We report for the first time an autosomal recessive inborn error of de novo purine synthesis (DNPS)-PAICS deficiency. We investigated two siblings from the Faroe Islands born with multiple malformations resulting in early neonatal death. Genetic analysis of affected individuals revealed a homozygous missense mutation in PAICS (c.158A>G; p.Lys53Arg) that affects the structure of the catalytic site of the bifunctional enzyme phosphoribosylaminoimidazole carboxylase (AIRC, EC 4.1.1.21)/phosphoribosylaminoimidazole succinocarboxamide synthetase (SAICARS, EC 6.3.2.6) (PAICS). The mutation reduced the catalytic activity of PAICS in heterozygous carrier and patient skin fibroblasts to approximately 50 and 10% of control levels, respectively. The catalytic activity of the corresponding recombinant enzyme protein carrying the mutation p.Lys53Arg expressed and purified from E. coli was reduced to approximately 25% of the wild-type enzyme. Similar to other two known DNPS defects-adenylosuccinate lyase deficiency and AICA-ribosiduria-the PAICS mutation prevented purinosome formation in the patient's skin fibroblasts, and this phenotype was corrected by transfection with the wild-type but not the mutated PAICS. Although aminoimidazole ribotide (AIR) and aminoimidazole riboside (AIr), the enzyme substrates that are predicted to accumulate in PAICS deficiency, were not detected in patient's fibroblasts, the cytotoxic effect of AIr on various cell lines was demonstrated. PAICS deficiency is a newly described disease that enhances our understanding of the DNPS pathway and should be considered in the diagnosis of families with recurrent spontaneous abortion or early neonatal death.

CZ OPENSCREEN Institute of Molecular Genetics Czech Academy of Sciences 140 00 Prague Czech Republic

Department of Biochemistry University of Cambridge CB2 1TN Cambridge UK

Department of Clinical Genetics Rigshospitalet 2100 Copenhagen Denmark

FarGen The Genetic Biobank of the Faroe Islands FO 100 Tórshavn Faroe Islands

Laboratory Embryology and Genetics of Congenital Malformation INSERM UMR1163 Imagine Institute Université de Paris F 75015 Paris France

Pediatric Unit Medical Department The Faroese Hospital System FO 100 Tórshavn Faroe Islands

Research Unit for Rare Diseases Department of Paediatrics and Adolescent Medicine 1st Faculty of Medicine Charles University and General University Hospital 12808 Prague Czech Republic

Research Unit for Rare Diseases Department of Paediatrics and Adolescent Medicine 1st Faculty of Medicine Charles University and General University Hospital 12808 Prague Czech Republic Section on Nephrology Wake Forest School of Medicine 271 03 Winston Salem NC USA

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