To elucidate the genotype-phenotype correlation in MELAS patients with low heteroplasmy to tailor  therapeutic regimens.

MELAS (Mitochondrial Encephalopathy Lactic Acidosis with Stroke-like episodes) is an intractable progressive neurodegenerative disease. Patients exhibit a broad and heterogeneous clinical spectrum. Most harbor the maternally inherited pathogenic variant, m.3243A>G, mapping in the MT-TL1 gene encoding the mitochondrial tRNA^Leu/UUR. It affects the oxidative phosphorylation (OXPHOS), causing ATP deficiency. Since this variant only affects a subset of the multi-copy mitochondrial genome, MELAS cells harbor a mixed population of functional and dysfunctional mitochondria, or heteroplasmy. Generally, patients become symptomatic when heteroplasmy exceeds a certain threshold, typically above 80%. However, some patients with low heteroplasmy present  neurological manifestations.

Ten patients with the m.3243A>G variant in blood were recruited to undergo a skin biopsy and segregated in three experimental groups: high, medium and low heteroplasmy. Healthy fibroblasts were commercially acquired. Heteroplasmy was measured by long-range PCR followed by Next-Generation Sequencing. Live-cell metabolic assays were performed using the Seahorse technology.

Our analysis revealed overall similar quantifications of the m.3243A>G heteroplasmy in blood and fibroblasts. We found  the baseline metabolic phenotype is influenced by the degree of heteroplasmy. Fibroblasts with high heteroplasmy consistently showed severe decrease in basal respiration, whereas some fibroblasts with low or medium heteroplasmy displayed  normal baseline metabolic signature. However, regardless of their heteroplamy, MELAS fibroblasts showed a stunted spare respiratory capacity. This is clinically relevant, as neurons solely rely on the spare respiratory capacity for high energy expenditure to avert an ATP crisis and chronic energy exhaustion. MELAS fibroblasts, irrespective of heteroplasmy, displayed an impaired metabolic switch from OXPHOS to glycolysis to sustain ATP synthesis.

Patients with low blood heteroplasmy should undergo a skin biopsy to define their metabolic signature in order to tailor their therapeutic regimen for abating their energy deficit and managing their neurological symptoms.