Chromosomal instability by beta-catenin/TCF transcription in APC or beta-catenin mutant cells

Adenomatous polyposis coli (APC/Apc) gene encodes a key tumor suppressor whose mutations activate beta-catenin/T-cell factor (TCF)-mediated transcription (canonical Wnt signaling). Here, we show that Wnt signaling can cause chromosomal instability (CIN). As an indicator of CIN, we scored anaphase bridge index (ABI) in mouse polyps and ES cells where Wnt signaling was activated by Apc or beta-catenin mutations. We found three to nine times higher ABI than in wild-type controls. Furthermore, karyotype analysis confirmed that the Wnt signal-activated ES cells produced new chromosomal aberrations at higher rates; hence CIN. Consistently, expression of dominant-negative TCFs in these cells reduced their ABI. We also found that Wnt signal activation increased phosphorylation of Cdc2 (Cdk1) that inhibited its activity, and suppressed apoptosis upon exposure of the cells to nocodazole or colcemid. The data suggest that Wnt signaling stimulates the cells to escape from mitotic arrest and apoptosis, resulting in CIN. In human gastric cancer tissues with nuclear beta-catenin, ABI was significantly higher than in those without. These results collectively indicate that beta-catenin/TCF-mediated transcription itself increases CIN through dysregulation of G2/M progression.

Optic cup and facial patterning defects in ocular ectoderm beta-catenin gain-of-function mice

Background

The canonical Wnt signaling pathway has a number of critical functions during embryonic development and, when activated aberrantly, in the genesis of cancer. Current evidence suggests that during eye development, regulation of Wnt signaling is critical for patterning the surface ectoderm that will contribute to multiple components of the eye. Wnt signaling loss-of-function experiments show that a region of periocular ectoderm will form ectopic lentoid bodies unless the Wnt pathway modifies its fate towards other structures. Consistent with this, Wnt signaling gain of function in the ocular region ectoderm results in a suppression of lens fate.

Results

Here we demonstrate that ectoderm-specific Wnt signaling gain-of-function embryos exhibit additional defects besides those noted in the lens. There are profound facial defects including a foreshortened snout, malformation of the nasal region, and clefting of the epidermis along the ocular-nasal axis. Furthermore, despite the restriction of Wnt pathway gain-of-function to the surface ectoderm, the optic cup is inappropriately patterned and ultimately forms a highly convoluted, disorganized array of epithelium with the characteristics of retina and retinal pigmented epithelium.

Conclusion

We suggest that activation of the Wnt pathway in surface ectoderm may disrupt the normal exchange of signals between the presumptive lens and retina that coordinate development of a functional eye.

Mucosal prolapse in the pathogenesis of Peutz-Jeghers polyposis

Germline mutations in LKB1 cause the rare cancer prone disorder Peutz-Jeghers syndrome (PJS). Gastrointestinal hamartomatous polyps constitute the major phenotypic trait in PJS. Hamartomatous polyps arising in PJS patients are generally considered to lack premalignant potential although rare neoplastic changes in these polyps and an increased gastrointestinal cancer risk in PJS are well documented. These conflicting observations are resolved in the current hypothesis by providing a unifying explanation for these contrasting features of PJS polyposis. We postulate that a genetic predisposition to epithelial prolapse underlies the formation of the polyps associated with PJS. Conventional sporadic adenomas arising in PJS patients will similarly show mucosal prolapse and carry the associated histological features.

Accelerated onsets of gastric hamartomas and hepatic adenomas/carcinomas in Lkb1+/−p53−/− compound mutant mice

Germline mutations in the LKB1 gene are responsible for Peutz-Jeghers syndrome (PJS), which is characterized by gastrointestinal hamartomas and increasing risk of cancer. Mice with Lkb1(+/-) mutation develop gastric hamartomas after >20 weeks of age, and hepatocellular adenomas and carcinomas >30 weeks. It has been reported that, in PJS patients, carcinomas progressed from hamartomas contain p53 mutations, and that LKB1 regulates p53-dependent apoptosis. To investigate the roles of LKB1 and p53 mutations in tumorigenesis, we constructed compound mutant mice of Lkb1 and p53 genes. In the Lkb1(+/-)p53(-/-) mice, formation of gastric hamartomas and hepatic tumors was accelerated. However, histopathology of hamartomas was similar between Lkb1(+/-)p53(-/-) and Lkb1(+/-) mice, and Lkb1 genotype remained heterozygous, suggesting that the p53 mutation affected hamartoma initiation. Contrary to the heterozygous hamartomas in the stomach and duodenum, the hepatic adenomas in Lkb1(+/-)p53(-/-) mice showed loss of Lkb1 heterozygosity (LOH), suggesting that lack of p53 stimulated Lkb1 LOH and tumor initiation in the liver. Taken together, these results indicate that lack of p53 causes earlier onsets of gastric hamartomas and hepatic tumors in Lkb1(+/-)p53(-/-) mice.

The duality of β-catenin function: A requirement in lens morphogenesis and signaling suppression of lens fate in periocular ectoderm

In the current analysis, we have investigated both the cytoskeletal and signaling roles of beta-catenin during the early phases of lens development using conditional loss- and gain-of-function strategies. Conditional loss of beta-catenin in the presumptive lens does not perturb the normal sequential appearance of lens fate markers but results in a dramatic failure of the coordinated epithelial cell behavior that constitutes lens morphogenesis. Similarly, loss-of-function for Lrp6, the Wnt pathway coreceptor expressed in the eye primordium, does not prevent expression of lens induction markers. Surprisingly, conditional deletion of beta-catenin in periocular ectoderm results in the formation of Prox-1 and beta-crystallin-positive ectopic lentoid bodies. Combined with the observation that the Wnt pathway reporter TOPGAL is expressed in nasal periocular ectoderm, these data suggest that, in this location, the canonical Wnt signaling pathway normally suppresses lens fate in favor of other structures. Consistent with this proposal, a dominant-active form of beta-catenin causes a loss of lens fate and a complete absence of lens development when expressed in the presumptive lens ectoderm.

Development of spontaneous tumours and intestinal lesions in Fhit gene knockout mice

The fragile histidine triad (FHIT) gene is frequently inactivated in various types of tumours. However, the system-wide pathology caused by FHIT inactivation has not been examined in detail. Here we demonstrate that Fhit gene knockout mice develop tumours in the lymphoid tissue, liver, uterus, testis, forestomach and small intestine, together with structural abnormalities in the small intestinal mucosa. These results suggest that Fhit plays important roles in systemic tumour suppression and in the integrity of mucosal structure of the intestines.

Simultaneous expression of COX-2 and mPGES-1 in mouse gastrointestinal hamartomas

Cyclo-oxygenase (COX)-2 is induced in various types of cancer tissues. Here, we demonstrate stromal expression of both COX-2 and microsomal prostaglandin E₂ synthase (mPGES)-1 in gastrointestinal hamartomas developed in Lkb1^+/−, Smad4^+/− and Cdx2^+/−mice. These results suggest that PGE₂ produced by COX-2 and mPGES-1 plays an important role in hamartoma development regardless of the mutated genes causing hamartomas.

CDX2 expression in the stomach with intestinal metaplasia and intestinal-type cancer: Prognostic implications

CDX2, a transcriptional factor expressed in the intestine, is implicated in the development and maintenance of the intestinal mucosa. Recent studies have demonstrated that CDX2 is expressed in the intestinal metaplasia of the stomach and intestinal-type gastric cancer, while it is not expressed in the normal gastric mucosa. To investigate the role of CDX2 in gastric cancer, we determined CDX2 expression and cell proliferation rate in various types of gastric cancer tissues by immunostaining. Surgically dissected gastric cancer tissues were collected from 40 patients. Consistent with previous reports, CDX2 was expressed in most gastric mucosa samples with intestinal metaplasia (89%, 16/18), although it was not found in the adjacent normal mucosa. CDX2 expression was also detected in 64% (18/28) of intestinal-type gastric cancer cases, whereas it was not observed in the diffuse-type gastric cancer (0/12). Moreover, the CDX2-positive gastric cancer samples showed significantly lower index for Ki-67 immunostaining, indicating reduced cell proliferation rates than in the CDX2-negative samples. Importantly, multivariate analysis for the overall survival rate revealed that the CDX2-positive gastric cancer patients survived significantly longer than the CDX2-negative patients. Even among the intestinal-type gastric cancer cases, the CDX2-positive group showed a lower Ki-67 index and longer postoperative survival than the CDX2-negative group. These results collectively indicate that CDX2 expression in gastric cancer tissues can be a novel prognostic marker for patient survival.

Morphologic and molecular analysis of estrogen-induced pituitary tumorigenesis in targeted disruption of transforming growth factor-beta receptor type II and/or p27 mice

The critical genes and products involved in estrogen-induced tumorigenesis of the pituitary gland were investigated in heterozygous transforming growth factor-beta (TGF-beta) receptor type II and p27 knockout mouse models. Tgfbr2(+/-), p27(+/-); Tgfbr2(+/-), and p27(+/-) mice and C57BL/6J wild-type mice received sc implantation of estrogen or placebo pellets for 16 or 25 wk, after which the mice were sacrificed and their pituitary glands removed for examination. The bromodeoxyuridine labeling indexes in tissue from both the anterior and intermediate pituitary lobes from p27 (+/-) and Tgfbr2(+/-); p27(+/-) mice were significantly higher than those from wild-type and Tgfbr2(+/-) mice after treatment with estrogen for 16 wk. Pituitary tumorigenesis was significantly accelerated in Tgfbr2(+/-), p27(+/-), and Tgfbr2(+/-); p27(+/-) mice compared with wild-type mice after treatment with estrogen for 16 wk. Pituitary tumorigenesis was not accelerated in Tgfbr2(+/-); p27(+/-) mice compared with Tgfbr2(+/-) or p27(+/-) mice. Expression of TGF-beta receptor type II mRNA was lower in the pituitary gland of Tgfbr2(+/-) mice than in wild-type mice before estrogen treatment and was significantly reduced after treatment. Pituitary tumorigenesis is accelerated in mice with severe TGF-beta resistance, and greatly accelerated in mice with TGF-beta resistance combined with decreased p27 expression compared with wild-type mice. Both the TGF-beta receptor type II gene and p27 gene and their products are involved in estrogen-induced tumorigenesis.

Acceleration of intestinal polyposis through prostaglandin receptor EP2 in Apc(Delta 716) knockout mice

Arachidonic acid is metabolized to prostaglandin H(2) (PGH(2)) by cyclooxygenase (COX). COX-2, the inducible COX isozyme, has a key role in intestinal polyposis. Among the metabolites of PGH(2), PGE(2) is implicated in tumorigenesis because its level is markedly elevated in tissues of intestinal adenoma and colon cancer. Here we show that homozygous deletion of the gene encoding a cell-surface receptor of PGE(2), EP2, causes decreases in number and size of intestinal polyps in Apc(Delta 716) mice (a mouse model for human familial adenomatous polyposis). This effect is similar to that of COX-2 gene disruption. We also show that COX-2 expression is boosted by PGE(2) through the EP2 receptor via a positive feedback loop. Homozygous gene knockout for other PGE(2) receptors, EP1 or EP3, did not affect intestinal polyp formation in Apc(Delta 716) mice. We conclude that EP2 is the major receptor mediating the PGE2 signal generated by COX-2 upregulation in intestinal polyposis, and that increased cellular cAMP stimulates expression of more COX-2 and vascular endothelial growth factor in the polyp stroma.

Somatic activation of beta-catenin bypasses pre-TCR signaling and TCR selection in thymocyte development

Mutation or ablation of T cell factor 1 and lymphocyte enhancer factor 1 indicated involvement of the Wnt pathway in thymocyte development. The central effector of the Wnt pathway is beta-catenin, which undergoes stabilization upon binding of Wnt ligands to frizzled receptors. We report here that conditional stabilization of beta-catenin in immature thymocytes resulted in the generation of single positive T cells that lacked the alpha beta TCR and developed in the absence of pre-TCR signaling and TCR selection. Although active beta-catenin induced differentiation in the absence of TCRs, its action was associated with reduced proliferation and survival when compared to developmental changes induced by the pre-TCR or the alpha beta TCR.

Chemoprevention of intestinal polyposis in the Apcdelta716 mouse by rofecoxib, a specific cyclooxygenase-2 inhibitor

Mutations in the human adenomatous polyposis (APC) gene are causative for familial adenomatous polyposis (FAP), a rare condition in which numerous colonic polyps arise during puberty and, if left untreated, lead to colon cancer. The APC gene is a tumor suppressor that has been termed the "gatekeeper gene" for colon cancer. In addition to the 100% mutation rate in FAP patients, the APC gene is mutated in >80% of sporadic colon and intestinal cancers. The Apc gene in mice has been mutated either by chemical carcinogenesis, resulting in the Min mouse Apcdelta850, or by heterologous recombination, resulting in the Apcdelta716 or Apedelta1368 mice (M. Oshima et al., Proc. Natl. Acad. Sci. USA, 92: 4482-4486, 1995). Although homozygote Apc-/- mice are embryonically lethal, the heterozygotes are viable but develop numerous intestinal polyps with loss of Apc heterozygosity within the polyps (M. Oshima et al., Proc. Natl. Acad. Sci. USA, 92: 4482-4486, 1995). The proinflammatory, prooncogenic protein cyclooxygenase (COX)-2 has been shown to be markedly induced in the Apcdelta716 polyps at an early stage of polyp development (M. Oshima et al., Cell, 87: 803-809, 1996). We demonstrate here that treatment with the specific COX-2 inhibitor rofecoxib results in a dose-dependent reduction in the number and size of intestinal and colonic polyps in the Apcdelta716 mouse. The plasma concentration of rofecoxib that resulted in a 55% inhibition of polyp number and an 80% inhibition of polyps > 1 mm in size is comparable with the human clinical steady-state concentration of 25 mg rofecoxib (Vioxx) taken once daily (A. Porras et al., Clin. Pharm. Ther., 67: 137, 2000). Polyps from both untreated and rofecoxib- or sulindac-treated Apcdelta716 mice expressed COX-1 and -2, whereas normal epithelium from all mice expressed COX-1 but minimal amounts of COX-2. Polyps from either rofecoxib- or sulindac-treated mice had lower rates of DNA replication, expressed less proangiogenic vascular endothelial-derived growth factor and more membrane-bound beta-catenin, but showed unchanged nuclear localization of this transcription factor. This study showing the inhibition of polyposis in the Apcdelta716 mouse suggests that the specific COX-2 inhibitor rofecoxib (Vioxx) has potential as a chemopreventive agent in human intestinal and colon cancer.

Mouse Wnt receptor gene Fzd5 is essential for yolk sac and placental angiogenesis

Wnts are secreted signaling molecules implicated in various developmental processes and frizzled proteins are the receptors for these Wnt ligands. To investigate the physiological roles of frizzled proteins, we isolated and characterized a novel mouse frizzled gene Fzd5. Fzd5 mRNA was expressed in the yolk sac, eye and lung bud at 9.5 days post coitum. Fzd5 specifically synergized with Wnt2, Wnt5a and Wnt10b in ectopic axis induction assays in Xenopus embryos. Using homologous recombination in embryonic stem cells, we have generated Fzd5 knockout mice. While the heterozygotes were viable, fertile and appeared normal, the homozygous embryos died in utero around 10.75 days post coitum, owing to defects in yolk sac angiogenesis. At 10.25 days post coitum, prior to any morphological changes, endothelial cell proliferation was markedly reduced in homozygous mutant yolk sacs, as measured by BrdU labeling. By 10.75 days post coitum, large vitelline vessels were poorly developed, and the capillary plexus was disorganized. At this stage, vasculogenesis in the placenta was also defective, although that in the embryo proper was normal. Because Wnt5a and Wnt10b co-localized with Fzd5 in the developing yolk sac, these two Wnts are likely physiological ligands for the Fzd5-dependent signaling for endothelial growth in the yolk sac.

Cytokeratins 8 and 19 in the mouse placental development

To investigate the expression and biological roles of cytokeratin 19 (K19) in development and in adult tissues, we inactivated the mouse K19 gene (Krt1-19) by inserting a bacterial beta-galactosidase gene (lacZ) by homologous recombination in embryonic stem cells, and established germ line mutant mice. Both heterozygous and homozygous mutant mice were viable, fertile, and appeared normal. By 7.5-8.0 days post coitum (dpc), heterozygous mutant embryos expressed lacZ in the notochordal plate and hindgut diverticulum, reflecting the fact that the notochord and the gut endoderm are derived from the axial mesoderm-originated cells. In the adult mutant, lacZ was expressed mainly in epithelial tissues. To investigate the possible functional cooperation and synergy between K19 and K8, we then constructed compound homozygous mutants, whose embryos died approximately 10 dpc. The lethality resulted from defects in the placenta where both K19 and K8 are normally expressed. As early as 9. 5 dpc, the compound mutant placenta had an excessive number of giant trophoblasts, but lacked proper labyrinthine trophoblast or spongiotrophoblast development, which apparently caused flooding of the maternal blood into the embryonic placenta. These results indicate that K19 and K8 cooperate in ensuring the normal development of placental tissues.

Targeted deletion of keratins 18 and 19 leads to trophoblast fragility and early embryonic lethality

It has been reported previously that keratin 8 (K8)-deficient mice of one strain die from a liver defect at around E12.5, while those of another strain suffer from colorectal hyperplasia. These findings have generated considerable confusion about the function of K8, K18 and K19 that are co-expressed in the mouse blastocyst and internal epithelia. To resolve this issue, we produced mice doubly deficient for K18 and K19 leading to complete loss of keratin filaments in early mouse development. These embryos died at around day E9.5 with 100% penetrance. The absence of keratins caused cytolysis restricted to trophoblast giant cells, followed by haematomas in the trophoblast layer. Up to that stage, embryonic development proceeded unaffected in the absence of keratin filaments. K18/19-deficient mouse embryos die earlier than any other intermediate filament knockouts reported so far, suggesting that keratins, in analogy to their well established role in epidermis, are essential for the integrity of a specialized embryonic epithelium. Our data also offer a rationale to explore the involvement of keratin mutations in early abortions during human pregnancies.

Gastrointestinal tumorigenesis in Smad4 (Dpc4) mutant mice

The SMAD4 (Dpc4) gene plays a key role in the TGF-beta signaling pathway. We recently inactivated the mouse homolog Smad4. The homozygous mutants were embryonic lethals, whereas the heterozygotes were viable and fertile. Although young heterozygotes were normal, old mice developed gastric and duodenal polyps similar to those found in human juvenile polyps characterized by abundant stroma and eosinophilic infiltrations. These data are consistent with the reports that a subset of human juvenile polyposis kindreds carry germline mutations in the SMAD4 gene. We then introduced the Smad4 mutation into the Apc delta 716 knockout mice, a model for human familial adenomatous polyposis. Because both Apc and Smad4 are located on mouse chromosome 18, we constructed by meiotic recombination, compound heterozygotes carrying both mutations on the same chromosome. In such mice, intestinal polyps developed into more malignant tumors than those in the simple Apc delta 716 heterozygotes, showing an extensive stromal cell proliferation and strong submucosal invasion. These results indicate that mutations in SMAD4 play a significant role in the malignant progression of colorectal tumors.

Multiple functional defects in peripheral autonomic organs in mice lacking muscarinic acetylcholine receptor gene for the M3 subtype

Muscarinic acetylcholine receptors consist of five distinct subtypes and have been important targets for drug development. In the periphery, muscarinic acetylcholine receptors mediate cholinergic signals to autonomic organs, but specific physiological functions of each subtype remain poorly elucidated. Here, we have constructed and analyzed mutant mice lacking the M(3) receptor and have demonstrated that this subtype plays key roles in salivary secretion, pupillary constriction, and bladder detrusor contractions. However, M(3)-mediated signals in digestive and reproductive organs are dispensable, likely because of redundant mechanisms through other muscarinic acetylcholine receptor subtypes or other mediators. In addition, we have found prominent urinary retention only in the male, which indicates a considerable sex difference in the micturition mechanism. Accordingly, this mutant mouse should provide a useful animal model for investigation of human diseases that are affected in the peripheral cholinergic functions.

Cloning and expression of human B cell-specific transcription factor BACH2 mapped to chromosome 6q15

The transcription factor Bach2, a member of the BTB-basic region leucine zipper (bZip) factor family, binds to a 12-O-tetradecanoylphorbol-13-acetate (TPA)-responsive element and the related Maf-recognition element (MARE) by forming homodimers or heterodimers with Maf-related transcription factors. Bach2 regulates transcription by binding to these elements. To understand the function in hematopoiesis, we isolated a cDNA clone for human Bach2 (BACH2) encoding a protein of 841 amino acid residues with a deduced amino acid sequence having 89.5% identity to mouse homolog. Among human hematopoietic cell lines, BACH2 is expressed abundantly only in some B-lymphocytic cell lines. RT-PCR analysis of hematopoietic cells revealed that BACH2 mRNA is expressed in primary B-cells. Enforced expression of BACH2 in a human Burkitt cell line, RAJI that does not express endogenous BACH2, resulted in marked reduction of clonogenic activity, indicating that BACH2 possesses an inhibitory effect on cell proliferation. By fluorescent in situ hybridization, the BACH2 gene was localized to chromosome 6q15. Because deletion of the long arm of chromosome 6 (6q) is one of the commonest chromosomal alterations in human B-cell lymphoma, we examined for the loss of heterozygosity (LOH) of the BACH2 gene in human B-cell non-Hodgkin's lymphomas (NHL). Among 25 informative cases, five (20%) showed LOH. These results indicate that BACH2 plays important roles in regulation of B cell development.

Involvement of caspase 3 in apoptotic death of cortical neurons evoked by DNA damage

Previous reports have shown that DNA-damage-evoked death of embryonic cortical neurons is delayed by general caspase inhibitors and is accompanied by an increase in DEVD-AFC cleavage activity. We show here that this cleavage activity is lacking in camptothecin-treated caspase 3-deficient neurons. Moreover, we report that death of camptothecin-treated caspase 3-deficient neurons cultured from E16 embryos is delayed and that no significant increase in survival is observed with cotreatment with the general caspase inhibitor BAF. These results indicate that caspase-dependent death of camptothecin-treated cortical neurons requires caspase 3 activity. The delay in death is accompanied by impairment of DNA fragmentation. However, Bax-dependent cytochrome c release still occurs in camptothecin-treated caspase 3-deficient cortical neurons. Accordingly, we hypothesize that the delayed death which occurs in the absence of caspase 3 activity may be due to mitochondrial dysfunction. Finally, we show that the delay in death observed with E16 caspase 3-deficient neurons does not occur in neurons cultured from E19 embryos. This suggests that the requirement for caspase 3 in death of neurons evoked by DNA damage may differ depending upon the developmental state of the cell.

Gastro-intestinal tumorigenesis in Smad4 mutant mice

The SMAD4 gene plays a key role in the TGF-beta signaling pathway. We inactivated its mouse homolog Smad4. The homozygous mutants were embryonically lethal, whereas the heterozygotes were viable and fertile. Although young heterozygotes appeared normal, old mice developed gastric and duodenal polyps similar to human juvenile polyps characterized by abundant stroma and eosinophilic infiltrations. These data are consistent with the reports that a subset of human juvenile polyposis kindreds carry germline mutations in the SMAD4 gene. We then introduced the Smad4 mutation into the Apc(Delta716) knockout mice, a model for human familial adenomatous polyposis. Because both Apc and Smad4 are located on mouse chromosome 18, we constructed by meiotic recombination compound heterozygotes carrying both mutations on the same chromosome. In such mice, intestinal polyps developed into more malignant tumors than those in the simple Apc(Delta716) heterozygotes, showing an extensive stromal cell proliferation and strong submucosal invasion. These results indicate that mutations in SMAD4 play a significant role in the malignant progression of colorectal tumors.

Gene structure and chromosome mapping of mouse transcription elongation factor S-II (Tcea1)

We report the organization and chromosome localization of the mouse transcription elongation factor S-II gene (Tcea1). This gene was found to be a single copy gene consisting of 10 exons spanning approximately 30kb. Its organization was the same as those of the mouse testis-specific S-II gene (Tcea2) and Xenopus general S-II gene (xTFIIS.oA), but different from that of the human S-II gene family. We also identified a processed pseudogene (Tcea1-ps1) with a sequence highly homologous to those of S-II cDNAs but containing a translation termination codon within its open reading frame. Linkage analysis showed that Tcea1 and Tcea1-ps1 are mapped on mouse chromosomes 1 and 15, respectively. Relationships between Tcea1 and S-II cDNAs isolated so far are discussed.

Gastric and duodenal polyps in Smad4 (Dpc4) knockout mice

The SMAD4 (DPC4) gene was initially isolated as a candidate tumor suppressor from the convergent site of homozygous deletions on 18q in a panel of pancreatic carcinoma cell lines. It encodes a common cytoplasmic signaling molecule shared by the transforming growth factor-beta, activin, and bone morphogenic pathways. We recently inactivated its mouse homologue Smad4 and demonstrated its role in the malignant progression of benign adenomas to invasive adenocarcinomas by analyzing mice with Apc and Smad4 compound mutations. Although simple Smad4 homozygotes were embryonically lethal, the heterozygotes were fertile and appeared normal up to the age of 1 year. Upon further investigation, however, they have developed inflammatory polyps in the glandular stomach and duodenum. By PCR genotyping and immunohistochemical staining, the wild-type Smad4 allele has been lost in the polyp epithelial cells, ie., loss of heterozygosity. On the other hand, we have not found any mutations in such genes as K-Ras, H-Ras, N-Ras, p53, or PTEN. Histologically, the polyps are similar to human juvenile polyps showing moderate stromal cell proliferation and infiltrations by eosinophils and plasma cells. In addition, foci of adenocarcinoma with signet ring cells are also found. These results are consistent with a recent report that germ-line SMAD4 mutations are found in a subset of familial juvenile polyposis.

Intestinal polyposis in mice with a dominant stable mutation of the beta-catenin gene

Ectopic expression of certain Wnt genes in mouse mammary tissue is tumorigenic, and mutations that stabilize beta-catenin are found in various human cancers including colorectal cancer. To determine the role of stabilized beta-catenin in intestinal tumorigenesis in mice, we constructed by embryonic stem (ES) cell-mediated homologous recombination, a mutant beta-catenin allele whose exon 3 was sandwiched by loxP sequences. When the germline heterozygotes were crossed with mice expressing Cre recombinase in the intestines, the serines and threonine encoded by exon 3 and to be phosphorylated by glycogen synthase kinase 3beta (GSK3beta) were deleted in the offspring intestines, which caused adenomatous intestinal polyps resembling those in Apc(Delta716) knockout mice. Some nascent microadenomas were also found in the colon. These results present experimental genetic evidence that activation of the Wnt signaling pathway can cause intestinal and colonic tumors.

Colonic hamartoma development by anomalous duplication in Cdx2 knockout mice

To determine the biological role of caudal-like homeobox gene CDX2, we constructed knockout mice in which its mouse homologue Cdx2 was inactivated by homologous recombination, placing a bacterial lacZ gene under the control of the Cdx2 promoter. Although the homozygous mutants died in utero around implantation, the heterozygotes were viable and fertile and expressed lacZ in the caudal region in early embryos and in the gut tissues in adults. The heterozygotes developed cecal and colonic villi by anteriorization and formed hamartomatous polyps in the proximal colon. The hamartoma started to develop at 11.5 days of gestation as an outpocket of the gut epithelium, which ceased to express the remaining Cdx2 allele. The outpocket then expanded as a partially duplicated gut but was contained as a hamartoma after birth. In adult mice, these hamartomas grew very slowly and took a benign course. None of them progressed into invasive adenocarcinomas, even at 1.5 years of age. Whereas the cecal and colonic villi expressed lacZ, the hamartoma epithelium did not, nor did it express Cdx2 mRNA from the wild-type allele. However, genomic DNA analysis of the polyp epithelium did not show a loss of heterozygosity of the Cdx2 gene, suggesting a mechanism of biallelic Cdx2 inactivation other than loss of heterozygosity. These results indicate that the Cdx2 haploin-sufficiency caused cecal and colonic villi, whereas the biallelic inactivation of Cdx2 triggered anomalous duplications of the embryonic gut epithelium, which were contained as hamartomas after birth.

Retrovirus integration site Mintb encoding the mouse homolog of hnRNP U

Retroviral genes are not usually expressed in mouse embryonal carcinoma (EC) cells, but they are readily expressed upon differentiation of these cells. We previously reported the isolation of EC cell lines that express a neomycin resistance (neo) gene introduced by a recombinant transducing Moloney murine leukemia virus from specific integration sites, Minta, Mintb, Mintc, or Mintd. In some of these clones, the entire 5' long terminal repeat (LTR) was deleted, and the neo gene was expressed by read-through transcription from upstream cellular promoters in a "promoter-trap" fashion. One such promoter ("promoter B" at the Mintb locus) was found in a CpG island, associated with an upstream enhancer ("enhancer B"). Although enhancer B caused expression of the neo gene in the transductant EC cell line, no endogenous transcription from promoter B was detected in the parental EC or NIH3T3 cells. In contrast, we found a strong counter-flow endogenous transcription unit ("R" for reverse), which apparently interfered with transcription from promoter B. Promoter R turned out to have a bidirectional activity in transfection assays. In normal tissues, promoter R activates gene R, which encodes an 800-residue protein that is highly homologous to the rat and human heterogeneous nuclear ribonucleoprotein U (hnRNP U). Northern and in situ hybridization analyses revealed that gene R was abundantly expressed in the testis, especially in the pachytene spermatocytes and round spermatids.

Impaired extrapyramidal function caused by the targeted disruption of retinoid X receptor RXRgamma1 isoform

BACKGROUND

Retinoid X receptors RXRalpha, beta and gamma exert multiple functions in the genetic regulation of mammalian signalling systems by forming heterodimeric complexes with several nuclear ligand receptors. In contrast to the widespread expression of RXRalpha and RXRbeta, the expression of RXRgamma is restricted to particular tissues in which RXRgamma1 is the major isoform expressed in the mouse corpus striatum.

RESULTS

To investigate the function of this particular isoform RXRgamma1, we generated RXRgamma1 gene-knockout mice by homologous recombination in ES cells. Both heterozygous and homozygous mice showed severe runting after birth, which often resulted in the early death of mice of the 129/C57BL-6 genetic background. Independent of genetic background, however, the expression of choline acetyltransferase (ChAT) in the cholinergic interneurones in the striatum (caudal putamen) was markedly reduced in the RXRgamma1 gene-null mice. Furthermore, the mutant exhibited an altered response to the administration of dopamine receptor antagonists, haloperidol and chlorpromazine, which normally induce catalepsy in mice.

CONCLUSIONS

These results strongly suggest that RXRgamma1 plays an important role in either the development or activation of cholinergic neurones in nigrostriatal extrapyramidal pathways.

Mapping of five subtype genes for muscarinic acetylcholine receptor to mouse chromosomes

Muscarinic acetylcholine receptors in mammals consist of five subtypes (M1-M5) encoded by distinct genes. They are widely expressed throughout the body and play a variety of roles in the peripheral and central nervous systems. Although their pharmacological properties have been studied extensively in vitro, colocalization of the multiple subtypes in each tissue and lack of subtype-specific ligands have hampered characterization of the respective subtypes in vivo. We have mapped mouse genomic loci for all five genes (Chrm1-5) by restriction fragment length variant (RFLV) analyses in interspecific backcross mice. Chrm1, Chrm2, and Chrm3 were mapped to chromosome (Chr) 19, 6, and 13, respectively. Both Chrm4 and Chrm5 were mapped to Chr 2. Although a comparison of their map positions with other mutations in their vicinities suggested a possibility that the El2 (epilepsy 2) allele might be a mutation in Chrm5, sequencing analyses of the Chrm5 gene in the El2 mutant mice did not support such a hypothesis.

Cyclooxygenase-2 inhibitors in tumorigenesis (Part II)

The rate-limiting step in arachidonate metabolism is mediated by enzymes known as cyclooxygenases (COXs). These enzymes catalyze the biosynthesis of prostaglandin H2, the precursor of molecules such as prostaglandins, prostacyclin, and thromboxanes. The COX enzyme family consists of the classical COX-1 enzyme, which is constitutively expressed in many tissues, and a second isozyme, i.e., COX-2, which is induced by various stimuli, such as mitogens and cytokines, and is involved in many inflammatory reactions. Because nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit both COX-1 and COX-2, these drugs also cause unwanted side effects, exemplified by gastrointestinal bleeding. Accumulating evidence indicates that NSAIDs can reduce the incidence of colorectal cancers in human and experimental animals and can reduce the number and size of polyps in patients with familial adenomatous polyposis. This Part II (of a two-part review) focuses on the growing clinical and experimental evidence that NSAIDS and COX-2 inhibitors can influence the risk of colon (and possibly of other) cancers.

Size- and invasion-dependent increase in cyclooxygenase 2 levels in human colorectal carcinomas

Nonsteroidal anti-inflammatory drugs reduce the incidence and mortality of colorectal carcinoma. Their chemopreventive effects appear to be due to inhibition of cyclooxygenase (COX)-2. Here, we have studied the relationship between the COX-2 mRNA levels and pathological characteristics in 43 primary colorectal carcinomas. COX-2 levels were significantly higher in tumors with larger sizes and in those with deeper invasions but were not correlated with whether the patients had metastasis or not. These results suggest that larger carcinomas produce more COX-2 to support their own growth and that COX-2 inhibitors may be effective agents of carcinoma growth suppression.

Cyclooxygenase-2 inhibitors in tumorigenesis (part I)

The rate-limiting enzyme in arachidonate metabolism is mediated by enzymes known as cyclooxygenases (COXs). These enzymes catalyze the biosynthesis of prostaglandin H2, the precursor of molecules, such as prostaglandins, prostacyclin, and thromboxanes. The COX enzyme family consists of the classical COX-1 enzyme, which is constitutively expressed in many tissues, and a second enzyme, i.e., COX-2, which is induced by various stimuli, such as mitogens and cytokines, and is involved in many inflammatory reactions. Because nonsteroidal anti-inflammatory drugs inhibit both COX-1 and COX-2, these drugs also cause unwanted side effects, exemplified by gastrointestinal bleeding. Accumulating evidence indicates that nonsteroidal anti-inflammatory drugs can reduce the incidence of colorectal cancers in human and experimental animals and can reduce the polyp number and size in patients with familial adenomatous polyposis. This Part I (of a two-part review) focuses on the discovery of the COXs; their biochemical, molecular, and structural properties; and on the discovery of isozyme-specific inhibitors of COX activity.

COX-2 and colon cancer

The role of cyclooxygenase-2 (COX-2) in colorectal tumorigenesis in mice was studied by Oshima et al. to determine the effects of COX-2 gene knockouts and a new COX-2 inhibitor. In the study, heterozygous Apcdelta716 knockout mice, a mouse model of human familial adenomatous polyposis (FAP), were either crossed to COX-2 gene knockout mice, or fed chow containing the COX-2-selective inhibitor. Apcdelta716 litter mates were used as positive controls, which developed 652+/-198 (SD) polyps at 10 weeks. Introduction of a COX-2 gene mutation, or feeding with the COX-2-selective inhibitor to the Apcdelta716 knockout mice, reduced the number and size of intestinal polyps dramatically. The results provide direct genetic evidence that COX-2 plays a key role in tumorigenesis, and indicate that COX-2-selective inhibitors can be a new class of therapeutic agents for colorectal polyposis and cancer.

Estrogen-induced tumorigenesis in the pituitary gland of TGF-beta(+/-) knockout mice

Transforming growth factor (TGF)-beta receptor type II gene (Tgfbr2) knockout and wild type mice underwent chronic estrogen exposure using estradiol pellets. Histological examination of the pituitary glands found 38 adenomas in 14 Tgfbr2(+/-) mice but only one tumor in ten wild type mice. Pituitary tumorigenesis is greatly accelerated in Tgfbr2(+/-) mice by estrogen treatment.

Characterization of mouse retinoid X receptor (RXR)-beta gene promoter: negative regulation by tumor necrosis factor (TNF)-alpha

A genomic clone of mouse retinoid X receptor (RXR)-beta (Rxrb) has recently been isolated and mapped within the H2-K region of the mouse major histocompatibility complex. A putative 250-bp promoter, which is located between Rxrb and H2-Ke4, and may possibly be their common promoter, has also been identified. In order to study the gene regulation of Rxrb, we analyzed the transcriptional function of the Rxrb promoter with chimeric constructs containing the Rxrb promoter fragments fused upstream of a firefly luciferase cDNA, which were transiently transfected into rat GH3 cells. We found that 1) a part of the H2-Ke4 genomic region (1.9-kb), as well as the 250-bp promoter, was transcriptionally active as an Rxrb promoter; 2) tumor necrosis factor (TNF)-alpha significantly repressed the activity of the 250-bp promoter although thyroid hormone, 9-cis retinoic acid, interleukin (IL)-1beta, and IL-6 did not affect the activity; 3) either the change in orientation or point mutations of a consensus NF-kappaB site located in the 250-bp promoter did not affect the repression; 4) SB 203580, a highly specific inhibitor of p38 mitogen-activated protein (MAP) kinase, completely abolished the repression by TNF-alpha. These data suggest that TNF-alpha represses the promoter activity of the 250-bp region, and the repression is mediated by p38 MAP kinase independent of NF-kappaB. We thus have first shown a relation between the retinoic acid receptor and a cytokine TNF-alpha.

Nuclear translocation of beta-catenin in hereditary and carcinogen-induced intestinal adenomas

The physical interaction between beta-catenin and the adenomatous polyposis coli (APC) gene, and the ability of APC to regulate cytoplasmic levels of beta-catenin suggest a role for beta-catenin in colorectal carcinogenesis. In this study, we found that beta-catenin immunoreactivity was detected exclusively in the cell membrane and cytoplasm of morphologically normal intestinal epithelial cells with predominant distribution in the differentiated nonproliferative cell population. In contrast, beta-catenin was localized predominantly in the nucleus of adenomas from Min/+ mice and transgenic mice expressing a mutant truncated form of the APC gene (Apc(delta716) mice). Beta-catenin was expressed predominantly at the cell membrane and cytoplasm of the nontransformed rat intestinal epithelial (RIE-1) cells in culture, whereas predominantly nuclear localization of beta-catenin was observed in the human colon cancer cell line SW480. In the azoxymethane (AOM) treated rats, overexpression and nuclear localization of beta-catenin was observed in all adenomas. Previous studies have indicated the incidence of APC mutations amongst AOM-induced tumors to be 15% or less. These results demonstrate that nuclear localization of beta-catenin is a common event in colorectal tumorigenesis.

Intestinal tumorigenesis in compound mutant mice of both Dpc4 (Smad4) and Apc genes

The DPC4 (SMAD4) gene plays a key role in the TGFbeta signaling pathway. We inactivated its mouse homolog Dpc4 (Smad4). The homozygous mutants were embryonic lethal, whereas the heterozygotes showed no abnormality. We then introduced the Dpc4 mutation into the Apc(delta716) knockout mice, a model for human familial adenomatous polyposis. Because both Apc and Dpc4 are located on chromosome 18, we constructed compound heterozygotes carrying both mutations on the same chromosome by meiotic recombination. In such mice, intestinal polyps developed into more malignant tumors than those in the simple Apc(delta716) heterozygotes, showing an extensive stromal cell proliferation, submucosal invasion, cell type heterogeneity, and in vivo transplantability. These results indicate that mutations in DPC4 (SMAD4) play a significant role in the malignant progression of colorectal tumors.

Mapping of eight testis-specific genes to mouse chromosomes

We previously identified eight testis-specific genes using antibodies raised against testicular germ cells. They are expressed during spermatogenesis and are presumed to be involved in testicular germ cell differentiation and sperm formation. We have mapped the genomic loci for these testis-specific genes using restriction fragment length variants in interspecific backcross mice. The calmegin gene (Clgn) was mapped to Chr 8. The synaptonemal complex protein gene 1 (Sycp1) probe hybridized with two sequences on different chromosomes; Sycp1-rs2 was mapped to Chr 3, whereas Sycp1-rs3 was mapped to Chr 7. The relaxin-like factor gene (Rlnl) was mapped to Chr 8, and collapsin response mediator protein 1 (Crmp1) was mapped to Chr 5. Three novel genes encoding testis-specific proteins A2 (Tsga2), A8 (Tsga8), and A12 (Tsga12) were mapped to chromosomes 3, X, and 10, respectively.

Suppression of intestinal polyp development by low-fat and high-fiber diet in Apc(delta716) knockout mice

Most epidemiological and animal studies show a positive correlation of the dietary intake of fat with the incidence of colon cancer, whereas an inverse correlation of the dietary intake of fiber. In rats fed a diet low in fat and high in wheat bran fiber and calcium, a significant decrease was reported in the number of azoxymethane-induced aberrant crypt foci compared with those fed a high-fat, low-fiber and low-calcium diet. Mutations in the human APC gene play a key role, not only in familial adenomatous polyposis, but also in many sporadic cancers of the entire digestive tract. We previously constructed a mouse strain Apc(delta716), carrying a truncation mutation at codon 716 of the Apc gene, the homolog of human APC (10). The heterozygous mice developed numerous intestinal polyps, and all microadenomas dissected from the earliest polyps had already lost the wild-type allele, indicating the loss of heterozygosity. Using these Apc(delta716) knockout mice, we have investigated the effect of a low-fat and high-fiber diet (LRD for 'low-risk' diet) on intestinal polyposis, and compared it with that of a high-fat and low-fiber diet (HRD for 'high-risk' diet). The mice were fed either diet for 7 weeks, and the number and size of intestinal polyps were scored. The LRD-fed mice had fewer polyps than the HRD-fed mice, by 36% in the small intestine and by 64% in the colon. As for the polyp size distribution, there was no significant difference between the HRD- and LRD-fed mice. These results indicate that LRD can suppress intestinal polyposis compared with HRD which does not, and suggest that its suppression is at the initiation of polyp formation. This is likely to be due to a decreased frequency of loss of heterozygosity, rather than a retarded growth of the polyp adenomas.

Tertiary hypothyroidism and hyperglycemia in mice with targeted disruption of the thyrotropin-releasing hormone gene

Thyrotropin-releasing hormone (TRH) is a brain hypothalamic hormone that regulates thyrotropin (TSH) secretion from the anterior pituitary and is ubiquitously distributed throughout the brain and other tissues including pancreas. To facilitate studies into the role of endogenous TRH, we have used homologous recombination to generate mice that lack TRH. These TRH-/- mice are viable, fertile, and exhibit normal development. However, they showed obvious hypothyroidism with characteristic elevation of serum TSH level and diminished TSH biological activity. Their anterior pituitaries exhibited an apparent decrease in TSH immunopositive cells that was not due to hypothyroidism. Furthermore, this decrease could be reversed by TRH, but not thyroid hormone replacement, suggesting a direct involvement of TRH in the regulation of thyrotrophs. The TRH-/- mice also exhibited hyperglycemia, which was accompanied by impaired insulin secretion in response to glucose. These findings indicate that TRH-/- mice provide a model of exploiting tertiary hypothyroidism, and that TRH gene abnormalities cause disturbance of insulin secretion resulting in marked hyperglycemia.

Mesp2: a novel mouse gene expressed in the presegmented mesoderm and essential for segmentation initiation

We isolated a novel bHLH protein gene Mesp2 (for mesoderm posterior 2) that cross-hybridizes with Mesp1 expressed in the early mouse mesoderm. Mesp2 is expressed in the rostral presomitic mesoderm, but down-regulated immediately after the formation of the segmented somites. To determine the function of MesP2 protein (MesP2) in somitogenesis, we generated Mesp2-deficient mice by gene targeting. The homozygous Mesp2 (-/-) mice died shortly after birth and had fused vertebral columns and dorsal root ganglia, with impaired sclerotomal polarity. The earliest defect in the homozygous embryos was a lack of segmented somites. Their disruption of the metameric features, altered expression of Mox-1, Pax-1, and Dll1, and lack of expression of Notch1, Notch2, and FGFR1 suggested that MesP2 controls sclerotomal polarity by regulating the signaling systems mediated by notch-delta and FGF, which are essential for segmentation.

Early embryonic lethality caused by targeted disruption of the mouse selenocysteine tRNA gene (Trsp)

Selenoprotein biosynthesis is mediated by tRNASec, which inserts selenocysteine at UGA codons in a complex, context-specific manner. This opal suppressor serves in the conversion of serine to selenocysteine as well. The mouse tRNASec gene (Trsp) maps to a proximal segment of chromosome 7. We constructed mice carrying a targeted deletion of the Trsp gene. The heterozygous mutants were viable, fertile, and appeared normal. Although the level of tRNASec was reduced to about 50%-80% of the wild type in most organs, one of the selenoproteins, glutathione peroxidase, remained unaffected in the levels of its mRNA, protein, and enzyme activity, indicating that the haploid amount of tRNASec is not limiting in its biosynthesis. In contrast, the homozygous mutants died shortly after implantation, and the embryos were resorbed before 6.5 days post coitum. When the preimplantation embryos were placed in culture, however, the trophoectoderm cells showed outgrowths and the inner cell mass cells of the homozygous embryos were able to proliferate. These results indicate that Trsp expression is essential for early development of the embryo, and its lack causes peri-implantation lethality. However, the lethality does not appear to be due to a cell-autonomous function of tRNASec.

Morphological and molecular processes of polyp formation in Apc(delta716) knockout mice

Mutations in the human adenomatous polyposis coli (APC) gene are responsible for not only familial adenomatous polyposis but also many sporadic cancers of the digestive tract. Using homologous recombination in embryonic stem cells, we recently constructed Apc gene knockout mice that contained a truncation mutation at codon 716 (Apc(delta716)). The heterozygous mice developed numerous intestinal polyps. All microadenomas dissected from nascent polyps had already lost the wild-type allele, indicating the loss of heterozygosity (M. Oshima et al., Proc. Natl. Acad. Sci. USA, 92: 4482-4486, 1995). We also demonstrated that cyclooxygenase 2 is induced in the polyps at an early stage and plays a key role in polyp development (M. Oshima et al., Cell 87: 803-809, 1996). We have analyzed the process of polyp development in these mice both at morphological and molecular levels. A small intestinal microadenoma is initiated as an outpocketing pouch in a single crypt and develops into the inner (lacteal) side of a neighboring villus forming a double-layer nascent polyp. The microadenoma then enlarges and gets folded inside the villus. When it fills the intravillous space, it expands downward and extends into adjoining villi, rather than rupturing into the intestinal lumen. During this course of development, the basement membrane remains intact, and the labeling index of the microadenoma cells is similar to that of the normal crypt epithelium. As in the crypt cells, neither transforming growth factor beta1 nor its receptor type II is expressed in the microadenoma cells. No hot spot mutations in the K-ras gene are found in the microadenoma tissue during these early stages of polyp development. Essentially, the same results have been obtained for the colonic polyps as well. These results suggest that early adenomas in the Apc(delta716) polyps are very similar to the normal proliferating cells of the crypt except for the lack of directed migration along the crypt-villus axis.

Suppression of intestinal polyposis in Apc delta716 knockout mice by inhibition of cyclooxygenase 2 (COX-2)

Two cyclooxygenase isozymes catalyze conversion of arachidonic acid to prostaglandin H2: constitutive COX-1 and inducible COX-2. To assess the role of COX-2 in colorectal tumorigenisis, we determined the effects of COX-2 gene (Ptgs2) knockouts and a novel COX-2 inhibitor on Apc delta716 knockout mice, a model of human familial adenomatous polyposis. A Ptgs2 null mutation reduced the number and size of the intestinal polyps dramatically. Furthermore, treating Apc delta716 mice with a novel COX-2 inhibitor reduced the polyp number more significantly than with sulindac, which inhibits both isoenzymes. These results provide direct genetic evidence that COX-2 plays a key role in tumorigenesis and indicate that COX-2-selective inhibitors can be a novel class of therapeutic agents for colorectal polyposis and cancer.

Genomic organization, expression, and chromosomal mapping of the mouse adrenomedullin gene

We have isolated and characterized the mouse adrenomedullin (AM) gene (Adm) and determined its chromosomal location. The gene spans approximately 2.1 kb and is organized into four exons separated by three introns. The transcription start site was determined to be the adenine nucleotide at -618. The mouse AM 5'-flanking region contains a TATA box-like sequence and several cis-acting regulatory elements. Analysis of the nucleotide and deduced amino acid sequences revealed that mouse preproAM is a 184-amino-acid polypeptide, from which AM and proAM N-terminal 20 peptide are cleaved. Using restriction fragment length variants on a DNA panel of interspecific backcross mice, we mapped Adm to a distal region of mouse chromosome 7.