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 f rom injecting drug users and the entire long terminal repeat (LTR) reg ion was sequenced. Restriction analysis showed that all the Italian is olates are of the IIb subtype, whereas the British isolates are of the IIa subtype. To understand whether the further differentiation of eac h two principal HTLV-II subtypes in several subgroups could be statist ically supported by phylogenetic analysis, the neighbor-joining, parsi mony, 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 w ithin the HTLV-IIb subtype. In the present analysis, no statistical su pport was obtained for additional phylogroups. Two particular subgroup s seem interesting because they include all European and North America n injecting drug user strains within the IIa and IIb subtypes, respect ively. These data confirm that European HTLV-II infection among drug u sers is probably derived from North America. They also suggest that th ough a certain differentiation by restriction analysis in different su bgroups is possible, carefully interpreted phylogenetic analyses remai n necessary. Using the likelihood ratio test, a molecular clock for th e 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 ca lculated for the IIa and IIb injecting drug user strains. This is the lowest fixation rate so far reported for RNA viruses, including for HI V, which typically range between 10(-2) and 10(-4).