Safe and sound

Abstract

The use of retroviruses as vectors for gene therapy is restricted by their inability to replicate in non-dividing cells. Lentiviruses, such as HIV-1, have this capability, but their use for human gene targeting has been limited because of biosafety concerns. The report by Dull et al.[ 1 Dull T. et al. A third-generation lentivirus vector with a conditional packaging system. J. Virol. 1998; 72: 8463-8471 Crossref PubMed Google Scholar ]represents the latest attempt to generate a replication-defective vector in which only a few essential elements from the HIV-1 genome remain. Four components comprise this lentivirus-based delivery system. A conditional packaging construct contains gag, pol and a rev-responsive element (RRE) from HIV-1. Expression of Gag and Pol is driven from a cytomegalovirus (CMV) promoter in the presence of a second plasmid expressing rev. A heterologous envelope gene is provided by a third plasmid encoding vesicular stomatitis virus (VSV) G protein. The fourth plasmid contains the transgene. The authors have made major modifications to improve biosafety and limit the ability of the vector to propagate beyond a single round of replication. Previous constructs with the HIV-1 long terminal repeat (LTR) required the HIV-1 tat gene for efficient expression. However, Dull et al.[ 1 Dull T. et al. A third-generation lentivirus vector with a conditional packaging system. J. Virol. 1998; 72: 8463-8471 Crossref PubMed Google Scholar ]show that the requirement for tat can be bypassed by replacing part of the HIV 5′ LTR with the enhancer/promoter region of the Rous sarcoma virus (RSV) LTR. The chimeric LTR is constitutively active in the absence of tat. In addition, most of the U3 region of the 3′ LTR in this construct has been deleted, including the TATA box. As the 3′ LTR is the template for LTR synthesis during proviral integration, both newly generated LTRs will be transcriptionally inactivated. The authors show that this vector can drive transgene expression in rat neurons. It is hoped that such a multi-component delivery system will ultimately prove to be both efficacious and safe for gene therapy in humans.