Epstein-Barr virus

The Epstein-Barr virus (EBV), also called Human herpesvirus 4 (HHV-4), is a virus of the herpes family (which includes Herpes simplex virus and Cytomegalovirus), and is one of the most common viruses in humans. Most people become infected with EBV, which is often asymptomatic but commonly causes infectious mononucleosis.

EBV is named after Michael Epstein and Yvonne Barr, who together with Bert Achong, discovered the virus in 1964.^[1]

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Biology

On infecting the B-lymphocyte, the linear virus genome circularizes and the virus subsequently persists within the cell as an episome.

The virus can execute several distinct programs of gene expression which can be broadly categorised as being lytic cycle or latent cycle.

The lytic cycle or productive infection results in staged expression of a host of viral proteins with the ultimate objective of producing infectious virions. Formally, this phase of infection does not inevitably lead to lysis of the host cell as EBV virions are produced by budding from the infected cell.

The latent cycle (lysogenic) programs are those that do not result in production of virions. A very limited, distinct set of viral proteins are produced during latent cycle infection. These include Epstein-Barr nuclear antigen (EBNA)-1, EBNA-2, EBNA-3A, EBNA-3B, EBNA-3C, EBNA-leader protein (EBNA-LP) and latent membrane proteins (LMP)-1, LMP-2A and LMP-2B and the Epstein-Barr encoded RNAs (EBERs). In addition, EBV codes for at least twenty microRNAs which are expressed in latently infected cells.^[3]

From studies of EBV gene expression in cultured Burkitt's lymphoma cell lines, at least three programmes exist:

• EBNA1 only (group I)
• EBNA1 + EBNA2 (group II)
• Latent cycle proteins (group III).

It is also postulated that a program in which all viral protein expression is shut off exists.

When EBV infects B-lymphocytes in vitro, lymphoblastoid cell lines eventually emerge that are capable of indefinite growth. The growth transformation of these cell lines is the consequence of viral protein expression.

EBNA-2, EBNA-3C and LMP-1 are essential for transformation while EBNA-LP and the EBERs are not. The EBNA-1 protein is essential for maintenance of the virus genome.^[4]

It is postulated that following natural infection with EBV, the virus executes some or all of its repertoire of gene expression programmes to establish a persistent infection. Given the initial absence of host immunity, the lytic cycle produces large amounts of virus to infect other (presumably) B-lymphocytes within the host.

The latent programs reprogram and subvert infected B-lymphocytes to proliferate and bring infected cells to the sites at which the virus presumably persists. Eventually, when host immunity develops, the virus persists by turning off most (or possibly all) of its genes, only occasionally reactivating to produce fresh virions. A balance is eventually struck between occasional viral reactivation and host immune surveillance removing cells that activate viral gene expression.

The site of persistence of EBV may be bone marrow. EBV-positive patients who have had their own bone marrow replaced with bone marrow from an EBV-negative donor are found to be EBV-negative after transplantation.^[5]

EBV latent antigens

All EBV nuclear proteins are produced by alternative splicing of a transcript starting at either the Cp or Wp promoters at the left end of the genome (in the conventional nomenclature). The genes are ordered EBNA-LP/EBNA-2/EBNA-3A/EBNA-3B/EBNA-3C/EBNA-1 within the genome.

The initiation codon of the EBNA-LP coding region is created by an alternate splice of the nuclear protein transcript. In the absence of this initiation codon, EBNA-2/EBNA-3A/EBNA-3B/EBNA-3C/EBNA-1 will be expressed depending on which of these genes is alternatively spliced into the transcript.

• EBNA-1

EBNA-1 protein binds to a replication origin (oriP) within the viral genome and mediates replication and partitioning of the episome during division of the host cell. It is the only viral protein expressed during group I latency. EBNA-1 possesses a glycine-alanine repeat that impairs antigen processing and MHC class I-restricted antigen presentation thereby inhibiting the CD8-restricted cytotoxic T-cell response against virus infected cells.^[6]

EBNA-1 was initially identified as the target antigen of sera from rheumatoid arthritis patients (rheumatoid arthritis-associated nuclear antigen; RANA).

• EBNA-2

EBNA-2 is the main viral transactivator, switching transcription from the Wp promoters used during initially after infection to the Cp promoter. Together with EBNA-3C, it also activates the LMP-1 promoter. It is known to bind the host RBP-Jκ protein that is a key player in the Notch pathway. EBNA-2 is essential for EBV-mediated growth transformation.

• EBNA-3A/EBNA-3B/EBNA-3C

These genes also bind the host RBP-Jκ protein.

• EBNA-3C

EBNA-3C is also a ubiquitin-ligase and has been shown to target cell cycle regulators like pRb^[7][8]

• LMP-1

LMP-1 is a six-span transmembrane protein that is also essential for EBV-mediated growth transformation. LMP-1 mediates signalling through the Tumor necrosis factor-alpha/CD40 pathway.

• LMP-2A/LMP-2B

LMP-2A/LMP-2B are transmembrane proteins that act to block tyrosine kinase signalling. it is believed that they act to inhibit activation of the viral lytic cycle. It s unknown whether LMP-2B is required for EBV-mediated growth transformation, while different groups have reported that LMP-2A alternatively is, or is not needed for transformation.

• EBER-1/EBER-2

EBER-1/EBER-2 are small nuclear RNAs of an unknown role. They are not required for EBV-mediated growth transformation.

• miRNAs

EBV microRNAs are encoded by two transcripts, one set in the BART gene and one set near the BHRF1 cluster. The three BHRF1 miRNAS are expressed during type III latency while the large cluster of BART miRNAs (up to 20 miRNAs) are expressed during type II latency. The functions of these miRNAs are currently unknown.

Pathology

Infectious mononucleosis

Epstein-Barr can cause infectious mononucleosis, also known as 'glandular fever', 'Mono' and 'Pfeiffer's disease'. Infectious mononucleosis is caused when a person is first exposed to the virus during or after adolescence. Though once deemed "The Kissing Disease," recent research has shown that transmission of Mono not only occurs from exchanging saliva, but also from contact with the airborne virus. It is predominantly found in the developed world, and most children in the developing world are found to have already been infected by around 18 months of age. EBV antibody tests turn up almost universally positive. In the United States up to 95% of adults between 35 and 40 years of age have been infected.^[9]

EBV-associated malignancies

The strongest evidence linking EBV and cancer formation is found in Burkitt's lymphoma and nasopharyngeal carcinoma. It has been postulated to be a trigger for a subset of chronic fatigue syndrome patients^[10] as well as multiple sclerosis and other autoimmune diseases.^[11]

Burkitt's lymphoma is a type of Non-Hodgkin's lymphoma [2] and is most common in equatorial Africa and is co-existent with the presence of malaria. Malaria infection causes reduced immune surveillance of EBV immortalised B cells, so allowing their proliferation. This proliferation increases the chance of a mutation to occur. Repeated mutations can lead to the B cells escaping the body's cell-cycle control, so allowing the cells to proliferate unchecked, resulting in the formation of Burkitt's lymphoma. Burkitt's lymphoma commonly affects the jaw bone, forming a huge tumour mass. It responds quickly to chemotherapy treatment, namely cyclophosphamide, but recurrence is common.

Other B cell lymphomas arise in immunocompromised patients such as those with AIDS or who have undergone organ transplantation with associated immunosuppression (Post-Transplant Lymphoproliferative Disorder (PTLPD)). Smooth muscle tumors are also associated with the virus in malignant patients.^[12]

Nasopharyngeal carcinoma is a cancer found in the upper respiratory tract, most commonly in the nasopharynx, and is linked to the EBV virus. It is found predominantly in Southern China and Africa, due to both genetic and environmental factors. It is much more common in people of Chinese ancestry (genetic), but is also linked to the Chinese diet of a high amount of smoked fish, which contain nitrosamines, well known carcinogens (environmental).^[13]

Chronic fatigue syndrome

In the late 1980s and early 1990s, EBV became the favored explanation for chronic fatigue syndrome. It was noted that people with chronic exhaustion had EBV, although it was also noted EBV was present in almost everyone. In a four year study, the Centers for Disease Control and Prevention found that the virus did not adhere to Koch's Postulates and therefore had no definitive association between CFS and EBV but it is still being studied by researchers.

Diseases associated with EBV

• Infectious mononucleosis
• Stevens-Johnson syndrome
• Hepatitis
• Herpes
• Alice in Wonderland syndrome
• Several Non-Hodgkin's lymphomas, including primary cerebral lymphoma
• Hodgkin's disease
• Post-transplant lymphoproliferative disorder
• Herpangina
• Multiple Sclerosis
• Hairy leukoplakia
• Common variable immunodeficiency (CVID)
• Kikuchi's disease
• Nasopharyngeal cancer
• Subepithelial Infiltrates

References