2. Introduction
• Retinal pigment epithelium-specific 65 kDa protein, also known as retinoid isomer
hydrolase, is an enzyme of the vertebrate visual cycle that is encoded in humans by
the RPE65 gene.
• RPE65 is an isomer hydrolase expressed in retinal pigment epithelium. It is critical
for the regeneration of the visual pigment necessary for both rod and cone-mediated
vision. Mutations in human RPE65 cause Leber’s congenital amaurosis type2 and
other forms of autosomal recessive retinitis pigmentosa which are associated with
early-onset blindness.
• The visual cycle (or retinoid cycle) is the process by which 11-c/s-retinal is
regenerated from all-trans-retinal after a photo-isomerization event. This cycle takes
place in the photoreceptors and retinal pigment epithelium (RPE) and involves a
series of enzymatic reactions.
• RPE65 is a key isomerase in this process and is responsible for converting all-trans-
retinyl ester to 11-c/s-retinol. Without RPE65, 11-c/s-retinal levels are substantially
reduced and retinyl esters accumulate in RPE. RPE65 is an evolutionarily conserved
61-kDa membrane associated protein found in the smooth endoplasmic reticulum of
the RPE cells.
• Mutations in RPE65 are associated with recessive blinding diseases and models of
RPE65 deficiency have been a common target for gene therapy studies over the past
several years
External IDs OMIM: 180069 MGI: 98001 HomoloGene: 20108 GeneCards: RPE65
The reaction completed by RPE65 in the retinoid cycle
3. RPE65 functions
• RPE65 is a critical enzyme in the vertebrate visual cycle found the retinal pigmented
epithelium. It is also found in rods and cones.
• The photo-isomerization of 11-cis-retinal to all-trans-retinal initiates the photo
transduction pathway through which the brain detects light. All-trans-retinol is not
photoactive and therefore must be reconverted to 11-cis-retinal before it can recombine
with opsin to form an active visual pigment.
• RPE65 reverses the photo-isomerization by converting an all-trans-retinyl ester to 11-
cis-retinol. Most commonly, the ester substrate is retinyl palmitate.The other enzymes
of the visual cycle complete the reactions necessary to oxidize and esterify all-trans-
retinol to a retinyl ester (RPE65's substrate) and to oxidize 11-cis-retinol to 11-cis-
retinal (the required photoactive visual pigment component)
• RPE65 has two forms; a soluble form called sRPE65, and a palmitoylated, membrane-
bound form known as mRPE65. Although it was proposed in 2004 that the
palmitoylation state of RPE65 could serve as a molecular switch mechanism to help
regulate retinoid recycling and transport (as a result of the differential binding affinity
of the two forms of RPE65 for different retinoid), subsequent experiments have shown
that this is not likely to be the case.
• It was originally thought that the role of RPE65 was restricted to the binding and
mobilization of all-trans-retinyl esters for processing by the isomer-hydrolase
responsible for the critical conversion of all-trans- to 11-cis- retinoid
• Although RPE65 was originally identified only in RPE, it is also expressed in both
amphibian and mammalian cone photoreceptors but not rods. The role of RPE65 in
cones is unknown, but this observation (and others) provides support for the hypothesis
that mammalian cones may utilize a different retinoid processing cycle in addition to
the traditional one used by rod.
4. Clinical Significance
• Mutations in this gene have been associated with Leber’s congenital amaurosis type 2
(LCA2) and retinitis pigmentosa (RP). RPE65 mutations are the most commonly
detected mutations in LCA patients in Europe. The vast majority of RPE65 mutations
in patients with LCA2 and RP occur in the beta-propeller regime and are believed to
inhibit proper protein folding and iron cofactor binding. Particularly common propeller
mutation sites are Tyr368 and His182. Substitution at Arg91 is also common and have
been shown to impact RPE65 membrane interactions and substrate uptake.
• Though complete loss of function is associated with diseases such as LCA2 and RP,
partial inhibition of RPE65 has been proposed as a treatment for age-related macular
degeneration (AMD).
• RPE65 is a critical enzyme in the vertebrate visual cycle found the retinal pigmented
epithelium. It is also found in rods and cones. The photo-isomerization of 11-cis-retinal
to all-trans-retinal initiates the photo-transduction pathway through which the brain
detects light. All-trans-retinol is not photoactive and therefore must be reconverted to
11-cis-retinal before it can recombine with opsin to form an active visual pigment.
RPE65 reverses the photo-isomerization by converting an all-trans-retinyl ester to 11-
cis-retinol. Most commonly, the ester substrate is retinyl palmitate. The other enzymes
of the visual cycle complete the reactions necessary to oxidize and esterify all-trans-
retinol to a retinyl ester (RPE65's substrate) and to oxidize 11-cis-retinol to 11-cis-
retinal (the required photoactive visual pigment component)
5. RPE65 deficiency disorders
1. Leber congenital amaurosis type2 :
• Leber congenital amaurosis is an eye disorder that primarily affects the retina, which is
the specialized tissue at the back of the eye that detects light and color. People with this
disorder typically have severe visual impairment beginning in infancy. The visual
impairment tends to be stable, although it may worsen very slowly over time.
• It is also associated with other vision problems, including an increased sensitivity to
light (photophobia), involuntary movements of the eyes (nystagmus), and extreme
farsightedness (hyperopia)The pupils, which usually expand and contract in response to
the amount of light entering the eye, do not react normally to light. Instead, they
expand and contract more slowly than normal, or they may not respond to light at all.
2. Fundus albipunctatus :
• Fundus albipunctatus is an eye disorder characterized by an impaired ability to see in
low light (night blindness) and the presence of whitish-yellow flecks in the retina,
which is the specialized light-sensitive tissue in the inner lining of the back of the eye
(the fundus). The flecks are detected during an eye examination.
• Individuals with fundus albipunctatus experience night blindness from an early age. In
particular, they have delayed dark adaptation, which means they have trouble adapting
from bright light to dark conditions, such as when driving into a dark tunnel on a sunny
day. It often takes hours for adaptation to occur. Their vision in bright light is usually
normal.
3. Retinitis pigmentosa :
• Retinitis pigmentosa is a group of related eye disorders that cause progressive vision
loss. These disorders affect the retina, which is the layer of light-sensitive tissue at the
back of the eye. In people with retinitis pigmentosa, vision loss occurs as the light-
sensing cells of the retina gradually deteriorate.
• The first sign of retinitis pigmentosa is usually a loss of night vision, which becomes
apparent in childhood. Problems with night vision can make it difficult to navigate in
low light. Later, the disease causes blind spots to develop in the side (peripheral)
vision. Over time, these blind spots merge to produce tunnel vision. The disease
progresses over years or decades to affect central vision, which is needed for detailed
tasks such as reading, driving, and recognizing faces. In adulthood, many people with
retinitis pigmentosa become legally blind.
6. Prognosis and Research
• The progressive nature of and lack of a definitive cure contribute to the inevitably
discouraging outlook for patients with this disease. While complete blindness is
rare, the person's visual acuity and visual field will continue to decline as initial rod
photoreceptor and later cone photoreceptor degradation proceeds
• Studies indicate that children carrying the disease genotype benefit from
presymptomatic counseling in order to prepare for the physical and social
implications associated with progressive vision loss. While the psychological
prognosis can be slightly alleviated with active counseling the physical implications
and progression of the disease depend largely on the age of initial symptom
manifestation and the rate of photoreceptor degradation, rather than access to
prospective treatments. Corrective visual aids and personalized vision therapy
provided by Low Vision Specialists may help patient correct slight disturbances in
visual acuity and optimize their remaining visual field. Support groups, vision
insurance, and lifestyle therapy are additional useful tools for those managing
progressive visual decline.
• The treatments may involve retinal transplants, artificial retinal implants, gene
therapy, stem cells, nutritional supplements and/or drug therapies.
• In 2012: Scientists at the University of Miami Bascom Palmer Eye Institute
presented data showing protection of photoreceptors in an animal model when eyes
were injected with mesencephalic astrocyte-derived neurotrophic factor (MANF).
Researchers at the University of California, Berkeley were able to restore vision to
blind mice by exploiting a "photoswitch" that activates retinal ganglion cells in
animals with damaged rod and cone cells.
• In 2015: A study by Bakondi et al. at Cedars-Sinai Medical Center showed that
CRISPR/Cas9 can be used to treat rats with the autosomal dominant form of
retinitis pigmentosa. Researchers find that two molecules, rod-derived cone viability
factor (RdCVF) and Nrf2, can protect cone photoreceptors in mouse models of
retinitis pigmentosa.
• In 2016: RetroSense Therapeutics aimed to inject viruses with DNA from light-
sensitive algae into the eyes of several blind people (who have retinitis pigmentosa).
If successful, they will be able to see in black and white.
• In 2017 the FDA approved the gene therapy voretigene neparvovec to treat people
with biallelic RPE65 mutation-associated retinal dystrophy.
• Luxturna containing voretigene neparvovec is the only treatment currently
available which does not cure for the condition, but substantially improves vision in
those treated.
7. Regional Analysis
• The RPE65-based retinal dystrophy is a genetic condition; though the defect is
recessive it only occurs when a child inherits copies of the defective gene from both
parents the mutation is more common in the Middle East, partly due to social factors
such as intermarriage.
• The consanguineous marriage ultimately end up with most genetic related disorders
which is more prevalent over Middle Eastern region especially in UAE, Qatar and
KSA.
• In a consanguineous UAE family two sibs were affected with Goldmann syndrome and
both had poor vision and night blindness since early childhood.
• In addition, a large UAE family with six children in four branches affected with retinitis
pigmentosa starting in early childhood was found to have a homozygous deletion of
exons 1-7 of the RPE65 gene.
• Two families from the UAE were also diagnosed with achromatopsia, which is also
known as rod monochromacy or total color blindness.
8. • The prevalence of Leber congenital amaurosis type2 is from 2:100,000 to
3:100,000 and for Retinitis Pigmentosa it is from 1 in 4,000 people.
• LCA2 is more prevalent in KSA compared to other Arab World but RP is more
prevalent in Qatar and Lebanon.
• More than 200 causative mutations of RP have been previously discovered in more
than 50 different genes, the molecular defect is distinguishable in just about half of
the affected patients.
9. Methodology
• Treatment:
LUXTURNA (voretigene neparvovec-rzyl) is an adeno-associated virus vector-
based gene therapy indicated for the treatment of patients with confirmed
biallelic RPE65 mutation-associated retinal dystrophy.
The recommended dose for each eye is 1.5 x 1011 vector genomes, administered
by sub retinal injections in total volume of 0.3 mL on separate days within a
close interval of not fewer than 6 days apart.
• Cost:
The cost of LUXTURNA medication itself is around 545,000USD and rest of
the treatment including consultation, procedures, testing and medication is
approximately 11,000USD (Patient cost yearly without medical insurance
coverage).
• Market:
LUXTURNA is manufactured and marketed in USA by Spark Therapeutics and
outside USA its marketed by Novartis, which is the only and current key player
in the world.
The growth of market size for RPE65 deficiency associated disorders is
attributed to support treatment regimens, the only approved therapy (Luxturna),
as well as, emerging therapies. The market size was found to be USD 133.82
million in 2017.
In UAE, the treatment for the UAE national having the disorders is done along
with collaboration of the Government of UAE.
The RPE65 deficiency has a potential increase in number of cases in the
UAE/GCC region and also across the world. Due to no side effects and
complete visual restoration within a period of time will maximize the
availability of the gene related medications in the near future.
Research for the RPE65 deficiency has been going for decades with
government and private funded entities and more trial has reached end of phase
two, some others awaiting marketing approval which imply there is sufficient
demand for the medication in the world market.
10. Cost Analysis
• Average annual total health care costs were significantly greater for the RP and LCA2
patients (avg. USD 11000).
• Patients with RP and LCA2 had annual costs that were higher by USD 1000 for
inpatient, USD 5000 for outpatient, USD 500 for pharmacy, and USD 7000 for other
health care costs.
• Total mean annual costs per patient were estimated to be 9900 USD in United States.
• The total mean annual costs for RP and LCA2 patients in the Europe were estimated to
be 10000 Euros.
• Variation in costs was attributed to age such as children, adults, and for old age.
Children annual cost is 35% higher than average total annual cost.
The above table represents the marginal effect of RP and LCA2 patients on costs and
resource utilization compare to non-RP and LCA2 patients.
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
0 to 12
yrs
13 to 17 18 to 24 25 to 34 35 to 44 45 to 54 55 to 64 65 to 74 75 to 84 85 above
AGE GROUP VS EXPENSES IN AED
Age group
11. • The Chart in the left side indicating the direct medical cost for the RP and LCA2
patients and the chart in the right side indicates the direct non-medical cost for RP
and LCA2 patients across the world.
• Indirect costs annually estimates in losses of productivity and absenteeism costs in
US, Canada and EU ranges from 4.1 Billion to 13.3 Billion in US$.
• Direct medical costs occurred mostly due to hospitalization and the use of medical
services whereas, productivity loss and loss of income demonstrated high
expenditures related to indirect costs. Higher resource utilization reported for
children and working-age adults compared with the elderly blinds.
• Although evidence suggest that RP and LCA2 may pose similar economic burden
as blindness on the healthcare systems and society, further research is needed to
generate evidence on economic burden associated with RPE65 mutations.
12. Summary
• The RPE65 gene provides instructions for making a protein that is essential for normal
vision. The RPE65 protein is produced in a thin layer of cells at the back of the eye
called the retinal pigment epithelium (RPE).
• The RPE65 protein is involved in a multi-step process called the visual cycle, which
converts light entering the eye into the electrical signals that are transmitted to the
brain.
• The RPE65 gene deficiency associated disorders such as LCA2, RP, etc. that causes
progressive degeneration of the retina.
• The disease is accompanied by early degeneration of the highly sensitive rod
photoreceptors, followed by a progressive decline in daylight central vision, due to loss
of function of the less-sensitive cone photoreceptors.
• GCC region has more prevalence than other parts of the world due to higher rates of
consanguineous marriage which leads to higher number of gene related disorders in the
progenies. Number of affected patients will be less compared to the population ratio in
EU and US (US has more than 2000 affected patients).
• Luxturna is the only AAV vector treatment in the world marketed by Novartis. FDA
approved the marketing of drug in 2017 and only 10 countries have the infrastructure to
perform the procedure. UAE is the third country in the world who successfully
performed the procedure.
• Emerging drug companies includes Horama, Meira GTX, Biogen, Neurotech,
Reneuron, etc. either in clinical trials phase 2/3 or waiting for the approval from FDA.
