Concepts and strategies for human gene therapy

Abstract

Beginning in the late 1960s and early 1970s, the availability of new molecular-biological tools and concepts began to suggest that designed genetic changes may eventually provide a new and effective form for the therapy of human diseases. In 1972, the medical need, as well as the technical and conceptual issues involved in efficient virally mediated gene transfer for therapeutic purposes, were outlined (Friedmann and Roblin, 1972). In the 20 years since that time, progress has been so great that human experiments are now underway to test the efficacy of foreign genes in defective human cells as a form of gene therapy. There are a very large number of human diseases that represent potential targets for this kind of manipulation. More than 4500 human diseases are currently classified as genetic (McKusick, 1988). Until now, only a very small minority of these diseases has been associated with specific mutations in the human genome. Recessive genetic diseases like cystic fibrosis and adenosine deaminase (ADA) deficiency require mutations to be present in both alleles of a gene in order to generate the disease phenotype, while dominant diseases like Huntington’s disease can be caused by the presence of only one mutated copy of a gene. The mere presence of a mutant allele overrides the remaining normal allele and leads to disease. With the application of the tools of molecular genetics, and with new approaches to the identification and characterization of disease-related defects, therapy through the correction of genetic defects has come within reach (Friedmann, 1989; Anderson 1992). Most modern gene-therapeutic approaches are based on the introduction of functional copies of defective genes into cells.

Methods of modern molecular genetics have been developed that allow stable transfer and expression of foreign DNA sequences in human and other mammalian somatic cells. It is therefore no surprise that the methods have been applied in attempts to complement genetic defects and correct disease phenotypes. Two decades of research have now led to the first clinically applicable attempts to introduce genetically modified cells into human beings to cure diseases caused at least partially by genetic defects. We discuss here some of the strategies being followed for both in vitro and in vivo application of therapeutic gene transfer and summarize some of the technical and conceptual difficulties associated with somatic-cell gene therapy.