A comprehensive picture of the mutations associated with aromatic amino acid decarboxylase deficiency: from molecular mechanisms to therapy implications

Abstract

Dopa decarboxylase (DDC), or aromatic amino acid decarboxylase (AADC), is a pyridoxal 5'-phosphate enzyme responsible for the production of the neurotransmitters dopamine and serotonin. Deficit of this enzyme causes AADC deficiency, an inherited neurometabolic disorder. To date, 18 missense homozygous mutations have been identified through genetic screening in ∼80 patients. However, little is known about the mechanism(s) by which mutations cause disease. Here we investigated the impact of these pathogenic mutations and of an artificial one on the conformation and the activity of wild-type DDC by a combined approach of bioinformatic, spectroscopic and kinetic analyses. All mutations reduce the kcat value, and, except the mutation R347Q, alter the tertiary structure, as revealed by an increased hydrophobic surface and a decreased near-UV circular dichroism signal. The integrated analysis of the structural and functional consequences of each mutation strongly suggests that the reason underlying the pathogenicity of the majority of disease-causing mutations is the incorrect apo-holo conversion. In fact, the most remarkable effects are seen upon mutation of residues His70, His72, Tyr79, Phe80, Pro81, Arg462 and Arg447 mapping to or directly interacting with loop1, a structural key element involved in the apo-holo switch. Instead, different mechanisms are responsible for the pathogenicity of R347Q, a mere catalytic mutation, and of L38P and A110Q mutations causing structural-functional defects. These are due to local perturbation transmitted to the active site, as predicted by molecular dynamic analyses. Overall, the results not only give comprehensive molecular insights into AADC deficiency, but also provide an experimental framework to suggest appropriate therapeutic treatments.

Liver peroxisomal alanine:glyoxylate aminotransferase and the effects of mutations associated with Primary Hyperoxaluria Type I: An overview

“…Among them, 79 are missense mutations that lead to single amino acid substitutions and concern residues spread over the entire protein structure. As already found for many other disease-causing mutations [8,30], they often map highly conserved residues that are supposed to be crucial for the enzyme structure and function. All the known pathogenic amino acid substitutions of AGT present in PH1 patients are listed in Fig.…”

“…In many cases the genetic defects underlying the disease are missense mutations, whose impact at the protein level is not always easy to predict. Several efforts have been made to delineate the clinical features, to improve the diagnostic tests and to develop therapeutic strategies for these diseases as well as to understand the mechanism(s) by which the mutations induce the loss of functionality of PLP-enzymes [3][4][5][6][7][8][9]. Primary Hyperoxaluria Type 1 (PH1), an autosomal recessive disorder caused by the deficiency of alanine:glyoxylate aminotransferase (AGT) (EC 2.6.1.44) is one of the most investigated.…”

Oligomeric State and Thermal Stability of Apo- and Holo- Human Ornithine δ-Aminotransferase

“…1c), thus indicating that the apo-form exposes more hydrophobic patches with respect to the holo-form. Thus, unlike other PLP-dependent enzymes, for which significant changes in the tertiary structure associated with the apo-to-holo transition were reported [26, 27], the binding of PLP to apo-hOAT does not seem to alter the microenvironment of aromatic amino acids of the enzyme, but only induce a surface alteration of the protein, as suggested by the ANS emission fluorescence spectra.…”

Consensus guideline for the diagnosis and treatment of aromatic l-amino acid decarboxylase (AADC) deficiency

“…More recently, a patient with other variants affecting the L-Dopa binding site p.[R347Q];[R160W], c.[1040G > A];[478C > T], also showed a good response to L-Dopa [19]. Currently, therapeutic implications of other DDC gene variants in AADCD are being investigated [48]. …”