To elucidate the pathogenic molecular mechanisms of MELAS and LHON-Plus to design shared biochemical endpoints for a basket clinical trial design.

MELAS (Mitochondrial encephalopathy, lactic acidosis, stroke-like episodes) and LHON-Plus (Leber’s hereditary optic neuropathy-Plus) are progressive neurodegenerative diseases with overlapping and divergent clinical neurological symptoms. The divergence in their onset has ramifications on phenotypic manifestations and developmental milestones. MELAS has a predominant childhood onset, whereas LHON-Plus has a predominant adulthood onset. MELAS has no gender bias while LHON-Plus has a female predominance. Both exhibit a broad and heterogeneous clinical spectrum, even among their overlapping neurological symptoms. Patients harbor distinct maternally inherited pathogenic variants affecting the oxidative phosphorylation (OXPHOS) system. The most frequent MELAS variant m.3243A>G and LHON-Plus variant m.11778G>A map in the mitochondrial tRNA^Leu/UUR and ND4 subunit of Complex I, respectively. The MELAS variants only affect a subset of the multi-copy mitochondrial genome and are heteroplasmic, while the LHON-Plus variants are near-homoplasmic.

20 MELAS and 20 LHON-Plus patients were recruited to undergo a skin biopsy. Their clinical parameters and history were recorded and their molecular parameters, heteroplasmy and mitochondrial energy metabolism, were measured by long-range PCR followed by Next-Generation Sequencing and live-cell Seahorse assays.

OXPHOS is influenced by the heteroplasmic load of the MELAS variant, as only high heteroplasmy severely decreased the basal OXPHOS. All the LHON-Plus fibroblasts had a reduced basal OXPHOS. Regardless of their heteroplasmic load and mitochondrial genotype, MELAS and LHON-Plus fibroblasts showed a stunted spare respiratory capacity. This is clinically relevant to the neurological manifestations, as neurons rely on the spare respiratory capacity for high energy expenditure to avert chronic energy exhaustion.

MELAS and LHON-plus share a molecular etiology with a deficient spare respiratory capacity causing chronic energy deficit that can be exploited to design a basket clinical trial.