Distinct Urea Cycle Dysfunction Profiles Differentiate Acute Metabolic Decompensation in TMEM70 and MT-ATP6-Related Mitochondrial ATP Synthase Defects.

ATP synthase defects, including TMEM70 and MT-ATP6 deficiencies, cause severe mitochondrial encephalo-(cardio)-myopathies complicated by acute metabolic decompensations (AMDs) often associated with hyperammonaemia. However, detailed biochemical characterisation of these events remains limited. The aim of the study was to evaluate the metabolic profiles associated with TMEM70 and MT-ATP6 deficiencies during AMDs in comparison to stable metabolic conditions, assessing frequency and severity of hyperammonaemia, and exploring the mechanisms linking impaired mitochondrial ATP production to the urea cycle by in vivo ureagenesis studies, using [15N] ammonium chloride as stable isotope and assessed by high-resolution mass-spectrometry coupled with liquid chromatography. We retrospectively analysed clinical and biochemical profiles from two genetically confirmed cohorts. Patients with TMEM70 deficiency experienced more frequent AMDs, often with hyperammonaemia and requiring extracorporeal detoxification, while the MT-ATP6 cohort had more prominent neurological symptoms and a lower incidence of hyperammonaemia. Biochemically, both groups showed elevated lactate, alanine and glutamine, with orotic aciduria and abnormalities in purine/pyrimidine metabolism. Plasma citrulline levels were divergent in the two cohorts, with a consistent reduction in patients with MT-ATP6 deficiency and normal or borderline elevated levels in the TMEM70 cohort. In vivo stable isotope studies pointed to the differential impact of TMEM70 and MT-ATP6 deficiency on ureagenesis and on the enrichment of individual urea cycle-related amino acids. This study reveals that TMEM70 and MT-ATP6 deficiencies share features of mitochondrial dysfunction but present distinct metabolic profiles, highlighting a different impact on the urea cycle and its related metabolites, and providing novel insights on our understanding of mitochondrial pathophysiology.