Insights into Gene Expression Changes Impacting B-Cell Transformation: Cross-Species Microarray Analysis of Bovine Leukemia Virus Tax-Responsive Genes in Ovine B Cells

Abstract

Large-animal models for leukemia have the potential to aid in the understanding of networks that contribute to oncogenesis. Infection of cattle and sheep with bovine leukemia virus (BLV), a complex retrovirus related to human T-cell leukemia virus type 1 (HTLV-1), is associated with the development of B-cell leukemia. Whereas the natural disease in cattle is characterized by a low tumor incidence, experimental infection of sheep leads to overt leukemia in the majority of infected animals, providing a model for studying the pathogenesis associated with BLV and HTLV-1. Tax BLV , the major oncoprotein, initiates a cascade of events leading toward malignancy, although the basis of transformation is not fully understood. We have taken a cross-species ovine-to-human microarray approach to identify Tax BLV -responsive transcriptional changes in two sets of cultured ovine B cells following retroviral vector-mediated delivery of Tax BLV . Using cDNA-spotted microarrays comprising 10,336 human genes/expressed sequence tags, we identified a cohort of differentially expressed genes, including genes related to apoptosis, DNA transcription, and repair; proto-oncogenes; cell cycle regulators; transcription factors; small Rho GTPases/GTPase-binding proteins; and previously reported Tax HTLV-1 -responsive genes. Interestingly, genes known to be associated with human neoplasia, especially B-cell malignancies, were extensively represented. Others were novel or unexpected. The results suggest that Tax BLV deregulates a broad network of interrelated pathways rather than a single B-lineage-specific regulatory process. Although cross-species approaches do not permit a comprehensive analysis of gene expression patterns, they can provide initial clues for the functional roles of genes that participate in B-cell transformation and pinpoint molecular targets not identified using other methods in animal models.DNA microarray technology has facilitated the identification of a large number of genes involved in the complex deregulation of cell homeostasis taking place in cancer (25, 59). There exists, however, a significant limitation in the variety of organisms for which microarrays have been developed because of a lack of genomic sequence data. Although species-specific small-scale application-targeted arrays are useful for monitoring specific networks of genes (9, 66), they do not address the broad spectrum of genes involved in the complex deregulations taking place in cancer. Larger-scale microarrays for domestic species are currently under investigation, but these emerging tools suffer from a lack of annotated genes. A solution to this limitation is to use microarrays designed for one species to analyze RNA samples from closely related species. The assumption is that the conservation of gene sequences between species will be sufficient to generate a reasonable amount of good-quality data. While there have been relatively few previously published reports that described the use of microarrays for cross-species hybridization (8,24,26,31,47)...

Bovine Leukemia Virus Gene Segment Detected in Human Breast Tissue

“…Several laboratory trials showed that Tax could transform cells in in vitro cultures and being oncogenic in animal models where cells can be transformed when Tax is combined with ha-ras oncogene to produce tumor cells in athymic mice and fibroblasts immortalization coming from rat embryo [24,25]; 2) Viral genome integration process close to highly active cellular genome segments associated with cell division [26]. This is how the Avian Sarcoma Leukosis Virus (ASLV) can integrate in many places of cellular genome, but when there is tumor in the host, the virus has been found upstream of the c-myc cellular gene, which codifies for the Myc proteins implicated in the cellular DNA synthesis control [26,27]. A similar situation happens with MMTV that locates in cellular genome places that induce cellular proliferation by insertional mutagenesis or transcriptional activation of close oncogenes [28,30]; 3) Blocking of tumor suppressors genes and apoptosis activation genes processes [14].…”

Suppression of Viral Gene Expression in Bovine Leukemia Virus-Associated B-Cell Malignancy: Interplay of Epigenetic Modifications Leading to Chromatin with a Repressive Histone Code

“…BLV shares a number of structural and functional similarities with HTLV-1. Unlike simple retroviruses, which induce tumors by expressing viral products or by proviral insertional mutagenesis, complex oncoretroviruses such as HTLV-1 and BLV exert their oncogenic potential using poorly understood mechanisms which involve Tax, the viral transactivator/oncoprotein (16,19,42). Although Tax is an essential contributor to the oncogenic potential of both viruses, mainly through the transcriptional modification of host genes (19,32,42), there is compelling evidence that expression of Tax is not sufficient for transformation.…”

“…Unlike simple retroviruses, which induce tumors by expressing viral products or by proviral insertional mutagenesis, complex oncoretroviruses such as HTLV-1 and BLV exert their oncogenic potential using poorly understood mechanisms which involve Tax, the viral transactivator/oncoprotein (16,19,42). Although Tax is an essential contributor to the oncogenic potential of both viruses, mainly through the transcriptional modification of host genes (19,32,42), there is compelling evidence that expression of Tax is not sufficient for transformation. Furthermore, the presence of mutations in tumor-associated proviral sequences, including that of tax, suggests that neither virus nor Tax expression is required for the maintenance of the transformed phenotype (18,(45)(46)(47).…”

Deep sequencing reveals abundant noncanonical retroviral microRNAs in B-cell leukemia/lymphoma

“…However, what differentiates the emerging from the other clones remains largely unknown. Although Tax, the viral transactivator, is an essential contributor to the oncogenic potential of the virus, there is compelling evidence that expression of Tax is not sufficient for transformation (6)(7)(8). The identification of mutations in tumor-associated proviruses further proves that neither virus nor Tax expression is required for maintaining the transformed state (9)(10)(11).…”