LONP1 and lactic acidosis: By contrast, we identified rare biallelic variants in LONP1, c.1693T > C, p.(Tyr565His) and c.2197G > A, p.(Glu733Lys), in a patient presenting with congenital lactic acidosis, muscle weakness and brain MRI typical of Leigh syndrome, who had profound OXPHOS deficiencies and severe mtDNA depletion in skeletal muscle.