Characterization of SSBP1-related optic atrophy and foveopathy

Structures of the mitochondrial single-stranded DNA binding protein with DNA and DNA polymerase γ

The mitochondrial single-stranded DNA (ssDNA) binding protein, mtSSB or SSBP1, binds to ssDNA to prevent secondary structures of DNA that could impede downstream replication or repair processes. Clinical mutations in the SSBP1 gene have been linked to a range of mitochondrial disorders affecting nearly all organs and systems. Yet, the molecular determinants governing the interaction between mtSSB and ssDNA have remained elusive. Similarly, the structural interaction between mtSSB and other replisome components, such as the mitochondrial DNA polymerase, Polγ, has been minimally explored. Here, we determined a 1.9-Å X-ray crystallography structure of the human mtSSB bound to ssDNA. This structure uncovered two distinct DNA binding sites, a low-affinity site and a high-affinity site, confirmed through site-directed mutagenesis. The high-affinity binding site encompasses a clinically relevant residue, R38, and a highly conserved DNA base stacking residue, W84. Employing cryo-electron microscopy, we confirmed the tetrameric assembly in solution and capture its interaction with Polγ. Finally, we derived a model depicting modes of ssDNA wrapping around mtSSB and a region within Polγ that mtSSB binds.

Mitochondria in Retinal Ganglion Cells: Unraveling the Metabolic Nexus and Oxidative Stress

This review explored the role of mitochondria in retinal ganglion cells (RGCs), which are essential for visual processing. Mitochondrial dysfunction is a key factor in the pathogenesis of various vision-related disorders, including glaucoma, hereditary optic neuropathy, and age-related macular degeneration. This review highlighted the critical role of mitochondria in RGCs, which provide metabolic support, regulate cellular health, and respond to cellular stress while also producing reactive oxygen species (ROS) that can damage cellular components. Maintaining mitochondrial function is essential for meeting RGCs' high metabolic demands and ensuring redox homeostasis, which is crucial for their proper function and visual health. Oxidative stress, exacerbated by factors like elevated intraocular pressure and environmental factors, contributes to diseases such as glaucoma and age-related vision loss by triggering cellular damage pathways. Strategies targeting mitochondrial function or bolstering antioxidant defenses include mitochondrial-based therapies, gene therapies, and mitochondrial transplantation. These advances can offer potential strategies for addressing mitochondrial dysfunction in the retina, with implications that extend beyond ocular diseases.

Maternal mosaicism in SSBP1 causing optic atrophy with retinal degeneration: implications for genetic counseling

BACKGROUND

Optic atrophy-13 with retinal and foveal abnormalities (OPA13) (MIM #165510) is a mitochondrial disease in which apparent bilateral optic atrophy is present and sometimes followed by retinal pigmentary changes or photoreceptors degeneration. OPA13 is caused by heterozygous mutation in the SSBP1 gene, associated with variable mitochondrial dysfunctions.

RESULTS

We have previously reported a 16-year-old Taiwanese male diagnosed with OPA13 and SSBP1 variant c.320G>A (p.Arg107Gln) was identified by whole exon sequence (WES). This variant was assumed to be de novo since his parents were clinically unaffected. However, WES and Sanger sequencing further revealed the proband's unaffected mother carrying the same SSBP1 variant with a 13% variant allele frequency (VAF) in her peripheral blood. That finding strongly indicates the maternal gonosomal mosaicism contributing to OPA13, which has not been reported before.

CONCLUSIONS

In summary, we described the first case of OPA13 caused by maternal gonosomal mosaicism in SSBP1. Parental mosaicism could be a serious issue in OPA13 diagnosis, and appropriate genetic counseling should be considered.

Maternal Mosaicism in SSBP1 Causing Optic Atrophy with Retinal Degeneration: Implications for Genetic Counseling

Background: Optic atrophy-13 with retinal and foveal abnormalities (OPA13) (MIM #165510) is a mitochondrial disease in which apparent bilateral optic atrophy is present and sometimes followed by retinal pigmentary changes or photoreceptors degeneration. OPA13 is caused by heterozygous mutation in the SSBP1 gene, associated with variable mitochondrial dysfunctions. Results: We have previously reported a 16-year-old Taiwanese male diagnosed with OPA13 and SSBP1 variant c.320G>A (p.Arg107Gln) was identified by whole exon sequence (WES). This variant was assumed to be de novo since his parents were clinically unaffected. However, WES and Sanger sequencing further revealed the proband’s unaffected mother carrying the same SSBP1 variant with a 13% variant allele frequency (VAF) in her peripheral blood. That finding strongly indicates the maternal gonosomal mosaicism contributing to OPA13, which has not been reported before. Conclusions: In summary, we described the first case of OPA13 caused by maternal gonosomal mosaicism in SSBP1 . Parental mosaicism could be a serious issue in OPA13 diagnosis, and appropriate genetic counseling should be considered.

The importance of genome sequencing: unraveling SSBP1 variant missed by exome sequencing

BACKGROUND

Single-stranded DNA-binding protein 1 (SSBP1) plays an essential role in mitochondrial DNA (mtDNA) replication and maintenance, as well as development of retina. Here, we describe the clinical findings and genetic basis of a family with two members affected with bilateral optic atrophy.

MATERIALS AND METHODS

Clinical data were retrospectively collected from an electronic medical record system. Genetic results were obtained using exome sequencing (ES) and genome sequencing (GS).

RESULTS

A 36-year-old man presented with low vision in both eyes since early childhood, with a best-corrected visual acuity of 20/500 in both eyes. He exhibited generalized optic atrophy and diffuse retinal nerve fiber layer thinning without retinal degeneration in both eyes. The family history was consistent with autosomal dominant traits. ES was performed; however, we did not identify any pathogenic variants in the known dominant optic atrophy genes. Subsequently, GS was performed, and it revealed a novel heterozygous c.364A>G p.(Lys122Glu) variant in SSBP1. In silico prediction supported it as deleterious, while segregation analysis detected it in his affected mother and his unaffected sister. No foveopathy or retinal degeneration was observed in the patient's family members.

CONCLUSIONS

We report a novel pathogenic heterozygous SSBP1 variant in a family with autosomal dominant optic atrophy and incomplete penetrance. Furthermore, we demonstrated that GS is advantageous over ES even for the discovery of coding variants, providing uniform coverage. Therefore, GS should be emphasized to improve the molecular diagnostic rate of inherited optic neuropathy.

Photoreceptor Manifestations of Primary Mitochondrial Optic Nerve Disorders

Purpose

To compare the manifestations of photoreceptors (PRs) in three hereditary optic neuropathies affected by primary mitochondrial dysfunction and discuss whether the retinal ganglion cells (RGCs) or the PRs are preferentially affected.

Methods

A retrospective analysis of patients with genetically confirmed diagnoses of optic neuropathies associated with mitochondrial dysfunction was performed. This cohort included Leber's hereditary optic neuropathy (LHON), autosomal dominant optic atrophy type 1 (OPA1), and optic atrophy type 13 (OPA13). Patient chart evaluations included clinical characteristics, best-corrected visual acuity (BCVA), fundus photography, spectral-domain optical coherence tomography (SD-OCT), electroretinogram (ERG), and visual evoked potential data.

Results

This analysis included seven patients with LHON, six with OPA1, and one with OPA13 from a tertiary medical center. Thirteen of the 14 individuals were male. The average BCVA at diagnosis was 20/285 and 20/500 in the right and left eyes, respectively. Five of the seven patients with LHON, and three of the six patients with OPA1 also showed a mild amplitude reduction or delayed latency on light-adapted ERG and 30-Hz flicker responses; however, SD-OCT imaging did not show correlated PR abnormalities. Notably, a 7-year follow-up of a patient with OPA13 revealed degeneration of RGCs prior to the degeneration of PRs. Follow-up data also demonstrated continuous loss of cone outer segment tips on SD-OCT imaging.

Conclusions

RGCs are, in general, affected by mitochondrial dysfunction, whereas variable PR dysfunction exists in patients with LHON and OPA1, especially with respect to the cone responses. Involvement of PRs is particularly evident in OPA13 after RGC degenerations.

Mitochondrial Neurodegeneration

Mitochondria are cytoplasmic organelles, which generate energy as heat and ATP, the universal energy currency of the cell. This process is carried out by coupling electron stripping through oxidation of nutrient substrates with the formation of a proton-based electrochemical gradient across the inner mitochondrial membrane. Controlled dissipation of the gradient can lead to production of heat as well as ATP, via ADP phosphorylation. This process is known as oxidative phosphorylation, and is carried out by four multiheteromeric complexes (from I to IV) of the mitochondrial respiratory chain, carrying out the electron flow whose energy is stored as a proton-based electrochemical gradient. This gradient sustains a second reaction, operated by the mitochondrial ATP synthase, or complex V, which condensates ADP and Pi into ATP. Four complexes (CI, CIII, CIV, and CV) are composed of proteins encoded by genes present in two separate compartments: the nuclear genome and a small circular DNA found in mitochondria themselves, and are termed mitochondrial DNA (mtDNA). Mutations striking either genome can lead to mitochondrial impairment, determining infantile, childhood or adult neurodegeneration. Mitochondrial disorders are complex neurological syndromes, and are often part of a multisystem disorder. In this paper, we divide the diseases into those caused by mtDNA defects and those that are due to mutations involving nuclear genes; from a clinical point of view, we discuss pediatric disorders in comparison to juvenile or adult-onset conditions. The complementary genetic contributions controlling organellar function and the complexity of the biochemical pathways present in the mitochondria justify the extreme genetic and phenotypic heterogeneity of this new area of inborn errors of metabolism known as ‘mitochondrial medicine’.