HTLV-I generally integrates at least one full-length copy in adult T-cell leukemia (ATL) cells. A group of patients without full-length provirus have a unique conserved truncation of the provirus which retains env-pX-3′LTR. Tumor cells of a patient from this group were genetically analyzed. Analysis of the 5′ and 3′ cellular flanking region adjacent to the provirus suggest that the defective provirus was integrated immediately downstream of a promoter of an unknown cellular gene. The activity of the promoter was weak but was responsive to Tax-like HTLV-I LTR. The provirus may have utilized it as a substitute for the 5′LTR and thus 3′LTR may have become an alternative promoter for the cellular gene, which may give similar viral-cellular interactions to that of general cases with full-length proviruses. Surprisingly, the 3′ cellular flanking region which is thought to be controlled originally by the promoter is constitutively expressed specifically in an HTLV-I producing ATL cell line HUT102G, in which the corresponding region is not modified by provirus. The detection of this HTLV-I-induced transcript provides a probe to find an HTLV-I inducible unknown cellular gene that may be related to the pathogenesis of ATL.
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