Succinic semialdehyde dehydrogenase deficiency

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Succinic semialdehyde dehydrogenase deficiency (SSADHD), also known as 4-hydoxybutyric aciduria or gamma-hydoxybutyric aciduria, is a rare autosomal recessive disorder[1] of the degradation pathway of the inhibitory neurotransmitter ?-aminobutyric acid, or GABA. The disorder has been identified in approximately 350 families, with a significant proportion being consanguineous families.[2] The first case was identified in 1981 and published in a Dutch clinical chemistry journal that highlighted a patient that suffered from a number of neurological conditions such as delayed intellectual, motor, speech, and language as the most common manifestations. Later cases reported in the early 1990s began to show that hypotonia, hyporreflexia, seizures, and a nonprogressive ataxia were frequent clinical features as well.[3]

SSADH deficiency is caused by an enzyme deficiency in GABA degradation. Under normal conditions, SSADH works with the enzyme GABA transaminase to convert GABA to succinic acid. Succinic acid can then be utilized for energy production via the Krebs cycle. However, because of the deficiency, the final intermediate of the GABA degradation pathway, succinic semialdehyde, accumulates and cannot be oxidized to succinic acid and is therefore reduced to gamma-hydroxybutyric acid (GHB) by gamma-hydroxybutyric dehydrogenase. This causes elevations in GHB and is believed to be the trademark of this disorder and cause for the neurological manifestations seen.[2]

The symptoms of SSADH deficiency fall into three primary categories: neurological, psychiatric, and ocular. The most constant features seen are developmental delay, hypotonia, and mental retardation. Nearly half of patients seen manifest ataxia, behavior problems, seizures, and hyporreflexia.[2]

The age of onset ranges from newborn period to 25 years. Problems unique to neonates can include prematurity, lethargy, decreased sucking, respiratory difficulty and hypoglycemia. Gastrointestinal symptoms have been seen primarily in this population and are usually related to increased feeding.

Ocular problems related to the disorder include strabismus, nystagmus, retinitis, disc pallor, and oculomotor apraxia.[4]

Nearly half of the patients with SSADH deficiency have seizures. These include absence, tonic clonic, and convulsive status epilepticus. It is unclear whether decreased levels of GABA or elevated levels of GHB are responsible for these seizures but alterations in these neurotransmitters and their receptor binding or neurotransmitter transport is hypothesized to play a role in the pathogenesis of the seizures in this population.[5]

SSADH deficiency is inherited in an autosomal recessive fashion. Such diseases are caused by an error in a single DNA gene. Because the disease is autosomal, the defective gene is found on an autosome (chromosome 6), rather than the sex-linked 23rd chromosome. Being a recessive disorder, the disease can only be inherited from both parents since the disorder can only occur when a person has two copies of the gene.

It is believed that the genetic basis for SSADH deficiency resides in the SSADH human ALDH5A1 gene which maps to chromosome 6p22. More than 47 disease-causing mutations have been identified for the disorder, all of which lead to absence of functional proteins through missense, nonsense, or splicing errors; no hotspots have been identified. Consanguinity is frequent; this suggests the occurrence of rare disease causing alleles in the general population.[6]

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