistochemistry for CDX2, a homeobox protein whose expression in normal adult tissues is restricted to intestinal and colonic epithelium. Markedly reduced or absent CDX2 expression was noted in 13 of 15 (87%) LCMDCs, whereas only 1 of the 25 (4%) DACs showed reduced CDX2 expression (P < 0.001). Nine of 15 (60%) LCMDCs had the high-frequency microsatellite instability phenotype, but only 2 of 25 (8%) DACs had the high-frequency microsatellite instability phenotype (P = 0.002). Our findings provide support for the hypothesis that the molecular pathogenesis of LCMDCs is distinct from that of most DACs. CDX2 alterations and DNA mismatch repair defects have particularly prominent roles in the development of LCMDCs.

Features of colorectal cancers with high-level microsatellite instability occurring in familial and sporadic settings: parallel pathways of tumorigenesis

High-level microsatellite instability (MSI-H) is demonstrated in 10 to 15% of sporadic colorectal cancers and in most cancers presenting in the inherited condition hereditary nonpolyposis colorectal cancer (HNPCC). Distinction between these categories of MSI-H cancer is of clinical importance and the aim of this study was to assess clinical, pathological, and molecular features that might be discriminatory. One hundred and twelve MSI-H colorectal cancers from families fulfilling the Bethesda criteria were compared with 57 sporadic MSI-H colorectal cancers. HNPCC cancers presented at a lower age (P < 0.001) with no sporadic MSI-H cancer being diagnosed before the age of 57 years. MSI was less extensive in HNPCC cancers with 72% microsatellite markers showing band shifts compared with 87% in sporadic tumors (P < 0.001). Absent immunostaining for hMSH2 was only found in HNPCC tumors. Methylation of hMLH1 was observed in 87% of sporadic cancers but also in 55% of HNPCC tumors that showed loss of expression of hMLH1 (P = 0.02). HNPCC cancers were more frequently characterized by aberrant beta-catenin immunostaining as evidenced by nuclear positivity (P < 0.001). Aberrant p53 immunostaining was infrequent in both groups. There were no differences with respect to 5q loss of heterozygosity or codon 12 K-ras mutation, which were infrequent in both groups. Sporadic MSI-H cancers were more frequently heterogeneous (P < 0.001), poorly differentiated (P = 0.02), mucinous (P = 0.02), and proximally located (P = 0.04) than HNPCC tumors. In sporadic MSI-H cancers, contiguous adenomas were likely to be serrated whereas traditional adenomas were dominant in HNPCC. Lymphocytic infiltration was more pronounced in HNPCC but the results did not reach statistical significance. Overall, HNPCC cancers were more like common colorectal cancer in terms of morphology and expression of beta-catenin whereas sporadic MSI-H cancers displayed features consistent with a different morphogenesis. No individual feature was discriminatory for all HNPCC cancers. However, a model based on four features was able to classify 94.5% of tumors as sporadic or HNPCC. The finding of multiple differences between sporadic and familial MSI-H colorectal cancer with respect to both genotype and phenotype is consistent with tumorigenesis through parallel evolutionary pathways and emphasizes the importance of studying the two groups separately.

WNT1 inducible signaling pathway protein 3, WISP-3, a novel target gene in colorectal carcinomas with microsatellite instability

BACKGROUND & AIMS

Microsatellite instability (MSI) is the phenotype of colorectal carcinomas with defect mismatch repair. Genes with repetitive sequences within their coding regions are targets for mutations in these tumors. We have evaluated 2 novel candidate genes for potential involvement in development of MSI colorectal carcinomas and compared them with alterations in known target genes.

METHODS

The MSI status was determined by multiplex polymerase chain reactions (PCRs) of 5-17 markers in a Norwegian series of 275 colorectal carcinomas. All MSI tumors were analyzed for gene mutations using fluorescence PCR followed by capillary electrophoresis. Two novel candidate genes, WNT1-inducible signaling pathway protein 3 (WISP-3) and caspase-1, and 9 known target genes were analyzed.

RESULTS

Thirteen percent of the tumors were MSI-high (H) and 12% were MSI-low (L). Thirty-three of 37 MSI-H vs. 1 of 34 MSI-L tumors showed mutations in the target genes (P < 0.001). WISP-3 was mutated in 31% of the MSI-H tumors. The frequencies of frameshift mutations in the known target genes were comparable with other studies.

CONCLUSIONS

The relative high frequency of mutation, higher than those seen for other known target genes, the predicted truncation of the protein product, and the homology with WISP-1 and WISP-2, 2 proteins induced downstream of WNT1 signaling, strongly suggest WISP-3 as a novel target in development of MSI-H colorectal carcinomas.

New aspects of Wnt signaling pathways in higher vertebrates

The development of tissues and organs in embryos is controlled by an interplay of several signaling pathways that cross-talk to provide positional information and induce cell fate specification. One of the major signaling systems is the Wnt pathway which was recently shown to split into several intracellular branches which regulate multiple cellular functions. In the present review, we discuss novel members and their role in the diversification of the Wnt pathway. Many of these components were studied in model organisms such as C.elegans, Drosophila and Xenopus. Here we focus on recent studies of mutant phenotypes in Mouse and Zebrafish which implicate members of the Wnt pathway in processes such as axis and mesoderm formation, initiation of organ development and stem cell differentiation.

APC, signal transduction and genetic instability in colorectal cancer

Colorectal cancer arises through a gradual series of histological changes, each of which is accompanied by a specific genetic alteration. In general, an intestinal cell needs to comply with two essential requirements to develop into a cancer: it must acquire selective advantage to allow for the initial clonal expansion, and genetic instability to allow for multiple hits in other genes that are responsible for tumour progression and malignant transformation. Inactivation of APC--the gene responsible for most cases of colorectal cancer--might fulfil both requirements.

Genomic structure, chromosome mapping and expression analysis of the human AXIN2 gene

Conductin is a Wnt signalling protein and serves as a negative regulator of beta-catenin stability. We have previously isolated the human homolog (AXIN2) of the murine conductin gene and shown that it is mutated in colorectal cancer (CRC) with defective mismatch repair (MMR). Here we report the detailed genomic structure of this gene by analysis of cDNA and genomic clones. The gene spans > or =25 kb containing ten exons ranging from 96 bp to 904 bp. All splice donor and acceptor sites conform to the GT/AG rule. FISH (Fluorescence in situ Hybridization) analysis localized this gene to human chromosome band 17q24 and showed that it exists as a single copy in the human genome. Northern blot analysis from different human organs demonstrated that the AXIN2 gene is highly expressed in human thymus, prostate, testis, small intestine and ovarian tissues but expressed at a lower level in colon. The data reported here provides a framework for further analysis of this important Wnt signalling protein in vertebrate development and tumorigenesis.

Beta-catenin activation through mutation is rare in rectal cancer

Increased transcriptional activation through beta-catenin stabilization plays a central role in colorectal tumorigenesis. Alterations of phosphorylation sites within the CTNNB1 gene, which codes for beta-catenin has been reported to occur in about one-half of colorectal tumors without APC-gene mutations. We assessed the importance of mutations in the regulatory domain, located within exon 3 of CTNNB1, in 103 rectal carcinomas and correlated these data with presence of microsatellite instability, somatic frame-shift alterations of the TCF-4 gene, and APC-gene mutations in the tumors. No mutation was detected in exon 3 of the CTNNB1 gene and our results thus demonstrate that beta-catenin activation through mutation rarely contributes to the development of sporadic and microsatellite instability stable rectal cancer.

Superficial fibromatoses are genetically distinct from deep fibromatoses

Whereas deep fibromatoses (abdominal, extra-abdominal, mesenteric) display locally aggressive behavior, superficial fibromatoses typically remain small and less likely to recur despite essentially identical morphology. Somatic beta-catenin or APC gene mutations have been reported in < or =74% of sporadic deep fibromatoses and in virtually 100% of Gardner syndrome-associated fibromatoses, whereas genetic events in superficial fibromatoses remain less well characterized. We performed immunohistochemical staining for beta-catenin on 29 superficial fibromatoses (22 palmar, 5 plantar, 1 penile, and 1 infantile digital fibromatosis) and 5 deep fibromatoses. Mutations of beta-catenin and APC genes were analyzed in cases of superficial fibromatoses by direct DNA sequencing of the beta-catenin gene on Exon 3 encompassing the GSK-3 36 phosphorylation region and of the APC gene on the mutation cluster region. Nuclear accumulation of beta-catenin was present in 86% (25/29) of superficial fibromatosis cases ranging from 5 to 100% of nuclei (mean, 13%; median, 10%), though in a minority of nuclei in most examples. Deep fibromatoses had 60 to 100% nuclear staining in all five cases. No somatic mutations of beta-catenin or APC genes were identified in any of the superficial fibromatoses. In contrast to deep fibromatoses, superficial fibromatoses lack beta-catenin and APC gene mutations; the significance of focal nuclear beta-catenin accumulation is unclear. This difference may account inpart for their divergent clinical manifestations despite their morphologic resemblance to deep fibromatoses.

Germline and somatic mutation analyses in the DNA mismatch repair gene MLH3: Evidence for somatic mutation in colorectal cancers

DNA mismatch repair is of considerable scientific and medical importance because of its essential role in maintaining genomic integrity, and its association with hereditary non-polyposis colon cancer (HNPCC). Germline mutations in five mismatch repair genes (MLH1, MSH2, PMS1, PMS2, and MSH6) have been associated with HNPCC susceptibility. Our laboratory recently identified MLH3, a novel DNA mismatch repair gene. We screened the MLH3 coding sequence in 60 probands with increased genetic risk factors for colorectal cancer susceptibility and no mutations in the other candidate genes. No definite MLH3 germline mutations were found. We subsequently screened 36 colon tumors, and discovered an appreciable frequency of somatic MLH3 coding mutations in MSI-H tumors (25%). In four of six tumors, evidence of biallelic inactivation was noted. Furthermore, MLH3 nonsense mutations were identified in two of 12 microsatellite stable (MSS) tumors with 14q24 loss of heterozygosity. While our analyses do not exclude the existence of germline MLH3 mutations in patients with increased genetic risk factors for colorectal cancer susceptibility, they suggest such mutations are uncommon in this patient population. The finding of an appreciable frequency of somatic MLH3 mutations is consistent with a possible role for this gene in the progression of colorectal cancer tumorigenesis. Hum Mutat 17:389-396, 2001. Published 2001 Wiley-Liss, Inc.

Cadherin sequences that inhibit beta-catenin signaling: a study in yeast and mammalian cells

Drosophila Armadillo and its mammalian homologue beta-catenin are scaffolding proteins involved in the assembly of multiprotein complexes with diverse biological roles. They mediate adherens junction assembly, thus determining tissue architecture, and also transduce Wnt/Wingless intercellular signals, which regulate embryonic cell fates and, if inappropriately activated, contribute to tumorigenesis. To learn more about Armadillo/beta-catenin's scaffolding function, we examined in detail its interaction with one of its protein targets, cadherin. We utilized two assay systems: the yeast two-hybrid system to study cadherin binding in the absence of Armadillo/beta-catenin's other protein partners, and mammalian cells where interactions were assessed in their presence. We found that segments of the cadherin cytoplasmic tail as small as 23 amino acids bind Armadillo or beta-catenin in yeast, whereas a slightly longer region is required for binding in mammalian cells. We used mutagenesis to identify critical amino acids required for cadherin interaction with Armadillo/beta-catenin. Expression of such short cadherin sequences in mammalian cells did not affect adherens junctions but effectively inhibited beta-catenin-mediated signaling. This suggests that the interaction between beta-catenin and T cell factor family transcription factors is a sensitive target for disruption, making the use of analogues of these cadherin derivatives a potentially useful means to suppress tumor progression.

The many faces of the tumor suppressor gene APC

Inactivation of the tumor suppressor adenomatous polyposis coli (APC) protein is a critical early step in the development of familial and sporadic colon cancer. Close examination of the function of APC has shown that it is a multifunctional protein involved in a wide variety of processes, including regulation of cell proliferation, cell migration, cell adhesion, cytoskeletal reorganization, and chromosomal stability. Tantalizing clues to the different functions of APC have been provided by the identification of proteins interacting with several discrete motifs within APC. Each of these putative functions could link APC inactivation with tumorigenesis. Here, we will summarize recent findings regarding the diverse role of APC. We will emphasize the interaction of APC with different binding partners, the role of these complex interactions for normal functioning of the cell, and how disruption of these interactions may play a role in tumor development. The rapid progress made recently shows the many faces of APC, leading to a constant reappreciation of this multitasking tumor suppressor protein.