FAD synthase deficiency: a severe mitochondrial myopathy involving a secondary reduction of RFVT2 expression

The redox cofactor FAD is essential for mitochondrial functionality: in the inner- membrane it ensures the activity of the respiratory chain complex II and of the ETF/ETFQO system, in the matrix the oxidation of pyruvate and other α-oxoacids as well as of some amino acids. Matrix located FAD-dependent dehydrogenases are also involved in β-oxidation of fatty acil-CoAs. The last step of the metabolic pathway converting the vitamin riboflavin (Rf) into FAD is catalyzed by FAD synthase (FADS), coded by human FLAD1 [1]. FLAD1 variations were identified as a cause of a severe lipid storage myopathy resembling Multiple Acyl CoA Dehydrogenase Deficiency, named LSMFLAD (OMIM #255100). Patients’ symptoms can sometimes favourably respond to Rf therapy. In the frame of the structural and functional characterisation of the different hFADS isoforms, we describe here some morphological and biochemical alterations in patients’ fibroblasts expressing FLAD1 pathological variants. In these cells we observed an impairment of mitochondrial bioenergetics, mainly due to reduction in the level of succinate dehydrogenase flavoprotein subunit, accompanied by, at least in one patient, an increase in cellular ROS and a decrease of mtDNA content. In the same patient’s cells, increased PGC-1α and PrxIII levels were also observed, suggesting an active response to stress conditions. Interestingly enough, in patient’s fibroblasts we proved a drastic reduction of the levels of both the transcripts and the protein product of SLC52A2 gene, i.e., RFVT2, the main Rf transporter in muscle [1]. The decreased levels of all the flavin species in the cell extracts are, thus, explainable as due to the impairment of Rf flux from outside. In conclusion, a proposal is made here that RFVT2 may be the primary target of Rf-based (and possibly alternative) therapeutic strategies against LSMFLAD. [1] M. Tolomeo, A. Nisco, P. Leone, M. Barile, Development of Novel Experimental Models to Study Flavoproteome Alterations in Human Neuromuscular Diseases: The Effect of Rf Therapy, International journal of molecular sciences, 21 (2020).