Gene Therapy for Malignant Gliomas

Retroviruses are enveloped RNA viruses that possess the ability to integrate into the host cell genome by transcribing DNA from their RNA template via viral reverse transcriptase. The transcribed viral DNA is then integrated into the host cell genome nonspecifically. There is relative specificity for tumors because, except for lentiviruses, retroviral DNA preferentially integrates into the genome of dividing cells. Retroviral vectors have been derived from the Moloney murine leukemia virus (M-MuLV) established by Baltimore and colleagues. The replication-defective retroviral vector is constructed as follows. DNA plasmid constructs containing long-terminal repeats (LTRs) and the packaging signal y together with the transgene of interest are transfected into modified cultured cells termed vector-producer cells (VPCs). These VPCs, which are usually derived from the murine fibroblast 3T3 cell line, have been stably transfected with a plasmid with the entire retroviral genome, save the packaging signal y. Thus the VPCs are able to transcribe and translate viral RNA but are unable to package viral genomic RNA into virions. Nevertheless, they efficiently complement the plasmid construct (containing the packaging signal y), leading to packaging of the transgene of interest into virions, which can then be harvested from the medium of VPCs. Advantages of retroviral vectors include (1) integration into dividing cells, which is particularly advantageous for tumor therapy, and (2) low toxicity because of replication deficiency. Disadvantages include (1) low transgene capacity, (2) the necessity of implanting transfected VPCs, which may not survive long in the host, and (3) a risk of insertional mutagenesis in host cells (e.g., by insertion at a proto-oncogene locus). In the attempt to improve transduction efficiency, replication-competent retroviruses (RCR) have recently been developed. RCR are able to transduce human and rat glioma cell lines in vitro much more effectively than replication-defective retroviral vectors at the same dose. In addition, RCR capably and selectively transduce established U-87 gliomas in vivo. To date, RCR have not been used in clinical trials. Devin K. Binder