Riboflavin-responsive and Non-responsive Mutations in the FAD Synthase Gene Cause Multiple Acyl-CoA Dehydrogenase and Combined Respiratory Chain Deficiency

Signe Mosegaard, BSc, Research Unit for Molecular Medicine, Aarhus University and Aarhus University Hospital, Aarhus, Denmark

Email: signe.mosegaard@clin.au.dk

Phone: +45 20835507



Riboflavin is an essential vitamin and precursor to FMN and FAD, essential cofactors to numerous dehydrogenases involved within cellular metabolism, antioxidant defense, protein folding and apoptosis. Riboflavin is transformed to FMN by riboflavin kinase (RFK) and to FAD by FAD synthase (FADS).

The FADS protein, encoded by FLAD1, consists of a molydopterin-binding domain (MPTb) in the N-terminus and a phosphoadenosine 5’-phosphosuldate (PAPS) reductase domain responsible for the synthesis of FAD in the C-terminal.

The biochemical profile of riboflavin deficiency is elevated multiple acyl carnitines and organic aciduria that resembles the biochemistry of Multiple Acyl-CoA Dehydrogenation Deficiency (MADD) caused by defects in the electron transfer flavoprotein genes that support mitochondrial acyl-CoA dehydrogenation reactions.

In recent years the number of MADD associated genes have increased by the identification of defects in genes responsible for transport of riboflavin or mitochondrial FAD transport, yet the genetic basis of many cases remains unexplained.

In this multicenter study we present patients from seven unrelated families suffering from MADD and respiratory chain deficiencies, caused by mutations in FLAD1. In four families we identified biallelic frameshift variants in the MPTb domain and we would expect a non-functional PAPS domain and thereby no FAD synthesis, but surprisingly we found FAD synthesis. By using protein mass spectrometry and RNAseq analysis we identified isoforms of FLAD1, only encoding the PAPS domain and with the ability to produce FAD. Three patients with riboflavin-responsive disease all harbored missense mutations within the PAPS domain. In vitro reconstitution with FAD rescued the stability of one of these mutant proteins.

Please note that this research project has been accepted as an article in American Journal of Human Genetics.

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