Flavin Adenine Dinucleotide Depletion Caused by electron transfer flavoprotein subunit alpha Haploinsufficiency Leads to Hepatic Steatosis and Injury in Zebrafish

The electron transfer flavoprotein (ETF) complex, made up of the ETF alpha subunit (ETFA), ETF beta subunit (ETFB), and ETF dehydrogenase (ETFDH), regulates fatty acid β-oxidation activity while scavenging leaked electrons through flavin adenine dinucleotide (FAD)/reduced form FAD (FADH₂) redox reactions in mitochondria. Here, we hypothesized that ETF dysfunction-mediated FAD deficiency may result in increased mitochondrial oxidative stress and steatosis and subsequent liver injury. We report that etfa haploinsufficiency caused hyperlipidemia, hypercholesterolemia, and hepatic steatosis and injury in adult zebrafish. Further, etfa^+/- mutant livers had reduced levels of FAD and glutathione and an increase in reactive oxygen species. Because FAD depletion might be critical in the pathogenesis of the liver lesion identified in etfa^+/- mutants, we used riboflavin to elevate FAD levels in the liver and found that riboflavin supplementation significantly suppressed hepatic steatosis and injury in etfa^+/- mutants through suppression of oxidative stress and de novo lipogenesis in the liver. Additionally, we found that adenosine triphosphate-linked mitochondrial oxygen consumption and mitochondrial membrane potential were reduced in etfa^+/- primary hepatocytes and that riboflavin supplementation corrected these defects. Conclusion: FAD depletion caused by etfa haploinsufficiency plays a key role in hepatic steatosis and oxidative stress-mediated hepatic injury in adult zebrafish. This raises the possibility that people with ETFA haploinsufficiency have a high risk for developing liver disease.