Succinic semialdehyde dehydrogenase deficiency

Consensus guidelines for the diagnosis and management of succinic semialdehyde dehydrogenase deficiency

Succinic semialdehyde dehydrogenase deficiency (SSADHD) (OMIM #271980) is a rare autosomal recessive metabolic disorder caused by pathogenic variants of ALDH5A1. Deficiency of SSADH results in accumulation of γ-aminobutyric acid (GABA) and other GABA-related metabolites. The clinical phenotype of SSADHD includes a broad spectrum of non-pathognomonic symptoms such as cognitive disabilities, communication and language deficits, movement disorders, epilepsy, sleep disturbances, attention problems, anxiety, and obsessive-compulsive traits. Current treatment options for SSADHD remain supportive, but there are ongoing attempts to develop targeted genetic therapies. This study aimed to create consensus guidelines for the diagnosis and management of SSADHD. Thirty relevant statements were initially addressed by a systematic literature review, resulting in different evidence levels of strength according to the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) criteria. The highest level of evidence (level A), based on randomized controlled trials, was unavailable for any of the statements. Based on cohort studies, Level B evidence was available for 12 (40%) of the statements. Thereupon, through a process following the Delphi Method and directed by the Appraisal of Guidelines for Research and Evaluation (AGREE II) criteria, expert opinion was sought, and members of an SSADHD Consensus Group evaluated all the statements. The group consisted of neurologists, epileptologists, neuropsychologists, neurophysiologists, metabolic disease specialists, clinical and biochemical geneticists, and laboratory scientists affiliated with 19 institutions from 11 countries who have clinical experience with SSADHD patients and have studied the disorder. Representatives from parent groups were also included in the Consensus Group. An analysis of the survey's results yielded 25 (83%) strong and 5 (17%) weak agreement strengths. These first-of-their-kind consensus guidelines intend to consolidate and unify the optimal care that can be provided to individuals with SSADHD.

Clinical and molecular outcomes from the 5-Year natural history study of SSADH Deficiency, a model metabolic neurodevelopmental disorder

BACKGROUND

Succinic semialdehyde dehydrogenase deficiency (SSADHD) represents a model neurometabolic disease at the fulcrum of translational research within the Boston Children's Hospital Intellectual and Developmental Disabilities Research Centers (IDDRC), including the NIH-sponsored natural history study of clinical, neurophysiological, neuroimaging, and molecular markers, patient-derived induced pluripotent stem cells (iPSC) characterization, and development of a murine model for tightly regulated, cell-specific gene therapy.

METHODS

SSADHD subjects underwent clinical evaluations, neuropsychological assessments, biochemical quantification of γ-aminobutyrate (GABA) and related metabolites, electroencephalography (standard and high density), magnetoencephalography, transcranial magnetic stimulation, magnetic resonance imaging and spectroscopy, and genetic tests. This was parallel to laboratory molecular investigations of in vitro GABAergic neurons derived from induced human pluripotent stem cells (hiPSCs) of SSADHD subjects and biochemical analyses performed on a versatile murine model that uses an inducible and reversible rescue strategy allowing on-demand and cell-specific gene therapy.

RESULTS

The 62 SSADHD subjects [53% females, median (IQR) age of 9.6 (5.4-14.5) years] included in the study had a reported symptom onset at ∼ 6 months and were diagnosed at a median age of 4 years. Language developmental delays were more prominent than motor. Autism, epilepsy, movement disorders, sleep disturbances, and various psychiatric behaviors constituted the core of the disorder's clinical phenotype. Lower clinical severity scores, indicating worst severity, coincided with older age (R= -0.302, p = 0.03), as well as age-adjusted lower values of plasma γ-aminobutyrate (GABA) (R = 0.337, p = 0.02) and γ-hydroxybutyrate (GHB) (R = 0.360, p = 0.05). While epilepsy and psychiatric behaviors increase in severity with age, communication abilities and motor function tend to improve. iPSCs, which were differentiated into GABAergic neurons, represent the first in vitro neuronal model of SSADHD and express the neuronal marker microtubule-associated protein 2 (MAP2), as well as GABA. GABA-metabolism in induced GABAergic neurons could be reversed using CRISPR correction of the pathogenic variants or mRNA transfection and SSADHD iPSCs were associated with excessive glutamatergic activity and related synaptic excitation.

CONCLUSIONS

Findings from the SSADHD Natural History Study converge with iPSC and animal model work focused on a common disorder within our IDDRC, deepening our knowledge of the pathophysiology and longitudinal clinical course of a complex neurodevelopmental disorder. This further enables the identification of biomarkers and changes throughout development that will be essential for upcoming targeted trials of enzyme replacement and gene therapy.

4‐Hydroxy‐trans‐2‐nonenoic acid is a γ‐hydroxybutyrate receptor ligand in the cerebral cortex and hippocampus

Elevated production of 4-hydroxy-trans-2-nonenal (HNE) occurs in numerous neurological disorders involving oxidative damage. HNE is metabolized to the non-toxic 4-hydroxy-trans-2-nonenoic acid (HNEAcid) by aldehyde dehydrogenases in the rat cerebral cortex. Based upon the structural similarity of HNEAcid to ligands of the gamma-hydroxybutyrate (GHB) receptor, we hypothesized that HNEAcid is an endogenous ligand for the GHB receptor. HNEAcid displaced the specific binding of the GHB receptor ligand (3)H-NCS382 (30 nm) in membrane preparations of human frontal cerebral cortex and whole rat cerebral cortex with IC(50s) of 3.9 +/- 1.1 and 5.6 +/- 1.2 micro m, respectively. Inhibition was attenuated when the carboxyl group of HNEAcid was replaced with an aldehyde or an alcohol. HNEAcid (300 micro m) did not displace the binding of beta-adrenergic receptor and GABA(B) receptor antagonists, demonstrating the selectivity of HNEAcid for the GHB receptor. HNEAcid is formed in homogenates of human frontal cortical gray matter in an NAD(+)-dependent (V(Max), 0.71 nmol/min/mg) and NADP(+)-dependent (V(Max), 0.12 nmol/min/mg) manner. Lastly, (3)H-NCS382 binding is elevated 2.7-fold with age in the cerebral cortex of rats. Our data demonstrate that an HNE metabolite, formed in rat and human brain, is a signaling molecule analogous to other bioactive lipid peroxidation products.

The gamma-hydroxybutyrate signalling system in brain: organization and functional implications

gamma-Hydroxybutyrate is a metabolite of GABA which is synthesized and accumulated by neurons in brain. This substance is present in micromolar quantities in all brain regions investigated as well as in several peripheral organs. Neuronal depolarization releases gamma-hydroxybutyrate into the extracellular space in a Ca(2+)-dependent manner. Gamma-hydroxybutyrate high-affinity receptors are present only in neurons, with a restricted specific distribution in the hippocampus, cortex and dopaminergic structures of rat brain (the striatum in general, olfactory bulbs and tubercles, frontal cortex, dopaminergic nuclei A9, A10 and A12). Stimulation of these receptors with low amounts of gamma-hydroxybutyrate induces in general hyperpolarizations in dopaminergic structures with a reduction of dopamine release. However, in the hippocampus and the frontal cortex, it seems that gamma-hydroxybutyrate induces depolarization with an accumulation of cGMP and an increase in inositol phosphate turnover. Some of the electrophysiological effects of GHB are blocked by NCS-382, a gamma-hydroxybutyrate receptor antagonist while some others are strongly attenuated by GABAB receptors antagonists. Gamma-hydroxybutyrate penetrates freely into the brain when administered intravenously or intraperitoneally. This is a unique situation for a molecule with signalling properties in the brain. Thus, the gamma-hydroxybutyrate concentration in brain easily can be increased more than 100 times. Under these conditions, gamma-hydroxybutyrate receptors are saturated and probably desensitized and down-regulated. It is unlikely that GABAB receptors could be stimulated directly by GHB. Most probably, GABA is released in part under the control of GHB receptors in specific pathways expressing GABAB receptors. Alternatively, GABAB receptors might be specifically stimulated by the GABA formed via the metabolism of gamma-hydroxybutyrate in brain. In animals and man, these GHBergic and GABAergic potentiations induce dopaminergic hyperactivity (which follows the first phase of dopaminergic terminal hyperpolarization), a strong sedation with anaesthesia and some EEG changes with epileptic spikes. It is presumed that, under pathological conditions (hepatic failure, alcoholic intoxication, succinic semialdehyde dehydrogenase defects), the rate of GHB synthesis or degradation in the peripheral organ is modified and induces increased GHB levels which could interfere with the normal brain mechanisms. This pathological status could benefit from treatments with gamma-hydroxybutyric and/or GABAB receptors antagonists. Nevertheless, the regulating properties of the endogenous gamma-hydroxybutyrate system on the dopaminergic pathways are a cause for the recent interest in synthetic ligands acting specifically at gamma-hydroxybutyrate receptors and devoid of any role as metabolic precursor of GABA in brain.

Gammahydroxybutyrate: an endogenous regulator of energy metabolism

Gammahydroxybutyrate is a naturally occurring metabolite of many mammalian tissues. Although its administration produces a wide range of pharmacological effects, its normal function has never been clearly defined. GHB can induce NREM and REM sleep, anaesthesia, hypothermia, and a trance-like state which has been considered a model for petit mal epilepsy. It markedly increases brain dopamine levels. It has been touted as a central neurotransmitter or neuromodulator, and high affinity brain receptors, as well as central mechanisms for its synthesis, uptake and release have been demonstrated in support of this. But GHB is also found in many peripheral tissues and in some of these in higher concentrations than in the brain. No explanation has been offered for its presence in these tissues. A number of studies indicate that GHB can reduce energy substrate consumption in both brain and peripheral tissues, and that it can protect these tissues from the damaging effects of anoxia or excessive metabolic demand. Indeed there is some evidence to suggest that endogenous GHB levels rise under these circumstances. GHB appears to act through the endogenous opioid system, since in the brain, at least, GHB raises dynorphin levels and its metabolic and pharmacological effects can be blocked by naloxone. These, and other observations detailed in this review, suggest that GHB may function naturally in the induction and maintenance of physiological states, like sleep and hibernation, in which energy utilization is depressed. GHB may also function naturally as an endogenous protective agent when tissue energy supplies are limited.

Demonstration of 4-aminobutyric acid aminotransferase deficiency in lymphocytes and lymphoblasts

Lysates of lymphocytes, isolated from whole blood, and Epstein-Barr virus transformed cultured lymphoblasts catalysed the transamination of 4-aminobutyric acid with 2-oxoglutaric acid as co-substrate. 4-Aminobutyric acid aminotransferase activity in lymphocyte and lymphoblast sonicates derived from 12 unrelated control individuals (6 each) was 39 +/- 19 pmol min(-1) (mg protein (-1] (mean +/- 1 SD). Activities in lysates of both types of cell derived from a Flemish patient were less than 3% of control. 4-Aminobutyric acid aminotransferase activity in sonicates derived from the parents and a healthy sibling were 15-37% of the control mean for lymphocytes and 13-20% of the control mean in lymphoblasts, respectively. Km values in a control lymphoblast sonicate were 0.63 and 0.08 mmol L(-1) for 4-aminobutyric and 2-oxoglutaric acids, respectively. These data indicate that the parents and healthy sibling are heterozygous and the patient is homozygous for a defective gene responsible for 4-aminobutyric acid aminotransferase deficiency, and that inheritance is autosomal recessive.