Ó³»­´«Ã½

Management of patients with mitochondrial respiratory chain diseases is challenging, in part because of our incomplete understanding of pathogenesis and a lack of biomarkers. Unknown metabolites account for >90% of detected features in modern metabolomics experiments and hold immense untapped promise for new basic and biomedical research. We recently used mass spectrometry-based metabolomics to identify and validate 19 circulating blood-based biomarkers for patients with the mitochondrial DNA (mtDNA) m.3243A>G pathogenic variant, which is the most frequent cause of the mitochondrial disorder MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes). However, the most significantly changing biomarker corresponded to an "unknown" metabolite. Here, we combine cheminformatics with analytical chemistry and identify that feature as 4,5-dihydroxyhexanoic acid (4,5-DHHA), a metabolite previously associated with inherited defects of gamma-aminobutyric acid (GABA) catabolism, but with no prior links to mitochondrial respiratory chain disorders. We validate this finding in an independent MELAS cohort and further show that 4,5-DHHA levels correlate with disease severity and are elevated in patients with other forms of mitochondrial disease and sepsis. Furthermore, brain 4,5-DHHA levels were elevated in two genetic mouse models of mitochondrial disease. and tissue culture experiments indicate that 4,5-DHHA is generated when the GABA catabolite succinic semialdehyde reacts with an intermediate of the pyruvate dehydrogenase reaction and is sensitive to mitochondrial complex I function. Our work identifies 4,5-DHHA as a robust plasma and urine marker of mitochondrial dysfunction in humans and reveals new connections between the respiratory chain and GABA metabolism.