p21X mRNA is expressed as a singly spliced pX transcript from defective provirus genomes having a partial deletion of the pol-env region in human T-cell leukemia virus type 1-infected cells

Elucidation of the Mechanism of Host NMD Suppression by HTLV-1 Rex: Dissection of Rex to Identify the NMD Inhibitory Domain

The human retrovirus human T-cell leukemia virus type I (HTLV-1) infects human T cells by vertical transmission from mother to child through breast milk or horizontal transmission through blood transfusion or sexual contact. Approximately 5% of infected individuals develop adult T-cell leukemia/lymphoma (ATL) with a poor prognosis, while 95% of infected individuals remain asymptomatic for the rest of their lives, during which time the infected cells maintain a stable immortalized latent state in the body. It is not known why such a long latent state is maintained. We hypothesize that the role of functional proteins of HTLV-1 during early infection influences the phenotype of infected cells in latency. In eukaryotic cells, a mRNA quality control mechanism called nonsense-mediated mRNA decay (NMD) functions not only to eliminate abnormal mRNAs with nonsense codons but also to target virus-derived RNAs. We have reported that HTLV-1 genomic RNA is a potential target of NMD, and that Rex suppresses NMD and stabilizes viral RNA against it. In this study, we aimed to elucidate the molecular mechanism of NMD suppression by Rex using various Rex mutant proteins. We found that region X (aa20-57) of Rex, the function of which has not been clarified, is required for NMD repression. We showed that Rex binds to Upf1, which is the host key regulator to detect abnormal mRNA and initiate NMD, through this region. Rex also interacts with SMG5 and SMG7, which play essential roles for the completion of the NMD pathway. Moreover, Rex selectively binds to Upf3B, which is involved in the normal NMD complex, and replaces it with a less active form, Upf3A, to reduce NMD activity. These results revealed that Rex invades the NMD cascade from its initiation to completion and suppresses host NMD activity to protect the viral genomic mRNA.

HTLV-1 Rex Tunes the Cellular Environment Favorable for Viral Replication

Human T-cell leukemia virus type-1 (HTLV-1) Rex is a viral RNA binding protein. The most important and well-known function of Rex is stabilizing and exporting viral mRNAs from the nucleus, particularly for unspliced/partially-spliced mRNAs encoding the structural proteins essential for viral replication. Without Rex, these unspliced viral mRNAs would otherwise be completely spliced. Therefore, Rex is vital for the translation of structural proteins and the stabilization of viral genomic RNA and, thus, for viral replication. Rex schedules the period of extensive viral replication and suppression to enter latency. Although the importance of Rex in the viral life-cycle is well understood, the underlying molecular mechanism of how Rex achieves its function has not been clarified. For example, how does Rex protect unspliced/partially-spliced viral mRNAs from the host cellular splicing machinery? How does Rex protect viral mRNAs, antigenic to eukaryotic cells, from cellular mRNA surveillance mechanisms? Here we will discuss these mechanisms, which explain the function of Rex as an organizer of HTLV-1 expression based on previously and recently discovered aspects of Rex. We also focus on the potential influence of Rex on the homeostasis of the infected cell and how it can exert its function.

HTLV-1 Rex: the courier of viral messages making use of the host vehicle

The human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus causing an aggressive T-cell malignancy, adult T-cell leukemia (ATL). Although HTLV-1 has a compact RNA genome, it has evolved elaborate mechanisms to maximize its coding potential. The structural proteins Gag, Pro, and Pol are encoded in the unspliced form of viral mRNA, whereas the Env protein is encoded in singly spliced viral mRNA. Regulatory and accessory proteins, such as Tax, Rex, p30II, p12, and p13, are translated only from fully spliced mRNA. For effective viral replication, translation from all forms of HTLV-1 transcripts has to be achieved in concert, although unspliced mRNA are extremely unstable in mammalian cells. It has been well recognized that HTLV-1 Rex enhances the stability of unspliced and singly spliced HTLV-1 mRNA by promoting nuclear export and thereby removing them from the splicing site. Rex specifically binds to the highly structured Rex responsive element (RxRE) located at the 3' end of all HTLV-1 mRNA. Rex then binds to the cellular nuclear exporter, CRM1, via its nuclear export signal domain and the Rex-viral transcript complex is selectively exported from the nucleus to the cytoplasm for effective translation of the viral proteins. Yet, the mechanisms by which Rex inhibits the cellular splicing machinery and utilizes the cellular pathways beneficial to viral survival in the host cell have not been fully explored. Furthermore, physiological impacts of Rex against homeostasis of the host cell via interactions with numerous cellular proteins have been largely left uninvestigated. In this review, we focus on the biological importance of HTLV-1 Rex in the HTLV-1 life cycle by following the historical path in the literature concerning this viral post-transcriptional regulator from its discovery to this day. In addition, for future studies, we discuss recently discovered aspects of HTLV-1 Rex as a post-transcriptional regulator and its use in host cellular pathways.

Mycosis fungoides in European Russia: No Antibodies to Human T Cell Leukemia Virus Type I Structural Proteins, but Virus-Like Sequences in Blood and Saliva

OBJECTIVE

Mycosis fungoides (MF) is the most frequent form of cutaneous T cell lymphoma (CTCL). Human T cell leukemia virus type 1 (HTLV-1) involvement in MF progression is a matter of debate. The goal of the investigation was to search for HTLV-1 markers in a group of MF patients from a nonendemic area to HTLV-1.

MATERIALS AND METHODS

Fifty MF patients and 60 healthy donors from Moscow and the Moscow region were examined for HTLV-1 markers by Western blot, PCR, nested PCR, PCR/Southern hybridization, TaqMan real-time PCR and sequencing.

RESULTS

Plasma samples from MF patients were repeatedly negative for antibodies to HTLV-1 structural proteins. HTLV-1 tax-related sequences (corresponding to the second exon) were found in blood from 20 of 50 MF patients and in 3 of 5 saliva specimens. Three of 8 sequenced tax-like amplimers were identical and 5 of 8 contained 1-2 substitutions. tax transcripts and antibodies to p40(tax) were detected in some 'PCR-tax'-positive MF patients. Defective HTLV-1 genomes were demonstrated in 2 of 50 MF patients. Phylogenetic analysis of the defective genome 5'-LTR sequence revealed a relationship with HTLV-1a sequences from the transcontinental subgroup of HTLV-1.

CONCLUSIONS

HTLV-1 tax-like sequences were revealed in blood and for the first time in saliva from MF patients living in an HTLV-1 nonendemic region. Expression of tax-like sequences was confirmed by both reverse transcription PCR and Western blot.

Construction of a full-length human T cell leukemia virus type I genome from MT-2 cells containing multiple defective proviruses using overlapping polymerase chain reaction

Human T cell leukemia virus type I (HTLV-I), the etiological agent of adult T cell leukemia, integrates into the host genome as a provirus. Multiple defective copies of the integrated provirus are often present in the host genome. For this reason it is difficult to clone the intact provirus from HTLV-I-infected cells using conventional techniques. Here, we used overlapping polymerase chain reaction (PCR) to construct a full-length provirus of HTLV-I directly from an HTLV-I-transformed cell line, MT-2, which contains multiple defective proviruses. First, four overlapping proviral HTLV-I fragments (1.4-3.9 kb each) were constructed from genomic MT-2 DNA using PCR. Next, the complete HTLV-I proviral DNA (9 kb) was generated from these fragments using asymmetric PCR and cloned into a plasmid vector. 293 T cells transfected with this plasmid produced virus-like particles, and we show that these particles are capable of infecting a human T cell line. We propose that this cloning technique constitutes a powerful tool for constructing infectious molecular clones from cells of patients infected with HTLV-I or other viruses.

Comparisons of defective HTLV-I proviruses predict the mode of origin and coding potential of internally deleted genomes

Cell lines infected with a variety of HTLV-I isolates were examined for the presence of defective proviruses that contain deletions spanning the gag, pol, and env genes. Internally deleted proviruses were identified by Southern blotting and by PCR amplification with 5' and 3' primers complementary to gag and tax sequences, respectively. PCR products representing eight defective proviruses from seven different cell lines were subsequently cloned and sequenced. The objectives of this study were twofold: first, we sought to determine whether nucleotide sequences surrounding sites of deletion shared common features that might reveal the mechanisms by which the defective genomes originated. Second, we asked whether deleted proviruses encode Gag fusion proteins with related C-terminal residues derived from open reading frames in the pX region. While most of the defective proviruses had incurred a single, large deletion, two of them displayed a more complex pattern of multiple rearrangements. Alignments of bases flanking the 5' and 3' deletion endpoints within each provirus showed tracts of sequence identity consistent with a mechanism involving aberrant intramolecular strand-transfer events during replication. We suggest that the amount or activity of HTLV-I polymerase in virions may contribute both to the poor infectivity of the virus and to the high deletion frequency. Two of the eight proviruses that were examined encoded a gag gene joined to an extended open reading frame; the other six had very short open reading frames (one to six amino acids) derived from pX or env regions joined to gag that showed no apparent amino acid sequence similarity.

Assessment of bovine leukemia virus transcripts in vivo

Reverse transcriptase PCR (RT-PCR) consistently detected bovine leukemia virus transcripts in fresh cells, and competitive RT-PCR enumerated these transcripts. The detection of transcripts in limited numbers of tumor cells indicated that expression occurs in a minority of cells. The data suggest that individual cells contain hundreds of copies of the tax/rex transcript in vivo.

Defective and wild-type human T-cell leukemia virus type I proviruses: characterization of gene products and trans-interactions between proviruses

Defective provirus genomes of human T-cell leukemia virus type I are frequently detected in lymphocytes from infected individuals and in infected cell lines. One type of defective provirus contains internal deletions spanning gag, pol, and env genes but retains portions of open reading frames for trans-regulatory proteins. The deleted proviruses could potentially contribute to viral pathology by producing novel gene products that directly affect cell metabolism or that modulate expression of resident, wild-type proviruses. Virus gene products and the control of their expression were examined in cells transfected with defined molecular clones of wild-type and defective proviruses. Internally deleted provirus clones, which are unable to produce functional Tax and Rex proteins, were transcriptionally inactive in transfected cells. Ectopic expression of p40Tax activated transcription of the deleted provirus, resulting in the accumulation of a two-exon mRNA that yields a truncated form of Rex (p21Rex). Although this two-exon mRNA also has a potential initiation codon in the tax frame, a truncated form of Tax was not detected by immunoblotting or in transactivation assays. When complemented with p40Tax and p27Rex, cells transfected with deleted proviruses accumulated an unspliced mRNA that could potentially encode gag-pX fusion proteins. Although expression of deleted proviruses was dependent on trans-acting factors produced from intact proviruses, gene products from defective proviruses did not significantly affect expression of a cotransfected, full-length provirus.

Cytoplasmic forms of human T-cell leukemia virus type 1 Tax induce NF-kappaB activation

Human T-cell leukemia virus type 1 (HTLV-1) Tax targets I-kappaB alpha and I-kappaB beta for phosphorylation, ubiquitination, and proteasome-mediated degradation, causing the nuclear translocation of NF-kappaB/Rel proteins and transcription induction of many cellular genes. The mechanism by which a nuclear protein such as Tax stimulates I-kappaB phosphorylation and degradation remains unclear. Here, we describe two cytoplasmic mutants of Tax, designated TaxDeltaN81 and TaxDeltaN109, from which the domains important for cyclic AMP response element binding factor (CREB) and serum response factor (SRF) binding and nuclear transport have been removed. These mutants were unable to trans activate from the HTLV-1 21-bp repeats or the serum response element in the c-fos promoter. In contrast, they activated NF-kappaB reporters, suggesting that activation of NF-kappaB by Tax occurs in the cytoplasm. Incorporation of the nuclear localization signal (NLS) of the simian virus 40 large T antigen into TaxDeltaN81 and TaxDeltaN109 redirected both proteins predominantly to the nucleus yet did not restore trans activation via CREB or SRF. The NLS fusion had little effect on TaxDeltaN81 but reduced NF-kappaB trans activation by TaxDeltaN109, possibly because of its proximity to the NF-kappaB-activating domain of Tax. In contrast to wild-type Tax, the cytoplasmic TaxDeltaN mutants are not cytotoxic. Stable expression of TaxDeltaN109 in HeLa cells resulted in a significant reduction in the intracellular level of I-kappaB alpha, with the constitutive presence of NF-kappaB in the nucleus and concomitant activation of the NF-kappaB enhancer. These results are suggestive of a potential application of the TaxDeltaN109-like mutants in targeting I-kappaB degradation and NF-kappaB activation. Interestingly, a Tax species with a molecular mass similar to that of TaxDeltaN109 was identified in many HTLV-1-transformed T cells, suggesting that TaxDeltaN109-like species might play a role in HTLV-1-induced leukemogenesis.

De novo reverse transcription of HTLV-1 following cell-to-cell transmission of infection

Analogous to transmission of human T-cell leukemia virus type 1 (HTLV-1) in vivo, an in vitro cell-to-cell infection model was established by coculturing MT-2 cells as virus donors and HUT78 cells as recipients. At a donor:recipient ratio of 1:2, cell fusion occurred and a new round of HTLV-1 genome replication was initiated in the cocultured cells. Newly synthesized unintegrated viral DNA was detected by Southern blot within 4-8 h and then increased between 8 and 48 h following cell mixing. The most dominant species of unintegrated viral DNA was 3.7 kb in size which hybridized to a full-length HTLV-1 DNA probe but not to a Kpnl viral DNA fragment that is absent from a defective proviral genome that has been previously identified in MT-2 cells. Northern blot analysis showed large amounts of viral RNA in the virus donor cells and in the cocultured cells, with a 3.4-kb species being the most abundant. This 3.4-kb RNA gave a pattern identical to that of the 3.7-kb unintegrated viral DNA in hybridization studies using the two probes. It seems likely that the unspliced RNA transcript from the defective proviral genome in MT-2 cells was effectively reverse transcribed upon initiation of cell-to-cell viral transmission to susceptible HUT78 cells. Despite active de novo reverse transcription, however, viral RNA levels remained unchanged following cell-to-cell transmission of HTLV-1 infection and no viral antigen production could be attributed to the newly initiated round of viral genome replication. As an abortive infection model this simple cell-to-cell infection system warrants more detailed study as it has the potential to provide reliable information regarding the early events in HTLV-1 transmission and infection.

cis-Acting inhibitory elements within the pol-env region of human T-cell leukemia virus type 1 possibly involved in viral persistence

Human T-cell leukemia virus type 1 (HTLV-1) remains latent throughout the life of the carrier, with cells containing the provirus and viral gene expression efficiently down-regulated. On a molecular level, exactly how viruses are down-regulated in vivo remains unresolved. We described here the possibility that down-regulation results from the presence of inhibitory elements within the gag-env region of the provirus in fresh peripheral blood mononuclear cells from carriers. In vitro experiments then revealed that potent cis-acting inhibitory elements (CIEs) are indeed contained in two discrete fragments from the pol region and weaker ones in the env region. The effect of CIEs is relieved by the HTLV-1 posttranscriptional regulator Rex through binding to the Rex-responsive element (RxRE), suggesting that Rex might interfere with pre-mRNA degradation and/or activate the export of mRNA molecules harboring both of the inhibitory elements and RxRE on the same RNA molecule. Thus, we propose the hypothesis that such functions of CIEs may be involved in HTLV-1 persistence.

Presence of antibodies to p21X and/or p27rex proteins in sera from human T-cell leukemia virus type I-infected individuals

The human T-cell leukemia virus type I (HTLV-I) pX gene encodes three nonstructural proteins, p40tax, p27rex and p21X. So far, natural antibodies to p27rex and/or p21X have not been found in sera from HTLV-I-infected individuals, although antibodies to p40tax have been found. Recently, the viral transcripts specific for these proteins were detected in fresh peripheral blood mononuclear cells from HTLV-I-infected individuals by the polymerase chain reaction coupled to reverse transcription, showing the in vivo expression of these proteins. We detected antibodies to p21X and p27rex by an enzyme-linked immunosorbent assay (ELISA) system using a recombinantly produced p21X protein as a common antigen, because p21X is identical to the C-terminal portion of p27rex. The sensitivity of the ELISA was determined to be approximately 100 times greater than that of Western blotting. From the analyzed sera of 31 ATL patients, 30 asymptomatic carriers, 18 HAM patients and 100 healthy donors, three specimens from one ATL patient and two carriers were found to be positive for anti-p21X/p27rex antibodies. The specificity of the ELISA reaction was confirmed by the competitive ELISA test with the highly purified recombinant p21X protein. As of result, we first determined the presence of anti-p21X/p27rex antibodies in a small percentage (3.8%) of the sera from HTLV-I-infected individuals. Even sera from the ATL patients, whose fresh PBMCs contained the transcripts for these proteins, were not found to contain these antibodies, suggesting that the immune response to these proteins is low in HTLV-I-infected humans.

The cutaneous T cell lymphoma, mycosis fungoides, is a human T cell lymphotropic virus-associated disease. A study of 50 patients

For nearly two decades it has been suspected that the cutaneous T cell lymphoma, mycosis fungoides (MF), and its leukemic variant, the Sézary syndrome, are caused by the human T lymphotropic virus (HTLV-I/II). Arguments against this concept included the finding that only a small number of MF patients have antibodies to HTLV-I/II and that attempts to detect proviral sequences by mere Southern hybridization of extracted DNA usually met with failure. However, we have reported repeatedly that HTLV-like particles emerge in blood mononuclear cell (PBMC) cultures of practically all patients with this disease. In several instances, the particles were identified as HTLV by immunoelectron microscopy as well as biomolecular analysis. With the assumptions that the virus in MF patients may have become detection by Southern hybridization alone, the extracts of freshly isolated PBMC of 50 consecutive patients were subjected to combined PCR/Southern analysis. Here we report the presence of HTLV pol and/or tax proviral sequences in 46 out of 50 (92%) of the patients tested. In addition, five of the patients, who lacked antibodies to HTLV-I/II structural proteins, were found to be seropositive for tax. It thus seems reasonable to conclude that MF/Sézary syndrome is an HTLV-associated disease and that lack of an immune response does not preclude infection with this type of virus.

Human T-cell leukemia virus type II nucleotide sequences between env and the last exon of tax/rex are not required for viral replication or cellular transformation

Human T-cell leukemia virus types I (HTLV-I) and II (HTLV-II) and bovine leukemia virus contain a region of approximately 600 nucleotides located 3' to the env gene and 5' to the last exon of the tax and rex regulatory genes. This region was originally termed nontranslated or untranslated (UT) since it did not appear to be expressed. Several studies have identified novel mRNAs in HTLV-I-, HTLV-II-, a bovine leukemia virus-infected cells that splice into open reading frames (ORFs) contained in the UT region and, thus, have the potential to produce proteins that might contribute to the biological properties of these viruses. The HTLV-II infectious molecular clone pH6neo has several ORFs in the UT region (nucleotides 6641 to 7213) and a large ORF which overlaps the third exon of tax/rex. To investigate the importance of these ORF-containing sequences on viral replication and transformation in cell culture, proviral clones containing deletions in UT (pH6neo delta UT) or a stop codon insertion mutation (pH6neoST) were constructed. Lymphoid cells were transfected with mutant proviral constructs, and stable cell clones, designated 729pH6neo delta UT and 729pH6neoST, were characterized. Viral protein production, reverse transcriptase activity, and the capacity to induce syncytia were indistinguishable from cells transfected with the wild-type clone. Finally, 729pH6neo delta UT- and 729pH6neoST-producer cells cocultured with primary blood T lymphocytes resulted in cellular transformation characteristic of HTLV. These results indicate that putative protein-coding sequences between env and the last exon of tax/rex are not required for viral replication or transformation in cell culture.