GABA Transaminase Deficiency With Survival Into Adulthood

Histidine-containing dipeptide deficiency links to hyperactivity and depression-like behaviors in old female mice

Carnosine, anserine, and homocarnosine are histidine-containing dipeptides (HCDs) abundant in the skeletal muscle and nervous system in mammals. To date, studies have extensively demonstrated effects of carnosine and anserine, the predominant muscular HCDs, on muscular functions and exercise performance. However, homocarnosine, the predominant brain HCD, is underexplored. Moreover, roles of homocarnosine and its related HCDs in the brain and behaviors remain poorly understood. Here, we investigated potential roles of endogenous brain homocarnosine and its related HCDs in behaviors by using carnosine synthase-1-deficient (Carns1-/-) mice. We found that old Carns1-/- mice (female 12 months old) exhibited hyperactivity- and depression-like behaviors with higher plasma corticosterone levels on light-dark transition and forced swimming tests, but had no defects in spontaneous locomotor activity, repetitive behavior, olfactory functions, and learning and memory abilities, as compared with their age-matched wild-type (WT) mice. We confirmed that homocarnosine and its related HCDs were deficient across brain areas of Carns1-/- mice. Homocarnosine deficiency exhibited small effects on its constituent γ-aminobutyric acid (GABA) in the brain, in which GABA levels in hypothalamus and olfactory bulb were higher in Carns1-/- mice than in WT mice. In WT mice, homocarnosine and GABA were highly present in hypothalamus, thalamus, and olfactory bulb, and their brain levels did not decrease in old mice when compared with younger mice (3 months old). Our present findings provide new insights into roles of homocarnosine and its related HCDs in behaviors and neurological disorders.

Computational Exploration of the Effects of Mutations on GABA Aminotransferase in GABA Aminotransferase Deficiency

Gamma-aminobutyric acid (GABA) transaminase-also called GABA aminotransferase (GABA-AT)-deficiency is a rare autosomal recessive disorder characterized by a severe neonatal-infantile epileptic encephalopathy with symptoms such as seizures, hypotonia, hyperreflexia, developmental delay, and growth acceleration. GABA transaminase deficiency is caused by mutations in GABA-AT, the enzyme responsible for the catabolism of GABA. Mutations in multiple locations on GABA-AT have been reported and their locations have been shown to influence the onset of the disease and the severity of symptoms. We examined how GABA-AT mutations influence the structural stability of the enzyme and GABA-binding affinity using computational methodologies such as molecular dynamics simulation and binding free energy calculation to understand the underlying mechanism through which GABA-AT mutations cause GABA-AT deficiency. GABA-AT 3D model depiction was carried out together with seven individual mutated models of GABA-AT. The structural stability of all the predicted models was analyzed using several tools and web servers. All models were evaluated based on their phytochemical values. Additionally, 100 ns MD simulation was carried out and the mutated models were evaluated using RMSD, RMSF, R_g, and SASA. gmxMMPBSA free energy calculation was carried out. Moreover, RMSD and free energy calculations were also compared with those obtained using online web servers. Our study demonstrates that P152S, Q296H, and R92Q play a more critical role in the structural instability of GABA-AT compared with the other mutated models: G465R, L211F, L478P, and R220K.

Neurotransmission-related gene expression in the frontal pole is altered in subjects with bipolar disorder and schizophrenia

The frontal pole (Brodmann area 10, BA10) is the largest cytoarchitectonic region of the human cortex, performing complex integrative functions. BA10 undergoes intensive adolescent grey matter pruning prior to the age of onset for bipolar disorder (BP) and schizophrenia (SCHIZ), and its dysfunction is likely to underly aspects of their shared symptomology. In this study, we investigated the role of BA10 neurotransmission-related gene expression in BP and SCHIZ. We performed qPCR to measure the expression of 115 neurotransmission-related targets in control, BP, and SCHIZ postmortem samples (n = 72). We chose this method for its high sensitivity to detect low-level expression. We then strengthened our findings by performing a meta-analysis of publicly released BA10 microarray data (n = 101) and identified sources of convergence with our qPCR results. To improve interpretation, we leveraged the unusually large database of clinical metadata accompanying our samples to explore the relationship between BA10 gene expression, therapeutics, substances of abuse, and symptom profiles, and validated these findings with publicly available datasets. Using these convergent sources of evidence, we identified 20 neurotransmission-related genes that were differentially expressed in BP and SCHIZ in BA10. These results included a large diagnosis-related decrease in two important therapeutic targets with low levels of expression, HTR2B and DRD4, as well as other findings related to dopaminergic, GABAergic and astrocytic function. We also observed that therapeutics may produce a differential expression that opposes diagnosis effects. In contrast, substances of abuse showed similar effects on BA10 gene expression as BP and SCHIZ, potentially amplifying diagnosis-related dysregulation.

Combined exome analysis and exome depth assessment achieve a high diagnostic yield in an epilepsy case series, revealing significant genomic heterogeneity and novel mechanisms

OBJECTIVES

Genetics of epilepsy are highly heterogeneous and complex. Lesions detected involve genes encoding various types of channels, transcription factors, and other proteins implicated in numerous cellular processes, such as synaptogenesis. Consequently, a wide spectrum of clinical presentations and overlapping phenotypes hinders differential diagnosis and highlights the need for molecular investigations toward delineation of underlying mechanisms and final diagnosis. Characterization of defects may also contribute valuable data on genetic landscapes and networks implicated in epileptogenesis.

METHODS

This study reports on genetic findings from exome sequencing (ES) data of 107 patients with variable types of seizures, with or without additional symptoms, in the context of neurodevelopmental disorders.

RESULTS

Multidisciplinary evaluation of ES, including ancillary detection of copy number variants (CNVs) with the ExomeDepth tool, supported a definite diagnosis in 59.8% of the patients, reflecting one of the highest diagnostic yields in epilepsy.

CONCLUSION

Emerging advances of next-generation technologies and 'in silico' analysis tools offer the possibility to simultaneously detect several types of variations. Wide assessment of variable findings, specifically those found to be novel and least expected, reflects the ever-evolving genetic landscape of seizure development, potentially beneficial for increased opportunities for trial recruitment and enrollment, and optimized, even personalized, medical management.

Gamma-Aminobutyric Acid Transaminase (GABA-T) Deficiency in a Consanguineous Saudi Family: A Case Report and Literature Review

Gamma-aminobutyric acid transaminase (GABA-T) deficiency is a rare, autosomal recessive disorder caused by mutations in the 4-aminobutyrate aminotransferase (ABAT) gene, which encodes an enzyme involved in GABA catabolism. It is characterized by severe psychomotor retardation, early-onset epileptic encephalopathy, intractable seizures, hypotonia, hyperreflexia, movement disorder, hypersomnolence, and early childhood mortality. It is associated with elevated free GABA in cerebrospinal fluid (CSF), GABA-T deficiency in cultured lymphoblasts, hypomyelination on brain magnetic resonance imaging (MRI), and elevated GABA level in the basal ganglia on proton magnetic resonance spectroscopy (MRS). Only 14 cases have been published in the literature. A rare case of infantile epileptic encephalopathy caused by GABA-T deficiency resulting from a previously unreported homozygous missense mutation in the ABAT gene is described. Our findings add to the phenotypic, neuroradiological, and genetic spectrum of ABAT mutations.

Coupling of GABA Metabolism to Mitochondrial Glucose Phosphorylation

Glucose and oxygen (O₂) are vital to the brain. Glucose metabolism and mitochondria play a pivotal role in this process, culminating in the increase of reactive O₂ species. Hexokinase (HK) is a key enzyme on glucose metabolism and is coupled to the brain mitochondrial redox modulation by recycling ADP for oxidative phosphorylation (OXPHOS). GABA shunt is an alternative pathway to GABA metabolism that increases succinate levels, a Krebs cycle intermediate. Although glucose and GABA metabolisms are intrinsically connected, their interplay coordinating mitochondrial function is poorly understood. Here, we hypothesize that the HK and the GABA shunt interact to control mitochondrial metabolism differently in the cortex and the hypothalamus. The GABA shunt stimulated mitochondrial O₂ consumption and H₂O₂ production higher in hypothalamic synaptosomes (HSy) than cortical synaptosomes (CSy). The GABA shunt increased the HK coupled to OXPHOS activity in both population of synaptosomes, but the rate of activation was higher in HSy than CSy. Significantly, malonate and vigabatrin blocked the effects of the GABA shunt in the HK activity coupled to OXPHOS. It indicates that the glucose phosphorylation is linked to GABA and Krebs cycle reactions. Together, these data shed light on the HK and SDH role on the metabolism of each region fed by GABA turnover, which depends on the neurons' metabolic route.

Epilepsy in inherited neurotransmitter disorders: Spotlights on pathophysiology and clinical management

Inborn errors of neurotransmitter metabolism are ultrarare disorders affecting neurotransmitter biosynthesis, breakdown or transport or their essential cofactors. Neurotransmitter dysfunctions could also result from the impairment of neuronal receptors, intracellular signaling, vesicle release or other synaptic abnormalities. Epilepsy is the main clinical hallmark in some of these diseases (e.g. disorders of GABA metabolism, glycine encephalopathy) while it is infrequent in others (e.g. all the disorders of monoamine metabolism in exception for dihydropteridine reductase deficiency). This review analyzes the epileptogenic mechanisms, the epilepsy phenotypes and the principle for the clinical management of epilepsy in primary and secondary inherited disorders of neurotransmitter metabolism (disorders of GABA, serine and glycine metabolism, disorders of neurotransmitter receptors and secondary neurotransmitter diseases).

The characterization of psychotic symptoms in succinic semialdehyde dehydrogenase deficiency: a review

Succinic semialdehyde dehydrogenase (SSADH) deficiency is an ultra-rare inborn error of metabolism that results in disrupted gamma-amino butyric acid (GABA) catabolism. In addition to developmental delay, intellectual disability, hypotonia, ataxia, and seizures, a variety of neuropsychiatric symptoms may occur, including psychosis. By highlighting all available and relevant case reports/series, this qualitative review seeks to characterize the prevalence, clinical manifestation, pathophysiology, and treatment of psychotic symptoms in this population. Psychosis occurs in a minority of SSADH-deficient individuals, and most commonly presents as auditory or visual hallucinations with an onset in adolescence or young adulthood. Although the pathophysiology underlying the development of psychosis in this context is not fully understood, it likely in part relates to increased GABA and/or gamma hydroxybutyric acid activity. Although antipsychotic medications should be used cautiously in SSADH deficiency, they may be effective at treating emergent psychotic symptoms.