Functional Loss of the HTLV-I Tax Protein Encoded by Variant Proviruses in Infected Individuals

Accumulating evidence has indicated the presence of HTLV-I quasispecies in infected individuals. To elucidate their biological consequences, we amplified the whole Tax open reading frame (ORF) of HTLV-I proviruses by nested PCR from six infected individuals, including three HAM/TSP patients, and a cloned HTLV-I DNA, pMT2, and the products were introduced into an expression vector. The potential for transcriptional transactivation of protein products of independent 20-39 tax clones derived from each sample was evaluated by transfecting into pA18G-BHK-21 cells containing the HTLV-I LTR-driven lacZ gene. While all of 30 clones derived from pMT2 gave positive results, significant proportions, ranged between 16.0 and 35.0%, of the tax clones from the infected indiv iduals were functionally defective. The functional loss of these tax clones was confirmed by chloramphenicol acetyltransferase (CAT) assay in cells cotransfected with an HTLV-I LTR-CAT reporter. DNA sequence analysis revealed that the defective clones contained at least one nonsynonymous nucleotide substitutions from the consensus sequences of the individual. These findings strongly suggested that the accumulation of HTLV-I proviruses with defective tax was a common feature among infected individuals. Since the Tax protein is indispensable for viral replication, these defective viruses were likely to be generated in individuals after the event of infection. It is conceivable that the quasispecies plays a key role in the latency of HTLV-I infection and possibly in HTLV-I-related pathogenesis.