Superactivity of human phosphoribosyl pyrophosphate synthetase due to altered regulation by nucleotide inhibitors and inorganic phosphate

Contribution of Model Organisms to Investigating the Far-Reaching Consequences of PRPP Metabolism on Human Health and Well-Being

Phosphoribosyl pyrophosphate synthetase (PRS EC 2.7.6.1) is a rate-limiting enzyme that irreversibly catalyzes the formation of phosphoribosyl pyrophosphate (PRPP) from ribose-5-phosphate and adenosine triphosphate (ATP). This key metabolite is required for the synthesis of purine and pyrimidine nucleotides, the two aromatic amino acids histidine and tryptophan, the cofactors nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+), all of which are essential for various life processes. Despite its ubiquity and essential nature across the plant and animal kingdoms, PRPP synthetase displays species-specific characteristics regarding the number of gene copies and architecture permitting interaction with other areas of cellular metabolism. The impact of mutated PRS genes in the model eukaryote Saccharomyces cerevisiae on cell signalling and metabolism may be relevant to the human neuropathies associated with PRPS mutations. Human PRPS1 and PRPS2 gene products are implicated in drug resistance associated with recurrent acute lymphoblastic leukaemia and progression of colorectal cancer and hepatocellular carcinoma. The investigation of PRPP metabolism in accepted model organisms, e.g., yeast and zebrafish, has the potential to reveal novel drug targets for treating at least some of the diseases, often characterized by overlapping symptoms, such as Arts syndrome and respiratory infections, and uncover the significance and relevance of human PRPS in disease diagnosis, management, and treatment.

A novel mutation in gene of PRPS1 in a young Chinese woman with X-linked gout: a case report and review of the literature

Pyrophosphate synthetase-1(PRS-1) is a crucial enzyme that catalyzes the synthesis of phosphoribosylpyrophosphate (PRPP) with substrate: adenosine triphosphate (ATP) and ribose-5-phophate(R5P) in the de novo pathways of purine and pyrimidine nucleotide synthesis. Mutation in PRPS1 can result in a series of diseases of purine metabolism, which includes PRS-1 superactivity. The common clinical phenotypes are hyperuricemia and hyperuricosuria. We identified a novel missense mutation in X-chromosomal gene PRPS1 in a young Chinese woman while her mother has heterogeneous genotype and phenotype. A 24-year-old Chinese female patient suffered hyperuricemia, gout, and recurrent hyperpyrexia for more than 6 years, and then was diagnosed with hyperandrogenism, insulin resistance (IR), and polycystic ovary syndrome (PCOS). A novel missense mutation, c.521(exon)G>T, p.(Gly174Val) was detected by next-generation sequencing (NGS) and confirmed by Sanger sequencing in the patient and her parents. Interestingly, her mother has the same heterozygous missense mutation but without uric acid overproduction which can be explained by the phenomenon of the skewed X-chromosome inactivation. The substituted amino acid Val for Gly174 is positioned in the pyrophosphate (PPi) binding loop, and this mutation impacts the binding rate of Mg²⁺-ATP complex to PRS-1, thus the assembling of homodimer is affected by changed Val174 leading to the instability of the allosteric site. Our report highlights the X-linked inheritance of gout in females caused by mutation in PRPS1 accompanied with severe metabolic disorders and recurrent hyperpyrexia.

Mutations in PRPS1 causing syndromic or nonsyndromic hearing impairment: intrafamilial phenotypic variation complicates genetic counseling

BACKGROUND

PRPS1 encodes isoform I of phosphoribosylpyrophosphate synthetase (PRS-I), a key enzyme in nucleotide biosynthesis. Different missense mutations in PRPS1 cause a variety of disorders that include PRS-I superactivity, nonsyndromic sensorineural hearing impairment, Charcot-Marie-Tooth disease, and Arts syndrome. It has been proposed that each mutation would result in a specific phenotype, depending on its effects on the structure and function of the enzyme.

METHODS

Thirteen Spanish unrelated families segregating X-linked hearing impairment were screened for PRPS1 mutations by Sanger sequencing. In two positive pedigrees, segregation of mutations was studied, and clinical data from affected subjects were compared.

RESULTS

We report two novel missense mutations in PRPS1, p.Ile275Thr and p.Gly306Glu, which were found in the propositi of two unrelated Spanish families, both subjects presenting with nonsyndromic hearing impairment. Further investigation revealed syndromic features in other hemizygous carriers from one of the pedigrees. Sequencing of genes that are functionally related to PRPS1 did not reveal any candidate variant that might act as a phenotype modifier.

CONCLUSION

This case of intrafamilial phenotypic variation associated with a single PRPS1 mutation complicates the genotype-phenotype correlations, which makes genetic counseling of mutation carriers difficult because of the wide spectrum of severity of the associated disorders.

Association of PRPS1 Mutations with Disease Phenotypes

Phosphoribosylpyrophosphate synthetase 1 (PRPS1) codes for PRS-I enzyme that catalyzes the first step of nucleotide synthesis. PRPS1 gene mutations have been implicated in a number of human diseases. Recently, new mutations in PRPS1 have been identified that have been associated with novel phenotypes like diabetes insipidus expanding the spectrum of PRPS1-related diseases. The purpose of this review is to evaluate current literature on PRPS1-related syndromes and summarize potential therapies. The overexpression of PRPS1 results in PRS-I superactivity resulting in purine overproduction. Patients with PRS-I superactivity demonstrate uric acid overproduction, hypotonia, ataxia, neurodevelopment abnormalities, and postlingual hearing impairment. On the other hand, decreased activity leads to X-linked nonsyndromic sensorineural deafness (DFNX-2), Charcot-Marie-Tooth disease-5 (CMTX5), and Arts syndrome depending on the residual activity of PRS-I. Mild PRS-I deficiency (DFNX-2) results in non-syndromic progressive hearing loss whereas moderate PRS-I deficiency (CMTX5) and severe PRS-I deficiency (Arts syndrome) present with peripheral or optic neuropathy, prelingual progressive sensorineural hearing loss, and central nervous system impairment. Currently, purine replacement via S-adenosylmethionine (SAM) supplementation in patients with Arts syndrome appears to improve their condition. This suggests that SAM supplementation can alleviate symptoms of PRPS1 deficient patients and open new avenues of therapeutic intervention.

PRPS1 mutations: four distinct syndromes and potential treatment

Phosphoribosylpyrophosphate synthetases (PRSs) catalyze the first step of nucleotide synthesis. Nucleotides are central to cell function, being the building blocks of nucleic acids and serving as cofactors in cellular signaling and metabolism. With this in mind, it is remarkable that mutations in phosphoribosylpyrophosphate synthetase 1 (PRPS1), which is the most ubiquitously expressed gene of the three PRS genes, are compatible with life. Mutations described thus far in PRPS1 are all missense mutations that result in PRS-I superactivity or in variable levels of decreased activity, resulting in X-linked Charcot-Marie-Tooth disease-5 (CMTX5), Arts syndrome, and X-linked nonsyndromic sensorineural deafness (DFN2). Patients with PRS-I superactivity primarily present with uric acid overproduction, mental retardation, ataxia, hypotonia, and hearing impairment. Postlingual progressive hearing loss is found as an isolated feature in DFN2 patients. Patients with CMTX5 and Arts syndrome have peripheral neuropathy, including hearing impairment and optic atrophy. However, patients with Arts syndrome are more severely affected because they also have central neuropathy and an impaired immune system. The neurological phenotype in all four PRPS1-related disorders seems to result primarily from reduced levels of GTP and possibly other purine nucleotides including ATP, suggesting that these disorders belong to the same disease spectrum. Preliminary results of S-adenosylmethionine (SAM) supplementation in two Arts syndrome patients show improvement of their condition, indicating that SAM supplementation in the diet could alleviate some of the symptoms of patients with PRPS1 spectrum diseases by replenishing purine nucleotides (J.C., unpublished data).

Pediatric neurological syndromes and inborn errors of purine metabolism

This review is devised to gather the presently known inborn errors of purine metabolism that manifest neurological pediatric syndromes. The aim is to draw a comprehensive picture of these rare diseases, characterized by unexpected and often devastating neurological symptoms. Although investigated for many years, most purine metabolism disorders associated to psychomotor dysfunctions still hide the molecular link between the metabolic derangement and the neurological manifestations. This basically indicates that many of the actual functions of nucleosides and nucleotides in the development and function of several organs, in particular central nervous system, are still unknown. Both superactivity and deficiency of phosphoribosylpyrophosphate synthetase cause hereditary disorders characterized, in most cases, by neurological impairments. The deficiency of adenylosuccinate lyase and 5-amino-4-imidazolecarboxamide ribotide transformylase/IMP cyclohydrolase, both belonging to the de novo purine synthesis pathway, is also associated to severe neurological manifestations. Among catabolic enzymes, hyperactivity of ectosolic 5'-nucleotidase, as well as deficiency of purine nucleoside phosphorylase and adenosine deaminase also lead to syndromes affecting the central nervous system. The most severe pathologies are associated to the deficiency of the salvage pathway enzymes hypoxanthine-guanine phosphoribosyltransferase and deoxyguanosine kinase: the former due to an unexplained adverse effect exerted on the development and/or differentiation of dopaminergic neurons, the latter due to a clear impairment of mitochondrial functions. The assessment of hypo- or hyperuricemic conditions is suggestive of purine enzyme dysfunctions, but most disorders of purine metabolism may escape the clinical investigation because they are not associated to these metabolic derangements. This review may represent a starting point stimulating both scientists and physicians involved in the study of neurological dysfunctions caused by inborn errors of purine metabolism with the aim to find novel therapeutical approaches.

Arts syndrome is caused by loss-of-function mutations in PRPS1

Arts syndrome is an X-linked disorder characterized by mental retardation, early-onset hypotonia, ataxia, delayed motor development, hearing impairment, and optic atrophy. Linkage analysis in a Dutch family and an Australian family suggested that the candidate gene maps to Xq22.1-q24. Oligonucleotide microarray expression profiling of fibroblasts from two probands of the Dutch family revealed reduced expression levels of the phosphoribosyl pyrophosphate synthetase 1 gene (PRPS1). Subsequent sequencing of PRPS1 led to the identification of two different missense mutations, c.455T-->C (p.L152P) in the Dutch family and c.398A-->C (p.Q133P) in the Australian family. Both mutations result in a loss of phosphoribosyl pyrophosphate synthetase 1 activity, as was shown in silico by molecular modeling and was shown in vitro by phosphoribosyl pyrophosphate synthetase activity assays in erythrocytes and fibroblasts from patients. This is in contrast to the gain-of-function mutations in PRPS1 that were identified previously in PRPS-related gout. The loss-of-function mutations of PRPS1 likely result in impaired purine biosynthesis, which is supported by the undetectable hypoxanthine in urine and the reduced uric acid levels in serum from patients. To replenish low levels of purines, treatment with S-adenosylmethionine theoretically could have therapeutic efficacy, and a clinical trial involving the two affected Australian brothers is currently underway.

Purine and pyrimidine salvage in whole rat brain. Utilization of ATP-derived ribose-1-phosphate and 5-phosphoribosyl-1-pyrophosphate generated in experiments with dialyzed cell-free extracts

The object of this work stems from our previous studies on the mechanisms responsible of ribose-1-phosphate- and 5-phosphoribosyl-1-pyrophosphate-mediated nucleobase salvage and 5-fluorouracil activation in rat brain (Mascia, L., Cappiello M., Cherri, S., and Ipata, P. L. (2000) Biochim. Biophys. Acta 1474, 70-74; Mascia, L., Cotrufo, T., Cappiello, M., and Ipata, P. L. (1999) Biochim. Biophys. Acta 1472, 93-98). Here we show that when ATP at "physiological concentration" is added to dialyzed extracts of rat brain in the presence of natural nucleobases or 5-fluorouracil, adenine-, hypoxanthine-, guanine-, uracil-, and 5-fluorouracil-ribonucleotides are synthesized. The molecular mechanism of this peculiar nucleotide synthesis relies on the capacity of rat brain to salvage purine and pyrimidine bases by deriving ribose-1-phosphate and 5-phosphoribosyl-1-pyrophosphate from ATP even in the absence of added pentose or pentose phosphates. The levels of the two sugar phosphates formed are compatible with those of synthesized nucleotides. We propose that the ATP-mediated 5-phosphoribosyl-1-pyrophosphate synthesis occurs through the action of purine nucleoside phosphorylase, phosphopentomutase, and 5-phosphoribosyl-1-pyrophosphate synthetase. Furthering our previous observations on the effect of ATP in the 5-phosphoribosyl-1-pyrophosphate-mediated 5-fluorouracil activation in rat liver (Mascia, L., and Ipata, P. L. (2001) Biochem. Pharmacol. 62, 213-218), we now show that the ratio [5-phosphoribosyl-1-pyrophosphate]/[ATP] plays a major role in modulating adenine salvage in rat brain. On the basis of our in vitro results, we suggest that massive ATP degradation, as it occurs in brain during ischemia, might lead to an increase of the intracellular 5-phosphoribosyl-1-pyrophosphate and ribose-1-phosphate pools, to be utilized for nucleotide resynthesis during reperfusion.

Phosphoribosylpyrophosphate synthetase and the regulation of phosphoribosylpyrophosphate production in human cells

between purine nucleoside diphosphate inhibition and inorganic phosphate (Pi) activation; and intracellular concentration of the PRS1 isoform. The operation of additional determinants of rates of PRPP synthesis in human cells is suggested by: (1) multiple PRS isoforms with distinctive physical and kinetic properties; (2) nearly immediate activation of intracellular PRPP synthesis in response to mitogens, growth-promoters, and increased intracellular Mg2+ concentrations; (3) tissue-specific differences in PRS1 and PRS2 transcript and isoform expression; and (4) reversible association of PRS subunits with one another and/or with PRS-associated proteins (PAPs), as a result of which the catalytic and perhaps regulatory properties of PRS isoforms are modified.

Accelerated transcription of PRPS1 in X-linked overactivity of normal human phosphoribosylpyrophosphate synthetase

Phosphoribosylpyrophosphate (PRPP) synthetase (PRS) superactivity is an X-linked disorder characterized by gout with overproduction of purine nucleotides and uric acid. Study of the two X-linked PRS isoforms (PRS1 and PRS2) in cells from certain affected individuals has shown selectively increased concentrations of structurally normal PRS1 transcript and isoform, suggesting that this form of the disorder involves pretranslational dysregulation of PRPS1 expression and might be more appropriately termed overactivity of normal PRS. We applied Southern and Northern blot analyses and slot blotting of nuclear runoffs to delineate the process underlying aberrant PRPS1 expression in fibroblasts and lymphoblasts from patients with overactivity of normal PRS. Neither PRPS1 amplification nor altered stability or processing of PRS1 mRNA was identified, but PRPS1 transcription was increased relative to GAPDH (3- to 4-fold normal in fibroblasts; 1.9- to 2.4-fold in lymphoblasts) and PRPS2. Nearly coordinate relative increases in each process mediating transfer of genetic information from PRPS1 transcription to maximal PRS1 isoform expression in patient fibroblasts further supported the idea that accelerated PRPS1 transcription is the major aberration leading to PRS1 overexpression. In addition, modulated relative increases in PRS activities at suboptimal Pi concentration and in rates of PRPP and purine nucleotide synthesis in intact patient fibroblasts indicate that despite an intact allosteric mechanism of regulation of PRS activity, PRPS1 transcription is a major determinant of PRPP and purine synthesis. The genetic basis of disordered PRPS1 transcription remains unresolved; normal- and patient-derived PRPS1s share nucleotide sequence identity at least 850 base pairs 5' to the consensus transcription initiation site.

The genetic and functional basis of purine nucleotide feedback-resistant phosphoribosylpyrophosphate synthetase superactivity

The genetic and functional basis of phosphoribosylpyrophosphate synthetase (PRS) superactivity associated with purine nucleotide inhibitor-resistance was studied in six families with this X chromosome-linked purine metabolic and neurodevelopmental disorder. Cloning and sequencing of PRS1 and PRS2 cDNAs, derived from fibroblast total RNA of affected male patients by reverse transcription and PCR amplification, demonstrated that each PRS1 cDNA contained a distinctive single base substitution predicting a corresponding amino acid substitution in the PRS1 isoform. Overall, the array of substitutions encompassed a substantial portion of the translated sequence of PRS1 cDNA. Plasmid-mediated expression of variant PRS1 cDNAs in Escherichia coli BL21 (DE3/pLysS) yielded recombinant mutant PRS1s, which, in each case, displayed a pattern and magnitude of purine nucleoside diphosphate inhibitor-resistance comparable to that found in cells of the respective patient. Kinetic analysis of recombinant mutant PRS1s showed that widely dispersed point mutations in the X chromosome-linked PRPS1 gene encoding the PRS1 isoform result in alteration of the allosteric mechanisms regulating both enzyme inhibition by purine nucleotides and activation by inorganic phosphate. The functional consequences of these mutations provide a tenable basis for the enhanced production of phosphoribosylpyrophosphate, purine nucleotides, and uric acid that are the biochemical hallmarks of PRS superactivity.

An HPLC-linked assay of phosphoribosylpyrophosphate synthetase activity in the erythrocytes of adults and children with neurological disorders

A two-step non-radioactive method that uses reverse-phase high-performance liquid chromatography (RP-HPLC) is described for the determination of phosphoribosylpyrophosphate synthetase (EC 2.7.6.1) activity in human erythrocytes. The method is accurate and easily reproducible in different chromatographic systems; it is based on the quantification of phosphoribosylpyrophosphate by conversion into orotidine monophosphate and uridine monophosphate. Phosphoribosylpyrophosphate synthetase activity was determined in the erythrocytes of healthy adults and children, the latter showing significantly higher activity than the former. The enzyme activity assayed in children with different neurological disorders was significantly lower in patients with Rett syndrome than in control children or in autistic or mentally retarded patients.

Clinical and biochemical aspects of uric acid overproduction

Purine nucleotides are synthesized and degraded through a regulated series of reactions which end in the formation of uric acid. Increased uric acid synthesis may be the result of two major pathophysiological disorders: increased de novo purine synthesis and enhanced purine nucleotide degradation, both of which may be the result of an increased or decreased enzyme activity. In addition, some conditions and disorders associated with uric acid overproduction have been recognized as the result of increased ATP degradation or decreased synthesis of ATP. The clinical manifestations of the diseases leading to excess uric acid synthesis are heterogenous, but symptoms related to uric acid overproduction are always secondary to the precipitation of crystals in soft tissues, joints, and the kidney excretory system. In clinical practice, serum urate concentration and urinary uric acid excretion are used to assess uric acid synthesis, taking into account that a purine-rich diet can be a confounding variable. Quantification of uric acid precursors, such as adenosine, inosine, guanosine, hypoxanthine, and xanthine, in biological fluids and intracellular nucleotides has provided further insight into the metabolic disturbances underlying disorders associated with uric acid overproduction. Additional studies are necessary to define precisely the metabolic derangement in idiopathic uric acid overproduction and to assess fully the consequences of increased purine nucleotide degradation, such as free-radical formation, increased adenosine synthesis, and reduced synthesis of signal transducers.

A simplified method for the determination of phosphoribosylpyrophosphate synthetase activity in hemolysates

Methods currently employed for measurement of phosphoribosylpyrophosphate synthetase (ribophosphate pyrophosphokinase; PRPPs) activity are cumbersome and expensive, requiring an auxiliary enzyme reaction, a radioactive nucleobase and multiple incubations, or do not allow a complete kinetic study. The aim of the present study is to describe a simplified, single step, non-isotopic method for determination of PRPPs in hemolysate, appropriate for a complete screening of PRPPs activity disorders. Briefly, the charcoal-treated hemolysate is incubated with saturating amounts of substrates and P1 P5 di(adenosine 5') pentaphosphate (Ap5A), an inhibitor of human adenylate kinase, to prevent conversion of AMP to ADP. AMP generated in this reaction is then measured by HPLC. Adenylate kinase activity was fully inhibited by Ap5A, allowing the accurate determination of AMP. The method was sensitive and reproducible and mean and variance values compared closely with those reported using other, more complicated, assay procedures. The hyperbolic curve relating Pi concentration to initial reaction velocity was shifted to sigmoidal by addition of 0.02 mmol/l GDP which inhibited PRPPs activities only at inorganic phosphate concentrations < 2 mmol/l. This suggests that this method should provide sensitive and accurate screening for regulatory, as well as catalytic, defects underlying PRPPs superactivity.

Promoter regions of the human X-linked housekeeping genes PRPS1 and PRPS2 encoding phosphoribosylpyrophosphate synthetase subunit I and II isoforms

The 5' regions of the human phosphoribosylpyrophosphate synthetase subunit I and II genes (PRPS1 and PRPS2, respectively) were isolated and sequenced. A comparison of the nucleotide sequences between human and rat PRPS1 genes revealed that the sequences around the transcription initiation sites were conserved over 56 nucleotides, and that a TATA-like sequence, a CCAAT box and three putative Sp1 binding sites were present at almost the same positions in the GC-rich sequences. Two major transcription initiation sites were localized in the human PRPS1, one of the two was located 27 nucleotides downstream from the TATA-like sequence, while the upstream initiation site was in the TATA-like sequence. The promoter region of the human PRPS2 gene was also GC-rich and contained a TATA-like sequence, four Sp1 binding sites and a homopyrimidine stretch. The initiation sites were localized at 90 nucleotides upstream from the ATG initiation codon. Chloramphenicol acetyltransferase (CAT)/promoter fusion assays showed that a 2.0 kb region (human PRPS1) and a 1.1 kb region (human PRPS2) possessed the promoter activities in four cell lines. The CAT activities in the three human cell lines tended to correlate with the steady-state mRNA levels of the PRPS1 and PRPS2 genes. These results suggest that the 5' flanking regions cloned contribute to the cell-differential expression of these two genes.

Anti-proliferative activity of L-651,582 correlates with calcium-mediated regulation of nucleotide metabolism at phosphoribosyl pyrophosphate synthetase

L-651,582, 5-amino-[4-(4-chlorobenzoyl)-3,5-dichlorobenzyl]-1, 2,3-triazole-4-carboxamide, is an antiproliferative and antiparasitic agent which inhibits nucleotide metabolism in mammalian cells. The drug equivalently inhibited 3H-hypoxanthine, 14C-adenine, and 14C-formate incorporation into nucleotide pools in Madin-Darby bovine kidney (MDBK) cells, suggesting depletion of the supply of phosphoribosyl pyrophosphate, (PRPP), required for each of these independent pathways. Inhibition of nucleotide metabolism correlated with inhibition of proliferation for three cell types with differing sensitivities toward the drug. L-651,582 inhibited incorporation of 3H-hypoxanthine into nucleotide pools with either glucose, uridine, or ribose as carbon source suggesting a block at PRPP synthetase, rather than a block in a pathway supplying ribose-5-phosphate. PRPP synthetase was not inhibited directly by the compound, indicating regulation of the enzyme in intact cells. Drug treatment did not kill cells but reduced the fraction of cells in S and G2/M while increasing the population in G1. Inhibition of uptake of 45Ca was demonstrated at concentrations identical to those required for inhibition of nucleotide metabolism or proliferation. Inhibition of cellular PRPP biosynthesis rates were also observed using EGTA to lower calcium levels. These data suggest a previously unrecognized link between calcium entry, the regulation of nucleotide biosynthesis at PRPP synthetase, and the rate of proliferation of mammalian cells.

Molecular cloning and sequencing of human cDNA for phosphoribosyl pyrophosphate synthetase subunit II

cDNA clones for human phosphoribosyl pyrophosphate synthetase subunit II (PRS II) were isolated. The five overlapping clones contained 2457 base pairs (bp) covering a 954-bp complete coding region for 318 amino acid residues. Homologies between human and rat PRS II were 99% of the amino acids and 88% of the nucleotides in the coding region. This amino acid homology seems to be the highest so far reported for enzymes involved in nucleotide metabolism and glycolysis. The highly conserved structure may be required for unique catalysis and rigid regulation of this enzyme.

Localization of human phosphoribosylpyrophosphate synthetase subunit I and II genes (PRPS1 and PRPS2) to different regions of the X chromosome and assignment of two PRPS1-related genes to autosomes

Complementary DNA clones for phosphoribosylpyrophosphate synthetase subunits I and II (PRS I and PRS II) were used to determine the chromosomal localization of the corresponding human genes. Southern blot analysis of genomic DNAs isolated from human placenta and a panel of human-mouse somatic cell hybrids revealed that the rat PRS I cDNA probe detected at least five human specific DNA segments (23, 20, 14.5, 6.7, and 4.3 kb) in BamHI digests. The 23-, 14.5-, and 6.7-kb DNA segments were detected only if the hybrids contained human chromosome X or translocation chromosome 7p+ (7qter greater than 7p22::Xq21 greater than Xqter), indicating the location of these segments to Xq21-qter (PRPS1). The 20- and 4.3-kb DNA segments did not cosegregate with the other three segments, and spot blot hybridization analysis using flow-sorted human chromosomes indicated that these are the PRPS1-related genes (PRPS1L1 and PRPS1L2) and could be assigned to chromosomes 7 and 9, respectively. The human-specific PRS II cDNA probe revealed a BamHI DNA segment (17 kb), which segregated condordantly with the X chromosome but not with the PRPS1 gene. We surmise that the gene for PRS II (PRPS2) is located at a different region of the X chromosome, namely Xpter-a21.

Inherited superactivity of phosphoribosylpyrophosphate synthetase: association of uric acid overproduction and sensorineural deafness

PURPOSE

Superactivity of 5-phosphoribosyl 1-pyrophosphate (PP-Rib-P) synthetase, inherited as an X chromosome-linked trait, has been reported in nearly 20 families in which overproduction of uric acid is invariably present in hemizygous affected males. Clinical manifestations of PP-Rib-P synthetase superactivity are mainly limited to gout in early adulthood. Neurologic deficits, including sensorineural deafness, have rarely been described. We herein document the association of PP-Rib-P synthetase superactivity, gout with excessive uric acid synthesis, and sensorineural deafness in an additional family.

PATIENTS AND METHODS

Two members of a Spanish family were studied: an eight-year-old boy (Patient 1) with tophaceous gout, purine nucleotide and uric acid overproduction, and sensorineural deafness, and his 27-year-old mother (Patient 2), who had gout. Fibroblast cultures were initiated from skin biopsy specimens, and measurements of PP-Rib-P and purine nucleotide metabolism in the fibroblasts were performed.

RESULTS

A labile but superactive PP-Rib-P synthetase was demonstrated in the fibroblasts cultured from both Patients 1 and 2. The kinetic basis of PP-Rib-P synthetase superactivity in this family was resistance to purine nucleotide inhibition of enzyme activity. More severe derangements in the enzyme and in PP-Rib-P and purine synthesis in Patient 1's cells than in Patient 2's cells suggest that Patient 1 is hemizygous and Patient 2 is heterozygous for an X chromosome-linked genetic defect. Limited pedigree data support this view. Compared with affected members of seven other families with PP-Rib-P synthetase superactivity, these patients are intermediate in the range of clinical expression and in the severity of the enzyme defect as measured by the degree of aberration of PP-Rib-P and purine nucleotide synthesis in fibroblasts. Metabolic abnormalities were more severe in Patient 1's cells than in the cells of most male patients (in whom clinical expression is limited to early adult-onset gout) but were less severe than in the cells of two patients in whom more complex enzyme defects were associated with uric acid overproduction and neurodevelopmental abnormalities (including deafness) in male children and adult women.

CONCLUSION

Certain defects resulting in PP-Rib-P synthetase superactivity may be causally related to neurologic impairment, most commonly sensorineural deafness.