Monoamine oxidase A activity in fibroblasts as a functional confirmation of MAOA variants

Tessa M.A. Peters, Irma Lammerts van Bueren, Ben P.B.H. Geurtz, Karlien L.M. Coene*, Nicole de Leeuw, Han G. Brunner, Jón Jóhannes Jónsson, Michèl A.A.P. Willemsen, Ron A. Wevers, Marcel M. Verbeek

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Monoamine oxidase A (MAO‐A) deficiency is a rare inborn error of metabolism with impaired degradation of biogenic amines including 5‐hydroxytryptamine (5‐HT), resulting in borderline intellectual disability and behavioral abnormalities. Genetic variants in MAOA need functional confirmation to enable a definite diagnosis. To this end, we developed an inexpensive, simple and nonradioactive MAO‐A activity assay based on the conversion of 5‐HT into 5‐hydroxyindoleacetic acid (5‐HIAA). Fibroblast cell lysates were incubated with 5‐HT and aldehyde dehydrogenase to allow 5‐HIAA production. 5‐HIAA was quantified using high‐performance liquid chromatography with fluorimetric detection. We optimized reaction mixture components, pH, and substrate concentration and tested linearity and specificity of the assay. We verified the functional validity of the enzyme assay using fibroblasts of controls, female mutation carriers and MAO‐A deficient patients. This included a newly described patient with a novel MAOA variant (c.1336G>A, p.(Glu446Lys)), who represents the fifth MAO‐A deficiency family so far. The optimized enzyme assay showed good linearity and specificity. Application to clinical samples showed a 100% differentiation of affected patients (with negligible MAO‐A enzyme activity) and controls or mutation carriers. In conclusion, the described MAO‐A activity assay is easy to implement and can readily be used to test the pathogenicity of variants in the MAOA gene in a clinical setting. Especially in this era of whole‐exome (and whole‐genome) sequencing, this functional assay fulfills a clinical need for functional confirmation of a suspected diagnosis of MAO‐A deficiency.
Original languageEnglish
Pages (from-to)114-121
Number of pages8
JournalJIMD Reports
Issue number1
Publication statusPublished - 28 Dec 2020

Bibliographical note

The authors would like to thank Frans van den Brandt, Saskia van der Velde-Visser, and Marlie H. M. Jacobs-Camps for their work in culturing the fibroblasts and Astrid van Rens for performing the protein assay. Furthermore, the authors thank Stichting Stofwisselkracht and for their financial support for this study. The study was funded by Stofwisselkracht under the project name of “Innovative diagnostics in cerebrospinal fluid of patients with neurometabolic disorders” (KLM Coene and MM Verbeek). The authors confirm independence from the sponsor; the content of the article has not been influenced by the sponsor. Lastly, the authors thank the The Society for the Study of Inborn Errors of Metabolism for covering the article processing charge.

Other keywords

  • enzyme assay
  • functional confirmation
  • HPLC
  • MAO-A deficiency
  • variant of uncertain significance


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