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.