The mechanism of vision requires a first step of light perception and encoding, performed by the retina, and a second step consisting in the transduction of this encoded information to the brain, which is performed by the optic nerve. Inherited Optic Neuropathies (ION) are neurodegenerative pathologies affecting specifically the Retinal Ganglion Cells (RGCs), which axons form the fibres of the optic nerve. They induce chronic or acute loss of visual acuity, often leading to legal blindness (visual acuity < 10/200). They are rare diseases, with a global incidence of 1/10.000 in occidental countries. To date, no treatment is available.

            Two clinical forms exist : the Leber Hereditary Optic Neuropathy (LHON), systematically maternally transmitted and caused by mutations on the mitochondrial genome, and the Autosomal Dominant Optic Atrophy (ADOA), transmitted by the nuclear genome of either the father or the mother, with 50% probability of transmission. The later are the object of our research at the Institut des Neurosciences de Montpellier, of clinical investigations at the CHU St Eloi and of active recruitment and exhaustive clinical diagnosis at the Centre de Référence Maladies Rares « Affections Neurosensorielles Génétiques » from the CHU Gui de Chauliac (Montpellier).

For the clinician, in addition to the loss of visual acuity, the ADOA clinical presentation includes a temporal palor of the optic disk in the fundus, a central scotoma (reduction of the central visual field), a dyschromatopsy (loss of color vision) and a severe reduction of the Visual Evoked Potentials (witnessing the loss of RGCs) without modifications of the ERG.

Normal (left) and pathological (right) papilla:

Pictures evidence the palor of the papilla (optic nerve emergence), corresponding to the degenerescence of the RGC axonal fibers in the affected patient.

Normal (left) and pathological (right) visual fields:

Note the predominant reduction of the central and peri-papillar visual fields.

Normal (left) and pathological (right) color tests (15-Hue):

The lines crossing the central surface witness a major dyschromatopsy in the red-green axis.

Normal (left) and pathological (right) Visual Evoked Potentials:

The electro-physiological signal, composed of a N (negative) wave, followed by a P (positive) wave is consistently reduced in NOH patients.

Research Objectives:

Our major research objectives consist in identifying the patho-physiological mechanisms of ADOA and propose therapeutic strategies.
Our research results and projects concern:

Identification of genes and mutations responsible for ION:

            Seven loci responsible of ION have been identified, 6 with a dominant transmission (OPA1 at 3q28, OPA2 at Xp11.4, OPA3 at 19q13.2, OPA4 at 18q12.2, OPA5 at 22q11.1 and OPA7 at 16q21) and one with a recessive transmission (ROA1 at 8q21.2). We have identified the only two genes yet known responsible for ADOA: OPA1 and OPA3. As for ION transmitted by the mitochondrial genome, OPA1 and OPA3 encode mitochondrial proteins from the inner mitochondrial membrane, ubiquitously expressed in all cells from the body. In order to increase our medical knowledge of ION and precise their diagnostic, we favour patient recruitment at the Centre de Référence Maladies Rares « Affections Sensorielles Génétiques » (CHU Gui de Chauliac, Montpellier).

Genealogic tree of an ADOA family:

Squares representent males, circles females. The black, or white, surfaces correspond to affected, or not, members of the family.

For each patient diagnosed with a ION, a genetic study consisting in the screening for mutations in OPA1 or OPA3, or for the LHON primary mutations are systematically performed (in collaboration with the CHU from Angers, Dr P. Bonneau). If no mutation is found, a genetic analysis is initiated to identify new morbid locus and mutations in candidate genes. As a result of this process, a novel gene OPA8 has recently been identified in 7 families, which is now under genetic and molecular characterization.

Functional characterization of genes responsible of ADOA :

            Our scientific objectives consist in identifying at a molecular level the roles of the different OPA proteins in the mitochondrial functions and address their dysfunction and pathological mechanisms when they are mutated.
            Two approaches are developed: one based on a fibroblast library derived from patients biopsies allow to characterize the dysfunctions induced by the mutations in ADOA genes. The second is based on HeLa cells and allow deciphering the localization of proteins encoded by the OPA genes inside the mitochondria and their functions in cell metabolism.
            Yet we have established the precise localization of OPA1 in the inter membrane space mainly associated to the cristae membrane, and characterized its fundamental role in structuring the cristae, in the mitochondrial network dynamic and in the control of apoptosis, by segregating cytochrome c in the cristae volume. Consequently, OPA1 functional perturbation dismantles the mitochondrial dynamic and respiration, and increase the susceptibility of cells to programmed cell death.

Physiological study of Retinal Ganglion Cells:

           
            Our goals consist in the characterization of the mitochondrial functions an dynamics in RGCs, and in understanding why these cells are so susceptible to mitochondrial dysfunctions, as the one evidenced in the former chapter. These aspects are addressed both in vitro on primary RGC cultures, and in vivo in mouse models.

RGC mitochondrial network (in vitro) :

Mitochondria are stained in red by the Mitotracker dye and the nucleus is stained in blue. Note the remarkable repartition of mitochondria in dendrites, which consist in neuronal expansions.

Mitochondrial distribution at the connection between the retina and the optic nerve:

The retina (in red) is composed of multiple cell layers among which the RGCs at the extreme left. The optic nerve (in green) is composed of the RGC axonal prolongations connecting the eye to the brain.

Development of animal models:

          We have generated mouse models for ADOA, with mutations in OPA1 and OPA8 genes. These mice are now processed to study the molecular and cellular basis of the pathology, and its progression in the course of the development until late adult stages. They are further used to study environmental factors favouring the onset of the pathology, and to evaluate neuro-protective strategies.

Group Investigators:

- Dr. Guy Lenaers, Group Leader, Directeur de recherche (CNRS),
- Pr. Christian Hamel, Ophtalmologiste (INSERM et Hôpital Gui de Chauliac),
- Pr. Agnès Muller, Professeur (UM1),
- Dr. Cécile Delettre, Chargée de Recherche (INSERM),
- Dr. Olivier Payet, Maître de Conférence (UM1),
- Dr. Valérie Matha, Maître de Conférence (UM1),
- Mlle Aurélie Saleur, Technicienne, (UM1),
- Mme Ghizlane ElAchouri, Doctorante, (Association),
- Mlle Laetitia Buret, Doctorante, (Association),
- Mlle Sandrine Albert, Doctorante (Fondation EADS),
- Mr Abdelkader Benyagoub, Doctorant (bourse MRT)

Main references:

- Delettre C et al., Nuclear gene OPA1, encoding a mitochondrial dynamin-related protein, is mutated in dominant optic atrophy. Nat Genet. 2000; 26(2):207-10.

- Olichon A et al., Loss of OPA1 perturbates the mitochondrial inner membrane structure and integrity, leading to cytochrome c release and apoptosis. J. Biol. Chem. 2003; 278(10):7743-6.

- Reynier P et al., OPA3 gene mutations responsible for autosomal dominant optic atrophy and cataract. J Med Genet. 2004;41(9):e110.

- Kamei S, et al.; Expression of the Opa1 mitochondrial protein in retinal ganglion cells; its down regulation causes aggregation of the mitochondrial network. IOVS, 2005; 46(11):4288-94.

- Amati-Bonneau P et al.; OPA1 R445H Mutation in Optic Atrophy associated with Sensorineural Deafness. Annals of Neurology, 2005; 20.

- Olichon A, et al.; Effects of OPA1 mutations on mitochondrial morphology and apoptosis: Relevance to ADOA pathogenesis. J Cell Physiol. 2007 May;211(2):423-30.

- Olichon A, et al.; OPA1 alternate splicing uncouples an evolutionary conserved function in mitochondrial fusion from a vertebrate restricted function in apoptosis. Cell Death Differ. 2007 Apr;14(4):682-92.

- Baricault L et al. OPA1 cleavage depends on decreased mitochondrial ATP level and bivalent metals. Exp Cell Res. 2007 Oct 15;313(17):3800-8.

- Cornille K et al.; Reversible optic neuropathy with OPA1 exon 5b mutation. Ann Neurol. 2008 May;63(5):667-71.

- Amati-Bonneau P. et al.; OPA1 mutations induce mitochondrial DNA instability and optic atrophy 'plus' phenotypes. Brain. 2008 Feb;131(Pt 2):338-51.

Institutional and Associatif supports:

INSERM, CNRS, Universités de Montpellier I et II,

We are deeply indebted to the following patient associations: Retina France, UNADEV, IRRP, Ouvrir les Yeux, and SOS Rétinite, for their remarkable support all along the last years.