芳香族L－アミノ酸脱炭酸酵素欠損症

Aromatic L-amino acid decarboxylase (AADC) deficiency (Online Mendelian Inheritance in Man #608643) is a rare autosomal recessive neurometabolic disorder caused by pathogenic variants in the DDC gene, leading to impaired enzyme activity. Affected individuals typically develop symptoms in early infancy, including truncal hypotonia, global developmental delay and oculogyric crises, and the condition is generally associated with a severe clinical course and poor prognosis.We report a patient with a mild and atypical phenotype of AADC deficiency complicated by sensorineural hearing loss and oculocutaneous albinism. The clinical course and response to treatment are described. This unusual presentation raises the possibility of coexisting conditions contributing to the phenotype.This case highlights the clinical variability of AADC deficiency and underscores the importance of comprehensive genetic and biochemical investigations to achieve an accurate diagnosis and inform personalised management.

The inherent reactivity of free- or enzyme-linked pyridoxal 5'-phosphate (PLP) in the presence of catechol compounds with an aminic group is known and leads, after the formation of a Schiff base, to an irreversible cyclic adduct, called the Pictet-Spengler condensation product. In PLP-dependent aromatic amino acid decarboxylase (AADC), the protein scaffold protects PLP, preventing its leakage as a cyclic adduct complexed with the L-Dopa substrate. Here, we demonstrate that a group of AADC deficiency enzyme variants can undergo this unproductive reaction at a high rate. By using computational modeling, spectroscopic and functional studies with twelve pathogenic variants, we uncovered the basis of the variants' ability to synthesize this adduct. Since the metabolic conditions present in patients (high PLP amount as medical treatment and high L-Dopa concentrations as a consequence of enzyme impairment) may trigger Pictet-Spengler adduct synthesis, we modeled the possible oxidative effects played by this compound on α-synuclein (Syn) in solution and in neuroblastoma SH-SY5Y cells, given the known sensitivity of this protein to dopamine or dopa-related compounds. By multiple experimental approaches including chromatographic analysis, colorimetric assays, native mass spectrometry, dynamic light scattering and isothermal titration calorimetry, transmission electron microscopy, as well as cytotoxicity assays, we determined that the Pictet-Spengler adduct is not toxic to cells, exhibits a dose dependent inhibition of Syn fibrillation, and promotes Syn oxidation. These observations suggest that the AADC pathogenic variants undergoing Pictet-Spengler condensation could synthesize the biologically active adduct enhancing oxidative stress in neuronal cells, thereby contributing to the worsening of the phenotype.

Background/Objectives: Aromatic L-amino acid decarboxylase deficiency (AADC-D; OMIM #608643) is a rare autosomal recessive neurometabolic disorder caused by pathogenic variants in the DDC gene, leading to impaired of monoamine neurotransmitter biosynthesis. AADC, a pyridoxal-5'-phosphate (PLP)-dependent enzyme, catalyzes the conversion of L-dopa and 5-hydroxytryptophan (5-HTP) to dopamine and serotonin, respectively. Early diagnosis remains challenging due to the limited specificity of current biochemical approaches. This study aimed to evaluate plasma AADC enzyme activity using these physiological substrates by High-Performance Liquid Chromatography (HPLC)-based method and assess its potential utility in the biochemical diagnosis of AADC deficiency. Methods: Plasma AADC activity was quantified using physiological substrates (L-dopa and 5-HTP) by HPLC with electrochemical and fluorescence detection. Sanger sequencing of the DDC gene was performed in two suspected patients to identify pathogenic variants. Results: Two genetically confirmed AADC-D patients demonstrated reduced enzyme activity. Using L-dopa as substrate, enzyme activity in patients was 12.4 and 26.1 pmol/min/mL, both below the published reference interval (36-129 pmol/min/mL). Using 5-HTP as substrate, enzyme activity was 1.5 and 5.1 pmol/min/mL; Patient 1 showed activity below the reference interval (2.0-7.1 pmol/min/mL), while Patient 2 demonstrated activity within the lower range of reported values. Reduced enzyme activity was consistent with the clinical features and molecular findings with identification of pathogenic variants in the DDC gene (c.175G>A and c.714+4A>T). Conclusions: Plasma AADC activity measurement demonstrates potential as a functional biochemical biomarker that augments molecular genetic testing in the biochemical evaluation of AADC deficiency. Further studies involving larger patient cohorts are required to further evaluate its diagnostic performance and broader clinical applicability.

Aromatic L-amino acid decarboxylase (AADC), encoded by DDC gene, catalyzes the final step in dopamine and serotonin biosynthesis. Pathogenic DDC variants cause AADC deficiency, a severe neurometabolic disorder. This study aimed to elucidate previously uninvestigated DDC variant combinations: p.Pro47Ala/p.Arg447Cys and p.Cys261Phe/p.Gly354Ser, with respect to structural and functional characteristics. Recombinant heterodimeric and homodimeric AADC proteins were expressed in Escherichia coli and purified. Enzymatic activity was quantified via ion-pair reversed-phase HPLC with photodiode detection, while structural changes were analyzed using circular dichroism and molecular modelling. Functional characterization revealed that p.Pro47Ala, p.Gly354Ser, and p.Arg447Cys homodimers showed marked loss of activity towards both L-DOPA (88%, 97% and >99%, respectively) and L-5-hydroxytryptophan (88%, 99%, >99%, respectively), whereas p.Cys261Phe retained substantial activity (82% for L-DOPA and 75% for L-5-hydroxytryptophan). Kinetic analysis indicated a mild to severe decrease in substrate affinity for all homodimers, particularly for p.Arg447Cys. Furthermore, heterodimeric combinations showed positive complementation effects on decarboxylase reaction for both heterodimers. Surprisingly, p.Cys261Phe/p.Gly354Ser also restored the wild-type activity with L-DOPA and regained 43% of L-5-hydroxytryptophan activity, whereas p.Pro47Ala/p.Arg447Cys was 15% and 10% active towards L-DOPA and L-5-hydroxytryptophan with respect to the wild-type. Our results indicate that the pathogenicity of AADC variants depends on structural combination, complementation effects in heterodimeric species, and residual activity of homodimers, highlighting the complexity of polypeptide chain interactions in compound heterozygotes.

Disorders of the phenylalanine-tyrosine-catecholamine axis are a clinically relevant group of neurometabolic conditions in which pathogenic variants in key enzymes impair dopamine and norepinephrine biosynthesis. Patients may present with movement disorders, autonomic dysfunction, developmental delay, and related neurobehavioral manifestations. In this narrative review, we synthesize the main enzymatic defects across the axis, focusing on phenylalanine hydroxylase, tyrosine hydroxylase, aromatic L-amino acid decarboxylase, and dopamine beta-hydroxylase. We describe how diagnostic practice has evolved from isolated biochemical assays to integrated approaches that link clinical phenotyping with targeted biochemical profiling and molecular confirmation. Genetic testing now supports diagnosis, treatment planning, and family counseling, while chromatographic and mass spectrometry-based methods remain essential for quantifying amino acids and neurotransmitter-related metabolites. We also discuss emerging biosensor-based strategies as a potential route to decentralized and minimally invasive monitoring.