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Research Fields
Team 01 :
Genetics and therapy of retinal blindness
Genetics
of retinitis pigmentosa |
Diseases
we are focusing on
Our project focuses on non syndromic pigmentary retinopathies,
which include retinitis pigmentosa (RP) and some cases of Leber
congenital amaurosis and macular dystrophies. Non syndromic
pigmentary retinopathies account for 70 % of all cases of inherited
retinal dystrophies (RD) with a prevalence of 1/4,000. They
are characterized by an irreversible loss of photoreceptors,
resulting to blindness in nearly 100 % of the cases. There
is currently no efficient treatment for these diseases.
RP (Hamel at http://www.ojrd.com/content/1/1/40) is typically
presenting as a rod-cone degeneration (RCD) affecting primarily
rod photoreceptors which results in night blindness, and secondarily
cone photoreceptors with a progressive reduction of the visual
field in day light and decrease in visual acuity. RP leads
to blindness after several decades of evolution. Alternatively,
the cone-rod dystrophies (CRD), much less frequent than RCD,
predominantly involve the central region of the retina with
an early decrease in visual acuity, due to cone photoreceptors
being more severely affected than rod photoreceptors (Hamel
at http://www.ojrd.com/content/2/1/7).
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| Right eye fundus with ritinitis pigmentosa (RCD) |
Normal left eye fundus |
Right eye fundus with cone rode dystrophy |
Why is there a need
for genetic research on these diseases
The first point is that we have only a partial knowledge of
the genes that cause RPs and CRDs. For non syndromic RPs, we
have estimated that the cloned genes account for about 50-60
% of dominant RP patients, 40 % of recessive RP and 80 % of
X-linked RP (Maubaret et Hamel, J Fr Ophtalmol 2005). Since
roughly half the genes remains unknown, providing a molecular
diagnosis for genetic counselling is still impossible in half
the cases, and generate frustration in patients and families.
The second point is that there is a tremendous genetic heterogeneity
of pigmentary retinopathies. In non syndromic RP, there are
currently 45 genes/loci identified, including 15 for autosomal
dominant- (14 cloned, 1 mapped), 24 for autosomal recessive-
(18 cloned, 6 mapped), 5 for X-linked- (2 cloned, 3 mapped)
inheritance, and 1 (ROM1) which has been found mutated only
in digenism with RDS. In CRD, there are 13 genes/loci identified
(10 clones, 3 mapped). We therefore anticipate that several
tenths of genes remain to be identified to reach 90-100 % of
known genes. As a consequence of this vast genetic heterogeneity,
the prevalence of the disease for each gene is low, especially
for the unidentified genes which probably account each for
a few families.
The third point is that the partial knowledge of the genes
involved in pigmentary retinopathies prevents having a clear
picture of the pathophysiology of these diseases. Genes from
several metabolisms of the photoreceptors and of the retinal
pigment epithelium, like visual transduction, visual cycle,
photoreceptor cytoskeleton, photoreceptor development, spliceosome…,
are involved. But in many instances, some actors of these metabolisms
are still missing, and the fundamental studies in photoreceptor
physiology, which rely on the human genetics approach, await
their discovery. In addition, it is likely that several apoptotic
pathways are involved in photoreceptor loss, sometimes concurrently,
and this also needs to be carefully studied. The understanding
of these mechanisms is really what is relevant to future therapies
based on photoreceptor survival and pharmacological approaches.
Finally, the identification of the responsible genes and systematic
genotyping is necessary to define homogenous groups of patients
ready for clinical trials and therapeutic studies.
What are we doing
Our group works in close partnership with the outpatient clinics
at the University Hospital of Montpellier (Reference Center
for Inherited Sensory Diseases) which recruits patients with
various types of pigmentary retinopathies. More than 1000 families
have been examined since 1989 and > 3000 DNA samples have
been obtained. We used a combination of techniques to identify
genes and mutations, such as direct sequencing and D-HPLC (see
INM’s neurogenetic facility).
Screening genes
of the visual cycle
We are focusing on genes of the visual cycle which are expressed in the retinal
pigment epithelium (see RPE group, Philippe Brabet). We previously showed that
mutations in RPE65 are responsible for cases of Leber congenital amaurosis and
RP (Marlhens et al., Nature Genet 1997; Marlhens et al., Eur J Hum Genet 1998).
In this condition, there is a dramatic visual dysfunction from early childhood
whereas the retinal photoreceptors remain present in the retina for many years,
presumably until teenage (Hamel et al., Br J Ophthalmol 2001; Jacobson et al.,
Proc Natl Acad Med 2005). This defines a therapeutic window during which gene
therapy could be successfully performed, as suggested by pre-clinical trials
(Acland et al., Nature Genet 2001, Le Meur et al., Gene Ther 2007). We believe
that conditions due to other genes of the visual cycle, such retinitis punctata
albescens, fundus albipunctatus, may also show an early dysfunction syndrome
followed by a late stage loss of photoreceptors, and thus be relevant for a gene
therapy approach. |
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We therefore are screening more than 300 unrelated patients
with RP and various types of flecked dystrophies to search
for mutations. Here are some of our recent results :
CRBP1 : It encodes the cellular retinol binding protein that
takes up all-trans retinol at the apical RPE microvilli. Screening
on 331 patients is currently ongoing.
RDH10 : This is a retinol dehydrogenase specifically expressed
in the retina. We found no mutation in 216 patients (Senechal
et al., Am J Ophthalmol, 2006).
LRAT : The lecithin retinol acyltransferase
transfers an acyl group from the membrane phospholipids to
all-trans retinol (vitamin A). This enzyme provides the substrate
(retinyl palmitate) to RPE65. Two mutations were previously
described (Thompson et al., Nature Genet 2001). We found a
new mutation in a young boy who present a typical “RPE65
phenotype” (Senechal
et al., Am J Ophthalmol, 2006).
LRAT sequence of a patient homozygote for an AT deletion (top
left, arrown) compared to the wild type (bottom left). Summary
of the mutaions found in LRAT so far are shown on top of the
above diagram.
RPE65 : This protein converts all-trans retinyl palmitate
to 11-cis retinol. In the past, we have found 2 patients with
Leber congenital amaurosis from 1 family (Marlhens et al.,
Nature Genetics, 1997) and later found 1/184 unrelated patients
with RP carrying RPE65 mutations (Marlhens et al., Eur J Hum
Genet, 1998). More recently, we found several additional patients.
RLBP1 : This gene encodes CRALBP (cellular retinaldehyde binding
protein), which leads to retinitis punctata albescens (RPA)
when mutated. We recently found a RPA patient carrying an homozygous
deletion of the last 3 exons due to Alu sequence-driven unequal
recombination (Humbert et al., Invest Ophthalmol Vis Sci 2006).
IRBP : This is the interphotoreceptor binding protein. Screening
on 331 patients is currently ongoing.
RGR : This is an opsin like protein of the RPE that could
play a role in the visual cycle. Screening on 331 patients
is currently ongoing.
Primary mapping of RP families
We are selecting RP families excluded for known genes. We are
currently studying 3 unrelated consanguineous families with
a novel locus.
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