RETINITIS PIGMENTOSA

Retinitis
pigmentosa (RP) is an inherited retinal dystrophy characterized by night
blindness, progressive visual field loss, and eventually blindness. (26) It can
be inherited as autosomal dominant, recessive, or X-linked, and is also highly
variable with some RP patients developing symptomatic visual loss in early
childhood whereas other patients may stay asymptomatic until mid-adulthood. (27,44)
The disease is caused by loss of rod photoreceptor cells, which lines the outer
edge of the retina and are responsible for night vision, light sensitivity, and
seeing peripherally. This is then followed by the loss of cone photoreceptor
cells, densely packed in the center of the fovea, that allow for color vision.

(27) Due to the mechanism of the disease and the order of degeneration, a common
symptom of the disease is described as one having “tunnel vision”.

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Epidemiology

RP occurs in one
out of every 3,500 people worldwide, with X-linked RP (XLRP) being most severe
and comprising 15% of the cases. (28) More than 20,000 people in the United
States and the European Union are noted to have XLRP. (29)

Mutations in more
than 60 genes have currently been shown to cause RP, with over 20 of these
genes being autosomal dominant, and mutations in the RHO gene being the most
common, accounting for 30% of all cases. (30) 35 genes have been associated
with the autosomal recessive form of RP, with 15% of mutations in USH2A. Six
genes cause XLRP, mainly due to mutations in RPGR and RP2 genes. Several other
common genes associated with RP are PDE6A, PDE6B, and PDE6G. (29)

 

Standard of Care

RP is diagnosed by
looking for presentations of:

1)    Sometimes
asymmetric bilateral involvement

2)    Impaired
night and peripheral vision

3)    Rod
dysfunction by elevated rod final threshold on dark background

4)    Reduced
or extinguished b-wave amplitude on Full-Field ERG testing

5)    Progressive
loss in photoreceptor function (31)

There is currently
no approved treatment for any form of RP, but studies have shown that
nutritional supplements can slow disease progression. Supplements like high-dose
vitamin A palmitate and vitamin E remain controversial and risky. (32) For patients who have little
to no vision, artificial vision devices like the Argus II Retinal Prosthesis
System are an option, and have been shown to restore some vision (33)

 

Current Gene Therapy

So far, there have been six clinical trials for
RP gene therapy drug studies, with five utilizing AAV vectors. I have compiled
important information from all past and current RP gene therapy clinical trials
utilizing AAV vectors (Table 3). This includes two studies looking at the
effects of AAV administration in patients with XLRP with a specific mutation in
RPGR, one trial studying effects of recombinant AAV8 in patients with mutation
in RLBP1, one trial studying effects of recombinant AAV2.7m8 in patients with
non-syndromic RP, and one trial monitoring the effects of AAV2/5 in patients
with 

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