Chargée de recherche à l'INSERM U1051 Institut des Neurosciences de Montpellier


Génération de poissons zèbres mutants dans le but de modéliser l’atteinte rétinienne liée aux mutations dans SSBP1

Generation of mutant zebrafishes to model the human retinal degeneration linked to SSBP1 mutation



Inherited optic neuropathies (ION) characterized by the atrophy of the optic disc, also called optic atrophies (OAs), are genetic disorders caused by the progressive loss of retinal ganglion cells (RGC) and are major causes of childhood blindness worldwide. There is no treatment to prevent blindness in these diseases. With the recent findings of new genes causing ION, the interest in these conditions grew up because what causes RGC death might also apply to other Central Nervous System neurons, making these diseases good models, because of the simplicity of light stimulation, to study neuronal degeneration. In particular, genetic studies showed that most ION are caused by mitochondrial defects, which are also found in diseases involving pyramidal, dopaminergic and motor neurons.

Our laboratory is one of the leader in the field of retinal and optic nerve genetics in Europe, with the finding of very important genes like RPE65 and OPA1, and more recently of other genes. Our goal is to take advantage of our gene discoveries to decipher pathophysiological mechanisms as we did using the Opa1 mutant mouse that we generated and to start gene therapy projects in this new field.

In the center of reference “maladies sensorielles génétiques” that we head in Montpellier’s hospital, we follow more than 200 patients with ION, half of them being Dominant Optic Atrophy (DOA) cases. About 20% of DOA cases are negative for known genes. Using a strategy of candidate gene analysis on our patient cohort, we identified heterozygous missense mutations in SSBP1 in five unrelated families, leading to the R38Q and R107Q amino-acid changes in the mitochondrial single-stranded DNA-binding protein, a crucial protein involved in mtDNA replication. All affected individuals presented optic atrophy, associated with foveopathy in half of the cases. SSBP1 mutations have also been found in patients with autosomal dominant and recessive phenotypes, which ranged from isolated optic atrophy to additional clinical features including sensorineural deafness, mitochondrial myopathy and kidney failure. A novel de novo SSPB1 mutation has been reported in a child with single large-scale mtDNA deletion clinically manifesting as Pearson, Kearns-Sayre and Leigh syndromes.

To uncover the structural features underlying SSBP1 mutations, we determined a new revised SSBP1 crystal structure. Structural analysis suggests that both mutations affect dimer interactions and presumably distort the DNA binding region. Using patient fibroblasts, we validated that the R38Q variant destabilizes SSBP1 dimer/tetramer formation, affects mtDNA replication and induces mtDNA depletion (Figure 1). Our study, showing that mutations in SSBP1 cause a novel form of dominant optic atrophy frequently accompanied with foveopathy, brings new insights into mtDNA maintenance disorders.

Our project is now to decipher the mechanisms associated to the dual phenotype of DOA with or without maculopathy, a unique phenotype to date and understand the role of SSBP1 in the specific ocular phenotype by using 2 point mutations (R38Q and R107Q) knock-in zebrafishes.

These models will also allow us to test therapeutic approaches.