C-MYC IS ACTIVATED BY BETA-CATENIN AND TCF-4

Abstract: The APC tumor suppressor protein binds to beta -catenin, a protein recently shown to interact with Tcf/Lef transcription factors. Here, the gene encoding a Tcf family member that is expressed in colonic epithelium (hTcf-4) was cloned and characterized. hTcf-4 transactivates transcription only when associated with beta -catenin. Nuclei of APC<-/-> colon carcinoma cells were found to contain a stable beta -catenin-hTCF-4 complex that was constitutively active, as measured by transcription of a Tcf reporter gene. Reintroduction of APC removed beta -catenin from hTcf4 and abrogated the transcriptional transactivation. Constitutive transcription of TCF target genes, caused by loss of APC function, may be a crucial event in the early transformation of colonic epithelium. It is also shown here that the products of mutant APC genes found in colorectal tumors are defective in regulating beta -catenin/Tcf-4 transcriptional activation. Furthermore, colorectal tumors with intact APC genes were shown to contain subtle activating mutations of beta -catenin that altered functionally significant phosphorylation sites. These results indicate that regulation of beta -catenin is critical to APC's tumor suppressive effect and that this regulation can be circumvented by mutations in either APC or beta -catenin.

Description: C-MYC IS ACTIVATED BY BETA-CATENTN AND TCF-4 TECHNICAL FIELD OF THE INVENTION This invention is related to the field of cancer diagnostics and therapeutics. More particularly it relates to methods for diagnosing and treating cancers associated with APC or β-catenin mutations. BACKGROUND OF THE INVENTION Mutations of the adenomatous polyposis coli (APC) gene are the most common disease-causing genetic events in humans; approximately 50% of the population will develop colorectal polyps initiated by such mutations during a normal life span (14). Individuals who inherit APC mutations develop thousands of colorectal tumors, consistent with APC's tumor suppressor or "gatekeeping" role in colorectal tumorigenesis (15,16). APC homodimerizes through its amino-terminus (17), and interacts with at least six other proteins: β-catenin (18), γ-catenin (plakoglobin) (19), tubulin (20), EB1 (21), hDLG, a homologue of a Drosophila tumor suppressor protein (22), and ZW3/GSK3β kinase (23). Whether any of these interacting proteins communicate APC growth-controlling signals is unknown. Thus there is a need in the art for a fuller understanding of how the tumor suppressor gene APC functions in cells. SUMMARY OF THE INVENTION It is an object of the present invention to provide methods of determining the presence or absence in a cell of wild-type APC or a downstream protein in the APC transcription regulatory pathway. Another object of the invention is to provide methods of identifying candidate drugs for use in Familial Adenomatous Polyposis (FAP) patients or patients with increased risk of developing cancer. It is yet another object of the invention to provide methods of identifying candidate drugs for the treatment of cancer patients, in particular those with APC or β-catenin mutations. These and other objects of the invention are achieved by providing one or more of the embodiments described below. In one embodiment of the invention a method is provided for determining the presence or absence in a cell of wild-type APC or a downstream protein in the APC transcription regulatory pathway. The method comprises the steps of: introducing a Tcf-responsive reporter gene comprising upstream genomic sequences of c-MYC into the cell; and measuring transcription of said reporter gene; wherein a cell which supports active transcription of said reporter gene does not have wild-type APC or does not have a wild-type downstream protein in the APC transcription regulatory pathway. According to yet another embodiment of the invention a method is provided for determining the presence or absence in a cell of wild-type APC. The method comprises the steps of: contacting a Tcf-responsive reporter gene comprising upstream genomic sequences of c-MYC with a lysate of the cell; and measuring transcription of said reporter gene; wherein a lysate which inhibits said transcription has wild-type APC. In still another embodiment of the invention a method of identifying candidate drugs is provided. The drugs may be useful for treatment of FAP or other cancer patients or patients with increased risk of developing cancer. The method comprises the steps of: contacting a cell having no wild-type APC or a mutant β-catenin with a test compound; measuring transcription of a Tcf-responsive reporter gene comprising upstream genomic sequences of c-MYC, wherein a test compound which inhibits the transcription of the reporter gene is a candidate drug for cancer therapy. According to yet another aspect of the invention another method is provided for identifying candidate drugs for use in for use in FAP patients, colon cancer patients, patients with mutations in β-catenin or APC, or patients with increased risk of developing cancer. The method, comprises the steps of: contacting a Tcf-responsive reporter gene comprising upstream genomic sequences of c-MYC with a test compound under conditions in which the reporter gene is transcribed in the absence of the test compound; and measuring transcription of the Tcf-responsive reporter gene; wherein a test compound which inhibits said transcription is a candidate drug for cancer therapy. The present invention thus provides the art with diagnostic, therapeutic and drug discovery methods especially useful for FAP and other cancers with APC or β- catenin mutations. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1. Sequence comparison of hTcf-4 and hTcf-1. Two alternative splice forms of hTcf-4 were identified, each encoding a different COOH-terminus. One form (hTcf-4E) was homologous to hTCF-lE (top) (7); the other form (hTcf-4B) was homologous to hTcf-lB (bottom). The highly conserved NH 2 -terminal interaction domain and the High Mobility Group (HMG) box DNA-binding region are boxed. Abbreviations for the amino acids are: A. Ala; C, Cys; D, Asp; E, Glu; F, Phe; G, Gly; H, His; I, lie; IC, Lys; L, Leu; M, Met; N, Asn; P, Pro; Q, Gin; P, Ar g; S, Ser; T, Thr; V, Val; W, Trp; and Y, Tyr. The nucleotide sequence has been deposited in GenBank (accession number: ) Fig. 2. Analysis oi hTcf-4 expression in colonic epithelium. (Fig. 2A) Northern blot analysis of hTcf-4, hTcf-1, hLef-I expression in Jurkat T cells (lane 1); colonic mucosa (lane 2); colon carcinoma cell lines DLD-1 (lane 3), HCT116 (lane 4); SW480 (lane 5); SW620 (lane 6); HT29 (lane 7). Lane 2 contains 5 μg total RNA; all others contain 15 μg total RNA. The positions of 18S and 28S ribosomal RNAs are shown. EtBr, ethidium bromide stain. (Fig. 2B) In situ hybridization of healthy human colon tissue to an hTcf-4 probe. (Fig. 2C) In situ hybridization to a negative control probe (a fragment of the E. coli neomycin resistance gene). Fig. 3. Transactivational properties of β-catenin/hTcf-4. All reporter assays were performed as duplicate transfections. For each condition, both values are shown. (Fig. 3A) Reporter gene assays in IIA1.6 B cells. Cells were transfected by electroporation with 1 μg luciferase reporter plasmid, 5 μg β-catenin expression plasmid, and 3 II- hTcf-4 expression plasmids. Empty pCDNA was added to a total of 10 μg, plasmid DNA. (Fig. 3B) Reporter gene assays in SW480 colon carcinoma cells. Cells were transfected with 0.3 μg, of the indicated luciferase reporter gene, 0.7 μg pCATCONTROL as internal control, the indicated amounts of pCMVNeoAPC, and empty PCDNA to a total of 2.5 μg plasmid DNA. Control CAT values are given in the right panel. Fig. 4. Constitutive presence of β-catenin-hTcf-4 complexes in APC " ' " cells. Gel retardation assays were performed on nuclear extracts from the indicated cell lines before and after a 20-hour exposure to Zn ++ . Samples in lanes 1, 4, 7, 10 were incubated under standard conditions. To the samples in lanes 2, 5, 8, 11, 0.25 μg, anti β-catenin was added. To the samples in lanes 3, 6, 9, 12, 0.25 μg of a control (human CD4) antibody was added. N.S., nonspecific band also observed with mutant (nonbinding) probe (lane Mt). Fig. 5. Effects of APC mutations on CRT. (F g. 5A) Schematics of wild-type (WT) and mutant APC. APC is a 2843-amino-acid (AA) protein (32) with contains armadillo (ARM) repeats in the amino-terminus (33), 15 and 20 AA β-catenin-binding repeats in the central region (18,19), and a basic region in the carboxyl-terminus (32). The carboxyl-terminus also contains a TXV sequence which mediates DLG binding (22). (Fig. 5B) Effects of WT and mutant APC on CRT. SW480 cells containing endogenous mutant APC were transfected with the APC expression vectors shown in (F g. 5A) and CRT was measured. Cells were transfected with increasing amounts of WT APC (0, 0.15 and 0.5 μg) or 0.5 μg mutant APC. CRT reporter activities are expressed relative to assays containing no WT APC and are the means of three replicates. Error bars represent standard deviations. Lipofectamine was used to cotransfect SW480 cells with an internal control (0.5 μg pCMV-βgal), a reporter construct (0.5 μg pTOPFLASH or pFOPFLASH) and the indicated amount of the various APC expression vectors. The pTOPFLASH reporter contained an optimized Tcf-binding site 5' of a luciferase reporter gene, whereas pFOPFLASH contained a mutated site that does not bind Tcf . The amount of DNA in each transfection was kept constant by addition of an appropriate amount of empty expression vector (pCEP4). Luciferase and β-galactosidase activities were determined 16 hours after transfection. Luciferase activity was corrected for transfection efficiency (using the control β-galactosidase activity) and nonspecific transcription (using the pFOPFLASH control). Fig. 6. Evaluation of CRT in colorectal cancer cell lines with WT APC. (Fig. 6A) Immunoblot of endogenous APC in the DLDl, SW480, HCT116, SW48 and 293 cell lines, developed with APC monoclonal antibody FE9 (34). (Fig. 6B) Effects of exogenous WT APC on CRT in cell lines with endogenous mutated or WT APC. Cells were transfected with increasing amounts (0, 0.15 μg, 0.5 μg for DLDl and SW48; 0, 0.5 μg, 5 μg for HCT116) of WT APC or APC1309Δ mutant (0.5 μg for DLDl and SW48; 5 μg for HCT116) and CRT was assessed as in Fig. 5. CRT reporter activities are expressed relative to activity in extracts without exogenous APC and are the means of three replicates. Error bars represent standard deviations. F g. 7. Evaluation of β-catenin in colorectal cancer cell lines with WT APC. (Fig. 7A) Immunoblot of the cell lines used in this study, developed with β-catenin monoclonal C19220 (Transduction Laboratories, Lexington, KY)(51). (Fig. 7B) Sequence of CTNNBl in HCT116 and SW48 . Overlapping segments constituting the entire CTNNBl were amplified by RT-PCR from SW480, DLDl, HCT116, and SW48 cells, and sequenced directly with ThermoSequenase (Amersham). In the case of HCT116, a PCR product containing the deleted region was also cloned into pCI-neo (Promega, Madison) and multiple clones corresponding to each allele were individually sequenced. The left panel (nts 121 to 143 from HCT116) reveals the presence of a deletion in addition to the WT sequence. The middle panel (antisense strand 156 to 113 of the WT and deleted alleles of HCT116) reveals the 3-bp deletion (ΔTCT) that removed codon 45 in half the clones. The right panel (nts 80 to 113 from SW48) reveals a C to A transition affecting codon 33 (T£T to TΔT). (Fig. 7C) Schematic of β-catenin illustrating the armadillo repeats (33) and negative regulatory domain. The residues in larger type fit the consensus sequence for GSK3β phosphorylation (29) and those in bold have been demonstrated to affect down regulation of β-catenin through GSK3β phosphorylation in Xenopus embryos (27). The five mutations found in human colon cancers are indicated at the top. Fig. 8. Functional evaluation of β-catenin mutants. (Fig. 8A) Constitutive nuclear complex of β-catenin and Tcf in HCTl 16 cells. The presence of nuclear β-catenin-Tcf complexes was assessed by gel shift assays. Lanes 1 to 3, optimal Tcf retardation probe shifted with nuclear extract from HCTl 16 cells with addition of no antibody (lane 1), anti β-catenin (0.25 μg, lane 2), or an irrelevant antibody (0.25 μg, lane 3). Lane 4, mutant Tcf retardation probe shifted with nuclear extract from HCTl 16 cells, n.s., nonspecific shifting seen with the mutant probe. (Fig. 8B) Effects of the β-catenin mutations on CRT. 293 cells were transfected with WT (WT) or mutant (Δ45, S33Y) β-catenin and CRT was assessed. CRT reporter activities are expressed relative to WT β-catenin and are the means of three replicates. Error bars represent standard deviations, β-catenin expression constructs were prepared as follows. WT CTNNBl was amplified by RT-PCR from SW480 cells and cloned into the mammalian expression vector pCI-neo (Promega) to produce pCI-neo-β-cat. The pCI-neo-β-cat Δ45 and S33Y were generated by replacing codons 1 to 89 in pCI-neo-β-cat with a PCR product encoding the equivalent region from HCTl 16 or SW48 cDNA, respectively. The structures of all constructs were verified by sequence analysis. Lipofectamine was used to cotransfect 293 cells with an internal control (0.1 μg CMV-βgal), a reporter (0.5 μg pTOPFLASH or pFOPFLASH), a Tcf-4 expression vector (0.5 μg pCDNA-TCF4), and β-catenin (0.5 μg) or dominant negative hTcf-4 1.0 μg) expression vectors. CRT was determined as described above. F g. 9. c-MYC expression after APC induction (A) Total RNA was isolated from the ZnCl 2 - treated cells at the indicated times and evaluated on Northern blots (10 μg RNA / lane) that were hybridized with a probe for c-MYC or a control probe for elongation factor lc mRNA (EF1). (B) Total cellular proteins from the same cells were separated by SDS-polyacrylamide gel electrophoresis and subjected to immunoblotting with a monoclonal antibody to c-MYC (9E10, Santa Cruz Biotechnology). An identical blot probed with a monoclonal antibody to p53 shows that equal amounts of proteins were loaded in each lane. F g. 10. APC- and β-catenin responsive regions within the c-MYC promoter. (A) Map of the c-MYC promoter showing the restriction sites used for generating nested deletions ("Del" constructs) and fragments ("Frag" constructs). The horizontal lines represent the sequences in each reporter construct, which were placed upstream of a minimal promoter and luciferase cassette. PI and P2 are start sites of transcription; P2 is the major start site. (B) and (C) SW480 cells were cotransfected with the indicated reporter plasmids plus an APC expression construct or a control plasmid. The bars represent luciferase activity in the cells transfected with APC relative to that in cells transfected with the control plasmid. Luciferase activity was measured in three separate experiments, with the bars and brackets representing means and standard deviations, respectively. The constitutive reporter activity (APC off) of the deletions constructs (Del 1 to Del-4) varied less than two fold ranging from 3520 to 6859 as expressed in arbitrary luciferase light units. The constitutive activity of the Frag-A, -B, -C, -D and -E, were 364, 3050, 1063, 1754 and 976, respectively. (D) 293 cells were cotransfected with the indicated reporters plus a β- catenin expression construct or a control plasmid. The increase in luciferase activity in the β-catenin transfectants compared to the control transfectants is plotted on the y-axis. Bars and brackets represent means and standard deviations determined from three separate transfections. Fig. 11. Tcf-4 Binding Elements (TBE) with the c-MYC promoter. (A) Map of the c-MYC promoter, indicating the 2.5-kb region containing the APC- and β- catenin-responsive elements. The fragment containing the wt sequence of the promoter ("TBE1/2") contains TBE sites near both ends. This fragment was engineered to contain mutations in either site 1 (TBElm/2) or site 2 (TBEl/2m) or both sites 1 and 2 (TBElm/2m), and each fragment was placed upstream of a minimal promoter and luciferase reporter. Reporters containing four copies of TBE1 (4xTBEl) or TBE2 (4xTBE2) or a mutant TBE2 (4xTBE2m), in the absence of any additional genomic sequences, were constructed similarly. (B) SW480 cells were cotransfected with the indicated reporter plasmids plus an APC expression construct or a control plasmid. Data are presented as in Fig. 2B. (C) 293 cells were cotransfected with the indicated reporters plus a β-catenin expression construct or a control plasmid. The increase in luciferase activity in the β-catenin transfectants compared to the control transfectants is plotted on the y-axis. Bars and brackets represent means and standard deviations determined from three separate transfections. (D) Electrophoresis mobility shift assays (EMS A). Oligonucleotides containing TBE1 or TBE2 sequences ("wt") or mutants ("mt") with nt substitutions at critical positions were end-labeled with γ- 32 P-ATP and incubated with 0.5 μg of a GST-fusion protein containing the DNA-binding domain of Tcf-4. DNA-protein complexes were separated by electrophoresis and detected as "shifts" from the position of free probe. Unlabeled oligonucleotides (250 ng) were used as competitors (Comp.) in some reactions. Fig. 12. Repression of c-MYC expression by a dominant-negative Tcf-4. Exponentially growing HCTl 16 and SW480 cells were mock infected (Control) or infected with adenovirus expressing dominant negative Tcf4 (DN-Tcf4) or β- galactosidase (β-Gal) at multiplicity of infection of 100, respectively. Total cellular proteins were isolated 24 hours after infection and subjected to immunoblotting with a monoclonal antibody to c-MYC (C-19, Santa Cruz Biotechnology). As expected, an identical blot probed with a monoclonal antibody to β-catenin (Transduction Laboratories) shows preservation of the β-catenin protein and that equal amounts of protein were loaded in each lane. The recombinant adenoviruses were constructed using the AdEasy system (25), the details of which are available upon request. DETAILED DESCRIPTION It is a discovery of the present invention that hTcf-4 binds to β-catenin and activates transcription in colorectal epithelial cells. Moreover, it has now been found that APC regulates this transcriptional activation, at least in part by binding to β- catenin. In colorectal cancer cells this regulation is frequently abrogated, either by mutation of APC or by mutation of β-catenin. Two alternative splice forms of human Tcf-4 have been found. One form (hTcf-4E) is homologous to hTcf-lE and the other (hTcf-4B) is homologous to hTcf- IB. The sequence of the nucleotide and amino acid sequences are shown in SEQ ID NOs: 1-4. The coding sequences and proteins can be used in assays as described below. Intron-free DNA molecules are provided which are originally made by reverse transcription of a mRNA molecule. They can be propagated in cells or amplified as is desired. Isolated Tcf-4 proteins can be provided substantially free of other human proteins if, for example, the nucleotide sequences are expressed in non- human cells. Methods and vectors for achieving such expression are well known in the art. Choice of such expression means is made by the skilled artisan according to the desired usage and convenience. Cells can be tested to determine if they have a wild-type APC or a wild-type downstream protein in the APC transcription regulatory pathway, called herein the CRT pathway ( β-catenin/Tcf-regulated transcription). One protein within the CRT pathway which has been identified as a target of mutations in human cancers is β- catenin (encoded by the CTNNBl gene). Other parts of the pathway are also likely to be targets. Although the target genes of the CRT pathway have not been identified, they can be readily identified using the system disclosed here. Genes which are differentially transcribed in the presence of wild-type and mutant CTNNBl, for example, can be identified. Tcf-responsive reporter genes are those constructs which comprise a readily detectable or assayable gene (such as luciferase, β- galactosidase, chloramphenicol acetyltransferase) linked in cis to a Tcf-responsive element. Such responsive elements are known in the art (7) and any such elements can be used. An optimal Tcf motif may contain the sequence CCTTTGATC. From one to twenty copies, and preferably from three to six copies, of the motif may be used. Mutation of the sequence to CCTTTGGCC abrogates responsiveness. One particular Tcf motif which can be used is from the 5' upstream region of the c-MYC gene. The binding motifs which have been identified in c-MYC are TBE1 (CTTTGAT), and TBE2 (ATCAAAG). Upstream genomic sequences comprise at least TBE1 or TBE2 plus an additional 3, 4, 5, 10, 20, 30, 50, 100, or 500 adjacent nucleotides. Another necessary part of such constructs is a minimal promoter, such as the c-Fos or the Herpes virus thymidine kinase promoter. Transcription of the reporter gene may be performed by any means known in the art, usually by assaying for the activity of the encoded gene, although immunological detection methods can also be used. In addition, transcription can be monitored by measuring the transcribed mRNA directly, typically using oligonucleotide probes. As shown below, a cell which has a wild-type APC protein will inhibit CRT. However, most mutations in APC render APC unable to inhibit CRT. Similarly, certain mutations in CTNNBl render β-catenin super-active and/or refractory to the inhibition by APC. Thus measuring Tcf-responsive reporter gene transcription is an indication of the status of APC and CTNNBl. Mutations in both of these genes are associated with cancers and therefore provides diagnostic and prognostic information. Assays for CRT can be accomplished in vitro or in cells. If the assay is to be accomplished in cells, then a Tcf-responsive reporter gene must be introduced into the cell. Any means for introducing genetic material into cells can be used, including but not limited to infection, transfection, electroporation. If the assay is to be performed in vitro then the components for transcription must be present. These include suitable buffers, RNA polymerase, as well as ribonucleotides. If the protein product is to be assayed, then the components for translation must also be present, such as ribosomes, and amino acids. These assays can also be used to screen compounds for potential as anti-cancer therapeutic agents. Using either the in vitro or cell form of the assay, test compounds can be introduced to determine whether they are able to mimic the effect of wild-type APC or to convert a mutant APC into a form which is able to inhibit CRT or a mutant β-catenin into a form which is regulatable by APC. In addition, compounds can be tested for the ability to inhibit the binding of β-catenin and Tcf-4, thus mimicking the action of APC. Such a test can be conducted in vitro or in vivo, for example using a two hybrid assay. A means for diagnosis of cancers is the result of the observation that CTNNBl mutations are found in tumor cells, especially those which have wild-type APC. Such mutations can be found, inter alia, by sequencing either the gene or the protein found in a sample. Functional assays can also be used, such as whether β-catenin binds to APC or Tcf-4, or whether it is capable of mediating CRT. Sequences can be compared to those found in a normal tissue of a human, especially the same human who provided the sample being tested. Suitable tumors for testing include, but are not limited to those which are associated with FAP. Suitable tumors include colorectal cancer, thyroid cancer, brain cancer, medulloblastoma, desmoid tumor, osteoma, breast cancer, and head and neck cancer. Because APC mutations are so frequent, and because it appears that APC mutations do not occur in the same tumors as CTNNBl mutations, one can prescreen samples for APC mutations before performing a CTNNBl determination. The portion of the APC gene which encodes the β-catenin binding site can be used in a gene therapy format. Suitable techniques are known in the art for administering genes to tumors, and any such technique can be used. Suitable expression vectors are also known in the art and it is within the skill of the artisan to select an appropriate one. Upon expression in a tumor cell of the β-catenin binding portion of APC, β-catenin will be bound and titrated away from binding to Tcf-4, thus preventing unregulated expression of the CRT target genes. Similarly, a polypeptide portion of APC containing the β-catenin binding site can be administered to cells to perform a titration of β-catenin. Techniques for such administration to cells is well known in the art. Cells which are treated with either the polynucleotide or the polypeptide can be used to study the interaction between APC and β-catenin, and for developing drugs which interfere with such binding. The above disclosure generally describes the present invention. A more complete understanding can be obtained by reference to the following specific examples which are provided herein for purposes of illustration only, and are not intended to limit the scope of the invention. EXAMPLE 1 This example identifies Tcf-4 as the expressed family member in colorectal epithelial cells and provides the complete sequence of the cloned cDNA. There are four known members of the Tcf/Lef family in mammals: the lymphoid- specific factors Tcf- 1 and Lef- 1 (7,8), and the less well characterized Tcf-3 and 4(9). We performed a qualitative Reverse Transcriptase-Polymerase Chain Reaction assay for expression of the four Tcf/Lef genes on 43 colon tumor cell lines. While most colon cell lines expressed more than one of the genes, only hTcf-4 mRNA was expressed in essentially all lines. We then screened a human fetal CDNA library and retrieved clones encoding full-length hTcf-4 (Fig. 1). A genomic fragment encoding, the HMG box region of hTcf-4 (7) was used to probe a human 12 week-fetal cDNA library in Lambda GT- 11. Positive clones were subcloned into pBluescriptSK and sequenced. See SEQ ID NOs: 1 and 3. The predicted sequence of hTcf-4 was most similar to that of hTcf-1. Alternative splicing yielded two COOH-termini that were conserved between hTcf- I and hTcf-4. The NH 2 -terminus, which in hTcf- 1, mLef-1 and Xenopus TCF-3 mediates binding to β-catenin (6), was also conserved in hTcf-4. Northern blot analysis of selected colon carcinoma cell lines revealed high-level expression of hTcf- 4 (Fig. 2A). Northern blot hybridizations (7) were performed with full-length hTcf- 1, hLef-I and hTcf-4 cDNA. Colon epithelial cells were freshly prepared from a mucosal preparation dissected from a healthy surgical colon sample. The sample was minced, and incubated with 1 mM dithiothreitol (DTT) in Hanks' medium to remove mucus. Single-cell suspensions were prepared by incubation at RT in 0.75 mM EDTA in Hanks' medium. Epithelial cells were separated from lymphocytes by Percoll gradient centrifugation. As evidenced by in situ hybridization (Fig. 2, B and C) and Northern blotting (Fig. 2A), hTcf-4 MRNA was readily detectable in normal colonic epithelium, whereas hTcf-1 and hLef-I were not detectable. In situ hybridization of 6 μ frozen sections of healthy colon biopsy samples were performed as described(lθ). hTcf-4 cDNA encoding amino acids 200 to 310 was amplified and labeled with Dig-11-dUTP (Boehringer Mannheim, Germany) by PCR. After hybridization and washing, the sections were sequentially incubated with mouse anti-Dig antibody (Boehringer) and a horseradish peroxidase conjugated rabbit antibody to mouse immunoglobulin (Dako, Glostrup, Denmark). The signal was visualized with diaminobenzidine, which produces a reddish-brown precipitate. Blue counterstining was performed with haematoxyline. EXAMPLE 2 This example demonstrates the interaction of Tcf-4 and β-catenin and their function as a transcriptional activating factor. To investigate whether hTcf-4 functionally interacts with β-catenin, we used two sets of reporter constructs in a β -catenin-Tcf reporter gene assay (7). One contained three copies of the optimal Tcf motif CCTTTGATC, or three copies of the mutant motif CCTTTGGCC, upstream of a minimal c-Fos promoter driven-luciferase expression (PTOPFLASH and PFOPFLASH). The second set contained three copies of the optimal motif, or three copies of the mutant motif, upstream of a minimal Herpes virus thymidine kinase promoter driven-Chloramphenicol Acetyl Transferase (CAT) expression (PTOPCAT and PFOPCAT, respectively). Reporter gene assays were performed as in (7). In brief, 2 x 10 6 cells were transfected with plasmids by electroporation. After 24 hours, cells were harvested and lysed in 1 mM DTT, 1 % Triton X-100, 15 % glycerol, 25 mM Tris pH 7.8 and 8 mM MgCl 2 . cDNAs encoding Myc-tagged versions of β-catenin and hTcf-4 were inserted into the mammalian expression vector pCDNA (Invitrogen). PCATCONTROL, encoding the CAT enzyme under the control of the SV40 promoter, was purchased from Promega. Epitope-tagged hTcf-4 and a deletion mutant lacking, the NH 2 - terminal 30 amino acids (ΔNhTcf-4) were cloned into the expression vector pCDNA. Transient transfections were performed in a murine B cell line (IIA1.6), that does not express any of the Tcf genes (6). The TOPFLASH reporter was strongly transcribed upon cotransfection with the combination of β-catenin and hTcf-4 plasmids, but not with the individual plasmids or with the combination of β-catenin and ΔNhTcf-4 plasmids. No enhanced transcription was detected in cells transfected with the negative control PFOPFLASH (Fig. 3A). These results show that interaction of the NH 2 -terminus of hTcf-4 with β-catenin results in transcriptional activation. EXAMPLE 3 This example demonstrates the functional regulation of CRT transcriptional activation by wild-type APC. In three APC " ' " carcinoma cell lines, SW480, SW620 and DLD-1 (Fig. 3B), the PTOPFLASH reporter was 5-20 fold more actively transcribed than PFOPFLASH. Importantly, transfection of SW480 cells with the reporter gene and an APC-expression vector abrogated the transcriptional activity in a dose-dependent manner (Fig. 3B). In contrast APC had no effect on a cotransfected internal control (pCATCONTROL), or on the basal transcription of PFOPFLASH (Fig. 3B). The use of PTOPCAT and PFOPCAT instead of PTOPFLASH and PFOPFLASH led to comparable observations. The constitutive transcriptional activity of Tcf reporter genes in APC " ' " colon carcinoma cells was in stark contrast to the inactivity of these genes in non-colonic cell lines, including HA1.6 B cells (Fig. 3A), the C57MG breast carcinoma cell line; the Jurkat and BW5147 T cell lines; the Daudi and NS1 B cell lines; the K562 erythromyeloid cell line; the HeLa cervical carcinoma line; the HepG2 hepatoma cell line; 3T3, 3T6, and Rat-I fϊbroblasts; and the kidney derived SV40-transformed COS cell line (7,16). EXAMPLE 4 This example demonstrates that a functional β-catenin-hTcf-4 complex exists constitutively in APC " ' " cells. We used HT29-APC " ' ' colon carcinoma cells (12), in which APC is controlled by a metallothionein promoter. Induction by Zn ++ restores wild-type levels of APC, and leads to apoptosis (12). HT29-Gal cells which carry a Zn + + -inducible LacZ gene were used as a control. The only Tcf family member expressed in HT29 is hTcf-4 (Fig. 2C). In nuclear extracts from uninduced HT29 derived transfectants, we readily detected hTcf-4 by gel retardation (Fig. 4). An additional band of slightly slower mobility was also observed. The addition of a β-catenin antibody resulted in the specific retardation of the latter band, indicating that it represented a β-catenin-hTcf-4 complex (Fig. 4) (12). After Zn ++ induction for 20 hours, the β-catenin-hTcf-4 complex was diminished sixfold relative to uncomplexed hTcf-4 in HT29-APC1, while no significant change was observed in HT29-Gal cells (Fig. 4). Importantly, the overall levels of cellular β-catenin do not change during the induction period in HT29-APC1 cells (12). Gel retardation assays were performed as described elsewhere (7). Extracts were prepared from intact nuclei that were washed four times to avoid contamination with cytoplasmic β-catenin. As the optimal Tcf/Lef probe, we used a double- stranded 15-mer CCCTTTGATCTTACC; the control probe was CCCTTTGGCCTTACC. (All oligonucleotides were from Isogene, Holland). The β-catenin antibody was purchased from Transduction Laboratories (Lexington, KY). A typical binding reaction contained 3 μg nuclear protein, 0. 1 ng radiolabeled probe, 100 ng of dldC, in 25 μl of binding buffer (60 mm KCl, 1 mM EDTA, 1 mM DTT, 10% glycerol). Samples were incubated for 20 min at room temperature, antibody was added, and the samples incubated 20 min further. On the basis of these data, we propose the following model. In normal colonic epithelium hTcf-4 is the only expressed member of the Tcf family. The interaction of β-catenin with hTcf-4 is regulated by APC. When appropriate extracellular signals are delivered to an epithelial cell, β-catenin accumulates in a form that is not complexed with GSK3β-APC, and that enables its nuclear transport and association with hTcf-4. The HMG domain of hTcf-4 binds in a sequence- specific fashion to the regulatory sequences of specific target genes; β-catenin supplies a transactivation domain. Thus, transcriptional activation of target genes occurs only when hTcf-4 is associated with β-catenin. The hTcf-4 target genes remain to be identified. However, the link with APC and catenin suggests that these genes may participate in the generation and turnover of epithelial cells. Upon loss of wild-type APC, monomeric β-catenin accumulates in the absence of extracellular stimuli, leading to uncontrolled transcription of the hTcf-4 target genes. The apparent de novo expression of other members of the Tcf family in some colon carcinoma cell lines might lead to a further deregulation of Tcf target gene expression by the same mechanism. The control of β-catenin -Tcf signaling is likely to be an important part of the gatekeeper function of APC (19), and its disruption an early step in malignant transformation. EXAMPLE 5 This example demonstrates that mutant APC protein does not regulate CRT and that a complete set of 20- AA repeats in APC is required to mediate inhibition of CRT. We tested four APC mutants (Fig. 5A) for their ability to inhibit β-ςatenin/Tcf-iegulated transcription (CRT) in transfection assays. The first mutant, APC331Δ represents a type of mutation found in the germline of Familial Adenomatous Polyposis (FAP) patients as well as in sporadic tumors (15). The APC331Δ protein is truncated at codon 331, amino-terminal to the three 15-amino-acid (AA) β-catenin-binding repeats between codons 1020 and 1169. The second mutant, APC1309Δ, is the most common germline APC mutation (15), a 5-bp deletion that produces a frameshift at codon 1309 and truncation of the protein. The APC1309Δ protein retains the 15-AA β-catenin repeats but lacks the seven 20- AA repeats between codons 1323 and 2075 that have been implicated in binding and phosphorylation of β-catenin (18). The third mutant, APC 1941Δ, represents one of the most distal somatic mutations observed in colorectal tumors (25). The APC 1941Δ protein is truncated at codon 1941 and therefore contains the 15-AA repeats and all but the last two 20- A A repeats. Finally, APC2644Δ represents a germline mutation resulting from a 4-bp deletion in codon 2644. Patients with this type of unusual carboxyl-terminal mutation develop few polyps (attenuated polyposis) but have pronounced extracolonic disease, particularly desmoid tumors (26). Each of the APC mutants was cotransfected with a CRT reporter into the SW480 colorectal cancer cell line. SW480 cells have truncated APC and constitutively active CRT which can be suppressed by exogenous WT APC. Although all four mutants produced comparable levels of APC protein after transfection, they varied in their CRT inhibitory activity. The three mutants found in patients with typical polyposis or cancer were markedly deficient in inhibition of CRT (Fig. 5B). The reduced activity of APC1309Δ and APC 1941Δ suggests that β-catenin binding is not sufficient for APC-mediated inhibition of CRT and that the complete set of 20-AA repeats is required. Interestingly, the inhibitory activity of the APC2644Δ mutant associated with attenuated polyposis was comparable to that of WT APC (Fig. 5B), suggesting that the DLG-binding domain at the carboxyl-terminus of APC is not required for down-regulation of CRT. WT and mutant APC constructs (2 μg) were transfected into 293, SW480, and HCTl 16 cells using Lipofectamine (GIBCO/BRL, Gaithersburg). Protein was harvested 24 hours later and subjected to immunoblot analysis with APC monoclonal antibody FE9 (23). In HCTl 16 and 293 cells, exogenous WT APC comigrated with the endogenous APC. In SW480 cells, APC 1309Δ comigrated with the endogenous mutant APC. In all other cases, the nonfunctional APC constructs (APC331Δ, APC 1309Δ, and APC1941Δ) produced as much or more protein than the CRT-functional forms of APC (APC WT and APC 2644Δ). EXAMPLE 6 This example demonstrates that other components of the APC-regulatory pathway are affected in some cancer cells. We evaluated CRT in two colorectal tumor cell lines (HCTl 16 and SW48) that express full-length APC (Fig. 6A). Both HCTl 16 and SW48 displayed constitutively active CRT and, in contrast to cell lines with truncated APC (DLDl and SW480), this activity was not inhibited by exogenous WT APC (Fig. 5B, 6B). Other (noncolorectal cancer) cell lines expressing WT APC do not display constitutive CRT activity. These transfection results suggested that the constitutive CRT in HCTl 16 and SW48 might be due to an altered downstream component of the APC tumor suppressor pathway. EXAMPLE 7 This example demonstrates a defect in the gene encoding β-catenin in some cancer cells, which affects CRT. We evaluated the status of a likely candidate for a downstream component of the APC tumor suppressor pathway, β-catenin, in the same four lines. All four lines expressed similar amounts of apparently intact β-catenin, as assessed by immunoblots (Fig. 7A). However, sequence analysis revealed that both HCTl 16 and SW48 harbored mutations in the β-catenin gene (CTNNBl) (Fig. 7B). HCTl 16 had a 3-bp deletion that removed one AA (Ser-45), and SW48 had a C to A missense mutation that changed Ser-33 to Tyr. Analysis of paraffin-embedded archival tissue from the HCTl 16 patient confirmed the somatic nature of this mutation and its presence in the primary tumor prior to culture. Interestingly, both mutations affected serines that have been implicated in the downregulation of β-catenin through phosphorylation by the ZW3/GSK3β kinase in Xenopus embryos (Fig. 7C) (27,28). Genomic DNA was isolated from paraffin-embedded normal and tumor tissue from the patient from whom the HCTl 16 cell line was derived. A 95 bp PCR product encompassing the mutation was then amplified by PCR and directly sequenced using THERMOSEQUENASE (Amersham). The 3 bp deletion was observed in tumor but not in normal tissue. To test the generality of this mutational mechanism, we evaluated five primary colorectal cancers in which sequencing of the entire coding region of APC revealed no mutations (25). Three of these five tumors were found to contain CTNNBl mutations (S45F, S45F, and T44A) that altered potential ZW3/GSK3β phosphorylation sites (Fig. 7C). Each mutation appeared to affect only one of the two CTNNBl alleles and to be somatic. Genomic DNA was isolated from frozen-sectioned colorectal cancers and a 1001 bp PCR product containing exon 3 of CTNNBl was then amplified by PCR and directly sequenced using ThermoSequenase (Amersham). An ΔCC to G.CC change at codon 41 (T41A) and a TCT to TIT at codon 45 (S45F) was observed in one and two tumors, respectively. EXAMPLE 8 This example demonstrates dominant mutations of CTNNBl that render CRT insensitive to the effects of WT APC. Because the β-catenin mutations were heterozygous, we hypothesized that the mutations might exert a dominant effect, rendering a fraction of cellular β-catenin insensitive to APC-mediated down regulation. To test this notion, we performed gel shift analyses with nuclear extracts from untransfected HCTl 16 cells. In contrast to noncolorectal cancer cell lines with intact APC, HCTl 16 cells contained a β-catenin/Tcf complex that gel-shifted an optimized Tcf-binding oligonucleotide, and this complex supershifted with anti-β-catenin (Fig. 8A). We also constructed β-catenin expression vectors and compared the biologic activity of the mutant β-catenin from HCTl 16 (β-Cat Δ45) and SW48 (β-Cat S33Y) with that of their WT counterpart. For these experiments, we used the 293 kidney epithelial cell line as it is highly transfectable, exhibits low endogenous CRT, and contains a high level of endogenous APC (Fig. 6A). In the presence of endogenous APC, both mutant β-catenins were at least 6-fold more active than the WT protein and this activity was inhibited by dominant-negative hTcf-4 (Fig. 8B). Together, these results indicate that disruption of APC-mediated regulation of CRT is critical for colorectal tumorigenesis. This is most commonly achieved by recessive inactivating mutations of both APC alleles but, as shown here, can also be achieved by dominant mutations of CTNNBl that render CRT insensitive to the effects of WT APC. Our results suggest that APC inhibition of CRT requires phosphorylation of β-catenin at multiple sites. These potential phosphorylation sites are consistent with the known specificity of ZW3/GSK3β (29) a serine kinase that negatively regulates β-catenin in Xenopus and Drosophila cells (27) and that interacts with APC and β-catenin in mammalian cells (23). These results also suggest a functional basis for the occasional CTNNBl mutations observed in other tumor types (30) and illustrate how a critical pathway in human disease can be illuminated by the discovery of mutations in different components of the pathway. The next step in understanding APC function will be the identification of the genes that are activated by hTcf-4/β-catenin complexes and inhibited by WT APC. These genes are likely to be related to APC's ability to induce apoptosis in colorectal cancer cells (31). REFERENCES (for preceeding pages) 1. B. Rubinfeld et al Science, 262, 1731 (1993); L. K. Su, B. Vogelstein, K.W. Kinzler, ibid 262, 1734 (1993). 2. B. Gumbiner, Curr. Opin. Cell Biol. 7, 634 (1995). 3. B. Rubinfeld et al, Science 272, 1023 (1996). 4. J. Papkoff, B. Rubinfeld, B. Schryver, P. Polakis, Mol. Cell. BioL 16, 2128 (1996). 5. S. Munemitsa, B. Souza, I. Albert, B. Rubinfeld, P. Polakis, Proc. Natl. Acad Sci. U.S.A. 92, 3046 (1995); B. Rubinfeld, B. Souza, I. Albert, S. Muneinitsa, P. Polakis, J Biol Chem. 270, 5549 (1995). 6. M. Molenaar et al, Cell 86, 396 (1996); J. Behrens et al, Nature 382, 638 (1996); 0. Huber et al, Mech. Dev. 59, 3 (1996). 7. M. van de Weterinc,, M. Oosterwegel, D. Dooijes, H. Clevers, EMBO J 10, 123 (1991); M. van de Wetering, J. Castrop, V. Korinek, Mol Cell Biol, 16, 745 (1996). 8. A. Travis et al . Genes Dev. 5, 880 (199 1); M.L. Waterman, W.H. Fischer, K.A. Jones ibid. p. 6562. H. Clevers and R. Grosschedl, Immunol. Today 17, 336 (1996). 9. J. Castrop, K. van Norren, H.C. Clevers. Nucleic Acids Res. 20, 611 (1992). 10. E. van Hoffen et al, Am. J Pathol 149, 1991 (1996). 11. M. van de Wetering, M. Oosterwegel, K. van Norren, H. Clevers, EMBO J. 12, 3847 (1993) 12. P. Morin, B. Vogelstein, K.W. Kinzler, Proc. Natl. Acad. Sci. U.S.A. 93, 7950 (1996). 13. K. W. Kinzler and B. Vogelstein, Cell 87, 159 (1996). 14. About 50% of the Western population develop colorectal adenomas by the age of 70 [D. Ransohoff and C. Lang, N. Eng.U. Med. 325, 37 (1991)] and at least 85% of these tumors contain APC mutations ; Y. Miyoshi et al, Hum Mol Genet 1, 229-33 (1992); J. Jen et al, Cancer Res. 54, 5523 (1994). 15. H. Νagase and Y. Νakamura, Hum. Mutation 2, 425 (1993). 16. K. W. Kinzler and B. Vogelstein, Cell 87, 159 (1996); S. M. Prescott and R. L. White, ibid. , p. 783. 17. G. Joslyn, D. S. Richardson, R. White, T. Alber, Proc. Natl. Acad. Sci. U. S. A. 90, 11109 (1993); L. K. Su et al , Cancer Res. 53, 2728 (1993). 18. B. Rubinfeld et al , Science 262, 1731 (1993); L. K. Su, B. Vogelstein, K. W. Kinzler, ibid. , p. 1734. 19. J. Hulsken, J. Behrens, W. Birchmeier, Curr. Opin. Cell Biol 6, 711 (1994); B. Rubinfeld, B. Souza, I. Albert, S. Munemitsu, P. Polakis, J. Biol. Chem. 270, 5549 (1995). 20. S. Munemitsu et al. , Cancer Res. 54, 3676 (1994); K. J. Smith et al , ibid. p. 3672. 21. L. K. Su et al. , Cancer Res. 55, 2972 (1995). 22. A. Matsumine et al , Science 272, 1020 (1996). 23. B. Rubinfeld et al , Science 272, 1023 (1996). 24. M. Molenaar et al, Cell 86, 391 (1996); J. Behrens et al, Nature 382, 638 (1996). 25. S. M. Powell et al. , Nature 359, 235 (1992). 26. D. M. Eccles et al , Am. J. of Hum. Genet. 59, 1193 (1996); W. Friedl et al , Hum Genet 97, 579 (1996); R. J. Scott et al , Human Molecular Genetics 5, 1921 (1996). 27. C. Yost et al, Genes Dev. 10, 1443 (1996). 28. S. Munemitsu, I. Albert, B. Rubinfeld, P. Polakis, Mol Cell Biol 16, 4088 (1996). 29. M. Peifer, L. M. Pai, M. Casey, Dev. Biol 166, 543 (1994). 30. D J. Kawanishi, et al, Mol. Cell Biol. 15, 1175 (1995); P. F. Robbins, et al , J. Exp. Med. 183, 1185 (1996). 31. P. J. Morin, B. Vogelstein, K. W. Kinzler, Proc. Natl. Acad. Sci. U. S. A. 93, 7950 (1996). 32. J. Groden et al. , Cell 66, 589 (1991); G. Joslyn et al. , ibid. , p. 601; K. W. Kinzler et al. , Science 253, 661 (1991); I. Nishisho et al. , ibid. , p. 665. 33. M. Peifer, S. Berg, A. B. Reynolds, Cell 76, 789 (1994). 34. K. J. Smith et al , Proc. Natl. Acad. Sci. U. S. A. 90, 2846 (1993). 35. S. E. Goelz, S. R. Hamilton, B. Vogelstein, Biochem. Biophys. Res. Commun. 130, 118 (1985) EXAMPLE 9 The adenomatous polyposis coli gene (APC) is a tumor suppressor gene that is inactivated in most colorectal cancers. Mutations of APC cause aberrant accumulation of β-catenin, which then binds T cell factor-4 (Tcf-4), causing increased transcriptional activation of unknown genes. Here, the c-MYC oncogene is identified as a target gene in this signaling pathway. Expression of c-MYC is repressed by wild- type APC and activated by β-catenin, and these effects are mediated through Tcf-4 binding sites in the c-MYC promoter. These results provide a molecular framework for understanding the previously enigmatic overexpression of c-MYC in colorectal cancers. Most human colorectal tumors are initiated by inactivation of the APC tumor suppressor gene, located on chromosome 5q21 (1). APC is a cytoplasmic protein which can bind to an promoter the degradation of β-catenin (2). Among β-catenin functions is the ability to bind members of the Tcf family of transcription factors and activate gene transcription (3). Accordingly, human colorectal tumors with APC or β-catenin mutations exhibit increased β-catenin/Tcf mediated transcription (4, 5). However, the downstream targets of this β-catenin/Tcf-4-regulated transcription are unknown. This study was undertaken to define those targets and thereby gain clues to the mechanisms through which APC affects cellular growth. To evaluate the transcription effects of APC, we studied a human colorectal cancer cell line (HT29-APC) containing a zinc-inducible APC gene and a control cell line (HT29-β-Gal) containing an analogous inducible lacZ gene (6). Both endogenous APC alleles in HT29 cells contain truncating mutations, and restoration of wild- type APC expression results in growth inhibition and apoptosis. Upon induction, APC protein is synthesized rapidly and reaches maximal levels by nine hours (7). By twelve hours, a significant fraction of the cells display morphological signs of apoptosis. Because we were interested in identifying changes in gene expression that directly relate to restoration of APC function and not apoptosis, we analyzed the HT29-APC cells nine hours after APC induction. To evaluate changes in gene expression, we used Serial Analysis of Gene Expression (SAGE), a technique that allows the quantitative evaluation of cellular mRNA in an unbiased manner (8). In brief, the method is based on the use of short sequence tags (15 bp) generated from defined positions within each transcript. Expression levels are deduced from the abundance of individual tags in a sample. SAGE analysis of 51,622 and 55,846 tags from APC-induced and control cells, respectively, allowed identification of 14,346 different transcripts (9), the majority of which were expressed at similar levels in the APC-induced and control cells. Of the 30 tags showing signficant differences in expression (10), 14 were overexpressed and 16 were repressed in APC-induced cells. Because biochemical studies have indicated that APC represses β-catenin/Tcf-4-mediated transcription (4,5), we focused on the latter transcripts. One of the three most highly repressed transcripts was a tag corresponding to the c-MYC oncogene (eight tags in HT29-β-Gal vs. zero in HT29- APC). This repression was confirmed at the mRNA and protein level by Northern blot (Fig. 1A) and immunoblot (Fig. IB) analysis, respectively. Repression of c- MYC mRNA and protein was evident within 6 hours after zinc induction and within 3 hours after the first detection of APC protein (Fig. 1). These results suggested that APC might directly modulate c-MYC transcription through β-catenin/Tcf-4. To assess this possibility, we isolated a 2.5-kb genomic fragment encompassing the c-MYC promoter, inserted it upstream of a luciferase reporter gene (11) and then tested the construct for responsiveness to APC (12). This c-MYC promoter region conferred signficiant transcriptional activity to the basal reporter gene when transfected into human colorectal cancer cells, and this activity was significantly repressed by APC (Fig. 2). Nested deletions of the promoter were used to map the APC-responsive region to a fragment containing nt -1194 to -484 relative to the TATA box at the c-MYC major transcription start site (Fig. 2 A and B). Testing of restriction fragments spanning the promoter revealed two responsive regions, one located in fragment B (nt - 1194 to -741) and the other in fragment C (nt -741 to -484) (Fig. 3A and C). If the effects of APC on c-MYC transcription were mediated through inhibition of β-caterιin/Tcf-4-regulated transcription, then the c-MYC promoter should be activated by β-catenin. It has previously been shown that β-catenin/Tcf-4 transcription can be activated by exogenous expression of a mutant β-catenin gene in the human kidney cell line 293. The β-catenin construct used for these experiments was mutated at codon 33, rendering it insensitive to downregulation by the endogenous wild-type APC in 293 cells (5). The c-MYC reporter was found to be significantly activated by β-catenin in this line. Using the nested deltion and restriction fragment constructs noted above, we found that the region of the c-MYC promoter that conferred β-catenin responsiveness was the same region (fragments B and C) shown to be APC-repressible in colorectal cancer cells (Fig. 2D). Analysis of the m-MYC promoter sequence revealed one potential Tcf-4 binding site (13) within fragment B (TBE1) and another within fragment C (TBE2) (Fig. 4A). To test the functional significance of these sites, we created fragments of the c-MYC promoter in which one or both binding sites were eliminated by nucleotide substitutions (14). Mutation of either TBE1 or TBE2 reduced the activity of the c- MYC promoter fragment by 50%. Importantly, deletion of both sites completely removed APC repression and β-catenin activation from the report, whereas deltion of either element alone did not abrogate responsiveness (Fig. 3A to C). We also tested the TBE1 and TBE2 elements in isolation. Constructs containing four tandem copies of either TBEl or TBE2 upstream of a minimal promoter (15) conferred β-catenin responsiveness and APC repression to a downstream luciferase reporter (Fig. 3 A to C). In all cases, the responsiveness of the reporter containing TBE2 sites was greater than that obtained with TBEl sites. Nucleotide substitutions within TBEl or TBE2 that would be expected to abolish Tcf- 4 binding abrogated responsiveness to APC and β-catenin (Fig. 3 A to C). Finally, to confirm the direct nature of the responsiveness, we tested the ability of Tcf-4 produced in bacteria to bind TBEl and TBE2 (16). Tcf-4 bound both TBEl and TBE2, as judged by electrophoresis mobility shift assays, and this binding was abrogated by the same nucleotide substitutions that eliminated transcriptional responses (Fig. 3D). On the basis of these data, we propose that in normal colorectal epithelial cells, wild-type APC prevents β-catenin from forming a complex with Tcf-4 and activating c-MYC. In colorectal tumors with APC mutations or activating β-catenin mutations, increased β-catenin/Tcf-4 activity leads to overexpression of c-MYC, which then promotes neoplastic growth. Consistent with this model, expression of a dominant negative Tcf-4 in colorectal cancer cells with mutant β-catenin (HCTl 16) or mutant APC (SW480) significantly reduced the endogenous levels of c-MYC (Fig. 4). This model is also consistent with c-MYC's powerful oncogenic activities (17) and provides an explanation for two long-standing quandaries. First, it has been extensively documented that c-MYC is overexpressed at the RNA and protein levels at both early and late stages of colorectal tumorigenesis (18). However, unlike the situation in some other cancers, where the c-MYC gene is rearranged or amplified (19), genetic alterations of c-MYC are rare in colorectal tumors and the cause of the overexpression has been unknown (20). The only clue to this mechanism has come from chromosome transfer experiments, in which it was shown that an extra copy of chromosome 5 can repress c-MYC transcription and inhibit neoplastic growth (21). This repression fits well with the molecular data presented here on APC (chromosome 5q21). The second enigma involves the cyclin-dependent kinase inhibitor pl6INK4a. Most tumor types exhibit genetic alterations of the pl6INK4a growth-inhibitory pathway, through direct mutation of pl6INK4a, its neighbor pl5INK4b, or its downstream targets Rb, cdk4, or cyclin Dl (22). Colorectal cancers are a unique and notable exception, in that few mutations of any of the genes in this pathway occur (22,23). The activation of c-MYC through APC inactivation would explain this, as c-MYC expression can bypass pl6INK4a- and pl5INK4b- mediated growth arrest (24). REFERENCES AND NOTES (for example 9) 1. J. Groden et al, Cell 66, 589 (1991); G. Joslyn et al, ibid., p. 601; K.W. Kinzler et al, Science 253, 661 (1991); I. Nishisho et al, ibid., p. 665; K.W. Kinzler and B. Vogelstein, Cell 87, 159 (1996. 2. B. Rubinfeld et al, Science 262, 1731 (1993); L.K. Su, B. Vogelstein, K.W. Kinzler, Science 262, 1734 (1993); S. Munemitsu, I. Albert, B. Souza, B. Rubinfeld, P. Polakis, Proc. Natl. Acad. Sci.XJ.S.A. 92, 3046 (1995); B. Rubinfeld et al , Science 275, 1790 (1997). 3. M. Molenaar et al, Cell 86, 391 (1996); J. Behrens et al , Nature 382, 638 (1996). 4. V. Korinek et al. , Science 275, 1784 (1997). 5. P.J. Morin et al, ibid. p. 1787. 6. P.J. Morin, B. Vogelstein, K.W. Kinzler, Proc. Natl. Acad. Sci. U.S.A. 93, 7950 (1996). 7. Gene expression was induced as in (6) except that 120 μM ZnCl 2 was used. 8. V.E. Velculescu, L. Zhang, B. Voglestien, K.W. Kinzler, Science 270, 484 (1995); L. Zhang et al, ibid. 276, 1268 (1997); V.E. Velculescu et al. Cell 88, 243 (1997). 9. SAGE was performed as in (8) on mRNA from exponentially growing HT29-APC and HT29-βGal cells nine hours after induction. A total of 55,233 and 59,752 tags were obtained from HT29-APC and HT29- βGal cells, respectively. Analysis of internal linker controls revealed a sequencing error rate of 0.065 per tag, corresponding to a sequencing error rate of 0.0067 per base. This was in good agreement with instrument specifications and previous estimates of SAGE tag errors based on analysis of the completed yeast genome (8). After correcting for sequencing mistakes, a total of 107,468 tags representing 51,622 and 55,846 from HT29-APC and HT29-βGal cells, respectively, were analyzed. These tages represented 14,346 unique transcripts, of which 7,811 transcripts appears at least twice. 10. Expression differences were considered significant if they had a P FALSE of < 0.1 as determined by Monte Carlo simulations and they were at least five fold in magnitude (8). 11. A low-basal activity reporter plasmid, pBV-Luc, was first constructed. The pDel-1, pDel-2, pDel-3, pDel-4, pFrag-A, pFrag-B, pFrag-C, pFrag-D, and pFrag-E reporters were constructed by cloning corresponding restriction fragments (illustrated in Fig. 2 A) of human c-MYC promoter into pBV-Luc. Details of vector construction are available upon request. 12. Explonentially growing SW480 and 293 cells were cultured in 12-well plates and transfected with 0.4 μg reporter, 0.2 μg pCMVβgal control and 0.9 μg effector plasmid using LipofectAmine (Life Technologies, Inc.). The APC [K.J. Smith et al, Cancer Res. 54, 3672 (1994)] and β-catenin (5) effector plasmids have been described. Luciferase assays were carried out 24 hours after transfection and normalized for transfection efficiency using β-galactosidase activity. Each assay was performed in triplicate. 13. Two TBE-binding elements were identified int the region conferring APC and β-catenin repsonsiveness. TBEl (CTTTGAT) was located 1156 bp upstream of the TATA box at the PI transcription start site and perfectly matched the consensus for Tcf-binding CTTTG(A/T)(A/T) [M. van de Wetering, M. Oosterwegel, D. Dooijes, H. Clevers, EMBO J. 10, 123 (1991); K. Giese, A. Amsterdam, R. Grosschedl, Genes Dev. 5 2567 (1991)]. TBE2 was located 589 bp upstream of the TATA box and contained an inverted perfect match (ATCAAAG). A third Tcf-binding site was located 1400 bp upstream of the TATA box but did not overlap with APC or β-catenin responsiveness. 14. To construct pTBEl/2 plasmid, we used PCR primers (5 1 - CTAGCTAGCCTAGCACCTTrGATTTCTCCC-3' and 5'- CGTGATATCCGCTTTGATCAAGAGTCCCAG-3') to amplifiy nt -576 to -1162 of the c-MYC promoter region. The PCR product was cloned into pBV-Luc. To construct ρTBEl/2m, pTBElm/2, and pTBElm 2m, and pTBElm/2m, we used a mutated TBEl primer (5'- CTAGCTAGCACTGGTGCATCTCCCAAACCCGGCAGCCCG-3') an d a mutated TBE2 primer (5 ' - CTGGATATCACTGGTGCATCCCAGGGAGAGTGGAGGAAAG- 3'), in combination with either of the wt primers, to amplify the same region, and subcloned the products into pBV-Luc. 15. To construct the four tandem repeats of TBEl, TBE2, and TBE2m, we dimerized oligo cassettes containing two copies of each site and cloned the products into pBV-Luc (for TBEl: 5'- CTAGCGCACCTΓΓGATΓTCTGCACCTTTGATTTCTG-3' and 5'- CTAGCAGAAATCAAAGGTGCAGAAATCAAAGGTGCG-3 ' ; for T B E 2 : 5 ' - CTAGCGGACTCTTGATCAAAGGACTCTTGATCAAAG-3' and 5'- CTAGCTTTGATC AAGAGTCCTTTGATCAAGAGTCCG-3 ' ; for T B E 2 m : 5 CTAGCGGACTCTTGGCCAAAGGACTCTTGGCCAAAG-3' AND S'-CTAGCTTTGGCCAAGAGTCCTTTGGCCAAGAGTCCG-S'). 16. A GST-Tcf-4 fusion protein was constructed by PCR amplification of the sequence encoding the DNA-binding domain (codons 265 to 496) of human Tcf-4 with the following primers: 5'- CGCGGATCCGCTTCCGTGTCCAGGTTCCCTC-3' and 5'- CGGGAATTCCTAGCCTAGC AGGTTCGGGGAGGG-3 ' . The PCR product was cloned into pGEX-2TK (Pharmacia). GST-Tcf-4 protein was purified from BL-21 cells and DNA-binding assays were performed as described [L. Zawel et al, Molecular Cell 1, 611 (1998)]. The probes used for TBEl, TBE2, and TBE2m consisted of the oligonucleotides used for construction of multimerized site reporter (15). For mutant TBElm, the following primers were used: 5'- CTAGCGCACCTTTGGCTTCTGCACCTTTGGCTTCTG-3' and 5'- CTAGC AGAACGCAAAGGTGCAGA ACGC A A AGGTGCG-3 ' . Each binding assay contained 0.5 μg protein and 0.5 ng probe end- labeled to 2 x 10 8 dpm/μg. The specificity of binding was tested by competition with unlabeled wt sites and lack of competition with mutant sites. 17. K.B. Marcu, S.A. Bossone, AJ. 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Claims: CLAIMS 1. A method of determining the presence or absence in a cell of wild-type APC or a downstream protein in the APC transcription regulatory pathway, comprising the steps of: introducing a Tcf-responsive reporter gene comprising upstream genomic sequences of c-MYC into the cell; and measuring transcription of said reporter gene; wherein a cell which supports active transcription of said reporter gene does not have wild-type APC or a downstream protein in the APC transcription regulatory pathway. 2. The method of claim 1 wherein the Tcf-responsive reporter gene comprises nucleotides -1194 to -484 of c-MYC. 3. The method of claim 1 wherein the Tcf-responsive reporter gene comprises nucleotides -1194 to -741 of c-MYC 4. The method of claim 1 wherein the Tcf-responsive reporter gene comprises nucleotides -741 to -484 of c-MYC. 5. A method of determining the presence or absence in a cell of wild-type APC, comprising the steps of: contacting a Tcf-responsive reporter gene comprising upstream genomic sequences of c-MYC with a lysate of the cell; and measuring transcription of said reporter gene; wherein a lysate which inhibits said transcription has wild-type APC. 6. The method of claim 5 wherein the Tcf-responsive reporter gene comprises nucleotides -1194 to -484 of c-MYC. 7. The method of claim 5 wherein the Tcf-responsive reporter gene comprises nucleotides -1194 to -741 of c-MYC 8. The method of claim 5 wherein the Tcf-responsive reporter gene comprises nucleotides -741 to -484 of c-MYC. 9. A method of identifying candidate drugs for use in FAP patients, patients with APC or β-catenin mutations, or patients with increased risk of developing cancer, comprising the steps of: contacting a cell having no wild-type APC or a mutant β-catenin with a test compound; measuring transcription of a Tcf-responsive reporter gene comprising upstream genomic sequences of c-MYC, wherein a test compound which inhibits the transcription of the reporter gene is a candidate drug for cancer therapy. 10. The method of claim 9 wherein the Tcf-responsive reporter gene comprises nucleotides -1194 to -484 of c-MYC. 11. The method of claim 9 wherein the Tcf-responsive reporter gene comprises nucleotides -1194 to -741 of c-MYC 12. The method of claim 9 wherein the Tcf-responsive reporter gene comprises nucleotides -741 to -484 of c-MYC. 13. The method of claim 9 wherein the cell produces an APC protein defective in β-catenin binding or regulation. 14. The method of claim 9 wherein the cell produces a β-catenin protein which is super-active, or which is defective in APC binding or resistant to APC regulation. 15. The method of claim 9 wherein the cell produces no detectable APC protein. 16. A method of identifying candidate drugs for use in for use in FAP patients, patients with APC or β-catenin mutations, or patients with increased risk of developing cancer, comprising the steps of: contacting a Tcf-responsive reporter gene comprising upstream genomic sequences of c-MYC with a test compound under conditions in which the reporter gene is transcribed in the absence of the test compound; and measuring transcription of the Tcf-responsive reporter gene; wherein a test compound which inhibits said transcription is a candidate drug for cancer therapy. 17. The method of claim 16 wherein the Tcf-responsive reporter gene comprises nucleotides -1194 to -484 of c-MYC. 18. The method of claim 16 wherein the Tcf-responsive reporter gene comprises nucleotides -1194 to -741 of c-MYC 19. The method of claim 16 wherein the Tcf-responsive reporter gene comprises nucleotides -741 to -484 of c-MYC. 20. The method of claim 16 wherein the step of contacting is performed in the presence of a lysate of a cell which has no wild-type APC. 21. The method of claim 16 wherein the step of contacting is performed in the presence of a lysate of a cell which has a mutant β-catenin defective in APC binding or resistant to APC regulation or which is super-active. 22. The method of claim 16 wherein the cell produces an APC protein defective in β-catenin binding or regulation.

Inventors: HE TONG-CHUAN, KINZLER KENNETH W, VOGELSTEIN BERT, HE, TONG-CHUAN, KINZLER, KENNETH, W., VOGELSTEIN, BERT
Applicants: HE TONG CHUAN, KINZLER KENNETH W, UNIV JOHNS HOPKINS MED, VOGELSTEIN BERT, HE, TONG-CHUAN, KINZLER, KENNETH, W., THE JOHNS HOPKINS UNIVERSITY SCHOOL OF MEDICINE, VOGELSTEIN, BERT
Application year: 1999
Main classification: C12N15
IPC codes: A61K45, A61P35, C12N15, C12Q1