Enzootic nasal tumor virus

The tumors that are induced by ENTV Env have characteristics of adenomas and adenocarcinoma due to well-differentiated epithelial appearance and a less well-differentiated appearance respectively.[9] Symptoms of nasal adenocarcinoma are similar in both goats and sheep. They include weight loss, nasal discharge with the nasal fluid being viscous and containing pus. Tumors form in the upper nasal cavity and have a gelatinous consistency with caudal areas being having hemorrhage causing a brownish-red color.[4] Symptoms of enzootic nasal adenocarcinoma do not appear until it is too late. An ovine that grows a tumor will eventually die of suffocation.[6]

Tumors form in nose and are contagious. ENTV targets the respiratory system in ovine, specifically the upper-airway epithelial cells.[6] Oncogenesis occurs in the nasal turbinate cells [8] A typical oncogenic virus will cause a mutation in a host cell, causing the transformation of host cells from a protooncogene into an oncogene. ENTV is unique among retroviruses because the envelope glycoprotein is an oncogene. It is able to induce tumors by itself.[10]  ENTV does not have a viremic stage where the virus enters the blood which is unusual.[11]

There is not a humoral immune response to the ENTV capsid protein meaning there are no antibodies produced by the host in response to the virus capsid.[12] However, recent studies have shown that there is sero-conversion in response to the envelope protein if a host is exposed to the virus after a long period of time. These antibodies have proven to be important in preventing further spread of the virus. The antibodies are more likely to respond to the ENTV envelope than the capsid because the envelope is located on the exterior of the virus whereas the capsid is not.[13] The lack of an extensive immune response may be due to the face that a large portion of the ovine genome is made up of sequences that are related to the retroviruses JSRV and ENTV.

ENTV gag encodes a chain of amino acids linked by peptide bonds, or a polypeptide that contains 613 amino acids. Gag is also responsible for encoding structural proteins including the capsid protein, the matrix shell and the nucleic acid binding protein.[2]

Pro encodes a 326 amino acid polypeptide and generally works with gag to form a gag-pro polypeptide due to ribosomal frame shifting. It encodes the viral protease which breaks down the viral proteins into peptides and amino acids.[2]

Pol encodes a 870 amino acid peptide. It is usually translated to make a gag-pro-pol polypeptide due to ribosomal frame shifting. Inside the pol gene is an open reading frame, Orf-x which is abnormal for simple retroviruses. Pol is important because it encodes reverse transcriptase and integrase.[2]

Env is generated by splicing and overlaps pol at the start. It encodes surface and transmembrane envelope proteins by being cleaved twice, a polypeptide precursor, and a hydrophobic region. Env plays a large role in the functionality of ENTV.[2] Env is important in ENVT tissue selectivity.[7]

LTR is 374 bases long and contains the primer binding site and the polypurine tract. It is made up of inverted repeat sequences GCAG and CTGC. tRNALys1,2 is the reverse transcriptase primer for ENVT.[2] The U3 region of LTR is important for replication and transcription regulatory signaling.[4] The U3 region is also important in tissue selectivity.[7]