Evolutionary rate and genetic heterogeneity of human T-cell lymphotropic virus type II (HTLV-II) using isolates from European injecting drug users

Seven new Italian and two new British HTLV-II isolates were obtained from injecting drug users and the entire long terminal repeat (LTR) region was sequenced. Restriction analysis showed that all the Italian isolates are of the IIb subtype, whereas the British isolates are of the IIa subtype. To understand whether the further differentiation of each two principal HTLV-II subtypes in several subgroups could be statistically supported by phylogenetic analysis, the neighbor-joining, parsimony, and maximum likelihood methods were used. The separation between IIa and IIb is very well supported by all three methods. At least two phylogenetic subgroups exist within the HTLV-IIa and at least three within the HTLV-IIb subtype. In the present analysis, no statistical support was obtained for additional phylogroups. Two particular subgroups seem interesting because they include all European and North American injecting drug user strains within the IIa and IIb subtypes, respectively. These data confirm that European HTLV-II infection among drug users is probably derived from North America. They also suggest that though a certain differentiation by restriction analysis in different subgroups is possible, carefully interpreted phylogenetic analyses remain necessary. Using the likelihood ratio test, a molecular clock for the drug user strains was calibrated. A fixation rate between 1.08 x 10(-4) and 2.7 x 10(-5) nucleotide substitutions per site per year was calculated for the IIa and IIb injecting drug user strains. This is the lowest fixation rate so far reported for RNA viruses, including for HIV, which typically range between 10(-2) and 10(-4).