cyclin-d1 and Rhabdoid-Tumor

SMARCB1 (INI1/SNF5/BAF47), a core subunit of the SWI/SNF (BAF) chromatin-remodeling complex, is inactivated in the large majority of rhabdoid tumors, and germline heterozygous SMARCB1 mutations form the basis for rhabdoid predisposition syndrome. Mouse models validated Smarcb1 as a bona fide tumor suppressor, as Smarcb1 inactivation in mice results in 100% of the animals rapidly developing cancer. SMARCB1 was the first subunit of the SWI/SNF complex found mutated in cancer. More recently, at least seven other genes encoding SWI/SNF subunits have been identified as recurrently mutated in cancer. Collectively, 20% of all human cancers contain a SWI/SNF mutation. Consequently, investigation of the mechanisms by which SMARCB1 mutation causes cancer has relevance not only for rhabdoid tumors, but also potentially for the wide variety of SWI/SNF mutant cancers. Here we discuss normal functions of SMARCB1 and the SWI/SNF complex as well as mechanistic and potentially therapeutic insights that have emerged.

Topics: Animals; Chromatin Assembly and Disassembly; Chromosomal Proteins, Non-Histone; Cyclin D1; Cyclin-Dependent Kinase 4; DNA-Binding Proteins; Enzyme Activation; Humans; Mice; Mutation; Nucleosomes; Rhabdoid Tumor; SMARCB1 Protein; Transcription Factors; Tumor Suppressor Proteins

Clear cell sarcoma of kidney (CCSK) is the second most common pediatric renal malignancy, constituting ∼3% of renal tumors. Due to its morphologic diversity, the diagnosis of CCSK is often challenging. Recent studies have identified internal tandem duplication of BCL6 corepressor (BCOR) gene in CCSKs which coupled with cyclin D1 immunoreactivity, is helpful in differentiating it from its mimics, particularly blastema-rich Wilms tumor (WT), malignant rhabdoid tumor (MRT), and congenital mesoblastic nephroma (CMN). We aimed to evaluate the utility of cyclin D1 and BCOR immunohistochemistry in differentiating CCSK from its morphologic mimics.. Our cohort comprised of 38 pediatric renal tumors which included CCSK (n=18), WT (n=10), MRT (n=5), and CMN (n=5) cases. A detailed clinicopathologic analysis was performed, and tissue microarray were constructed for CCSK and WT, while MRT and CMN tumors were individually stained.. The age ranged from 2 months to 16 years with male:female ratio of 3:1. Strong, diffuse nuclear immunoreactivity for cyclin D1 and BCOR was noted in 61% (n=11/18) and 83% (n=15/18) of CCSK, respectively, while it was significantly less in WT (n=3/10 for cyclin D1) (n=2/10 for BCOR). None of the MRT and CMN examples demonstrated any immunoreactivity. Interestingly, only the blastemal component of WTs showed distinct, rare nuclear immunoreactivity for cyclin D1 or BCOR and the combination of these was never positive in a given case.. Our results provide evidence that concurrent immunopositivity with cyclin D1 and BCOR is helpful in distinguishing CCSK from its morphologic mimics.

Topics: Adolescent; Biomarkers, Tumor; Child; Child, Preschool; Cyclin D1; Diagnosis, Differential; Female; Follow-Up Studies; Humans; Immunohistochemistry; Infant; Kidney Neoplasms; Male; Nephroma, Mesoblastic; Prognosis; Proto-Oncogene Proteins; Repressor Proteins; Rhabdoid Tumor; Sarcoma, Clear Cell; Wilms Tumor

Atypical teratoid rhabdoid tumor (ATRT) is a fatal pediatric malignancy of the central neural system lacking effective treatment options. It belongs to the rhabdoid tumor family and is usually caused by biallelic inactivation of SMARCB1, encoding a key subunit of SWI/SNF chromatin remodeling complexes. Previous studies proposed that SMARCB1 loss drives rhabdoid tumor by promoting cell cycle through activating transcription of cyclin D1 while suppressing p16. However, low cyclin D1 protein expression is observed in most ATRT patient tumors. The underlying mechanism and therapeutic implication of this molecular trait remain unknown. Here, we show that SMARCB1 loss in ATRT leads to the reduction of cyclin D1 expression by upregulating MIR17HG, a microRNA (miRNA) cluster known to generate multiple miRNAs targeting CCND1. Furthermore, we find that this cyclin D1 deficiency in ATRT results in marked in vitro and in vivo sensitivity to the CDK4/6 inhibitor palbociclib as a single agent. Our study identifies a novel genetic interaction between SMARCB1 and MIR17HG in regulating cyclin D1 in ATRT and suggests a rationale to treat ATRT patients with FDA-approved CDK4/6 inhibitors. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Topics: Cell Line, Tumor; Cell Survival; Cyclin D1; Gene Expression Regulation, Neoplastic; Humans; Proteins; Rhabdoid Tumor; SMARCB1 Protein; Teratoma; Up-Regulation

Distinguishing clear cell sarcoma of the kidney (CCSK) from other paediatric malignancies, particularly blastema-rich Wilms tumour (WT) and congenital mesoblastic nephroma (CMN), is challenging. Specific immunohistochemistry for CCSK does not exist, and diagnosis rests upon histopa thology. Recently, the YWHAE-FAM22 rearrange ment, identical to that in endometrial stromal sarcoma (ESS), has been identified in CCSKs. As this fusion results in overexpression of cyclin D1 in ESS, we postulated that overexpression would also occur in CCSK; cyclin D1 immunohistochemistry could then be used to differentiate CCSK from other tumours. The goal of this study was therefore to evaluate the utility of cyclin D1 immunohistochemistry in identifying CCSK and helping to differentiate it from its mimics.. Cyclin D1 expression was evaluated in 59 renal tumours-CCSK (14), WT (25), rhabdoid tumour (four), Ewing sarcoma (five), and CMN (11)-and four neuroblastomas. All 14 CCSKs showed diffuse and strong reactivity. In contrast, the blastematous component of most WTs showed only rare positive nuclei, that of rhabdoid tumours showed rare to focal immunoreactivity, and that of more than half of CMNs showed weak or focal immunoreactivity. Most Ewing sarcomas and all neuroblastomas showed diffuse moderate to strong staining.. Cyclin D1 is most helpful in distinguishing CCSK from WT, rhabdoid tumour, and some CMNs, but not from neuroblastoma or Ewing sarcomas.

Topics: Biomarkers, Tumor; Child, Preschool; Cyclin D1; Diagnosis, Differential; Female; Humans; Immunohistochemistry; Kidney Neoplasms; Male; Nephroma, Mesoblastic; Neuroblastoma; Rhabdoid Tumor; Sarcoma, Clear Cell; Sarcoma, Ewing; Wilms Tumor

Rhabdoid tumors (RTs) are rare, highly aggressive pediatric malignancies with poor prognosis and with no standard or effective treatment strategies. RTs are characterized by biallelic inactivation of the INI1 tumor suppressor gene. INI1 directly represses CCND1 and activates cyclin-dependent kinase (cdk) inhibitors p16(Ink4a) and p21(CIP). RTs are exquisitely dependent on cyclin D1 for genesis and survival. To facilitate translation of unique therapeutic strategies, we have used genetically engineered, Ini1(+/-) mice for therapeutic testing. We found that PET can be used to noninvasively and accurately detect primary tumors in Ini1(+/-) mice. In a PET-guided longitudinal study, we found that treating Ini1(+/-) mice bearing primary tumors with the pan-cdk inhibitor flavopiridol resulted in complete and stable regression of some tumors. Other tumors showed resistance to flavopiridol, and one of the resistant tumors overexpressed cyclin D1, more than flavopiridol-sensitive cells. The concentration of flavopiridol used was not sufficient to down-modulate the high level of cyclin D1 and failed to induce cell death in the resistant cells. Furthermore, FISH and PCR analyses indicated that there is aneuploidy and increased CCND1 copy number in resistant cells. These studies indicate that resistance to flavopiridol may be correlated to elevated cyclin D1 levels. Our studies also indicate that Ini1(+/-) mice are valuable tools for testing unique therapeutic strategies and for understanding mechanisms of drug resistance in tumors that arise owing to loss of Ini1, which is essential for developing effective treatment strategies against these aggressive tumors.

Topics: Animals; Cell Line, Tumor; Chromosomal Proteins, Non-Histone; Cyclin D1; Cyclin-Dependent Kinase Inhibitor p16; Cyclin-Dependent Kinase Inhibitor p21; DNA Primers; Drug Resistance, Neoplasm; Flavonoids; Gene Expression Regulation, Neoplastic; Gene Silencing; Histological Techniques; Immunoblotting; Immunohistochemistry; In Situ Hybridization, Fluorescence; Longitudinal Studies; Mice; Mice, Inbred C57BL; Mice, Transgenic; Microscopy, Electron, Transmission; Piperidines; Polymerase Chain Reaction; Positron-Emission Tomography; Rhabdoid Tumor; SMARCB1 Protein

Topics: Chromatin Assembly and Disassembly; Chromosomal Proteins, Non-Histone; Cyclin D1; DNA-Binding Proteins; Gene Expression Regulation, Neoplastic; Humans; Models, Biological; Receptors, G-Protein-Coupled; Rhabdoid Tumor; Signal Transduction; SMARCB1 Protein; Smoothened Receptor; Transcription Factors; Zinc Finger Protein GLI1

Rhabdoid tumors (RTs) are an extremely aggressive pediatric malignancy that results from loss of the INI1/hSNF5 tumor suppressor gene. Loss of INI1 results in aberrant expression of Cyclin D1, which supports rhabdoid tumorigenesis and survival. 4-HPR, a synthetic retinoid that down-modulates Cyclin D1, has shown promise in treating various tumors including RTs. In this study, we have generated a chemical library of peptidomimetic derivatives of 4-HPR in an attempt to create a more biologically active compound for use as a therapeutic agent against RTs and other tumors. We have synthesized novel peptidomimetic compound by substituting alkene backbone with a ring structure that retains the biological activity in cell culture models of rhabdoid tumors. We further identified derivative of peptidomimetic compound (11d, IC(50) approximately 3 microM) with approximately five times higher potency than 4-HPR (1, IC(50) approximately 15 microM) based on a survival assay against rhabdoid tumor cells. These studies indicate that peptidomimetic derivatives that retain the cytotoxic activity are promising novel chemotherapeutic agents against RTs and other tumors.

Topics: Alkenes; Cell Cycle; Cell Line, Tumor; Cell Proliferation; Chemistry, Pharmaceutical; Cyclin D1; Drug Design; Fenretinide; Humans; Inhibitory Concentration 50; Models, Chemical; Models, Statistical; Peptides; 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.

Topics: Animals; Cell Line, Tumor; Chromosomal Proteins, Non-Histone; Cisplatin; Cyclin D1; Diploidy; DNA Damage; DNA Repair; DNA-Binding Proteins; Epigenesis, Genetic; Etoposide; G2 Phase; Genomic Instability; Histones; Humans; Mice; Neoplasms; Phenotype; Polymorphism, Single Nucleotide; Protein Transport; Rhabdoid Tumor; SMARCB1 Protein; Transcription Factors

Rhabdoid tumors are aggressive and incurable pediatric malignancies. INI1/hSNF5, a tumor suppressor biallelically deleted/inactivated in rhabdoid tumors, directly represses cyclin D1. Rhabdoid tumors and cells are exquisitely dependent on cyclin D1 for genesis and survival, suggesting that targeting the cyclin/cyclin-dependent kinase (cdk) axis may be an effective therapeutic strategy for these tumors. Because cdk inhibitors have not been used for preclinical or clinical testing on rhabdoid tumors, we investigated the effect of flavopiridol, a pan-cdk inhibitor with promising clinical activity, on rhabdoid tumors.. The effect of flavopiridol on rhabdoid cells was tested in vitro using survival, cell cycle, and apoptosis assays. Its effect was assessed in vivo using xenografted rhabdoid tumor models. Immunoblot and immunohistochemical analysis was used to assess the effect of flavopiridol on cyclin D1 and p21 expression in vitro and in vivo, respectively.. Nanomolar concentrations of flavopiridol inhibited rhabdoid cell growth (IC(50) approximately 200 nmol/L), induced G(1) and G(2) arrest, and apoptosis in vitro in a concentration-dependent manner. These effects were correlated with the down-modulation of cyclin D1, up-regulation of p21, and induction of caspase 3/7 activities. Flavopiridol (at 7.5 mg/kg) significantly inhibited the growth of xenografted rhabdoid tumors, and its effect was correlated with the induction of p21 and down-modulation of cyclin D1.. Flavopiridol is effective in inducing cell cycle arrest and cytotoxicity in rhabdoid tumors. Its effects are correlated with the down-regulation of cyclin D1 and the up-regulation of p21. Flavopiridol is potentially a novel chemotherapeutic agent for rhabdoid tumors.

Topics: Animals; Antineoplastic Agents; Apoptosis; Caspases; Cell Line, Tumor; Cell Proliferation; Cyclin D1; Cyclin-Dependent Kinase Inhibitor p21; Cyclin-Dependent Kinases; Drug Screening Assays, Antitumor; Flavonoids; Humans; Mice; Mice, SCID; Neoplasm Transplantation; Piperidines; Protein Kinase Inhibitors; Rhabdoid Tumor; Xenograft Model Antitumor Assays

Rhabdoid tumors (RTs) are aggressive and currently incurable pediatric malignancies. INI1/hSNF5 is a tumor suppressor biallelically inactivated in RTs. Our previous studies have indicated that cyclin D1 is a key downstream target of INI1/hSNF5 and genesis and/or survival of RTs in vivo is critically dependent on the presence of cyclin D1. In this report, we have tested the hypothesis that therapeutic targeting of cyclin D1 is an effective means of treating RTs. We found that RNA interference of cyclin D1 in rhabdoid cells was sufficient to induce G1 arrest and apoptosis. Furthermore, we found that pharmacological intervention with low micromolar concentrations of N-(4-hydroxyphenyl)retinamide (4-HPR), which downmodulates cyclin D1, induced G1 arrest and apoptosis in rhabdoid cell lines. 4-HPR in combination with 4-hydroxy-tamoxifen (4OH-Tam), synergistically inhibited survival as well as anchorage-dependent and -independent growth of rhabdoid cells and caused synergistic induction of cell cycle arrest and apoptosis. 4-HPR and tamoxifen exhibited synergistic growth inhibition of RTs in xenograft models in vivo. The effects of combination of drugs were correlated to the depletion of cyclin D1 levels both in in vitro and in vivo tumor models. These results demonstrate that 4-HPR and tamoxifen are effective chemotherapeutic agents for RTs. We propose that downmodulation of cyclin D1 is a novel and effective therapeutic strategy for RTs.

Topics: Animals; Cell Proliferation; Chromosomal Proteins, Non-Histone; Cyclin D1; DNA-Binding Proteins; Drug Synergism; Fenretinide; Humans; Mice; Rhabdoid Tumor; RNA, Small Interfering; SMARCB1 Protein; Tamoxifen; Transcription Factors

SNF5 is a core subunit of the SWI/SNF chromatin-remodeling complex. Mammalian SNF5 is essential for normal cell viability, and loss or mutation of the human SNF gene is the molecular basis for familial malignant rhabdoid tumorigenesis. Previous studies have suggested that SNF5 suppresses cancer by signaling through the p16Ink4a and retinoblastoma tumor suppressors to negatively regulate cell cycle progression from G0/G1 into S phase. A recent paper in Genes & Development (Vries et al., 2005) reports that human SNF5 also signals via the p16INK4a-Rb-E2F pathway to regulate chromosomal stability, suggesting a new function for this chromatin remodeling protein in tumor suppression.

Topics: Aneuploidy; Animals; Cell Cycle; Cell Cycle Proteins; Chromatin Assembly and Disassembly; Chromosomal Instability; Chromosomal Proteins, Non-Histone; Cyclin D1; Cyclin-Dependent Kinase 4; Cyclin-Dependent Kinase Inhibitor p16; Cyclin-Dependent Kinases; DNA-Binding Proteins; E2F Transcription Factors; Humans; Models, Biological; Phosphorylation; Polyploidy; Proto-Oncogene Proteins; Retinoblastoma Protein; Rhabdoid Tumor; SMARCB1 Protein; Transcription Factors; Tumor Suppressor Proteins

Although atypical teratoid/rhabdoid tumor (AT/RT) is known to generate through inactivation of the hSNF5/INI1 gene on chromosome 22q, the downstream molecular mechanism remains unclear. We histologically and molecularly reviewed our pediatric brain tumors for unrecognized AT/RTs and evaluated the role of cyclin D1, a potential molecular target of hSNF5/INI1.. We analyzed 16 tumors under three years of age: seven medulloblastomas, three anaplastic ependymomas (E IIIs), two each of supratentorial primitive neuroectodermal tumors (sPNETs) and choroid plexus carcinomas (CPCs), and one each of neuroblastoma and pineoblastoma. Immunohistochemistry for glial fibrillary acidic protein, vimentin, epithelial membrane antigen, smooth muscle actin and cyclin D1 was performed. Polymerase chain reaction (PCR)-single-strand conformation polymorphism analysis with direct sequencing, differential PCR and microsatellite analysis were conducted for hSNF5/INI1mutation, homozygous deletion and loss of heterozygosity (LOH) on 22q, respectively. Because of the presence of rhabdoid cells and the polyimmunophenotypic features, the diagnosis was revised to AT/RT in five (31%) tumors, namely, two E IIIs and one each of medulloblastoma, CPC and pineoblastoma. Three of them harbored such hSNF5/INI1 aberrations as germline single base deletion (492/6 delC) and missense mutation (C157T) together with LOH 22q or homozygous deletion. Cyclin D1 was overexpressed in those three tumors but not in the two that lacked hSNF5/INI1 inactivation.. AT/RT can be misdiagnosed as a variety of tumors, including ependymoma that potentially harbors LOH 22q. Our data indicate that cyclin D1 is a target of hSNF5/INI1in primary tumors.

Topics: Central Nervous System Neoplasms; Child, Preschool; Choroid Plexus Neoplasms; Chromosomal Proteins, Non-Histone; Chromosomes, Human, Pair 22; Cyclin D1; Cytogenetic Analysis; DNA-Binding Proteins; Female; Humans; Infant; Loss of Heterozygosity; Male; Medulloblastoma; Mutation; Neuroblastoma; Pinealoma; Polymorphism, Single Nucleotide; Rhabdoid Tumor; SMARCB1 Protein; Teratoma; Transcription Factors

Rhabdoid tumors are aggressive pediatric malignancies for which, currently, there are no effective or standard treatment strategies. Rhabdoid tumors arise because of the loss of the tumor suppressor gene INI1. We have previously demonstrated that INI1 represses Cyclin D1 transcription in rhabdoid cells by directly recruiting histone deacetylase 1 complex to its promoter, leading to G(0)-G(1) arrest. Expression of Cyclin D1 overcomes cell cycle arrest mediated by INI1 and Cyclin D1 overexpression in human rhabdoid tumors is a common phenomenon. However, it is not clear whether Cyclin D1 is a critical downstream target of INI1 in vivo and whether the derepression of this gene is essential for rhabdoid tumorigenesis. To determine the requirement of Cyclin D1 for genesis of rhabdoid tumors in vivo, we developed Ini1 heterozygous mice by targeted disruption. We found that the tumors developed in these Ini1+/- mice are rhabdoid, defective for Ini1 protein, and like the human tumors, express Cyclin D1. We crossed Ini1+/- mice to Cyclin D1-/- mice and found that Ini1+/- mice with Cyclin D1 deficiency did not develop any spontaneous tumors, in contrast to the parental Ini1+/- mice. These results strongly support the hypothesis that Cyclin D1 is a key mediator in the genesis of rhabdoid tumors. Our results provide an in vivo proof of concept that drugs that target Cyclin D1 expression or activity could be potentially effective as novel therapeutic agents for rhabdoid tumors.

Topics: Animals; Blotting, Western; Cell Cycle; Chromosomal Proteins, Non-Histone; Crosses, Genetic; Cyclin D1; DNA Primers; DNA-Binding Proteins; Gene Deletion; Gene Expression Regulation, Neoplastic; Gene Targeting; Immunohistochemistry; Mice; Mice, Mutant Strains; Reverse Transcriptase Polymerase Chain Reaction; Rhabdoid Tumor; SMARCB1 Protein; Transcription Factors

The hSNF5 chromatin-remodeling factor is a tumor suppressor that is inactivated in malignant rhabdoid tumors (MRTs). A number of studies have shown that hSNF5 re-expression blocks MRT cell proliferation. However, the pathway through which hSNF5 acts remains unknown. To address this question, we generated MRT-derived cell lines in which restoration of hSNF5 expression leads to an accumulation in G(0)/G(1), induces cellular senescence and increased apoptosis. Following hSNF5 expression, we observed transcriptional activation of the tumor suppressor p16(INK4a) but not of p14(ARF), repression of several cyclins and CD44, a cell surface glycoprotein implicated in metastasis. Chromatin immunoprecipitations indicated that hSNF5 activates p16(INK4a) transcription and CD44 down-regulation by mediating recruitment of the SWI/SNF complex. Thus, hSNF5 acts as a dualistic co-regulator that, depending on the promoter context, can either mediate activation or repression. Three lines of evidence established that p16(INK4a) is an essential effector of hSNF5-induced cell cycle arrest. 1) Overexpression of p16(INK4a) mimics the effect of hSNF5 induction and leads to cellular senescence. 2) Expression of a p16(INK4a)-insensitive form of CDK4 obstructs hSNF5-induced cell cycle arrest. 3) Inhibition of p16(INK4a) activation by siRNA blocks hSNF5-mediated cellular senescence. Collectively, these results indicate that in human MRT cells, the p16(INK4a)/pRb, rather than the p14(ARF)/p53 pathway, mediates hSNF5-induced cellular senescence.

Topics: Apoptosis; Cell Cycle; Cell Division; Cell Line, Tumor; Cellular Senescence; Chromatin; Chromosomal Proteins, Non-Histone; Cyclin D1; Cyclin-Dependent Kinase Inhibitor p16; DNA-Binding Proteins; Down-Regulation; Electrophoresis, Polyacrylamide Gel; G1 Phase; HeLa Cells; Humans; Hyaluronan Receptors; Immunoblotting; Lentivirus; Neoplasm Metastasis; Precipitin Tests; Promoter Regions, Genetic; Resting Phase, Cell Cycle; Retinoblastoma Protein; Rhabdoid Tumor; RNA, Messenger; RNA, Small Interfering; SMARCB1 Protein; Time Factors; Transcription Factors; Transcriptional Activation; Tumor Suppressor Protein p14ARF; Tumor Suppressor Protein p53

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.

Topics: Cell Cycle; Cell Separation; Cell Size; Chromosomal Proteins, Non-Histone; Cyclin D1; DNA-Binding Proteins; Flow Cytometry; Gene Expression Regulation; Histone Deacetylase 1; Histone Deacetylases; Humans; Recombinant Fusion Proteins; Rhabdoid Tumor; SMARCB1 Protein; Transcription Factors; Transcription, Genetic; Tumor Cells, Cultured