Clastogenic Effect of the Human T-cell Leukemia Virus Type I Tax Oncoprotein Correlates with Unstabilized DNA Breaks*

Expression of the human T-cell leukemia virus type I (HTLV-I) Tax oncoprotein rapidly engenders DNA damage as reflected in a significant increase of micronuclei (MN) in cells. To understand better this phenomenon, we have investigated the DNA content of MN induced by Tax. Using an approach that we termed FISHI,fluorescent in situhybridization and incorporation, we attempted to characterize MN with centric or acentric DNA fragments for the presence or absence of free 3′-OH ends. Free 3′-OH ends were defined as those ends accessible to in situ addition of digoxigenin-dUTP using terminal deoxynucleotidyl transferase. MN were also assessed for centromeric sequences using standard fluorescent in situhybridization (FISH). Combining these results, we determined that Tax oncoprotein increased the frequency of MN containing centric DNA with free 3′-OH and decreased the frequency of MN containing DNA fragments that had incorporation-inaccessible 3′-ends. Recently, it has been suggested that intracellular DNA breaks without detectable 3′-OH ends are stabilized by the protective addition of telomeric caps, while breaks with freely detectable 3′-OH are uncapped and are labile to degradation, incomplete replication, and loss during cell division. Accordingly, based on increased detection of free 3′-OH-containing DNA fragments, we concluded that HTLV-I Tax interferes with protective cellular mechanism(s) used normally for stabilizing DNA breaks.