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.