Quantitative measurement of 3-O-methyldopa from dried blood spot for screening of aromatic L-amino acid decarboxylase (AADC) deficiency — a rare but treatable neurotransmitter disorder.
This method screens for aromatic L-amino acid decarboxylase (AADC) deficiency, a rare autosomal recessive neurotransmitter disorder affecting the synthesis of dopamine, serotonin, and other critical catecholamines. The test quantifies 3-O-methyldopa (3-OMD) from a dried blood spot, which accumulates when AADC enzyme activity is impaired.
AADC deficiency is profoundly important to identify because eladocagene exuparvovec (Upstaza) — a recently approved gene therapy — is now available as a disease-modifying treatment. Early diagnosis enables access to this transformative therapy and prevents years of disability.
Clinical presentations of AADC deficiency include:
Critically, AADC deficiency is frequently misdiagnosed as cerebral palsy, epilepsy, autism spectrum disorder, or idiopathic developmental delay, leading to years of missed opportunities for effective treatment. A simple DBS biomarker test can fundamentally change a patient's clinical trajectory.
Aromatic L-amino acid decarboxylase (AADC) is a pyridoxal phosphate-dependent enzyme encoded by the DDC gene. It catalyses two essential metabolic steps: the conversion of L-DOPA to dopamine in the catecholamine synthesis pathway, and the conversion of 5-hydroxytryptophan (5-HTP) to serotonin in the serotonin synthesis pathway. AADC also participates in the synthesis of histamine and trace amines. Thus, AADC deficiency causes simultaneous depletion of dopamine, serotonin, and other neurotransmitters critical for motor control, mood regulation, and autonomic function.
Eladocagene exuparvovec (Upstaza) received regulatory approval from the EMA and FDA, offering the first disease-modifying treatment
Symptoms mimic cerebral palsy, epilepsy, and autism, resulting in many patients going undiagnosed for years
A single 3-OMD measurement from a heel prick can identify affected individuals and enable early access to therapy
Mutations in the DDC gene impair AADC protein synthesis or function, leading to reduced enzyme activity. In affected individuals, neurotransmitter precursors (L-DOPA and 5-HTP) accumulate and are shunted into alternative metabolic pathways. One major pathway is methylation by catechol-O-methyltransferase (COMT), producing 3-O-methyldopa. This accumulation makes 3-O-methyldopa an excellent biomarker for AADC deficiency: blood levels are dramatically elevated in affected individuals and normal in unaffected controls.
Differential diagnosis includes other causes of neurotransmitter disorders such as PNPO (pyridoxal phosphate oxidase) deficiency, various B6-responsive conditions, monoamine oxidase deficiencies, and vesicular monoamine transporter defects. However, AADC deficiency can be specifically identified through 3-OMD measurement combined with evaluation of other neurotransmitter metabolites (biopterin, homovanillic acid, 5-hydroxyindoleacetic acid) by expanded metabolomics.
The availability of gene therapy has transformed AADC deficiency from a progressive neurodevelopmental disorder into a treatable condition. Gene therapy restores AADC enzyme expression in the central nervous system, enabling normalisation of dopamine and serotonin synthesis. Early treatment, particularly in infants identified through expanded newborn screening, has demonstrated the capacity to prevent neurological deterioration and enable normal or near-normal development.
3-O-methyldopa is extracted from dried blood spots and quantified by high-performance liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). The assay uses stable isotope-labelled internal standard to correct for variation in extraction efficiency and ionisation. The method is highly sensitive, with the ability to detect even modest elevation in 3-OMD levels that may distinguish heterozygous carriers from affected individuals in some cases.
3-O-methyldopa is a stable metabolite that does not degrade significantly in dried blood spots at ambient temperature, making DBS collection ideal for newborn screening and population-based testing. The dried format enables convenient home-based collection and postal transit without loss of analytical sensitivity.
Dried blood spot collection from a heel prick is rapid, minimally invasive, and suitable for all ages including newborns. Samples are stable at room temperature during collection, transport, and storage, facilitating integration into universal newborn screening programmes and point-of-care testing systems.
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