Designing Our Descendants: The Promises and Perils of Genetic Modifications

7 Germ-Line Gene Therapy: Can We Do It, Do We Need It, Where Do We Start, and Where Might It Lead?

c h a p t e r s e v e n Germ-Line Gene Therapy Can We Do It, Do We Need It, Where Do We Start, and Where Might It Lead? Christopher H. Evans, Ph.D. Despite several setbacks,somatic cell gene therapy is finally beginning to record some solid clinical successes.1 There is growing optimism that such an approach to the treatment of disease will indeed prove fruitful, and somatic gene therapies for a variety of disorders, both genetic and nongenetic, lethal and nonlethal, are at advanced stages of development.As this former frontier of genetic medicine becomes increasingly commonplace, the leading edge of novel therapeutic intent will continue to expand and challenge new boundaries. One of these,germ-line gene therapy,is already beginning to evolve,in certain quarters , from the realm of the undoable and unthinkable, to that of the possible and desirable. Indeed, it is increasingly suffused by an air of inevitability. This chapter,written during the early stages of this transition,examines whether intentional human inheritable genetic modification (IGM) is indeed scientifically possible and medically necessary, and attempts to identify clinical settings where it might be applied, both initially and ultimately. No attempt is made to address societal, moral, religious, or other nonscientific elements of IGM, which are dealt with elsewhere in this volume. 94 Christopher H. Evans Can We Do It? IGM is clearly possible—scientists have been doing it in mice for more than twenty years.2 The real question is whether it can be achieved in a manner that may be responsibly applied to people for noble purposes. Germ-line gene therapy places severe constraints on the technologies that should be used to achieve the required germ-line modifications. Existing approaches to gene transfer and gene modification have been reviewed in Chapters 3 and 4 in this volume. Somatic cell gene therapy traditionally involves gene supplementation rather than gene replacement or repair. In gene supplementation the offending mutations are not eliminated, but instead their effects are masked. This approach is workable for most recessive loss-of-function disorders.It could also be useful in dominant gain-of-function disorders if reliable methods could be developed to silence mutant alleles. However, there are two good reasons why gene supplementationapproachesarenotappropriateforgerm -lineapplications.Oneisthe persistence of the mutant allele, which could segregate from the transferred therapeutic gene in subsequent generations so that the disorder would reappear. The other reflects the probability that the foreign genetic material introduced into the modified genome will prove harmful.This is a particular concern when using vectors, such as retroviruses, which integrate into the genome at random locations. The nature and incidence of the abnormalities that may result from such events is hard to predict,but large percentages of transgenic mice have mutations and associated problems. More precise data may be forthcoming with the increasing use and scrutiny of genetically modified crops and farm animals3 because, unlike mice, we eat them or their products. Even when the additional genetic information is introduced as an episome or artificial chromosome,there are concerns about long-term, transgenerational safety. Given such uncertainties , it would seem prudent to restrict IGM, if indeed it is to go forward, initially to gene replacement or correction. Strategies for achieving this include homologous recombination and gene repair.4 Kenneth Culver, in Chapter 6 in this volume, reaches a similar conclusion and reviews the present state of the art regarding gene correction technology. This remains at an early stage of development, but there is the probability that efficient methods will emerge. Once they have done so, and been shown to work safely in human somatic cell gene therapy, there will be pressure to use them for intentional IGM. If IGM were to be used in humans, it is possible that genes would initially be modified in zygotes by ex vivo strategies coupled with in vitro fertilization (IVF) procedures.In this case,techniques for achieving efficient gene correction or replacement will need to be adapted from their present development in somatic cells to the rather different circumstances of the fertilized egg. Germinal stem cells5 provide an alternative and possibly simpler target of use when the father’s genome carries the targeted mutation. Because these cells can be grown in tissue culture and divide indefinitely, cells containing the appropriate genetic...