Herpesviruses are a large group of DNA viruses that cause disease in many organisms [1, 2]. The ability of herpesviruses to occupy diverse biological niches, the wide range of cell types targeted for primary infection and latency, along with acute and chronic infection symptoms, contribute to the significance and relevance of herpesvirus research. In particular, human herpesviruses are a major public health concern due to the prevalence of such viruses within and among populations worldwide and the associated pathologies. Two of the eight disease-causing human herpesviruses, Epstein-Barr virus (EBV) and Kaposi's sarcoma virus (KSHV), are associated with neoplastic transformation. A closely related avian herpesvirus, Marek's disease virus (MDV; family Herpesviridae, genus Mardivirus, species Gallid herpesvirus 2) also exhibits oncogenic properties inducing lymphoid tumors in chickens.
MDV rapidly infects B cells and subsequently transforms T cells during pathogenesis of the highly contagious neoplastic disease of chickens known as Marek's disease (MD). Since discovery , MD has been extensively studied and utilized as a valuable biomedical model for human herpesvirus infection and viral induced malignancies [4, 5]. MDV is of great concern in agriculture, causing large economic losses as well as impacting animal well-being. Despite current control through the use of vaccines, viral evolution leads to increasing virulence and vaccine breaks on a cyclical basis [6, 7].
A fascinating biological property that MDV shares with all herpesviruses is the ability to persist indefinitely in the host and evade immune response by establishing a latent infection. The molecular mechanisms controlling lytic replication and latent persistence of herpesviruses have been the subject of intense study in recent years, aided largely by advances made in the field of viral genetics. Although new information has emerged from these studies regarding mechanisms of the lytic cycle, factors controlling latency and tumorigenesis remain poorly understood. MDV presents a unique model system in which to explore the mechanisms of herpesvirus latency and virus-induced tumor formation.
Another intriguing aspect of herpesvirus infection is that both episomal and linear-integrated forms of the virus are present in the host . One consideration is that latency, and potential reactivation of the virus, could be achieved in part through genomic interactions between the host and virus, evasion through integration and recurring infection through emergence/release. Evidence for herpesvirus integration into human chromosomes was reported for Human Herpesvirus 6 (HHV-6) and EBV [9–14]. Since Delecluse and Hammerschmidt (1993) first reported on the potential for MDV to exist in an integrated state, it has been a subject of debate as to whether the virus integrates stably (and where) into the chicken genome or is merely associated and how such associations might be germane to pathogenesis and oncogenesis. Since that early report, complete genome sequences have been reported for several MDV strains and BAC vectors containing the full-length MDV genome have been cloned [15–20]. The sequencing provided data supportive of the hypothesis that MDV interacts with the chicken genome, as it was discovered that the circular viral genome encodes several orthologous host genes including the telomerase RNA (vTR) gene as well as telomeric repeats (TTAGGG) [17, 18, 21].
To resolve whether and where MDV integrates in the host chicken genome we employed high-resolution multi-color fluorescence in situ hybridization (FISH) using the tools developed from both the MDV and chicken sequencing efforts. We investigated the details of integration on both intra-and inter-chromosomal levels by mapping the location of MDV integrations. In depth analysis of MDV integration sites allowed us to investigate several specific objectives, including the clonal composition of MDV tumors and integration site preference of the virus. We show here that MDV specifically integrates into the telomeric regions of chicken chromosomes in MDV-induced clonal lymphomas.