Nature Genetics
25, 245 - 246 (2000)
doi:10.1038/76976
Restoration of compact discsThomas P SakmarHoward Hughes Medical Institute, Laboratory of Molecular
Biology and Biochemistry, Rockefeller University, New York,
New York 10021, USA.
sakmar@rockvax.rockefeller.edu
More than 100 genes have been mapped that cause inherited blinding
diseases. The majority affect the outer layer of the retina, which is composed
of photoreceptor cells and retinal pigment epithelium. Gene therapy is a potential
means of correcting defects in specialized cellular structures that mediate
phototransduction or visual pigment regeneration, as illustrated by a paper
in this issue.It took me a while to develop the knack of using an ophthalmoscope, but
the first time I saw the optic fundus (the central retina and its blood vessels),
I was hooked. To visualize the retina is to get a glimpse of the brain—or
at least that part of the brain that has evolved to detect light. Unfortunately,
and all too often, one of a variety of inherited retinal dystrophies (such
as retinitis pigmentosa (RP)) is evident. More than 100 human genetic diseases
include some form of retinal dystrophy. The optic fundus of someone with RP
has a classic appearance—it is dotted with jet-black pigment spicules
and has attenuated retinal vessels and a 'waxy' yellow disc atrophy
(Fig. 1). But this is at the late stage, after the photoreceptor
cells have degenerated and died. Can photoreceptor degeneration be arrested
before programmed cell death is triggered? Can the splendid outer segment,
which contains the specialized biochemical photoreceptor apparatus, be restored
to good health?
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On page 306 Robin Ali
et al.1 present data indicating that the answers to both
questions may be "yes". They used the 'retinal degeneration
slow' (or rds) mouse, a strain with an interesting and long-studied
form of RP. The outer segments of the rod and cone photoreceptor cells of
homozygotes fail to develop normally in rds mice; eventually these
cells degenerate and die2. Heterozygotes have stunted outer
segments or distorted stubs that resemble membrane whorls. As the single allele
of rds is a null mutation in the gene Prph2, these phenotypes
are caused by absence and haploinsufficiency of Prph2, respectively.
And so the rds mouse provides a useful model to investigate whether
gene transfer can restore the development of functional outer segments.
The outer segment is a magnificent structure. It is cylindrical, and about
50−60  m long and 1.5−8 m wide, depending on the species.
In sagittal cross-section, hundreds of intracellular discs can be observed,
stacked like a column of coins. These are packed with rhodopsin, the visual
pigment that converts light into chemical signal. Each disc consists of two
closely spaced membrane bilayers (Fig. 2), closed at
each end by a membrane hairpin loop. The hairpin is presumably kept taut by
the protein encoded by Prph2, peripherin, which stabilizes contacts
between juxtaposed disc membranes and between the disc membrane and plasma
membrane5. Peripherin is also expressed in cone cells, which
have a less complicated outer segment structure.
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Mutations in RDS are associated with numerous dominant retinal dystrophies,
including RP, macular dystrophy and the so-called 'pattern' dystrophies.
This spectrum of disease is consistent with defective development of the outer
segments in both rods and cones, but the manner in which specific RDS
mutations lead to retinal degeneration is unknown.
A point of no return?
Some years ago, Gabriel Travis and colleagues6 demonstrated
that transgenic mice expressing retinal Prph2 against a Prph2-null
genetic background developed normal retinas—in other words, they were
able to prevent the degeneration of the photoreceptor cells. Ali et al
.1 wondered whether the outer segment of the rds
mouse could be restored by triggering the expression of functional peripherin
after the point at which the outer segment normally develops. They injected
a recombinant adeno-associated virus containing Prph2 under the control
of the opsin promoter into the outer retinas of 10-day-old rds mice.
Similar 'rescue' experiments7,
8 involving a gene
that mediates signal transduction in another mouse model of retinal degeneration
have proved successful, but the complex architecture of the outer segment,
together with a general ignorance of its requirements for development, obscured
an obvious outcome.
So how did the rds mice respond to 'therapy'? One of
their first responses was a dramatic increase in rhodopsin synthesis, with
concomitant correction of the rod outer segment. Restoration of function is
also indicated by evidence of increased interaction between the outer segments
and cells of the retinal pigment epithelium (RPE), against which the outer
segments abut. The juxtaposition of these two cell types is required for vision9,
10,
11; the RPE recycles 11-trans retinal, a 'waste'
product exported by the photoreceptor, into 11-cis retinal, which is
shuttled back into the photoreceptor where it joins up with opsin to form
rhodopsin. In addition to a biochemical interdependence, these two cell types
have a curious physical relationship. The supply of rhodopsin as well as the
outer segment disc stack must be continually replenished. As new discs are
generated at the proximal end of the outer segment, effete discs are phagocytosed
by the RPE. With respect to downstream effect, partial recovery of the electroretinogram
b-wave in the treated mice indicates that the photoreceptor cells couple with
the neurons of the inner nuclear layer of the retina.
It should be noted that these assays of outer-segment function are crude.
For example, whereas the interaction between outer segments and the RPE is
inferred by the presence of microvilli, and shedding discs in RPE cells is
interpreted as evidence of phagocytosis, direct evidence of a functional biochemical
retinoid regeneration pathway is yet to be had. Prolonged exposure to light
may effect degeneration of the treated cells if they cannot exchange retinoids
with the RPE.
How does expression of the transgene activate the developmental program
that restores the outer segment, both in terms of its architecture and its
functional interactions with both the RPE and retinal circuitry? It might
be argued that gene therapy works in the rds mouse because the outer
segment continually regenerates, even in the adult animal. If this is the
case, it will be key to introduce the transgene before apoptosis of the photoreceptor
cell—the probable point of no return for therapeutic approaches that
don't involve retinal transplantation12.
How is gene therapy as applied to the rds mouse relevant to understanding
retinal degenerative disorders in general? The study by Ali et al.
indicates that replacement of the missing gene may restore function relatively
late in the disease process, even in cases where the lack of a structural
gene prevents complete rod cell development. This may contrast with some genetically
dominant disorders in which the outer segment initially develops but then
degenerates over time, after which the photoreceptor cell undergoes apoptosis.
The question of whether a damaged outer segment can be restored to 'full
health' remains open. Ribozyme-targeted degradation of mutant RNA may
be one approach to treat dominant retinal degenerations13.
The complex ontogeny, anatomy and physiology of the retina make it a common
site of genetic disease, not to mention a fascinating structure. With the
advent of gene therapy and appropriate models of retinal dystrophies come
new ways to probe its development, form and function. Its tantalizing accessibility
makes it even harder to resist as an object of wonder and awe.
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