Walsh & Hoyt: Oncoviruses

Two closely related viruses in the Retroviridae subfamily of oncoviruses infect humans and cause human disease. Because these viruses were first isolated from patients with Tcell leukemia and were the first identified viral agents believed to produce human cancer, they were called the human T-cell leukemia viruses types I and II or HTLV-I and HTLVII. These viruses share about 65% nucleotide similarity. The viral structure and life cycle of HTLV-I and HTLV-II are similar to those of HIV-1 and HIV-2, but HTLVs contain two additional regulatory proteinstax and rexthat are not present in most other retroviruses. Tax upregulates viral RNA transcription, and rex both downregulates the expression of spliced mRNA that encodes for the regulatory proteins and upregulates the expression of structural proteins. Of the two human oncoviruses, HTLV-I is associated with the majority of human infections. Nevertheless, increasing evidence is accumulating that infection with HTLV-II may also cause significant human infection.