Liver mitochondrial DNA damage and genetic variability of Cytochrome b – a key component of the respirasome – drive the severity of fatty liver disease

Abstract

Background and aims: The progression of nonalcoholic fatty liver disease (NAFLD) into severe histological forms (steatohepatitis – NASH) is paralleled by the occurrence of complex molecular processes. Mitochondrial dysfunction is a hallmark feature of advanced disease. Mitochondrially encoded cytochrome B (cytochrome b, MT-CYB), a member of the oxidative phosphorylation system, is a key component of the respirasome supercomplex. Here, we hypothesized that NAFLD severity is associated with liver tissue cytochrome b mutations and damaged mitochondrial DNA (mtDNA). Methods: We included 252 liver specimens of NAFLD patients – in whom histological disease ranged from mild to severe – which were linked to clinical and biochemical information. Tissue molecular explorations included MT-CYB sequencing and analysis of differential mtDNA damage. Profiling of circulating Krebs cycle metabolites and global liver transcriptome was performed in a subsample of patients. Tissue levels of 4-hydroxynonenal – a product of lipid peroxidation and 8-hydroxy-2’-deoxyguanosine, a marker of oxidative damage – were measured. Results: Compared to simple steatosis, NASH is associated with a higher level of MT-CYB variance, 12.1 vs. 15.6 substitutions per 103 bp (P = 5.5e-10). The burden of variants was associated with increased levels of 2-hydroxyglutarate, branched-chain amino acids, and glutamate, and changes in the global liver transcriptome. Liver mtDNA damage was associated with advanced disease and inflammation. NAFLD severity was associated with increased tissue levels of DNA oxidative adducts and lipid peroxyl radicals. Conclusion: NASH is associated with genetic alterations of the liver cellular respirasome, including high cytochrome b variation and mtDNA damage, which may result in broad cellular effects.