Sequence and chromosomal mapping of the mouse homolog (Madh4) of the human DPC4/MADH4 gene

Intestinal tumours induced in Apc(Min/+) mice by X-rays and neutrons

PURPOSE

To compare the development of intestinal adenomas following neutron and X-ray exposure of Apc(Min/+) mice (Apc - adenomatous polyposis coli; Min - multiple intestinal neoplasia).

MATERIALS AND METHODS

Adult mice were exposed to acute doses of X-rays or fission neutrons. Tumour counting was undertaken 200 days later and samples were taken for Loss of Heterozygosity (LOH) analysis.

RESULTS

Tumour numbers (adenomas and microadenomas) increased by 1.4-fold, 1.7-fold, 2.7-fold and 9-fold, after 0.5, 1, 2 and 5 Gy X-rays, respectively, and by 2.4-fold and 5.7-fold, after 0.5 and 1 Gy fission neutrons, respectively. LOH analysis of tumours from neutron-exposed mice showed that 63% had lost Apc and 90% (cf. 53% in controls) had lost D18mit84, a marker for Epb4.1l4a/NBL4 (erythrocyte protein band 4.1-like 4a/novel band 4.1-like 4), known to be involved in the Wnt (wingless-related mouse mammary tumour virus integration site) pathway. Some tumours from neutron-exposed mice appeared to have homozygous loss of some chromosomal markers.

CONCLUSIONS

X-ray or fission neutron irradiation results in strongly enhanced tumour multiplicities. Comparison of tumour yields indicated a low Relative Biological Effectiveness of around 2-8 for fission neutrons compared with X-rays. LOH in intestinal tumours from neutron-exposed mice appeared to be more complex than previously reported for tumours from X-irradiated mice.

TGF-β superfamily signaling is essential for tooth and hair morphogenesis and differentiation

Members of the transforming growth factor beta (TGF-beta) superfamily of signaling molecules are involved in the regulation of many developmental processes that involve the interaction between mesenchymal and epithelial tissues. Smad7 is a potent inhibitor of many members of the TGF-beta family, notably TGF-beta and activin. In this study, we show that embryonic overexpression of Smad7 in stratified epithelia using a keratin 5 promoter, results in severe morphogenetic defects in skin and teeth and leads to embryonic and perinatal lethality. To further analyze the functions of Smad7 in epithelial tissues of adult mice, we used an expression system that allowed a controlled overexpression of Smad7 in terms of both space and time. Skin defects in adult mice overexpressing Smad7 were characterized by hyper-proliferation and missing expression of early markers of keratinocyte differentiation. Upon Smad7-mediated blockade of TGF-beta superfamily signaling, ameloblasts failed to produce an enamel layer in incisor teeth. In addition, TGF-beta blockade in adult mice altered the pattern of thymic T cell differentiation and the number of thymic T cells was significantly reduced. This study shows that TGF-beta superfamily signaling is essential for development of hair, tooth and T-cells as well as differentiation and proliferation control in adult tissues.

Cloning of Smad2, Smad3, Smad4, and Smad7 from the goldfish pituitary and evidence for their involvement in activin regulation of goldfish FSHbeta promoter activity

Follicle-stimulating hormone (FSH), a glycoprotein consisting of an alpha subunit and a unique beta subunit, is essential for gonadal development and function in vertebrates including teleosts. FSH is regulated by a variety of neuroendocrine and endocrine factors, and its biosynthesis is primarily determined by the expression of the beta subunit. Although the regulation of FSH biosynthesis has been well documented in mammals, the molecular mechanisms underlying the regulation are poorly understood. Our previous studies demonstrated that activin stimulated goldfish FSHbeta expression in the primary pituitary cell culture and enhanced its promoter activity in the mouse gonadotrope cell line LbetaT-2 cells. However, little is known about the signal transduction pathway involved in the transcriptional activation of this gene by activin. To assess the involvement of intracellular signaling protein Smads in regulating goldfish FSHbeta promoter, we first cloned full-length cDNAs for goldfish Smad2, Smad3, Smad4, and Smad7 from the pituitary. All Smads cloned show high sequence conservation with their mammalian counterparts. The spatial expression of these Smads overlapped with that of activin subunits and its receptors in various tissues examined. In addition, we demonstrated that activin induced Smad3 and Smad7 expression, but not Smad2 and Smad4. Co-transfection of Smad2 or Smad3 cDNA into the LbetaT-2 cells with the reporter construct of goldfish FSHbeta promoter significantly enhanced basal and activin-stimulated reporter (SEAP, secreted alkaline phosphatase) expression, while Smad7 completely blocked basal and Smad2/3-stimulated FSHbeta activity. Interestingly, the effect of Smad3 was much higher than that of Smad2, suggesting that Smad3 is likely the principal signal transducing molecule involved in activin stimulation of FSHbeta expression in the goldfish. This work lays a foundation for further analysis of goldfish FSHbeta promoter for the cis-regulatory elements involved in activin signaling.

Loss of heterozygosity in spontaneous and X-ray-induced intestinal tumors arising in F1 hybrid min mice: evidence for sequential loss of APC(+) and Dpc4 in tumor development

Min (multiple intestinal neoplasia) mice carry a mutant allele of the murine Apc (adenomatous polyposis coli) locus and are predisposed to intestinal adenoma formation in the intestinal tract. Early studies have shown complete loss of function of Apc by whole chromosome loss on the tumor-sensitive C57BL/6J genetic background and in AKR x B6 F1 hybrids. Gamma-radiation-induced chromosomal losses focus the critical region on wt Apc, but because of the limited number of polymorphic markers used, no other critical regions of loss on chromosome 18 were identified. Using intestinal tumors arising spontaneously and induced by X-rays in CBA/H x C57BL/6J F1 hybrid mice and high-resolution microsatellite loss of heterozygosity (LOH) techniques, we provide mapping data for wt Apc loss, which confirms and extends earlier observations. In addition, high-frequency loss events at the Dpc4 locus were found in both spontaneous and radiation-induced tumors. These data identified LOH of Dpc4 as a critical secondary event following complete functional loss of Apc. LOH across the Trp53 genomic region of chromosome 11 was not observed. No LOH was recorded for the Mom1 candidate gene Pla2g2a or for 9 out of 10 polymorphic markers from the Mom1 genomic region on murine chromosome 4. One marker mapping distal to Pla2g2a showed LOH in a small minority of spontaneous tumors. These data support the contention that Mom1 does not act as a classical tumor suppressor. Overall, our data indicates a significant role for Dpc4 mutation in intestinal tumor progression in the mouse and provides further evidence for the importance of interstitial chromosome losses in radiation tumorigenesis.

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.

Cloning and studies of the mouse cDNA encoding Smad3

Following stimulation by the transforming growth factor-beta (TGF-beta) family in the cytoplasm, the Smad family is phosphorylated and translocated to the nucleus and activates several gene transcriptions. In this study, the mouse Smad3 cDNA including the open reading frame (ORF) was cloned from the mouse brain using a RACE (rapid amplification of cDNA ends) technique, and its expression pattern was analyzed in mouse tissue using northern blot. The predicted amino acid (aa) sequences of mouse Smad3 showed a high homology with human Smad3 (99.3%) and mouse Smad2 (85.4%). It revealed that this protein may be highly conserved in different species of mammals. Northern blot analyses revealed that Smad3 was highly expressed in the brain and ovary, and that the size of major transcript was about 5.7 kb. In situ hybridization analyses revealed the high expression of Smad3 was detected in the pyramidal cells of the hippocampus, the granule cells of the dentate gyrus, the granular cells of the cerebral cortex and the granulosa cells of the ovary. Smad3 may be essential transducer of signals from TGF-beta and activin in these cells.

Ectopic expression of the transforming growth factor beta type II receptor disrupts mesoderm organisation during mouse gastrulation

Transforming growth factor beta (TGFbeta) regulates the cell cycle and extracellular matrix (ECM) deposition of many cells in vitro. We have analysed chimaeric mouse embryos generated from embryonic stem cells with abnormal receptor expression to study the effect of TGFbeta on these processes in vivo and the consequences for normal development. The binding receptor for TGFbeta, TbetaRII, is first detected in the embryo proper around day 8.5 in the heart. Ectopic expression of TbetaRII from the blastocyst stage onward resulted in an embryonic lethal around 9.5 dpc. Analysis of earlier stages revealed that the primitive streak of TbetaRII chimaeras failed to elongate. Furthermore, although cells passed through the streak and initially formed mesoderm, they tended to accumulate within the streak. These defects temporally and spatially paralleled the expression of the TGFbeta type I receptor, which is first expressed in the node and primitive streak. We present evidence that classical TGFbeta-induced growth inhibition was probably the cause of insufficient mesoderm being available for paraxial and axial structures. The results demonstrate that (1) TGFbeta mRNA and protein detected previously in early postimplantation embryos is present as a biologically active ligand; and (2) assuming that ectopic expression of TbetaRII results in no other changes in ES cells, the absence of TbetaRII is the principle reason why the embryo proper is unresponsive to TGFbeta ligand until after gastrulation.

At least four loci and gender are associated with susceptibility to the chemical induction of lung adenomas in A/J x BALB/c mice

Four putative quantitative trait loci (QTLs) that influence susceptibility to the induction of lung adenomas by urethane in an F2 cross between A/J and BALB/cOlaHsd have been mapped. Following microsatellite typing of mice with resistant and susceptible phenotypes at 97 microsatellite marker loci, a major locus was identified on chromosome 18 with a lod score of 15. This was responsible for an 8- to 10-fold increase in tumor multiplicity in males and females, respectively, having the AA and CC genotypes at the D18Mit188 marker locus. It mapped close to Dcc (deleted in colorectal cancer). A locus on chromosome 4 (lod score 6.5) had the resistant allele in strain A/J and the susceptible allele in BALB/c, with a 14-fold difference in tumor multiplicity between mice of the AA and CC genotypes. This mapped close to the Cdkn2a (cyclin-dependent kinase inhibitor 2A) locus, which is commonly deleted in mouse lung tumors. Two loci with smaller effects (lod scores 3.03 and 3.25) were identified on chromosomes 1 and 11. There was also significant sexual dimorphism in tumor multiplicity both among 151 F2 hybrids and among 52 mice resulting from a backcross to strain A/J, with males having higher tumor counts than females.

The tumor suppressor SMAD4/DPC4 is essential for epiblast proliferation and mesoderm induction in mice

Members of the transforming growth factor (TGF)-beta superfamily have been shown to play a variety of important roles in embryogenesis, including dorsal and ventral mesoderm induction. The tumor suppressor SMAD4, also known as DPC4, is believed to be an essential factor that mediates TGF-beta signals. To explore functions of SMAD4 in development, we have mutated it by truncating its functional C-domain. We show that Smad4 is expressed ubiquitously during murine embryogenesis. Mice heterozygous for the Smad4(ex8/+) mutation are developmentally normal, whereas homozygotes die between embryonic day 6.5 (E6.5) and 8.5. All Smad4(ex8/ex8) mutants are developmentally delayed at E6 and show little or no elongation in the extraembryonic portion of late egg cylinder stage embryos. Consistent with this, cultured Smad4(ex8/ex8) blastocyst outgrowths suffer cellular proliferation defects and fail to undergo endoderm differentiation. Although a portion of mutant embryos at E8.5 show an increase in the embryonic ectoderm and endoderm, morphological and molecular analyses indicate that they do not form mesoderm. Altogether, these data demonstrate that SMAD4-mediated signals are required for epiblast proliferation, egg cylinder formation, and mesoderm induction.

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.