Neurologic aspects of adenylosuccinate lyase deficiency

Electroclinical features and phenotypic differences in adenylosuccinate lyase deficiency: Long-term follow-up of seven patients from four families and appraisal of the literature

OBJECTIVE

Adenylosuccinate lyase (ADSL) deficiency is a rare inherited metabolic disorder with a wide phenotypic presentation, classically grouped into three types (neonatal, type I, and type II). We aim to better delineate the pathological spectrum, focusing on the electroclinical characteristics and phenotypic differences of patients with ADSL deficiency.

PATIENTS AND METHODS

Seven patients, from four different families, underwent serial electroencephalogram (EEG), clinical assessment, and neuroimaging. We also performed a systematic review of the cases published in the literature, summarizing the available clinical, neurophysiological, and genetic data.

RESULTS

We report seven previously unreported ADSL deficiency patients with long-term follow-up (10-34 years). From the literature review, we collected 81 previously reported cases. Of the included patient population, 58 % (51/88) were classified as having ADSL deficiency type I, 28% (25/88) as having type II, and 14% (12/88) as having neonatal. The most frequently reported pathogenic variants are p.R426H homozygous (19 patients), p.Y114H in compound heterozygosity (13 patients), and p.D430N homozygous (6 patients). In the majority (89.2%), disease onset was within the first year of life. Epilepsy is present in 81.8% of the patients, with polymorphic and often intractable seizures. EEG features seem to display common patterns and developmental trajectories: (i) poor general background organization with theta-delta activity; (ii) hypsarrhythmia with spasms, usually adrenocorticotropic hormone-responsive; (iii) generalized epileptic discharges with frontal or frontal temporal predominance; and (iv) epileptic discharge activation in sleep with an altered sleep structure. Imaging features present consistent findings of cerebral atrophy with frontal predominance, cerebellar atrophy, and white matter abnormalities among the three types.

SIGNIFICANCE

ADSL deficiency presents variable phenotypic expression, whose severity could be partially attributed to residual activity of the mutant protein. Although a precise phenotype-genotype correlation was not yet feasible, we delineated a common pattern of clinical, neuroradiological, and neurophysiological features.

Inborn Errors of Metabolism Associated With Autism Spectrum Disorders: Approaches to Intervention

Increasing evidence suggests that the autism spectrum disorder (ASD) may be associated with inborn errors of metabolism, such as disorders of amino acid metabolism and transport [phenylketonuria, homocystinuria, S-adenosylhomocysteine hydrolase deficiency, branched-chain α-keto acid dehydrogenase kinase deficiency, urea cycle disorders (UCD), Hartnup disease], organic acidurias (propionic aciduria, L-2 hydroxyglutaric aciduria), cholesterol biosynthesis defects (Smith-Lemli-Opitz syndrome), mitochondrial disorders (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes-MELAS syndrome), neurotransmitter disorders (succinic semialdehyde dehydrogenase deficiency), disorders of purine metabolism [adenylosuccinate lyase (ADSL) deficiency, Lesch-Nyhan syndrome], cerebral creatine deficiency syndromes (CCDSs), disorders of folate transport and metabolism (cerebral folate deficiency, methylenetetrahydrofolate reductase deficiency), lysosomal storage disorders [Sanfilippo syndrome, neuronal ceroid lipofuscinoses (NCL), Niemann-Pick disease type C], cerebrotendinous xanthomatosis (CTX), disorders of copper metabolism (Wilson disease), disorders of haem biosynthesis [acute intermittent porphyria (AIP)] and brain iron accumulation diseases. In this review, we briefly describe etiology, clinical presentation, and therapeutic principles, if they exist, for these conditions. Additionally, we suggest the primary and elective laboratory work-up for their successful early diagnosis.

Reduced purine biosynthesis in humans after their divergence from Neandertals

We analyze the metabolomes of humans, chimpanzees, and macaques in muscle, kidney and three different regions of the brain. Although several compounds in amino acid metabolism occur at either higher or lower concentrations in humans than in the other primates, metabolites downstream of adenylosuccinate lyase, which catalyzes two reactions in purine synthesis, occur at lower concentrations in humans. This enzyme carries an amino acid substitution that is present in all humans today but absent in Neandertals. By introducing the modern human substitution into the genomes of mice, as well as the ancestral, Neandertal-like substitution into the genomes of human cells, we show that this amino acid substitution contributes to much or all of the reduction of de novo synthesis of purines in humans.

Clinical and molecular characterization of patients with adenylosuccinate lyase deficiency

Background

Adenylosuccinate lyase deficiency (ADSLD) is an ultrarare neurometabolic recessive disorder caused by loss-of-function mutations in the ADSL gene. The disease is characterized by wide clinical variability. Here we provide an updated clinical profiling of the disorder and discuss genotype–phenotype correlations.

Results

Data were collected through "Our Journey with ADSL deficiency Association" by using a dedicated web survey filled-in by parents.

Clinical and molecular data were collected from 18 patients (12 males, median age 10.9 years ± 7.3), from 13 unrelated families. The age at onset ranged from birth to the first three years (median age 0.63 years ± 0.84 SD), and age at diagnosis varied from 2 months to 17 years, (median age 6.4 years ± 6.1 SD). The first sign was a psychomotor delay in 8/18 patients, epilepsy in 3/18, psychomotor delay and epilepsy in 3/18, and apneas, hypotonia, nystagmus in single cases. One patient (sibling of a previously diagnosed child) had a presymptomatic diagnosis. The diagnosis was made by exome sequencing in 7/18 patients. All patients were definitively diagnosed with ADSL deficiency based on pathogenic variants and/or biochemical assessment. One patient had a fatal neonatal form of ADSL deficiency, seven showed features fitting type I, and nine were characterized by a milder condition (type II), with two showing a very mild phenotype. Eighteen different variants were distributed along the entire ADSL coding sequence and were predicted to have a variable structural impact by impairing proper homotetramerization or catalytic activity of the enzyme. Six variants had not previously been reported. All but two variants were missense.

Conclusions

The study adds more details on the spectrum of ADSLD patients’ phenotypes and molecular data.

Untargeted Metabolomics for Autism Spectrum Disorders: Current Status and Future Directions

Autism spectrum disorders (ASDs) are a group of neurodevelopment disorders characterized by childhood onset deficits in social communication and interaction. Although the exact etiology of most cases of ASDs is unknown, a portion has been proposed to be associated with various metabolic abnormalities including mitochondrial dysfunction, disorders of cholesterol metabolism, and folate abnormalities. Targeted biochemical testing like plasma amino acid and acylcarnitine profiles have demonstrated limited utility in helping to diagnose and manage such patients. Untargeted metabolomics has emerged, however, as a promising tool in screening for underlying biochemical abnormalities and managing treatment and as a means of investigating possible novel biomarkers for the disorder. Here, we review the principles and methodology behind untargeted metabolomics, recent pilot studies utilizing this technology, and areas in which it may be integrated into the care of children with this disorder in the future.

The nonessentiality of essential genes in yeast provides therapeutic insights into a human disease

Essential genes refer to those whose null mutation leads to lethality or sterility. Theoretical reasoning and empirical data both suggest that the fatal effect of inactivating an essential gene can be attributed to either the loss of indispensable core cellular function (Type I), or the gain of fatal side effects after losing dispensable periphery function (Type II). In principle, inactivation of Type I essential genes can be rescued only by re-gain of the core functions, whereas inactivation of Type II essential genes could be rescued by a further loss of function of another gene to eliminate the otherwise fatal side effects. Because such loss-of-function rescuing mutations may occur spontaneously, Type II essential genes may become nonessential in a few individuals of a large population. Motivated by this reasoning, we here carried out a systematic screening for Type II essentiality in the yeast Saccharomyces cerevisiae Large-scale whole-genome sequencing of essentiality-reversing mutants reveals 14 cases whereby the inactivation of an essential gene is rescued by loss-of-function mutations on another gene. In particular, the essential gene encoding the enzyme adenylosuccinate lyase (ADSL) is shown to be Type II, suggesting a loss-of-function therapeutic strategy for the human disorder ADSL deficiency. A proof-of-principle test of this strategy in the nematode Caenorhabditis elegans shows promising results.

Adenylosuccinate lyase deficiency

Adenylosuccinate lyase ADSL) deficiency is a defect of purine metabolism affecting purinosome assembly and reducing metabolite fluxes through purine de novo synthesis and purine nucleotide recycling pathways. Biochemically this defect manifests by the presence in the biologic fluids of two dephosphorylated substrates of ADSL enzyme: succinylaminoimidazole carboxamide riboside (SAICAr) and succinyladenosine (S-Ado). More than 80 individuals with ADSL deficiency have been identified, but incidence of the disease remains unknown. The disorder shows a wide spectrum of symptoms from slowly to rapidly progressing forms. The fatal neonatal form has onset from birth and presents with fatal neonatal encephalopathy with a lack of spontaneous movement, respiratory failure, and intractable seizures resulting in early death within the first weeks of life. Patients with type I (severe form) present with a purely neurologic clinical picture characterized by severe psychomotor retardation, microcephaly, early onset of seizures, and autistic features. A more slowly progressing form has also been described (type II, moderate or mild form), as having later onset, usually within the first years of life, slight to moderate psychomotor retardation and transient contact disturbances. Diagnosis is facilitated by demonstration of SAICAr and S-Ado in extracellular fluids such as plasma, cerebrospinal fluid and/or followed by genomic and/or cDNA sequencing and characterization of mutant proteins. Over 50 ADSL mutations have been identified and their effects on protein biogenesis, structural stability and activity as well as on purinosome assembly were characterized. To date there is no specific and effective therapy for ADSL deficiency.

Reversal of autism-like behaviors and metabolism in adult mice with single-dose antipurinergic therapy

Autism spectrum disorders (ASDs) now affect 1-2% of the children born in the United States. Hundreds of genetic, metabolic and environmental factors are known to increase the risk of ASD. Similar factors are known to influence the risk of schizophrenia and bipolar disorder; however, a unifying mechanistic explanation has remained elusive. Here we used the maternal immune activation (MIA) mouse model of neurodevelopmental and neuropsychiatric disorders to study the effects of a single dose of the antipurinergic drug suramin on the behavior and metabolism of adult animals. We found that disturbances in social behavior, novelty preference and metabolism are not permanent but are treatable with antipurinergic therapy (APT) in this model of ASD and schizophrenia. A single dose of suramin (20 mg kg(-1) intraperitoneally (i.p.)) given to 6-month-old adults restored normal social behavior, novelty preference and metabolism. Comprehensive metabolomic analysis identified purine metabolism as the key regulatory pathway. Correction of purine metabolism normalized 17 of 18 metabolic pathways that were disturbed in the MIA model. Two days after treatment, the suramin concentration in the plasma and brainstem was 7.64 μM pmol μl(-1) (±0.50) and 5.15 pmol mg(-1) (±0.49), respectively. These data show good uptake of suramin into the central nervous system at the level of the brainstem. Most of the improvements associated with APT were lost after 5 weeks of drug washout, consistent with the 1-week plasma half-life of suramin in mice. Our results show that purine metabolism is a master regulator of behavior and metabolism in the MIA model, and that single-dose APT with suramin acutely reverses these abnormalities, even in adults.

Metabolic causes of epileptic encephalopathy

Epileptic encephalopathy can be induced by inborn metabolic defects that may be rare individually but in aggregate represent a substantial clinical portion of child neurology. These may present with various epilepsy phenotypes including refractory neonatal seizures, early myoclonic encephalopathy, early infantile epileptic encephalopathy, infantile spasms, and generalized epilepsies which in particular include myoclonic seizures. There are varying degrees of treatability, but the outcome if untreated can often be catastrophic. The importance of early recognition cannot be overemphasized. This paper provides an overview of inborn metabolic errors associated with persistent brain disturbances due to highly active clinical or electrographic ictal activity. Selected diseases are organized by the defective molecule or mechanism and categorized as small molecule disorders (involving amino and organic acids, fatty acids, neurotransmitters, urea cycle, vitamers and cofactors, and mitochondria) and large molecule disorders (including lysosomal storage disorders, peroxisomal disorders, glycosylation disorders, and leukodystrophies). Details including key clinical features, salient electrophysiological and neuroradiological findings, biochemical findings, and treatment options are summarized for prominent disorders in each category.

Novel proton MR spectroscopy findings in adenylosuccinate lyase deficiency

Adenylosuccinate lyase (ADSL) deficiency is a rare inborn error of metabolism resulting in accumulation of metabolites including succinylaminoimidazole carboxamide riboside (SAICAr) and succinyladenosine (S-Ado) in the brain and other tissues. Patients with ADSL have progressive psychomotor retardation, neonatal seizures, global developmental delay, hypotonia, and autistic features, although variable clinical manifestations may make the initial diagnosis challenging. Two cases of the severe form of the disease are reported here: an 18-month-old boy with global developmental delay, intractable neonatal seizures, progressive cerebral atrophy, and marked hypomyelination, and a 3-month-old girl presenting with microcephaly, neonatal seizures, and marked psychomotor retardation. In both patients in vivo proton magnetic resonance spectroscopy (MRS) showed the presence of S-Ado signal at 8.3 ppm, consistent with a prior report. Interestingly, SAICAr signal was also detectable at 7.5 ppm in affected white matter, which has not been reported in vivo before. A novel splice-site mutation, c.IVS12 + 1/G > C, in the ADSL gene was identified in the second patient. Our findings confirm the utility of in vivo proton MRS in suggesting a specific diagnosis of ADSL deficiency, and also demonstrate an additional in vivo resonance (7.5 ppm) of SAICAr in the cases of severe disease.

Structural and biochemical characterization of human adenylosuccinate lyase (ADSL) and the R303C ADSL deficiency-associated mutation

Adenylosuccinate lyase (ADSL) deficiency is a rare autosomal recessive disorder, which causes a defect in purine metabolism resulting in neurological and physiological symptoms. ADSL executes two nonsequential steps in the de novo synthesis of AMP: the conversion of phosphoribosylsuccinyl-aminoimidazole carboxamide (SAICAR) to phosphoribosylaminoimidazole carboxamide, which occurs in the de novo synthesis of IMP, and the conversion of adenylosuccinate to AMP, which occurs in the de novo synthesis of AMP and also in the purine nucleotide cycle, using the same active site. Mutation of ADSL's arginine 303 to a cysteine is known to lead to ADSL deficiency. Interestingly, unlike other mutations leading to ADSL deficiency, the R303C mutation has been suggested to more significantly affect the enzyme's ability to catalyze the conversion of succinyladenosine monophosphate than that of SAICAR to their respective products. To better understand the causation of disease due to the R303C mutation, as well as to gain insights into why the R303C mutation potentially has a disproportional decrease in activity toward its substrates, the wild type (WT) and the R303C mutant of ADSL were investigated enzymatically and thermodynamically. Additionally, the X-ray structures of ADSL in its apo form as well as with the R303C mutation were elucidated, providing insight into ADSL's cooperativity. By utilizing this information, a model for the interaction between ADSL and SAICAR is proposed.

Magnetic resonance imaging of the brain in adenylosuccinate lyase deficiency: a report of seven cases and a review of the literature

Adenylosuccinate lyase (ADSL) deficiency is a rare autosomal recessive disorder of purine metabolism. Patients may present with a wide range of neurological symptoms. Head imaging abnormalities have been reported only rarely in the scientific literature and include atrophy of the cerebral cortex, corpus callosum, cerebellar vermis, lack of myelination, delayed myelination, anomalies of the white matter, and lissencephaly. The pathogenesis of abnormalities remains unknown. To further the understanding of the spectrum of brain abnormalities associated with ADSL deficiency, we examined the magnetic resonance findings in seven Polish patients with different clinical phenotypes and genotypes. Head MRI showed impaired white matter myelination with various degrees of global supra- and infratentorial white matter loss including widening of the lateral ventricles, enlargement of the subarachnoid spaces, atrophy of the cerebrum, hypoplasia of the cerebellar hemispheres and enlargement of the cisterna magna, and white matter abnormal hyperintense signal on T(2)-weighted sequences. We recommend performing a detailed analysis of urine and plasma purine metabolites in patients who have neurological findings, including developmental delay, microcephaly, autistic features, neonatal encephalopathy, and seizures especially if MRI findings such as delayed or lack of myelination, white matter abnormal signal, and atrophy of the cerebrum and/or cerebellum are also present. Greater awareness of adenylosuccinate lyase deficiency among general pediatricians, neonatologists, pediatric neurologists, and also radiologists is the key to identifying the disorder at an early stage.

Syndromic autism: causes and pathogenetic pathways

BACKGROUND

Autism is a severe neurodevelopmental disorder known to have many different etiologies. In the last few years, significant progresses have been made in comprehending the causes of autism and their multiple impacts on the developing brain. This article aims to review the current understanding of the etiologies and the multiple pathogenetic pathways that are likely to lead to the autistic phenotype.

DATA SOURCES

The PubMed database was searched with the keywords "autism" and "chromosomal abnormalities", "metabolic diseases", "susceptibility loci".

RESULTS

Genetic syndromes, defined mutations, and metabolic diseases account for less than 20% of autistic patients. Alterations of the neocortical excitatory/inhibitory balance and perturbations of interneurons' development represent the most probable pathogenetic mechanisms underlying the autistic phenotype in fragile X syndrome and tuberous sclerosis complex. Chromosomal abnormalities and potential candidate genes are strongly implicated in the disruption of neural connections, brain growth and synaptic/dendritic morphology. Metabolic and mitochondrial defects may have toxic effects on the brain cells, causing neuronal loss and altered modulation of neurotransmission systems.

CONCLUSIONS

A wide variety of cytogenetic abnormalities have been recently described, particularly in the low functioning individuals with dysmorphic features. Routine metabolic screening studies should be performed in the presence of autistic regression or suggestive clinical findings. As etiologies of autism are progressively discovered, the number of individuals with idiopathic autism will progressively shrink. Studies of genetic and environmentally modulated epigenetic factors are beginning to provide some clues to clarify the complexities of autism pathogenesis. The role of the neuropediatrician will be to understand the neurological basis of autism, and to identify more homogenous subgroups with specific biologic markers.

Recent advances in the pathogenesis of syndromic autisms

Background. Current advances in genetic technology continue to expand the list of medical conditions associated with autism. Clinicians have to identify specific autistic-related syndromes, and to provide tailored counseling. The aim of this study is to elucidate recent advances in autism research that offer important clues into pathogenetic mechanisms of syndromic autism and relevant implications for clinical practice. Data Sources. The PubMed database was searched with the keywords “autism” and “chromosomal abnormalities,” “metabolic diseases,” “susceptibility loci.” Results. Defined mutations, genetic syndromes, and metabolic diseases account for up to 20% of autistic patients. Metabolic and mitochondrial defects may have toxic effects on the brain cells, causing neuronal loss and altered modulation of neurotransmission systems. Alterations of the neocortical excitatory/inhibitory balance and perturbations of interneurons' development represent the most probable pathogenetic mechanisms underlying the autistic phenotype in Fragile X-Syndrome and Tuberous Sclerosis Complex. Chromosomal abnormalities and potential candidate genes are strongly implicated in the disruption of neural connections, brain growth, and synaptic/dendritic morphology. Conclusion. Metabolic testing may be appropriate if specific symptoms are present. High-resolution chromosome analysis may be recommended if a specific diagnosis is suspected because of obvious dysmorphisms. Identifying cryptic chromosomal abnormalities by whole genome microarray analysis can increase the understanding of the neurobiological pathways to autism.

D-ribose therapy in four Polish patients with adenylosuccinate lyase deficiency: absence of positive effect

Deficiency of adenylosuccinate lyase (ADSL) (OMIM 103050) is an autosomal recessive disorder of the purine de novo synthesis pathway and purine nucleotide cycle, diagnosed so far in approximately 50 patients. The clinical presentation is characterized by severe neurological involvement including hypotonia, seizures, developmental delay and autistic features. Epilepsy in ADSL deficiency is frequent and occurs in approximately two-thirds of patients, beginning either early in the neonatal period or after the first year of life. At present there is no treatment of proven clinical efficacy. Despite of the increasing number of ADSL-deficient patients reported, there are only a few communications of therapeutic considerations or efforts. Among them only two showed some beneficial effects in ADSL-deficient patients. D-ribose, a simple and relatively cheap therapy, has been associated with improvement of behaviour and progressive reduction of the seizure frequency in one 13-year-old patient with ADSL deficiency. In this study we have re-examined D-ribose treatment in four ADSL-deficient patients. Assessments consisted of biochemical markers and neurological outcome. The 12-month trial of D-ribose failed to show any clinical benefit in ADSL patients with both milder and severe phenotype. D-ribose administration was accompanied by neither reduction in seizure frequency nor growth enhancement. Additionally, patients with milder type II presented the first seizure after 4 and 8 months of the D-ribose treatment. Therefore, we could not confirm a positive effect of D-ribose as previously reported.

The characterization of mutant Bacillus subtilis adenylosuccinate lyases corresponding to severe human adenylosuccinate lyase deficiencies

Adenylosuccinate lyase is a homotetramer that catalyzes two discrete reactions in the de novo synthesis of purines: the cleavage of adenylosuccinate and succinylaminoimidazole carboxamide ribotide (SAICAR). Several point mutations in the gene encoding the enzyme have been implicated in human disease. Bacillus subtilis adenylosuccinate lyase was used as a model system in which mutations were constructed corresponding to those mutations associated with severe human adenylosuccinate lyase deficiency. Site-directed mutagenesis was utilized to construct amino acid substitutions in B. subtilis adenylosuccinate lyase; Met(10), Ile(123), and Thr(367) were replaced by Leu, Trp, and Arg, respectively, and the altered enzymes were expressed in Escherichia coli. These purified enzymes containing amino acid substitutions were found to have substantial catalytic activity and exhibit relatively small changes in their kinetic parameters. The major deviations from the wild-type-like behavior were observed upon biophysical characterization. All of these enzymes with amino acid replacements are associated with marked thermal instability. I123W adenylosuccinate lyase exhibits notable changes in the circular dichroism spectra, and a native gel electrophoresis pattern indicative of some protein aggregation. T367R also exhibits alterations at the quarternary level, as reflected in native gel electrophoresis. Experimental results, combined with homology modeling, suggest that the altered enzymes are primarily structurally impaired. The enzyme instability was found to be lessened by subunit complementation with the wild-type enzyme, under mild conditions; these studies may have implications for the in vivo behavior of adenylosuccinate lyase in heterozygous patients. Residues Met(10), Ile(123), and Thr(367) appear to be located in regions of the enzyme important for maintaining the structural integrity required for a stable, functional enzyme.