Dominant mutations in mtDNA maintenance gene SSBP1 cause optic atrophy and foveopathy
Camille Piro-Mégy, … , Maria Sola, Cécile Delettre
Camille Piro-Mégy, … , Maria Sola, Cécile Delettre
Published September 24, 2019
Citation Information: J Clin Invest. 2020;130(1):143-156. https://doi.org/10.1172/JCI128513.
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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 3

SSBP1 protein expression in human retina and fibroblasts.

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SSBP1 protein expression in human retina and fibroblasts. (A) SSBP1 prot...
(A) SSBP1 protein expression in human retina. Immunofluorescence labeling was done on retinal cross-sections from healthy human donor. Hoechst was added to label nuclei (blue). SSBP1 localization was done using a specific antibody (green). An antibody against the ATP synthase subunit 5A was used to target mitochondria (red). RPE, retinal pigment epithelium; POS, photoreceptor outer segment; ONL, outer nuclear layer; INL, inner nuclear layer; GCL, ganglion cell layer. Scale bars: 20 μm. (B) Quantification of SSBP1 transcript levels in cultured skin fibroblasts from both controls and affected individuals (patient 1, patient 2, and patient 3). mRNA levels were normalized to the reference gene L27. (C) Western blot in lysates from controls (control 1 and control 2) and patient fibroblasts and densitometric analysis of SSBP1 protein abundance in lysates from both controls and patient fibroblasts. GAPDH was used as a loading control. Data are shown as mean ± SEM. ***P < 0.001. (D) Representative ultrastructure of the mitochondria from both controls (C1, C2) and patient fibroblasts by TEM. Scale bar: 1 μm. Single arrows show abnormal mitochondria. Double arrows show lipid droplets. Quantification analyses of abnormal mitochondria (large vacuoles, disturbed cristae) in fibroblasts from both controls and patients. Data are represented as mean percentage of abnormal mitochondria ± SEM in total examined mitochondria. **P < 0.01. All data are representative of 3 independent experiments. One-way ANOVA with Dunnett’s correction was used.