Gene transfer is a novel area of therapeutics in which the active agent is a nucleic acid rather than a protein or small molecule. Because delivery of naked DNA or RNA to a cell is an inefficient process, most gene transfer is carried out using a vector, or gene delivery vehicle. These vehicles have generally been engineered from viruses by deleting some or all of the viral genome and replacing it with the therapeutic gene of interest under the control of a suitable promoter (Table 68–1). Gene transfer strategies can be described in terms of three essential elements: (1) a vector; (2) a gene to be delivered, sometimes called the transgene; and (3) a relevant target cell to which the DNA or RNA is delivered. The series of steps in which the donated DNA enters the target cell and expresses the transgene is referred to as transduction. Gene delivery can take place in vivo, in which the vector is directly injected into the patient or, in the case of hematopoietic and some other target cells, ex vivo, with removal of the target cells from the patient, followed by return of the modified autologous cells after gene transfer in the laboratory. The latter approach offers opportunities to integrate gene transfer techniques with cellular therapies (Chap. 67).
Table 68–1 Characteristics of Gene Delivery Vehicles |Favorite Table|Download (.pdf)
Table 68–1 Characteristics of Gene Delivery Vehicles
|Features||Retroviral||Lentiviral||Adenoviral||AAV||Human Foamy Virus||HSV-1||SV-40||Alphaviruses|
|Cell division requirement||Yes||G1 phase||No||No||No||No||No||No|
|Packaging limitation||8 kb||8 kb||8–30 kb||5 kb||8.5 kb||40–150 kb||5 kb||5 kb|
|Immune responses to vector||Few||Few||Extensive||Few||Few||Few in recombinant virus||Few||Few|
|Main advantages||Persistent gene transfer in dividing cells||Persistent gene transfer in transduced tissues||Highly effective in transducing various tissues||Elicits few inflammatory responses, nonpathogenic||Persistent gene expression in both dividing and nondividing cells||Large packaging capacity with persistent gene transfer||Wide host cell range; lack of immunogenicity||Limited immune responses against the vector|
|Main disadvantages||Theoretical risk of insertional mutagenesis (occurred in three cases)||Might induce oncogenesis in some cases||Viral capsid elicits strong immune responses||Limited packaging capacity||In need of a stable packaging system||Residual cytotoxicity with neuron specificity||Limited packaging capacity||Transduced gene expression is transient|
Gene transfer technology is still under development, although licensing applications for gene therapy products have been filed. Gene therapy is one of the most complex therapeutic modalities yet attempted, and each new disease represents a therapeutic problem for which dosing, safety, and efficacy must be defined. ...