6598

SMARCB1

BAF47|INI1|RDT|RTPS1|SNF5|SNF5L1|Sfh1p|Snr1|hSNFS;SWI/SNF related, matrix associated, actin dependent regulator of chromatin, subfamily b, member 1;SMARCB1;SWI/SNF related, matrix associated, actin dependent regulator of chromatin, subfamily b, member 1

BRG1-associated factor 47|SNF5 homolog|SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily B member 1|hSNF5|integrase interactor 1 protein|malignant rhabdoid tumor suppressor|sucrose nonfermenting, yeast, homolog-like 1

22q11.23|22q11

protein-coding

Count: 4; Pubmed_search,TAG,UniProt,Generif

In T-cell prolymphocytic leukemia (T-PLL), chromosomal imbalances affecting the long arm of chromosome 22 are regarded as typical chromosomal aberrations secondary to a TCRAD-TCL1A fusion due to inv(14) or t(14;14). We analyzed recently obtained data from conventional karyotyping, SNP-chip array copy number mapping, genome-wide expression profiling, and interphase fluorescence in situ hybridization (FISH) of inv(14)-positive T-PLL with respect to structural aberrations on chromosome 22. Combined gene chip and interphase FISH analyses revealed interstitial deletions on 22q in 4 of 12 cases, with one case additionally showing a terminal copy number gain. A minimally deleted region of approximately 9.1 Mb was delineated, from 16.2 Mb (22cen) to 25.3 Mb (22q12.1). The distal borders of copy number alterations spread over a region of approximately 8.8 Mb, from 25.2 Mb (22q12.1) to 34 Mb (22q12.3). mutation screening of candidate tumor suppressor genes SMARCB1 and CHEK2 mapping to the minimally deleted and the breakpoint regions, respectively, in cases with hemizygous deletion, revealed no inactivating mutations. With gene expression profiling, no significantly downregulated genes were identified in the minimally deleted region. We therefore assume that haploinsufficiency or alternative pathomechanisms underlie chromosome 22 aberrations in T-PLL.

PURPOSE: A high-resolution genomic profiling and comprehensive targeted analysis of INI1/SMARCB1 of a large series of pediatric rhabdoid tumors was done. The aim was to identify regions of copy number change and loss of heterozygosity (LOH) that might pinpoint additional loci involved in the development or progression of rhabdoid tumors and define the spectrum of genomic alterations of INI1 in this malignancy. EXPERIMENTAL DESIGN: A multiplatform approach using Illumina single nucleotide polymorphism-based oligonucleotide arrays, multiplex ligation-dependent probe amplification, fluorescence in situ hybridization, and coding sequence analysis was used to characterize genome-wide copy number changes, LOH, and genomic alterations of INI1/SMARCB1 in a series of pediatric rhabdoid tumors. RESULTS: The biallelic alterations of INI1 that led to inactivation were elucidated in 50 of 51 tumors. INI1 inactivation was shown by a variety of mechanisms, including deletions, mutations, and LOH. The results from the array studies highlighted the complexity of rearrangements of chromosome 22 compared with the low frequency of alterations involving the other chromosomes. CONCLUSIONS: The results from the genome-wide single nucleotide polymorphism array analysis suggest that INI1 is the primary tumor suppressor gene involved in the development of rhabdoid tumors with no second locus identified. In addition, we did not identify hotspots for the breakpoints in sporadic tumors with deletions of chromosome 22q11.2. By employing a multimodality approach, the wide spectrum of alterations of INI1 can be identified in the majority of patients, which increases the clinical utility of molecular diagnostic testing.

SMARCB1 is deleted in rhabdoid tumor, an aggressive paediatric malignancy affecting the kidney and CNS. We hypothesized that the oncogenic pathway in rhabdoid tumors involved epigenetic silencing of key cell cycle regulators as a consequence of altered chromatin-remodelling, attributable to loss of SMARCB1, and that this hypothesis if proven could provide a biological rationale for testing epigenetic therapies in this disease. We used an inducible expression system to show that the imprinted cell cycle inhibitor CDKN1C is a downstream target for SMARCB1 and is transcriptionally activated by increased histone H3 and H4 acetylation at the promoter. We also show that CDKN1C expression induces cell cycle arrest, CDKN1C knockdown with siRNA is associated with increased proliferation, and is able to compete against the anti-proliferative effect of restored SMARCB1 expression. The histone deacetylase inhibitor (HDACi), Romidepsin, specifically restored CDKN1C expression in rhabdoid tumor cells through promoter histone H3 and H4 acetylation, recapitulating the effect of SMARCB1 on CDKNIC allelic expression, and induced cell cycle arrest in G401 and STM91-01 rhabdoid tumor cell lines. CDKN1C expression was also shown to be generally absent in clinical specimens of rhabdoid tumor, however CDKN1A and CDKN1B expression persisted. Our observations suggest that maintenance of CDKN1C expression plays a critical role in preventing rhabdoid tumor growth. Significantly, we report for the first time, parallels between the molecular pathways of SMARCB1 restoration and Romidepsin treatment, and demonstrate a biological basis for the further exploration of histone deacetylase inhibitors as relevant therapeutic reagents in the treatment of rhabdoid tumor.

There is a growing appreciation of the role that epigenetic alterations can play in oncogenesis. However, given the large number of genetic anomalies present in most cancers, it has been difficult to evaluate the extent to which epigenetic changes contribute to cancer. SNF5 (INI1/SMARCB1/BAF47) is a tumor suppressor that regulates the epigenome as a core member of the SWI/SNF chromatin remodeling complex. While the SWI/SNF complex displays potent tumor suppressor activity, it is unknown whether this activity is exerted genetically via maintenance of genome integrity or epigenetically via transcriptional regulation. Here we show that Snf5-deficient primary cells do not show altered sensitivity to DNA damaging agents, defects in gamma-H2AX induction, or an abrogated DNA damage checkpoint. Further, the aggressive malignancies that arise following SNF5 loss are diploid and genomically stable. Remarkably, we demonstrate that most human SNF5-deficient cancers lack genomic amplifications/deletions and, aside from SNF5 loss, are indistinguishable from normal cells on single-nucleotide polymorphism arrays. Finally, we show that epigenetically based changes in transcription that occur following SNF5 loss correlate with the tumor phenotype. Collectively, our results provide novel insight into the mechanisms of oncogenesis by demonstrating that disruption of a chromatin remodeling complex can largely, if not completely, substitute for genomic instability in the genesis of aggressive cancer.

Malignant rhabdoid tumors (MRT) are extremely aggressive pediatric tumors caused by the inactivation of the hSNF5/INI1 tumor suppressor gene, which encodes a core member of the SWI/SNF chromatin remodeling complex. Roles for hSNF5/INI1 in cell cycle and differentiation have been documented. Based on the observation that MRTs are highly invasive, we investigated a role for hSNF5/INI1 in cell migration. MRT cell lines exhibit high migration properties that are dramatically reduced upon hSNF5/INI1 expression. This effect is associated with the disorganization of the actin stress fiber network and is mediated by the inhibition of the activity of the small GTPase RhoA, through a nuclear, SWI/SNF-dependent transcriptional mechanism. We further show that the knockdown of hSNF5/INI1 in epithelial 293T or MCF7 cells results in increased cell size, loss of cell-cell adhesions, and enhanced migration, associated with an increased RhoA activity. Finally, we show that the SNF5 homology domain is required for hSNF5/INI1-mediated inhibition of migration, and that a missense mutation (S284L) associated with cancer is sufficient to impair hSNF5/INI1 function in migration. We conclude that the inhibition of migration is another crucial tumor suppressor function of hSNF5/INI1, in addition to its previously described functions in proliferation and differentiation, and that its loss-of-function in MRTs may account for the high invasiveness and metastatic potential of these tumors.

Schwannomatosis is a third major form of neurofibromatosis that has recently been linked to mutations in the SMARCB1 (hSnf5/INI1) tumor suppressor gene. We analyzed the coding region of SMARCB1 by direct sequencing and multiplex ligation-dependent probe amplification (MLPA) in genomic DNA from 19 schwannomatosis kindreds. Microsatellite markers in the SMARCB1 region were developed to determine loss of heterozygosity (LOH) in associated tumors. We detected four alterations in conserved splice acceptor or donor sequences of exons 3, 4 and 6. Two alterations that likely affect splicing were seen in introns 4 and 5. An additional four alterations of unclear pathogenicity were found to segregate on the affected allele in eight families including two non-conservative missense alterations in three families. No constitutional deletions or duplications were detected by MLPA. Nine of 13 tumors examined showed partial LOH of the SMARCB1 region consistent with 'second hits.' Alterations were detected in tumors both with and without somatic NF2 gene changes. These findings support the hypothesis that SMARCB1 is a tumor suppressor for schwannomas in the context of familial disease. Further work is needed to determine its role in other multiple and single tumor syndromes.

BACKGROUND: The transgenic adenocarcinoma of the mouse prostate (TRAMP) model closely mimics PC-progression as it occurs in humans. However, the timing of disease incidence and progression (especially late stage) makes it logistically difficult to conduct experiments synchronously and economically. The development and characterization of androgen depletion independent (ADI) TRAMP sublines are reported. METHODS: Sublines were derived from androgen-sensitive TRAMP-C1 and TRAMP-C2 cell lines by androgen deprivation in vitro and in vivo. Epithelial origin (cytokeratin) and expression of late stage biomarkers (E-cadherin and KAI-1) were evaluated using immunohistochemistry. Androgen receptor (AR) status was assessed through quantitative real time PCR, Western blotting, and immunohistochemistry. Coexpression of AR and E-cadherin was also evaluated. Clonogenicity and invasive potential were measured by soft agar and matrigel invasion assays. Proliferation/survival of sublines in response to androgen was assessed by WST-1 assay. In vivo growth of subcutaneous tumors was assessed in castrated and sham-castrated C57BL/6 mice. RESULTS: The sublines were epithelial and displayed ADI in vitro and in vivo. Compared to the parental lines, these showed (1) significantly faster growth rates in vitro and in vivo independent of androgen depletion, (2) greater tumorigenic, and invasive potential in vitro. All showed substantial downregulation in expression levels of tumor suppressor, E-cadherin, and metastatis suppressor, KAI-1. Interestingly, the percentage of cells expressing AR with downregulated E-cadherin was higher in ADI cells, suggesting a possible interaction between the two pathways. CONCLUSIONS: The TRAMP model now encompasses ADI sublines potentially representing different phenotypes with increased tumorigenicity and invasiveness.

Members of the ING family of tumor suppressors regulate cell cycle progression, apoptosis, and DNA repair as important cofactors of p53. ING1 and ING3 are stable components of the mSin3A HDAC and Tip60/NuA4 HAT complexes, respectively. We now report the purification of the three remaining human ING proteins. While ING2 is in an HDAC complex similar to ING1, ING4 associates with the HBO1 HAT required for normal progression through S phase and the majority of histone H4 acetylation in vivo. ING5 fractionates with two distinct complexes containing HBO1 or nucleosomal H3-specific MOZ/MORF HATs. These ING5 HAT complexes interact with the MCM helicase and are essential for DNA replication to occur during S phase. Our data also indicate that ING subunits are crucial for acetylation of chromatin substrates. Since INGs, HBO1, and MOZ/MORF contribute to oncogenic transformation, the multisubunit assemblies characterized here underscore the critical role of epigenetic regulation in cancer development.

Allelic losses of chromosome 22 found in oligodendrogliomas suggest that at least one tumor suppressor gene on chromosome 22 is inactivated during the multistep process of tumorigenesis in this glial tumor. INI1hSNF5 (HUGO symbol: SMARCB1), located at 22q11, encodes a component of the ATP-dependent chromatin remodeling hSWI-SNF complex; it is a tumor suppressor gene that is mutated in several malignant tumors. The PARVG gene, located at 22q13, has been found to exhibit reduced expression in some cancer lines. Both genes are thus candidate tumor suppressors, potentially involved in the pathogenesis of gliomas. We performed mutation analyses of INI1hSNF5 and PARVG in a series of 40 oligodendrogliomas, but only sequence polymorphic variations were identified. Accordingly, INI1hSNF5 and PARVG do not seem to be the tumor suppressor genes involved in oligodendroglioma development and progression.

Epithelioid sarcoma is a rare soft tissue neoplasm of uncertain lineage that usually arises in the distal extremities of adults, presents a high rate of recurrences and metastases and frequently poses diagnostic dilemmas. The recently reported large-cell "proximal-type" variant is characterized by increased aggressiveness, deep location, preferential occurrence in proximal/axial regions of older patients, and rhabdoid features. Previous cytogenetic studies indicated that the most frequent alterations associated with this tumor entity affect chromosome 22. In this study, combined spectral karyotyping, fluorescence in situ hybridization, and array-based comparative genomic hybridization analyses of two proximal-type cases harboring a rearrangement involving 10q26 and 22q11 revealed that the 22q11 breakpoints were located in a 150-kb region containing the SMARCB1/INI1 gene, and that homozygous deletion of the gene was present in the tumor tissue. The SMARCB1/INI1 gene encodes for an invariant subunit of SWI/SNF chromatin remodeling complex and has been previously reported to act as a tumor suppressor gene frequently inactivated in infantile malignant rhabdoid tumors. We analyzed SMARCB1/INI1 gene status in nine additional epithelioid sarcoma cases (four proximal types and five conventional types) and altogether we identified deletions of SMARCB1/INI1 gene in 5 of 11 cases, all proximal types. We confirmed and further extended the number of cases with SMARCB1/INI1 inactivation to 6 of 11 cases, by real-time quantitative PCR analysis of mRNA expression and by SMARCB1/INI1 immunohistochemistry. Overall, these results point to SMARCB1/INI1 gene involvement in the genesis and/or progression of epithelioid sarcomas. Analysis of larger series of epithelioid sarcomas will be necessary to highlight putative clinically relevant features related to SMARCB1/INI1 inactivation.

Malignant rhabdoid tumors (MRT) of the liver are rare. A few liver MRT cell lines have been established but none has been characterized in detail. Here we describe a new MRT cell line from the liver, which is designated MP-MRT-AN, and describe it in detail. Immunohistochemical assays detected the expression of vimentin and cytokeratin but they were negative for neurofilament, desmin, alpha-smooth muscle actin, alpha-sarcomeric actin, and smooth muscle myosin heavy chains SM1 and SM2. RT-PCR assays revealed that this cell line did not express smooth muscle myosin heavy chain isoforms or MyoD1. No aberration was identified in 22q by G-banded analysis; however, the hSNF5/INI1 gene, a suppressor gene of MRT that maps to 22q11.2, was homozygously deleted from exons 1 to 5 in this cell line. Furthermore, the expression of another tumor suppressor gene, p16 (CDKN2A), was not detected by RT-PCR. This raises the possibility that the aggressive phenotype of malignant rhabdoid tumors is caused by the loss of two or more tumor suppressor genes.

hSNF5/INI1, which encodes a component of the ATP-dependent chromatin remodeling hSWI-SNF complex, is a tumor suppressor gene mutated in malignant rhabdoid tumors. We have developed a tetracycline-based hSNF5/INI1-inducible system in a hSNF5/INI1-deficient malignant rhabdoid tumor cell line and studied time course variation of 22,000 genes/expressed sequence tags upon hSNF5/INI1 induction. A total of 482 responsive genes were identified and further clustered into 9 groups of coregulated genes. Among genes with early and strong inductions, the use of a fusion protein with the hormone-binding domain of the estrogen receptor enabled the identification of a subset of direct targets regulated independently of de novo protein synthesis. We show that the G(1) arrest induced by hSNF5/INI1 is reversible and associated with the down-regulation of components of the DNA replication complex. We also identify an unsuspected role of hSNF5/INI1 in cytoskeleton organization. Indeed, induction of hSNF5/INI1 induces dramatic modifications of the cell shape including complete disruption of the actin stress fiber network and disappearance of focal adhesions associated with up-regulation of genes involved in the organization of the actin cytoskeleton. We document a strong decrease of Rho activity upon hSNF5/INI1 expression, suggesting that the regulation of this activity constitutes a crucial step of the hSNF5/INI1-induced reorganization of the actin network. This study identifies hSNF5/INI1 target genes and provides evidence that hSNF5/INI1 may modulate the cell cycle control and cytoskeleton organization through the regulation of the retinoblastoma protein-E2F and Rho pathways.

The INI1/hSNF5 tumor suppressor is an integral component of mammalian SWI/SNF chromatin remodeling enzymes that contain SNF2 family ATPases BRM (Brahma) or BRG1 (Brahma Related Gene 1) and that contribute to the regulation of many genes. Genetic studies of yeast SWI/SNF enzyme revealed similar phenotypes when single or multiple components of the enzyme were deleted, indicating a requirement for each subunit. To address the contribution of INI1 in the regulation of SWI/SNF-dependent genes in mammalian cells, we examined the expression of multiple BRG1-dependent, constitutively expressed genes in INI1-deficient cancer cell lines. At least one INI1-deficient line expressed each gene, and reintroduction of INI1 had negligible effects on expression levels. Lack of INI1 also did not prevent interferon gamma (IFNgamma)-mediated induction of CIITA, which is BRG1 dependent, and GBP-1, which is BRG1 enhanced, and reintroduction of INI1 had minimal effects. Chromatin immunoprecipitation experiments revealed that BRG1 inducibly binds to the CIITA promoter despite the absence of INI1. Unlike yeast deleted for the INI1 homologue, SWI/SNF enzymes in INI1-deficient cells were largely intact. Thus in human cells, SWI/SNF enzyme complex formation and the expression of many BRG1-dependent genes are independent of INI1.CI - Copyright 2004 Nature Publishing Group

The human transforming acidic coiled-coil 2 (TACC2) gene has been suggested recently to be a putative breast tumor suppressor. Now we can report the cloning of full length TACC2 cDNAs corresponding to the major isoforms expressed during development. The TACC2 gene is encoded by 23 exons, and spans 255 kb of chromosome 10q26. In breast cancer cell lines, TACC2 is expressed as a 120 kDa protein corresponding to the major transcript expressed in the mammary gland. Although only slight differences in the expression of TACC2 in normal versus breast tumors were observed, overexpression of TACC2 can alter the in vitro cellular dynamics of some breast cancer cell lines. Significantly, we demonstrate that TACC2 interacts with GAS41 and the SWI/SNF chromatin remodeling complex. This suggests that defects in TACC2 expression may affect gene regulation, thus contributing to the pathogenesis of some tumors.

The INI1 gene, which encodes a functionally uncharacterized protein component of the hSWI/SNF chromatin remodeling complex, is often mutated or deleted in malignant rhabdoid tumor (MRT). Two isoforms of INI1, that differ by the variable inclusion of nine amino acids, potentially are produced by differential RNA splicing. To determine the effect of the two INI1 isoforms on cell growth, INI1-devoid (MRT) and INI1-expressing cell lines were transfected separately with mammalian expression vectors or transduced with adenoviruses. Transfection of the short form of INI1 into either INI1-deficient or expressing cell lines resulted in complete suppression of cell growth in colony formation assays. The longer splice variant induced moderate to severe growth suppression of MRT cells, but had a far milder effect on non-MRT cells. Transduction of MRT cells with adenoviruses expressing either isoform of INI1 led to a dramatic change in morphology, growth suppression, and cell cycle arrest. Furthermore, senescence-associated proteins were up-regulated after transduction, while levels of proteins implicated in cell cycle progression were down-regulated. Adenoviral delivery of INI1 into a non-MRT cell line, however, had no demonstrable effect on any of these parameters. These results support the genetic evidence that INI1 is a tumor suppressor gene gone awry in MRT cells, and also suggest that delivery of the INI1 gene to MRT cells by adenoviruses may lead to a more effective treatment of this highly aggressive malignancy.CI - Copyright 2003 Wiley-Liss, Inc.

INI1/hSNF5 is a component of the ATP-dependent chromatin remodeling hSWI/SNF complex and a tumor suppressor gene of aggressive pediatric atypical teratoid and malignant rhabdoid tumors (AT/RT). To understand the molecular mechanisms underlying its tumor suppressor function, we studied the effect of reintroduction of INI1/hSNF5 into AT/RT-derived cell lines such as MON that carry biallelic deletions of the INI1/hSNF5 locus. We demonstrate that expression of INI1/hSNF5 causes G(0)-G(1) arrest and flat cell formation in these cells. In addition, INI1/hSNF5 repressed transcription of cyclin D1 gene in MON, in a histone deacetylase (HDAC)-dependent manner. Chromatin immunoprecipitation studies revealed that INI1/hSNF5 was directly recruited to the cyclin D1 promoter and that its binding correlated with recruitment of HDAC1 and deacetylation of histones at the promoter. Analysis of INI1/hSNF5 truncations indicated that cyclin D1 repression and flat cell formation are tightly correlated. Coexpression of cyclin D1 from a heterologous promoter in MON was sufficient to eliminate the INI1-mediated flat cell formation and cell cycle arrest. Furthermore, cyclin D1 was overexpressed in AT/RT tumors. Our data suggest that one of the mechanisms by which INI1/hSNF5 exerts its tumor suppressor function is by mediating the cell cycle arrest due to the direct recruitment of HDAC activity to the cyclin D1 promoter thereby causing its repression and G(0)-G(1) arrest. Repression of cyclin D1 gene expression may serve as a useful strategy to treat AT/RT.

The SWI/SNF complex is required for the transcription of several genes and has been shown to alter nucleosome structure in an ATP-dependent manner. The tumor suppressor protein p53 displays growth and transformation suppression functions that are frequently lost in mutant p53 proteins detected in various cancers. Using genetic and biochemical approaches, we show that several subunits of the human SWI/SNF complex bind to the tumor suppressor protein p53 in vivo and in vitro. The transactivation function of p53 is stimulated by overexpression of hSNF5 and BRG-1 and dominant forms of hSNF5 and BRG-1 repress p53-dependent transcription. Chromatin immunoprecipitation assay shows that hSNF5 and BRG-1 are recruited to a p53-dependent promoter in vivo. Overexpression of dominant negative forms of either hSNF5 or BRG-1 inhibited p53-mediated cell growth suppression and apoptosis. Molecular connection between p53 and the SWI/SNF complex implicates that (i) the SWI/SNF complex is necessary for p53-driven transcriptional activation, and (ii) the SWI/SNF complex plays an important role in p53-mediated cell cycle control.

Sin3 is an evolutionarily conserved corepressor that exists in different complexes with the histone deacetylases HDAC1 and HDAC2. Sin3-HDAC complexes are believed to deacetylate nucleosomes in the vicinity of Sin3-regulated promoters, resulting in a repressed chromatin structure. We have previously found that a human Sin3-HDAC complex includes HDAC1 and HDAC2, the histone-binding proteins RbAp46 and RbAp48, and two novel polypeptides SAP30 and SAP18. SAP30 is a specific component of Sin3 complexes since it is absent in other HDAC1/2-containing complexes such as NuRD. SAP30 mediates interactions with different polypeptides providing specificity to Sin3 complexes. We have identified p33ING1b, a negative growth regulator involved in the p53 pathway, as a SAP30-associated protein. Two distinct Sin3-p33ING1b-containing complexes were isolated, one of which associates with the subunits of the Brg1-based Swi/Snf chromatin remodeling complex. The N terminus of p33ING1b, which is divergent among a family of ING1 polypeptides, associates with the Sin3 complex through direct interaction with SAP30. The N-terminal domain of p33 is present in several uncharacterized human proteins. We show that overexpression of p33ING1b suppresses cell growth in a manner dependent on the intact Sin3-HDAC-interacting domain.

Aberrant activation of the Hedgehog (Hh) pathway can drive tumorigenesis. To investigate the mechanism by which glioma-associated oncogene family zinc finger-1 (GLI1), a crucial effector of Hh signaling, regulates Hh pathway activation, we searched for GLI1-interacting proteins. We report that the chromatin remodeling protein SNF5 (encoded by SMARCB1, hereafter called SNF5), which is inactivated in human malignant rhabdoid tumors (MRTs), interacts with GLI1. We show that Snf5 localizes to Gli1-regulated promoters and that loss of Snf5 leads to activation of the Hh-Gli pathway. Conversely, re-expression of SNF5 in MRT cells represses GLI1. Consistent with this, we show the presence of a Hh-Gli-activated gene expression profile in primary MRTs and show that GLI1 drives the growth of SNF5-deficient MRT cells in vitro and in vivo. Therefore, our studies reveal that SNF5 is a key mediator of Hh signaling and that aberrant activation of GLI1 is a previously undescribed targetable mechanism contributing to the growth of MRT cells.

Epigenetic alterations have been increasingly implicated in oncogenesis. Analysis of Drosophila mutants suggests that Polycomb and SWI/SNF complexes can serve antagonistic developmental roles. However, the relevance of this relationship to human disease is unclear. Here, we have investigated functional relationships between these epigenetic regulators in oncogenic transformation. Mechanistically, we show that loss of the SNF5 tumor suppressor leads to elevated expression of the Polycomb gene EZH2 and that Polycomb targets are broadly H3K27-trimethylated and repressed in SNF5-deficient fibroblasts and cancers. Further, we show antagonism between SNF5 and EZH2 in the regulation of stem cell-associated programs and that Snf5 loss activates those programs. Finally, using conditional mouse models, we show that inactivation of Ezh2 blocks tumor formation driven by Snf5 loss.CI - Copyright (c) 2010 Elsevier Inc. All rights reserved.

Alterations in chromatin play an important role in oncogenic transformation, although the underlying mechanisms are often poorly understood. The SWI/SNF complex contributes to epigenetic regulation by using the energy of ATP hydrolysis to remodel chromatin and thus regulate transcription of target genes. SNF5, a core subunit of the SWI/SNF complex, is a potent tumor suppressor that is specifically inactivated in several types of human cancer. However, the mechanism by which SNF5 mutation leads to cancer and the role of SNF5 within the SWI/SNF complex remain largely unknown. It has been hypothesized that oncogenesis in the absence of SNF5 occurs due to a loss of function of the SWI/SNF complex. Here, we show, however, distinct effects for inactivation of Snf5 and the ATPase subunit Brg1 in primary cells. Further, using both human cell lines and mouse models, we show that cancer formation in the absence of SNF5 does not result from SWI/SNF inactivation but rather that oncogenesis is dependent on continued presence of BRG1. Collectively, our results show that cancer formation in the absence of SNF5 is dependent on the activity of the residual BRG1-containing SWI/SNF complex. These findings suggest that, much like the concept of oncogene addiction, targeted inhibition of SWI/SNF ATPase activity may be an effective therapeutic approach for aggressive SNF5-deficient human tumors.

Snf5 (Ini1/Baf47/Smarcb1), a core member of the Swi/Snf chromatin remodeling complex, is a potent tumor suppressor whose mechanism of action is largely unknown. Biallelic loss of Snf5 leads to the onset of aggressive cancers in both humans and mice. We have developed an innovative and widely applicable analytical technique for cross-species validation of cancer models and show that the gene expression profiles of our Snf5 murine models closely resemble those of human Snf5-deficient rhabdoid tumors. We exploit this system to produce what we believe to be the first report documenting the effects on gene expression of inactivating a Swi/Snf subunit in normal mammalian cells and to identify the transcriptional pathways regulated by Snf5. We demonstrate that the tumor suppressor activity of Snf5 depends on its regulation of cell cycle progression; Snf5 inactivation leads to aberrant up-regulation of E2F targets and increased levels of p53 that are accompanied by apoptosis, polyploidy, and growth arrest. Further, conditional mouse models demonstrate that inactivation of p16Ink4a or Rb (retinoblastoma) does not accelerate tumor formation in Snf5 conditional mice, whereas mutation of p53 leads to a dramatic acceleration of tumor formation.

The gene encoding INI1, a component of the mammalian SWI/SNF ATP-dependent chromatin remodeling enzymes, has been classified as a tumor suppressor in humans. Gene-targeting experiments confirmed that Ini1 also functions as a tumor suppressor in mice. Although Ini1-null mice are embryonic lethal, 15-30% of mice heterozygous for Ini1 presented with poorly differentiated tumors with variable rhabdoid features. All tumors examined showed loss of heterozygosity at the Ini1 locus. We report here that cells and tissues heterozygous for the Ini1 tumor suppressor express levels of Ini1 protein and message roughly equivalent to the levels observed in wild type counterparts. Compensation of Ini1 is mediated by an increase in the rate of transcription from the Ini1 promoter. Moreover, when Ini1 is expressed exogenously, transcription from the endogenous promoter is reduced, suggesting that Ini1 levels are tightly regulated. This is the first report describing transcriptional compensation for haploinsufficiency of a tumor suppressor gene.

Recent data suggest the SWI/SNF chromatin remodeling complex may also act as a tumor suppressor. Utilizing a reversibly inactivating conditional allele, we demonstrate that loss of Snf5/Ini1/Baf47/SmarcB1, a core subunit of SWI/SNF, results in highly penetrant cancer predisposition with 100% of mice developing mature CD8(+) T cell lymphoma or rare rhabdoid tumors with a median onset of only 11 weeks. Notably, while loss of Snf5 predisposes to aggressive cancers, it is also required for survival of virtually all nonmalignant cells in vivo. Reversible gene targeting demonstrates a critical and specific role for Snf5 in tumor suppression, provides a novel system in which to explore the genetic pathways involved in tumor suppression by Swi/Snf, and should be of wide use in evaluating other essential tumor suppressor genes.