Aminoacylase 1 is a zinc-binding enzyme which hydrolyzes N-acetyl amino acids into the free amino acid and acetic acid. Deficiency of aminoacylase 1 due to mutations in the aminoacylase 1 (ACY1) gene follows an autosomal-recessive trait of inheritance and is characterized by accumulation of N-acetyl amino acids in the urine. In affected individuals neurological findings such as febrile seizures, delay of psychomotor development and moderate mental retardation have been reported. Except for one missense mutation which has been studied in Escherichia coli, mutations underlying aminoacylase 1 deficiency have not been characterized so far. This has prompted us to approach expression studies of all mutations known to occur in aminoacylase 1 deficient individuals in a human cell line (HEK293), thus providing the authentic human machinery for posttranslational modifications. Mutations were inserted using site directed mutagenesis and aminoacylase 1 enzyme activity was assessed in cells overexpressing aminoacylase 1, using mainly the natural high affinity substrate N-acetyl methionine. Overexpression of the wild type enzyme in HEK293 cells resulted in an approximately 50-fold increase of the aminoacylase 1 activity of homogenized cells. Most mutations resulted in a nearly complete loss of enzyme function. Notably, the two newly discovered mutations p.Arg378Trp, p.Arg378Gln and the mutation p.Arg393His yielded considerable residual activity of the enzyme, which is tentatively explained by their intramolecular localization and molecular characteristics. In contrast to aminoacylase 1 variants which showed no detectable aminoacylase 1 activity, aminoacylase 1 proteins with the mutations p.Arg378Trp, p.Arg378Gln and p.Arg393His were also detected in Western blot analysis. Investigations of the molecular bases of additional cases of aminoacylase 1 deficiency contribute to a better understanding of this inborn error of metabolism whose clinical significance and long-term consequences remain to be elucidated.

Aminoacylase 1 (ACY1) deficiency is a recently described inborn error of metabolism. Most of the patients reported so far have presented with rather heterogeneous neurologic symptoms. At this moment, it is not clear whether ACY1 deficiency represents a true metabolic disease with a causal relationship between the enzyme defect and the clinical phenotype or merely a biochemical abnormality. Here we present a patient identified in the course of selective screening for inborn errors of metabolism (IEM). The patient was diagnosed with autistic syndrome and admitted to the Children's Memorial Health Institute (CMHI) for metabolic evaluation. Organic acid analysis using gas chromatography-mass spectrometry (GC-MS) revealed increased urinary excretion of several N-acetylated amino acids, including the derivatives of methionine, glutamic acid, alanine, glycine, leucine, isoleucine, and valine. In Epstein-Barr virus (EBV)-transformed lymphoblasts, ACY1 activity was deficient. The mutation analysis showed a homozygous c.1057C>T transition, predicting a p.Arg353Cys substitution. Both parents were heterozygous for the mutation and had normal results in the organic acid analysis using GC-MS. This article reports the findings of an ACY1-deficient patient presenting with autistic features.

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