Mutations in the SMAD4/DPC4 gene in juvenile polyposis

Familial juvenile polyposis is an autosomal dominant disease characterized by a predisposition to hamartomatous polyps and gastrointestinal cancer. Here it is shown that a subset of juvenile polyposis families carry germ line mutations in the gene SMAD4 (also known as DPC4), located on chromosome 18q21.1, that encodes a critical cytoplasmic mediator in the transforming growth factor-beta signaling pathway. The mutant SMAD4 proteins are predicted to be truncated at the carboxyl-terminus and lack sequences required for normal function. These results confirm an important role for SMAD4 in the development of gastrointestinal tumors.

A genetic screen for modifiers of Drosophila decapentaplegic signaling identifies mutations in punt, Mothers against dpp and the BMP-7 homologue, 60A

decapentaplegic (dpp) is a Transforming Growth Factor beta (TGF-beta)-related growth factor that controls multiple developmental processes in Drosophila. To identify components involved in dpp signaling, we carried out a genetic screen for dominant enhancer mutations of a hypomorphic allele of thick veins (tkv), a type I receptor for dpp. We recovered new alleles of tkv, punt, Mothers against dpp (Mad) and Medea (Med), all of which are known to mediate dpp signaling. We also recovered mutations in the 60A gene which encodes another TGF-beta-related factor in Drosophila. DNA sequence analysis established that all three 60A alleles were nonsense mutations in the prodomain of the 60A polypeptide. These mutations in 60A caused defects in midgut morphogenesis and fat body differentiation. We present evidence that when dpp signaling is compromised, lowering the level of 60A impairs several dpp-dependent developmental processes examined, including the patterning of the visceral mesoderm, the embryonic ectoderm and the imaginal discs. These results provide the first in vivo evidence for the involvement of 60A in the dpp pathway. We propose that 60A activity is required to maintain optimal signaling capacity of the dpp pathway, possibly by forming biologically active heterodimers with Dpp proteins.

The Drosophila Medea gene is required downstream of dpp and encodes a functional homolog of human Smad4

The Transforming Growth Factor-beta superfamily member decapentaplegic (dpp) acts as an extracellular morphogen to pattern the embryonic ectoderm of the Drosophila embryo. To identify components of the dpp signaling pathway, we screened for mutations that act as dominant maternal enhancers of a weak allele of the dpp target gene zerknLllt. In this screen, we recovered new alleles of the Mothers against dpp (Mad) and Medea genes. Phenotypic analysis of the new Medea mutations indicates that Medea, like Mad, is required for both embryonic and imaginal disc patterning. Genetic analysis suggests that Medea may have two independently mutable functions in patterning the embryonic ectoderm. Complete elimination of maternal and zygotic Medea activity in the early embryo results in a ventralized phenotype identical to that of null dpp mutants, indicating that Medea is required for all dpp-dependent signaling in embryonic dorsal-ventral patterning. Injection of mRNAs encoding DPP or a constitutively activated form of the DPP receptor, Thick veins, into embryos lacking all Medea activity failed to induce formation of any dorsal cell fates, demonstrating that Medea acts downstream of the thick veins receptor. We cloned Medea and found that it encodes a protein with striking sequence similarity to human SMAD4. Moreover, injection of human SMAD4 mRNA into embryos lacking all Medea activity conferred phenotypic rescue of the dorsal-ventral pattern, demonstrating conservation of function between the two gene products.

The Drosophila gene Medea demonstrates the requirement for different classes of Smads in dpp signaling

Signals from transforming growth factor-beta (TGF-beta) ligands are transmitted within the cell by members of the Smad family, which can be grouped into three classes based on sequence similarities. Our previous identification of both class I and II Smads functioning in a single pathway in C. elegans, raised the issue of whether the requirement for Smads derived from different classes is a general feature of TGF-beta signaling. We report here the identification of a new Drosophila class II Smad, Medea, a close homolog of the human tumor-suppressor gene DPC4. Embryos from germline clones of both Medea and Mad (a class I Smad) are ventralized, as are embryos null for the TGF-beta-like ligand decapentaplegic (dpp). Loss of Medea also blocks dpp signaling during later development, suggesting that Medea, like Mad, is universally required for dpp signaling. Furthermore, we show that the necessity for these two closely related, non-redundant Smads, is due to their different signaling properties - upon activation of the Dpp pathway, Mad is required to actively translocate Medea into the nucleus. These results provide a paradigm for, and distinguish between, the requirement for class I and II Smads in Dpp/BMP signaling.

Medea is a Drosophila Smad4 homolog that is differentially required to potentiate DPP responses

Mothers against dpp (Mad) mediates Decapentaplegic (DPP) signaling throughout Drosophila development. Here we demonstrate that Medea encodes a MAD-related protein that functions in DPP signaling. MEDEA is most similar to mammalian Smad4 and forms heteromeric complexes with MAD. Like dpp, Medea is essential for embryonic dorsal/ventral patterning. However, Mad is essential in the germline for oogenesis whereas Medea is dispensable. In the wing primordium, loss of Medea most severely affects regions receiving low DPP signal. MEDEA is localized in the cytoplasm, is not regulated by phosphorylation, and requires physical association with MAD for nuclear translocation. Furthermore, inactivating MEDEA mutations prevent nuclear translocation either by preventing interaction with MAD or by trapping MAD/MEDEA complexes in the cytosol. Thus MAD-mediated nuclear translocation is essential for MEDEA function. Together these data show that, while MAD is essential for mediating all DPP signals, heteromeric MAD/MEDEA complexes function to modify or enhance DPP responses. We propose that this provides a general model for Smad4/MEDEA function in signaling by the TGF-beta family.

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.

Mutations of the DPC4/Smad4 gene in biliary tract carcinoma

A candidate tumor suppressor gene, DPC4, located at 18q21.1, has recently been shown to be inactivated in half of pancreatic adenocarcinomas. The close developmental relationship of the pancreas and biliary tract prompted us to determine the role of DPC4 in the multistep carcinogenesis of biliary tract carcinoma. A search for mutations in the genomic sequence of the highly conserved COOH-terminal domain of DPC4 (exons 8-11) was performed by single-strand conformational polymorphism analysis. Five of 32 (16%) primary biliary tract carcinomas had point mutations in the DPC4 sequence. Interestingly, inactivation of DPC4 was especially common in carcinomas originating from the common bile duct (four of eight specimens analyzed), suggesting an important role for DPC4 in the development of this subtype of biliary tract tumor.

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.

Targeted deletion of Smad4 shows it is required for transforming growth factor beta and activin signaling in colorectal cancer cells

Smad4 (DPC4) is a candidate tumor suppressor gene that has been hypothesized to be critical for transmitting signals from transforming growth factor (TGF) beta and related ligands. To directly test this hypothesis, the Smad4 gene was deleted through homologous recombination in human colorectal cancer cells. This deletion abrogated signaling from TGF-beta, as well as from the TGF-beta family member activin. These results provide unequivocal evidence that mutational inactivation of Smad4 causes TGF-beta unresponsiveness and provide a basis for understanding the physiologic role of this gene in tumorigenesis.

Presence of multiple incontiguous deleted regions at the long arm of chromosome 18 in head and neck cancer

The 18q chromosomal region is frequently lost in head and neck squamous cell carcinomas (HNSCCs). Several candidate tumor suppressor genes have been mapped to this chromosomal region, including DCC, DPC4, and MADR2. The latter two genes are members of the Smad family, key downstream mediators in the transforming growth factor beta signaling pathway, and their alterations could confer resistance to transforming growth factor beta and contribute to tumorigenesis. Nevertheless, genetic alterations of DCC and DPC4 in HNSCC have not been frequently reported. To further investigate the extent and significance of the loss of the 18q chromosomal region in HNSCC, we performed detailed mapping at this region in a set of 50 primary HNSCCs using 19 highly polymorphic microsatellite markers. We detected loss of heterozygosity in 84% of the tumors tested and were able to identify three minimal deleted regions encompassing markers D18S467-D18S474 at 18q12 (4 cM), D18S1099-D18S487 at 18q21.1 (3 cM), and D18S69-41 at 18q21.1-q21.2 (2 cM). Of these minimal deleted regions, only one harbors a known candidate tumor suppressor gene, DCC, which maps telomeric to D18S46. In addition, the role of the MADR2 gene in HNSCCs was investigated by examining nine HNSCC cell lines for alterations of the gene by reverse transcription-PCR and direct sequencing analysis. No mutations or polymorphisms were detected, making this gene an unlikely target of the frequent loss at 18q in HNSCC. Our data indicate high frequency of loss of heterozygosity at 18q in HNSCC and the presence of at least two as yet unidentified tumor suppressor genes in this chromosomal region. Additional efforts to identify these putative tumor suppressor genes are warranted.

Human Smad3 and Smad4 are sequence-specific transcription activators

Mounting evidence indicates that Smad proteins are required for TGF beta signaling, but the way(s) in which Smad proteins propagate this signal is unclear. We found that two human Smad proteins (Smad3 and Smad4) could specifically recognize an identical 8 bp palindromic sequences (GTCTAGAC). Tandem repeats of this palindrome conferred striking TGF beta responsiveness to a minimal promoter. This responsiveness was abrogated by targeted deletion of the cellular Smad4 gene. These results define a novel biochemical property of Smad proteins that is likely to play a direct role in the biologic responses to TGF beta and related ligands.

Abrogation of Smad3 and Smad2 or of Smad4 gene expression positively regulates murine embryonic lung branching morphogenesis in culture

Smad genes are recently identified intracellular effectors for receptor signaling in the BMP/activin/TGF-beta pathway. Since TGF-beta ligands are known to inhibit embryonic lung branching morphogenesis, we tested the hypothesis that Smad genes negatively regulate lung organogenesis. Antisense oligodeoxynucleotides were designed to attenuate Smad3 and Smad2 gene expression in embryonic (E11) mouse lungs over 4 days in culture. Endogenous Smad3 and Smad2 mRNA levels were suppressed by 97 and 91%, respectively, in cultured embryonic lungs when antisense oligodeoxynucleotide (40 microM) to Smad was added, compared to scrambled and sense sequence controls. The corresponding Smad3 and Smad2 protein amounts were also decreased respectively by 86 and 90% in lungs treated with Smad3 and Smad2 antisense oligodeoxynucleotide. Phenotypically, Smad antisense oligodeoxynucleotides resulted in a concentration-dependent increase in lung branching: embryonic lung branching was stimulated by up to 53% in culture with 40 microM antisense oligodeoxynucleotide, whereas both scrambled and sense controls showed no stimulatory effect. Thus, inhibition of endogenous Smad3 and Smad2 gene expression resulted in stimulation of embryonic lung branching similar to that caused by inhibition of TGF-beta type II receptor expression and signaling (J. Zhao et al., 1996, Dev. Biol. 180, 242-257). Abrogation of Smad4 (DPC4), the downstream mediator of Smad3 and Smad2 proteins, with antisense oligodeoxynucleotide, also resulted in increased branching morphogenesis. Furthermore, while TGF-beta alone inhibited lung branching morphogenesis in culture, addition of exogenous TGF-beta 1 could not overcome the stimulatory effect on lung branching of Smad antisense oligodeoxynucleotide treatment. By immunohistochemistry, Smad proteins were localized mainly to the epithelial cells lining the branching distal airways, indicating that Smad genes could regulate lung morphogenesis through mesoderm-endoderm interaction. Our results demonstrate, for the first time, that abrogation of Smad2 and Smad3 or of Smad4 gene expression stimulated early mouse embryonic lung branching morphogenesis in culture, possibly through reversing the negative influence of endogenous TGF-beta signaling upon lung branching morphogenesis.

Smad5 and DPC4 are key molecules in mediating BMP-2-induced osteoblastic differentiation of the pluripotent mesenchymal precursor cell line C2C12

Since the bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta (TGF-beta) superfamily that induce the differentiation of mesenchymal precursor cells into the osteogenic cells, we identified the relevant signaling molecules responsible for mediating BMP-2 effects on mesenchymal precursor cells. BMP-2 induces osteoblastic differentiation of the pluripotent mesenchymal cell line C2C12 by increasing alkaline phosphatase activity and osteocalcin production. As recent studies have demonstrated that cytoplasmic Smad proteins are involved in TGF-beta superfamily signaling, we plan to isolate the relevant Smad family members involved in osteoblastic differentiation. We identified human Smad5, which is highly homologous to Smad1. BMP-2 caused serine phosphorylation of Smad5 as well as Smad1. In contrast, TGF-beta failed to cause serine phosphorylation of Smad1 and Smad5. We found Smad5 is directly activated by BMP type Ia or Ib receptors through physical association with these receptors. Following phosphorylation, Smad5 bound to DPC4, another Smad family member, and the complex was translocated to the nucleus. Overexpression of point-mutated Smad5 (G419S) or a C-terminal deletion mutant DPC4 (DPC4 delta C) blocked the induction of alkaline phosphatase activity, osteocalcin production, and Smad5-DPC4 signaling cascades upon BMP-2 treatment in C2C12 cells. These data suggest that activation of Smad5 and subsequent Smad5-DPC4 complex formation are key steps in the BMP signaling pathway, which mediates BMP-2-induced osteoblastic differentiation of the C2C12 mesenchymal cells.

Smad6 inhibits BMP/Smad1 signaling by specifically competing with the Smad4 tumor suppressor

Bone morphogenetic protein (BMP) receptors signal by phosphorylating Smad1, which then associates with Smad4; this complex moves into the nucleus and activates transcription. Here we report the existence of a natural inhibitor of this process, Smad6, a longer version of the previously reported JV15-1. In Xenopus embryos and in mammalian cells, Smad6 specifically blocks signaling by the BMP/Smad1 pathway. Smad6 inhibits BMP/Smad1 signaling without interfering with receptor-mediated phosphorylation of Smad1. Smad6 specifically competes with Smad4 for binding to receptor-activated Smad1, yielding an apparently inactive Smad1-Smad6 complex. Therefore, Smad6 selectively antagonizes BMP-activated Smad1 by acting as a Smad4 decoy.

DPC4 splice variants in neuroblastoma

One of the loci for neuroblastoma suppressor genes is chromosome 18q21 where the DPC4 tumor suppressor gene, as well as the DCC and MADR2 genes, is located. DPC4 is a molecule of the TGF-beta signal which regulates differentiation of the neural crest precursor cells from which neuroblastoma originates. During the search for the significance of DPC4 as a candidate neuroblastoma suppressor gene, we found that there are at least two variant forms of the DPC4 transcripts by using the reverse-transcriptase-PCR procedure. The subsequent sequencing analysis has revealed that one is missing exons 5 and 6 and the other is missing exons 4-6. Both splice variants were frequently observed in neuroblastomas and at low levels in normal tissues. Though the functional role of the DPC4 splice variants is unknown, they might be important in regulating the TGF-beta signaling not only in neuroblastomas but also in other tumors and normal tissues.

The tumor suppressor gene Smad4/Dpc4 is required for gastrulation and later for anterior development of the mouse embryo

Mutations in the SMAD4/DPC4 tumor suppressor gene, a key signal transducer in most TGFbeta-related pathways, are involved in 50% of pancreatic cancers. Homozygous Smad4 mutant mice die before day 7.5 of embryogenesis. Mutant embryos have reduced size, fail to gastrulate or express a mesodermal marker, and show abnormal visceral endoderm development. Growth retardation of the Smad4-deficient embryos results from reduced cell proliferation rather than increased apoptosis. Aggregation of mutant Smad4 ES cells with wild-type tetraploid morulae rescues the gastrulation defect. These results indicate that Smad4 is initially required for the differentiation of the visceral endoderm and that the gastrulation defect in the epiblast is secondary and non-cell autonomous. Rescued embryos show severe anterior truncations, indicating a second important role for Smad4 in anterior patterning during embryogenesis.

Dual role of the Smad4/DPC4 tumor suppressor in TGFbeta-inducible transcriptional complexes

Upon ligand binding, the receptors of the TGFbeta family phosphorylate Smad proteins, which then move into the nucleus where they activate transcription. To carry out this function, the receptor-activated Smads 1 and 2 require association with the product of deleted in pancreatic carcinoma, locus 4 (DPC4), Smad4. We investigated the step at which Smad4 is required for transcriptional activation. Smad4 is not required for nuclear translocation of Smads 1 or 2, or for association of Smad2 with a DNA binding partner, the winged helix protein FAST-1. Receptor-activated Smad2 takes Smad4 into the nucleus where they form a complex with FAST-1 that requires these three components to activate transcription. Smad4 contributes two functions: Through its amino-terminal domain, Smad4 promotes binding of the Smad2/Smad4/FAST-1 complex to DNA; through its carboxy-terminal domain, Smad4 provides an activation function required for Smad1 or Smad2 to stimulate transcription. The dual function of Smad4 in transcriptional activation underscores its central role in TGFbeta signaling.

Tumor suppressor Smad4 is a transforming growth factor beta-inducible DNA binding protein

Members of the Smad family of proteins are thought to play important roles in transforming growth factor beta (TGF-beta)-mediated signal transduction. In response to TGF-beta, specific Smads become inducibly phosphorylated, form heteromers with Smad4, and undergo nuclear accumulation. In addition, overexpression of specific Smad combinations can mimic the transcriptional effect of TGF-beta on both the plasminogen activator inhibitor 1 (PAI-1) promoter and the reporter construct p3TP-Lux. Although these data suggest a role for Smads in regulating transcription, the precise nuclear function of these heteromeric Smad complexes remains largely unknown. Here we show that in Mv1Lu cells Smad3 and Smad4 form a TGF-beta-induced, phosphorylation-dependent, DNA binding complex that specifically recognizes a bipartite binding site within p3TP-Lux. Furthermore, we demonstrate that Smad4 itself is a DNA binding protein which recognizes the same sequence. Interestingly, mutations which eliminate the Smad DNA binding site do not interfere with either TGF-beta-dependent transcriptional activation or activation by Smad3/Smad4 cooverexpression. In contrast, mutation of adjacent AP1 sites within this context eliminates both TGF-beta-dependent transcriptional activation and activation in response to Smad3/Smad4 cooverexpression. Furthermore, concatemerized AP1 sites, in isolation, are activated by Smad3/Smad4 cooverexpression and, to a certain extent, by TGF-beta. Taken together, these data suggest that the Smad3/Smad4 complex has at least two separable nuclear functions: it forms a rapid, yet transient sequence-specific DNA binding complex, and it potentiates AP1-dependent transcriptional activation.

Deletion of the M6P/IGF2r gene in primary hepatocellular carcinoma

To evaluate the different alteration patterns of the mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2r) gene in hepatocellular carcinoma (HCC), 41 HCCs were screened for homozygous deletion and loss of heterozygosity (LOH) at the M6P/IGF2r gene with a dinucleotide repeat polymorphic marker. Of these, three (8.8%) were heterozygous and LOH was observed in two (66.7%) of these informative cases. Five (14.7%) out of 34 informative cases showed homozygous deletions for the dinucleotide repeat polymorphic marker. The frequent allelic loss and homozygous deletion of the M6P/ IGF2r gene suggest that the M6P/IGF2r gene functions as a tumor suppressor gene in the development of HCC.

Cellular interpretation of multiple TGF-beta signals: intracellular antagonism between activin/BVg1 and BMP-2/4 signaling mediated by Smads

During early embryogenesis of Xenopus, dorsoventral polarity of the mesoderm is established by dorsalizing and ventralizing agents, which are presumably mediated by the activity of an activin/BVg1-like protein and Bone Morphogenetic Proteins (BMP), respectively. Interestingly, these two TGF-beta subfamilies are found in overlapping regions during mesoderm patterning. This raises the question of how the presumptive mesodermal cells recognize the multiple TGF-beta signals and differentially interpret this information to assign a particular cell fate. In this study, we have exploited the well characterized model of Xenopus mesoderm induction to determine the intracellular interactions between BMP-2/4 and activin/BVg1 signaling cascades. Using a constitutively active BMP-2/4 receptor that transduces BMP-2/4 signals in a ligand-independent fashion, we demonstrate that signals provided by activin/BVg1 and BMP modulate each other's activity and that this crosstalk occurs through intracellular mechanisms. In assays using BMP-2/4 and activin/BVg1-specific reporters, we determined that the specificity of BMP-2/4 and activin/BVg1 signaling is mediated by Smad1 and Smad2, respectively. These Smads should be considered as the mediators of the intracellular antagonism between BMP-2/4 and activin/BVg1 signaling possibly through sequestration of a limited pool of Smad4. Consistent with such a mechanism, Smad4 interacts functionally with both Smad1 and −2 to potentiate their signaling activities, and a dominant negative variant of Smad4 can inhibit both activin/BVg1 and BMP-2/4 mediated signaling Finally, we demonstrate that an activin/BVg1-dependent transcriptional complex contains both Smad2 and Smad4 and thereby provides a physical basis for the functional involvement of both Smads in TGF-beta-dependent transcriptional regulation. Thus, Smad4 plays a central role in synergistically activating activin/BVg1 and BMP-dependent transcription and functions as an intracellular sensor for TGF-beta-related signals.

TbetaRI phosphorylation of Smad2 on Ser465 and Ser467 is required for Smad2-Smad4 complex formation and signaling

Mothers against Dpp-related or Smad proteins are essential components of serine/threonine kinase receptor signaling pathways that are regulated by phosphorylation. Recently, it was demonstrated that Smad2 interacts transiently with and is a direct substrate of the transforming growth factor-beta (TGF-beta) type I receptor, TbetaRI. Phosphorylation sites on Smad2 were localized to a carboxyl-terminal fragment containing three serine residues at positions 464, 465, and 467. In this report, we show that TbetaRI specifically phosphorylates Smad2 on serines 465 and 467. Serine 464 is not a site of phosphorylation, but is important for efficient phosphorylation of Smad2. Phosphorylation at both sites is required to mediate association of Smad2 with Smad4 in mammalian cells, while in yeast, Smad2 interacts directly with Smad4 and does not require phosphorylation. Mutation of either serine residue 465 or 467 prevents dissociation of Smad2 from activated TbetaRI and blocks TGF-beta-dependent signaling and Smad2 transcriptional activity. These results indicate that receptor-dependent phosphorylation of Smad2 on serines 465 and 467 is required in mammalian cells to permit association with Smad4 and to propagate TGF-beta signals.

Induction of apoptosis by DPC4, a transcriptional factor regulated by transforming growth factor-beta through stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) signaling pathway

Many of the actions of serine/threonine kinase receptors for the transforming growth factor-beta (TGFbeta) are mediated by DPC4, a human MAD-related protein identified as a tumor suppressor gene in pancreatic carcinoma. Overexpression of DPC4 is sufficient to induce the activation of gene expression and cell cycle arrest, characteristic of the TGFbeta response. The stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) is also one of the downstream targets required for TGFbeta-mediated signaling. Here we report that expression of the dominant-interfering mutant of various components of the SAPK/JNK cascade specifically blocked both TGFbeta and DPC4-induced gene expression. These dominant-interfering mutants also inhibited TGFbeta-stimulated DPC4 transcriptional activity. Moreover, we find that overexpression of DPC4 causes transfected cells to undergo the morphological changes typical of apoptosis. These findings define a mechanism whereby TGFbeta signals mediated by DPC4 and SAPK/JNK cascade are integrated in the nucleus to activate gene expression and identify a new cellular function for DPC4.

Mapping of a breast cancer tumor suppressor gene locus to a 4-cM interval on chromosome 18q21

DPC4 and DCC, putative tumor suppressor genes implicated in the genesis of several types of human cancer, lie on the long arm of human chromosome 18. We examined 200 primary breast cancers for allelic losses on chromosome 18, using 15 microsatellite markers distributed along the long arm. Allelic loss was detected most frequently (29-30%) at loci mapped to 18q21. Deletion mapping of the 34 tumors showing partial or interstitial deletions identified a commonly deleted region within the 4-cM interval flanked by D18S474 and D18S487 at 18q21.1-q21.3. Although this interval included the DPC4 and DCC genes, we excluded DPC4 from candidacy when polymerase chain reaction-single-strand conformation polymorphism analysis of each exon failed to detect abnormalities in any of the 54 breast cancers that exhibited loss of heterozygosity involving 18q. Allelic loss on 18q was found more frequently in tumors of the solid tubular histological type (24 of 55, 44%) than in other types (24 of 113, 21%) (P = 0.0049). The results suggest that a tumor suppressor gene located within the 4-cM region at 18q21, either DCC or another gene not yet identified, may play a role in the development of some sporadic breast cancers, particularly those of the solid tubular type.

Inactivation of Smad4 in gastric carcinomas

Allelic loss of chromosome 18q has been noted in intestinal type gastric adenocarcinomas. Smad4 is a gene located at 18q that was recently cloned in humans and found to be significantly altered in pancreatic cancers. We sought to determine whether Smad4 genetic alterations played a significant role in gastric tumorigenesis by studying 35 gastric adenocarcinomas of all histopathological types and pathological stages. Microdissected specimens were used for mutational analysis of Smad4 at the nucleotide level, including the entire coding region and intron/exon boundaries. Allelic imbalance was also analyzed at the Smad4 locus using two nearby microsatellite markers. One case of apparent biallelic inactivation of Smad4 was found in our study of 35 gastric carcinomas. A nonsense point mutation at codon 334 was demonstrated, which, similar to other Smad4 mutations, is predicted to truncate the conserved COOH-terminal domain of this protein. This Smad4 C to T transition mutation was proven to be somatically acquired. Allelic loss was also noted on chromosome 18q at a marker near Smad4 in this mutated gastric cancer, apparently producing complete inactivation of Smad4 in this tumor. Significant 18q allelic loss (56% of 34 informative cases) was noted in our gastric carcinomas using microsatellite markers near the Smad4 locus, regardless of histological subtype or pathological stage. Additionally, three cases of microsatellite instability were observed. Thus, Smad4 inactivation was noted in our gastric carcinomas; however, this event was rare. The frequent loss of chromosomal arm 18q observed in gastric cancers suggests the presence of other tumor suppressor genes in this region that are involved in gastric tumorigenesis. Further studies are needed to identify these other targets of inactivation during gastric cancer development.

Induction of p21waf1 expression and growth inhibition by transforming growth factor beta involve the tumor suppressor gene DPC4 in human pancreatic adenocarcinoma cells

The tumor suppressor gene deleted in pancreatic cancer locus 4 (DPC4) is inactivated in about 50% of pancreatic adenocarcinomas. DPC4 was found to be homologous to Smad4 and may function as a transcription factor in the transforming growth factor beta (TGF-beta) receptor-mediated signal transduction pathway. We have investigated the role of DPC4 in the TGF-beta receptor-mediated signal transduction cascade in five human pancreatic cancer cell lines (Panc-1, MDAPanc-28, HS766T, Capan-1, and MiaPaCa-2). Our results demonstrate that the loss of responsiveness to TGF-beta-induced growth inhibition correlates with the loss of expression of DPC4. We have shown that TGF-beta induces p21waf1 expression in Panc-1 cells, whereas no induction of p21waf1 expression by TGF-beta was detected in the other four cell lines lacking either DPC4 expression or the TGF-beta type II receptor. No increase in p21waf1 mRNA stability was observed after treatment with TGF-beta, which suggests that the induction of p21waf1 in Panc-1 cells is transcriptionally regulated by TGF-beta. Our data also demonstrate that the expression of DPC4 is directly involved in TGF-beta-mediated induction of the 3TP-lux reporter gene, which contains a known TGF-beta-inducible plasminogen activator inhibitor promoter. These data suggest that: (a) TGF-beta-mediated induction of p21waf1 and subsequent growth inhibition require the expression of DPC4; (b) p21waf1 is a downstream target gene of DPC4; and (c) transfection of the DPC4 gene restores the TGF-beta-inducible gene expression. Inactivation of the tumor suppressor gene DPC4 and other components of the TGF-beta signal cascades may abolish one of the key negative controls of cell proliferation in pancreatic adenocarcinomas.