High frequency expression of integrated proviruses derived from enhancer trap retroviruses

Since retroviruses integrate preferentially into transcriptionally active loci, the provirus may come under the control of regulatory elements of the gene into which it integrated and thus become a functional tag for that gene. In order to determine the frequency of retroviral integration near active endogenous enhancer elements, a retroviral enhancer trap vector was constructed. Lacking the long terminal repeat enhancer, expression of the neomycin resistance (neo) gene, used as a reporter, is dependent upon endogenous enhancer elements able to activate the long terminal repeat promoter. Infection of murine fibroblast cells indicated that a high proportion of the proviral copies expressed the neo gene. Infection of hematopoietic lines confirmed this high frequency of expression of integrated proviruses. Overall, between 43 and 74% of proviruses integrated into several different cell lines expressed the neo gene. These data suggest that retroviral integration is not only dependent upon transcriptional activity of the genomic target sites, but, more specifically, retroviruses preferentially integrate near active enhancer elements which are often associated with developmentally regulated genes.