Succinic semialdehyde dehydrogenase deficiency

Did You Know?

• Over half of patients with SSADH deficiency have seizures, including absence, tonic-clonic, and convulsive status epilepticus.
• Ocular problems associated with the disorder include strabismus, nystagmus, retinitis, disc pallor, and oculomotor apraxia.
• More than 47 disease-causing mutations in the ALDH5A1 gene on chromosome 6p22 have been identified, with no hotspots.

Mechanism

GABA, a major inhibitory neurotransmitter, is metabolized via transamination to succinic semialdehyde, which is normally oxidized to succinic acid by SSADH. In SSADH deficiency, succinic semialdehyde is instead reduced to gamma-hydroxybutyric acid (GHB) by succinic semialdehyde reductase, leading to a 30-fold increase in GHB and a 2-4 fold increase in GABA in the brain. GHB elevation can induce spike-and-wave activity similar to generalized absence epilepsy in animal models. The GABAB receptor is particularly important in this disorder, as it mediates release of both GABA and GHB through presynaptic effects involving voltage-dependent inhibition of calcium channels. Altered function of the GABAB receptor due to elevated GABA and GHB may contribute to tonic-clonic seizures observed in patients.

Diagnosis

Neuroimaging modalities such as cranial computed tomography, magnetic resonance imaging, and fluorodeoxyglucose positron emission tomography have been used in diagnosis. Common abnormalities in 29 published cases include increased T2-weighted signal in the globus pallidi bilaterally and symmetrically, as well as subcortical white matter changes, with similar findings in the brainstem and cerebellar dentate nucleus. T2 signal intensity may reflect edema, inflammation, or demyelination; the globus pallidi contain myelinated axons, and repeated GHB exposure has been shown to affect myelin expression via MAP kinase.

Urine organic acid analysis can detect high GHB levels, though GHB's volatility may complicate gas chromatography or mass spectrometry. Other GABA metabolites such as glycine may also appear. SSADH enzyme levels can be measured in cultured leukocytes, where accumulation of 4,5-dihydroxyhexanoic acid—normally undetectable—is characteristic of the disorder.

Treatments

Pharmacological treatments have been tested in Aldh5a1-/- mice and humans, but controlled studies in patients are scarce. Vigabatrin, an irreversible inhibitor of GABA transaminase, reduces GHB levels while elevating GABA; clinical results vary, with some patients showing improved ataxia and speech and others worsening. Lower doses (30–50 mg/kg per day) are associated with fewer side effects, and high doses have enhanced survival in mice. Sodium valproate enhances GABA synthesis and release, and successful interventions have been noted, though no clinical trials have been conducted; valproate is usually contraindicated as it may inhibit residual SSADH activity.

The GABAB antagonist CGP-35348 reduced absence seizure frequency in mice but sometimes worsened convulsive seizures. Baclofen, which acts on presynaptic GABAB receptors in the spinal cord, has been considered due to progressive GABAB receptor loss in mice, but no efficacy data in mice or humans have been reported.

Research

Research on SSADH deficiency has been led by Phillip L. Pearl of Boston Children's Hospital and K. Michael Gibson of Washington State University. Pearl has conducted natural-history studies defining neurological progression, including developmental, epileptic, neuropsychiatric, and sleep-related manifestations, and has helped establish international patient registries and multidisciplinary clinical frameworks. His work has examined biomarkers, age-related phenotypic changes, and GABAergic abnormalities, contributing to therapeutic strategies such as ketogenic diets, enzyme replacement, and gene-based therapies.

Although the metabolic pathway is known, how enzyme deficiency and accumulation of GABA and GHB produce the clinical phenotype remains unclear, and genotype does not correlate with phenotypic severity. Future strategies may focus on decreasing GHB production, inhibiting the GHB receptor, mTOR inhibitors, and combination therapies. A patient registry has been established by the International Working Group on Neurotransmitter Related Disorders (iNTD).