SELF-DELETING RETROVIRUS VECTORS FOR GENE-THERAPY

A new generation of retrovirus vectors for gene therapy has been devel oped. The vectors have the ability to excise themselves after insertin g a gene into the genome, thereby avoiding problems encountered with c onventional retrovirus vectors, such as recombination with helper viru ses or transcriptional repression of transduced genes. The strategy ex ploited (i) the natural life cycle of retroviruses, involving duplicat ion of terminal control regions U5 and U3 to generate long terminal re peats (LTRs) and (ii) the ability of the pi phage site-specific recomb inase (Cre) to excise any sequences positioned between two loxP target sequences from the mammalian genome. Thus, an independently expressed selectable marker gene flanked by a loxP target sequence was cloned i nto the U3 region of a Moloney murine leukemia virus vector. A separat e cassette expressing the Cre recombinase was inserted between the LTR s into the body of the virus. LTR-mediated duplication placed vector s equences, including Cre, between loxP sites in the integrated provirus . This enabled Cre to excise from the provirus most of the viral and n onviral sequences unrelated to transcription of the U3 gene.