Effect of D-ribose on purine synthesis and neurological symptoms in a patient with adenylosuccinase deficiency

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.

Aberrant energy metabolism and redox balance in seizure onset zones of epileptic patients

Epilepsy is a disorder that affects around 1% of the population. Approximately one third of patients do not respond to anti-convulsant drugs treatment. To understand the underlying biological processes involved in drug resistant epilepsy (DRE), a combination of proteomics strategies was used to compare molecular differences and enzymatic activities in tissue implicated in seizure onset to tissue with no abnormal activity within patients. Label free quantitation identified 17 proteins with altered abundance in the seizure onset zone as compared to tissue with normal activity. Assessment of oxidative protein damage by protein carbonylation identified additional 11 proteins with potentially altered function in the seizure onset zone. Pathway analysis revealed that most of the affected proteins are involved in energy metabolism and redox balance. Further, enzymatic assays showed significantly decreased activity of transketolase indicating a disruption of the Pentose Phosphate Pathway and diversion of intermediates into purine metabolic pathway, resulting in the generation of the potentially pro-convulsant metabolites. Altogether, these findings suggest that imbalance in energy metabolism and redox balance, pathways critical to proper neuronal function, play important roles in neuronal network hyperexcitability and can be used as a primary target for potential therapeutic strategies to combat DRE. SIGNIFICANCE: Epileptic seizures are some of the most difficult to treat neurological disorders. Up to 40% of patients with epilepsy are resistant to first- and second-line anticonvulsant therapy, a condition that has been classified as refractory epilepsy. One potential therapy for this patient population is the ketogenic diet (KD), which has been proven effective against multiple refractory seizure types However, compliance with the KD is extremely difficult, and carries severe risks, including ketoacidosis, renal failure, and dangerous electrolyte imbalances. Therefore, identification of pathways disruptions or shortages can potentially uncover cellular targets for anticonvulsants, leading to a personalized treatment approach depending on a patient's individual metabolic signature.

Safety of d-ribose as a novel food pursuant to Regulation (EU) 2015/2283

Following a request from the European Commission, the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) was asked to deliver an opinion on D-ribose as a novel food (NF) pursuant to Regulation (EU) 2015/2283. The applicant intends to market the NF as ingredient in a variety of foods, food supplements and in certain foods for specific groups. The NF is produced by fermentation using a transketolase-deficient strain of Bacillus subtilis and marketed as Bioenergy Ribose™. The information provided on the batch-to-batch variability, specifications, stability, production process and history of the organism used as a source of the NF is sufficient and does not raise safety concerns. The Panel considers that the effects observed in a subchronic toxicity study in rats could be the consequence of nutritional imbalances, but toxicological effects could not be ruled out; from this study, the Panel derived a No observed adverse effect level (NOAEL) of 3.6 g/kg body weight (bw) per day. From the human studies indicating a potential decrease in glucose levels and/or the occurrence of transient symptomatic hypoglycaemia at intakes of 10 g of d-ribose, the Panel defined 70 mg/kg bw per day as the NOAEL with respect to hypoglycaemia that can be considered applicable for adults. For children, the Panel acknowledges the lack of human data directly relevant for this population group. Based on the NOAEL derived from the subchronic toxicity study in rats, an acceptable level of intake of 36 mg/kg bw per day was defined that would also take into account the potentially increased sensitivity of certain population groups to hypoglycaemia. The Panel concludes that the NF is safe for the general population at intake levels up to 36 mg/kg bw per day and considers that the safety of the NF at the intended uses and use levels as proposed by the applicant has not been established.

Antiproliferation effect of guanosine on HCT 116 cells involves MAPK and AMPK pathways

This study aims to investigate the mechanisms associated with the antiproliferation effect of guanosine on human colon carcinoma HCT 116 cells. In this study, guanosine induced more drastic cell cycle arrest effect than cell death effect on HCT 116 cells. The cell cycle arrest effect of guanosine on HCT 116 cells appeared to be associated with the increased activation of mitogen-activated protein kinases (MAPK) such as ERK1/2, p38 and JNK. The decrease of AMP-activated protein kinase (AMPK) activation and cyclin D1 expression was also involved. Thus, the antiproliferation of colon cancer cells of guanosine could be mediated by the disruption of MAPK and AMPK pathways.

Diagnosis of adenylosuccinate lyase deficiency by metabolomic profiling in plasma reveals a phenotypic spectrum

Adenylosuccinate lyase (ADSL) deficiency is a rare autosomal recessive neurometabolic disorder that presents with a broad-spectrum of neurological and physiological symptoms. The ADSL gene produces an enzyme with binary molecular roles in de novo purine synthesis and purine nucleotide recycling. The biochemical phenotype of ADSL deficiency, accumulation of SAICAr and succinyladenosine (S-Ado) in biofluids of affected individuals, serves as the traditional target for diagnosis with targeted quantitative urine purine analysis employed as the predominate method of detection. In this study, we report the diagnosis of ADSL deficiency using an alternative method, untargeted metabolomic profiling, an analytical scheme capable of generating semi-quantitative z-score values for over 1000 unique compounds in a single analysis of a specimen. Using this method to analyze plasma, we diagnosed ADSL deficiency in four patients and confirmed these findings with targeted quantitative biochemical analysis and molecular genetic testing. ADSL deficiency is part of a large a group of neurometabolic disorders, with a wide range of severity and sharing a broad differential diagnosis. This phenotypic similarity among these many inborn errors of metabolism (IEMs) has classically stood as a hurdle in their initial diagnosis and subsequent treatment. The findings presented here demonstrate the clinical utility of metabolomic profiling in the diagnosis of ADSL deficiency and highlights the potential of this technology in the diagnostic evaluation of individuals with neurologic phenotypes.

Inborn errors of purine metabolism: clinical update and therapies

Inborn errors of purine metabolism exhibit broad neurological, immunological, haematological and renal manifestations. Limited awareness of the phenotypic spectrum, the recent descriptions of newer disorders and considerable genetic heterogeneity, have contributed to long diagnostic odysseys for affected individuals. These enzymes are widely but not ubiquitously distributed in human tissues and are crucial for synthesis of essential nucleotides, such as ATP, which form the basis of DNA and RNA, oxidative phosphorylation, signal transduction and a range of molecular synthetic processes. Depletion of nucleotides or accumulation of toxic intermediates contributes to the pathogenesis of these disorders. Maintenance of cellular nucleotides depends on the three aspects of metabolism of purines (and related pyrimidines): de novo synthesis, catabolism and recycling of these metabolites. At present, treatments for the clinically significant defects of the purine pathway are restricted: purine 5'-nucleotidase deficiency with uridine; familial juvenile hyperuricaemic nephropathy (FJHN), adenine phosphoribosyl transferase (APRT) deficiency, hypoxanthine phosphoribosyl transferase (HPRT) deficiency and phosphoribosyl-pyrophosphate synthetase superactivity (PRPS) with allopurinol; adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) deficiencies have been treated by bone marrow transplantation (BMT), and ADA deficiency with enzyme replacement with polyethylene glycol (PEG)-ADA, or erythrocyte-encapsulated ADA; myeloadenylate deaminase (MADA) and adenylosuccinate lyase (ADSL) deficiencies have had trials of oral ribose; PRPS, HPRT and adenosine kinase (ADK) deficiencies with S-adenosylmethionine; and molybdenum cofactor deficiency of complementation group A (MOCODA) with cyclic pyranopterin monophosphate (cPMP). In this review we describe the known inborn errors of purine metabolism, their phenotypic presentations, established diagnostic methodology and recognised treatment options.

Novel features in the evolution of adenylosuccinate lyase deficiency

BACKGROUND

Adenylosuccinate lyase (ADSL) deficiency is an autosomal recessive disorder of the purine synthesis which results in accumulation of succinylpurines (succinyladenosine (S-Ado) and succinylamino-imidazole carboxamide riboside (SAICAr)) in body fluids. Patients present developmental delay, often accompanied by epilepsy and autistic spectrum disorders.

OBJECTIVES

To describe atypical neurological features in the evolution of three novel unrelated cases of ADSL deficiency.

PATIENTS

A 9-year-old boy with severe cognitive impairment and autistic behaviour received d-ribose therapy for one year. Drug withdrawal was associated with acute neurological deterioration, severe brain atrophy and demyelination on MRI. The second patient is a 5.5-year-old girl with mild developmental delay who presented a benign course with moderate cognitive impairment as the only feature in her evolution. The final patient is a 14-year-old boy with severe cognitive impairment who developed drug-resistant epilepsy and bathing reflex seizures, progressive spasticity in the lower limbs and thoracic deformity.

METHODS

SAICAr and S-Ado in urine were analysed by HPLC with diode array detection. Diagnosis was confirmed by molecular analysis of the ADSL gene.

RESULTS

An elevation of S-Ado and SAICAr excretion in urine was detected in all three patients. The patients were homozygous for the missence change p.I369L and for the novel change p.M389V.

CONCLUSION

Drug-resistant epilepsy and specific therapeutic interventions may modify the neurological outcome in ADSL deficiency. d-ribose must be considered with caution as, in our experience, it returns no clinical benefit and drug withdrawal can precipitate status epilepticus and acute neurological deterioration.

Study of inborn errors of metabolism in urine from patients with unexplained mental retardation

Mental retardation (MR) is a common disorder frequently of unknown origin. Because there are few studies regarding MR and inborn errors of metabolism (IEM), we aimed to identify patients with IEM from a cohort of 944 patients with unexplained MR. Biochemical examinations such as determination of creatine (Cr) metabolites, acylcarnitines, purine, and pyrimidines in urine were applied. We found seven patients with IEM [three with cerebral Cr deficiency syndromes (CCDS)], one with adenylosuccinate lyase (ADSL) deficiency, and three, born before the neonatal metabolic screening program in Catalonia, with phenylketonuria (PKU). All told, they represent 0.8% of the whole cohort. All of them had additional symptoms such as epilepsy, movement disorders, autism, and other psychiatric disturbances. In conclusion, in patients with MR, it is essential to perform a thorough appraisal of the associated signs and symptoms, and in most disorders, it is necessary to apply specific analyses. In some cases, it is important to achieve an early diagnosis and therapy, which may reduce the morbimortality, and to offer genetic counselling.

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.

Identification and characterization of human ribokinase and comparison of its properties withE. coliribokinase and human adenosine kinase

The gene responsible for ribokinase (RK) in human/eukaryotic cells has not yet been identified/characterized. Blast searches with E. coli RK have identified a human protein showing significant similarity to the bacterial RK. The cDNA for this protein was expressed in E. coli and the recombinant protein efficiently phosphorylated ribose to ribose-5-phosphate using ATP, confirming its identity as RK. In contrast to ribose, the enzyme exhibited very little to no phosphorylation of D-arabinose, D-xylose, D-fructose and D-galactose. The catalytic activity of human RK was dependent upon the presence of inorganic phosphate, as observed previously for E. coli RK and mammalian adenosine kinases (AK). A number of activators and inhibitors of human AK, produced very similar effects on the human and E. coli RKs, indicating that the catalytic mechanism of RK is very similar to that of the AKs.

Pentose phosphates in nucleoside interconversion and catabolism

Ribose phosphates are either synthesized through the oxidative branch of the pentose phosphate pathway, or are supplied by nucleoside phosphorylases. The two main pentose phosphates, ribose-5-phosphate and ribose-1-phosphate, are readily interconverted by the action of phosphopentomutase. Ribose-5-phosphate is the direct precursor of 5-phosphoribosyl-1-pyrophosphate, for both de novo and 'salvage' synthesis of nucleotides. Phosphorolysis of deoxyribonucleosides is the main source of deoxyribose phosphates, which are interconvertible, through the action of phosphopentomutase. The pentose moiety of all nucleosides can serve as a carbon and energy source. During the past decade, extensive advances have been made in elucidating the pathways by which the pentose phosphates, arising from nucleoside phosphorolysis, are either recycled, without opening of their furanosidic ring, or catabolized as a carbon and energy source. We review herein the experimental knowledge on the molecular mechanisms by which (a) ribose-1-phosphate, produced by purine nucleoside phosphorylase acting catabolically, is either anabolized for pyrimidine salvage and 5-fluorouracil activation, with uridine phosphorylase acting anabolically, or recycled for nucleoside and base interconversion; (b) the nucleosides can be regarded, both in bacteria and in eukaryotic cells, as carriers of sugars, that are made available though the action of nucleoside phosphorylases. In bacteria, catabolism of nucleosides, when suitable carbon and energy sources are not available, is accomplished by a battery of nucleoside transporters and of inducible catabolic enzymes for purine and pyrimidine nucleosides and for pentose phosphates. In eukaryotic cells, the modulation of pentose phosphate production by nucleoside catabolism seems to be affected by developmental and physiological factors on enzyme levels.

Autismus und Stoffwechselerkrankungen - was ist gesichert?

Zusammenfassung: Die Ursachen für Autismus sind heterogen und ganz überwiegend genetischer Natur. Eine exakte benennbare Ätiologie wird in weniger als 10% der Fälle gefunden. Die Enttäuschung über den geringen Erfolg bei der Ursachenfindung und zahlreiche Berichte über die Assoziation von Autismus mit Stoffwechselerkrankungen sowie über «Wunderheilungen» bei unterschiedlichsten medikamentösen oder diätetischen Therapien haben bei vielen Ärzten und Eltern zu einer zunehmenden Unsicherheit über die sinnvolle Diagnostik und Behandlung geführt. Diese Arbeit gibt einen Überblick über seltene angeborene Stoffwechselerkrankungen («inborn errors of metabolism»), die nachweislich (z.B. Phenylketonurie, Smith-Lemli-Opitz Syndrom) oder wahrscheinlich (z.B. Succinat-Semialdehyd-Dehydrogenase-Mangel) mit Autismus-spezifischen Symptomen vergesellschaftet sind. In aller Regel weisen betroffene Patienten zusätzliche neurologische Symptome auf. Es werden die zur Diagnostik dieser angeborenen Stoffwechselerkrankungen notwendigen Untersuchungen und mögliche therapeutische Maßnahmen dargestellt. Neben diesen gut definierten Stoffwechselerkrankungen mit der Möglichkeit einer rationalen Therapie wird auch auf Hypothesen über die Entstehung von Autismus durch «Stoffwechselveränderungen» eingegangen, die entweder nicht bewiesen oder nachweislich falsch sind.

Pathways for alpha-D-ribose utilization for nucleobase salvage and 5-fluorouracil activation in rat brain

Recently, interest has increased in the use of alpha-D-ribose (Rib) as a naturally occurring nutriceutical for enhancement of cardiac and muscular performance. Most likely the elevation of available PRPP, following Rib administration, plays a key role in the salvage of purine nucleobases, thus, accelerating the restitution of ATP pool. In addition, administration of Rib improves some of the neurological symptoms in patients with adenylosuccinase deficiency. In this paper, we show that rat brain extract can catalyze the Rib-mediated salvage of both adenine and uracil, as well as the activation of the pyrimidine pro-drug, 5-fluorouracil (5-FU). The results strongly support that the pentose may be converted to both PRPP and Rib1-P for the salvage of the adenine and uracil, respectively. Most likely two-reaction pathway, composed of ribokinase and PRPP synthetase, is responsible of the PRPP formation, needed to salvage adenine to adenine nucleotides. A two-reaction pathway, composed of ribokinase and phosphopentomutase, appears to be responsible of the Rib1-P formation, needed to salvage uracil to uracil-nucleotides and to activate 5-FU to cytotoxic 5-FU-ribonucleotides. alpha-D-2'-Deoxyribose (deoxyRib) has a negligible effect on both the salvage of natural nucleobases to deoxyribonucleotides and on the activation of 5-FU to cytotoxic 5-FU-deoxynucleotides.

Neurologic aspects of adenylosuccinate lyase deficiency

Adenylosuccinate lyase deficiency is an autosomal-recessive disorder of the purine de novo synthesis pathway, diagnosed up to now in approximately 40 patients. The clinical presentation is characterized by severe neurologic involvement including seizures, developmental delay, hypotonia, and autistic features. Neonatal seizures and a severe infantile epileptic encephalopathy are often the first manifestations of this disorder. The existence of genetic heterogeneity for the adenylosuccinate lyase defect could account for variability of the clinical presentation. Deficiency of purine nucleotides, impairment of energy metabolism, and toxic effects are potential mechanisms of cerebral damage. Laboratory investigations show the presence in urine and cerebrospinal fluid of succinylpurines, which are normally undetectable. Currently, no effective treatment is available for adenylosuccinate lyase deficiency. A search for this disorder should be included in the screening program of children with unexplained neonatal seizures or severe infantile epileptic encephalopathy.

Metabolic approaches to the treatment of autism spectrum disorders

Although the exact prevalence of metabolic abnormalities in autism spectrum disorders is unknown, several metabolic defects have been associated with autistic symptoms. These include phenylketonuria, histidinemia, adenylosuccinate lyase deficiency, dihydropyrimidine dehydrogenase deficiency, 5'-nucleotidase superactivity, and phosphoribosylpyrophosphate synthetase deficiency. When the metabolic consequences of an enzyme defect are well defined (e.g., phenylketonuria, 5'-nucleotidase superactivity), treatment with diet, drugs, or nutritional supplements may bring about a dramatic reduction in autistic symptoms. This review evaluates evidence for metabolic etiologies in autism spectrum disorders, as well as for the efficacy of dietary and vitamin treatments. The relationship between gastrointestinal abnormalities and autism spectrum disorders is also considered.