Human T-lymphotropic type 1 virus p30 inhibits homologous recombination and favors unfaithful DNA repair

Viral Modulation of the DNA Damage Response and Innate Immunity: Two Sides of the Same Coin

The DDR consists of multiple pathways that sense, signal, and respond to anomalous DNA. To promote efficient replication, viruses have evolved to engage and even modulate the DDR. In this review, we will discuss a select set of diverse viruses and the range of mechanisms they evolved to interact with the DDR and some of the subsequent cellular consequences. There is a dichotomy in that the DDR can be both beneficial for viruses yet antiviral. We will also review the connection between the DDR and innate immunity. Previously believed to be disparate cellular functions, more recent research is emerging that links these processes. Furthermore, we will discuss some discrepancies in the literature that we propose can be remedied by utilizing more consistent DDR-focused assays. By doing so, we hope to obtain a much clearer understanding of how broadly these mechanisms and phenotypes are conserved among all viruses. This is crucial for human health since understanding how viruses manipulate the DDR presents an important and tractable target for antiviral therapies.

Tampering of Viruses and Bacteria with Host DNA Repair: Implications for Cellular Transformation

A reduced ability to properly repair DNA is linked to a variety of human diseases, which in almost all cases is associated with an increased probability of the development of cellular transformation and cancer. DNA damage, that ultimately can lead to mutations and genomic instability, is due to many factors, such as oxidative stress, metabolic disorders, viral and microbial pathogens, excess cellular proliferation and chemical factors. In this review, we examine the evidence connecting DNA damage and the mechanisms that viruses and bacteria have evolved to hamper the pathways dedicated to maintaining the integrity of genetic information, thus affecting the ability of their hosts to repair the damage(s). Uncovering new links between these important aspects of cancer biology might lead to the development of new targeted therapies in DNA-repair deficient cancers and improving the efficacy of existing therapies. Here we provide a comprehensive summary detailing the major mechanisms that viruses and bacteria associated with cancer employ to interfere with mechanisms of DNA repair. Comparing these mechanisms could ultimately help provide a common framework to better understand how certain microorganisms are involved in cellular transformation.

p30 protein: a critical regulator of HTLV-1 viral latency and host immunity

The extraordinarily high prevalence of HTLV-1 subtype C (HTLV-1C) in some isolated indigenous communities in Oceania and the severity of the health conditions associated with the virus impress the great need for basic and translational research to prevent and treat HTLV-1 infection. The genome of the virus’s most common subtype, HTLV-1A, encodes structural, enzymatic, and regulatory proteins that contribute to viral persistence and pathogenesis. Among these is the p30 protein encoded by the doubly spliced Tax-orf II mRNA, a nuclear/nucleolar protein with both transcriptional and post-transcriptional activity. The p30 protein inhibits the productive replication cycle via nuclear retention of the mRNA that encodes for both the viral transcriptional trans-activator Tax, and the Rex proteins that regulate the transport of incompletely spliced viral mRNA to the cytoplasm. In myeloid cells, p30 inhibits the PU-1 transcription factor that regulates interferon expression and is a critical mediator of innate and adaptive immunity. Furthermore, p30 alters gene expression, cell cycle progression, and DNA damage responses in T-cells, raising the hypothesis that p30 may directly contribute to T cell transformation. By fine-tuning viral expression while also inhibiting host innate responses, p30 is likely essential for viral infection and persistence. This concept is supported by the finding that macaques, a natural host for the closely genetically related simian T-cell leukemia virus 1 (STLV-1), exposed to an HTLV-1 knockout for p30 expression by a single point mutation do not became infected unless reversion and selection of the wild type HTLV-1 genotype occurs. All together, these data suggest that inhibition of p30 may help to curb and eventually eradicate viral infection by exposing infected cells to an effective host immune response.

Silencers of HTLV-1 and HTLV-2: the pX-encoded latency-maintenance factors

Of the members of the primate T cell lymphotropic virus (PTLV) family, only the human T-cell leukemia virus type-1 (HTLV-1) causes disease in humans—as the etiological agent of adult T-cell leukemia/lymphoma (ATLL), HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), and other auto-inflammatory disorders. Despite having significant genomic organizational and structural similarities, the closely related human T-cell lymphotropic virus type-2 (HTLV-2) is considered apathogenic and has been linked with benign lymphoproliferation and mild neurological symptoms in certain infected patients. The silencing of proviral gene expression and maintenance of latency are central for the establishment of persistent infections in vivo. The conserved pX sequences of HTLV-1 and HTLV-2 encode several ancillary factors which have been shown to negatively regulate proviral gene expression, while simultaneously activating host cellular proliferative and pro-survival pathways. In particular, the ORF-II proteins, HTLV-1 p30^II and HTLV-2 p28^II, suppress Tax-dependent transactivation from the viral promoter—whereas p30^II also inhibits PU.1-mediated inflammatory-signaling, differentially augments the expression of p53-regulated metabolic/pro-survival genes, and induces lymphoproliferation which could promote mitotic proviral replication. The ubiquitinated form of the HTLV-1 p13^II protein localizes to nuclear speckles and interferes with recruitment of the p300 coactivator by the viral transactivator Tax. Further, the antisense-encoded HTLV-1 HBZ and HTLV-2 APH-2 proteins and mRNAs negatively regulate Tax-dependent proviral gene expression and activate inflammatory signaling associated with enhanced T-cell lymphoproliferation. This review will summarize our current understanding of the pX latency-maintenance factors of HTLV-1 and HTLV-2 and discuss how these products may contribute to the differences in pathogenicity between the human PTLVs.

Intrinsically Disordered Human T Lymphotropic Virus Type 1 p30 Protein: Experimental and Computational Evidence

Human T lymphotropic virus type 1 (HTLV-1) causes adult T cell leukemia and lymphoma and other neuroinflammatory diseases. The pX region of HTLV-1 genome encodes an accessory protein p30 that is required for viral persistence and spread in the host. p30 regulates viral gene expression at the transcription level by competing with Tax for p300 binding and at posttranscriptional level by nuclear retention of tax/rex messenger RNA (mRNA). In addition, p30 modulates the host cellular environment by binding to various host proteins such as ATM, REGγ, and PRMT5. However, the low expression levels of p30 has been a major hurdle in studying its structure-function relationship in the context of HTLV-1 pathobiology, which is most likely due to its intrinsically disordered nature. To investigate the unstable nature of p30, flow cytometric analysis of p30-GFP fusion protein expressed in Escherichia coli was conducted and bioinformatics analysis of p30 was performed. The bacterial cells were green fluorescent protein (GFP) positive, indicating that p30-GFP was in the soluble fraction. Induction, particularly at higher temperature, reduced the expression of p30-GFP. Moreover, p30-GFP was detected exclusively in insoluble fraction upon cell lysis, suggesting its unstable and disordered nature. The bioinformatics analysis of p30 protein sequence and amino acid content revealed that p30 has highly disordered regions from amino acids 75-155 and 197-241. Furthermore, p30 has regions for macromolecular interactions that could stabilize it and these regions coincide with the unstable regions. Collectively, the study indicates that HTLV-1 p30 is an intrinsically disordered protein.

Genomic exaptation enables Lasius niger adaptation to urban environments

Background

The world is rapidly urbanizing, and only a subset of species are able to succeed in stressful city environments. Efficient genome-enabled stress response appears to be a likely prerequisite for urban adaptation. Despite the important role ants play in the ecosytem, only the genomes of ~13 have been sequenced so far. Here, we present the draft genome assembly of the black garden ant Lasius niger – the most successful urban inhabitant of all ants – and we compare it with the genomes of other ant species, including the closely related Camponotus floridanus.

Results

Sequences from 272 M Illumina reads were assembled into 41,406 contigs with total length of 245 MB, and N50 of 16,382 bp, similar to other ant genome assemblies enabling comparative genomic analysis. Remarkably, the predicted proteome of L. niger is significantly enriched relative to other ant genomes in terms of abundance of domains involved in nucleic acid binding, DNA repair, and nucleotidyl transferase activity, reflecting transposable element proliferation and a likely genomic response. With respect to environmental stress, we note a proliferation of various detoxification genes, including glutatione-S-transferases and those in the cytochrome P450 families. Notably, the CYP9 family is highly expanded with 19 complete and 21 nearly complete members - over twice as many compared to other ants. This family exhibits the signatures of strong directional selection, with eleven positively selected positions in ligand-binding pockets of enzymes. Gene family contraction was detected for several components of the olfactory system, accompanied by instances of both directional selection and relaxation.

Conclusions

Our results suggest that the success of L. niger in urbanized areas may be the result of fortuitous coincidence of several factors, including the expansion of the CYP9 cytochrome family due to coevolution with parasitic fungi, the diversification of DNA repair systems as an answer to proliferation of retroelements, and the reduction of olfactory system and behavioral preadaptations from non-territorial subdominant life strategies found in natural environments. Diversification of cytochromes and DNA repair systems along with reduced odorant communication are the basis of L. niger pollutant resistance and polyphagy, while non-territorial and mobilization strategies allows more efficient exploitation of large but patchy food sources.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0867-x) contains supplementary material, which is available to authorized users.

DNA Repair--A Double-Edged Sword in the Genomic Stability of Cancer Cells--The Case of Chronic Myeloid Leukemia

Genomic instability is a common feature of cancer cells, which can result from aberrant DNA damage reaction (DDR). We and others showed that the well-known BCR-ABL1 fusion oncogene, the cause of chronic myeloid leukemia, induced an increased production of reactive oxygen species (ROS) and conferred therapeutic drug resistance by suppression of apoptotic signaling, prolonged G2/M arrest and stimulation of several pathways of DNA repair. However, to protect from apoptosis, cancer cells may tolerate some DNA lesions, which may increase genomic instability. Moreover, BCR/ABL1-stimulated DNA repair might be faulty, especially non-homologous end joining in its alternative forms. Normal DNA repair can remove DNA damage and prevent mutations, reducing genome instability, but on the other hand, due to its imprecise nature, it may increase genomic instability by increasing the ratio of mutagenic DNA lesions. The example of BCR-ABL1-expressing cells shows that DNA repair can both increase and decrease genomic instability of cancer cells and understanding the mechanism of the regulation of these opposite effects would be helpful in anticancer strategies.

HTLV-I Tax-Mediated Inactivation of Cell Cycle Checkpoints and DNA Repair Pathways Contribute to Cellular Transformation: "A Random Mutagenesis Model"

To achieve cellular transformation, most oncogenic retroviruses use transduction by proto-oncogene capture or insertional mutagenesis, whereby provirus integration disrupts expression of tumor suppressors or proto-oncogenes. In contrast, the Human T-cell leukemia virus type 1 (HTLV-I) has been classified in a separate class referred to as "transactivating retroviruses". Current views suggest that the viral encoded Tax protein transactivates expression of cellular genes leading to deregulated growth and transformation. However, if Tax-mediated transactivation was indeed sufficient for cellular transformation, a fairly high frequency of infected cells would eventually become transformed. In contrast, the frequency of transformation by HTLV-I is very low, likely less than 5%. This review will discuss the current understanding and recent discoveries highlighting critical functions of Tax in cellular transformation. HTLV-I Tax carries out essential functions in order to override cell cycle checkpoints and deregulate cellular division. In addition, Tax expression is associated with increased DNA damage and genome instability. Since Tax can inhibit multiple DNA repair pathways and stimulate unfaithful DNA repair or bypass checkpoints, these processes allow accumulation of genetic mutations in the host genome. Given this, a "Random Mutagenesis" transformation model seems more suitable to characterize the oncogenic activities of HTLV-I.

Modes of Human T Cell Leukemia Virus Type 1 Transmission, Replication and Persistence

Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that causes cancer (Adult T cell Leukemia, ATL) and a spectrum of inflammatory diseases (mainly HTLV-associated myelopathy—tropical spastic paraparesis, HAM/TSP). Since virions are particularly unstable, HTLV-1 transmission primarily occurs by transfer of a cell carrying an integrated provirus. After transcription, the viral genomic RNA undergoes reverse transcription and integration into the chromosomal DNA of a cell from the newly infected host. The virus then replicates by either one of two modes: (i) an infectious cycle by virus budding and infection of new targets and (ii) mitotic division of cells harboring an integrated provirus. HTLV-1 replication initiates a series of mechanisms in the host including antiviral immunity and checkpoint control of cell proliferation. HTLV-1 has elaborated strategies to counteract these defense mechanisms allowing continuous persistence in humans.

Modulation of DNA damage and repair pathways by human tumour viruses

With between 10% and 15% of human cancers attributable to viral infection, there is great interest, from both a scientific and clinical viewpoint, as to how these pathogens modulate host cell functions. Seven human tumour viruses have been identified as being involved in the development of specific malignancies. It has long been known that the introduction of chromosomal aberrations is a common feature of viral infections. Intensive research over the past two decades has subsequently revealed that viruses specifically interact with cellular mechanisms responsible for the recognition and repair of DNA lesions, collectively known as the DNA damage response (DDR). These interactions can involve activation and deactivation of individual DDR pathways as well as the recruitment of specific proteins to sites of viral replication. Since the DDR has evolved to protect the genome from the accumulation of deleterious mutations, deregulation is inevitably associated with an increased risk of tumour formation. This review summarises the current literature regarding the complex relationship between known human tumour viruses and the DDR and aims to shed light on how these interactions can contribute to genomic instability and ultimately the development of human cancers.

Acetylation of the c-MYC oncoprotein is required for cooperation with the HTLV-1 p30(II) accessory protein and the induction of oncogenic cellular transformation by p30(II)/c-MYC

The human T-cell leukemia retrovirus type-1 (HTLV-1) p30(II) protein is a multifunctional latency-maintenance factor that negatively regulates viral gene expression and deregulates host signaling pathways involved in aberrant T-cell growth and proliferation. We have previously demonstrated that p30(II) interacts with the c-MYC oncoprotein and enhances c-MYC-dependent transcriptional and oncogenic functions. However, the molecular and biochemical events that mediate the cooperation between p30(II) and c-MYC remain to be completely understood. Herein we demonstrate that p30(II) induces lysine-acetylation of the c-MYC oncoprotein. Acetylation-defective c-MYC Lys→Arg substitution mutants are impaired for oncogenic transformation with p30(II) in c-myc(-/-) HO15.19 fibroblasts. Using dual-chromatin-immunoprecipitations (dual-ChIPs), we further demonstrate that p30(II) is present in c-MYC-containing nucleoprotein complexes in HTLV-1-transformed HuT-102 T-lymphocytes. Moreover, p30(II) inhibits apoptosis in proliferating cells expressing c-MYC under conditions of genotoxic stress. These findings suggest that c-MYC-acetylation is required for the cooperation between p30(II)/c-MYC which could promote proviral replication and contribute to HTLV-1-induced carcinogenesis.

DNA repair mechanisms and human cytomegalovirus (HCMV) infection

Herpesvirus infections, such as those induced by human cytomegalovirus (HCMV), induce specific DNA damages. DNA damages can lead to cell mutation, death, apoptosis and immune system activation. Various types of DNA damage are repaired through multiple repair pathways, such as base excision, nucleotide excision, homologous recombination and nonhomologous end joining. Changes in the activity of DNA repair proteins during viral infection can cause disturbances in the DNA repair system and change its mechanisms. This report reviews results from studies, assaying a DNA repair system in HCMV infection.

Modulation of homology-directed repair in T98G glioblastoma cells due to interactions between wildtype p53, Rad51 and HCMV IE1-72

Human cytomegalovirus (HCMV) is a ubiquitous pathogen capable of causing life threatening consequences in neonates and immune-compromised individuals. HCMV inflicts site-specific double strand breaks (DSBs) in the cellular genome. DNA damage infliction raises the corollary question of virus modulation of DNA repair. We recently reported HDR was stimulated in wt human foreskin fibroblasts (HFFs) during fully permissive infection or expression of the HCMV protein IE1-72 (IE72). These studies have been extended into semi-permissive T98G glioblastoma cells. T98Gs encode a mutant p53, which may contribute to their high baseline rate of HDR. We fully expected HCMV infection to increase HDR in T98Gs, similar to its effects in HFFs. Surprisingly in T98Gs HCMV infection, or sole expression of IE72, decreased HDR by two-fold. Transient expression of wt p53 in T98Gs also reduced HDR by two-fold. Dual transient expression of wt p53 and IE72 restored high baseline HDR levels. GST pulldown experiments revealed that both IE72 and wt p53 bound the important HDR protein, Rad51. We conclude that the expression of certain HCMV proteins can modulate HDR in an infected cell, dependent upon p53 status. We propose a model of the protein interactions explaining this behavior.

Animal models of bovine leukemia virus and human T-lymphotrophic virus type-1: insights in transmission and pathogenesis

Bovine leukemia virus (BLV) and human T-lymphotrophic virus type-1 (HTLV-1) are related retroviruses associated with persistent and lifelong infections and a low incidence of lymphomas within their hosts. Both viruses can be spread through contact with bodily fluids containing infected cells, most often from mother to offspring through breast milk. Each of these complex retroviruses contains typical gag, pol, and env genes but also unique, nonstructural proteins encoded from the pX region. These nonstructural genes encode the Tax and Rex regulatory proteins, as well as novel proteins essential for viral spread in vivo. Improvements in the molecular tools to test these viral determinants in cellular and animal models have provided new insights into the pathogenesis of each virus. Comparisons of BLV and HTLV-1 provide insights into mechanisms of spread and tumor formation, as well as potential approaches to therapeutic intervention against the infections.

The need to accessorize: molecular roles of HTLV-1 p30 and HTLV-2 p28 accessory proteins in the viral life cycle

Extensive studies of human T-cell leukemia virus (HTLV)-1 and HTLV-2 over the last three decades have provided detailed knowledge on viral transformation, host–viral interactions and pathogenesis. HTLV-1 is the etiological agent of adult T cell leukemia and multiple neurodegenerative and inflammatory diseases while HTLV-2 disease association remains elusive, with few infected individuals displaying neurodegenerative diseases similar to HTLV-1. The HTLV group of oncoretroviruses has a genome that encodes structural and enzymatic proteins Gag, Pro, and Env, regulatory proteins Tax and Rex, and several accessory proteins from the pX region. Of these proteins, HTLV-1 p30 and HTLV-2 p28 are encoded by the open reading frame II of the pX region. Like most other accessory proteins, p30 and p28 are dispensable for in vitro viral replication and transformation but are required for efficient viral replication and persistence in vivo. Both p30 and p28 regulate viral gene expression at the post-transcriptional level whereas p30 can also function at the transcriptional level. Recently, several reports have implicated p30 and p28 in multiple cellular processes, which provide novel insight into HTLV spread and survival and ultimately pathogenesis. In this review we summarize and compare what is known about p30 and p28, highlighting their roles in viral replication and viral pathogenesis.

Human T-lymphotropic virus type 1 non-structural proteins: Requirements for latent infection

It has been more than 30 years since the discovery of human T-lymphotropic virus type 1 (HTLV-1), the first human retrovirus identified. Human T-lymphotropic virus type 1 infects 15-20 million people worldwide causing two major diseases: adult T-cell leukemia/lymphoma and HTLV-1-associated myelopathy/tropical spastic paraparesis. Human T-lymphotropic virus type 1 establishes several decades of latent infection, during which viral-host interaction determines disease segregation. This review highlights non-structural proteins that are encoded on the viral genome and manage latent infection. Latent infection is a key in HTLV pathology, so that effective inhibition of these proteins might lead to successful disease management.

Overview on HTLV-1 p12, p8, p30, p13: accomplices in persistent infection and viral pathogenesis

The human T-lymphotropic virus type-1 (HTLV-1) is etiologically linked to adult T cell leukemia/lymphoma and tropical spastic paraparesis/HTLV-1-associated myelopathy. While the role of Tax and Rex in viral replication and pathogenesis has been extensively studied, recent evidence suggests that additional viral proteins are essential for the virus life cycle in vivo. In this review, we will summarize possible molecular mechanisms evoked in the literature to explain how p12, p8, p30, and p13 facilitate persistent viral infection of the host. We will explore several stratagems used by HTLV-1 accessory genes to escape immune surveillance, to establish latency, and to deregulate cell cycle and apoptosis to participate in virus-mediated cellular transformation.

HCMV-infected cells maintain efficient nucleotide excision repair of the viral genome while abrogating repair of the host genome

Many viruses subvert the host cell's ability to mount and complete various DNA damage responses (DDRs) after infection. HCMV infection of permissive fibroblasts activates host DDRs at the time of viral deposition and during replication, but the DDRs remain uncompleted without arrest or apoptosis. We believe this was in part due to partitioning of the damage response and double strand break repair components. After extraction of soluble proteins, the localization of these components fell into three groups: specifically associated with the viral replication centers (RCs), diffused throughout the nucleoplasm and excluded from the RCs. Others have shown that cells are incapable of processing exogenously introduced damage after infection. We hypothesized that the inability of the cells to process damage might be due to the differential association of repair components within the RCs and, in turn, potentially preferential repair of the viral genome and compromised repair of the host genome. To test this hypothesis we used multiple strategies to examine repair of UV-induced DNA damage in mock and virus-infected fibroblasts. Comet assays indicated that repair was initiated, but was not completed in infected cells. Quantitative analysis of immunofluorescent localization of cyclobutane pyrimidine dimers (CPDs) revealed that after 24 h of repair, CPDs were significantly reduced in viral DNA, but not significantly changed in the infected host DNA. To further quantitate CPD repair, we developed a novel dual-color Southern protocol allowing visualization of host and viral DNA simultaneously. Combining this Southern methodology with a CPD-specific T4 endonuclease V alkaline agarose assay to quantitate repair of adducts, we found efficient repair of CPDs from the viral DNA but not host cellular DNA. Our data confirm that NER functions in HCMV-infected cells and almost exclusively repairs the viral genome to the detriment of the host's genome.

Human cytomegalovirus (HCMV) is a leading cause of birth defects. This may be due in part to this virus' ability to inflict specific damage to its host's DNA, combined with the disruption of an infected cell's ability to repair damage. Earlier studies found that components of the cell's repair machinery were differentially associated with the HCMV viral replication centers in the nucleus. Experiments here extend this observation to include components of the machinery involved in UV lesion repair. We hypothesized that association of components of the DNA repair machinery within the viral replication centers could favor the repair of viral DNA, but more importantly, be detrimental to the repair of cellular DNA. Infected cells were irradiated and examined for repair by three different methods. In the course of this study, we developed a new technique allowing simultaneous evaluation of both the viral and host genomes in an infected cell. These experiments found rapid, selective removal of UV lesions from the viral and not the cellular DNA within infected cells. Our results indicate the differential association of certain cellular repair proteins with this virus may have far-reaching implications in the disease pathogenesis of HCMV infection.

PA28γ is a novel corepressor of HTLV-1 replication and controls viral latency

The establishment of a latent reservoir by human tumor viruses is a vital step in initiating cellular transformation and represents a major shortcoming to current therapeutic strategies and the ability to eradicate virus-infected cells. Human T-cell leukemia virus type 1 (HTLV-1) establishes a lifelong infection and is linked to adult T-cell leukemia lymphoma (ATLL). Here, we demonstrate that HTLV-1 p30 recruits the cellular proteasome activator PA28γ onto the viral tax/rex mRNA to prevent its nuclear export and suppress virus replication. Interaction of p30 with a PA28γ retaining fully functional proteasome activity is required for p30's ability to repress HTLV-1. Consistently, HTLV-1 molecular clones replicate better and produce more virus particles in PA28γ-deficient cells. These results define a unique and novel role for the cellular factor PA28γ in the control of nuclear RNA trafficking and HTLV-1–induced latency. Importantly, knockdown of PA28γ expression in ATLL cells latently infected with HTLV-1 reactivates expression of viral tax/rex RNA and the Tax protein. Because Tax is the most immunogenic viral antigen and triggers strong CTL responses, our results suggest that PA28γ-targeted therapy may reactivate virus expression from latently infected cells and allow their eradication from the host.

KEY POINTS

PA28γ acts as a co-repressor of HTLV-1 p30 to suppress virus replication and is required for the maintenance of viral latency. HTLV-1 has evolved a unique function mediated by its posttranscriptional repressor p30, which is not found in HTLV-2.

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.

Comparative host protein interactions with HTLV-1 p30 and HTLV-2 p28: insights into difference in pathobiology of human retroviruses

Background

Human T lymphotropic virus type-1 (HTLV-1) and type 2 (HTLV-2) are closely related human retroviruses, but have unique disease associations. HTLV-1 is the causative agent of an aggressive T-cell leukemia known as adult T-cell leukemia (ATL), HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP), and other inflammatory diseases. HTLV-2 infection has not been clearly associated with any disease condition. Although both viruses can transform T cells in vitro, the HTLV-1 provirus is mainly detected in CD4+ T cells whereas HTLV-2 is mainly detected in CD8+ T cells of infected individuals. HTLV-1 and HTLV-2 encode accessory proteins p30 and p28, respectively, which share partial amino acid homology and are required for viral persistence in vivo. The goal of this study was to identify host proteins interacting with p30 and p28 in order to understand their role in pathogenesis.

Results

Affinity-tag purification coupled with mass spectrometric (MS) analyses revealed 42 and 22 potential interacting cellular partners of p30 and p28, respectively. Of these, only three cellular proteins, protein arginine methyltransferase 5 (PRMT5), hnRNP K and 60 S ribosomal protein L8 were detected in both p30 and p28 fractions. To validate the proteomic results, four interacting proteins were selected for further analyses using immunoblot assays. In full agreement with the MS analysis two cellular proteins REGγ and NEAF-interacting protein 30 (NIP30) selectively interacted with p30 and not with p28; heterogeneous nuclear ribonucleoprotein H1 (hnRNP H1) bound to p28 and not to p30; and PRMT5 interacted with both p30 and p28. Further studies demonstrated that reduced levels of PRMT5 resulted in decreased HTLV-2 viral gene expression whereas the viral gene expression of HTLV-1 was unchanged.

Conclusion

The comparisons of p30 and p28 host protein interaction proteome showed striking differences with some degree of overlap. PRMT5, one of the host proteins that interacted with both p30 and p28 differentially affected HTLV-1 and HTLV-2 viral gene expression suggesting that PRMT5 is involved at different stages of HTLV-1 and HTLV-2 biology. These findings suggest that distinct host protein interaction profiles of p30 and p28 could, in part, be responsible for differences in HTLV-1 and HTLV-2 pathobiology. This study provides new avenues of investigation into mechanisms of viral infection, tropism and persistence.

Mechanisms of human T-lymphotropic virus type 1 transmission and disease

Human T-lymphotrophic virus type-1 (HTLV-1) infects approximately 15-20 million people worldwide, with endemic areas in Japan, the Caribbean, and Africa. The virus is spread through contact with bodily fluids containing infected cells most often from mother to child through breast milk or via blood transfusion. After prolonged latency periods, approximately 3-5% of HTLV-1 infected individuals will develop either adult T-cell leukemia/lymphoma, or other lymphocyte-mediated disorders such as HTLV-1-associated myelopathy/tropical spastic paraparesis. The genome of this complex retrovirus contains typical gag, pol, and env genes, but also unique nonstructural proteins encoded from the pX region. These nonstructural genes encode the Tax and Rex regulatory proteins, as well as novel proteins essential for viral spread in vivo such as p30, p12, p13 and the antisense-encoded HTLV-1 basic leucine zipper factor (HBZ). While progress has been made in knowledge of viral determinants of cell transformation and host immune responses, host and viral determinants of HTLV-1 transmission and spread during the early phases of infection are unclear. Improvements in the molecular tools to test these viral determinants in cellular and animal models have provided new insights into the early events of HTLV-1 infection. This review will focus on studies that test HTLV-1 determinants in context to full-length infectious clones of the virus providing insights into the mechanisms of transmission and spread of HTLV-1.

Molecular determinants of human T-lymphotropic virus type 1 transmission and spread

Human T-lymphotrophic virus type-1 (HTLV-1) infects approximately 15 to 20 million people worldwide, with endemic areas in Japan, the Caribbean, and Africa. The virus is spread through contact with bodily fluids containing infected cells, most often from mother to child through breast milk or via blood transfusion. After prolonged latency periods, approximately 3 to 5% of HTLV-1 infected individuals will develop either adult T-cell leukemia/lymphoma (ATL), or other lymphocyte-mediated disorders such as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The genome of this complex retrovirus contains typical gag, pol, and env genes, but also unique nonstructural proteins encoded from the pX region. These nonstructural genes encode the Tax and Rex regulatory proteins, as well as novel proteins essential for viral spread in vivo such as, p30, p12, p13 and the antisense encoded HBZ. While progress has been made in the understanding of viral determinants of cell transformation and host immune responses, host and viral determinants of HTLV-1 transmission and spread during the early phases of infection are unclear. Improvements in the molecular tools to test these viral determinants in cellular and animal models have provided new insights into the early events of HTLV-1 infection. This review will focus on studies that test HTLV-1 determinants in context to full length infectious clones of the virus providing insights into the mechanisms of transmission and spread of HTLV-1.

Orf-I and orf-II-encoded proteins in HTLV-1 infection and persistence

The 3′ end of the human T-cell leukemia/lymphoma virus type-1 (HTLV-1) genome contains four overlapping open reading frames (ORF) that encode regulatory proteins. Here, we review current knowledge of HTLV-1 orf-I and orf-II protein products. Singly spliced mRNA from orf-I encodes p12, which can be proteolytically cleaved to generate p8, while differential splicing of mRNA from orf-II results in production of p13 and p30. These proteins have been demonstrated to modulate transcription, apoptosis, host cell activation and proliferation, virus infectivity and transmission, and host immune responses. Though these proteins are not essential for virus replication in vitro, p8, p12, p13, and p30 have an important role in the establishment and maintenance of HTLV-1 infection in vivo.