Abstract Inosine triphosphate pyrophosphohydrolase (ITPase) deficiency is a common inherited condition char- acterized by the abnormal accumulation of inosine tri- phosphate (ITP) in erythrocytes. The genetic basis and pathological consequences of ITPase deficiency are un- known. We have characterized the genomic structure of the ITPA gene, showing that it has eight exons. Five single nucleotide polymorphisms were identified, three silent (138G→A, 561G→A, 708G→A) and two associated with ITPase deficiency (94C→A, IVS2+21A→C). Homozy- gotes for the 94C→A missense mutation (Pro32 to Thr) had zero erythrocyte ITPase activity, whereas 94C→A heterozygotes averaged 22.5% of the control mean, a level of activity consistent with impaired subunit association of a dimeric enzyme. ITPase activity of IVS2+21A→C ho- mozygotes averaged 60% of the control mean. In order to explore further the relationship between mutations and enzyme activity, we examined the association between genotype and ITPase activity in 100 healthy controls. Ten subjects were heterozygous for 94C→A (allele frequency: 0.06), 24 were heterozygotes for IVS2+21A→C (allele frequency: 0.13) and two were compound heterozygous for these mutations. The activities of IVS2+21A→C het- erozygotes and 94C→A/IVS2+21A→C compound het- erozygotes were 60% and 10%, respectively, of the nor- mal control mean, suggesting that the intron mutation af- fects enzyme activity. In all cases when ITPase activity was below the normal range, one or both mutations were found. The ITPA genotype did not correspond to any iden- tifiable red cell phenotype. A possible relationship be- tween ITPase deficiency and increased drug toxicity of purine analogue drugs is proposed. Introduction Inosine triphosphate pyrophosphohydrolase (ITPase; EC3.6.1.19) catalyzes the pyrophosphohydrolysis of ino- sine triphosphate (ITP) to inosine monophosphate (IMP; Fig. 1). IMP synthesized by the de novo or salvage purine pathways occupies a pivotal position in purine metabo- lism. In nucleated cells, IMP is converted to ATP via adenosine monophosphate (AMP) or to GTP via guano- sine monophosphate (GMP). However, IMP may also be phosphorylated to ITP. The putative role of ITPase is to recycle purines trapped in the form of ITP and to protect the cell from the accumulation of “rogue” nucleotides, such as ITP, dITP or xanthosine triphosphate (XTP), that may be incorporated into RNA and DNA. The first patient with an accumulation of ITP in eryth- rocytes (RBCs) was reported by Vanderheiden (1964). Sub- sequent population studies showed (1) that 7 of 6,000 RBC samples contain high levels of ITP (Vanderheiden 1969) and (2) a biphasic distribution for ITPase activity in Cau- casian populations (Duley et al. 1990; Fraser et al. 1975; Holmes et al. 1979; Soder et al. 1976; van Waeg et al. 1988) with average activities of the lower group (5% of the population studied and presumed to be heterozygous- deficient) ranging from 20% to 50% of the high activity (presumed wildtype) group. Fraser et al. (1975) reported that the incidence of decreased ITPase activity was higher in schizophrenics and in a severely mentally retarded pop- ulation but this was not supported by a later study (Holmes et al. 1979). Another enigma has been the finding of pa- Satoshi Sumi · Anthony M. Marinaki · Monica Arenas · Lynette Fairbanks · Monsor Shobowale-Bakre · David C. Rees · Swee Lay Thein · Azhar Ansari · Jeremy Sanderson · Ronney A. De Abreu · H. Anne Simmonds · John A. Duley Genetic basis of inosine triphosphate pyrophosphohydrolase deficiency Hum Genet (2002) 111 : 360–367 DOI 10.1007/s00439-002-0798-z Received: 21 May 2002 / Accepted: 21 June 2002 / Published online: 15 August 2002 ORIGINAL INVESTIGATION S. Sumi · A.M. Marinaki (✉) · M. Arenas · L. Fairbanks · M. Shobowale-Bakre · H.A. Simmonds · J.A. Duley Purine Research Unit, Department of Chemical Pathology, 5th Floor Thomas Guy House, Guy’s and St. Thomas’ Hospitals, London, SE1 9RT, UK e-mail: tony.marinaki@kcl.ac.uk, Tel.: +44-20-79555000 ext. 3392, Fax: +44-20-79554015 D.C. Rees · S.L. Thein Department of Haematology, King’s College Hospital, Denmark Hill, London, UK A. Ansari · J. Sanderson Department of Gastroenterology, Guy’s and St. Thomas’ Hospitals, London, UK R.A. De Abreu Center for Pediatric Oncology S.E. Netherlands, University Medical Center St. Radboud, Department of Pediatrics, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands © Springer-Verlag 2002