The mammalian SWI/SNF complexes mediate ATPdependent chromatin remodeling processes that are critical for differentiation and proliferation. Not surprisingly, loss of SWI/SNF function has been associated with malignant transformation, and a substantial body of evidence indicates that several components of the SWI/SNF complexes function as tumor suppressors. This review summarizes the evidence that underlies this conclusion, with particular emphasis upon the two catalytic subunits of the SWI/SNF complexes, BRM, the mammalian ortholog of SWI2/SNF2 in yeast and brahma in Drosophila, and Brahma-related gene-1 (BRG1).

The Epstein-Barr viral (EBV) genome of -170 kilobase pairs (kbp) is maintained as a plasmid in human B lymphoblasts transformed by the virus. We have identified a cis-acting element within 1.8 kbp of the viral genome that allows recombinant plasmids carrying it to be selected at high frequency and maintained as plasmids in cells latently infected by EBV. This functional element(s) requires a segment of DNA at least 800 bp and at most 1800 bp long, which contains a family of 30-bp tandem repeats at one end. Since this region confers efficient stable replication only to plasmids transfected into cells containing EBV genomes, its function probably requires trans-acting products encoded elsewhere in the viral genome.Epstein-Barr virus (EBV) is a human lymphotropic herpesvirus that causes infectious mononucleosis and lymphoproliferative disorders in immunosuppressed individuals. It is also probably a causative agent of Burkitt lymphoma and nasopharyngeal carcinoma in some parts of the world (for a review, see ref. 1). The causal association of EBV to diseases is likely a result of its capacity to induce unlimited proliferation in infected cells (2). Progress in understanding cell transformation by EBV has been limited by its large genome [172 kilobase pairs (kbp)], the likelihood that more than one gene is required for transformation (3), and the difficulty of introducing DNA stably into lymphocytes.Our approach in studying transformation by EBV has been to seek to define the genomic location of functions required for viral DNA replication in transformed cells. Since the viral genome is maintained as an independently replicating plasmid (4) in most cells transformed in vivo or in vitro (4-7), plasmid replication functions are probably required for efficient transformation. In addition, mapping the location of viral replication functions would allow the construction of a small EBV plasmid replicon suitable for the manipulations of molecular cloning, which in turn could aid in locating the remaining genes involved in transformation. Using a method described below, we scanned the EBV genome for cis-acting functions that would permit stable plasmid replication, and we found one such element in the short unique region of the genome. MATERIALS AND METHODSCell Lines. D98/Raji Cl 5 and D98/HR1 Cl 1 are hybrids between the human epithelial cell line D98 and the EBV genome-positive lymphoma lines Raji and P3JHR1, respectively (8,9). Both contain multiple copies of the EBV genome (ref. 10; unpublished results). These lines were grown in RPMI 1640 medium except during transfections, when they were in Dulbecco's modified Eagle's medium.Human fibrosarcoma 143 cells (11) and mouse BALB 3T3 fibroblasts (12) were grown in Dulbecco's modified Eagle's medium. Both media were supplemented with 10% fetal bovine serum.Transfections. About 1 x 106 cells in 6-cm dishes were transfected without carrier DNA, essentially as described by Graham and Van der Eb (13), with plasmid DNAs at 10 ,g/ml in Hepes-buffered saline (14). After 4 to ...

A genetic element of Epstein-Barr virus, oriP, when present on recombinant plasmids allows those plasmids to replicate and to be maintained in cells that express the Epstein-Barr virus-encoded nuclear antigen EBNA-1.Here we define the DNA sequences required for oriP activity. Two noncontiguous regions of oriP are required in cis for activity. One consists of approximately 20 tandem, imperfect copies of a 30-base-pair (bp) sequence. The other required region, approximately 1,000 bp away, is at most 114 bp in length and contains a 65-bp region of dyad symmetry. When present together on a plasmid, these two components supported plasmid replication even when the distance between them was varied or their relative orientation was altered, or both. When present alone on a plasmid that expresses a selectable marker, the family of 30-bp repeats efficiently conferred a transient drug-resistant phenotype in human 143 cells that is dependent on the presence of EBNA-1. This result leads us to suggest that EBNA-1 interacts with the 30-bp repeated sequence to activate oriP. To test whether the 30-bp repeats might cause the increased transient expression of drug resistance by enhancing transcription, the family of 30-bp repeats was tested for the ability to activate the simian virus 40 early promoter present in plasmid pAjoCAT2 (Laimins, et Epstein-Barr virus (EBV) is a human lymphotropic herpesvirus which is the causative agent of infectious mononucleosis and is associated with two human cancers, Burkitt's lymphoma and nasopharyngeal carcinoma (for a review see reference 43). B-lymphoid cells that have been transformed in vitro by EBV, or lymphoid cells derived from Burkitt's lymphoma biopsies, generally express the nuclear antigen EBNA (27) and contain multiple copies of the EBV genome (usually 1 to 100 copies per cell). Most or all of the EBV DNA is present as supercoiled DNA plasmids of approximately 172,000 base pairs (bp) (20). These multiple copies appear to arise soon after infection, by amplification of the viral DNA relative to cell DNA (36).For the most part, the study of DNA replication and the structure of origins of DNA replication in higher eucaryotic cells has been limited to lytic DNA viruses such as herpes simplex virus type 1 (HSV-1), simian virus 40 (SV40), polyomavirus, and the adenoviruses (3,4,25,29,33). Recently, however, studies of DNA replication have been extended to include stable extrachromosomal genetic elements such as those derived from bovine papillomavirus type 1 (23) and EBV (41). A cis-acting element, designated oriP, that allows replication and maintenance of recombinant plasmids in cells harboring either the EBV genome or the EBNA-1-encoding sequence from EBV has been isolated from the EBV genome (41, 42). In the presence of EBNA-1, oriP permits plasmid replication in a variety of mammalian cells that EBV cannot infect in culture (42).To determine the DNA sequence requirements for oriP, we generated and tested deletion derivatives of oriP. Our analysis revealed two cis-acting regions with...

Expression of the p53 gene protects cells against malignant transformation. Whereas control of p53 degradation has been a subject of intense scrutiny, little is known about the factors that regulate p53 synthesis. Here we show that p53 messenger RNA levels are low in a large proportion of breast tumours. Seeking potential regulators of p53 transcription, we found consensus HOX binding sites in the p53 promoterS. Transient transfection of Hox/HOXA5 activated the p53 promoter. Expression of HOXA5 in epithelial cancer cells expressing wild-type p53, but not in isogenic variants lacking the p53 gene, led to apoptotic cell death. Moreover, breast cancer cell lines and patient tumours display a coordinate loss of p53 and HOXA5 mRNA and protein expression. The HOXA5 promoter region was methylated in 16 out of 20 p53-negative breast tumour specimens. We conclude that loss of expression of p53 in human breast cancer may be primarily due to lack of expression of HOXA5.

Two regions of the Epstein-Barr virus (EBV) genome together make up an element, oriP, which acts in cis to support plasmid replication in cells that express the EBV nuclear antigen 1 (EBNA-1). The two components of oriP are a region containing a 65-base-pair (bp) dyad symmetry and a region containing 20 copies of a 30-bp direct repeat. Here we show that the 30-bp family of repeats of oriP can function as a transcriptional enhancer that is activated in trans by the EBNA-1 gene product. In either EBV-genome-positive cells or in cells that express EBNA-1, the 30-bp family of repeats, when positioned in either orientation upstream or downstream, enhances expression of the chloramphenicol acetyltransferase (CAT) gene expressed from either the simian virus 40 early promoter or the herpes simplex virus type 1 thymidine kinase promoter. The extent of transcriptional enhancement varies with the promoter and cell type. This enhanced CAT expression reflects an increased level of CAT mRNA and does not result from amplffication of the plasmids expressing CAT. In addition, plasmids carrying the gene for resistance to hygromycin B and the 30-bp family of repeats yielded 10 to 100 times more hygromycin B-resistant colonies than the vector lacking the 30-bp family of repeats in both EBV-genome-positive cells and cells that express EBNA-1. EBNA-1 is known to bind to sequences within the 30-bp family of repeats (D. R. Rawlins, G. Milman, S. D. Hayward, and G. S. Hayward, Cell 42:859468, 1985), and these trans-and cis-acting elements together have at least two functional roles: (i) they are required for DNA replication dependent upon oriP, and (ii) they can enhance expression of genes linked to the 30-bp family of repeats of oriP.Epstein-Barr virus (EBV) is a human herpesvirus that infects B lymphocytes and transforms them into cells capable of indefinite proliferation in culture (for a recent review, see reference 31). B cells that have been transformed by EBV usually express nuclear antigens known as EBNAs and contain multiple copies of the EBV genome (35,41). In general, the viral genome is maintained as a supercoiled DNA plasmid of approximately 172,000 base pairs (bp) (2, 27). Three regions of the viral genome are known to be transcribed into poly(A)+ mRNA in transformed cells (1,45). One region codes for a 62,000-dalton membrane protein (11), and the two others code for the nuclear antigens 18,43). Little is known about the regulation of expression of these genes, and apart from the role of EBNA-1 in EBV plasmid replication (29, 48), little is known about the role of the other expressed EBV genes in B-cell transformation.A cis-acting element, oriP, isolated from the EBV genome, allows the replication and maintenance of recombinant plasmids in cells that express EBNA-1 (29,47,48 To characterize further the properties of this transcriptional enhancer element, we measured its activity in different cell types and its effect on RNA synthesis from two promoters, the SV40 early promoter and the herpes simplex virus type 1 thymidine...

The SWI/SNF chromatin-remodeling complex serves as a master switch that directs and limits the execution of specific cellular programs, such as differentiation and growth control. SWI/SNF function requires one of two paralogous ATPase subunits, Brahma (BRM) or BRMrelated gene 1 (BRG1), which we previously found are lost together in cancer cell lines and primary lung cancers. Although BRG1 has been found to be mutated in cancer cell lines, the mechanisms underlying BRM silencing are not known. To address this question, we sequenced BRM in 10 BRM/BRG1-deficient cancer cell lines and found that BRM was devoid of abrogating mutations. Moreover, histone deacetylase (HDAC) inhibitors restored BRM expression in each of these BRG1/BRM-deficient cancer cell lines, indicating that epigenetic silencing is a major mechanism underlying the loss of BRM expression. Despite their ability to restore BRM expression, these HDAC inhibitors also blocked BRM function when present. However, after their removal, we observed that BRM expression remained elevated for several days, and during this period, BRM activity was detected. We also found that the suppression of BRM occurs in a broad range of human tumor types and that loss of one or both BRM alleles potentiated tumor development in mice. Thus, BRG1 and BRM are silenced by different mechanisms, and it may be possible to clinically target and reexpress BRM in a number of tumor types, potentially impacting tumor development.

Truncating mutations and homozygous deletions in the hSNF5/INI1/BAF47 subunit of human SWI/SNF complexes occur in most malignant rhabdoid tumors and some other malignancies. How loss of hSNF5 contributes to tumorigenesis remains unknown. Because the SWI/ SNF subunit BRG1 is required for RB-mediated cell cycle arrest, we hypothesized that hSNF5 deficiency disrupts RB signaling. Here we demonstrate that unlike BRG1, hSNF5 deficient cells retain functional RB since ectopic expression of either p16ink4a or a constitutively active form of RB (PSM -RB) led to cell cycle arrest. To determine how hSNF5 loss might contribute to tumorigenesis, we used a retrovirus to introduce hSNF5 into multiple deficient cell lines. In all cases, re-expression inhibited colony formation and induced cell cycle arrest characterized by a flattened morphology. Flow cytometry revealed that these cells accumulated in G 0 /G 1 . Importantly, arrested cells exhibited strong induction of p16ink4a, hypophosphorylated RB, and down-regulation of cyclin A, suggesting that hSNF5 signals upstream of RB to induce growth arrest. Co-expression of SV40 T/t abolished hSNF5-induced G 1 arrest and activation of RB. Likewise, HPV-16 E7 was sufficient to partially overcome cell cycle arrest. These results suggest that hSNF5 loss is not equivalent to BRG1/ BRM loss in human tumor cell lines. Furthermore, hSNF5-induced cell cycle arrest of deficient cells is mediated in part through activation of p16ink4a expression. These findings provide insight into mechanisms of hSNF5-mediated tumor suppression.

Mammalian cells express two homologs of the SWI2 subunit of the SWI/SNF chromatin-remodeling complex called BRG1 and BRM. Whether the SWI/SNF complexes formed by these two subunits perform identical or dierent functions remains an important question. In this report, we show concomitant downregulation of BRG1 and BRM in six human tumor cell lines. This down-regulation occurs at the level of mRNA abundance. We tested whether BRM could aect aberrant cellular functions attributed to BRG1 in tumor cell lines. By transient transfection, we found that BRM can restore RB-mediated cell cycle arrest, induce expression of CD44 protein and suppress Cyclin A expression. Therefore, BRM may be consistently downregulated with BRG1 during neoplastic progression because they share some redundant functions. However, assorted tissues from BRM null/BRG1-positive mice lack CD44 expression, suggesting that BRM-containing SWI/SNF complexes regulate expression of this gene under physiological conditions. Our studies further de®ne the mechanism by which chromatin-remodeling complexes participate in RB-mediated cell cycle arrest and provide additional novel evidence that the functions of SWI/SNF complexes containing BRG1 or BRM are not completely interchangeable.