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Flavin adenine dinucleotide (FAD) synthase (EC 2.7.7.2), encoded by human flavin adenine dinucleotide synthetase 1 ( FLAD1 ), catalyzes the last step of the pathway converting riboflavin (Rf) into FAD.  FLAD1  variations were identified as a cause of LSMFLAD (lipid storage myopathy due to FAD synthase deficiency, OMIM #255100), resembling Multiple Acyl-CoA Dehydrogenase Deficiency, sometimes treatable with high doses of Rf; no alternative therapeutic strategies are available. We describe here cell morphological and mitochondrial alterations in dermal fibroblasts derived from a LSMFLAD patient carrying a homozygous truncating  FLAD1  variant (c.745C &gt; T) in exon 2. Despite a severe decrease in FAD synthesis rate, the patient had decreased cellular levels of Rf and flavin mononucleotide and responded to Rf treatment. We hypothesized that disturbed flavin homeostasis and Rf-responsiveness could be due to a secondary impairment in the expression of the Rf transporter 2 (RFVT2), encoded by  SLC52A2,  in the frame of an adaptive retrograde signaling to mitochondrial dysfunction. Interestingly, an antioxidant response element (ARE) is found in the region upstream of the transcriptional start site of  SLC52A2 . Accordingly, we found that abnormal mitochondrial morphology and impairments in bioenergetics were accompanied by increased cellular reactive oxygen species content and mtDNA oxidative damage. Concomitantly, an active response to mitochondrial stress is suggested by increased levels of PPARγ-co-activator-1α and Peroxiredoxin III. In this scenario, the treatment with high doses of Rf might compensate for the secondary RFVT2 molecular defect, providing a molecular rationale for the Rf responsiveness in patients with loss of function variants in  FLAD1  exon 2.HIGHLIGHTSFAD synthase deficiency alters mitochondrial morphology and bioenergetics;FAD synthase deficiency triggers a mitochondrial retrograde response;FAD synthase deficiency evokes nuclear signals that adapt the expression of RFVT2. FAD synthase deficiency alters mitochondrial morphology and bioenergetics; FAD synthase deficiency triggers a mitochondrial retrograde response; FAD synthase deficiency evokes nuclear signals that adapt the expression of RFVT2.

Retrograde response to mitochondrial dysfunctions associated to LOF variations in FLAD1 exon 2: unraveling the importance of RFVT2