Aberrant TSG101 transcripts in acute myeloid leukaemia

Epstein-Barr Virus Enhances Cancer-Specific Aberrant Splicing of TSG101 Pre-mRNA

Tumor viruses gain control of cellular functions when they infect and transform host cells. Alternative splicing is one of the cellular processes exploited by tumor viruses to benefit viral replication and support oncogenesis. Epstein-Barr virus (EBV) participates in a number of cancers, as reported mostly in nasopharyngeal carcinoma (NPC) and Burkitt lymphoma (BL). Using RT-nested-PCR and Northern blot analysis in NPC and BL cells, here we demonstrate that EBV promotes specific alternative splicing of TSG101 pre-mRNA, which generates the TSG101∆154-1054 variant though the agency of its viral proteins, such as EBNA-1, Zta and Rta. The level of TSG101∆154-1054 is particularly enhanced upon EBV entry into the lytic cycle, increasing protein stability of TSG101 and causing the cumulative synthesis of EBV late lytic proteins, such as VCA and gp350/220. TSG101∆154-1054-mediated production of VCA and gp350/220 is blocked by the overexpression of a translational mutant of TSG101∆154-1054 or by the depletion of full-length TSG101, which is consistent with the known role of the TSG101∆154-1054 protein in stabilizing the TSG101 protein. NPC patients whose tumor tissues express TSG101∆154-1054 have high serum levels of anti-VCA antibodies and high levels of viral DNA in their tumors. Our findings highlight the functional importance of TSG101∆154-1054 in allowing full completion of the EBV lytic cycle to produce viral particles. We propose that targeting EBV-induced TSG101 alternative splicing has broad potential as a therapeutic to treat EBV-associated malignancies.

Adding Some "Splice" to Stress Eating: Autophagy, ESCRT and Alternative Splicing Orchestrate the Cellular Stress Response

Autophagy is a widely studied self-renewal pathway that is essential for degrading damaged cellular organelles or recycling biomolecules to maintain cellular homeostasis, particularly under cellular stress. This pathway initiates with formation of an autophagosome, which is a double-membrane structure that envelopes cytosolic components and fuses with a lysosome to facilitate degradation of the contents. The endosomal sorting complexes required for transport (ESCRT) proteins play an integral role in controlling autophagosome fusion events and disruption to this machinery leads to autophagosome accumulation. Given the central role of autophagy in maintaining cellular health, it is unsurprising that dysfunction of this process is associated with many human maladies including cancer and neurodegenerative diseases. The cell can also rapidly respond to cellular stress through alternative pre-mRNA splicing that enables adaptive changes to the cell's proteome in response to stress. Thus, alternative pre-mRNA splicing of genes that are involved in autophagy adds another layer of complexity to the cell's stress response. Consequently, the dysregulation of alternative splicing of genes associated with autophagy and ESCRT may also precipitate disease states by either reducing the ability of the cell to respond to stress or triggering a maladaptive response that is pathogenic. In this review, we summarize the diverse roles of the ESCRT machinery and alternative splicing in regulating autophagy and how their dysfunction can have implications for human disease.

Cancer-Specifically Re-Spliced TSG101 mRNA Promotes Invasion and Metastasis of Nasopharyngeal Carcinoma

TSG101 (Tumor susceptibility 101) gene and its aberrantly spliced isoform, termed TSG101∆154-1054, are tightly linked to tumorigenesis in various cancers. The aberrant TSG101∆154-1054 mRNA is generated from cancer-specific re-splicing of mature TSG101 mRNA. The TSG101∆154-1054 protein protects the full-length TSG101 protein from ubiquitin-mediated degradation, implicating TSG101∆154-1054 protein in the progression of cancer. Here, we confirmed that the presence of TSG101∆154-1054 mRNA indeed caused an accumulation of the TSG101 protein in biopsies of human nasopharyngeal carcinoma (NPC), which was recapitulated by the overexpression of TSG101∆154-1054 in the NPC cell line TW01. We demonstrate the potential function of the TSG101∆154-1054 protein in the malignancy of human NPC with scratch-wound healing and transwell invasion assays. By increasing the stability of the TSG101 protein, TSG101∆154-1054 specifically enhanced TSG101-mediated TW01 cell migration and invasion, suggesting the involvement in NPC metastasis in vivo. This finding sheds light on the functional significance of TSG101∆154-1054 generation via re-splicing of TSG101 mRNA in NPC metastasis and hints at its potential importance as a therapeutic target.

TSGΔ154-1054 splice variant increases TSG101 oncogenicity by inhibiting its E3-ligase-mediated proteasomal degradation

Tumor susceptibility gene 101 (TSG101) elicits an array of cellular functions, including promoting cytokinesis, cell cycle progression and proliferation, as well as facilitating endosomal trafficking and viral budding. TSG101 protein is highly and aberrantly expressed in various human cancers. Specifically, a TSG101 splicing variant missing nucleotides 154 to 1054 (TSGΔ154-1054), which is linked to progressive tumor-stage and metastasis, has puzzled investigators for more than a decade. TSG101-associated E3 ligase (Tal)- and MDM2-mediated proteasomal degradation are the two major routes for posttranslational regulation of the total amount of TSG101. We reveal that overabundance of TSG101 results from TSGΔ154-1054 stabilizing the TSG101 protein by competitively binding to Tal, but not MDM2, thereby perturbing the Tal interaction with TSG101 and impeding subsequent polyubiquitination and proteasomal degradation of TSG101. TSGΔ154-1054 therefore specifically enhances TSG101-stimulated cell proliferation, clonogenicity, and tumor growth in nude mice. This finding shows the functional significance of TSGΔ154-1054 in preventing the ubiquitin-proteasome proteolysis of TSG101, which increases tumor malignancy and hints at its potential as a therapeutic target in cancer treatment.

Identification of TSG101 functional domains and p21 loci required for TSG101-mediated p21 gene regulation

TSG101 (tumor susceptibility gene 101) is a multi-domain protein known to act in the cell nucleus, cytoplasm, and periplasmic membrane. Remarkably, TSG101, whose location within cells varies with the stage of the cell cycle, affects biological events as diverse as cell growth and proliferation, gene expression, cytokinesis, and endosomal trafficking. The functions of TSG101 additionally are recruited for viral and microvesicle budding and for intracellular survival of invading bacteria. Here we report that the TSG101 protein also interacts with and down-regulates the promoter of the p21^CIP1/WAF1tumor suppressor gene, and identify a p21 locus and TSG101 domains that mediate this interaction. TSG101 deficiency in Saos-2 human osteosarcoma cells was accompanied by an increased abundance of p21 mRNA and protein and the retardation of cell proliferation. A cis-acting element in the p21 promoter that interacts with TSG101 and is required for promoter repression was located using chromatin immunoprecipitation (ChIP) analysis and p21-driven luciferase reporter gene expression, respectively. Additional analysis of TSG101 deletion mutants lacking specific domains established the role of the central TSG101 domains in binding to the p21 promoter and demonstrated the additional essentiality of the TSG101 C-terminal steadiness box (SB) in the repression of p21 promoter activity. Neither binding of TSG101 to the p21 promoter nor repression of this promoter required the TSG101 N-terminal UEV domain, which mediates the ubiquitin-recognition functions of TSG101 and its actions as a member of ESCRT endocytic trafficking complexes, indicating that regulation of the p21 promoter by TSG101 is independent of its role in such trafficking.

Malfunctions within the Cbl interactome uncouple receptor tyrosine kinases from destructive transport

Proteins of the Cbl family are adaptor molecules and ubiquitin ligases with major functions in the regulation, intracellular transport and degradation of receptor tyrosine kinases (RTKs). Due to this central role, mutations that cause malfunctions of Cbl or their associated proteins - termed the Cbl interactome - easily lead to the transformation of affected cells and eventually the development of cancer. This review intends to give an overview on the mechanisms of Cbl-mediated cell transformation in light of the dysregulated intracellular trafficking of RTKs.

Overexpression of tumor susceptibility gene TSG101 in human papillary thyroid carcinomas

Functional inactivation of tumor susceptibility gene tsg101 leads to cellular transformation and tumorigenesis in mice. While human TSG101 is located in a region where frequent loss of heterozygosity can be detected in a variety of cancers, no genomic deletion in TSG101 gene has been reported, casting a doubt on the role of TSG101 as a classical tumor suppressor. Some studies have revealed that TSG101 is a frequent target of splicing defects, which correlate with cellular stress and p53 status. Furthermore, recent reports have identified TSG101 as a part of the MDM2/p53 regulatory circuitry, a well-recognized circuitry that upon deregulation results in tumorigenesis. Interestingly, overexpression of tsg101 from an adventitious promoter also leads to neoplastic transformation. On the basis of this information, we have analysed TSG101 gene expression in 20 human papillary thyroid carcinomas (PTCs) by immunohistochemistry and demonstrated that the overexpression of TSG101 protein is closely associated with human PTCs. Further sequence analysis reveals no mutation in cDNA region encoding steadiness box in these PTC specimens, indicating that the upregulation of TSG101 protein is not caused by the alteration of this region. In situ hybridization analysis confirms that overexpression of TSG101 also occurs at the transcriptional level. In addition, semi-quantitative RT-PCR and subsequent Southern hybridization verify that the amounts of TSG101 transcripts are indeed lower in three normal thyroid tissues than in PTC specimens. Here we report the upregulation of TSG101 expression in PTC cells, providing the first evidence of the association of TSG101 overexpression with human tumors and suggesting that upregulation of TSG101 steady-state level might play a role in mediating tumorigenesis of human PTC.

Expression of hBUB1 in acute myeloid leukemia

hBUB1 gene is a component of the mitotic checkpoint that monitors proper assembly of the mitotic spindles and the alteration of the hBUB1 gene has been found to be associated with chromosomal instability in some tumor cell lines. We analyzed the coding region of the hBUB1 gene for mutations and its expression in 92 acute myeloid leukemia (AML) specimens and five hematopoietic cell lines. We also used Southern hybridization to analyze the genomic DNA of those cases, which had aberrant transcription to confirm the lesion. A thymine/cytosine polymorphism at 8 bp upstream of the 5' splice acceptor site of exon 10 was observed in Raji cell line and two AML specimens without a resultant change in the expression of hBUB1. Reduced expression and aberrant transcription of the hBUB1 gene, which may affect the control of mitotic checkpoint, were detected in AML specimens by reverse transcription-polymerase chain reaction (RT-PCR) analysis. Our study suggests that the mutation of the hBUB1 gene is a rare event in AML, and further studies are necessary to clarify its role in leukemia.

Inhibition of granulocyte-macrophage colony-stimulating factor receptor function by a splice variant of the common beta-receptor subunit

The receptors for human granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 are composed of a ligand-specific alpha-chain (eg, alpha-GM-CSF receptor [alpha-GMR]) and a common beta-subunit (beta-GMR). Ligand binding is believed to induce assembly or conformational changes in preformed complexes containing more than one alpha- and beta-subunit in the activated receptor complex. To analyze the function of a splice variant of beta-GMR with a truncation in the intracellular domain (beta-GMR(IT)), BaF-3 cells expressing human alpha-GMR plus beta-GMR were transfected with beta-GMR(IT). In these cells, coexpression of beta-GMR(IT) inhibits GM-CSF-mediated survival and proliferation in a GM-CSF concentration-dependent manner. To analyze the effect of cytoplasmic assembly of truncated and full-length intracellular beta-GMR sequences, beta-GMR and beta-GMR(IT) were coexpressed with different chimeric alpha/beta-GMR constructs. Whereas both beta-GMR and beta-GMR(IT) generate high-affinity GMR complexes in the presence of alpha/beta-GMR, beta-GMR(IT) inhibits while beta-GMR supports proliferation and cell survival mediated by alpha/beta-GMR. Correspondingly, beta-GMR, but not beta-GMR(IT), generates functional GMR complexes when coexpressed with a defective alpha/beta-GMR construct. These data indicate that beta-GMR(IT) can inhibit survival and mitogenic signaling of the wild-type GMR and demonstrate that recruitment of alternatively spliced receptor subunits may regulate the function of heteromeric cytokine receptors.

Mutation analysis of PTEN/MMAC1 in acute myeloid leukemia

Recently, a putative tumor suppressor gene, PTEN/MMAC1, has been identified at chromosome 10q23.3, which encodes a 403 amino acid dual-specificity phosphatase containing a region of homology to tensin and auxillin. Somatic mutations of the PTEN/MMAC1 gene have been identified in a number of cancer cell lines and primary cancers. Mutations in PTEN/MMAC1 are most frequently found in advanced cancers. To evaluate the role of the PTEN/MMAC1 gene in leukemia, bone marrow and/or peripheral blood from 62 acute myeloid leukemia (AML) patients, 5 hemopoietic cell lines (HL60, U937, Raji, KG-1, K562), and 30 normal controls were analyzed. The results showed aberrant PTEN/MMAC1 transcripts in 15 of the 62 (24%) AML patients, 4 of the 5 cell lines (80%), and 4 of the 30 (13%) normal controls. As in our previous study of TSG101, the abnormal transcripts may result from aberrant RNA splicing as evidenced by the presence of both these aberrant transcripts and normal full length transcripts in all specimens examined. Loss of heterozygosity (LOH) analysis and PCR-SSCP of the entire coding region showed that none of the AML cases had LOH or mutation. Only one frameshift mutation at codon 130 (insertion of CCCG) with premature termination of coding sequence was observed in the U937 cell line. Our results indicate that the PTEN/MMAC1 gene may play a role in a small percentage of AML, but its significance needs to be further evaluated.

Aberrant expression of TSG101 in Taiwan Chinese breast cancer

Functional inactivation of the tsg101 gene in mouse fibroblasts results in cell transformation and the ability to form metastatic tumors in nude mice. The human tsg101 gene was mapped to chromosome 11q15.1-2 and found to mutate in some cancer patients. To test the expression pattern of the tsg 101 gene in Chinese breast cancer patients, we analyzed the mRNA by RT-PCR in 51 breast cancer patients. The full-length tsg101 and 7 truncated transcripts were detected in both normal and matched tumor tissues. A short transcript with a deletion of nucleotides 154-1054 is frequently presented in late-stage breast cancers. TSG101 protein expression was also detected by Western blot analysis in 30 breast cancer patients. A predicted full-length 46 kDa and three proteins with smaller molecular weight were detected. The full-length 46 kDa protein was less expressed in tumor specimens. Immunohistochemical stains from 10 patients of each stage 0-4 revealed that TSG101 protein was predominantly present in the cytoplasm. Cell nuclei were occasionally immunopositive and the chromosomes were deeply stained during cell division. The intracellular location and the expression of TSG101 protein were both not stage-dependent in primary breast cancers. In addition, normal mammary glands were more homogenously immunopositive than invasive ductal carcinoma. These results support the notion that the aberrant expression of TSG101 in breast cancer is associated with altered cell growth.

Clinical implications of aberrant TSG101 transcripts in acute myeloblastic leukemia

Tsg101 is a mouse tumor suppressor gene whose homozygous deletion produces transformation of NIH3T3 cells and leads to metastases in nude mice. The human homologue of the gene, TSG101, is localized in chromosome 11p15.1-p15.2. Reduced TSG101 expression may cause the defect of the cell cycle checkpoint that leads to genetic instability and consequently to the progression of neoplasia. Aberrant TSG101 transcript have been identified in many types of cancers, and the relaxation of RNA splicing fidelity may be an onco-developmental marker in cancers and could play a general role in tumorigenesis. In our previous study, smaller TSG101 transcripts were found in AML specimens, hematopoietic cell lines and normal controls. The aberrant transcripts occurred more frequently in the AML cases and cell lines. The patients with aberrant TSG101 transcripts had higher initial white cell count, lower LDH level, and lower complete remission rate after induction chemotherapy. However, further multivariate analysis of clinical data revealed that there was no relationship to the TSG101 aberrant transcripts. The clinical significance of TSG101 aberrant transcript in AML needs further evaluation.

Stress-induced aberrant splicing of TSG101: association to high tumor grade and p53 status in breast cancers

The TSG101 gene, identified through insertional mutagenesis, is localized in a region that exhibits LOH in human cancers, suggesting that TSG101 might be a tumor suppressor gene. Numerous studies have then shown the presence of abnormal transcripts in various tumors which appear to result from aberrant splicing of the gene, rather than from intragenic deletions. Moreover, many studies demonstrated that these aberrantly spliced transcripts were not found in matched normal tissues. We have analysed TSG101 transcripts in 85 breast cancer samples and found that abnormal splicing of the gene is tightly correlated with tumor grade and p53 mutation. In addition, stress induced the appearance of these abnormal transcripts in primary lymphocytes. Hence, TSG101 splicing defects, while unrelated to the oncogenic process per se, could reflect the cellular environment of the tumor cells. The proposed role of stress and hypoxia to select p53 mutant cells could account for the tight association with p53 status.

Tumor susceptibility gene 101 protein represses androgen receptor transactivation and interacts with p300

BACKGROUND

Functional inactivation of the tsg101 gene in mouse fibroblasts leads to cell transformation and the ability to form metastatic tumors in nude mice. Abnormal TSG101 transcripts with highly-specific deletions in the protein-coding region have been identified in human tumor samples and cancer cell lines, including prostate and breast carcinomas, and have been attributed to alternative splicing of TSG101 mRNA. The function of the TSG101 protein is not known, although its predicted sequence has suggested that it may function as a transcription factor.

METHODS

Human TSG101 N-terminal (encoding amino acids 10-240) and C-terminal (encoding amino acids 230-391) fragments were cloned and used in both transient transfection and protein binding experiments. The transient transfections were carried in CV-1 cells. Protein-protein interactions were determined by both glutathione-S-transferase fusion protein binding and co-immunoprecipitation.

RESULTS

The N-terminal region of TSG101, when fused to the GAL4 DNA binding domain, can activate transcription; whereas the C-terminal region mediates transcriptional repression. Full-length TSG101 or its separated regions repressed ligand-dependent transcriptional activation by nuclear receptors, including androgen receptor and estrogen receptor, which play central roles in prostate carcinoma and breast carcinoma, respectively. In addition, a direct association between TSG101 and the transcriptional co-factor p300 was demonstrated in vitro and in vivo.

CONCLUSIONS

These results indicate that TSG101 can function as a transcription modulator to affect nuclear receptor-mediated transcriptional activation, which raises the possibility that the tumor suppression by TSG101 observed previously may be mediated at least in part by its effects on nuclear receptor function.

Expression of a new isoform of the tumor susceptibility TSG101 protein lacking a leucine zipper domain in Burkitt lymphoma cell lines

The tumor susceptibility gene, TSG101, has been identified as a candidate tumor suppressor gene. We have examined the expression of TSG101 in Burkitt lymphoma cell lines. Several aberrant messages were detected in all cell lines. Aberrant splice donor sites are located within exon 1 at positions 132, 154, 172 and 284. Splice acceptors are located at positions 847 and 1054 within exon 5. The aberrant messages are coexpressed with a normal message and could be the result of additional splicing reactions of the mature message that behaves as an intermediate. The normal message codes for 46 kDa protein (TSG101A). One aberrant message joins in frame nucleotides 283-1055 and codes for a protein isoform of 17 kDa (TSG101B), as demonstrated by in vitro translation assays. The TSG101B isoform lacks the leucine zipper near the C-terminus, a transcriptional repressor domain, and retains most of the N-terminal region which has homology to E2 ubiquitin regulatory enzymes and the CROC-1 transcriptional regulator. The TSG101B isoform was detected in sixteen out of twenty-two (72%) BL cell lines, but not in normal lymphoid populations. The presence of two TSG101 isoforms with different dimerization potential opens up a new level of regulation of the TSG101 proteins possibly affecting cell cycle regulation.