Chapter 065. Gene Therapy in Clinical Medicine (Part 2)

Indications in gene therapy clinical trials. The chart divides clinical gene transfer studies by disease classification. A majority of trials have addressed cancer, with monogenic disorders and cardiovascular diseases the next largestcategories. (Reproduced with permission from J Gene Med. New Jersey, Wiley, 2006.)Gene Transfer for Genetic DiseaseGene transfer strategies for genetic disease generally involve gene addition therapy. This approach most commonly involves transfer of the missing gene to a physiologically relevant target cell. ...
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Chapter 065. Gene Therapy in Clinical Medicine (Part 2) Chapter 065. Gene Therapy in Clinical Medicine (Part 2) Figure 65-1 Indications in gene therapy clinical trials. The chart divides clinical genetransfer studies by disease classification. A majority of trials have addressedcancer, with monogenic disorders and cardiovascular diseases the next largestcategories. (Reproduced with permission from J Gene Med. New Jersey, Wiley,2006.) Gene Transfer for Genetic Disease Gene transfer strategies for genetic disease generally involve gene additiontherapy. This approach most commonly involves transfer of the missing gene to aphysiologically relevant target cell. However, other strategies are possible,including supplying a gene that achieves a similar biologic effect through analternative pathway (e.g., factor VIIa for hemophilia A); supplying an antisenseoligonucleotide to splice out a mutant exon if the sequence is not critical to thefunction of the protein (as has been done with the dystrophin gene in Duchennemuscular dystrophy); or downregulating a harmful response through an siRNA.Two distinct strategies are used to achieve long-term gene expression: one is totransduce stem cells with an integrating vector, so that all progeny cells will carrythe donated gene; the other is to transduce long-lived cells, such as skeletal muscleor neural cells. In the case of long-lived cells, integration into the target cellgenome is unnecessary, provided the donated DNA can be stabilized in anepisomal form. Immunodeficiency Disorders: Proof of Principle Early attempts to provide gene replacement into hematopoietic stem cells(HSCs) were stymied by the relatively low transduction efficiency of retroviralvectors, which require dividing target cells for integration. Because HSCs arenormally quiescent, they are a formidable transduction target. However,identification of cytokines that induced cell division without promotingdifferentiation of stem cells, along with technical improvements in the isolationand transduction of HSCs, led to modest but real gains in transduction efficiency. The first convincing therapeutic effect from gene transfer occurred with X-linked severe combined immunodeficiency disease (SCID), which results frommutations in the gene (IL2RG) encoding the γc subunit of a cytokine receptorrequired for normal development of T and NK cells (Chap. 310). Affected infantspresent in the first few months of life with overwhelming infections and/or failureto thrive. In this disorder, it was recognized that the transduced cells, even if fewin number, would have a proliferative advantage compared to the non-transducedcells, which lack receptors for the cytokines required for lymphocyte developmentand maturation. Complete reconstitution of the immune system, includingdocumented responses to standard childhood vaccinations, clearing of infections,and remarkable gains in growth occurred in most of the treated children. However,two developed a syndrome similar to T cell acute lymphocytic leukemia, withsplenomegaly, rising white counts, and the emergence of a single clone of T cells.In these children, the retroviral vector had integrated within a gene, LMO-2 (LIMonly-2), which encodes a component of a transcription factor complex involved inhematopoietic development. Insertion of the retroviral long terminal repeat isthought to increase the expression of LMO-2. The X-linked SCID studies were a watershed event in the evolution of genetherapy. They demonstrated conclusively that gene therapy could cure disease; ofthe 16 infants eventually treated in these trials, 15 achieved correction of theimmunodeficiency disorder. Unfortunately, 3 later developed a leukemia-likedisorder, but 12 are alive and free of complications at time periods ranging up to 7years after initial treatment. These studies also demonstrated that insertionalmutagenesis leading to cancer was more than a hypothetical possibility. As a resultof the experience in these trials, all protocols using integrating vectors inhematopoietic cells must include a plan for monitoring sites of insertion and clonalproliferation. Strategies to overcome this complication have included employing asuicide gene cassette in the vector, so that errant clones can be quickly ablated;or using insulator elements in the cassette, which can limit the activation ofgenes surrounding the insertion site. More clear-cut success has been achieved in a gene therapy trial for anotherform of SCID, adenosine deaminase (ADA) deficiency (Chap. 310). ADA-SCIDis clinically similar to X-linked SCID, although it can be treated by enzymereplacement therapy with a pegylated form of the enzyme (PEG-ADA), whichl ...