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.

TGF-beta receptor type II deficiency results in defects of yolk sac hematopoiesis and vasculogenesis

TGF-beta signaling is mediated through two types of serine/threonin kinase-containing receptors, type I (TGF-betaRI) and type II (TGF-betaRII), which form a heteromeric complex. In this signaling complex, ligand binding TGF-betaRII phosphorylates and thereby activates the TGF-betaRI to signal downstream pathways. To determine the role of TGF-betaRII in embryogenesis, we have generated a TGF-betaRII gene (Tgfbr2) knockout mouse line. The heterozygous Tgfbr2 knockout mice are developmentally normal. The homozygous Tgfbr2 mutation causes defects in the yolk sac hematopoiesis and vasculogenesis, resulting in an embryonic lethality around 10.5 days of gestation. This phenotype is indistinguishable from the previously reported embryonic lethality by the homozygous TGF-beta1 gene (Tgfb1) null mutation. In addition, we have generated chimeric mice using a Tgfbr2 (-/-) embryonic stem cell line. Some chimeric mice showed several types of congenital anomalies, suggesting that TGF-beta II is important for normal development in a variety of organs.

MesP1: a novel basic helix-loop-helix protein expressed in the nascent mesodermal cells during mouse gastrulation

A subtractive hybridization strategy was used to isolate putative genes involved in the development of mouse primordial germ cells (PGC). Complimentary DNA was amplified on RNA isolated from the base of the allantois where PGC are located in the 7.5 days post coitum (dpc) mouse embryo. It was then subtracted by hybridization with cDNA amplified on RNA of the anterior region where PGC are absent. A novel gene thus isolated is designated as Mesp1 and encodes a possible transcription factor MesP1 containing a basic helix-loop-helix motif. Its earliest expression was observed at the onset of gastrulation, as early as 6.5 dpc, in the nascent mesodermal cells that first ingressed at the end of the primitive streak. These expressing cells in the lateral and extraembryonic mesoderm showed a wing-shaped distribution. Its initial expression was soon down-regulated at 7.5 dpc before the completion of gastrulation, except at the proximal end of the primitive streak which included the extraembryonic mesoderm and the base of allantois. At 8 dpc, the expression at the base of the allantois moved laterally. This distribution between 7.0 and 8.0 dpc was similar to that of PGC detected by the alkaline phosphatase activity. However, the expression of Mesp1 was down-regulated thereafter, when PGC entered in the migration stage. After birth, Mesp1 expression was detected only in mature testes, but in a different isoform from that expressed in the embryo. Mesp1 was mapped to the mid region of chromosome 7, near the mesodermal deficiency gene (mesd). However, a Southern hybridization study clearly showed that Mesp1 was distinctly different from mesd. The amino acid sequence and its expression pattern suggest that MesP1 plays an important role in the development of the nascent mesoderm including PGC.

Early embryonic lethality caused by targeted disruption of the mouse thioredoxin gene

Thioredoxins belong to a widely distributed group of small proteins with strong reducing activities mediated by a consensus redox-active dithiol (Cys-Gly-Pro-Cys). Thioredoxin was first isolated as a hydrogen donor for enzymatic synthesis of deoxyribonucleotides by ribonucleotide reductase in Escherichia coli. Recent studies have revealed a variety of roles that thioredoxin plays in transcription, growth control, and immune function. In this report, we describe the phenotype of mice carrying a targeted disruption of the thioredoxin gene (Txn). Heterozygotes are viable, fertile, and appear normal. In contrast, homozygous mutants die shortly after implantation, and the concepti were resorbed prior to gastrulation. When preimplantation embryos were placed in culture, the inner cell mass cells of the homozygous embryos failed to proliferate. These results indicate that Txn expression is essential for early differentiation and morphogenesis of the mouse embryo.

Mapping of the genes encoding mouse prostaglandin D, E, and F and prostacyclin receptors

Prostaglandins and prostacyclin are metabolites of arachidonic acid and exert a variety of actions to maintain local homeostasis in the body. Their actions are mediated by cell surface receptors specific to the respective ligands. Using a panel of interspecific back-cross mice, we have mapped the prostaglandin D receptor gene (Ptgdr), prostaglandin E receptor subtype EP(1) gene (Ptgerepl), prostaglandin F receptor gene (Ptgfr), and prostacyclin receptor gene (Ptgir). Ptgdr mapped to proximal Chr 14, Ptgfr mapped to distal Chr 3, Ptgerepl mapped to middle Chr 8, and Ptgir mapped to proximal Chr 7.

Effects of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine on intestinal polyp development in Apc delta 716 knockout mice

Damage to the human adenomatous polyposis coli (APC) gene is responsible for not only familial adenomatous polyposis but also many sporadic cancers of the entire digestive tract. Using homologous recombination in embryonic stem cells, we recently constructed gene knockout mice with a truncation mutation in the Apc gene. These heterozygous mice developed intestinal polyps. We found that all microadenomas dissected from the earliest polyps had already lost the wild-type allele, indicating loss of heterozygosity (LOH) (Oshima et al., Proc. Natl. Acad. Sci. USA 92:4482-4486, 1995). Using these knockout mice, we investigated the effects of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhlP), one of the most abundant heterocyclic amines found in cooked meat and fish. When PhIP was fed to these mice at 400 ppm for 8 wk, the polyp distribution shifted to a larger size range, although the total polyp number did not change significantly. Similar, but weaker, effects were observed with the other heterocyclic amines 2-amino-3-methylimidazo[4,5-f]quinoline and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline. On the other hand, intraperitoneal injections of 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-OH-PhlP) at a higher dose (50 mg/kg) for five consecutive days increased the polyp number significantly. This increment was not associated with mutations in the Apc gene; however, most polyps showed loss of the full-length Apc allele (LOH). These results suggest that PhIP affects intestinal polyp development by accelerating the growth rate of microadenomas. It is also possible that high doses of N-OH-PhIP increase the frequency of Apc gene LOH.

Effects of docosahexaenoic acid (DHA) on intestinal polyp development in Apc delta 716 knockout mice

Epidemiological studies and animal experiments show an association of dietary intake of fish oils and low incidence of several types of cancers. The active ingredients of fish oils appear to be polyunsaturated fatty acids of omega-3 type such as eicosapentaenoic acid and docosahexaenoic acid (DHA). We have investigated chemopreventive effects of DHA on mouse intestinal polyposis using adenomatous polyposis coli (Apc) gene knockout mice. Damage to the human APC gene is responsible for not only familial adenomatous polyposis but also many sporadic cancers of the entire digestive tract. Using homologous recombination in embryonic stem cells, we recently constructed gene knockout mice containing a truncation mutation in the Apc gene at codon 716 (Apc delta 716). 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 [Oshima et al. (1995), Proc. Natl Acad. Sci. USA, 92, 4482-4486]. We fed Apc delta 716 heterozygotes with AIN-76A purified diet containing 3% DHA for 7 weeks, and scored the number and size of intestinal polyps. Average DHA intakes per day were 4.1 and 4.3 g/kg body wt for males and females, respectively. DHA-fed females had only 31% of polyps compared with the control females that developed about 220 polyps, whereas DHA-fed females showed no significant decrease in the polyp number. As for the polyp size, the proportion of larger polyps decreased more significantly in females than in males. This is the first demonstration that DHA inhibits intestinal polyposis induced by an Apc mutation at both its formation and growth.

Evidence against dominant negative mechanisms of intestinal polyp formation by Apc gene mutations

Mutations in the adenomatous polyposis coli (APC) gene are responsible for not only familial adenomatous polyposis but also many sporadic cancers of the digestive tract. Most mutations found in familial adenomatous polyposis patients are of the truncation type, and the phenotype is affected by the mutation sites in the gene. Truncated APC proteins can associate with the wild-type protein. Accordingly, it has been proposed that the polyposis is caused by a dominant negative mechanism. To test this possibility, we constructed transgenic mice that contained mutant minigenes. They expressed the APC protein truncated either at codon 716 (Apc delta 716) or 1287 (Apc delta 1287) at high levels in the intestinal epithelium. Contrary to our expectation, no intestinal polyps or tumors were found in any of such mice, even after 7 months. These results rule out any dominant negative mechanisms in which the truncated APC protein is directly involved in the formation of intestinal polyps in the mouse.