(Investigative Ophthalmology and Visual Science. 2000;41:2821-2826.)
© 2000
by The Association for Research in Vision and Ophthalmology, Inc.
Ocular Gene Therapy: Quo Vadis?
William W. Hauswirth1 and
Laurent Beaufrere1,2
1 From the Department of Molecular Genetics and Microbiology, Department of Ophthalmology, and Center for Gene Therapy, University of Florida, Gainesville; and the
2 Laboratoire de Biochimie Genetique, CNRS ERS 155, Institut de Biologie and Service d’Ophthalmologie, CHU de Montpellier, Montpellier, France.
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Introduction
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The question of where gene therapy is going has almost as many
answers as there are practitioners in the field. Narrowing our focus to
diseases of the eye, this diversity of opinions does not contract
proportionately. Therefore, rather than forecasting the future, it is
more useful to assess the field today, including a consideration of
where problems remain before clinical trials would be generally
feasible. Progress is clearly most advanced in developing therapies for
retinal degenerative diseases, principally because the genetics of many
forms of retinitis pigmentosa (RP) are relatively well understood, and
many natural and transgenic animal models exist. However, given the
surgically simpler and less traumatic access afforded by any part of
the eye other than photoreceptors and retinal pigment epithelium (RPE),
the primary cellular targets of RP, it is not necessarily true that
gene therapy for outer retinal diseases will be the first or most
successful clinically. The brief opinion presented here will therefore
attempt to cover the status of gene therapy for all parts of the eye in
as balanced a manner as possible.
Ideally there are four basic prerequisites that should be met for any
genetic therapy targeted to an ocular disease. First, a gene delivery
technique must be available that is efficient and nontoxic. Second, the
genetic basis of the disease, or minimally its biochemical basis,
should be well characterized so that an appropriately matched
therapeutic approach can be selected. Third, expression of the
therapeutic gene needs to be properly controlled, both insofar as
levels of the gene product are concerned as well as which tissues or
cell types support or do not support expression. Finally, having an
experimental animal model of the disease available for preclinical
testing of the therapy is clearly key in demonstrating proof of
principle. The field of ocular gene therapy has been dealing with these
issues for about 5 years, but as yet no clear consensus has emerged for
any single approach,1
and, given the diversity of tissues
in the eye and the range of pathology they can experience, this should
not be surprising. On the other hand, over the past 2 years, there have
been major advances toward satisfying one or more of the four
prerequisites for several ocular diseases. It is this progress that
will be reviewed and then placed in the larger context of ocular gene
therapy, quo vadis? To logically review the eye as a gene therapy
target, issues regarding individual ocular tissues and conditions will
be discussed generally in the order in which they occur, from the back
of the eye forward.
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Outer Retina/RPE
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Three classes of therapy for photoreceptor/RPE diseases have seen
recent developments worth highlighting, each exhibiting photoreceptor
rescue in rodent models of RP. These include ribozyme therapy targeted
against mRNA from individual dominant negative alleles,
neurotrophin/growth factor gene therapy against a variety of genetic
forms of RP, and, more indirectly, therapy aimed at promoting avoidance
of photoreceptor apoptosis, the apparent common final cell death
pathway in RP.
Ribozyme Therapy
Morphologic and electrophysiological preservation of
photoreceptors was seen for 3 months in the P23H transgenic rat
using recombinant adeno-associated virus (rAAV)-vectored
ribozymes.2
This experiment has been recently repeated,
assessing the longevity of rAAV-mediated ribozyme therapy (M. LaVail,
A. Lewin, and W. Hauswirth, unpublished observations, 2000).
After 8 months, ribozyme-treated eyes in 12 rats retained on average 4
to 5 rows of photoreceptor nuclei, whereas contralateral,
phosphate-buffered saline (PBS)–injected eyes now had only
approximately 1 row. Rod electroretinographic (ERG) amplitudes
reflected this difference. Another key question is whether residual
photoreceptors in a retina with advanced RP are still responsive to
rescue by gene therapy. This situation was modeled by delaying
rAAV-ribozyme injections in the P23H rat until 4 rows of photoreceptor
nuclei had been lost. Ninety days later, treated right eyes retained an
average of 5 rows of photoreceptor nuclei compared with 2 rows in the
control partner eyes. It, therefore, appears that even intervention
after photoreceptor loss is well under way by AAV-vectored genes can
delay the loss of the remaining photoreceptors.
Neurotrophin/Growth Factor Genes
Neurotrophin gene therapy using recombinant adenovirus
carrying a CNTF (ciliary neurotrophic factor) cDNA has led to
structural rescue of photoreceptors for several months in the
rd3
and rds4
mouse
models of RP. In the latter, scotopic ERG amplitudes were also
enhanced, suggesting functional rescue as well. However, the level of
rescue was only modest and of limited duration. Using an AAV vector,
CNTF genes delivered to photoreceptors in the P23H opsin transgenic rat
model of RP resulted in morphologic and electrophysiological
preservation for at least several months (W. Peterson,
unpublished observations, 1999). Various other forms of
neurotrophins in AAV vectors are currently under testing, and, given
the more persistent expression of AAV passenger genes in
photoreceptors,5
6
this should be a critical test of its
value in the retina. On the downside, CNTF as an injected protein
clearly has toxic effects in the retina and brain. Phase I trials of
Regeneron Corporation’s Axokine in the central nervous system for
weight loss were halted after occasional herpes reactivations were
reported. Regeneron also halted its transgenic P347L opsin pig trial
due to a lack of photoreceptor rescue, occasional cataracts, and
CNTF-related ERG abnormalities. Whether secreted CNTF from virally
vectored genes will have similar side effects remains to be
established. This is an important contrast because there are likely to
be significant differences between CNTF as an ocular bolus of injected
protein and CNTF as a gene leading to lower but more sustained local
levels of the protein.
Basic fibroblast growth factor (bFGF, FGF2) has been of long-standing
therapeutic interest in the eye. When the bFGF gene is delivered by an
adenovirus vector, photoreceptor survival is prolonged for at least 2
months in the Royal College of Surgeons (RCS) rat.7
This
is significantly longer than the rescue duration reported for
intraocular administration of the protein, highlighting the relative
effectiveness of prolonged survival factor production locally through
transgene expression. However, the potentially toxic angiogenic
properties of bFGF have yet to be carefully evaluated in
vector-bFGF–treated eyes. There is currently a growing list of
molecules within the FGF family, many of which have little or no
angiogenic potential, and these appear to be prime candidates for
testing by gene delivery in animal models of RP (J. Flannery,
unpublished observations, 2000).
Anti-Apoptosis Therapy
Because apoptosis appears to be the common death pathway for
photoreceptors in RP animal models, its manipulation by genetic means
seems a logical therapeutic option. However, results of crosses between
RP mice and transgenic mice either overexpressing the anti-apoptotic
genes bcl-2 or c-fos, or a knockout of the
pro-apoptotic p53 gene, have been mixed at best. More
recently, the photoreceptor survival effect of bcl-2 gene
therapy was significantly enhanced in the rd/rd mouse if a
wild-type copy of the defective ßPDE (ß-subunit of
cGMP-phosphodiesterase) gene was also delivered,8
suggesting that anti-apoptotic therapy can be combined with gene
augmentation for an additive effect. In contrast, AAV-vectored
bcl-2 expression in ganglion cells leads to increased
glutamate-mediated cell damage.9
However, such toxicity
should be avoidable by limiting the cell type expressing the passenger
gene through precise local injection and tight, cell-specific
promoters. Two recent reports portend other gene therapy
approaches.10
11
In the S334ter transgenic rat, activation
of caspase 3, a primary mediator of apoptosis, accompanied
photoreceptor cell loss.10
This opens a variety of
anti-caspase gene strategies to evaluation. A major new inhibitor of
apoptosis (IAP), X-linked IAP, was described that inhibits caspase
3,11
thus its testing in animal models of RP seems
logical.
Gene Augmentation Therapy
Delivery of a normal gene to compensate for lack-of-function
mutations such as null mutations in the ßPDE gene has shown
structural and biochemical rescue in the rd mouse. Rescue
was mediated either by recombinant adenovirus,12
lentivirus,13
or encapsidated adenovirus
minichromosomes14
(EAMs, "gutless adenovirus"), and in
each case treatment preserved photoreceptor morphology to a modest
degree for 1 to 2 months. Why neither longer nor more complete rescue
was seen remains unresolved. Both problems could be vector-related or
model-related because the rd mouse loses photoreceptors very
quickly over the first 3 to 4 weeks of life, perhaps too quickly for
effective rescue by any vector.
Studies in Large Animals
AAV-vectored green fluorescent protein (GFP) under control
of an opsin promoter was stably expressed in rats for more than 3 years
(J. Flannery and W. Hauswirth, unpublished observations, 2000)
and with a cytomegalovirus promoter in the rhesus monkey for nearly 2
years (J. Bennett, unpublished observations, 2000) without
obvious toxicity in either species. Importantly, in the monkey, after
transduction of one retina and seroconversion to modest anti-AAV
titers, the contralateral retina could be efficiently transduced by the
same vector.5
Thus, the fact that 60% to 80% of humans
are seropositive for AAV should not limit this vector’s utility in the
retina. Because all current retinal gene therapy reports have involved
rodents, it is vital to determine whether the analogous therapy can be
equally effective in a human-sized eye. Retinal gene therapy
experiments are currently under way in the P347S transgenic pig using
ribozymes and in the rdc1 dog using ßPDE gene augmentation to answer
this key question. In neither species did subretinal injection of high
titer AAV vector result in noticeable inflammation. Therapeutic results
have been slow to come, however, because rates of retinal degeneration
are significantly slower than in rodents, yet still faster than in
human RP.
Viral Vector Toxicity
Several recent reports have refocused concern on the safety of
viral vectors, particularly the adenoviruses. In the retina, immune
reaction to adenoviral vectors appears to limit its effectiveness to 3
to 8 weeks in mice, suggesting that vector antigens are recognized and
responded to relatively normally in the subretinal space. Consistent
with this, coexpression of immune-modulators enhances the persistence
of adenoviral-vectored retinal gene expression.15
Without
doubt, the gene therapy outcome that has generated the most attention
over the past year was the fatality during an adenovirus-OTC (ornithine
transcarbamylase) human trial. The patient’s immune reaction to the
large systemic dose of vector administered (4 x
1013 particles) seems to have been the initial
pathogenic trigger. It is important to note, however, that at current
viral titers well less than 10% of this amount could be injected into
a human subretinal space. Nevertheless, this unfortunate phase I
outcome will certainly place a much greater safety burden not only on
all adenoviral vector protocols but on other vector systems as well. In
this regard it should be noted that AAV (unrelated to adenovirus),
although able to elicit a modest immune response, has shown no toxicity
at similar systemic or subretinal doses in several large animal
species, including monkeys.
Three other developments this past year highlight the growing
opportunities for new retinal gene therapy approaches. Enhanced
survival of photoreceptors was noted in several mouse RP models after
ocular administration of GDNF (glial-derived neurotrophic
factor),16
PEDF (pigmented epithelium–derived
factor),17
or a calcium-channel blocker,18
suggesting alternative, perhaps complimentary, retinal gene therapy
strategies.
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Retinal Ganglion Cells
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Glaucoma is a leading cause of blindness in the world, and several
mechanisms of retinal ganglion (RGC) cell death have been proposed. The
two predominant hypotheses currently involve neuronal damage due to
either long-term elevation in intraocular pressure or the presence of
genetic factors predisposing a patient to RGC loss with age without
significantly elevated intraocular pressure. Proximal causes of RGC
death appear to include glutamate toxicity, induction of nitric oxide
synthase (NOS-2) leading to reactive oxygen species damage, and/or loss
of trophic factor support.19
As a result, apoptotic cell
death ensues. Because the genetic basis of glaucoma is not well
understood, gene therapy must currently either target these biochemical
manifestations of glaucoma or, at a more downstream stage, help RGCs
avoid apoptosis. An additional problem has been the lack of
well-defined animal models. Currently, the most useful models are
produced by the induction of RGC death in rodents by axotomy, cautery
of episcleral tissue, or episcleral vein occlusion, with the latter two
procedures leading to increases in intraocular pressure before RGC
loss. In these systems, essentially any neuroprotective paradigm may be
useful, including neurotrophin/cytokine gene therapy or anti-apoptotic
approaches,20
and a number of pharmacological agents have
shown promise. For gene-based therapy, the observation that optic nerve
transection leads to activation of the tumor-suppressor protein p53
and, hence, to upregulation of the proapoptotic gene bax
prompted the evaluation of bax antisense oligonucleotides. Vitreal
inoculation of antisense oligonucleotides reduced axotomy-induced RGC
death,21
suggesting the potential utility of a broad range
of anti-apoptotic genes. RGC loss due to optic nerve axotomy can also
be slowed by adenoviral vector delivery of the BDNF (brain-derived
neurotrophic factor) gene.22
Interestingly, Müller
cells appeared to be the primary cell type transduced on vitreal
injection of the vector. This suggests that transduction of supporting
Müller cells with genes for secretable trophic factor genes or
with genes that promote their production may be another way to protect
adjacent RGCs. Thus, a variety of death-avoidance strategies may be
useful in gene-based glaucoma treatment.
For pressure-induced glaucoma in particular, an obvious target tissue
is the trabecular meshwork (TM), where intraocular outflow is
regulated. Modulation of the TM physiology by gene transfer could be a
way to achieve long duration pressure reduction. Cells of the TM appear
to be good targets for gene transduction, as demonstrated by
liposome-mediated delivery of reporter genes or oligonucleotides into
rat or primate TM.23
Recently, good levels of transduction
have been reported using adenovirus vectors as well.24
Viral vector transduction was achieved without affecting outflow
facility, damaging neither tissue architecture nor TM
morphology.25
Thus, TM manipulation by vectored gene
expression seems a viable alternative for some forms of glaucoma.
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Optic Nerve
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Recurrent and acute optic neuritises are vision-threatening
sequelae of several local and systemic maladies, including Leber’s
hereditary optic neuropathy and multiple sclerosis. Recently, viral
vectors achieved efficient gene transfer to the optic nerve after
intravitreal injection near the optic nerve head in the mouse and
guinea pig. AAV-mediated transfer of the catalase gene, a reactive
oxygen species scavenger, suppressed demyelination and blood–barrier
disruption at the foci of experimental autoimmune encephalomyelitis in
the mouse optic nerve.26
The long-term (>1 year) reporter
gene expression with AAV-vectored gene transfer observed in nerve
fibers, glial cells, and associated blood vessels suggests the
feasibility of this strategy for chronic or recurrent optic
neuropathies as well.27
The use of a replication-deficient
adenovirus rather than an AAV vector allowed more rapid transduction
with the catalase gene in the same model system,28
a
modification that could be helpful in the treatment of more acute forms
of optic neuritis.
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Retinoblastoma
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The most common primary ocular malignancy of childhood is
retinoblastoma. Large retinoblastomas are currently treated by
enucleation, often with long-term problems (visual loss and severe
cosmetic deformity secondary to enucleation and/or irradiation of the
orbital region). Radiation treatment often eliminates any residual
tumor, but an increased risk of second cancers in the radiation field
is a serious side effect. Because small tumors might be surgically
treated, thus avoiding enucleation, control of tumor burden may be an
important therapeutic goal. Introduction of the herpes simplex virus
thymidine kinase gene (HSV-tk) followed by ganciclovir
treatment to create a toxic nucleotide analog that can be incorporated
specifically into the DNA of dividing cells and thereby kill them has
proven useful in a variety of systems. This strategy was tested as
tumor-targeted suicide gene therapy in Y79Rb cell line–induced ocular
tumors in mice29
and was shown to be effective in reducing
the ocular tumor burden. A similar result was obtained on induced
subcutaneous tumors in mice when the Rb cell line had been transduced
ex vivo with HSV-tk in a retrovirus vector.30
An alternative strategy involved HSV mutants defective in the
virus-encoded ribonucleotide reductase gene. This virus can replicate
and kill dividing cells well, but it is ineffective against nondividing
cells. Such mutant viruses indeed inhibited the growth of subcutaneous
Y79–induced tumors in mice.31
Thus, in appropriate cases,
suicide gene therapy mediated either by viral vectors and local
ganciclovir treatment or by a suicide HSV virus infection could be
useful in avoiding the need for enucleation and radiotherapy, thus
allowing more conservative treatment (cryotherapy or laser
photocoagulation) as used currently for small ocular tumors.
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Retinal and Choroidal Vasculature
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Choroidal neovascularization (CNV) is the major cause of
severe loss of vision in patients with age-related macular degeneration
(AMD), and retinal neovascularization is the prime cause of vision loss
in diabetic retinopathy and in retinopathy of prematurity. Together,
AMD and diabetic retinopathy are by far the most frequent causes of
untreatable blindness in the world. Thus, the development of gene
strategies that counteract neovascularization stimuli would be a major
therapeutic advance. Because vascular endothelial growth factor (VEGF)
appears to be a major regulator of the ocular vasculature, its control
may be key. Unfortunately, antisense oligonucleotide therapy against
VEGF failed to prevent experimental CNV in a laser-induced mouse
model.32
However, because VEGF is constitutively expressed
in the vasculature of normal eyes,33
it is possible that
significant inhibition of VEGF activity may have adverse consequences.
AMD and diabetes are chronic diseases; therefore, long-term
expression of therapeutic genes is likely to be required. Toward this
end, retrovirus-mediated reporter gene transfer after intravitreal or
subretinal injection was tested in a laser photocoagulation–induced
CNV mouse model. Gene transfer and expression were seen selectively
near sites of CNV.34
Because retroviral vectors require
proliferating cells for effective transduction, such vector homing to
sites of neovascularization could be very useful in gene-based CNV
treatment. Clearly, however, we are at a very early stage in the
development of gene therapy for AMD-related CNV, and other viral
vectors (AAV, adenovirus) as well as obvious candidate therapeutic
genes (e.g., angiostatin and PEDF) need to be tested. Perhaps most
importantly for AMD, a better animal model of CNV is likely to be
needed before significant headway will be made.
In the diabetic patient, fibronectin synthesis associated with basement
membrane thickening is a key indicator of diabetic microangiopathy.
Fibronectin antisense oligonucleotides delivered into normal rat
retinal capillary cells after intravitreal injection reduced
fibronectin expression 47% to 87% over the first week after
treatment.35
Although potentially useful for acute
inhibition of retinal neovascularization, longer-term treatments are
likely to be needed, and the vector and therapeutic gene considerations
noted above for the choroidal vasculature would seem to apply to the
retinal vasculature as well. Significantly, the development of an opsin
promoter–regulated VEGF mouse that exhibits retinal
neovascularization36
provides an important new model for
evaluating ocular anti-neovascularization gene therapy.
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Retinal Detachment
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Proliferative vitreoretinopathy (PVR) is the most common cause of
failure in treating rhegmatogenous retinal detachment. Removal of
inappropriately dividing cells, such as proliferating RPE cells, using
suicide gene strategies is therefore worth testing. In a rabbit with
vitreally implanted fibroblasts that had been transduced ex vivo with
HSV-tk, the severity of experimental PVR was reduced after
ganciclovir treatment.37
Similar selective reductions in
proliferating RPE cells or in fibroblasts in culture have been reported
for infection with a ribonucleotide reductase mutant HSV38
or an E2F decoy oligonucleotide delivered by a liposome,39
respectively. An additional problem associated with retinal detachments
is the local loss of photoreceptor function, perhaps due to an
apoptotic mechanism related to a partial loss of trophic support from
the RPE. BDNF protein injected during experimental detachment in cats
led to diminished cell proliferation locally and to better
photoreceptor morphology,40
suggesting that BDNF or other
neurotrophin gene delivery may be useful at the site of detachment as
well. However, in this instance as well as for PVR, any gene therapy
strategy should be designed for limited duration because once
proliferating cells in the vitreous have been eliminated or the
detachment resolved, the therapeutic gene’s continued presence is
unnecessary and may be undesirable.
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Lens
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Posterior capsule opacification (PCO) is a common late
complication of cataract surgery. The current treatment for PCO, laser
capsulotomy, can cause serious complications such as macular edema or
retinal tears or detachment. The benefit of surgical techniques such as
cortical cleanup or removal of lens epithelial cells, the source of
inappropriately mitotic cells, has yet to be demonstrated. Similarly,
antimetabolites or toxins linked to antibodies against lens epithelial
cells have also been unsuccessfully tested. The removal of dividing
cells by suicide gene transfer as described above is therefore a
possibility for PCO as well.41
In a rabbit model,
intracameral injection of HSV-tk adenovirus followed by ganciclovir
treatment prevented experimental PCO. Thus, targeted destruction of
pathogenic mitotic cells seems a viable strategy for a variety of
ocular disorders.
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Conjunctival and Corneal Epithelia
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The conjunctival and corneal epithelia play important roles in the
pathogenesis of various eye diseases. Their external position offers
several alternative gene delivery paradigms not available to deeper
tissue such as the retina. In theory, transfer of genes expressing
antiinflammatory proteins or growth factors could be a valuable way to
prevent an inflammatory cascade or to supply components for healthy
epithelial turnover. This could be of particular use in the
wound-healing process. Gene delivery to a predetermined area of a
rabbit cornea was reported without evidence of corneal damage using
gene gun technology (high velocity gold beads coated with plasmid
DNA).42
Adenovirus vectors have also been used for gene
transfer to conjunctival and corneal epithelia.43
44
45
Because the corneal epithelium seems less sensitive to transduction by
adenovirus than the conjunctiva, topical application of low titer
vectors may be suitable for selective conjunctival gene transfer
without affecting the corneal epithelium. However, because of the rapid
turnover of corneal and conjunctival epithelia, in vivo expression with
either technology was sustained for only a few days after treatment.
This could be an advantage if a limited therapy is required. However,
if prolonged expression is needed, gene transfer to corneal and
conjunctival stem cells, localized at the limbus and fornix,
respectively, will be necessary. Because both nonviral plasmid DNA and
adenoviral vectors lead to relatively short-term transgene expression
in most tissue, extended-term gene therapy in the cornea has been
approached recently using a retroviral vector for an ex vivo
transduction of corneal epithelial progenitor cells.46
Retrovirus vectors stably integrate into the host genome of mitotic
cells, providing the potential for long-term passenger gene expression.
In this study, keratolimbal autografts transduced with retroviruses
continued to produce progeny cells that expressed the transgene for at
least 6 months in vivo. Because stem cells located in the limbus appear
to be responsible for the continuous repopulation of the corneal
epithelium, this could provide a long-term therapy for corneal
disorders either by direct in vivo transduction of stem cells or by ex
vivo corneal stem cell transduction and implantation into the limbal
area.47
A novel corneal gene delivery technology is low-voltage electroporation
for transferring genes as naked DNA to a relatively circumscribed area
of the corneal endothelium.48
With circular plasmid DNA,
gene expression persisted for at least 21 days. Such transient
expression could be an alternative or adjuvant to drug therapy in wound
healing or in acute diseases of the cornea. More recently, plasmid
DNA-encoding tissue plasminogen activator (tPA) was transferred to a
portion of the corneal endothelium with a similar low-voltage
electroporation strategy.49
Intracameral fibrin formation
after laser-induced bleeding and subsequent corneal cloudiness were
inhibited significantly. Thus, given the special external access
afforded by the cornea, physical gene delivery techniques have clear
potential for therapeutic application in the cornea.
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Concluding Remarks
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Although there remain many untested genes that have therapeutic
potential for ocular diseases, current proof-of-principle successes,
primarily in rodent models of RP, suggest that the next experimental
steps toward human trials are justified. There are two key goals
remaining for all ocular gene therapies at the preclinical
level.1
The given therapy must show therapeutic results in
a species with a human-sized eye. For many of the ocular diseases and
conditions discussed above, this will be a daunting challenge for two
reasons. First, animal models with the relevant ocular condition may
not exist and/or the technology for their creation may be unknown or
prohibitively expensive. Second, the question of ocular dimensions and
therapeutic coverage (i.e., number and topography of target cells that
are required to be transduced to yield a measurable vision-preserving
outcome) may not be trivial, and the issue can only be resolved by
experimental verification. In the process, noninvasive clinical
end-point criteria over longer time periods, more like those expected
in human trials, need to be developed as well.2
A Federal
Drug Administration–approved phase I/II ocular gene therapy protocol
will require detailed animal toxicity and biodistribution studies.
Potential for toxicity will be intimately tied to the vector used.
Current herpes virus vectors retain toxic cellular properties that have
yet to be fully evaluated in the eye. As noted, adenovirus vectors have
immune response and persistence problems that have been well documented
in ocular tissue. AAV vectors, in contrast, appear to be relatively
nontoxic at high doses in the eye and elsewhere and, with the
appropriate promoter, lead to persistent gene expression. Lentiviral
vectors appear to have many of the same favorable properties as AAV
vectors, at least in photoreceptors,50
but they have
received only limited attention to date in ocular tissue. Important
issues for lentivirus vector preparations that remain to be fully
resolved include the possibility of low-level infectious wild-type
(HIV) contaminants, relatively low vector titers, and expression shut
off/mutagenesis upon chromosomal integration. At the moment, therefore,
AAV vectors are best poised for initial clinical trials. Finally, after
any vector administration, systematic toxicity analysis of ocular and
adjacent tissues would be best carried out on the same animals used for
efficacy studies. In this way any unintended vector dissemination and
associated pathology could be accurately balanced against any
therapeutic effects.
In conclusion, the vision community stands on the brink of a revolution
in therapies for blinding and acuity-altering conditions. It is an
exciting time, because ocular gene therapy seems well poised to be
among the earliest successful applications of this new approach to
disease and pathology. On a more sobering note, the field is
well-advised to build strong preclinical cases for safety and efficacy
before proceeding to the clinic for any ocular applications of gene
therapy. To launch prematurely into a poorly supported trial could
delay this promising therapy for years. As always, the message is that
careful science, particularly when building a case for human trials, is
never a mistake.
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Footnotes
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Submitted for publication February 10, 2000; revised April 8, 2000; accepted April 11, 2000.
Commercial relationships policy: N.
Corresponding author: William W. Hauswirth, Department of Ophthalmology, Box 100284 JHMHC, Gainesville, FL 32610-0284. hauswrth{at}eye1.eye.ufl.edu
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Introduction
Outer Retina/RPE
