Divergent role of Mitochondrial Amidoxime Reducing Component 1 (MARC1) in human and mouse

Recent human genome-wide association studies have identified common missense variants in MARC1, p.Ala165Thr and p.Met187Lys, associated with lower hepatic fat, reduction in liver enzymes and protection from most causes of cirrhosis. Using an exome-wide association study we recapitulated earlier MARC1 p.Ala165Thr and p.Met187Lys findings in 540,000 individuals from five ancestry groups. We also discovered novel rare putative loss of function variants in MARC1 with a phenotype similar to MARC1 p.Ala165Thr/p.Met187Lys variants. In vitro studies of recombinant human MARC1 protein revealed Ala165Thr substitution causes protein instability and aberrant localization in hepatic cells, suggesting MARC1 inhibition or deletion may lead to hepatoprotection. Following this hypothesis, we generated Marc1 knockout mice and evaluated the effect of Marc1 deletion on liver phenotype. Unexpectedly, our study found that whole-body Marc1 deficiency in mouse is not protective against hepatic triglyceride accumulation, liver inflammation or fibrosis. In attempts to explain the lack of the observed phenotype, we discovered that Marc1 plays only a minor role in mouse liver while its paralogue Marc2 is the main Marc family enzyme in mice. Our findings highlight the major difference in MARC1 physiological function between human and mouse. Non-alcoholic fatty liver disease (NAFLD) affects almost a third population worldwide and is the main cause of chronic liver disease and cirrhosis. Recent human genetics studies identified several common variants in MARC1 gene, associated with lower hepatic fat, reduction in liver enzymes and protection from most causes of cirrhosis, pointing to MARC1 as a potential therapeutic target. Yet, the hepatoprotective mechanism of MARC1 loss of function is unknown. In this study, we expand human genetic analysis to 540,000 individuals. We confirm previous findings and identify novel rare variants in MARC1 that similarly associated with reduction in liver fat, liver enzymes and protection from cirrhosis. We also confirm that most common variant in MARC1 is a true loss of function. We knockdown Marc1 in mice and challenge them with NAFLD-causing diets to understand its role in liver disease. We find loss of Marc1 in mice did not lead to hepatoprotective effect observed in humans. To understand the discrepancy, we knockdown Marc1 and its paralogue Marc2 from primary hepatocytes and discover that Marc1 plays only a minor role in mouse liver while its paralogue Marc2 is the main Marc family enzyme in mice.