Regulation of nicotinamide-adenine dinucleotide synthesis in erythrocytes of patients with hypoxanthine-guanine phosphoribosyltransferase deficiency and a patient with phosphoribosylpyrophosphate synthetase superactivity

Inborn Errors of Purine Salvage and Catabolism

Cellular purine nucleotides derive mainly from de novo synthesis or nucleic acid turnover and, only marginally, from dietary intake. They are subjected to catabolism, eventually forming uric acid in humans, while bases and nucleosides may be converted back to nucleotides through the salvage pathways. Inborn errors of the purine salvage pathway and catabolism have been described by several researchers and are usually referred to as rare diseases. Since purine compounds play a fundamental role, it is not surprising that their dysmetabolism is accompanied by devastating symptoms. Nevertheless, some of these manifestations are unexpected and, so far, have no explanation or therapy. Herein, we describe several known inborn errors of purine metabolism, highlighting their unexplained pathological aspects. Our intent is to offer new points of view on this topic and suggest diagnostic tools that may possibly indicate to clinicians that the inborn errors of purine metabolism may not be very rare diseases after all.

Induced pluripotent stem cells from subjects with Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is an inherited disorder caused by pathogenic variants in the HPRT1 gene, which encodes the purine recycling enzyme hypoxanthine-guanine phosphoribosyltransferase (HGprt). We generated 6 induced pluripotent stem cell (iPSC) lines from 3 individuals with LND, along with 6 control lines from 3 normal individuals. All 12 lines had the characteristics of pluripotent stem cells, as assessed by immunostaining for pluripotency markers, expression of pluripotency genes, and differentiation into the 3 primary germ cell layers. Gene expression profiling with RNAseq demonstrated significant heterogeneity among the lines. Despite this heterogeneity, several anticipated abnormalities were readily detectable across all LND lines, including reduced HPRT1 mRNA. Several unexpected abnormalities were also consistently detectable across the LND lines, including decreases in FAR2P1 and increases in RNF39. Shotgun proteomics also demonstrated several expected abnormalities in the LND lines, such as absence of HGprt protein. The proteomics study also revealed several unexpected abnormalities across the LND lines, including increases in GNAO1 decreases in NSE4A. There was a good but partial correlation between abnormalities revealed by the RNAseq and proteomics methods. Finally, functional studies demonstrated LND lines had no HGprt enzyme activity and resistance to the toxic pro-drug 6-thioguanine. Intracellular purines in the LND lines were normal, but they did not recycle hypoxanthine. These cells provide a novel resource to reveal insights into the relevance of heterogeneity among iPSC lines and applications for modeling LND.

Lesch-Nyhan disease: A rare disorder with many unresolved aspects

Lesch-Nyhan Disease (LND) is a rare X-linked recessive metabolic and neurological syndrome due to the deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT). Besides its well known “housekeeping” function this purine salvage enzyme has revealed an unexpected role in neurodevelopment, unveiled by the peculiar neurological symptoms flanking hyperuricemia in LND: dystonia, choreoathetosis, compulsive self-injurious behaviour. Several lines of research have tried to find the molecular basis for the neurological phenotype after the disease was first described in 1964. Dopaminergic deficit was then found to underlie the neurologic symptoms but the aetiology for such alteration seemed inexplicable. A number of detailed studies in the last 50 years addressed the genetic, metabolic, cognitive, behavioral and anatomical features of this disease. Initial investigations seeked for accumulation of toxic metabolites or depletion of essential molecules to disclose potential connections between purine recycling and neuronal dysfunction. In the last two decades sophisticated biotechnological methods were used for a deeper insight in the genetic and molecular aspects, unveiling a network of combined gene dysregulations in neuronal development and differentiation producing neurotransmission defects. These studies, conducted with several different approaches, allowed consistent steps forward, demonstrating transcriptional aberrations affecting different metabolic pathways in HPRT deficiency, yet leaving many questions still unsolved.

Inborn errors of purine and pyrimidine metabolism: how much we owe to H. Anne Simmonds

Purines and pyrimidines, regarded for a long time merely as building blocks for nucleic acid synthesis and intermediates in the transfer of metabolic energy, have attracted increasing attention after genetically determined aberrations in their metabolism were linked to a range of symptoms from hyperuricemia and immunodeficiency to neurological disorders. The pathogenesis of such disorders involves cell or mitochondrial damage, but the molecular mechanisms underlying symptoms is often unclear. H. Anne Simmonds made major contributions to the metabolic, clinical, and molecular aspects of these disorders and the Purine Research Laboratory, which she established in London, became the world center for clinical and experimental studies in the field. We owe her gratitude not only for this direct contribution but also for her enthusiasm for purine and pyrimidine research that she transmitted to generations of scientists. Our research in this field stemmed from expertise in pyridine metabolism and its connection with purines, and from clinical involvement with biochemical diagnosis of enzyme deficiencies. We joined H. Anne Simmonds in studying the biochemical basis of altered NAD content in erythrocytes of PNP- and HPRT-deficient patients, discovering some alterations in NAD synthesis and breakdown.

Severe pyridine nucleotide depletion in fibroblasts from Lesch-Nyhan patients

The relationship between a complete deficiency of the purine enzyme hypoxanthine-guanine phosphoribosyltransferase and the neurobehavioural abnormalities in Lesch-Nyhan disease remains an enigma. In vitro studies using lymphoblasts or fibroblasts have evaluated purine and pyrimidine metabolism with conflicting results. This study focused on pyridine nucleotide metabolism in control and Lesch-Nyhan fibroblasts using radiolabelled salvage precursors to couple the extent of uptake with endocellular nucleotide concentrations. The novel finding, highlighted by specific culture conditions, was a marked NAD depletion in Lesch-Nyhan fibroblasts. ATP and GTP were also 50% of the control, as reported in lymphoblasts. A 6-fold greater incorporation of [(14)C]nicotinic acid into nicotinic acid- adenine dinucleotide by Lesch-Nyhan fibroblasts, with no unmetabolized substrate (20% in controls), supported disturbed pyridine metabolism, NAD depletion being related to utilization by poly(ADP-ribose) polymerase in DNA repair. Although pyrimidine nucleotide concentrations were similar to controls, Lesch-Nyhan cells showed reduced [(14)C]cytidine/uridine salvage into UDP sugars. Incorporation of [(14)C]uridine into CTP by both was minimal, with more than 50% [(14)C]cytidine metabolized to UTP, indicating that fibroblasts, unlike lymphoblasts, lack active CTP synthetase, but possess cytidine deaminase. Restricted culture conditions may be neccesary to mimic the situation in human brain cells at an early developmental stage. Cell type may be equally important. NAD plus ATP depletion in developing brain could restrict DNA repair, leading to neuronal damage/loss by apoptosis, and, with GTP depletion, affect neurotransmitter synthesis and basal ganglia dopaminergic neuronal systems. Thus aberrant pyridine nucleotide metabolism could play a vital role in the pathophysiology of Lesch-Nyhan disease.

Biochemical and molecular study of mentally retarded patient with partial deficiency of hypoxanthine-guanine phosphoribosyltransferase

Nucleotide metabolism was studied in erythrocytes of a mentally retarded child and family members. Partial hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency was found in the propositus and an asymptomatic maternal uncle. Studies in crude lysates demonstrated decreased apparent V(max) and slightly decreased apparent K(m) for hypoxanthine in both HPRT-deficient subjects. Genomic DNA analysis revealed a single nucleotide change with leucine-147 to phenylalanine substitution in both subjects; mother and grandmother were heterozygous carriers of the same defect. This new variant has been termed HPRT(Potenza). Increased erythrocyte concentration of NAD and rate of synthesis by intact erythrocytes were found in the patient; increased activities of nicotinic acid phosphoribosyltransferase (NAPRT) and NAD synthetase (NADs) were demonstrated in erythrocyte lysates, with normal apparent K(m) for their substrates and increased V(max). These alterations were not found in any member of the family, including the HPRT-deficient uncle. These findings show multiple derangement of nucleotide metabolism associated with partial HPRT deficiency. The enzyme alteration was presumably not the cause of neurological impairment since no neurological symptoms were found in the HPRT-deficient uncle, whereas they were present in the propositus' elder brother who had normal HPRT activity.

Salvage pathway for NAD biosynthesis in Brevibacterium ammoniagenes: regulatory properties of triphosphate-dependent nicotinate phosphoribosyltransferase

As the rate-limiting enzyme, catalyzing the first reaction in NAD salvage synthesis, nicotinate phosphoribosyltransferase (NAPRTase, EC 2.4.2.11) is of important interest for studies of intracellular pyridine nucleotide pool regulation. We have purified NAPRTase 520-fold from Brevibacterium ammoniagenes ATCC 6872 without using an over-expression system by applying acid treatment, salt fractionation, Ca-phosphate gel treatment, anion exchange column chromatography and size-exclusion gel filtration. Unlike this enzyme from other sources, B. ammoniagenes NAPRTase was found to be controlled by the feedback inhibition by the end product NAD with K(i)=0.7+/-0.1 mM. The reaction products, pyrophosphate and nicotinate mononucleotide, also decreased the enzyme activity, as did other intermediates of NAD synthesis, such as AMP, ADP and a NAD direct precursor, nicotinate adenine dinucleotide or deamido NAD. The enzyme was observed to require a nucleoside triphosphate for its activity and showed the maximum affinity for ATP. The specificity, however, turned out to be poor, and ATP could be substituted by other nucleoside triphosphates as well as by sodium triphosphate. The kinetic characteristics of the enzyme are reported. For the first time, our data have experimentally revealed such complicated stimulatory and inhibitory effects by the intermediates of NAD biosynthesis on one of its salvage enzymes, NAPRTase. On the basis of these data, the key role of NAPRTase is discussed in light of the regulation of NAD metabolism in B. ammoniagenes.

Hypoxanthine-guanine phosphoribosyltransferase deficiency and erythrocyte synthesis of pyridine coenzymes

Purine and pyridine metabolism were studied in ten Lesch-Nyhan patients, with virtually no hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity in erythrocytes. Increased NAD erythrocyte concentrations were found in all patients. Raised activities of two enzymes catalysing NAD synthesis from nicotinic acid (nicotinic acid phosphoribosyltransferase: NAPRT, and NAD synthetase: NADs) was found in erythrocyte lysates from all patients. The two enzymes had normal apparent Km for their substrates and increased Vmax. The rate of synthesis of pyridine nucleotides from nicotinic acid by intact erythrocytes in vitro was also increased in most patients. These findings suggest that raised NAD concentrations in HPRT- erythrocytes are due to enhanced synthesis as a result of increased enzyme activities.

Nicotinic acid phosphoribosyltransferase activity in human erythrocytes: studies using a new HPLC method

A non-radiochemical method linked to reverse-phase high-performance liquid chromatography was developed to determine the activity of nicotinic acid phosphoribosyltransferase (EC 2.4.2.11) in crude lysates of human red blood cells. The method is accurate and easily reproducible in different chromatographic systems. The enzyme activity was determined in erythrocytes of healthy subjects and in patients with different purine disorders showing altered NAD levels. Very low enzyme activity was found in a boy hemizygous for phosphoribosylpyrophosphate synthetase superactivity, consistent with the low erythrocyte NAD concentration.

HPLC determination of oxidized and reduced pyridine coenzymes in human erythrocytes

The nucleotide concentrations in acid and alkaline erythrocyte extracts have been measured by RP-HPLC in healthy controls and in patients bearing different inherited disorders, with altered erythrocyte NAD(P) levels. The objective was the simultaneous determination of the nucleotide profile and of the oxidative state of pyridine coenzymes by the most suitable extraction method. Both alkaline and acid extractions were necessary to obtain the complete pattern, due to defective recovery of the oxidized or reduced coenzymes, respectively, during the extraction procedures. Purine nucleotide quantification seemed to be reliable by all methods. High NADP+ levels were confirmed in two glucose-6-phosphate dehydrogenase deficient patients, coupled with raised NAD levels, lowered NADPH/NADP+ ratio and increased NADH/NAD+ ratio. Higher NAD+ and normal or lower NADH/NAD+ ratios were found in two hypoxanthine-phosphoribosyltransferase deficient patients, while a patient with superactive phosphoribosylpyrophosphate synthetase showed a decreased NADH level in addition to the low NAD+ level previously found.

Determination of nicotinamide phosphoribosyltransferase activity in human erythrocytes: high-performance liquid chromatography-linked method

A radiochemical reverse-phase-high-performance liquid chromatography-linked method to measure the activity of nicotinamide phosphoribosyltransferase (EC 2.4.2.12) in crude lysates of human red blood cells is described. The apparent Km for nicotinamide was in the micromolar range, much lower than that described in human erythrocytes in the past. The enzyme activity in crude hemolysates was found to be extremely low (21 +/- 3.5 nmol x h-1 x g-1 Hb); nevertheless, the low Km for nicotinamide might account for the production of pyridine nucleotides reported by us for intact erythrocytes incubated at low, physiological concentrations of this substrate.

Importance of nicotinamide as an NAD precursor in the human erythrocyte

The effect of variation in the concentration of inorganic phosphate and of the pyridine precursors nicotinamide (NAm) and nicotinic acid (NA) on pyridine nucleotide synthesis was studied using intact human erythrocytes. A wide range of incubation times was employed. The results showed that under physiological conditions the rate of synthesis of NAD from NAm exceeded that from NA twofold, while the reverse situation pertained at higher and unphysiological substrate levels. The two pathways had different regulation points. For NAm the rate-limiting factor was the initial step, namely its conversion into the mononucleotide, while for NA it lay at the second step, conversion of NA mononucleotide (NAMN) to its adenine dinucleotide. At physiological substrate levels the uptake of NA and conversion to NAMN were rapid, while the uptake and conversion of NAm were time dependent. This process was stimulated significantly by inorganic phosphate only for NAm. These results indicate that while NA is the predominant precursor of human erythrocyte NAD at high (unphysiological) substrate and phosphate levels, NAm is more efficient as an NAD precursor under physiological conditions, suggesting an important and hitherto unrecognized role for nicotinamide in NAD synthesis in vivo.