Z Nochi1, B Ryder2, M Tolomeo3, M Colella3, M Barile3, RKJ Olsen1, M Inbar-Feigenberg2
1Research Unit for Molecular Medicine, Department for Clinical Medicine, Aarhus University and Aarhus University Hospital, Aarhus, Denmark
2Division of Clinical and Metabolic Genetics, The Hospital for Sick Children, University of Toronto, Toronto, Ontario, Canada
3Department of Bioscience, Biotechnology and Biopharmaceutics, University of Bari, via Orabona, 4, I-70125 Bari, Italy
Zahra Nochi, PhD Postdoc
Research Unit for Molecular Medicine (MMF) Department of Clinical Medicine
Aarhus University Hospital Palle Juul-Jensens Boulevard 99 8200 Aarhus N, Denmark
MADD is a heterogeneous disorder affecting multiple flavoproteins involved in energy metabolism. Presentations range from severe to mild forms, with response to riboflavin. Genetic testing for MADD comprises analysis of ETFA, ETFB, ETFDH and genes involved in riboflavin transport and flavoprotein biosynthesis. Deficiency of FAD synthase (FADS) caused by FLAD1 variations was identified as a cause of MADD with potential effect of riboflavin treatment. We describe a novel, FLAD1 loss-of-function (LOF) variant causing riboflavin-responsive MADD myopathy in an 8-year-old boy.
The patient referred to the metabolic service with a positive newborn screen for medium-chain acyl-CoA dehydrogenase deficiency. Confirmatory biochemical analysis were diagnostic for MADD, but with negative findings in ETFA, ETFB and ETFDH genes. Riboflavin was initiated with biochemical and clinical response. By 8 years of age, the child had developed myopathy, and an expanded gene analysis revealed homozygosity for FLAD1 c.745C>T (p.Arg249*). Functional assays were performed to better characterize the variant.
Immunoblotting showed almost no detectable full-length FADS protein in patients fibroblasts, but revealed a 26 kDa band, corresponding to the recently characterized FADS isoform, which lacks the N-terminal region of the protein containing the c.745C>T variant, but with a functional FADS domain in the C-terminal. While there was almost total lack (<10 %) of FAD synthesis rate, cellular FAD content was 54% that of controls.
Compared to the previously reported cases with biallelic LOF FLAD1 variants, the present case shows a better response to riboflavin treatment, stressing the importance of early recognition and treatment. Our data support earlier speculations that FADS is not the rate-limiting step in FAD synthesis and that residual synthesis activity, possibly complemented by the 26 kDa truncated FADS protein, determine cellular FAD content.