The novel P330L pathogenic variant of aromatic amino acid decarboxylase maps on the catalytic flexible loop underlying its crucial role.

Aromatic amino acid decarboxylase (AADC) deficiency is a rare monogenic disease, often fatal in the first decade, causing severe intellectual disability, movement disorders and autonomic dysfunction. It is due to mutations in the gene coding for the AADC enzyme responsible for the synthesis of dopamine and serotonin. Using whole exome sequencing, we have identified a novel homozygous c.989C > T (p.Pro330Leu) variant of AADC causing AADC deficiency. Pro330 is part of an essential structural and functional element: the flexible catalytic loop suggested to cover the active site as a lid and properly position the catalytic residues. Our investigations provide evidence that Pro330 concurs in the achievement of an optimal catalytic competence. Through a combination of bioinformatic approaches, dynamic light scattering measurements, limited proteolysis experiments, spectroscopic and in solution analyses, we demonstrate that the substitution of Pro330 with Leu, although not determining gross conformational changes, results in an enzymatic species that is highly affected in catalysis with a decarboxylase catalytic efficiency decreased by 674- and 194-fold for the two aromatic substrates. This defect does not lead to active site structural disassembling, nor to the inability to bind the pyridoxal 5'-phosphate (PLP) cofactor. The molecular basis for the pathogenic effect of this variant is rather due to a mispositioning of the catalytically competent external aldimine intermediate, as corroborated by spectroscopic analyses and pH dependence of the kinetic parameters. Altogether, we determined the structural basis for the severity of the manifestation of AADC deficiency in this patient and discussed the rationale for a precision therapy.

The case of fatal acute hemorrhagic necrotizing encephalitis in a two-month-old boy with Covid-19.

SARS-CoV-2 infection in healthy children is usually benign. However, severe, life-threatening cases have previously been reported, notably in infants. We must be aware that data on the natural history of COVID-19 are still full of gaps, especially as far as the pediatric population is concerned. Therefore, it is important to describe rare manifestations of SARS-CoV-2 acute infection in children. Here we present the case of acute hemorrhagic necrotizing encephalitis (AHNE) in a previously healthy, 2-month-old male infant with SARS-CoV-2 infection. After 2 days of fever with signs of respiratory tract infection, neurological manifestations appeared: irritability, nystagmus, agitation then apathy. As a consequence of apnea, he required emergent intubation and was transferred to our PICU. Brain MRI revealed diffuse areas of oedema associated with numerous symmetrical changes with punctate hemorrhages in basal ganglia, thalami, brainstem, and cerebral gray matter. CSF was clear with pleocytosis 484 cells/µl, elevated lactic acid and protein. Despite broad microbiological testing, only SARS-CoV2 was detected in PCR nasal swab. Therefore, acute hemorrhagic necrotizing encephalitis (AHNE) as a result of COVID-19 was the most probable diagnosis. The outcome was unfavorable - brain death was confirmed, life support was withdrawn.