Clinical manifestations

Presentation and course

All patients presented with neonatal or early infantile onset developmental impairment, generalized tonic-clonic seizures, hypotonia, choreoathetosis, and subcortical myoclonus. Accelerated growth has been reported in 4 patients and delayed myelination was seen in 9 of 10 reported patients (34; 22).

Clinical findings in the index family were neonatal seizures, lethargy, hypotonia, hyperreflexia, poor feeding, severely retarded psychomotor development, and a high-pitched cry (16; 08; 15; 15; 06; 31; 18; 38). In both siblings, linear growth and head circumference were accelerated, with normal height and head circumference noted in both parents. Accelerated growth is consistent with the growth hormone-promoting effects of GABA (36). At 2 years of age, the female was 4 cm over the 97th percentile, and her head circumference was in the 75th percentile. The male showed a rapid increase in head circumference, from the 50th to 97th percentiles, during the last 6 weeks of his life. Adiposity and intermittent hepatomegaly were noted in the female proband. The first child of these parents died at the age of 5 days from an unknown cause; the second child is healthy. EEG in the female was normal at 2 weeks of age, revealed predominantly low-voltage beta-frequency activity with intermittent epileptiform discharges at 7 months, and showed generalized epileptiform paroxysms at 2 years. Visual, auditory, and somatosensory evoked potentials were absent at the age of 2 years. CT in both siblings revealed severe ventricular enlargement as well as increased cisternal and sulcal spaces.

The next reported patient was a 6745-gram female born to nonconsanguineous Japanese parents (44). Both parents and a 6-year-old sister were healthy, and there was no family history of neurologic disorders. The patient had an uncomplicated birth and normal early infancy but was evaluated at 7 months for hyperreflexia, psychomotor retardation, hypotonia, bilateral intermittent esotropia, and a positive Babinski reflex. No dysmorphic features were noted. At 8 months of age, the patient was hospitalized for decreased consciousness after the onset of a febrile illness, and she subsequently developed respiratory distress and required mechanical ventilation. She began to experience segmental myoclonic jerks that were not completely controlled by phenobarbital, clonazepam, valproate, or midazolam. EEG at 8 months revealed diffuse spike-and-wave activity with 1- to 2-second periods of background suppression. At 11 months, the patient was experiencing chronic respiratory failure, and the febrile illness resulted in neurodegeneration characterized by opisthotonic posturing with generalized dystonia and continued segmental myoclonus. Free GABA was elevated in serum, CSF, and cortical tissue, whereas GABA-transaminase activity was diminished. CT was normal, and the brain MRI suggested a mild delay in myelination, but no structural abnormalities. Follow-up was reported at 9 and a half years of age with profound neurodevelopmental impairment, active generalized tonic-clonic seizures, and a medication program of phenobarbital, clonazepam, diazepam, baclofen, dantrolene, and haloperidol.

Diagnostic findings include significantly elevated GABA (free and total) and beta-alanine levels in plasma and CSF, as well as diminished GABA-transaminase enzymatic activity. In both the female index patient and the unrelated case, CSF free-GABA was the most elevated biological marker at 60x and 16x elevated levels, respectively CSF bound, plasma bound, and plasma free GABA was also markedly elevated (at least 3 times normal levels) (16; 18; 44). Tsuji and colleagues used quantitative proton magnetic resonance spectroscopy (1H-MRS) to detect significantly increased GABA concentration in the basal ganglia of their living patient (44).

A 12-month-old male patient, diagnosed postmortem, was described in an abstract published in 2015 (25). He presented as a neonate with generalized tonic-clonic seizures, hypotonia, accelerated growth, lethargy, and cerebral atrophy without developmental progress. A 2015 paper discussed 2 siblings: a female who died at age 7 and a male who was alive at 7. Both presented at 3 months of age with profound impairment, seizures, hypotonia, lethargy, and cerebral atrophy (03). Free CSF GABA levels were more than twice the upper limit of normal (2.9 and 3.1 ULN = 1.4). A 2016 paper reported a third sibling of the above 2 patients, who presented at 2 months with a highly similar clinical presentation to the 2 siblings previously discussed and died at 12 months (02). This paper also reported a patient with a clinical presentation milder than the previously reported cases. The male patient presented at 6 months of age with hypotonia, hypersomnolence, developmental delay, and mild chorea. No structural brain abnormalities were noted, but a cranial MRI at 9 months showed delayed myelination. This patient had a milder presentation, without overt seizures, although EEG showed hypsarrhythmia. Unlike the previously reported patients, he reached developmental milestones and could control his head when sitting, track faces, smile appropriately, and eat and drink solid food by mouth by 18 months (02).

Nagappa and colleagues described a female neonate that presented with global developmental delay, hypersomnolence, and hypotonia (30). The patient also presented with a hyperkinetic movement disorder, including choreoathetosis, with movements that subsided during sleep. EEG initially was normal at 6 months, but showed multifocal epileptiform spikes at 18 months. MRI showed increased T2/FLAIR signal with T1 hypointensity, interpreted as dysmyelination, affecting the medulla, dorsal pons, cerebellar dentate nucleus, and internal capsule (anterior limb and genu). A trilaminated appearance was present based on central hypointensity and hyperintense margins involving the posterior limb of the internal capsule. Morales-Briceño and colleagues described previously unreported findings of paroxysmal dyskinesias associated with drowsiness and T2 thalamic hyperintensities in 2 adult siblings, 32- and 25-year-old sisters with history of mild developmental delays and seizures (29).

A series published in 2017 reported a boy born to consanguineous parents who presented at 5 months with developmental delay, hypotonia, hypersomnolence, choreoathetosis, and focal seizures (22). Seizure types expanded to include generalized tonic-clonic at 2 years and absence as well as generalized convulsive status epilepticus at 6 years. Cranial MRI showed progressive atrophy at 4 years.

In the case series by Koenig and colleagues, of the 10 patients reported with confirmed GABA-transaminase deficiency, 4 were deceased by age 25 months and 1 survived to age 7 years. The 5 surviving reported cases are ages 18 months to 9 and a half years. In recent years, milder phenotypes are emerging and are associated with higher residual enzymatic activity and survival into adulthood (13; 29).

In 2020, another genetically confirmed GABA-transaminase deficiency case was reported (32). The description of this case included a neonate who presented with severe neonatal epileptic encephalopathy, lethargy, hypotonia, hyperreflexia, and poor feeding. An MRI of this neonate revealed extensive intraventricular hemorrhage (which could be coincidental). At 14 days of age, he developed polyuria and hypernatremia and had a low antidiuretic hormone serum level, corresponding to diabetes insipidus. He was treated with desmopressin until his death at 7 months. A whole exome study detected homozygosity of the c.316G > A (p.Gly106Ser) variant in the ABAT gene. Both his parents, who were consanguineous, were heterozygous carriers of this variant.

Increasing the use of sequencing of the ABAT gene is expected to identify new cases. CSF quantification of GABA-free and total levels is recommended to confirm the pathogenic sequencing findings. Definitive diagnosis can also be made by measurement of GABA-T activity in the liver, lymphocytes isolated from whole blood, or Epstein-Barr virus-transformed cultured lymphoblasts (09).

Prognosis and complications

Patients have uniformly presented with neonatal or early infantile encephalopathy, often accompanied by hypotonia, lethargy, seizures, and extrapyramidal manifestations. In the first reported family, 1 child died at 5 days of age from an unknown cause without sufficient clinical history to establish a retrospective diagnosis. GABA-transaminase deficiency appears to be extremely rare, although it is possible that the inborn error is lethal in utero or very early in the neonatal period, before a metabolic diagnosis is made. Additionally, CSF GABA levels in CSF are not measured routinely measured in patients with epileptic encephalopathies, and the disorder may be undetected.

Clinical vignette

An 8-year-old boy born to first cousin parents presented at 5 months of age with hypotonia, excessive daytime somnolence, and virtually continuous movements in the awake state consistent with choreoathetosis. Generalized seizures ensued by age 2 years, and EEGs showed multifocal discharges and diffuse slowing. MRI demonstrated progressive atrophy. Whole exome sequencing demonstrated a novel homozygous variant in ABAT (c.1129C> T; p.R377W). Parents were confirmed as heterozygous carriers.

Biological basis

Etiology and pathogenesis

GABA-transaminase deficiency is inherited (E.C. 2.6.1.19) as an autosomal recessive disorder. The enzymatic defect involves the major GABA degradative pathway whereby GABA is converted to succinic semialdehyde, an unstable intermediate, by GABA-transaminase. Succinic semialdehyde is rapidly converted to succinic acid, which enters the tricarboxylic acid cycle (33). This process leads to the conversion of alpha-ketoglutarate to glutamate, which characterizes the GABA shunt whereby each molecule of GABA that is metabolized leads to the synthesis of a molecule of glutamate, which is then converted to GABA via glutamic acid decarboxylase. Hence, GABA-transaminase is a vital enzyme in the intricate balance between glutamate, the major excitatory neurotransmitter, and GABA, a major inhibitory neurotransmitter in the brain (04).

Compared to control levels, CSF concentrations of free GABA were 60-fold higher, homocarnosine 3-fold higher, and beta-alanine 10-fold elevated in the index female (18). CSF from this patient also contained elevated quantities of "unidentified" GABA conjugates, ornithine, and putrescine (16). In the same patient, free GABA and beta-alanine plasma concentrations were elevated 10- and 4-fold, respectively, compared to control. Fasting plasma growth hormone levels were increased (8 to 38 ng/mL; normal, less than 5 ng/mL). Increased concentrations of GABA, beta-alanine, beta-aminoisobutyric acid, and taurine were detected in the patient's urine (16). In the unrelated patient, metabolite findings revealed that free GABA was increased nearly 7-fold compared to control levels in plasma and 16-fold in CSF (44). 2-pyrrolidinone is a lactam cyclization product of GABA and has long been suggested as a transport form of GABA due to the ability to readily convert to GABA in the brain. 2-pyrrolidinone also crosses the blood-brain barrier more readily than GABA and is more stable in CSF and plasma. Koenig and Bonnen described elevated 2-pyrrolidinone in CSF and plasma (above 2 standard deviations from the control mean) in 2 patients. The administration of flumazenil did not alter the elevated 2-pyrrolidinone (21). Kennedy and colleagues described elevated 2-pyrrolidinone (average of over 4 standard deviations beyond the control mean) and succinamic acid in the plasma in 3 of 4 patients, with similar findings in CSF and urine. To validate results, this elevation of 2-pyrrolidinone was also observed in patients taking vigabatrin, an antiseizure medication that is an inhibitor of GABA-transaminase. Although 2-pyrrolidinone was elevated with the administration of other antiseizure medications, the difference was not significantly higher as was seen with GABA-T deficiency and vigabatrin (20). Thus, 2-pyrrolidinone has the potential to be a biomarker for GABA and can be utilized to identify elevations in GABA from noninvasive testing of a plasma or urine sample for diagnosis of GABA-transaminase deficiency.

GABA-transaminase deficiency was documented in the first proband from biopsied liver, lymphocytes isolated from whole blood, and Epstein-Barr virus-transformed cultured lymphoblasts (16; 09). The activity of GABA-transaminase in biopsied liver approximated 15% of the parallel mean activity for 10 control specimens. Mean GABA-transaminase activity was 1.2 protein pmol/h/mg (range 20 to 50) in the original female proband and 2 protein pmol/h/mg (range 23 to 64) in the unrelated patient (44).

The encephalopathy in patients with GABA-transaminase deficiency may well be a result of elevated GABA, and possibly beta-alanine, concentrations in central nervous system tissue. Sjaastad and associates presented a clinically normal adult whose CSF homocarnosine levels were about twice those of the female patient with GABA-transaminase deficiency (40). This suggests that homocarnosine, which was less elevated in the patient's CSF than GABA and beta-alanine, was not contributing to the neurologic manifestations. Length-growth acceleration, an unusual feature for a disorder associated with intellectual disability, was likely the effect of growth hormone hypersecretion, an observation consistent with the growth hormone-releasing effect of GABA (37; 36). The encephalopathic episodes in infancy and clinical severity of involuntary hyperkinetic movements may be correlated with the levels of GABA in the basal ganglia, with the highest levels of GABA/Cr levels at 1 to 2 years and subsequently decreasing with age (14).

In a male child who died from undisclosed reasons at 1 year of age, an autopsy revealed well-developed subcutaneous fat, edema and congestion of the brain, large thymus, bilateral bronchopneumonia, and small necrotic regions in the liver. In the same patient, a neuropathologic examination revealed poor or absent myelination in the white matter of the gyri. Even where well-myelinated, a spongy state of varying degrees was present throughout the white matter of the cerebral hemispheres, cerebellum, and brainstem, sparing the arcuate fibers (16). An autopsy was not performed on the patient’s sister.

Inheritance is autosomal recessive with affected male and female offspring and unaffected parents. Lymphocytes and cultured lymphoblasts derived from the parents and an unaffected sibling in the first described family had GABA-transaminase activities intermediate between the female patient and the control range (09).

Molecular investigations of the index proband were reported by Medina-Kauwe and colleagues (27; 28). In the index proband, an A-to-G transition at nucleotide 754 of the human GABA-transaminase gene was identified in lymphoblast cDNA (c.754A> G), resulting in substitution of an invariant arginine at amino acid 220 by lysine (p.Arg220Lys). This mutation results in destabilization of the binding of pyridoxal-5’-phosphate to GABA-transaminase, which is required for the transamination of GABA to succinic semialdehyde. The second allele in this patient was later identified as c.1433T>C, causing the substitution p.Leu478Pro. In the second family, the patient is also a compound heterozygote, with transition c.275G>A that causes the substitution p.Arg92Gl and an unspecific exon deletion around nucleotides 199 to 316 of the GABA-transaminase coding region (44).

Besse and colleagues demonstrated a dual-functionality of GABA-transaminase, manifested by both known catabolism of GABA as well as newly reported mitochondrial nucleoside salvage after localizing its activity to the mitochondrial matrix (03). Low mitochondrial DNA copy number was observed in fibroblast in vitro studies, which was corrected by delivering wild-type GABA-transaminase. Similar depletion of mitochondrial DNA was reported in murine retinal cells after pharmacological inhibition of GABA-transaminase by administering vigabatrin. Administering vigabatrin along with nucleotides prevented mitochondrial DNA depletion, presenting a potential therapeutic for vigabatrin-related retinopathy with associated visual field defects.

Interestingly, in recent years GABA-transaminase has also been found to have an association with other medical conditions. There is emerging evidence suggesting GABAergic dysfunction takes part in the pathogenesis of Alzheimer disease. In this regard, through reanalyzing the data of a genome-wide association study (GWAS) of Alzheimer disease, genetic variants in the 3’-UTR of ABAT were identified as the top Alzheimer-associated single nucleotide polymorphisms (SNPs) (45). ABAT gene has also been found to have a role in tumorigenesis and tumor immunity. A study discovered that ABAT expression is correlated with disease progression and poor prognosis in hepatocellular carcinoma (05). Furthermore, it was reported that primary medulloblastoma tumors display decreased expression of ABAT-encoded GABA transaminase and that metastatic medulloblastoma cells facilitate leptomeningeal metastasis formation by using ABAT to maintain GABA, which serves as their energy source substitute (26).

Epidemiology

GABA-T deficiency is an extremely rare autosomal recessive disease, with reports published thus far for only 17 individuals (8 male, 9 female) from 11 families (22; 14; 13; 29). The oldest identified patient is currently 32 years old (20; 29).

Prevention

Only parents who are asymptomatic heterozygote carriers for GABA-transaminase deficiency could give rise to an affected child, with each pregnancy having a 1 in 4 chance of producing an affected fetus. Genetic counseling for a family with a previously documented proband is recommended. The potential for prenatal diagnosis by means of chorionic villus sampling has been presented (42). The possibility of prenatal diagnosis by measuring amniotic fluid levels of GABA and beta-alanine may also exist (23).

Differential diagnosis

GABA-transaminase deficiency could be confused with hyper-beta-alaninemia, described in a single patient (39). Despite certain similarities in clinical and metabolic presentation, however, there are clinical and metabolic differences between the 2 disorders. Linear growth is increased in GABA-transaminase deficiency and was delayed in hyper-beta-alaninemia. CSF beta-alanine concentrations were 2 orders of magnitude higher in the patient with hyper-beta-alaninemia than the (index) patient with GABA-transaminase deficiency studied for beta-alanine levels. In addition, the urinary metabolite profile, including the concentrations of GABA, beta-alanine and beta-aminoisobutyric acid, were different in the 2 disorders. Impaired transamination of beta-alanine was suggested as the primary defect in the patient with hyper-beta-alaninemia, but definitive enzyme studies have not been presented (39; 38). It remains possible that hyper-beta-alaninemia is a variant form of GABA-transaminase deficiency.

Patients with putative combined semialdehyde dehydrogenase deficiency may present with elevated quantities of urinary beta-alanine and beta-aminoisobutyric acid, which may suggest GABA-transaminase deficiency in the differential diagnosis (35; 11; 07). However, these patients present with organic aciduria, often characterized by increased urinary 4-hydroxybutyric, 3-hydroxyisobutyric, 2-ethylhydracrylic, 3-hydroxypropionic, and 3,4-dihydroxyhexanoic acids. The patient with GABA-transaminase deficiency manifests amino aciduria, not organic aciduria.

Jaeken and coworkers noted that a qualitative biochemical profile similar to that seen in GABA-transaminase deficiency, including increases in GABA, GABA-conjugates and beta-alanine, is detected in the CSF of patients treated with the GABA-transaminase inhibitor gamma-vinyl-GABA (vigabatrin), an antiepileptic drug widely used in Europe (15). However, the relative increase in free GABA is much less pronounced in the patient receiving pharmacologic doses of vigabatrin than in the patient with GABA-transaminase deficiency (12).

GABA-transaminase deficiency shares certain clinical features with other disorders, including cerebral gigantism, many of the inherited leukodystrophies including globoid cell leukodystrophy, adult-onset progressive dominant leukodystrophy, and Pelizaeus-Merzbacher disease. However, these disorders can be differentiated from GABA-transaminase deficiency based on clinical and metabolite findings. For example, there is no biochemical marker in patients with cerebral gigantism (19). Krabbe disease is associated with galactocerebrosidase deficiency (41). The early onset of GABA-transaminase deficiency should exclude a diagnosis of adult-onset progressive dominant leukodystrophy (24). Pelizaeus-Merzbacher disease manifests an unusual pendular nystagmus with X-linked inheritance (10). In all instances, the correct differential diagnosis of GABA-transaminase deficiency is achieved by accurately determining GABA and beta-alanine concentrations in CSF, with significant elevations of each.

GABA-transaminase deficiency is characterized by elevated GABA, which is consistent with succinic semialdehyde dehydrogenase (SSADH) deficiency, another GABA metabolic pathway disorder. SSADH deficiency can be differentiated by the build-up of gamma-hydroxybutyrate (GHB), a metabolite of GABA, which does not accumulate in GABA-transaminase deficiency. Repeated organic acid analysis of urine is the most reliable method of determining excess GHB (34).

Diagnostic workup

GABA-transaminase deficiency is not associated with acidosis, hypoglycemia, ketosis or hyperammonemia, abnormalities often considered in the differential diagnosis of inborn errors of metabolism. It may be worthwhile to request quantitative plasma, and most importantly quantitative CSF amino acid analysis in patients presenting with the psychomotor deficit, hypotonia, accelerated length-growth, and hyperreflexia. The detection of increased GABA and beta-alanine in urine would warrant the analysis of plasma and CSF amino acid concentrations. A pitfall in determining free GABA concentrations in CSF is the enzymatic degradation of homocarnosine, a dipeptide of GABA and L-histidine.

Free GABA levels in CSF show artifactual increases unless samples are deep frozen within a few minutes and stored at -20°C short-term or -70°C long-term. It may also be of value to determine the fasting level of plasma growth hormone if GABA-transaminase deficiency is suspected. Elevated fasting plasma growth hormone is consistent with the growth hormone-releasing effect of GABA and may suggest GABA-transaminase deficiency in the differential diagnosis (36). Detection of increased GABA and beta-alanine in CSF would warrant an assessment of GABA-transaminase activity in isolated lymphocytes or Epstein-Barr virus-transformed lymphoblasts. Tsuji and colleagues utilized quantitative [1H]MR spectroscopy to measure GABA concentrations in brain parenchyma in vivo (44).

A molecular profiling technology termed untargeted metabolomics has also been suggested as a useful screening method for inherited metabolic disorders. A recently discovered automated computational diagnostic method has shown to have the capacity to connect pieces of information derived from metabolite perturbations observed in individual metabolomics profiling data and modules identified in disease-specific metabolite co‑perturbation networks. This has demonstrated the ability to reproduce an accurate diagnosis of 16 different inherited metabolic disorders, including GABA-transaminase deficiency (43).

Management

Trials with pyridoxine (up to 3 g/day) as well as picrotoxin (up to 0.12 mg/kg per day) did not improve the clinical situation in the index case. Two patients have received trials of flumazenil, antagonist of the benzodiazepine binding site of the GABA(A) receptor. One appears to have responded to a regimen involving initial bolus of 0.01 mg/kg intravenously in an intensive care unit setting and maintained on 1.7 mg/kg/day with an 11-month ongoing course of treatment (22). The other patient discontinued the medicine following a 2-month trial due to increased wakefulness with agitation and no observed benefit.