Research in the Lieberman laboratory centers on understanding how the cancer-associated viruses, like Epstein-Barr virus (EBV) and Kaposi’s Sarcoma Associated Herpesvirus (KSHV), persist in a latent state and increases the risk of cancer cell evolution. EBV and KSHV establish latent infections that are associated with several human malignancies, including Burkitt's lymphoma, nasopharyngeal carcinoma, Hodgkin's disease, and post-transplant lymphoproliferative disorder for EBV, and Kaposi’s Sarcoma for KSHV.
The researchers have recently found that viral DNA replication and maintenance is regulated by interactions with cellular telomere binding proteins. Telomeres are the repetitive DNA sequences found at the ends of chromosomes. Telomeres protect chromosomes from loss of genetic information, and a similar process is thought to preserve the virus during latency. The Lieberman research team has worked out several biochemical pathways that control the stability, replication, and gene expression patterns of the latent virus. They have found that changes in viral chromatin structure alters the cancer-risk associated with latent infection.
Recent Scientific Advances
Epstein-Barr Virus Latency--Control of DNA Replication and Plasmid Maintenance
During latency, EBV genomes replicate once and only once to maintain a stable copy number in the infected cell. One viral protein, EBNA1, is essential for the replication and maintenance of the viral genome. EBNA1 binds to a region of the genome, referred to as OriP, that can function as the site of initiation for DNA replication and is required for viral plasmid stability.
The researchers have found that OriP DNA replication and plasmid maintenance is regulated by interactions with telomere-associated proteins. Telomere repeat binding factors 1 and 2; hRAP1, telomere-associated poly-ADP ribose polymerase (Tankyrase); and PARP1 were isolated as OriP binding proteins by biochemical methods. Inhibition of PARP activity altered OriP function in plasmid maintenance and TRF2 dominant negative mutants interfered with OriP-dependent DNA replication. Telomere-associated proteins regulate OriP by an NAD-dependent post-translational modification of EBNA1 and other Ori P-associated proteins. These studies indicate that a cellular telomeric complex regulates OriP in response to genotoxic and metabolic stress and therefore links viral DNA persistence in latently infected cells to cellular viability.
Role of Chromatin in the Regulation of Gammaherpesvirus Latency
The Lieberman research team has been exploring the importance of chromatin structure in the regulation of oncogenic gammaherpesvirus latency. The gammaherpesviruses (EBV and Kaposi’s sarcoma associated herpesvirus (KSHV)) exist predominantly as latent viruses with very few of its genes expressed. During latency the majority of viral genes are silenced by an unknown mechanism.
The Lieberman lab has been investigating the compartmentalization of the viral genome by chromatin boundaries and related structures that separate active from inactive domains of the genome. They have found that a cellular chromatin boundary factor, referred to as CTCF, binds to specific regions of the viral genome that separate the active genes, from the silenced genes. Perturbations in this chromatin segregation lead to significant changes in viral-host interactions and pathogenesis. Activation of the LMP1 or EBNA2 gene leads to cellular proliferation and increased risk of carcinogenesis. Activation of lytic activator Zta or Rta leads to the disruption of latency and the production of infectious virus. The mechanisms controling the switch between latency and lytic infection.