Structural and functional analysis of long terminal repeats of Suncus murinus mammary tumor virus

Mouse mammary tumor virus with rearranged long terminal repeats causes murine lymphomas

Mouse mammary tumor virus (MMTV) is a slowly transforming retrovirus associated primarily with the induction of mammary tumors. It is widely accepted that T-cell lymphomas of various mouse strains are associated with extra proviruses of MMTV. These extra proviruses showed site-specific rearrangements in the U3 region of long terminal repeats (LTRs), consisting of about 400 nucleotide deletions and occasional substitution resulting in unique tandem repeats. However, the question of whether these mutant MMTVs cause lymphomas has not been experimentally resolved. Here we present distinct evidence that they do. We constructed chimeric MMTVs by replacing the LTR of the recently constructed pathogenic MMTV provirus clone with rearranged LTRs of MMTV proviruses obtained from two DBA/2 mouse lymphoma cell lines, MLA and DL-8, and inoculated them into BALB/c mice. These mice developed lymphomas, but no mammary tumors, 4 to 11 months postinoculation, whereas the original pathogenic MMTV clone alone induced mammary tumors. These results showed that the tissue specificity of MMTV tumorigenesis is determined by the LTR structures.

Long terminal repeats of dwarf hamster endogenous retrovirus are highly diverged and do not maintain efficient transcription

Recently we described a new endogenous proretrovirus of dwarf hamster Phodopus sungorus (MRS-Ps). Its sequence possesses evident homology with the endonuclease domain of the mouse mammary tumor virus pol gene. Here we present nucleotide sequence data on three clones of retroviral long terminal repeats. As many as 15% of substituted, deleted, and inserted base pairs were found while comparing these sequences. Hence, MRS-Ps seems to be rather an old genetic element which originated about 30 million years ago. One LTR is 877 bp long and contains numerous elements that control its transcriptional activity: TATA-box, glucocorticoid responsive element, NF1-binding site, etc. Nevertheless, this LTR does not govern efficient transcription of adjacent genes in a transient expression assay. In addition, we failed to find MRS-specific mRNA in adult, embryonic, and mammary tumor cells.

Identification of a novel mammary cell line-specific enhancer element in the long terminal repeat of mouse mammary tumor virus, which interacts with its hormone-responsive element

We analyzed the long terminal repeat (LTR) of mouse mammary tumor virus for sequences that influence its promoter activity by using the chloramphenicol acetyltransferase assay. A series of LTR deletion mutants and recombinants between LTR and simian virus 40 regulatory sequences were used for these studies. Through transfection experiments in three different human cell lines (T47D, MCF-7, and HeLa), we identified a novel mammary cell line-specific enhancer element on a 98-bp BanII fragment (from position -1075 to -978 upstream of the start site of transcription) which interacts with the hormone-responsive element of LTR. We also identified nuclear factors that specifically interacted with this BanII fragment in the nuclear extract from the mammary tumor cell line, T47D, but not from the HeLa cell line.

Distribution of mouse mammary tumor virus-related sequences does not correlate with the taxonomic position of their hosts

Sequences (MRS) distantly related to mouse mammary tumor virus (MMTV) were found in genomes of a wide range of mammalian species using blot hybridization. The number of MRS copies and the degree of their homology with the hybridization probe varied and did not correlate with the taxonomic position of the species. Nevertheless, within a genus the set of MRS was species specific and reflected the taxonomic relation between the species. MRS were also found in avian genomes and the degree of their homology did not correlate with the taxonomic position of the species either. The origin and distribution of MRS is discussed on the basis of the authors' and published data.

Extra mouse mammary tumor proviruses in DBA/2 mouse lymphomas acquire a selective advantage in lymphocytes by alteration in the U3 region of the long terminal repeat

We determined the nucleotide sequences of the long terminal repeats (LTRs) from mouse mammary tumor virus (MMTV) proviruses acquired in two DBA/2 mouse lymphoma cell lines, MLA and DL-8. Proviruses from MLA contained a 352-base-pair deletion from nucleotides 669 to 1020 in the U3 region of the LTR, whereas the LTR alteration of the DL-8 provirus involved both a similar 360-base-pair deletion and generation of a tandem repeat region consisting of sequences of flanking deletions. To assess the function of the rearranged LTRs, we constructed plasmids in which normal and rearranged LTRs drove the reporter chloramphenicol acetyltransferase gene and transfected them into T-cell lines (Jurkat, Molt-3, and DL-8) and the mammary tumor cell line T47D. Both rearranged LTRs were transcriptionally active, but normal LTRs were not active in either the presence or absence of glucocorticoids in all T-cell lines. In T47D cells, however, the MLA provirus LTR showed the same glucocorticoid- or progestin-dependent transcriptional activity as did normal LTRs. The DL-8 provirus LTR acquired a novel enhancer(s) by rearrangement and thus had a high basal transcriptional activity in T47D cells. The results of chloramphenicol acetyltransferase assays using plasmids with various chimeric MMTV LTRs revealed that the rearranged LTRs had lost their negative regulatory element and contained an enhancer element that was highly homologous to the enhancer A element of polyomavirus (from nucleotides 525 to 558). GR but not C3H mouse MMTV contained this enhancer. These results elucidate some of the molecular mechanisms involved in the selection of mutant MMTVs with rearranged LTRs in lymphoma cells.