Date Published: March 5, 2012
Publisher: Hindawi Publishing Corporation
Author(s): Sajal K. Ghosh, Susan P. Perrine, Douglas V. Faller.
Epstein-Barr virus (EBV) is the causal agent in the etiology of Burkitt’s lymphoma and nasopharyngeal carcinoma and is also associated with multiple human malignancies, including Hodgkin’s and non-Hodgkin’s lymphoma, and posttransplantation lymphoproliferative disease, as well as sporadic cancers of other tissues. A causal relationship of EBV to these latter malignancies remains controversial, although the episomic EBV genome in most of these cancers is clonal, suggesting infection very early in the development of the tumor and a possible role for EBV in the genesis of these diseases. Furthermore, the prognosis of these tumors is invariably poor when EBV is present, compared to their EBV-negative counterparts. The physical presence of EBV in these tumors represents a potential “tumor-specific”
target for therapeutic approaches. While treatment options for other types of herpesvirus infections have evolved and improved over the last two decades, however, therapies directed at EBV have lagged. A major constraint to pharmacological intervention is the shift from lytic infection to a latent pattern of gene expression, which persists in those tumors associated with the virus. In this paper we provide a brief account of new virus-targeted therapeutic approaches against EBV-associated malignancies.
Epstein-Barr virus (EBV) infection is ubiquitous in human populations worldwide. EBV infection in children and adolescents usually leads to a self-limiting lytic infection, designated as infectious mononucleosis (IM) [1, 2]. However, in immunocompromised individuals, such as those with X-linked lymphoproliferative disease (XLP) [3, 4], EBV infections often progress unchecked and are lethal. EBV is invariably associated with nasopharyngeal carcinoma (NPC) , African Burkitt’s lymphoma (BL) , posttransplantation lymphoproliferative disease (PTLD) [7–10], and less often with a number of other human malignancies such as Hodgkin’s lymphoma (HD) , and non-Hodgkin’s lymphomas (NHL). In addition, EBV is found in a fraction of gastric carcinomas [12, 13] and carcinomas of the breast [14–16]. Although EBV has been identified in these latter tumors, it remains controversial whether EBV is causally-related to their development. Nonetheless, multiple studies have clearly demonstrated that the presence of EBV in these tumors confers a poorer prognosis [17–22].
EBV transmission usually takes place through the mucosal secretions of the mouth of an infected individual. Primary infection of epithelial cells of the oropharynx leads to active production of virus particles with shedding of the virus in saliva. Although the EBV-epithelial cell attachment process is not fully understood, the neighboring B-cells subsequently become infected via interaction of the EBV surface protein gp350 with the lymphocyte receptor CD21, however, such infections are often nonproductive. Active or “lytic” replication of EBV induces lysis of infected cells concurrent with production of virus particles, whereas latent replication of EBV does not. EBV is a member of the gamma herpesvirus family, with a large 172 Kb double-stranded linear DNA genome encoding nearly 100 genes. Most of these genes are expressed during lytic-phase replication, whereas only a maximum of eleven viral genes are expressed during latent-phase replication. The up to eleven EBV gene products that are expressed in latently infected cells (the number depends upon the type of latency) include six nuclear antigens (EBNA1, EBNA2, EBNA3A-3B-3C, EBNA-LP), three latent membrane proteins (LMP1, LMP2A-2B), the BARF0 protein, generated from BART transcripts, and two small noncoding non-poly-A RNAs (EBER1 and EBER2). Primary EBV infection results in strong humoral and cellular immune responses. IgM antibodies against EBV surface protein (gp350) are easily detectable in the serum during primary infection, which is then eclipsed by a steady state level of IgG antibody over the ensuing months and beyond .
Existing therapeutic approaches for EBV-associated diseases are broadly categorized into three groups, as shown in Table 1.
In most EBV-associated malignancies, all or nearly all of the tumor cells contain the viral genome. Furthermore, at any given time, the number of EBV-infected nontumor cells present in other physiological compartments of the host is usually very low, and for B cells is on the order of one in a million. This provides a unique opportunity to develop therapeutic strategies utilizing the presence of the viral genome of EBV in the tumors as an essentially “tumor-specific” target. One of the virus-targeted therapeutic strategies is based on the concept that EBV-containing cells will die if lytic replication can be induced. Other strategies employ selective expression of toxins in EBV-infected cells or preventing the function of EBV latent gene products that are linked to oncogenesis (Table 2). Elimination of episomal EBV genomes by low dose hydroxyurea treatment has been shown to decrease the tumorigenic potential of Akata cells of BL origin, both in vitro and in SCID mice . When two patients with AIDS-related (EBV-positive) primary lymphoma of the central nervous system were treated with low dose hydroxyurea, their median survival compared to historical controls increased by almost 18 months . The effectiveness of this approach in a controlled clinical trial, however, has yet to be evaluated. Expression of antisense RNA against the EBV LMP-1 protein has been shown to reduce LMP-1 expression in LCLs and concomitantly inhibit cell proliferation and stimulate apoptosis . As EBNA1 is a viral transactivator expressed in all latently EBV-infected tumor cells and utilizes the OriP promotor for its activity, several studies have utilized an OriP-based vector to direct the expression of cellular toxins, such as driving cytosine deaminase expression (which converts the prodrug 5-flurocytosine to cytotoxic 5-flurouracil), or the herpes simplex virus TK, to make the cells susceptible to nucleoside analog antiviral drugs [53, 54]. Targeted delivery of these EBV-dependent vectors specifically to the tumors cells, however, remains a serious and unresolved challenge.
EBV-associated malignancies remain a significant health concern worldwide, with particularly higher incidences in southeast Asia and China. In Western countries, the incidence of EBV malignancies, and particularly PTLD, is also on the rise. The increasing use of solid organ or hematopoietic transplantation, especially in situations requiring intense immunosuppression, is most likely a major contributor to this increase in EBV-lymphomas. Furthermore, as referenced above, the presence of EBV in the common NHL and HD lymphomas confers a much poorer prognosis after conventional therapy, demonstrating the need for new therapeutic approaches.