Colorectal cancers in a new mouse model of familial adenomatous polyposis: influence of genetic and environmental modifiers

Murine models of familial adenomatous polyposis harbor a germinal heterozygous mutation on Apc tumor suppressor gene. They are valuable tools for studying intestinal carcinogenesis, as most human sporadic cancers contain inactivating mutations of APC. However, Apc(+/-) mice, such as the well-characterized Apc(Min/+) model, develop cancers principally in the small intestine, while humans develop mainly colorectal cancers. We used a Cre-loxP strategy to achieve a new model of germline Apc invalidation in which exon 14 is deleted. We compared the phenotype of these Apc(Delta14/+) mice to that of the classical Apc(Min/+). The main phenotypic difference is the shift of the tumors in the distal colon and rectum, often associated with a rectal prolapse. Thus, the severity of the colorectal phenotype is partly due to the particular mutation Delta14, but also to environmental parameters, as mice raised in conventional conditions developed more colon cancers than those raised in pathogen-free conditions. All lesions, including early lesions, revealed Apc LOH and loss of Apc gene expression. They accumulated beta-catenin, overexpressed the beta-catenin target genes cyclin D1 and c-Myc, and the distribution pattern of glutamine synthetase, a beta-catenin target gene recently identified in the liver, was mosaic in intestinal adenomas. The Apc(Delta14/+) model is thus a useful new tool for studies on the molecular mechanisms of colorectal tumorigenesis.     

Somatic Apc mutations are selected upon their capacity to inactivate the beta-catenin downregulating activity

The APC gene, originally identified as the gene for familial adenomatous polyposis (FAP), is now considered as the true "gatekeeper" of colonic epithelial proliferation. Its main tumor suppressing activity seems to reside in the capacity to properly regulate intracellular beta-catenin signaling. Most somatic APC mutations are detected between codons 1286 and 1513, the mutation cluster region (MCR). This clustering can be explained either by the presence of mutation-prone sequences within the MCR, or by the selective advantage provided by the resulting truncated polypeptides. Here, a Msh2-deficient mouse model (Msh2(delta 7N) ) was generated and bred with Apc(1638N) and Apc(Min) that allowed the comparison of the somatic mutation spectra along the Apc gene in the different allelic combinations. Mutations identified in Msh2(delta 7N/delta 7N) tumors are predominantly dinucleotide deletions at simple sequence repeats leading to truncated Apc polypeptides that partially retain the 20 a.a. beta-catenin downregulating motifs. In contrast, the somatic mutations identified in the wild type Apc allele of Msh2(delta 7N/delta 7N) /Apc(+/1638N) and Msh2(delta 7N/delta 7N) /Apc(+/Min) tumors are clustered more to the 5' end, thereby completely inactivating the beta-catenin downregulating activity of APC. These results indicate that somatic Apc mutations are selected during intestinal tumorigenesis and that inactivation of the beta-catenin downregulating function of APC is likely to represent the main selective factor.     

Strain-specific of alachlor on murine olfactory mucosal responses

Chloracetanilide herbicides are multisite carcinogens in rodents. Progression of alachlor-induced olfactory tumors in rats is accompanied by cytoplasmic accumulation and nuclear localization of beta-catenin, suggesting activation of Wint signaling. Female CD-1 mice were resistant to alachlor-induced olfactory carcinogenesis. The current studies were performed to determine whether Apc(Min/+) mice, which have activated Wnt signaling due to mutation of the second allele of Apc, would be susceptible to alachlor olfactory carcinogenesis. Female and male Apc(Min/+) mice, as well as Apc(+/+) littermates received alachlor in the diet (260 mg/kg/d) for up to 3 months. Female A/J and C57BL/6J wild-type mice were also treated (for 10 and 14 months, respectively), as these strains vary in sensitivity to many respiratory tract insults. No olfactory mucosal tumors were observed in any of the mice, although alachlor-treated Apc(Min/+) mice developed histological changes similar to those in alachlor-treated rats. Alachlor-treated A/J mice developed pronounced intracellular accumulation of amorphous eosinophilic material in the olfactory mucosa, foci of respiratory-like metaplasia,and hyperplasia of nasal mucus glands. A similar but less intense response was seen in C57BL/6J mice. Mice and rats had equivalent levels of the putative bioactivating enzyme (CYP2A) in olfactory mucosa. and mice had induced hepatic CYP3A and CYP2B enzymes with alachlor treatment, which may increase alachlor elimination. These studies extend previous observations by describing alachlor-induced olfactory mucosal changes in mice and suggest that hepatic metabolic enzyme induction may be responsible for resistance of mice to alachlor-induced olfactory carcinogenesis.     

Adenomatous polyposis coli (<Emphasis Type="Italic">Apc</Emphasis>) tumor suppressor gene as a multifunctional gene

The adenomatous polyposis coli (Apc) gene is mutated in familial adenomatous polyposis and in sporadic colorectal tumors. The Apc gene product (APC), basically a cytoplasmic protein, blocks cell cycle progression and plays crucial roles in development. The APC binds to beta-catenin, axin and glycogen synthase kinase 3beta to form a large protein complex, in which beta-catenin is phosphorylated and broken down, resulting in negative regulation of the Wnt signaling pathway. Most of the mutated Apc genes in colorectal tumors lack beta-catenin-binding regions and fail to inhibit Wnt signaling, leading to overproliferation of tumor cells. The APC, having some nuclear localizing signals in its molecule, can also be localized in the nucleus. The nuclear APC exports excess beta-catenin to the cytoplasm. Through its C-terminus, APC binds to post-synaptic density discs large zonula occludens domain-containing proteins, such as discs large (DLG) and post-synaptic density (PSD)-95, and may play important roles in epithelial morphogenesis, brain development and neuronal functions. In addition, APC is involved in cell motility through its association with microtubules and APC-stimulated guanine nucleotide exchange factor. Colocalization of APC and DLG is dependent on microtubules. The Apc gene is highly expressed in the embryonic and postnatal developing brain. Recently, we found that APC is required for the activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors by facilitating the clustering of PSD-95 and these receptors at the postsynapse. In addition, APC is present in astrocytes, although its role in astrocytes is, as yet, unknown.     

The C-terminus of Apc does not influence intestinal adenoma development or progression

Adenomatous polyposis coli (APC ) mutations are found in most colorectal tumours. These mutations are almost always protein-truncating, deleting both central domains that regulate Wnt signalling and C-terminal domains that interact with the cytoskeleton. The importance of Wnt dysregulation for colorectal tumourigenesis is well characterized. It is, however, unclear whether loss of C-terminal functions contributes to tumourigenesis, although this protein region has been implicated in cellular processes--including polarity, migration, mitosis, and chromosomal instability (CIN)—that have been postulated as critical for the development and progression of intestinal tumours. Since almost all APC mutations in human patients disrupt both central and C-terminal regions, we created a mouse model to test the role of the C-terminus of APC in intestinal tumourigenesis. This mouse (Apc(ΔSAMP)) carries an internal deletion within Apc that dysregulates Wnt by removing the beta-catenin-binding and SAMP repeats, but leaves the C-terminus intact. We compared Apc(ΔSAMP) mice with Apc(1322T) animals. The latter allele represented the most commonly found human APC mutation and was identical to Apc(ΔSAMP) except for absence of the entire C-terminus. Apc(ΔSAMP) mice developed numerous intestinal adenomas indistinguishable in number, location, and dysplasia from those seen in Apc(1322T) mice. No carcinomas were found in Apc(ΔSAMP) or Apc(1322T) animals. While similar disruption of the Wnt signalling pathway was observed in tumours from both mice, no evidence of differential C-terminus functions (such as cell migration, CIN, or localization of APC and EB1) was seen. We conclude that the C-terminus of APC does not influence intestinal adenoma development or progression.     

Helicobacter hepaticus infection promotes colon tumorigenesis in the BALB/c-Rag2(-/-) Apc(Min/+) mouse

Adenomatous polyposis coli (APC) mutations are linked to human and mouse colorectal cancers. The Apc multiple intestinal neoplasia (Min) mouse mutation causes adenomas to develop throughout the small and large intestines. The BALB-Min (C.B6-Apc(Min/+)) congenic strain was generated by backcrossing into BALB/c the Apc(Min) allele from C57BL/6J-Apc(Min/+) mice. BALB-Min mice have a low tumor multiplicity (27.4 small intestine tumors/mouse) and a relatively long life span (>1 year) that makes them amenable to long-term studies. To investigate the interplay of the adaptive immune system and intestinal tumorigenesis, the immunodeficient compound mutant strain BALB-RagMin (C.Cg-Rag2(-/-) Apc(Min/+)) was generated. BALB-RagMin mice had a significant increase in tumors in the small, but not large, intestine relative to their BALB-Min counterparts (43.0 versus 24.0 tumors/mouse, respectively). The results suggest that the adaptive immune system plays a role in either the elimination or the equilibrium phase of cancer immunoediting in the small intestine in this model. We investigated the effect of the enterohepatic bacterial pathogen Helicobacter hepaticus on liver and intestine tumorigenesis in BALB-RagMin mice. H. hepaticus-infected BALB-RagMin mice developed moderate hepatitis, moderate typhlitis, and mild colitis. There were no differences in small intestine and cecal tumor multiplicity, regionality, or size relative to that in uninfected mice. However, H. hepaticus-infected BALB-RagMin mice had a significant increase in colon tumor incidence relative to uninfected BALB-RagMin mice (23.5% versus 1.7%, respectively). The data suggest that H. hepaticus, which is present in many research colonies, promotes colon tumorigenesis in the BALB-RagMin mouse and that it has the potential to confound colon tumorigenesis studies.     

Cross-species comparison of human and mouse intestinal polyps reveals conserved mechanisms in adenomatous polyposis coli (APC)-driven tumorigenesis

Expression profiling is a well established tool for the genome-wide analysis of human cancers. However, the high sensitivity of this approach combined with the well known cellular and molecular heterogeneity of cancer often result in extremely complex expression signatures that are difficult to interpret functionally. The majority of sporadic colorectal cancers are triggered by mutations in the adenomatous polyposis coli (APC) tumor suppressor gene, leading to the constitutive activation of the Wnt/beta-catenin signaling pathway and formation of adenomas. Despite this common genetic basis, colorectal cancers are very heterogeneous in their degree of differentiation, growth rate, and malignancy potential. Here, we applied a cross-species comparison of expression profiles of intestinal polyps derived from hereditary colorectal cancer patients carrying APC germline mutations and from mice carrying a targeted inactivating mutation in the mouse homologue Apc. This comparative approach resulted in the establishment of a conserved signature of 166 genes that were differentially expressed between adenomas and normal intestinal mucosa in both species. Functional analyses of the conserved genes revealed a general increase in cell proliferation and the activation of the Wnt/beta-catenin signaling pathway. Moreover, the conserved signature was able to resolve expression profiles from hereditary polyposis patients carrying APC germline mutations from those with bi-allelic inactivation of the MYH gene, supporting the usefulness of such comparisons to discriminate among patients with distinct genetic defects.     

Apc1638T: a mouse model delineating critical domains of the adenomatous polyposis coli protein involved in tumorigenesis and development

The adenomatous polyposis coli (APC) gene is considered as the true gatekeeper of colonic epithelial proliferation: It is mutated in the majority of colorectal tumors, and mutations occur at early stages of tumor development in mouse and man. These mutant proteins lack most of the seven 20-amino-acid repeats and all SAMP motifs that have been associated with down-regulation of intracellular beta-catenin levels. In addition, they lack the carboxy-terminal domains that bind to DLG, EB1, and microtubulin. APC also appears to be essential in development because homozygosity for mouse Apc mutations invariably results in early embryonic lethality. Here, we describe the generation of a mouse model carrying a targeted mutation at codon 1638 of the mouse Apc gene, Apc1638T, resulting in a truncated Apc protein encompassing three of the seven 20 amino acid repeats and one SAMP motif, but missing all of the carboxy-terminal domains thought to be associated with tumorigenesis. Surprisingly, homozygosity for the Apc1638T mutation is compatible with postnatal life. However, homozygous mutant animals are characterized by growth retardation, a reduced postnatal viability on the B6 genetic background, the absence of preputial glands, and the formation of nipple-associated cysts. Most importantly, Apc1638T/1638T animals that survive to adulthood are tumor free. Although the full complement of Apc1638T is sufficient for proper beta-catenin signaling, dosage reductions of the truncated protein result in increasingly severe defects in beta-catenin regulation. The SAMP motif retained in Apc1638T also appears to be important for this function as shown by analysis of the Apc1572T protein in which its targeted deletion results in a further reduction in the ability of properly controlling beta-catenin/Tcf signaling. These results indicate that the association with DLG, EB1, and microtubulin is less critical for the maintenance of homeostasis by APC than has been suggested previously, and that proper beta-catenin regulation by APC appears to be required for normal embryonic development and tumor suppression.     

Expression of CD44 in Apc and Tcf mutant mice implies regulation by the WNT pathway

Overexpression of cell surface glycoproteins of the CD44 family is an early event in the colorectal adenoma-carcinoma sequence. This suggests a link with disruption of APC tumor suppressor protein-mediated regulation of beta-catenin/Tcf-4 signaling, which is crucial in initiating tumorigenesis. To explore this hypothesis, we analyzed CD44 expression in the intestinal mucosa of mice and humans with genetic defects in either APC or Tcf-4, leading to constitutive activation or blockade of the beta-catenin/Tcf-4 pathway, respectively. We show that CD44 expression in the non-neoplastic intestinal mucosa of Apc mutant mice is confined to the crypt epithelium but that CD44 is strongly overexpressed in adenomas as well as in invasive carcinomas. This overexpression includes the standard part of the CD44 (CD44s) as well as variant exons (CD44v). Interestingly, deregulated CD44 expression is already present in aberrant crypt foci with dysplasia (ACFs), the earliest detectable lesions of colorectal neoplasia. Like ACFs of Apc-mutant mice, ACFs of familial adenomatous polyposis (FAP) patients also overexpress CD44. In sharp contrast, Tcf-4 mutant mice show a complete absence of CD44 in the epithelium of the small intestine. This loss of CD44 concurs with loss of stem cell characteristics, shared with adenoma cells. Our results indicate that CD44 expression is part of a genetic program controlled by the beta-catenin/Tcf-4 signaling pathway and suggest a role for CD44 in the generation and turnover of epithelial cells.     

Intestinal tumours induced by the food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in multiple intestinal neoplasia mice have truncation mutations as well as loss of the wild-type Apc(+) allele.

C57BL/6J-Min/+ (multiple intestinal neoplasia) is a murine model for familial adenomatous polyposis (FAP), where the mice are heterozygous for a nonsense Apc(Min) (adenomatous polyposis coli) mutation, and therefore develop numerous spontaneous adenomas in the small intestine and colon. Neonatal exposure of Min/+ mice to the food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) (eight subcutaneous injections of 25 or 50 mg/kg PhIP to pups or 50 mg/kg PhIP to lactating dams) markedly increased (2--9-fold) the number of intestinal tumours, especially in the small intestine. We examined whether the Apc gene was affected in small intestinal and colonic tumours induced by PhIP. In spontaneous tumours formed in these mice, the main mechanism for tumour induction is loss of the wild-type Apc(+) allele, i.e. loss of heterozygosity (LOH). Also in the PhIP-induced tumours, this is a major mechanism, since large fractions of PhIP-induced tumours had LOH in APC: However, mechanisms other than LOH must also prevail, since a lower frequency of LOH was found in the small intestinal tumours from male mice exposed to PhIP either via breast milk (65%) or by direct injection (68%), compared with the untreated controls (92%). Tumours that had retained the wild-type Apc(+) allele were further analysed for presence of truncated Apc proteins with in vitro synthesized protein (IVSP) assay. Truncated Apc proteins, indicating truncation mutations in exon 15 of the Apc gene, were detected in 20% (8 of 40) of the tumours not showing LOH from the small intestine after PhIP exposure, all in segment 2 (codons 686--1217). Seventeen percent (2 of 12) of the colonic tumours had a truncated Apc protein in segment 3 (codons 1099--1693). Importantly, no truncated proteins were detected in tumours from unexposed mice with apparently retained wild-type Apc(+) allele. These results show that PhIP induces intestinal tumours in the Min/+ mice both by causing LOH and truncation mutations in the wild-type Apc(+) allele.     

Putative tumor suppressor EDD interacts with and up-regulates APC

Adenomatous polyposis coli (APC), whose mutation causes colorectal cancers, is a key player in the Wnt signaling pathway. While the role of APC in inhibition of beta-catenin/LEF1-dependent activation of transformation-inducing genes has been intensively studied and well established, regulation of APC expression at the protein level is only partially understood. Here we report that APC is up-regulated by EDD, the mammalian orthologue of Drosophila melanogaster"hyperplastic discs" gene (hyd) that is considered to be a putative tumor suppressor. Screening of APC immunocomplexes by mass spectrometry identified EDD as a putative APC-interacting protein. Exogenously expressed and endogenous APC interacted with EDD in vivo. Indirect immunofluorescent analyses demonstrated that APC and EDD co-localized in the cytoplasm of the cell. Over-expression of EDD enhanced the protein expression level of APC and its binding partner Axin, resulting in inhibition of Wnt signaling downstream of beta-catenin. Conversely, siRNA knock-down of EDD down-regulated APC at the protein level without altering its mRNA level, causing enhanced protein expression of beta-catenin. Thus, through protein-protein interaction, EDD stabilizes APC and up-regulates APC's function to inhibit beta-catenin, suggesting that EDD could act as a colorectal tumor suppressor.     

Intestinal epithelial cell-specific CD98 expression regulates tumorigenesis in Apc(Min/+) mice

The transmembrane glycoprotein CD98 regulates integrin signaling that in turn controls cell proliferation and survival. CD98 expression is upregulated in various carcinomas, including colorectal cancer. Recently, by generating gain- and loss-of-function mouse models featuring genetic manipulation of CD98 expression specifically in intestinal epithelial cells (IECs), we have explored the crucial role of CD98 in the regulation of intestinal homeostasis and inflammation-associated tumorigenesis. In the present study, we investigated the contribution of CD98 to intestinal tumorigenesis in Apc(Min/+) mice and the underlying mechanism of action. Mice featuring IEC-specific CD98 overexpression (Tg animals) were crossed with Apc(Min/+) mice, and the characteristics of intestinal adenoma formation were assessed. Compared with Apc(Min/+) mice, Tg/Apc(Min/+) animals exhibited increases in both intestinal tumor incidence and tumor size; these parameters correlated with enhanced proliferation and decreased apoptosis of IECs. IEC-specific CD98 overexpression resulted in increased synthesis of the oncogenic proteins c-myc and cyclin-D1 in Apc(Min/+) mice, independently of the Wnt-APC-β-catenin pathway, suggesting the implication of CD98 overexpression-mediated Erk activation. IEC-specific CD98 overexpression enhanced the production of proinflammatory cytokines and chemokines that are crucial for tumorigenesis. We validated our results in mice exhibiting IEC-specific CD98 downregulation (CD98(flox/+)VillinCre animals). IEC-specific CD98 downregulation efficiently attenuated tumor incidence and growth in Apc(Min/+) mice. The reduction of intestinal tumorigenesis upon IEC-specific CD98 downregulation was caused by the attenuation of IEC proliferation and cytokine/chemokine production. In conclusion, we show that CD98 exerts an oncogenic activity in terms of intestinal tumorigenesis, via an ability to regulate tumor growth and survival.     

Wnt/β-catenin信号通路在实验性大鼠肝癌形成过程中的作用

目的:探讨Wnt/β-catenin信号转导通路与肿瘤发生的关系,为肝癌的防治提供新的思路。方法:选取Wnt/β-catenin信号转导通路中上下游关键因子wnt1、β-catenin、APC、cyclinD1以及c-myc等,应用RT-PCR法观察它们在正常肝脏,不典型增生肝脏和肝癌组织中的转录水平。用免疫组化染色法研究β-catenin、APC和cyclinD1等3个因子有无表达。结果:在正常肝脏中,用RT-PCR未检测到wnt1、cyc-linD1以及c-myc的mRNA转录,免疫组化染色也只观察到β-catenin在细胞膜处有比较弱的表达。诱癌14周后,在发生不典型增生的肝组织中,检测到β-catenin、APC和cyclinD13个基因的转录。通过免疫组化染色也观察到β-catenin蛋白在胞质中的积累,APC和cyclinD1在部分细胞中出现表达。诱癌16周后,在肝癌组织中,除wnt1mRNA外,其他几个因子的mRNA都有转录,免疫组化也印证了上述各个发生转录的因子在蛋白水平都有不同程度的表达。结论:Wnt/β-catenin信号转导通路在大鼠的肝癌形成过程中被激活,其可能是实验性肝癌发生的原因之一。     

A urokinase-type plasminogen activator deficiency diminishes the frequency of intestinal adenomas in ApcMin/+ mice

The interaction of urokinase-type plasminogen activator (uPA) and its receptor, uPAR, on cell surfaces facilitates the generation of cell-bound plasmin, thus allowing cells to establish a proteolytic front that enables their migration through protein barriers. This complex also activates cell signalling pathways that influence cell functions. Clinical studies have identified uPA as an indicator of poor overall survival in patients with colorectal cancer. In the current study, a mouse model of colon cancer, Apc(Min/+), with an additional deficiency of uPA (Apc(Min/+)/Plau-/-) was used to determine the effects of uPA on tumour initiation and growth. Utilizing this model, it was found that the number of tumours was diminished in these mice relative to Apc(Min/+) mice, which correlated with the decreased leukocyte infiltration in the tumours. However, tumour growth was not impeded in Apc(Min/+)/Plau-/- mice, and proliferation and tumour vascularization were, in fact, enhanced in Apc(Min/+)/Plau-/- mice. These latter effects are consistent with a mechanism involving up-regulation of COX-2 expression and Akt pathway activation in Apc(Min/+)/Plau-/- mice. The results from this study suggest that uPA plays dual and opposing roles in regulating lesion development: one early, during the transition from normal epithelia to dysplastic lesions, and another later during tumour growth.     

Hepatic adenomas: analysis of sex steroid receptor status and the Wnt signaling pathway.

Hepatic adenomas are strongly linked to excess hormonal exposure, but little else is known about their pathogenesis. The Wnt signaling pathway, which is activated in both hepatocellular carcinomas and hepatoblastomas, has not been studied in hepatic adenomas. Fifteen hepatic adenomas were studied by immunohistochemistry for estrogen, progesterone, and androgen receptors (ER, PR, AR, respectively) and correlated with the results of immunostaining for beta-catenin. Direct sequencing was performed to look for mutations in key genes involved in the Wnt signaling pathway: Exon 3 of beta-catenin encompassing the glycogen synthase kinase 3beta (GSK-3beta) phosphorylation region and the mutational cluster region of the adenomatosis polyposis coli protein (APC). Analysis for loss of heterozygosity (LOH) at chromosome 5q was also performed. Immunostaining for both ER and PR was present in 11/15 (73%) adenomas, and staining with one hormone receptor was positively associated with staining for the other receptor. AR positivity was present in 3/15 cases. Nuclear accumulation of beta-catenin was present in 7/15 (46%) of adenomas, indicating activation of the Wnt signaling pathway. However, no beta-catenin mutations, no APC mutations in the mutational cluster region, and no 5q LOH were detected. Two APC polymorphisms of unknown significance were seen. No clear association between beta-catenin nuclear accumulation and hormone receptor positivity was discerned. Activation of the Wnt signaling pathway appears to be important in a subset of hepatic adenomas but does not result from common beta-catenin or APC mutations and does not appear to be directly linked to hormonal receptor status.     

The modifier of Min 2 (Mom2) locus: embryonic lethality of a mutation in the Atp5a1 gene suggests a novel mechanism of polyp suppression

Inactivation of the APC gene is considered the initiating event in human colorectal cancer. Modifier genes that influence the penetrance of mutations in tumor-suppressor genes hold great potential for preventing the development of cancer. The mechanism by which modifier genes alter adenoma incidence can be readily studied in mice that inherit mutations in the Apc gene. We identified a new modifier locus of ApcMin-induced intestinal tumorigenesis called Modifier of Min 2 (Mom2). The polyp-resistant Mom2R phenotype resulted from a spontaneous mutation and linkage analysis localized Mom2 to distal chromosome 18. To obtain recombinant chromosomes for use in refining the Mom2 interval, we generated congenic DBA.B6 ApcMin/+, Mom2R/+ mice. An intercross revealed that Mom2R encodes a recessive embryonic lethal mutation. We devised an exclusion strategy for mapping the Mom2 locus using embryonic lethality as a method of selection. Expression and sequence analyses of candidate genes identified a duplication of four nucleotides within exon 3 of the alpha subunit of the ATP synthase (Atp5a1) gene. Tumor analyses revealed a novel mechanism of polyp suppression by Mom2R in Min mice. Furthermore, we show that more adenomas progress to carcinomas in Min mice that carry the Mom2R mutation. The absence of loss of heterozygosity (LOH) at the Apc locus, combined with the tendency of adenomas to progress to carcinomas, indicates that the sequence of events leading to tumors in ApcMin/+ Mom2R/+ mice is consistent with the features of human tumor initiation and progression.