Human Herpesvirus 8

Kaposi’s Sarcoma Herpesvirus (KSHV; also human herpesvirus 8), the most recently described member of the human herpesvirus family, is the etiologic agent of all forms of KS, as well as two lymphoproliferative disorders, primary effusion lymphoma (PEL), and the plasma cell variant of Multicentric Castleman’s disease (MCD). KSHV is classified as a g2-herpesvirus in the rhadinovirus subfamily. KSHV seroprevalence varies geographically and demographically, and both sexual and non-sexual routes of transmission have been proposed. In the USA, prevalence ranges from approximately 5% among random blood donors to as high as 80% among groups of homosexual men. Worldwide, KSHV antibodies are more frequent in regions in Europe and the Middle East ranging from 10-30%. In sub-Saharan Africa seroprevalence ranges from 30-100%.

Kaposi’s sarcoma:

KS is an angioproliferative tumor characterized by neoangiogenesis, inflammatory cell infiltration, erythrocyte extravasation and proliferation of spindle cells of endothelial origin. The majority of KS spindle cells harbor latent KSHV genomes and express a limited repertoire of latency-associated genes necessary for episomal maintenance and tumorigenesis. A small percentage of spindle cells express lytic proteins that contribute not only to production of viral progeny, but also to immune modulation and tumorigenesis. KS has become the most common overall cancer in many parts of Africa, and remains the most common AIDS-related malignancy in the USA and Western Europe. KSHV-seropositive individuals with other forms of immunodeficiency (genetic, iatrogenic, idiopathic) are also at increased risk for developing KS. 

Lymphoproliferative disorders (PEL and MCD):

KSHV sequences were also found in primary effusion lymphoma (PEL), a distinct sub-set of AIDS-associated B cell lymphomas, and plasma cell multicentric castleman’s disease (MCD), a rare variant that is almost always AIDS-associated. PEL typically occur as pleural, pericardial or peritoneal effusions with phenotypic and genotypic characteristics indicating a tumor of mature B cell origin. Lymphadenopathy, hyperimmunoglobulinemia and high serum IL-6 levels characterize MCD.  Both diseases are associated with considerably morbidity and mortality in the setting of immunodeficiency. Similar to KS, the presence of KSHV in PEL cells is universal and the majority of PEL cells express only latent proteins. In contrast, only a subset of plasmablasts in MCD tumors are KSHV-infected and the proportion of lytically infected cells is higher.  

Therapeutic Approaches:

Apart from HAART (for AIDS-KS), standard therapies include local radiotherapy or conventional chemotherapy. Recently, the mTOR inhibitor rapamycin has shown efficacy against iatrogenic KS and PEL, and drugs inhibiting angiogenesis, MMPs and kinase pathways are currently in clinical trails. Despite these advances, development of cost-effective and efficacious drugs against either KSHV or associated malignancies has yet to be realized. Rational approaches to anti-viral drug design will be greatly facilitated by having the structures of key viral molecules as well as their complexes with host cell proteins available. 

Shown are latency-associated transcripts (bold) and KSHV-specific (K) ORF and their respective RRV homologs.

The KSHV genome is approximately 165-kb, comprising a 145-kb unique coding region flanked by multiple terminal repeats. There are over 85 open reading frames (ORFS) arranged as seven highly conserved gene blocks separated by regions containing unique or subfamily-specific genes.The unique genes (denoted by a ‘K’ prefix) are of special interest with regard to KSHV-associated diseases. Within the viral capsid, KSHV DNA exists in a linear double-stranded form. Upon infection, the genome circularizes and is maintained as episomal DNA in the host nucleus.

There are two patterns of infection:

• a latent phase where a restricted set of viral genes is expressed and
• a lytic phase where viral replication is induced and new virions are released.

Both latent and lytic gene products interfere with aspects of host cell function, including cell cycle regulation, growth, signaling, and regulation of apoptosis and immune recognition. Some of these proteins are unique to the virus while others are homologs of cellular proteins. The ability of KSHV to hide from the immune system enables this virus to spread and maintain its infection in immune competent individuals. While these infections are usually asymptomatic, any attempt to eradicate KSHV will have to include asymptomatic carriers of KSHV. We hypothesize that counter-acting these immune-modulatory mechanisms will be an ideal treatment for KSHV for several reasons:

1. The virus becomes exposed to the immune system which will perform the elimination of the virus and virus-infected cells;
2. This exposure of virus to immune attack will stimulate a strong and long-lasting immune response to the virus.

This “drug-induced vaccination” will limit viral replication, if not ultimately eliminate the virus from the host. Thus, a major goal of this application is to determine the structures of important immune modulators of KSHV to facilitate the development of specific inhibitors of their function. In immune suppressed individuals however, the main problem is the oncogenic transformation of endothelial cells to KS spindle cells which results in their proliferation and generation of KS tumors. The main driving forces behind this transformation process are viral proteins that are expressed during the latent stage of viral replication.

The  second set of targets is therefore the latent proteins. Despite over a decade of active research, the molecular mechanisms supporting viral infection in immune competent individuals and leading to development of KSHV-associated malignancies in the immune suppressed remain unresolved. At present only a limited amount of structural information is available for KSHV.  Of the 85 ORFs, the structures for only 6 full-length proteins have been solved: the viral IL6 homologue (ORF K2), the IL-8 related chemokine vMIP-II (vCCL2, ORF K4) and vMIP-1 (vCCL-1, ORF K6), the Bcl-2 homologue (ORF 16), the viral protease (ORF 17), a complex of vCyclin (ORF 72) with Cdk6 and the INK4 inhibitor p18(INK4c). In addition, partial structures are available for tow ORFs: a 23 amino-acid peptide of LANA-1 (ORF 73) bound to nucleosomes and the RING-domain of MIR1 (ORF K3). The goal of this application is to add considerable structural information to the proteome of KSHV. Knowledge gained through structural studies of KSHV/RRV proteins, their interactions with host ligands and their feasibility as therapeutic targets should accelerate our understanding of KSHV biology and effective anti-viral and/or anti-tumor drug design.