A. RNA retroviruses

II. Carcinogenic viruses

B. Retrovirus terminology

Capsid--proteins that form the protein core of the virus and are coded by the viral gag gene.

Envelope --the outer lipid and protein bilayer of the virus that is formed from the cell membrane of the previously infected cell plus the proteins coded by the viral env gene. For a specific type of cell to be infected, it must have a specific membrane receptor that «recognizes» the envelope of a particular retrovirus. For example, CD4 («helper T4») cells have specific receptor sites for HIV capsid antigens.

Long-terminal repeats (LTRs) --DNA transcripts of the ends of a provirus that help the virus to become incorporated into cell chromosome DNA. LTRs signal the start and stop points for transcription of proviral RNA.

Provirus --the double-stranded DNA copy of a retroviral RNA.

Replication competent retroviruses --retroviruses with the full complement of sequences necessary for reproduction.

Replication deficient viruses --retroviruses that have lost some of the normal coding for proteins necessary for viral replication. These retroviruses typically have a v-onc and rapidly transform cells in animal tissues and in culture into malignant phenotypes.

Reverse transcriptase --a DNA polymerase that is part of the retroviral core and uses the viral RNA template to make a double strand of complementary DNA. This enzyme is coded by the viral pol gene.

Short-terminal repeats --located at both ends of the entire viral RNA sequence. These are reverse transcribed after viral infection into DNA LTR.

Viral oncogenes (v-onc) --transforming genes with close homology to normal cell genes.

Viral RNA --two identical strands are present in each virus and consist of several coding sequences. The special sequences pol, gag, and env are unique to certain retroviruses that regulate viral gene expression and reproduction. Examples of unique genes include the TAX gene of HTLV-I and the TAT gene of HIV. See II.A.3.a. and II.A.3.c. of this appendix.

Several different types of virus cause cancer to develop in animals and transform cells in tissue culture. Viral infection contributes to the development of a few human tumors, but cell gene mutations, host immune deficiency or stimulation, and other viral infections are required before cells actually become malignant.

RNA retroviruses cause cancers in animals and cause malignant transformation of human and animal cells in culture. As a rule, retroviruses do not kill the host cell but allow it to survive. Studies of these viruses have been essential in understanding growth control in normal and malignant human cells.

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