Figure 1.

Retroviral vector development for increased efficiency and targeting. (A) Structure of a simple retroviral genome containing coding sequences for gag, pro, pol, and env for replication. (B) Structure of a simple γ-retroviral gene therapy vector with replication coding sequences removed and transgene inserted. Note retention of the dual long-terminal repeats (LTRs), primer binding site (PBS), signal Ψ, attachment sites (att), and polypurine tract (PPT). The U3 component of the 5′LTR is used as a promoter to drive transgene expression. A second heterologous or tissue-specific promoter (P) has been inserted to drive an ampicillin gene to facilitate ex vivo selection of transduced cells. (C) Structure of an enhanced self-inactivating retroviral gene therapy vector (note substitution of the 5′LTR U3 component). The internal promoter (P) is tissue-specific to limit transgene expression. Additional genetic elements—such as chromatin insulators (CI), chromatin structure regulators (CR), a woodchuck hepatitis virus post-transcriptional regulatory element (WPRE), scaffold or matrix attachment regions (S/MAR), or resistance genes—are incorporated to enhance site specificity and integration efficiency while limiting gene silencing.