Characterization of new RNA polymerase III and RNA polymerase II transcriptional promoters in the Bovine Leukemia Virus genome

A complex network of transcription factors and epigenetic regulators involved in bovine leukemia virus transcriptional regulation

Bovine Leukemia Virus (BLV) is the etiological agent of enzootic bovine leukosis, a disease characterized by the neoplastic proliferation of B cells in cattle. While most European countries have introduced efficient eradication programs, BLV is still present worldwide and no treatment is available. A major feature of BLV infection is the viral latency, which enables the escape from the host immune system, the maintenance of a persistent infection and ultimately the tumoral development. BLV latency is a multifactorial phenomenon resulting in the silencing of viral genes due to genetic and epigenetic repressions of the viral promoter located in the 5' Long Terminal Repeat (5'LTR). However, viral miRNAs and antisense transcripts are expressed from two different proviral regions, respectively the miRNA cluster and the 3'LTR. These latter transcripts are expressed despite the viral latency affecting the 5'LTR and are increasingly considered to take part in tumoral development. In the present review, we provide a summary of the experimental evidence that has enabled to characterize the molecular mechanisms regulating each of the three BLV transcriptional units, either through cis-regulatory elements or through epigenetic modifications. Additionally, we describe the recently identified BLV miRNAs and antisense transcripts and their implications in BLV-induced tumorigenesis. Finally, we discuss the relevance of BLV as an experimental model for the closely related human T-lymphotropic virus HTLV-1.

Role of the cellular factor CTCF in the regulation of bovine leukemia virus latency and three-dimensional chromatin organization

Bovine leukemia virus (BLV)-induced tumoral development is a multifactorial phenomenon that remains incompletely understood. Here, we highlight the critical role of the cellular CCCTC-binding factor (CTCF) both in the regulation of BLV transcriptional activities and in the deregulation of the three-dimensional (3D) chromatin architecture surrounding the BLV integration site. We demonstrated the in vivo recruitment of CTCF to three conserved CTCF binding motifs along the provirus. Next, we showed that CTCF localized to regions of transitions in the histone modifications profile along the BLV genome and that it is implicated in the repression of the 5′Long Terminal Repeat (LTR) promoter activity, thereby contributing to viral latency, while favoring the 3′LTR promoter activity. Finally, we demonstrated that BLV integration deregulated the host cellular 3D chromatin organization through the formation of viral/host chromatin loops. Altogether, our results highlight CTCF as a new critical effector of BLV transcriptional regulation and BLV-induced physiopathology.

The variation in promoter sequences of the Akt3 gene between cow and buffalo revealed different responses against mastitis

BACKGROUND

Serine/threonine kinase 3 (AKT3) is a protein-coding gene that is associated with several cattle immune diseases including different tumors and cancers. The objective of this study was to investigate the differences in structures and functions of AKT3 of cow and buffalo cattle.

METHODS

The sequence differences of gene-coding sequence (CDS) and core promoter region of AKT3 in cow and buffalo were analyzed by using bioinformatics tools and PCR sequencing. Also, the functional analysis of promoter regulating gene expression by RT-PCR was performed using 500 Holstein cows and buffalos. And, evaluation of AKT3 inflammatory response to the lipopolysaccharide (LPS)-induced mastitis was performed between both species.

RESULTS

The results revealed the variation in 6 exons out of 13 exons of the two species of CDS. Also, 4 different regions in 3-kb promoters of the AKT3 gene were significantly different between cow and buffalo species, in which cow's AKT3 promoter sequence region was started from - 371 to - 1247, while in buffalo, the sequence was started from - 371 to - 969 of the promoter crucial region. Thus, the promoter was overexpressed in cows compared to buffaloes. As a result, significant differences (P < 0.05) between the two species in the AKT3 gene expression level related to the LPS stimulation in their mammary epithelial cell line.

CONCLUSIONS

This study emphasized the great importance of the structural differences of AKT3 between the animal species on their different responses against immune diseases like mastitis.

Expression of Viral microRNAs in Serum and White Blood Cells of Cows Exposed to Bovine Leukemia Virus

Bovine leukemia virus (BLV) affects the health and productivity of cattle. The virus causes abnormal immune function and immunosuppression. MicroRNAs (miRNAs) are involved in gene expression, having been associated with stress and immune response, tumor growth, and viral infection. The objective of this study was to determine the expression of circulating miRNAs produced by BLV in animals exposed to the virus. Sera from 14 animals were collected to establish IgG reactivity to BLV by ELISA, where seven animals were seropositive and seven were seronegative for BLV exposure. White blood cells (WBC) from each animal were also collected and miRNAs were identified by sequencing from sera and WBC. The seropositive group had higher counts of BLV miRNAs when compared to seronegative group in sera and WBC. Blv-miR-1-3p, blv-miR-B2-5p, blv-miR-B4-3p, and blv-miR-B5-5p were statistically significant (P < 0.00001) in serum with an average of 7 log2 fold difference between seropositive and seronegative groups. Blv-miR-B1-3p, blv-miR-B1-5p, blv-miR-B3, blv-miR-B4-3p, blv-miR-B4-5p, blv-miR-B5-5p were statistically significant (P < 1.08e⁻⁹) in WBC with an average of 7 log2 fold difference between the seropositive and the seronegative groups. Blv-miR-B2-3p and blv-miR-B2-5p were also statistically significant in WBC (P < 2.79e-17), with an average of 27 log2 fold difference between the seropositive and the seronegative groups. There were 18 genes identified as being potential targets for blv-miR-B1-5p, and 3 genes for blv-miR-B4-5p. Gene ontology analysis indicated that the target genes are mainly involved in the response to stress and in the immune system process. Several of the identified genes have been associated with leukemia development in humans and cattle. Differential expression of genes targeted by BLV miRNAs should be evaluated to determine their effect in BLV replication.

Leukocytosis and Expression of Bovine Leukemia Virus microRNAs in Cattle

Bovine Leukemia Virus (BLV) is an established model for studying retroviral infections, in particular the infection by the human T-cell leukemia type 1 (HTLV-1) virus. Here, we quantified gene expression of several BLV-related genes: effector protein of T and NK-killer cells NK-lysin (Nklys), reverse BLV transcriptase pol, BLV receptor (blvr), and also key enzymes of the microRNA maturation, Dicer (dc1) and Argonaut (ago2). The differences in the expression of the above genes were compared between five groups: (1) BLV infected cows with high and (2) low lymphocyte count, (3) with and (4) without BLV microRNA expressions, and (5) cows without BLV infections (control group). As compared to control, infected cows with high lymphocyte count and BLV microRNA expression had significantly decreased Nklys gene expression and increased dc1 and ago2 gene expressions. Few infected animals without pol gene expression nevertheless transcribed BLV microRNA, while others with pol gene expression didn't transcribe BLV microRNA. Notably, Pol expression significantly (P < 0.05) correlated with dc1 expression. For infected animals, there were no direct correlations between the number of leukocytes and pol, Nklys, and BLV microRNA gene expressions. Blvr gene expression is typical for juvenile lymphocytes and decreases during terminal differentiation. Our data suggest that BLV infects primarily juvenile lymphocytes, which further divide into two groups. One group expresses BLV DNA and another one expressed BLV microRNA that decreases host immune response against cells, expressing BLV proteins. It is suspected that regulatory microRNAs play a significant role in the bovine leukemia infections, yet the precise mechanisms and targets of the microRNAs remain poorly defined. Vaccines that are currently in use have a low response rate. Understanding of microRNA regulatory mechanisms and targets would allow to develop more effective vaccines for retroviral infections.

Ablation of non-coding RNAs affects bovine leukemia virus B lymphocyte proliferation and abrogates oncogenesis

Viruses have developed different strategies to escape from immune response. Among these, viral non-coding RNAs are invisible to the immune system and may affect the fate of the host cell. Bovine leukemia virus (BLV) encodes both short (miRNAs) and long (antisense AS1 and AS2) non-coding RNAs. To elucidate the mechanisms associated with BLV non-coding RNAs, we performed phenotypic and transcriptomic analyzes in a reverse genetics system. RNA sequencing of B-lymphocytes revealed that cell proliferation is the most significant mechanism associated with ablation of the viral non-coding RNAs. To assess the biological relevance of this observation, we determined the cell kinetic parameters in vivo using intravenous injection of BrdU and CFSE. Fitting the data to a mathematical model provided the rates of cell proliferation and death. Our data show that deletion of miRNAs correlates with reduced proliferation of the infected cell and lack of pathogenesis.

BLV is a retrovirus that integrates into the genomic DNA of B-lymphocytes from a series of ruminant species (cattle, sheep, zebu, water buffalo and yack). Expression of viral proteins is almost undetectable in infected animals. In contrast, the BLV genome contains a cluster of 10 microRNAs that are abundantly transcribed in BLV-infected cells in vivo. In this report, we show that these microRNAs primarily regulate host cell proliferation. Ablation of the viral microRNAs affects BLV replication and suppresses leukemia development.

Bovine leukemia virus pre-miRNA genes' polymorphism

MicroRNAs (miRNAs) are small non-coding RNAs with a wide distribution in nature among the living things. They play a key role both in normal signaling pathways and in pathological ones. Bovine leukemia virus (BLV) is an oncogenic retrovirus of Deltaretrovirus genus causing persistent infection in its natural hosts - cattle, zebu and water buffalo with diverse clinical manifestations through the defeat of B-lymphocytes (B-cells). Ten BLV encoded miRNAs (further miRs-B) transcribed from five different pre-miRNA (further pre-miR-B) genes are abundantly detected in BLV infected B-cells. Here we report about several alleles of each of pre-miRs-B' genes, some of which have a highly significant association with an increase or a decrease of the number of leukocytes (WBCs - white blood cells) in BLV-infected cows.

Detection and molecular characterization of bovine leukemia virus in various regions of Iran

Purpose. Bovine leukemia virus (BLV) infects cattle worldwide, imposing an economic impact on the dairy cattle industry. The purpose of this study was to evaluate the molecular epidemiology of BLV in Iran.Methodology. Blood samples taken from 280 cows aged over 2 years old from 13 provinces of Iran were used for leukocyte count and blocking ELISA. Genomic DNA was extracted from the peripheral blood leukocytes of BLV-infected samples and fetal lamb kidney cells to perform PCR of partial env, rex and tax genes and long-terminal-repeat region. The PCR products were sequenced, the phylogenetic tree of each gene was constructed, and nucleotide and amino acid sequence pair distances were calculated.Results. The frequency of BLV infection was 32.8 % among animals and was 80 % among provinces. In BLV seropositive animals, the rate of persistent lymphocytosis was 36.9 %. The constructed phylogenetic trees showed the presence of two BLV genotypes (1 and 4) in Iranian strains. As previous studies, our results showed that the env gene was more variable than previously thought, the Rex protein could withstand more amino acid changes compared to the Tax protein, and no significant differences were observed in average changes of the nucleotide of these genes between clinical stages.Conclusions. Our data indicates an increase in the frequency of this infection in Iran. This is the first study report of the presence of BLV genotype 4 in Iranian farms. These findings may have an important role in the control and prevention of BLV infection in Iran and other countries.

MiRNAs: Biology, Biogenesis, their Web-based Tools, and Databases

INTRODUCTION

MicroRNAs (miRNAs), which are evolutionarily conserved, and endogenous non-coding RNAs, participate in the post-transcriptional regulation of eukaryotic genes. The biogenesis of miRNAs occurs in the nucleus. Then, in the cytoplasm, they are assembled along with some proteins in a ribonucleoprotein complex called RISC. miRNA component of the RISC complex binds to the complementary sequence of mRNA target depending on the degree of complementarity, and leads to mRNA degradation and/or inhibition of protein synthesis. miRNAs have been found in eukaryotes and some viruses play a role in development, metabolism, cell proliferation, growth, differentiation, and death.

OBJECTIVE

A large number of miRNAs and their targets were identified by different experimental techniques and computational approaches. The principal aim of this paper is to gather information about some miRNA databases and web-based tools for better and quicker access to relevant data.

RESULTS

Accordingly, in this paper, we collected and introduced miRNA databases and some webbased tools that have been developed by various research groups. We have categorized them into different classes including databases for viral miRNAs, and plant miRNAs, miRNAs in human beings, mice and other vertebrates, miRNAs related to human diseases, and target prediction, and miRNA expression. Also, we have presented relevant statistical information about these databases.

Functional characterization of the bovine foamy virus miRNA expression cassette and its dumbbell-shaped pri-miRNA

Foamy viruses are unconventional and complex retroviruses distinct from the other members of the Retroviridae family. Currently, no disease has been firmly linked to persistent foamy virus infection of their cognate host including simians, bovines, felines, and equines or upon zoonotic transmission of different simian foamy viruses to humans. Bovine and simian foamy viruses have been recently shown to encode a RNA polymerase-III-driven micro RNA cluster which likely modulates and regulates host-virus interactions at different levels. Using sub-genomic bovine foamy virus micro RNA expression plasmids and dual luciferase reporter assays as readout, the requirements for expression and processing of the bovine foamy virus micro RNAs have been analyzed. Here, we report that the minimal BFV micro RNA cassette is significantly weaker than a U6 promoter-based construct and strongly suppressed by flanking sequences. The primary micro RNA sequence can be manipulated and chimerized as long as the dumbbell-like folding of the primary micro RNA is maintained. Since more subtle changes are associated with reduced functionality, the overall structure and shape, but possibly individual elements and residues also, are important for the expression and processing of the bovine foamy virus micro RNAs.

MicroRNAs Encoded by Bovine Leukemia Virus (BLV) Are Associated with Reduced Expression of B Cell Transcriptional Regulators in Dairy Cattle Naturally Infected with BLV

Bovine leukemia virus (BLV) is estimated to infect over 83% of dairy herds and over 40% of all dairy cows in the United States. While, BLV only causes leukemia in a small proportion of animals, research indicates that BLV+ cattle exhibit reduced milk production and longevity that is distinct from lymphoma development. It is hypothesized that BLV negatively affects production by interfering with cattle immunity and increasing the risk of secondary infections. In particular, BLV+ cows demonstrate reduced circulating levels of both antigen-specific and total IgM. This study investigated possible mechanisms by which BLV could interfere with the production of IgM in naturally infected cattle. Specifically, total plasma IgM and the expression of genes IGJ, BLIMP1, BCL6, and PAX5 in circulating IgM+ B cells were measured in 15 naturally infected BLV+ and 15 BLV− cows. In addition, BLV proviral load (PVL) (a relative measurement of BLV provirus integrated into host DNA) and the relative expression of BLV TAX and 5 BLV microRNAs (miRNAs) were characterized and correlated to the expression of selected endogenous genes. BLV+ cows exhibited lower total plasma IgM and lower expression of IGJ, BLIMP1, and BCL6. While, BLV TAX and BLV miRNAs failed to correlate with IGJ expression, both BLV TAX and BLV miRNAs exhibited negative associations with BLIMP1 and BCL6 gene expression. The results suggest a possible transcriptional pathway by which BLV interferes with IgM production in naturally infected cattle.

How Does HTLV-1 Undergo Oncogene-Dependent Replication Despite a Strong Immune Response?

In 1987, Mitsuaki Yoshida proposed the following model (Yoshida and Seiki, 1987): “... T-cells activated through the endogenous p40x would express viral antigens including the envelope glycoproteins which are exposed on the cell surface. These glycoproteins are targets of host immune surveillance, as is evidenced by the cytotoxic effects of anti-envelope antibodies or patient sera. Eventually all cells expressing the viral antigens, that is, all cells driven by the p40x would be rejected by the host. Only those cells that did not express the viral antigens would survive. Later, these antigen-negative infected cells would begin again to express viral antigens, including p40x, thus entering into the second cycle of cell propagation. These cycles would be repeated in so-called healthy virus carriers for 20 or 30 years or longer....” Three decades later, accumulated experimental facts particularly on intermittent viral transcription and regulation by the host immune response appear to prove that Yoshida was right. This Hypothesis and Theory summarizes the evidences that support this paradigm.

Epidemiology and genetic diversity of bovine leukemia virus

Bovine leukemia virus (BLV), an oncogenic member of the Deltaretrovirus genus, is closely related to human T-cell leukemia virus (HTLV-I and II). BLV infects cattle worldwide and causes important economic losses. In this review, we provide a summary of available information about commonly used diagnostic approaches for the detection of BLV infection, including both serological and viral genome-based methods. We also outline genotyping methods used for the phylogenetic analysis of BLV, including PCR restriction length polymorphism and modern DNA sequencing-based methods. In addition, detailed epidemiological information on the prevalence of BLV in cattle worldwide is presented. Finally, we summarize the various BLV genotypes identified by the phylogenetic analyses of the whole genome and env gp51 sequences of BLV strains in different countries and discuss the distribution of BLV genotypes worldwide.

Cis-drivers and trans-drivers of bovine leukemia virus oncogenesis

The bovine leukemia virus (BLV) is a retrovirus inducing an asymptomatic and persistent infection in ruminants and leading in a minority of cases to the accumulation of B-lymphocytes (lymphocytosis, leukemia or lymphoma). Although the mechanisms of oncogenesis are still largely unknown, there is clear experimental evidence showing that BLV infection drastically modifies the pattern of gene expression of the host cell. This alteration of the transcriptome in infected B-lymphocytes results first, from a direct activity of viral proteins (i.e. transactivation of gene promoters, protein-protein interactions), second, from insertional mutagenesis by proviral integration (cis-activation) and third, from gene silencing by microRNAs. Expression of viral proteins stimulates a vigorous immune response that indirectly modifies gene transcription in other cell types (e.g. cytotoxic T-cells, auxiliary T-cells, macrophages). In principle, insertional mutagenesis and microRNA-associated RNA interference can modify the cell fate without inducing an antiviral immunity. Despite a tight control by the immune response, the permanent attempts of the virus to replicate ultimately induce mutations in the infected cell. Accumulation of these genomic lesions and Darwinian selection of tumor clones are predicted to lead to cancer.

Repression of Human T-lymphotropic virus type 1 Long Terminal Repeat sense transcription by Sp1 recruitment to novel Sp1 binding sites

Human T-lymphotropic Virus type 1 (HTLV-1) infection is characterized by viral latency in the majority of infected cells and by the absence of viremia. These features are thought to be due to the repression of viral sense transcription in vivo. Here, our in silico analysis of the HTLV-1 Long Terminal Repeat (LTR) promoter nucleotide sequence revealed, in addition to the four Sp1 binding sites previously identified, the presence of two additional potential Sp1 sites within the R region. We demonstrated that the Sp1 and Sp3 transcription factors bound in vitro to these two sites and compared the binding affinity for Sp1 of all six different HTLV-1 Sp1 sites. By chromatin immunoprecipitation experiments, we showed Sp1 recruitment in vivo to the newly identified Sp1 sites. We demonstrated in the nucleosomal context of an episomal reporter vector that the Sp1 sites interfered with both the sense and antisense LTR promoter activities. Interestingly, the Sp1 sites exhibited together a repressor effect on the LTR sense transcriptional activity but had no effect on the LTR antisense activity. Thus, our results demonstrate the presence of two new functional Sp1 binding sites in the HTLV-1 LTR, which act as negative cis-regulatory elements of sense viral transcription.

Characterization of novel Bovine Leukemia Virus (BLV) antisense transcripts by deep sequencing reveals constitutive expression in tumors and transcriptional interaction with viral microRNAs

Background

Bovine Leukemia Virus (BLV) is a deltaretrovirus closely related to the Human T cell leukemia virus-1 (HTLV-1). Cattle are the natural host of BLV where it integrates into B-cells, producing a lifelong infection. Most infected animals remain asymptomatic but following a protracted latency period about 5 % develop an aggressive leukemia/lymphoma, mirroring the disease trajectory of HTLV-1. The mechanisms by which these viruses provoke cellular transformation remain opaque. In both viruses little or no transcription is observed from the 5′LTR in tumors, however the proviruses are not transcriptionally silent. In the case of BLV a cluster of RNA polymerase III transcribed microRNAs are highly expressed, while the HTLV-1 antisense transcript HBZ is consistently found in all tumors examined.

Results

Here, using RNA-seq, we demonstrate that the BLV provirus also constitutively expresses antisense transcripts in all leukemic and asymptomatic samples examined. The first transcript (AS1) can be alternately polyadenylated, generating a transcript of ~600 bp (AS1-S) and a less abundant transcript of ~2200 bp (AS1-L). Alternative splicing creates a second transcript of ~400 bp (AS2). The coding potential of AS1-S/L is ambiguous, with a small open reading frame of 264 bp, however the transcripts are primarily retained in the nucleus, hinting at a lncRNA-like role. The AS1-L transcript overlaps the BLV microRNAs and using high throughput sequencing of RNA-ligase-mediated (RLM) 5′RACE, we show that the RNA-induced silencing complex (RISC) cleaves AS1-L. Furthermore, experiments using altered BLV proviruses with the microRNAs either deleted or inverted point to additional transcriptional interference between the two viral RNA species.

Conclusions

The identification of novel viral antisense transcripts shows the BLV provirus to be far from silent in tumors. Furthermore, the consistent expression of these transcripts in both leukemic and nonmalignant clones points to a vital role in the life cycle of the virus and its tumorigenic potential. Additionally, the cleavage of the AS1-L transcript by the BLV encoded microRNAs and the transcriptional interference between the two viral RNA species suggest a shared role in the regulation of BLV.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0267-8) contains supplementary material, which is available to authorized users.