The role of the LTR in feline leukemia virus-mediated oncogenesis
Description
FeLV proviruses isolated from a subset of splenic lymphosarcomas exhibit a unique structure in the LTR which is unusual in 2 respects: (1) the LTR contains only a single copy of the enhancer and (2) the LTR contains a 21-bp tandem triplication beginning downstream of the enhancer. This is unusual, since LTRs from proviruses cloned directly from tumors often contain duplicated enhancers while non-pathogenic isolates contain only a single enhancer. The repeated finding of the triplication-containing LTR in proviruses inserted at flvi-1 in non-B-cell non-T-cell lymphosarcomas of the spleen suggests that the unique LTR is an essential participant in the development of tumors of this particular phenotype. The present study was based on this hypothesis The triplication-containing LTR has been demonstrated in all tumors with flvi-1 interruptions by hybridization of a triplication-specific oligomer to proviral DNA fragments amplified by PCR To examine the contribution of the triplication to LTR function, a study was performed to measure the ability of the triplication-containing LTR (FeLV-A/945), and two other structurally distinct LTRs, to modulate gene expression using reporter gene assays. The two other LTRs were (i) LC-FeLV, which was isolated from a thymic lymphosarcoma and contains a duplication of the enhancer and (ii) FeLV-A/61E which contains only a single copy of the enhancer. Studies of LTR-directed reporter gene expression in FEA and Jurkat cells indicate that the 21-bp triplication provides transcriptional enhancer function to the LTR that contains it, and that it substitutes at least in part for the duplication of enhancer. Data obtained from transfection of K-562 cells were consistent with these indications, but provided key additional information as well. The activity of the triplication-containing LTR is 19-fold more active than that of FeLV-A/61E and 3-fold more active than that of LC-FeLV. This is relevant because the K-562 line may be representative of the cell-type targeted for transformation, a hematopoietic stem cell or lymphoid progenitor The mechanism by which the 21-bp triplication contributes enhancer function to the LTR that contains it was also investigated. The 21-bp triplication confers a bona fide enhancer function upon the LTR-directed reporter gene expression, however, a possibility of a spacer function was not eliminated The findings from LTR-directed reporter gene studies predict that the transcription and replication of FeLV containing the 21-bp triplication would be enhanced in infected cells relative to FeLV lacking the triplication. A recombinant FeLV bearing the 21-bp triplication in U3 was constructed to test this hypothesis. This recombinant, 61g945L exhibited higher reverse transcriptase activity and more abundant transcripts relative to the parental FeLV-A/61E in infected cells On the basis of these findings we hypothesize that the 21-bp triplication plays a role in the malignant transformation of the appropriate target cell in the spleen by modulating the expression of viral and host genes at the site of integration