Lack of correlation between basal expression levels and susceptibility to transcriptional shutdown among single-gene murine leukemia virus vector proviruses.

Integrated retroviruses or retroviral vectors may be transcriptionally inactive although their promoter-enhancer regions are able to direct transcription in the host cell. We have used single-gene retroviral vectors with a long terminal repeat-directed neo marker gene to determine if the level of transcription relates to the susceptibility of a provirus to inactivation. We used two isogenic vectors, carrying long terminal repeats with a strong and a weak transcriptional enhancer derived from SL3-3 and Akv murine leukemia viruses, respectively. Nonselected cell clones of the murine lymphoid cell line L691 with single integrated vector proviruses exhibiting a 20-fold range of initial expression levels were studied. The basal expression level of a given cell clone with a single provirus did not show any pattern of correlation with the long-term stability of expression, as monitored for periods up to 150 days. Our results thus indicate that the inactivation mechanism operates independently of the initial transcriptional activity of the provirus.