FAD synthase deficiency: a severe mitochondrial myopathy searching for novel therapeutic strategies

Background Multiple Acyl CoA deficiency (MADD, OMIM #231680) is a heterogeneous disorder affecting multiple mitochondrial flavoproteins, which involves energy metabolism, and particularly β-oxidation of fatty acids. Presentations range from severe forms with multiple organ failure, to later-onset lipid storage myopathy, with excellent response to riboflavin (vitamin B2), the precursor of the redox flavin cofactors, FMN and FAD. Genetic testing for MADD comprises analysis of ETFA, ETFB, ETFDH and genes involved in riboflavin transport and flavoprotein biosynthesis. Deficiency of FAD synthase caused by FLAD1 variations was recently identified as a cause of MADD (now lipid storage myophaty, LSMFLAD, OMIM #255100) with potential effect of riboflavin treatment, especially in patients with milder FLAD1 variations (Olsen RKJ, et al. AJHG 2016; 98:1130-1145). Materials and Methods Human fibroblasts were cultured as in (Ryder B, et al. JIMD Rep. 2019;45:37-44). Flavin homeostasis was studied by HPLC. Protein and transcript levels were measured by Western Blotting and PCR. Bioenergetics, ROS measurements and worm flad1 silencing were performed, as in (Liuzzi VC, et al. BBA. 2012;1820:521-31). Results The derangements of flavin homeostasis were studied in fibroblasts of patients expressing FLAD1 pathological variants. An impairment of mitochondrial bioenergetics, mainly due to reduction in the level of succinate dehydrogenase flavoprotein subunit, and an increase in cellular ROS and peroxiredoxin 3 levels were observed, suggesting an active response to stress conditions. Interestingly, these changes were accompanied by a secondary transcriptional regulation of the riboflavin transporter (RFVT2). A similar behavior was found in a C. elegans model mimicking FAD synthase deficiency, obtained by silencing the worm orthologue of FLAD1. Conclusions FAD synthase deficiency is a pathological condition which severely alters mitochondrial flavoproteome and bioenergetics in both human cells and nematodes. Secondary derangements of RFVT2 might explain the therapeutic response to riboflavin treatment. The silenced worm is a promising model for testing alternative therapeutic strategies for this mitochondrial myopathy.