HERPES VIRUSES

Read Also

These large viruses (diameter 125–200 nm) contain a linear genome of double-stranded DNA (125–240 kbp) and have a capsid with icosahedral symmetry surrounded by an outer lipid-containing envelope. Herpesviruses typically cause acute infections followed by latency and eventual recurrence in each host, including humans.

In humans, herpesviruses have been linked to several specific types of tumors. Epstein-Barr (EB) herpesvirus causes acute infectious mononucleosis when it infects B lymphocytes of susceptible humans. Normal human lymphocytes have a limited life span in vitro, but EB virus can immortalize such lymphocytes into lymphoblast cell lines that grow indefinitely in culture.

EB virus is etiologically linked to Burkitt's lymphoma, a tumor most commonly found in children in central Africa; to nasopharyngeal carcinoma (NPC), more common in Cantonese Chinese and Alaskan Eskimos than other populations; to posttransplant lymphomas; and to Hodgkin's disease. These tumors usually contain EB viral DNA (both integrated and episomal forms) and viral antigens.

EB virus encodes a viral oncogene protein (LMP1) that mimics an activated growth factor receptor. LMP1 is able to transform rodent fibroblasts and is essential for transformation of B lymphocytes. Several EB virus-encoded nuclear antigens (EBNAs) are necessary for immortalization of B cells; EBNA1 is the only viral protein consistently expressed in Burkitt's lymphoma cells. EB virus is very successful at avoiding immune elimination; this may be due in part to the function of EBNA1 in inhibition of antigen processing to allow infected cells to escape killing by cytotoxic T lymphocytes.

Malaria may be a cofactor of African Burkitt's lymphoma. Most of those tumors also show characteristic chromosomal translocations between the c-myc gene and immunoglobulin loci, leading to the constitutive activation of myc expression. Consumption of salted or dried fish may be a dietary cofactor in EB virus-related NPC.

Kaposi's sarcoma-associated herpesvirus, also known as human herpesvirus 8 (KSHV/HHV8), is not as ubiquitous as most other human herpesviruses. It is suspected of being the cause of Kaposi's sarcoma, primary effusion lymphoma, and a particular lymphoproliferative disorder. KSHV has a number of genes that may stimulate cellular proliferation and modify host defense mechanisms.