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

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

Figure 65-1

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 largest
categories. (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 addition
therapy. This approach most commonly involves transfer of the missing gene to a
physiologically relevant target cell. However, other strategies are possible,
including supplying a gene that achieves a similar biologic effect through an
alternative pathway (e.g., factor VIIa for hemophilia A); supplying an antisense
oligonucleotide to splice out a mutant exon if the sequence is not critical to the
function of the protein (as has been done with the dystrophin gene in Duchenne
muscular dystrophy); or downregulating a harmful response through an siRNA.
Two distinct strategies are used to achieve long-term gene expression: one is to
transduce stem cells with an integrating vector, so that all progeny cells will carry
the donated gene; the other is to transduce long-lived cells, such as skeletal muscle
or neural cells. In the case of long-lived cells, integration into the target cell
genome is unnecessary, provided the donated DNA can be stabilized in an
episomal 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 retroviral
vectors, which require dividing target cells for integration. Because HSCs are
normally quiescent, they are a formidable transduction target. However,

identification of cytokines that induced cell division without promoting
differentiation of stem cells, along with technical improvements in the isolation
and 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 from
mutations in the gene (IL2RG) encoding the γc subunit of a cytokine receptor
required for normal development of T and NK cells (Chap. 310). Affected infants
present in the first few months of life with overwhelming infections and/or failure
to thrive. In this disorder, it was recognized that the transduced cells, even if few
in number, would have a proliferative advantage compared to the non-transduced
cells, which lack receptors for the cytokines required for lymphocyte development
and maturation. Complete reconstitution of the immune system, including
documented 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, with
splenomegaly, 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 (LIM
only-2), which encodes a component of a transcription factor complex involved in
hematopoietic development. Insertion of the retroviral long terminal repeat is
thought to increase the expression of LMO-2.
The X-linked SCID studies were a watershed event in the evolution of gene
therapy. They demonstrated conclusively that gene therapy could cure disease; of
the 16 infants eventually treated in these trials, 15 achieved correction of the
immunodeficiency disorder. Unfortunately, 3 later developed a leukemia-like
disorder, but 12 are alive and free of complications at time periods ranging up to 7
years after initial treatment. These studies also demonstrated that insertional
mutagenesis leading to cancer was more than a hypothetical possibility. As a result
of the experience in these trials, all protocols using integrating vectors in
hematopoietic cells must include a plan for monitoring sites of insertion and clonal
proliferation. Strategies to overcome this complication have included employing a

"suicide" 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 of
genes surrounding the insertion site.
More clear-cut success has been achieved in a gene therapy trial for another
form of SCID, adenosine deaminase (ADA) deficiency (Chap. 310). ADA-SCID
is clinically similar to X-linked SCID, although it can be treated by enzyme
replacement therapy with a pegylated form of the enzyme (PEG-ADA), which
leads to immune reconstitution but not always to normal T cell counts. Enzyme
replacement therapy is expensive (annual costs: $200,000–$300,000 in U.S.
dollars). Gene therapy protocols have evolved to include the use of HSCs rather
than T cells as the target for transduction; discontinuation of PEG-ADA at the
time of vector infusion, so that the transduced cells have a proliferative advantage
over the non-transduced; and the use of a mild conditioning regimen to facilitate
engraftment of the transduced cells. There have been no complications in the six
children treated on this protocol, with a median follow-up of >4 years. Based on
current data, the efficacy of gene transfer for ADA-SCID is convincing, but longer
term follow-up will be required to determine whether this approach is sufficiently
safe to be routinely recommended as an alternative to PEG-ADA.
Other diseases likely to be amenable to transduction of HSCs include
Wiskott-Aldrich syndrome (trials underway), chronic granulomatous disease,
sickle cell disease, and thalassemia.