セピアプテリン還元酵素（ＳＲ）欠損症

Sepiapterin reductase deficiency (SPD) is an extremely rare autosomal recessive neurotransmitter disorder caused by mutations in the sepiapterin reductase gene. Clinical features include motor and cognitive manifestations, and L-DOPA/Carbidopa is the main therapy available. To date, it is not known if prenatal diagnosis of SPD may improve the motor and cognitive outcomes by prompt correction of dopamine and serotonin deficiency after birth. We describe the motor and cognitive profiles of 2 siblings with SPD with a follow-up of 9 and 6 years, respectively. The diagnosis of the second patient was prenatally performed and the patient was treated by the neonatal age. Conversely, the first patient was treated by 10 months of age. Early treatment with L-DOPA/Carbidopa seems to be effective in improving motor outcomes but had no impact on cognitive impairment. Protocols for further treatment attempts, potentially also started before birth, are needed to provide conclusive results.

Sepiapterin reductase (SPR) catalyzes several key steps in the biosynthesis of tetrahydrobiopterin (BH4), an essential cofactor for nitric oxide synthases and aromatic amino acid hydroxylases, and therefore for neurotransmitter production. Although several reductases-including carbonyl reductase 1 (CBR1), aldose reductase (AKR1B1), and AKR1C3-can substitute for SPR activity in vitro, their physiological significance remains unresolved. This study examines AKR1C3 as a component of an alternative BH4-generating pathway and evaluates its capacity to compensate for BH4 loss under diminished SPR activity. In vitro assays identified 2'-OXPH4 as the primary product of AKR1C3, redirecting pathway flux away from the canonical 1'-OXPH4 intermediate and the sepiapterin-salvage pathway. To assess the occurrence and efficiency of this route in cells, we generated SPR-knockout (SPR-KO) cell and evaluated pathway products in SPR-KO and wild-type (WT) backgrounds. In WT cells neither AKR1C3 nor SPR overexpression altered BH4 synthesis, indicating that neither enzyme is rate-limiting. In contrast, AKR1C3 increased BH4 levels in SPR-KO cells, while inhibiting sepiapterin production, revealing that AKR1C3 becomes functionally engaged only when SPR activity is decreased. Based on relative enzyme abundance, AKR1C3 and SPR exhibited comparable catalytic efficiency in this context. Importantly, AKR1C3-mediated BH4 production was sufficient to sustain tyrosine hydroxylase (TH) activity in SPR-KO cells, as demonstrated by L-DOPA formation. These findings establish AKR1C3-driven 2'-OXPH4 synthesis as a bona fide, inducible pathway capable of maintaining BH4 levels when SPR activity is limiting. This alternative path provides a compelling therapeutic target and introduces a new diagnostic consideration for patients with diminished SPR activity.

BACKGROUND: Neurotransmitter disorders are a group of heterogeneous conditions that comprise defects in synthesis, transport, receptor binding, and degradation of neurochemical messengers. These rare disorders range from mild intermittent dystonia to lethal encephalopathies. The natural history and clinical presentation remain far from established. OBJECTIVES: The study was conducted between October 2015 and September 2024. This study aims to describe the spectrum of clinical presentation, laboratory, imaging features, and genetic profiles of children diagnosed with neurotransmitter disorders and to assess the treatment modalities and clinical outcomes in these children. RESULTS: Among 29 patients, the median age was 12 months, with a male predominance. Positive family history was noted in 9 cases. The most frequent presentation was global developmental delay (GDD), dystonia, and seizures with autonomic disturbances, with diurnal variation. Various subcategories of neurotransmitter disorders are aromatic L amino acid decarboxylase deficiency-7 cases, tyrosine hydroxylase deficiency-3 cases, dopamine transporter deficiency syndrome-1 case, vesicular monoamine transporter 2 deficiency (VMAT2)-2 cases, GTP cyclohydrolase type deficiency-1 case, 6-pyruvoyl-tetrahydropterin synthase deficiency-1 case, dihydropteridine reductase deficiency-3, sepiapterin reductase deficiency-1 case, glycine encephalopathy-1 case, FOLR1-related cerebral folate transport deficiency-3 cases, and succinic semialdehyde dehydrogenase deficiency-5 cases. Metabolic workups were normal in all cases, with elevated phenylalanine levels in tandem mass spectrometry (TMS) in 5 children. Neuroimaging and electroencephalogram (EEG) were abnormal in 7 and 5 children, respectively. Multi-pronged and early treatment ensured better outcomes in these children. CONCLUSION: The most common type of neurotransmitter disorder in our series was aromatic L-amino acid decarboxylase deficiency, with the most common presentation being global developmental delay and dystonia.

BACKGROUND: Sepiapterin reductase deficiency (SRD) is a very rare psychomotor disorder related to enzyme defects in synthesizing tetrahydrobiopterin (BH4) with a spectrum of symptoms. The most common of which are developmental delay and hypotonia. To elucidate the genetic cause of SRD, the patient was analyzed by whole-exome sequencing (WES) followed by mutation analysis. METHODS: A complete clinical examination was performed by a pediatric neurologist. Brain imaging and a thorough neuro-metabolic investigation were applied along with biochemical tests including high-performance liquid chromatography (HPLC) to quantify the concentrations of cerebrospinal fluid (CSF) pterins. Genomic DNA was extracted and evaluated through WES. This was followed by bioinformatic analysis of mutated sepiapterin reductase (SPR) protein structure and identification of the functional protein amino acids. RESULTS: Biochemical analysis of biogenic amines of CSF with HPLC showed very low levels of homovanillic acid and 5-hydroxyindolacetic acid in favor of neurotransmitter metabolism disorder. WES analysis displayed a homozygous nonsynonymous variant in exon 3 of the SPR gene (C.655C>T, p.Arg219Ter). The molecular graphic of SPR protein structure with 4HWK PDB code was generated by MOE software in comparison with P.Arg219* mutated protein which is predicted by a Swiss model homology modeling server determining the ligand-binding site residues using COFACTOR software and indicated that the (R219*) mutation destroyed the ligand-binding site from the position of 219. Another important codon including 254 and 256 positions is omitted. CONCLUSION: Whole exome sequencing and bioinformatic analysis could overcome lengthy, expensive, and emotional diagnostic adventures of challenging neurodevelopmental cases leading to improved management and prevention of irreversible side effects.