Enhancer activity correlates with the oncogenic potential of avian retroviruses.

Abstract

Avian retroviruses lacking an oncogene, such as Rousassociated virus 1 (RAV-1), RAV-2, and td mutants of Rous sarcoma virus (RSV), can nevertheless cause leukemias and other neoplastic diseases. During this process, viral DNA integrates near a cellular proto-oncogene, such as c-myc, and thus de-regulates its expression. The virus RAV-0, on the other hand, is known to be non-oncogenic even in long-term in vivo infections of domestic chickens. The major difference between oncogenic and non-oncogenic viruses is found within the U3 region of the long terminal repeat (LTR) which is known to harbor the promoter and enhancer elements. We therefore wanted to see whether viral oncogenicity was correlated with enhancer activity. Using a variety of techniques (including the SV40 'enhancer trap' from which we obtained RSV-SV40 recombinant viruses), we demonstrate that a strong enhancer exists within the LTRs of both RSV and RAV-1. In contrast, no enhancer is present in RAV-0, although RAV-0 has functional promoter elements. Our data therefore strongly support a concept of oncogenesis by enhancer insertion.

Self-Inactivating Alpharetroviral Vectors with a Split-Packaging Design

“…By quantifying transcript levels of upstream-initiated and internal transcripts, we showed the level of upstream-initiated transcription to be approximately 80-fold lower for the SIN vector than for its precursor containing an intact U3 region. Additionally, we performed transient reporter gene assays, by which we confirmed the strong transcriptional activity of the wild-type U3 region (13,42) in human cells. While we were still able to detect upstream-initiated transcription in cells transduced with the SIN vector, the transient assay showed a reversion of LTR transcriptional activity to background levels.…”

A very strong enhancer is located upstream of an immediate early gene of human cytomegalovirus

“…The procedure has also been used to identify and characterize heterologous enhancer sequences from various other viruses. In independent experiments we have identified and/or further characterized enhancers in the genome of Herpesvirus saimiri (S. Schirm, F. Weber, W. Schaffner, and B. Fleckenstein, unpublished), murine cytomegalovirus (K. DorschH&sler, G. Keil, W. Schaffner, and U. H. Koszinowski, unpublished), Rous sarcoma virus (F. Weber and W. Schaffner, 1985), and hepatitis B virus (A. Tognoni, R. Cattaneo, and W. Schaffner, unpublished). Among these strong viral enhancers, the one from HCMV has the highest activity not only in primate cells but also in cell lines from other species, including frog kidney cells; thus it is the strongest enhancer we have found so far.…”

A transcription enhancer in the Herpesvirus saimiri genome.

“…Two of these clones, designated SVHS-14 and SVHS-7, were mapped with restriction endonuclease and the inserts were sequenced. In the many previous enhancer trap experiments the SV40 recombinants had incorporated heterologous enhancers without any internal sequence rearrangements (Weber et al, 1984;Boshart et al, 1985;Weber and Schaffner, 1985 SVHS-7 have a similar internal deletion as compared with the genomic H. saimiri DNA (Figure 3). One of the end points of the internal deletion in the H. saimiri segment is the same for both recombinants.…”

“…The H. saimiri segment is located 7 kb upstream of an immediateearly coding region of the virus and is present not only in all lytically growing virus particles but also in the severely truncated viral genomes persisting in transformed cells. Unlike any other of the various enhancers selected with the 'enhancer trap' (Weber et al, 1984;Boshart et al, 1985;Weber and Schaffner, 1985;Dorsch-Hasler et al, 1985; E.Serfling, A.Lubbe, K.Dorsch-Hasler and W.Schaffner, in preparation) two SV40-H. saimiri recombinants have been isolated which have suffered a very similar deletion within the enhancer. By this, the enhancer activity apparently is improved, suggesting that the deleted segments exert negative effects within the recombinants.…”