The FeLV-945 LTR Confers a Replicative Advantage Dependent on the Presence of a Tandem Triplication

Abstract

Feline leukemia virus (FeLV), like other naturally occurring retroviruses, is characterized by a high degree of genetic diversity. FeLV-945 is a natural isolate derived from non-B-cell non-T-cell lymphomas classified anatomically as multicentric. FeLV-945 exhibits a unique structural motif in the LTR composed of a 21-bp tandem triplication downstream of a single copy of enhancer. The unique FeLV-945 LTR is precisely conserved among eight independent multicentric lymphomas collected in a geographic cluster. Previous studies using reporter gene constructs predict that the FeLV-945 LTR would confer a replicative advantage on the virus that contains it, particularly in primitive hematopoietic cells. Such an advantage may account for the precise conservation of the unique LTR sequence. To test that prediction, a set of recombinant, infectious FeLVs was developed that are isogenic other than the presence of the FeLV-945 LTR or mutations of it. Replication assays show that the FeLV-945 LTR confers a distinct growth advantage in K-562, FEA, and 3201 cells and implicate the 21-bp triplication in that function. Replacement of two copies of the triplicated element with random sequence greatly diminished the replicative capacity, thus implicating the triplicated sequence itself in LTR function. The 21-bp triplication was shown to contain specific nuclear protein binding sites, which may account for the selective pressure to conserve the sequence.

Advances in understanding molecular determinants in FeLV pathology

“…Regarding the mechanism by which the FeLV-945 LTR confers a replicative advantage, our studies identified binding sites for the transcription factor, c-Myb, that cross the repeat junctions of the 21-bp triplication (Prabhu et al, 1999;Finstad et al, 2004). It was intriguing to identify binding sites that cross the repeat junctions, because such sites would not occur in the absence of the repeat; thus, a requirement for c-Myb binding to the repeat junctions of the triplication might represent a selective pressure to conserve its sequence precisely.…”

“…FeLV LTR sequence variation in the cohort was examined using PCR amplification and nucleotide sequence analysis. Specifically, genomic DNA from diseased tissues was amplified by PCR using primers specific for the U3 region of exogenous FeLV LTR (Prabhu et al, 1999;Chandhasin et al, 2004). Amplification products of multiple sizes were detected from most animals, indicating the presence of FeLV LTR sequence variation within the tissues of individuals.…”

“…A series of studies of the 21-bp triplication showed that it contributes transcriptional enhancer function to the FeLV-945 LTR, that it functions preferentially in a cell type-specific manner and that it confers a replicative advantage to the virus in those cells (Levesque et al, 1990;Prabhu et al, 1999;Chandhasin et al, 2004). In one such study (Chandhasin et al, 2004), recombinant FeLV LTRs were constructed and substituted into infectious FeLV-A/61E in order to compare the replication of virus with one, two, three or four copies of the 21-bp repeat element.…”

“…The FeLV-945 SU gene encodes distinctive mutational differences as compared to FeLV-A By sequence analysis, the SU protein of FeLV-945 is most closely related to natural horizontally-transmissible FeLV-A, but was observed to differ from an FeLV-A prototype to a greater extent than the known FeLV-A isolates differ among themselves Prabhu et al, 1999). The sequence differences in FeLV-945 SU included point mutations restricted largely to domains that play essential roles in receptor recognition and entry, i.e., VRA, VRB and PRR (Battini et al, 1992;Boomer et al, 1997;Tailor and Kabat, 1997;Sugai et al, 2001).…”

“…al., 1999;Chandhasin et al, 2004). The SU protein of FeLV-945 was shown to be most closely related to natural horizontally-transmissible FeLV-A, but was observed to differ from an FeLV-A prototype to a greater extent than the known FeLV-A isolates differ among themselves Prabhu et al, 1999). The following review describes studies designed to understand the selective advantages that may have led to the predominance of FeLV-945 in a naturally infected population, and the consequences of its predominance on infection and disease progression.…”