Gene transfer is a novel area of therapeutics in which the active agent is a nucleic acid sequence 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 458-1). Gene transfer strategies can thus be described in terms of three essential elements: (1) a vector; (2) a gene to be delivered, sometimes called the transgene; and (3) a physiologically relevant target cell to which the DNA or RNA is delivered. The series of steps in which the vector and donated DNA enter the target cell and express 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 gene-modified autologous cells to the patient after manipulation in the laboratory. The latter approach effectively combines gene transfer techniques with cellular therapies (Chap. 473).
TABLE 458-1Characteristics of Gene Delivery Vehicles ||Download (.pdf) TABLE 458-1 Characteristics of Gene Delivery Vehicles
|Viral Vectors |
|Features ||Retroviral ||Lentiviral ||Adenoviral ||AAV ||Human Foamy Virus ||HSV-1 ||Alpha Viruses |
|Viral genome ||RNA ||RNA ||DNA ||DNA ||RNA ||DNA ||RNA |
|Cell division requirement ||Yes ||G1 phase ||No ||No ||No ||No ||No |
|Packaging limitation ||8 kb ||8 kb ||8–30 kb ||5 kb ||8.5 kb ||40–150 kb ||5 kb |
|Immune responses to vector ||Few ||Few ||Extensive ||Few ||Few ||Few in recombinant virus ||Few |
|Genome integration ||Yes ||Yes ||Poor ||Poor ||Yes ||No ||No |
|Long-term expression ||Yes ||Yes ||No ||Yes ||Yes ||No ||No |
|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 ||Limited immune responses against the vector |
|Main disadvantages ||Theoretical risk of insertional mutagenesis (occurred in multiple cases) ||Might induce oncogenesis in some cases (not yet observed) ||Viral capsid elicits strong immune responses ||Limited packaging capacity ||In need of a stable packaging system ||Residual cytotoxicity with neuron specificity ||Transduced gene expression is transient |
Gene transfer is one of the most powerful concepts in modern molecular medicine and has the potential to address a host of diseases for which there are currently no available treatments. Clinical trials of gene therapy have been under way since 1990; the first gene therapy product to be licensed in the United States or Europe ...