Proposed uses or targets of human gene therapy trials. During the past 5 years in the United States more than 500 human gene therapy clinical trials have been approved by the Office of Biotechnology Activities of the National Institutes of Health.
gies) should also be considered. Two fundamental approaches underlie the basis of gene therapy. In the first, genetic material is introduced into cells to alter the cellular phenotype but not the genotype. This is typified by the transfer of unintegrated DNA, antisense oligomers, and ribozymes. In this regard, gene therapy has many of the attributes and problems of conventional endocrine or antimicrobial therapy with respect to efficiency of targeting and the duration of effect. A second approach seeks permanent alteration of the genotype of the cell, leading to a modified phenotype that prevents or alters a disease state. In this setting, gene therapy will permanently modify organ function.
Theoretically, mutated or nonfunctional genes could be excised and replaced, and new genes with desired functions could be permanently inserted into the genome. Stable integration of an antisense DNA might also be desirable in some circumstances. Because of the technical difficulties associated with the delivery of nucleic acid-based products selectively to specific target cells in vivo, more experimental information is available for ex vivo human gene therapy.
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