Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy
Camille Piro-Mégy, Emmanuelle Sarzi, Aleix Tarrés-Solé, Marie Péquignot, Fenna Hensen, Mélanie Quilès, Gaël Manes, Arka Chakraborty, Audrey Sénéchal, Béatrice Bocquet, Chantal Cazevieille, Agathe Roubertie, Agnès Müller, Majida Charif, David Goudenège, Guy Lenaers, Helmut Wilhelm, Ulrich Kellner, Nicole Weisschuh, Bernd Wissinger, Xavier Zanlonghi, Christian Hamel, Johannes N. Spelbrink, Maria Sola, Cécile Delettre
Camille Piro-Mégy, Emmanuelle Sarzi, Aleix Tarrés-Solé, Marie Péquignot, Fenna Hensen, Mélanie Quilès, Gaël Manes, Arka Chakraborty, Audrey Sénéchal, Béatrice Bocquet, Chantal Cazevieille, Agathe Roubertie, Agnès Müller, Majida Charif, David Goudenège, Guy Lenaers, Helmut Wilhelm, Ulrich Kellner, Nicole Weisschuh, Bernd Wissinger, Xavier Zanlonghi, Christian Hamel, Johannes N. Spelbrink, Maria Sola, Cécile Delettre
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Research Article Genetics Ophthalmology

Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy

Abstract

Mutations in genes encoding components of the mitochondrial DNA (mtDNA) replication machinery cause mtDNA depletion syndromes (MDSs), which associate ocular features with severe neurological syndromes. Here, we identified heterozygous missense mutations in single-strand binding protein 1 (SSBP1) in 5 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. To uncover the structural features underlying SSBP1 mutations, we determined a revised SSBP1 crystal structure. Structural analysis suggested 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. Our study showing that mutations in SSBP1 cause a form of dominant optic atrophy frequently accompanied with foveopathy brings insights into mtDNA maintenance disorders.

Authors

Camille Piro-Mégy, Emmanuelle Sarzi, Aleix Tarrés-Solé, Marie Péquignot, Fenna Hensen, Mélanie Quilès, Gaël Manes, Arka Chakraborty, Audrey Sénéchal, Béatrice Bocquet, Chantal Cazevieille, Agathe Roubertie, Agnès Müller, Majida Charif, David Goudenège, Guy Lenaers, Helmut Wilhelm, Ulrich Kellner, Nicole Weisschuh, Bernd Wissinger, Xavier Zanlonghi, Christian Hamel, Johannes N. Spelbrink, Maria Sola, Cécile Delettre

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Figure 5

Functional effect of SSBP1 mutation on mtDNA replication.

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Functional effect of SSBP1 mutation on mtDNA replication. (A) Quantifica...
(A) Quantification of mtDNA. mtDNA content was measured by qPCR and normalized to a nuclear gene (β-hemoglobin) in both controls and patient fibroblasts (patient 1, patient 2, patient 3). (B) Representative confocal imaging of mitochondria using ATPsynthase antibody (green) and mitochondrial nucleoids visualized by DNA antibody (mtDNA, red) in control and patient fibroblasts. Scale bars: 15 μm. (C) Quantification of area and volume of mtDNA nucleoids in pooled controls and patient fibroblasts. (D) Assessment of mtDNA replication efficiency on cultured skin fibroblasts from control and patient 1. EdU labeling was used to detect and quantify mtDNA synthesis, while anti-DNA antibody was used to quantify all visible mtDNA foci. EdU and total mtDNA foci were counted in both controls and the 3 patient cell lines and pooled for statistical analysis. Scale bars: 10 μm (E) Quantification of mtDNA replication rate in controls and patients. Data are shown as mean ± SEM. *P < 0.05; **P < 0.01; **P < 0.001 unpaired t test. (F) Mutation rates in mtDNA from control and patient blood (left) and urine (right). All data are representative of 3 independent experiments. Mann-Whitney U test was used.