Somatic Gene Therapy

post-translational modification of the protein. Often it is desirable to have expression of a gene controlled by its normal genetic elements to achieve “normal” patterns of expression. For example, it is likely that gene therapy for congenital hypothyroidism will require gene replacement into thyroid follicular cells as well as highly regulated expression to establish normal levels of thyroid hormone. In such cases the vector can be constructed using genetic elements that normally direct expression of particular gene. Other times, however, it is desirable to express supraphysiological amounts of protein in a smaller number of cells or to express the protein in a heterotopic location where it is not normally expressed. For example, gene therapy for type I diabetes will require expression of insulin from cells other than the B cell. This requires the recombination of the gene with novel control elements in a chimeric vector containing genetic control elements derived from different genes.

The limiting factor in gene therapy is often achieving sufficient amounts of the product to exert a therapeutic effect. For this reason, many of the vectors currently being considered as potential therapeutic use genetic elements from viruses that are known to provide high levels of expression during viral infections. Another important factor is often the location of gene expression. Many genes in the body are normally expressed in a tissue-specific manner. The genetic elements that normally cause a gene to be expressed in one tissue, for example the liver, and not other tissues, do so by controlling the transcription of the DNA into mRNA. Thus, there are elements associated with tissue-specific genes such as phenylalanine hydroxylase of ornithine transcarbamylase that restrict transcription of these genes to the liver and othTer which restrict the expression of hemoglobin to red blood cell precursors. These genetic elements, and others, have been identified and may be combined with vitually any gene to achieve restricted expression in certain tissuces. Restricting gene expression to certain tissues and cells may be important to achieve a reproducible effect of gene transfer, since different tissues have different metabolic pathways and may respond differently to expression of a therapeutic protein.

It is also possible to design vectors to control the duration of gene expression. While gene therapy is often perceived to be a means for achieving a permanent cure for disease, it is in creasingly recogniced that permanent genetic manipulation may introduce unnecessary clinical risk in light of the inevitable complication of clinical care such as misdiagnosis, poor compliance, and reports of adverse events. Some vectors are designed to permanently integrate genes into the chromosomes of the host cell to as achieve indefinite expression of the therapeutic product. This is usually accomplished by incorporating viral elements into the vector that allow the vector to be inserted into the chromosomes of the host cell in a random fashion. Other vectors may be designed to maintain the therapeutic gene as an extrachromosomal (episomal) element within the nucleous that can be replicated and repaired like a normal gene withouth integrating into the host‘s chromosome. It is possible in certain cell and animal models to insert genes into select locations using a technique termed homologous recombination based on the fact that a gene sequence introduced into a cell will preferentially integrate into cells at locations having matching (homologous) sequences. The low prequency of these events makes this approach to therapy currently impractical for gene therapy in vivo.

Increasingly, attention has turned away from the development of permanent gene therapies to the use of genes which do not persist indefinitely within the body, but rather will be eliminated from the body like conventional medicines, providing a finite duration of action and predictable pharmacokinetic properties. Such therapies might be administered repetitively, allowing the physician to adjust the prequency or dose of therapy or even change or terminate therapy, based on the patient’s individual needs over time. For such therapies to be effective, they must have relatively low toxicity and be nonimmunogenic and the prequency as well as the route of administration must be compatible with achieving reasonable compliance.