OPA1 haploinsufficiency due to a novel splicing variant resulting in mitochondrial dysfunction without mitochondrial DNA depletion.

Background: To identify and investigate the effects of a novel splicing variant, c.1444-2A>C of OPA1, on its transcript, translation, and mitochondrial function, which was found in an 8-year-old patient with dominantly inherited optic atrophy (DOA). Materials and Methods: The clinical evaluations were performed at the Eye Center. Lymphoblast cell lines were generated from the patient, mother, and a normal control with the same haplotype of mitochondrial genome. The novel variant was confirmed by Sanger sequencing. The splicing alteration of cDNA was checked by both Sanger sequencing and agarose gel. OPA1 expression was carried out by RT-PCR and Western blotting. Transmission electron microscopy was used for mitochondrial morphology. Mitochondrial functions, including the rates of oxygen consumption, ATP generation, ROS product and membrane potential were assayed in lymphoblast cells. Results: The novel OPA1 splicing variant, c.1444-2A>C, led to a deletion of the 15th exon in mRNA transcript. Approximately 50% reduction of mRNA and protein expression was present in mutant cells as compared with controls. No marked depletion of mtDNA nor mitochondrial mass was caused by the splicing variant. However, defects that the impaired capacity of OXPHOS, reduced ATP generation, increased ROS and decreased membrane potential were observed in the mutant cells, which promoted a ubiquitin-binding mitophagy instead of apoptosis. Conclusions: The novel splicing variant, c.1444-2A>C resulted in OPA1 haploinsufficiency effect on its expression and mitochondrial function without mtDNA depletion. Our findings may provide new insights into the understanding of pathophysiology of DOA.

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

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.

Homozygous Mutation in TWNK Cases Ataxia, Sensorineural Hearing Loss and Optic Nerve Atrophy.

The TWNK (C10orf2) gene encodes Twinkle, an essential helicase for mtDNA replication. Homozygous mutations in TWNK can lead to mitochondrial DNA depletion syndrome 7 (MTDPS7) that usually manifests as Infantile onset spinocerebellar ataxia (IOSCA). Here, we report a 15-year-old Iranian boy with three main symptoms; ataxia, sensorineural hearing loss and optic nerves atrophy which were accompanied by other symptoms including flexion contracture, dysarthric speech, nystagmus, dystonia and borderline intellectual disability. Whole exome sequencing (WES) revealed a homozygous mutation in his TWNK gene. The mutation was a transversion which replaced a C with A (NM_021830.4 (TWNK):c.874C>A). This nucleotide substitution results in replacing a Threonine with Proline in codon 292 of Twinkle protein (p.Pro292Thr). In silico analyses showed that this amino acid change in Twinkle could be deleterious and disease-causing; therefore, we attribute the symptoms of our patient to this mutation. Our study extended the homozygous mutation spectrum of the TWNK gene that leads to IOSCA.

The C. elegans Opa1 homologue EAT-3 is essential for resistance to free radicals.

The C. elegans eat-3 gene encodes a mitochondrial dynamin family member homologous to Opa1 in humans and Mgm1 in yeast. We find that mutations in the C. elegans eat-3 locus cause mitochondria to fragment in agreement with the mutant phenotypes observed in yeast and mammalian cells. Electron microscopy shows that the matrices of fragmented mitochondria in eat-3 mutants are divided by inner membrane septae, suggestive of a specific defect in fusion of the mitochondrial inner membrane. In addition, we find that C. elegans eat-3 mutant animals are smaller, grow slower, and have smaller broodsizes than C. elegans mutants with defects in other mitochondrial fission and fusion proteins. Although mammalian Opa1 is antiapoptotic, mutations in the canonical C. elegans cell death genes ced-3 and ced-4 do not suppress the slow growth and small broodsize phenotypes of eat-3 mutants. Instead, the phenotypes of eat-3 mutants are consistent with defects in oxidative phosphorylation. Moreover, eat-3 mutants are hypersensitive to paraquat, which promotes damage by free radicals, and they are sensitive to loss of the mitochondrial superoxide dismutase sod-2. We conclude that free radicals contribute to the pathology of C. elegans eat-3 mutants.

The molecular mechanisms of OPA1-mediated optic atrophy in Drosophila model and prospects for antioxidant treatment.

Mutations in optic atrophy 1 (OPA1), a nuclear gene encoding a mitochondrial protein, is the most common cause for autosomal dominant optic atrophy (DOA). The condition is characterized by gradual loss of vision, color vision defects, and temporal optic pallor. To understand the molecular mechanism by which OPA1 mutations cause optic atrophy and to facilitate the development of an effective therapeutic agent for optic atrophies, we analyzed phenotypes in the developing and adult Drosophila eyes produced by mutant dOpa1 (CG8479), a Drosophila ortholog of human OPA1. Heterozygous mutation of dOpa1 by a P-element or transposon insertions causes no discernable eye phenotype, whereas the homozygous mutation results in embryonic lethality. Using powerful Drosophila genetic techniques, we created eye-specific somatic clones. The somatic homozygous mutation of dOpa1 in the eyes caused rough (mispatterning) and glossy (decreased lens and pigment deposition) eye phenotypes in adult flies; this phenotype was reversible by precise excision of the inserted P-element. Furthermore, we show the rough eye phenotype is caused by the loss of hexagonal lattice cells in developing eyes, suggesting an increase in lattice cell apoptosis. In adult flies, the dOpa1 mutation caused an increase in reactive oxygen species (ROS) production as well as mitochondrial fragmentation associated with loss and damage of the cone and pigment cells. We show that superoxide dismutase 1 (SOD1), Vitamin E, and genetically overexpressed human SOD1 (hSOD1) is able to reverse the glossy eye phenotype of dOPA1 mutant large clones, further suggesting that ROS play an important role in cone and pigment cell death. Our results show dOpa1 mutations cause cell loss by two distinct pathogenic pathways. This study provides novel insights into the pathogenesis of optic atrophy and demonstrates the promise of antioxidants as therapeutic agents for this condition.

Mutation spectrum and splicing variants in the OPA1 gene.

Optic atrophy type 1 (OPA1, MIM 165500) is a dominantly inherited optic neuropathy that features low visual acuity leading in many cases to legal blindness. We have recently shown, with others, that mutations in the OPA1 gene encoding a dynamin-related mitochondrial protein, underlie the dominant form of optic atrophy. Here we report that OPA1 has eight mRNA isoforms as a result of the alternative splicing of exon 4 and two novel exons named 4b and 5b. In addition, we screened a cohort of 19 unrelated patients with dominant optic atrophy by direct sequencing of the 30 OPA1 exons (including exons 4b and 5b) and found mutations in 17 (89%) of them of which 8 were novel. A majority of these mutations were truncative (65%) and located in exons 8 to 28, but a number of them were amino acid changes predominantly found in the GTPase domain (exons 8 to 15). We hypothesize that at least two modifications of OPA1 may lead to dominant optic atrophy, that is alteration in GTPase activity and loss of the last seven C-terminal amino acids that putatively interact with other proteins.