HIGH-FREQUENCY EXPRESSION OF INTEGRATED PROVIRUSES DERIVED FROM ENHANCER TRAP RETROVIRUSES

Since retroviruses integrate preferentially into transcriptionally act ive loci, the provirus may come under the control of regulatory elemen ts of the gene into which it integrated and thus become a functional t ag for that gene. In order to determine the frequency of retroviral in tegration near active endogenous enhancer elements, a retroviral enhan cer trap vector was constructed. Lacking the long terminal repeat enha ncer, expression of the neomycin resistance (neo) gene, used as a repo rter, is dependent upon endogenous enhancer elements able to activate the long terminal repeat promoter. Infection of murine fibroblast cell s indicated that a high proportion of the proviral copies expressed th e neo gene. Infection of hematopoietic lines confirmed this high frequ ency of expression of integrated proviruses. Overall, between 43 and 7 4% of proviruses integrated into several different cell lines expresse d the neo gene. These data suggest that retroviral integration is not only dependent upon transcriptional activity of the genomic target sit es, but, more specifically, retroviruses preferentially integrate near active enhancer elements which are often associated with developmenta lly regulated genes.