In vivo genomic footprinting of the human T-cell leukemia virus type 1 (HTLV-1) long terminal repeat enhancer sequences in HTLV-1-infected human T-cell lines with different levels of Tax I activity

Low Proviral Load is Associated with Indeterminate Western Blot Patterns in Human T-Cell Lymphotropic Virus Type 1 Infected Individuals: Could Punctual Mutations be Related?

BACKGROUND

indeterminate Western blot (WB) patterns are a major concern for diagnosis of human T-cell lymphotropic virus type 1 (HTLV-1) infection, even in non-endemic areas.

OBJECTIVES

(a) to define the prevalence of indeterminate WB among different populations from Argentina; (b) to evaluate if low proviral load (PVL) is associated with indeterminate WB profiles; and (c) to describe mutations in LTR and tax sequence of these cases.

RESULTS

Among 2031 samples, 294 were reactive by screening. Of them, 48 (16.3%) were WB indeterminate and of those 15 (31.3%) were PCR+. Quantitative real-time PCR (qPCR) was performed to 52 HTLV-1+ samples, classified as Group 1 (G1): 25 WB+ samples from individuals with pathologies; Group 2 (G2): 18 WB+ samples from asymptomatic carriers (AC); and Group 3 (G3): 9 seroindeterminate samples from AC. Median PVL was 4.78, 2.38, and 0.15 HTLV-1 copies/100 PBMCs, respectively; a significant difference (p=0.003) was observed. Age and sex were associated with PVL in G1 and G2, respectively. Mutations in the distal and central regions of Tax Responsive Elements (TRE) 1 and 2 of G3 were observed, though not associated with PVL.The 8403A>G mutation of the distal region, previously related to high PVL, was absent in G3 but present in 50% of WB+ samples (p = 0.03).

CONCLUSIONS

indeterminate WB results confirmed later as HTLV-1 positive may be associated with low PVL levels. Mutations in LTR and tax are described; their functional relevance remains to be determined.

Upregulation of human T-cell leukemia virus type 1 antisense transcription by the viral tax protein

Several studies have recently demonstrated the existence of human T-cell leukemia virus type 1 (HTLV-1) antisense transcripts, which allow the synthesis of the newly described HBZ protein. Although previous reports have been aimed at understanding the potential role of the HBZ protein in HTLV-1 pathogenesis, little is known as to how this viral gene is regulated. Here, using our K30-3'asLuc reporter construct, we show that the viral Tax protein upregulates antisense transcription through its action on the TRE sequences located in the 3' long terminal repeat. Generation of stable clones in 293T cells demonstrated that Tax-induced HBZ expression is importantly influenced by the integration site in the host genome. The cellular DNA context could thus affect the level of HBZ mRNA expression in infected cells.

Transcriptional control of spliced and unspliced human T-cell leukemia virus type 1 bZIP factor (HBZ) gene

The human T-cell leukemia virus type 1 (HTLV-1) basic leucine zipper factor (HBZ) gene is encoded by the minus strand of the HTLV-1 provirus and transcribed from the 3' long terminal repeat (LTR). HBZ gene expression not only inhibits the Tax-mediated activation of viral gene transcription through the 5' LTR but also promotes the proliferation of infected cells. However, the HBZ promoter region and the transcriptional regulation of the gene have not been studied. In this study, we characterize the promoters of the spliced version of the HBZ gene (sHBZ) and the unspliced version of the HBZ gene (usHBZ) by luciferase assay. Both promoters were TATA-less and contained initiators and downstream promoter elements. Detailed studies of the promoter for the sHBZ gene showed that Sp1 sites were critical for its activity. The activities of the sHBZ and usHBZ gene promoters were upregulated by Tax through Tax-responsible elements in the 3' LTR. We compared the functions of the proteins derived from the sHBZ and usHBZ transcripts. sHBZ showed a stronger suppression of Tax-mediated transcriptional activation through the 5' LTR than did usHBZ; the level of suppression correlated with the level of protein produced. The expression of sHBZ had a growth-promoting function in a T-cell line, while usHBZ expression did not. This study demonstrates that Sp1 is critical for sHBZ transcription, which accounts for the constitutive expression of the sHBZ gene. Functional differences between sHBZ and usHBZ suggest that the sHBZ gene plays a significant role in the proliferation of infected cells.

De novo human T-cell leukemia virus type 1 infection of human lymphocytes in NOD-SCID, common gamma-chain knockout mice

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia, a disease that is triggered after a long latency period. HTLV-1 is known to spread through cell-to-cell contact. In an attempt to study the events in early stages of HTLV-1 infection, we inoculated uninfected human peripheral blood mononuclear cells and the HTLV-1-producing cell line MT-2 into NOD-SCID, common gamma-chain knockout mice (human PBMC-NOG mice). HTLV-1 infection was confirmed with the detection of proviral DNA in recovered samples. Both CD4+ and CD8+ T cells were found to harbor the provirus, although the latter population harbored provirus to a lesser extent. Proviral loads increased with time, and inverse PCR analysis revealed the oligoclonal proliferation of infected cells. Although tax gene transcription was suppressed in human PBMC-NOG mice, it increased after in vitro culture. This is similar to the phenotype of HTLV-1-infected cells isolated from HTLV-1 carriers. Furthermore, the reverse transcriptase inhibitors azidothymidine and tenofovir blocked primary infection in human PBMC-NOG mice. However, when tenofovir was administered 1 week after infection, the proviral loads did not differ from those of untreated mice, indicating that after initial infection, clonal proliferation of infected cells was predominant over de novo infection of previously uninfected cells. In this study, we demonstrated that the human PBMC-NOG mouse model should be a useful tool in studying the early stages of primary HTLV-1 infection.

Versatile reporter systems show that transactivation by human T-cell leukemia virus type 1 Tax occurs independently of chromatin remodeling factor BRG1

Potent activation of human T-cell leukemia virus type 1 (HTLV-1) gene expression is mediated by the virus-encoded transactivator protein Tax and three imperfect 21-bp repeats in the viral long terminal repeats. Each 21-bp repeat contains a cAMP-responsive-element core flanked by 5' G-rich and 3' C-rich sequences. Tax alone does not bind DNA. Rather, it interacts with basic domain-leucine zipper transcription factors CREB and ATF-1 to form ternary complexes with the 21-bp repeats. In the context of the ternary complexes, Tax contacts the G/C-rich sequences and recruits transcriptional coactivators CREB-binding protein (CBP)/p300 to effect potent transcriptional activation. Using an easily transduced and chromosomally integrated reporter system derived from a self-inactivating lentivirus vector, we showed in a BRG1- and BRM1-deficient adrenal carcinoma cell line, SW-13, that Tax- and 21-bp repeat-mediated transactivation does not require BRG1 or BRM1 and is not enhanced by BRG1. With a similar reporter system, we further demonstrated that Tax- and tumor necrosis factor alpha-induced NF-kappaB activation occurs readily in SW-13 cells in the absence of BRG1 and BRM1. These results suggest that the assembly of stable multiprotein complexes containing Tax, CREB/ATF-1, and CBP/p300 on the 21-bp repeats is the principal mechanism employed by Tax to preclude nucleosome formation at the HTLV-1 enhancer/promoter. This most likely bypasses the need for BRG1-containing chromatin-remodeling complexes. Likewise, recruitment of CBP/p300 by NF-kappaB may be sufficient to disrupt histone-DNA interaction for the initiation of transcription.

Envelope is a major viral determinant of the distinct in vitro cellular transformation tropism of human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are related deltaretroviruses but are distinct in their disease-inducing capacity. These viruses can infect a variety of cell types, but only T lymphocytes become transformed, which is defined in vitro as showing indefinite interleukin-2-independent growth. Studies have indicated that HTLV-1 has a preferential tropism for CD4+ T cells in vivo and is associated with the development of leukemia and neurological disease. Conversely, the in vivo T-cell tropism of HTLV-2 is less clear, although it appears that CD8+ T cells preferentially harbor the provirus, with only a few cases of disease association. The difference in T-cell transformation tropism has been confirmed in vitro as shown by the preferential transformation of CD4+ T cells by HTLV-1 versus the transformation of CD8+ T cells by HTLV-2. Our previous studies showed that Tax and overlapping Rex do not confer the distinct T-cell transformation tropisms between HTLV-1 and HTLV-2. Therefore, for this study HTLV-1 and HTLV-2 recombinants were generated to assess the contribution of LTR and env sequences in T-cell transformation tropism. Both sets of proviral recombinants expressed p19 Gag following transfection into cells. Furthermore, recombinant viruses were replication competent and had the capacity to transform T lymphocytes. Our data showed that exchange of the env gene resulted in altered T-cell transformation tropism compared to wild-type virus, while exchange of long terminal repeat sequences had no significant effect. HTLV-2/Env1 preferentially transformed CD4+ T cells similarly to wild-type HTLV-1 (wtHTLV-1), whereas HTLV-1/Env2 had a transformation tropism similar to that of wtHTLV-2 (CD8+ T cells). These results indicate that env is a major viral determinant for HTLV T-cell transformation tropism in vitro and provides strong evidence implicating its contribution to the distinct pathogenesis resulting from HTLV-1 versus HTLV-2 infections.

The human T-cell leukemia virus type 1 (HTLV-1): New insights into the clinical aspects and molecular pathogenesis of adult t-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-associated myelopathy (TSP/HAM)

Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus to be identified in the early 1980s. The isolation and identification of a related virus, HTLV-2, and the distantly related human immunodeficiency virus (HIV) immediately followed. Of the three retroviruses, two are associated definitively with specific diseases, HIV, with acquired immune deficiency syndrome (AIDS) and HTLV-1, with adult T-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). While an estimated 10-20 million people worldwide are infected with HTLV-I, infection is endemic in the Caribbean, parts of Africa, southwestern Japan, and Italy. Approximately 4% of HTLV-I infected individuals develop ATLL, a disease with a poor prognosis. The clinical manifestations of infection and the current biology of HTLV viruses with emphasis on HTLV-1 are discussed in detail. The implications for improvements in diagnosis, treatment, intervention, and vaccination are included, as well as a discussion of the emergence of HTLV-1 and -2 as copathogens among HIV-1-infected individuals.

Enhanced activation of tax-dependent transcription of human T-cell leukemia virus type I (HTLV-I) long terminal repeat by TORC3

Tax, a protein encoded by the env-pX gene of human T-cell leukemia virus type I (HTLV-I), interacts with various host cell transcription factors. Tax activates transcription from the long terminal repeat (LTR) of HTLV-I through association with cyclic AMP-responsive element-binding protein (CREB). Here, we present evidence that transducer of regulated cyclic AMP-response element-binding protein 3 (TORC3), a co-activator of CREB, is involved in Tax-induced transcriptional activation from the HTLV-I LTR. By using a luciferase assay system, we show that TORC3 alone can enhance transcription from the HTLV-I LTR, as well as from a cellular cyclic AMP-response element (CRE). Interestingly, we find that co-expression of TORC3 and Tax dramatically increased transcriptional activation at the HTLV-I LTR. We also show by glutathione S-transferase pull-down and co-immunoprecipitation experiments that TORC3 interacts with Tax. Using deletion mutant analysis, we identify the Tax interaction domain of TORC3 as a region spanning from amino acid 1 to 103, which contains a coiled-coil domain. These results provide important clues toward understanding the molecular mechanism of Tax-dependent transcriptional activation of the HTLV-I LTR.

Tax relieves transcriptional repression by promoting histone deacetylase 1 release from the human T-cell leukemia virus type 1 long terminal repeat

Expression of human T-cell leukemia virus type 1 (HTLV-1) is regulated by the viral transcriptional activator Tax. Tax activates viral transcription through interaction with the cellular transcription factor CREB and the coactivators CBP/p300. In this study, we have analyzed the role of histone deacetylase 1 (HDAC1) on HTLV-1 gene expression from an integrated template. First we show that trichostatin A, an HDAC inhibitor, enhances Tax expression in HTLV-1-transformed cells. Second, using a cell line containing a single-copy HTLV-1 long terminal repeat, we demonstrate that overexpression of HDAC1 represses Tax transactivation. Furthermore, a chromatin immunoprecipitation assay allowed us to analyze the interaction of transcription factors, coactivators, and HDACs with the basal and activated HTLV-1 promoter. We demonstrate that HDAC1 is associated with the inactive, but not the Tax-transactivated, HTLV-1 promoter. In vitro and in vivo glutathione S-transferase-Tax pull-down and coimmunoprecipitation experiments demonstrated that there is a direct physical association between Tax and HDAC1. Importantly, biotinylated chromatin pull-down assays demonstrated that Tax inhibits and/or dissociates the binding of HDAC1 to the HTLV-1 promoter. Our results provide evidence that Tax interacts directly with HDAC1 and regulates binding of the repressor to the HTLV-1 promoter.

Genetic and epigenetic inactivation of tax gene in adult T-cell leukemia cells

To clarify the status of tax gene, we analyzed human T-cell leukemia virus type-I (HTLV-I) associated cell lines and fresh adult T-cell leukemia (ATL) cells. We compared 2 types of HTLV-I associated cell lines: one was derived from leukemic cells (leukemic cell line) and the other from nonleukemic cells (nonleukemic cell line). Although all nonleukemic cell lines expressed Tax, it could not be detected in 3 of 5 leukemic cell lines, in which nonsense mutation or deletion (60 bp) of tax genes, and DNA methylation in 5'-LTR were identified as the responsible changes. We found such genetic changes of the tax gene in 5 of 47 fresh ATL cases (11%). The tax gene transcripts could be detected in 14 of 41 fresh ATL cases (34%) by RT-PCR. In ATL cases with genetic changes that could not produce Tax protein, the tax gene was frequently transcribed, suggesting that such cells do not need the transcriptional silencing. Although DNA methylation of 5'-LTR was detected in the fresh ATL cases (19 of 28 cases; 68%), the complete methylation associated with transcriptional silencing was observed only in 4 cases. Since partial methylation could not silence the transcription, and the tax gene transcription was not detected in 27 of 41 cases (66%), the epigenetic change(s) other than DNA methylation is considered to play an important role in the silencing.

Seizing of T Cells by Human T-Cell Leukemia⧸Lymphoma Virus Type 1

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function. Viral proteins modulate the downstream effects of antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation are therefore important, as also suggested by epidemiological data. The ability of a given individual to respond to specific antigens is determined genetically. Thus, genetic and environmental factors, together with the virus, contribute to disease development. As in the case of other virus-associated cancers, HTLV-1-induced leukemia/lymphoma can be prevented by avoiding viral infection or by intervention during the asymptomatic phase with approaches able to interrupt the vicious cycle of virus-induced proliferation of a subset of T-cells. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells in vitro. The relevance of these laboratory findings will be related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Transcription factor binding and histone modifications on the integrated proviral promoter in human T-cell leukemia virus-I-infected T-cells

The human T-cell leukemia virus (HTLV-I)-encoded Tax protein is a potent transcriptional activator that stimulates expression of the integrated provirus. Biochemical studies indicate that Tax, together with cellular transcription factors, interacts with viral cAMP-response element enhancer elements to recruit the pleiotropic coactivators CREB-binding protein and p300. Histone acetylation by these coactivators has been shown to play a major role in activating HTLV-I transcription from chromatin templates in vitro. However, the extent of histone modification and the precise identity of the cellular regulatory proteins bound at the HTLV-I promoter in vivo is not known. Chromatin immunoprecipitation analysis was used to investigate factor binding and histone modification at the integrated HTLV-I provirus in infected T-cells (SLB-1). These studies reveal the presence of Tax, a variety of ATF/CREB and AP-1 family members (CREB, CREB-2, ATF-1, ATF-2, c-Fos, and c-Jun), and both p300 and CREB-binding protein at the HTLV-I promoter. Consistent with the binding of these coactivators, we observed histone H3 and H4 acetylation at three regions within the proviral genome. Histone deacetylases were also present at the viral promoter and, following their inhibition, we observe an increase in histone H4 acetylation on the HTLV-I promoter and a concomitant increase in viral RNA. Together, these results suggest that a variety of transcriptional activators, coactivators, and histone deacetylases participate in the regulation of HTLV-I transcription in infected T-cells.

5'-long terminal repeat-selective CpG methylation of latent human T-cell leukemia virus type 1 provirus in vitro and in vivo

CpG methylation of the human T-cell leukemia virus type 1 (HTLV-1) long terminal repeat (LTR) has been implicated in proviral latency, but there is presently little information available regarding the pattern of LTR methylation and its effect on viral gene expression. To gain insight into the mechanisms of HTLV-1 latency, we have studied methylation of individual CpG sites in the U3-R region of the integrated proviral LTR by using bisulfite genomic sequencing methods. Surprisingly, our results reveal selective hypermethylation of the 5' LTR and accompanying hypomethylation of the 3' LTR in both latently infected cell lines and adult T-cell leukemia (ATL) cells having a complete provirus. Moreover, we observed a lack of CpG methylation in the LTRs of 5'-defective proviruses recovered from ATL samples, which is consistent with the selective hypomethylation of the 3' LTR. Thus, the integrated HTLV-1 provirus in these carriers appears to be hypermethylated in the 5' LTR and hypomethylated in the 3' LTR. These results, together with the observation that proviral gene expression is reactivated by 5-azacytidine in latently infected cell lines, indicate that selective hypermethylation of the HTLV-1 5' LTR is common both in vivo and in vitro. Thus, hypermethylation of the 5' LTR appears to be an important mechanism by which HTLV-1 gene expression is repressed during viral latency.

Human T-lymphotropic virus type 1 oncoprotein tax promotes S-phase entry but blocks mitosis

Human T-lymphotropic virus type 1 (HTLV-1) Tax exerts pleiotropic effects on multiple cellular regulatory processes to bring about NF-kappaB activation, aberrant cell cycle progression, and cell transformation. Here we report that Tax stimulates cellular G(1)/S entry but blocks mitosis. Tax expression in naive cells transduced with a retroviral vector, pBabe-Tax, leads to a significant increase in the number of cells in the S phase, with an accompanying rise in the population of cells with a DNA content of 4N or more. In all cell types tested, including BHK-21, mouse NIH 3T3, and human diploid fibroblast WI-38, Tax causes an uncoupling of DNA synthesis from cell division, resulting in the formation of multinucleated giant cells and cells with decondensed, highly convoluted and lobulated nuclei that are reminiscent of the large lymphocytes with cleaved or cerebriform nuclei seen in HTLV-1-positive individuals. This contrasts with the Tax-transformed cell lines, PX1 (fibroblast) and MT4 (lymphocyte), which produce Tax at high levels, but without the accompanying late-stage cell cycle abnormalities. PX1 and MT4 may have been selected to harbor somatic mutations that allow a bypass of the Tax-induced block in mitosis.

Human T cell leukemia virus type I and neurologic disease: events in bone marrow, peripheral blood, and central nervous system during normal immune surveillance and neuroinflammation

Human T cell lymphotropic/leukemia virus type I (HTLV-I) has been identified as the causative agent of both adult T cell leukemia (ATL) and HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Although the exact sequence of events that occur during the early stages of infection are not known in detail, the initial route of infection may predetermine, along with host, environmental, and viral factors, the subset of target cells and/or the primary immune response encountered by HTLV-I, and whether an HTLV-I-infected individual will remain asymptomatic, develop ATL, or progress to the neuroinflammatory disease, HAM/TSP. Although a large number of studies have indicated that CD4(+) T cells represent an important target for HTLV-I infection in the peripheral blood (PB), additional evidence has accumulated over the past several years demonstrating that HTLV-I can infect several additional cellular compartments in vivo, including CD8(+) T lymphocytes, PB monocytes, dendritic cells, B lymphocytes, and resident central nervous system (CNS) astrocytes. More importantly, extensive latent viral infection of the bone marrow, including cells likely to be hematopoietic progenitor cells, has been observed in individuals with HAM/TSP as well as some asymptomatic carriers, but to a much lesser extent in individuals with ATL. Furthermore, HTLV-I(+) CD34(+) hematopoietic progenitor cells can maintain the intact proviral genome and initiate viral gene expression during the differentiation process. Introduction of HTLV-I-infected bone marrow progenitor cells into the PB, followed by genomic activation and low level viral gene expression may lead to an increase in proviral DNA load in the PB, resulting in a progressive state of immune dysregulation including the generation of a detrimental cytotoxic Tax-specific CD8(+) T cell population, anti-HTLV-I antibodies, and neurotoxic cytokines involved in disruption of myelin-producing cells and neuronal degradation characteristic of HAM/TSP.

HBV integrants of hepatocellular carcinoma cell lines contain an active enhancer

Hepatitis B virus (HBV) infection is a major risk factor worldwide for the development of hepatocellular carcinoma (HCC). Integrated HBV DNA fragments, often highly rearranged, are frequently detected in HCC. In woodchuck, the viral enhancer plays a central role in hepatocarcinogenesis, but in humans the mechanism of HBV oncogenesis has not been established. In this study we investigated the status of the viral enhancer in two human HCC cell lines, Hep3B and PLC/PRF/5 each containing one or more integrated HBV DNA fragments. Active enhancer was defined by virtue of its protein occupancy as determined by genomic in vivo DMS footprinting. In PLC/PRF/5 cells, the HBV DNA was integrated in a cellular gene at chromosome 11q13, at a locus reported to be amplified in many tumors. We show here that in both cell lines, the integrated HBV DNA fragments contain an active enhancer-I. In particular, the occupation of the two previously defined basic enhancer elements, E and EP, was prominent. While in both cell lines the same protein binds to the EP elements, the E element, however, is occupied in a cell-line specific manner. In PLC/PRF/5 but not Hep3B, the prominent binding of an undefined protein was detected. Our data suggest that this protein is likely to be the fetoprotein transcription factor (FTF). The finding that enhancer sequences are conserved and functional in different cell lines suggests a selection pressure for their long-term maintenance. We therefore propose that the HBV enhancer-I might play a role in hepatocellular carcinogenesis.

Induction of Tax i expression in MT-4 cells by 5-azacytidine leads to protein binding in the HTLV-1 LTR in vivo

The Tax I trans-activator protein of the human T-cell leukemia virus I (HTLV-I) enhances viral gene expression through enhancer sequences in the viral LTR. These sequences consist of three imperfect 21-bp repeats (TRE-1) and a region between the promoter central and promoter proximal TRE-1 (known as TRE-2). We have previously described the in vivo footprint of the HTLV-I TRE-1s and TRE-2 in two HTLV-I-infected cell lines, MT-2 and MT-4. MT-2 is a high-level producer of virus and shows significant DNA-protein interactions within the TRE-1s and TRE-2. In contrast, the proviral DNA in MT-4 cells is heavily methylated and produces no detectable virus. In this report, we describe the footprints of the TRE-1s and TRE-2 in MT-4 cells that were induced to express high levels of viral proteins by treatment with 5-azacytidine, a potent inhibitor of methylation. The footprints of the TRE-1s in 5-azacytidine-treated MT-4 cells were virtually identical to those observed in MT-2 cells. In contrast, the footprints within the TRE-2 region of 5-azacytidine-treated MT-4 cells did not resemble those in either MT-2 or MT-4 cells.