Mild/moderate phenotypes in AADC deficiency: Focus on the aromatic amino acid decarboxylase protein.

AADC deficiency is a severe neurometabolic inherited rare disorder due to the absence or decrease of dopamine and serotonin levels, causing deep motor and neurodevelopmental impairments. The disease is often fatal in the first decade of life, and pharmacological treatments (dopamine agonists, pyridoxine, and monoamine oxidase inhibitors as the first-line choices) can only alleviate the symptoms. Gene therapy surgery is now available for severe patients in the European Union and the United Kingdom, and follow-up data witness encouraging improvements. In the past few years, mostly due to the increased awareness and knowledge of AADC deficiency, together with newborn screening programs and advancements in methods for genetic diagnosis, the number of mild/moderate phenotypes of AADC deficiency patients has increased to 12% of the total. A review of the genotypes (homozygous/compound heterozygous) of AADC deficiency mild/moderate patients is presented here. The pathogenicity classification of each genetic variant is discussed. Then, we focused on the type of AADC protein possessed by patients and on the predictable structural score of the homodimeric/heterodimeric species of each protein variant. Since the terminology used for genetic and protein variants is the same, we highlighted how it could be misleading. We analyzed the loss-of-function as a fold-change decrease of activity of the recombinant purified AADC enzyme(s) theoretically synthesized by mild/moderate patients. A minimal residual kcat of 8% and/or kcat/Km of 1% seems necessary to avoid a severe disease manifestation. Overall, this cluster of mild/moderate patients needs consideration for a more appropriate and aimed therapeutic approach.

Exome sequencing data screening to identify undiagnosed Aromatic l-amino acid decarboxylase deficiency in neurodevelopmental disorders.

Aromatic l-amino acid decarboxylase (AADC) deficiency is a rare autosomal recessive neurometabolic disorder caused by biallelic pathogenic variants in the DDC gene and mainly characterized by developmental delay, hypotonia, and oculogyric crises. Early diagnosis is crucial for correct patient management; however, many patients remain misdiagnosed or undiagnosed due to the rarity and clinical heterogeneity of the disorder especially in the milder forms. Here, we applied exome sequencing approach by screening 2000 paediatric patients with neurodevelopmental disorders to identify possible new AADC variants and AADC deficiency patients. We identified five distinct DDC variants in two unrelated individuals. Patient #1 harboured two compound heterozygous DDC variants: c.436-12T > C and c.435 + 24A>C and presented with psychomotor delay, tonic spasms, and hyperreactivity. Patient #2 had three homozygous AADC variants: c.1385G > A; p.Arg462Gln, c.234C > T; p.Ala78 = , and c.201 + 37A > G and presented with developmental delay and myoclonic seizures. The variants were classified as benign class I variants and therefore non-causative according to the ACMG/AMP guidelines. Since the AADC protein is a structural and functional obligate homodimer, we evaluated the possible AADC polypeptide chain combinations in the two patients and determined the effects resulting from the amino acid substitution Arg462Gln. Our patients carrying DDC variants presented clinical manifestations not precisely overlapped to the classical symptoms exhibited by the most severe AADC deficiency cases. However, screening data derived from exome sequencing in patients featuring wide-range symptoms related to neurodevelopmental disorders may help to identify AADC deficiency patients, especially when applied to larger cohorts.

An attenuated, adult case of AADC deficiency demonstrated by protein characterization.

A case of an adult with borderline AADC deficiency symptoms is presented here. Genetic analysis revealed that the patient carries two AADC variants (NM_000790.3: c.1040G > A and c.679G > C) in compound heterozygosis, resulting in p.Arg347Gln and p.Glu227Gln amino acid alterations. While p.Arg347Gln is a known pathogenic variant, p.Glu227Gln is unknown. Combining clinical features to bioinformatic and molecular characterization of the AADC protein population of the patient (p.Arg347Gln/p.Arg347Gln homodimer, p.Glu227Gln/p.Glu227Gln homodimer, and p.Glu227Gln/p.Arg347Gln heterodimer), we determined that: i) the p.Arg347Gln/p.Arg347Gln homodimer is inactive since the alteration affects a catalytically essential structural element at the active site, ii) the p.Glu227Gln/p.Glu227Gln homodimer is as active as the wild-type AADC since the alteration occurs at the surface and does not change the chemical nature of the amino acid, and iii) the p.Glu227Gln/p.Arg347Gln heterodimer has a catalytic efficiency 75% that of the wild-type since only one of the two active sites is compromised, thus demonstrating a positive complementation. By this approach, the molecular basis for the mild presentation of the disease is provided, and the experience made can also be useful for personalized therapeutic decisions in other mild AADC deficiency patients. Interestingly, in the last few years, many previously undiagnosed or misdiagnosed patients have been identified as mild cases of AADC deficiency, expanding the phenotype of this neurotransmitter disease.