Regional localization of the gene for human phosphoribosylpyrophosphate synthetase on the X chromosome

Lesch-Nyhan syndrome and its variants: examining the behavioral and neurocognitive phenotype

PURPOSE OF REVIEW

Lesch-Nyhan Syndrome (LNS) is a metabolic disorder involving mutations in the HGPRT1 gene that result in hyperuricemia, intellectual disability, a dystonic movement disorder, and compulsive self-injury with self-mutilation. The aim of this review is to summarize recent research that documents the extended behavioral, neurologic, and neurocognitive phenotype in classic LNS, to describe milder variants of HGprt deficiency that do not self-injure and have less severe neurological and cognitive deficits, and to provide an update on treatment for associated psychiatric and behavioral disorders.

RECENT FINDINGS

Psychiatric management utilizes combined behavioral and pharmacological treatment in conjunction with protective equipment and dental management to avert self-injury. Pharmacological management focuses on stabilization of mood and anxiety management. S-adenosylmethionine (SAMe), a physiological intermediate in methylation and transsulfuration, has shown beneficial effects in carefully selected patients who can tolerate the drug. Deep brain stimulation is shown in several case reports and series to reduce or eliminate self-injury and aggression, and in some cases, modify dystonia.

SUMMARY

This review highlights progress in our understanding of the behavioral and neurocognitive phenotype of Lesch-Nyhan syndrome (HGprt deficiency) and its variants, describes psychiatric and behavioral management, and discusses prospects for new therapies.

Mammalian phosphoribosyl-pyrophosphate synthetase

PRPP synthetase from rat liver exists as large molecular weight aggregates composed of at least three different components. Cloning of cDNA for the catalytic subunit revealed the presence of two highly homologous isoforms of 34 kDa, designated as PRS I and PRS II. Northern blot analysis showed tissue-differential expression of the two isoform genes. cDNA was expressed in E. coli and studies on the recombinant isoforms showed differences in sensitivity to inhibition by ADP and GDP and to heat inactivation. The rat gene for PRS I has 22 kb and is split into 7 exons. cDNAs for human enzymes were also cloned. Human genes for PRS I and PRS II are localized at different regions on the X-chromosome and their promoter regions were examined. Another component, PRPP synthetase-associated protein of 39 kDa (PAP39), was cloned from cDNA library of the rat liver. The deduced amino acid sequence of PAP39 is remarkably similar to those of PRS I and PRS II. Evidence indicated molecular interaction between PAP39 and the catalytic subunits and an inhibitory effect of PAP39 on the catalytic activity. Expression of the PAP39 gene is tissue-differential like the PRS genes, indicating that the composition of PRPP synthetase may differ with the tissue, hence properties of the enzyme would differ. Further studies on these components and their interaction are expected to reveal various mechanisms governing mammalian PRPP synthetase.

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.

Mutation induction in human lymphoid cells by energetic heavy ions

One of the concerns for extended space flight outside the magnetosphere is exposure to galactic cosmic radiation. In the series of studies presented herein, the mutagenic effectiveness of high energy heavy ions is examined using human B-lymphoblastoid cells across an LET range from 32keV/micrometer to 190 keV/micrometer. Mutations were scored for an autosomal locus, thymidine kinase (tk), and for an X-linked locus, hypoxanthine phosphoribosyltransferase (hprt). For each of the radiations studied, the autosomal locus is more sensitive to mutation induction than is the X-linked locus. When mutational yields are expressed in terms of particle fluence, the two loci respond quite differently across the range of LET. The action cross section for mutation induction peaks at 61 keV/micrometer for the tk locus and then declines for particles of higher LET, including Fe ions. For the hprt locus, the action cross section for mutation is maximal at 95 keV/micrometer but is relatively constant across the range from 61 keV/micrometer to 190 keV/micrometer. The yields of hprt-deficient mutants obtained after HZE exposure to TK6 lymphoblasts may be compared directly with published data on the induction of hprt-deficient mutants in human neonatal fibroblasts exposed to similar ions. The action cross section for induction of hprt-deficient mutants by energetic Fe ions is more than 10-fold lower for lymphoblastoid cells than for fibroblasts.

Human ubiquitin-activating enzyme (E1): compensation for heat-labile mouse E1 and its gene localization on the X chromosome

We have constructed interspecific somatic cell hybrids between a temperature-sensitive (ts) mutant cell line of mouse FM3A cells, ts85, that has a heat-labile ubiquitin-activating enzyme (E1) and a human diploid fibroblast cell line, IMR-90. A hybrid clone that could grow stably at a nonpermissive temperature (39 degrees C) was obtained. Segregation of the hybrid cells at a permissive temperature (33 degrees C) gave rise to temperature-sensitive clones. The electrophoresis of extracted histones and karyotype analysis of the segregants revealed a close correlation of the ability to grow at 39 degrees C, the presence of uH2A (ubiquitin-H2A semihistone) at 39 degrees C, and the presence of the human X chromosome. One of the hybrid clones that could grow at the nonpermissive temperature contained the X chromosome as the only human chromosome. The sodium dodecyl sulfate-polyacrylamide gel electrophoretic pattern of affinity-purified E1 showed that this hybrid clone contained both human and mouse type E1. Thus we conclude that the functional gene for human E1 is located on the X chromosome.

Cloning of cDNAs for human phosphoribosylpyrophosphate synthetases 1 and 2 and X chromosome localization of PRPS1 and PRPS2 genes

Cloned cDNAs representing the entire, homologous (80%) translated sequences of human phosphoribosylpyrophosphate synthetase (PRS) 1 and PRS 2 cDNAs were utilized as probes to localize the corresponding human PRPS1 and PRPS2 genes, previously reported to be X chromosome linked. PRPS1 and PRPS2 loci mapped to the intervals Xq22-q24 and Xp22.2-p22.3, respectively, using a combination of in situ chromosomal hybridization and human x rodent somatic cell panel genomic DNA hybridization analyses. A PRPS1-related gene or pseudogene (PRPS1L2) was also identified using in situ chromosomal hybridization at 9q33-q34. Human HPRT and PRPS1 loci are not closely linked. Despite marked cDNA and deduced amino acid sequence homology, human PRS 1 and PRS 2 isoforms are encoded by genes widely separated on the X chromosome.

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.

Potent and specific inhibitors of mammalian phosphoribosylpyrophosphate (PRPP) synthetase

The monophosphates of the exocyclic amino ribonucleosides, 4-amino- and 4-methoxy-8-(D-ribofuranosylamino)pyrimido[5,4-d]pyrimidine, are potent and specific inhibitors of human erythrocyte and B-lymphoblast PRPP synthetase. The inhibition by MRPP monophosphate is competitive (Ki = 35 microM with the PRPP synthetase cofactor, Pi (Km = 2 mM). The nucleosides are phosphorylated to the active metabolite by adenosine kinase and these nucleoside monophosphates accumulate in the cell. beta-ARPP is a substrate, albeit poor, for adenosine deaminase and solutions of the beta-anomer of this nucleoside and its monophosphate anomerize over time to give alpha- and beta-mixtures. beta-MRPP is more resistant to adenosine deaminase and anomerization of the nucleoside and its monophosphate is negligible. The effect of treatment of cells with the nucleosides is a time-dependent and nearly universal reduction in the nucleotide content which appears to result from a reduction in the availability of PRPP for dependent metabolic pathways. In studies with the WI-L2 lymphoblasts, some of these pathways, de novo and salvage (hypoxanthine and guanine) synthesis of purine nucleotides, are more sensitive to a restriction of PRPP availability than others, i.e. de novo pyrimidine synthesis. The nucleosides have shown promise as therapeutic agents in a mouse leukemia evaluation system but may also have future use in unravelling the complex regulation of PRPP synthetase and the dependent nucleotide synthesis pathways.

Effects of chronic allopurinol therapy on purine metabolism in Duchenne muscular dystrophy

Adenine, adenosine, inosine, hypoxanthine, xanthosine, xanthine, guanine and guanosine blood levels in 11 Duchenne muscular dystrophy patients treated with allopurinol, 10 untreated patients and 8 healthy controls, were determined by HPLC. Serum ADA, PNP and 5'-NT were also determined. Untreated patients showed lower adenine (p less than 0.001) and higher adenosine, xanthine, ADA and PNP levels (p less than 0.01) than controls. Treated patients had lower adenine and higher xanthine levels (p less than 0.001), but higher hypoxanthine, xanthosine and guanine levels (p less than 0.001), than controls, with normal ADA and PNP. The changes observed in ADA and PNP levels suggest an involvement of these enzymes in accelerated degradation of purines in Duchenne dystrophy.

Fine structure of the human hypoxanthine phosphoribosyltransferase gene

The human hypoxanthine phosphoribosyltransferase (HPRT) gene has been characterized by molecular cloning, mapping, and DNA sequencing techniques. The entire gene, which is about 44 kilobases in length, is composed of nine exon elements. The positions of the introns within the coding sequence are identical to those of the previously-characterized mouse HPRT gene, although there are significant differences between intron sizes for the two genes. HPRT minigenes have been used in a transient expression assay involving microinjection into HPRT- cells to demonstrate functional promoter activity within a 234-base-pair region upstream from the ATG codon. The promoter of this gene resembles those of other recently characterized "housekeeping" genes in that it lacks CAAT- and TATA-like sequences, but contains several copies of the sequence GGGCGG. Both RNase protection and primer extension analysis indicate that human HPRT mRNA is heterogeneous at the 5' terminus, with transcription initiation occurring at sites located congruent to 104 to congruent to 169 base pairs upstream from the ATG codon. Comparison of the mouse and human HPRT 5'-flanking sequences indicates that there are only limited stretches of conserved sequence, although there are other shared features, such as an extremely high density of potential methylation sites, that may have functional significance.

Localization of MIC5 to the region between HPRT and G6PD on the human X chromosome

The X-linked gene, MIC5, encodes a human cell-surface antigen, R1. We have assigned MIC5 to the region between HPRT and G6PD on the long arm of the X chromosome. Regional localization was based on the pattern of reactivity of the R1 monoclonal antibody with human-rodent somatic cell hybrids which contained different fragments of the human X chromosome.

Phosphoribosylpyrophosphate synthetase superactivity. A study of five patients with catalytic defects in the enzyme

Superactive phosphoribosylpyrophosphate (PRPP) synthetases were characterized in fibroblasts and erythrocytes from 5 unrelated men with gout and/or hyperuricemia and uric acid overproduction. The kinetic basis of enzyme superactivity in all patients was increased maximal reaction velocity. Affinities of the enzymes for substrates and activators and responsiveness to inhibitors were normal, and levels of immunoreactive enzyme in patient and control fibroblast and erythrocyte extracts were comparable. Enzymes purified to homogeneity from 2 patients confirmed the presence of isolated catalytic defects. Altered physical properties of certain of the superactive enzymes suggested the presence of several distinctive structural defects among the aberrant forms. Fibroblasts from each affected patient showed increased PRPP concentration and generation, as well as accelerated rates of all PRPP-requiring purine nucleotide synthetic pathways. These findings support the concept that enzyme superactivity results in uric acid overproduction as a consequence of increased rates of PRPP and purine nucleotide synthesis. Cultured cells from female relatives of 2 patients showed evidence for the heterozygous carrier state, as measured both by enzyme activities and by rates of PRPP and purine synthesis. The clinical phenotype in 4 patients was limited to early adult-onset gout and its consequences, whereas the fifth patient expressed a familial constellation of hyperuricemia, sensorineural deafness, ataxia, and renal insufficiency. The severity of the derangements in PRPP synthetase and in PRPP and purine synthesis in cells from the 5 patients, however, was comparable. The neurologic accompaniments of enzyme superactivity found in 1 family described here, and in 2 others described previously, thus may not necessarily be consequences of primary defects in PRPP synthetase.

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

Phosphoribosyl pyrophosphate (PPRibP) synthetase activity was studied in cultured fibroblasts and lymphoblasts from a male child (patient 2-A) in whom inherited purine nucleotide and uric acid overproduction are accompanied by neurological deficits. Chromatographed or partially purified preparations of the child's enzyme showed 5-6-fold increased inhibitory constants (I0.5) for the noncompetitive inhibitors GDP and 6-methylthioinosine monophosphate but normal responsiveness to the competitive inhibitors ADP and 2,3-diphosphoglycerate. Activation of the PPRibP synthetase of patient 2-A by Pi was also abnormal with 3-4-fold reduced apparent KD values for Pi. Superactivity of the PPRibP synthetase of this child thus appeared to result from a combination of regulatory defects; selective resistance to noncompetitive inhibitors and increased responsiveness to Pi activation. Selective growth of the patient's fibroblasts in medium containing 6-methylthioinosine confirmed the functional significance of the in vitro inhibitor resistance of the aberrant enzyme. Fibroblasts and lymphoblasts derived from patient 2-A showed increased concentrations and rates of generation of PPRibP as well as increased rates of the pathways of purine base salvage and purine nucleotide synthesis de novo. The magnitudes of these increases in the child's cells exceeded those in cells with catalytically superactive PPRibP synthetases. These alterations as well as the in vitro kinetic abnormalities in the patient 2-A enzyme were expressed to a reduced degree in fibroblasts from the child's affected mother, supporting the proposal that this woman is a heterozygous carrier for X-linked enzyme superactivity.

Hyperuricemia and gout

Gout is a clinical syndrome encompassing a group of metabolic diseases that are all characterized by abnormal uric acid metabolism. In its fullest form, gout is defined by: an increase in the serum urate concentration; characteristic, recurrent, acute arthritic attacks, with monosodium urate monohydrate crystals demonstrable in synovial fluid leukocytes; tophi, usually in and around joints of the extremities, composed of monosodium urate monohydrate deposits; renal disease, often accompanied by hypertension with glomerular, tubular, interstitial, and vascular involvement; and uric acid nephrolithiasis. Any combination of these manifestations may occur, although tophi and urate nephropathy rarely antedate gouty arthritis.

Fine structure of the human hypoxanthine phosphoribosyltransferase gene

The human hypoxanthine phosphoribosyltransferase (HPRT) gene has been characterized by molecular cloning, mapping, and DNA sequencing techniques. The entire gene, which is about 44 kilobases in length, is composed of nine exon elements. The positions of the introns within the coding sequence are identical to those of the previously-characterized mouse HPRT gene, although there are significant differences between intron sizes for the two genes. HPRT minigenes have been used in a transient expression assay involving microinjection into HPRT- cells to demonstrate functional promoter activity within a 234-base-pair region upstream from the ATG codon. The promoter of this gene resembles those of other recently characterized "housekeeping" genes in that it lacks CAAT- and TATA-like sequences, but contains several copies of the sequence GGGCGG. Both RNase protection and primer extension analysis indicate that human HPRT mRNA is heterogeneous at the 5' terminus, with transcription initiation occurring at sites located congruent to 104 to congruent to 169 base pairs upstream from the ATG codon. Comparison of the mouse and human HPRT 5'-flanking sequences indicates that there are only limited stretches of conserved sequence, although there are other shared features, such as an extremely high density of potential methylation sites, that may have functional significance.

Selective expression of phosphoribosylpyrophosphate synthetase superactivity in human lymphoblast lines

Phenotypic expression of 5-phosphoribosyl 1-pyrophosphate (PRPP) synthetase superactivity was examined in lymphoblast lines derived from six unrelated male patients. Fibroblasts from these individuals have increased rates of PRPP and purine nucleotide synthesis and express four classes of kinetic derangement underlying enzyme superactivity: increased maximal reaction velocity (catalytic defect); inhibitor resistance (regulatory defect); increased substrate affinity (substrate binding defect); and combined catalytic and regulatory defects. Lymphoblast lines from three patients with catalytic defects and from three normal individuals were indistinguishable with respect to enzyme activities, PRPP concentrations and generation, and rates of purine synthesis. Enzyme in lymphoblasts from a patient with combined defects also showed normal maximal reaction velocity but expressed purine nucleotide inhibitor resistance. A second regulatory defect and a substrate binding defect were also demonstrable in lymphoblasts and were identical to the enzyme defects in fibroblasts from the respective patients. Regulatory and substrate binding defects in lymphoblasts were accompanied by increased rates of PRPP and purine nucleotide synthesis. Among explanations for selective expression of enzyme superactivity, reduced concentrations of catalytically superactive enzymes seemed unlikely: immunoreactive PRPP synthetase was comparable in normal-derived and patient-derived cells. Activation of normal enzyme in transformed lymphocytes was also unlikely because absolute specific activities of lymphoblast PRPP synthetases corresponded to those of normal fibroblast and erythrocyte enzymes. Abnormal electrophoretic mobilities and thermal stabilities, identified in certain catalytically superactive fibroblast PRPP synthetases, were not found in the corresponding lymphoblast enzymes. Thus, lymphoblast PRPP synthetases from patients with catalytic superactivity appeared to differ structurally and functionally from their fibroblast counterparts.

Regional localization of the phosphoglycerate kinase gene and pseudogene on the human X chromosome and assignment of a related DNA sequence to chromosome 19

We have used a cDNA clone for human phosphoglycerate kinase (PGK) to examine the chromosomal localization of three members of the human PGK gene family. Using somatic cell hybrids segregating portions of several X-autosome translocations as well as a clone panel of hybrids segregating radiation-induced fragments of the human X chromosome, we assign a PGK pseudogene to the region Xq11-Xq13, proximal to the functional X-linked PGK gene located in Xq13. In addition, using a panel of 24 somatic cell hybrids, we assign an autosomal PGK-related DNA sequence to human chromosome 19.

Clinical trials of allopurinol in Duchenne muscular dystrophy

A wide range of different disturbances characteristic of Duchenne muscular dystrophy (DMD) can be attributed to a fundamental chronic shortage of intracellular adenosine 5'-triphosphate (ATP), in turn arising from a basic lack of total adenylate. Purine conservation by the hypoxanthine isomer allopurinol, which promotes salvage and inhibits catabolism, greatly increases muscle ATP and total adenylate, with corresponding clinical benefit. Among subsequent confirmatory clinical trials some gave positive results, while others provided no information. Reasons given why these latter proved uninformative include asking questions either irrelevant and/or incapable of being answered, not least in older boys with too much shrinking fibrous tissue infiltrating too little remaining muscle. Informative results from any metabolic intervention can be expected only where sufficient muscle is left to respond, and this age-linked effect is everywhere evident in the positive trials. Thus, if an effect of allopurinol now seems apparent, and since it is extremely safe and does not enter the genetic material, it is suggested that it be administered shortly after birth before irreversible pathological changes occur. This implies neo-natal male mass screening, easily accomplished by a simple dried-blood spot test, and already carried out successfully elsewhere.

Inherited disorders of purine metabolism--underlying molecular mechanisms

An overview of inherited disorders of purine metabolism, concentrating on well established enzyme defects is given. Included are HPRT and the LNS, APRT and 2,8-dihydroxyadenine lithiasis, hyperactivity of PRPP synthetase, ADA and PNP and immunodeficiencies. Emphasis is put on underlying molecular mechanisms on the gene-, enzyme-, or metabolite level for a better understanding of the events leading from the genotype to the clinical phenotype. Finally some aspects of extracellular purine nucleotide metabolism catalyzed by cell surface-bound ectoenzymes are discussed.

Organization of the HPRT gene and related sequences in the human genome

Comparative Southern hybridization of cDNA probes to DNA from cells carrying either one or four X chromosomes has been used to distinguish sequences derived from the functional locus for hypoxanthine-guanine phosphoribosyltransferase (HPRT) on the X chromosome from four independent HPRT-like autosomal sequences in the human genome. Subfragments of cDNA were then used to orient fragments from the HPRT locus with respect to the mRNA sequence. The chromosomal origin of each of the autosomal sequences was determined by Southern analysis using DNA from a panel of human-Chinese hamster somatic cell hybrids. Two of the HPRT-like sequences were localized to chromosome 11, the third to chromosome 3, and the fourth to the region between p13 and q11 on chromosome 5. Three of these four autosomal sequences were isolated from genomic recombinant libraries and subcloned fragments from each were used as probes to study restriction fragment length polymorphisms (RFLP) at these loci. A RFLP for MspI was found at the HPRT-like locus on chromosome 5 with a 1.3-kb major allele (frequency = 0.8) and a 3.6-kb minor allele (frequency = 0.2).

Somatic cell genetic approaches to Down's syndrome

Somatic cell genetic analysis of mutants of Chinese hamster ovary cells with deficient purine synthesis and of hybrids between these mutants and human cells is described. Data are presented substantiating that two genes for enzymes of purine synthesis, AdeC and AdeG, can be coordinately regulated in mammalian cells. Analysis of a human-hamster hybrid cell, Ade C/21, which contains a normal complement of hamster chromosomes and human chromosome 21 as its only human genetic component recognizable by electrophoretic and immunogenetic techniques demonstrates that genes associated with the presence of human chromosome 21 and required for the synthesis of specific polypeptides and specific human lethal cell surface antigens can be detected in these hybrids.

Isolation of a genomic clone partially encoding human hypoxanthine phosphoribosyltransferase

Mouse cells deficient in the enzyme hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8) have been transfected with total human DNA, and cells producing human enzyme were isolated by growth in selective medium. DNA from several such cell lines has been used to generate secondary transfectants that make human HPRT. Blots of the DNA of these secondary cells have been hybridized with total human DNA probes or with cloned human Alu sequences, and one of several common bands has been cloned in pBR322. Colonies of transformed Escherichia coli containing human sequences were detected by their homology with human DNA, and subclones of resulting recombinant plasmids were prepared. Two subclones free of Alu sequences were found to contain human sequences that hybridized to human X chromosome DNA. One of these, pBR1.5, also hybridized to a single RNA band on gel blots of human and secondary transfectant cytoplasmic poly(A)+RNA but not to RNA from the parent mouse cell line. These results indicate that these clones represent human HPRT gene fragments. This has been confirmed by using pBR1.5 as a probe to isolate an authentic and expressible human HPRT cDNA clone from a library prepared by H. Okayama and P. Berg.

An X-linked syndrome characterised by hyperuricaemia, deafness, and neurodevelopmental abnormalities

Purine overproduction, with normal levels of the purine salvage enzymes, has been found in a 3-year-old boy and his mother. Both have high-frequency hearing loss from infancy. The child is hypotonic and shows locomotor delay. Two male siblings with similar neurodevelopmental problems died in early childhood. Biochemical studies suggest a combined syndrome associated with a superactive PP-ribose-P synthetase which in its severest form may produce neurodevelopmental complications, as well as uric-acid overproduction. Severe depletion of erythrocyte nicotinamide adenine dinucleotide and guanosine triphosphate appears to be associated with the neurological abnormalities and may be useful in diagnosis.

The mode of genetic transmission of gouty family with increased phosphoribosylpyrophosphate synthetase activity

The mode of genetic transmission of gout and increased activity of phosphoribosylphrophosphate synthetase (PRPPS) was studied in one family. Among 15 members of Family F, two male members had gout and had PRPPS activity of erythrocyte lysates three times higher than normal subjects. Five female members had activity 2.5 times higher than normal. The difference between the activities of male and female affected members was statistically significant (P less than 0.05). To examine the genetic trait of this abnormal PRPPS, the incorporation of 3H-adenine into erythrocytes or lymphocytes was studied using autoradiography. The number of grains which show the uptake of labeled adenine into cells revealed a normal distribution pattern in two normal persons and in two male patients, and a mixed pattern of the two cell populations in two female affected members. These results suggested mosaicism in female members and X-linked dominant transmission of this trait. Thermal inactivation of PRPPS of an affected female was intermediate between that from a normal subject and that from the affected males. This result showed the heterogeneity of the PRPPS from the hemolysate of an affected female. The genotype of PRPPS on the X-chromosome was assumed and the lod score between PRPPS and Xg was also estimated. From these findings and electrophoretical study, it was suggested that the abnormal enzyme was a mutant enzyme transmitted in an X-linked dominant trait, and that the mutation occurred on the structural gene of the PRPPS.

Assignment of the gene coding for phosphoribosylglycineamide formyltransferase to human chromosome 14

Purine-requiring Chinese hamster ovary cell auxotrophs of the complementation class ade-E were hybridized with various human cells, and hybrids were isolated under selective conditions in which the retention of the complementing gene on the human chromosome is necessary for survival. Synteny analysis in 72 primary and secondary hybrid clones using isozyme, karyotypic, and biochemical methods provides evidence for an assignment of the gene for phosphoribosylglycineamide formyltransferase (GART, EC 2.1.2.2), deficient in ade-E mutants, to human chromosome 14. The importance of this gene assignment to the development of hypotheses regarding the organization, structure, and regulation of genes involved in the same biosynthetic pathway in mammalian cells is discussed.

Metabolic basis for disorders of purine nucleotide degradation

Purine nucleotide degradation refers to a regulated series of reactions by which human purine ribonucleotides and deoxyribonucleotides are degraded to uric acid in humans. Two major types of disorders occur in this pathway. A block of degradation occurs with syndromes involving immune deficiency, myopathy or renal calculi. Increased degradation of nucleotides occurs with syndromes characterized by hyperuricemia and gout, renal calculi, anemia or acute hypoxia. Management of disorders of purine nucleotide degradation is dependent upon modifying the specific molecular pathology underlying each disease state.