Inherited optic neuropathies are the most common mitochondrial diseases, leading to neurodegeneration involving the irreversible loss of retinal ganglion cells, optic nerve degeneration and central visual loss. Importantly, properly regulated mitochondrial dynamics are critical for maintaining cellular homeostasis, and are further regulated by MIEF1 (mitochondrial elongation factor 1) which encodes for MID51 (mitochondrial dynamics protein 51), an outer mitochondrial membrane protein that acts as an adaptor protein to regulate mitochondrial fission. However, dominant mutations in MIEF1 have not been previously linked to any human disease. Using targeted sequencing of genes involved in mitochondrial dynamics, we report the first heterozygous variants in MIEF1 linked to disease, which cause an unusual form of late-onset progressive optic neuropathy characterized by the initial loss of peripheral visual fields. Pathogenic MIEF1 variants linked to optic neuropathy do not disrupt MID51’s localization to the outer mitochondrial membrane or its oligomerization, but rather, significantly disrupt mitochondrial network dynamics compared to wild-type MID51 in high spatial and temporal resolution confocal microscopy live imaging studies. Together, our study identifies dominant MIEF1 mutations as a cause for optic neuropathy and further highlights the important role of properly regulated mitochondrial dynamics in neurodegeneration.

Background/Objectives: Biallelic mutations in the PTRH2 gene are associated with a rare genetic disease known as infantile-onset multisystem neurologic, endocrine, and pancreatic disease (IMNEPD). In this study, we describe a new case carrying a previously identified mutation, provide an updated analysis of the relative frequencies of the clinical features across all published cases (including the three latest studies), and perform a bioinformatics analysis of the newly identified PTRH2 protein variants from a structural perspective. Methods: Clinical examination of the patients was carried out, and genetic testing was performed using a genome sequencing strategy. A bioinformatics analysis was carried out for the newly reported mutations using PYMOL that was utilized to view the structure and analyze the mutations. Additionally, the ThermoMPNN webserver was employed to check the effect of point mutations on the overall stability of the protein. Results: Our findings indicate that motor delay, neuropathy, intellectual disability, distal weakness, hearing impairment, and ataxia are the most common symptoms, while the other clinical features fall into two frequency categories: moderately common ones and the least common ones. The bioinformatics analysis revealed that the Q85 residue is highly conserved, suggesting that mutations at this position could disrupt key signaling pathways or cellular functions. Indeed, the Q85R mutation was shown to significantly impair the stability and functionality of the protein. Conclusions: The clinical presentation of IMNEPD remains highly variable in terms of both severity and progression. Mutations at the Q85 residue have been identified in nearly 50% of reported cases, highlighting this position as a potential mutational hotspot in the PTRH2 protein.