Summary

Glycine encephalopathy (GE) is an inborn error of glycine metabolism characterized by accumulation of glycine in body fluids and tissues, including the brain, resulting in neurometabolic symptoms of variable severity.

In Finland, an incidence at birth of 1/55,000 is reported and in British Columbia, Canada, 1/63,000, with a calculated carrier rate of 1/125.

Three forms of GE have been recognized based on the age of onset: neonatal, infantile and atypical glycine encephalopathy (see these terms). Most patients have the life-threatening neonatal form which presents within a few days of birth with manifestations such as lethargy or even coma, hypotonia, hiccups, myoclonic jerks, and breathing/swallowing disorders, with subsequent intellectual deficit, spasticity and intractable seizures. Some patients show developmental delay and generally mild seizures in the infantile period, while others do not develop symptoms until late infancy or adulthood. Although patients usually have either a mild or severe course, there is a continuous clinical spectrum. Some patients develop choreic movements. Atypical form indicates hyperglycinemic patients whose clinical presentations are different from those of neonatal or infantile form, for example, transient or late-onset hyperglycinemia and patients with spastic paraparesis.

Mutations inGLDC (9p22), AMT (3p21.2-p21.1) are known to cause GE. These genes code for the P- and T-protein components of the enzymatic glycine cleavage system (GCS). Although GCSH is another GCS gene, no mutation has been identified in neonatal or infantile form. Deficient GCS activity results in defective glycine metabolism and amino acid accumulation in tissues. In some patients with deficient GCS enzyme activity no mutation could be identified by exon sequencing analysis of any GCS gene. Most patients have no enzyme activity. Etiology of atypical forms remains elusive.

GE should be suspected in cases of elevated glycine levels in blood and cerebrospinal fluid (CSF). Increased CSF-to-plasma glycine ratios also suggest the diagnosis. Measurement of GCS activity of biopsied liver sample or by 13C-glycine breath test and genetic testing may confirm diagnosis. Brain MRI may reveal hypogenesis of corpus callosum, abnormal gyrus, and hypogenesis of cerebellum in the neonatal form. Suppression burst and hypsarrhythmia are common in EEG. Subsequently, delayed myelination and atrophy may be observed.

Differential diagnosis includes organic acidemias that may present hyperglycinemia such as D-glyceric acidemia, propionic acidemia, methylmalonic acidemia, isovaleric acidemia, and ketoacidosis due to beta-ketothiolase deficiency (see these terms). Conditions characterized by neonatal seizures should also be considered. Valproate treatment may cause hyperglycinemia.

Prenatal diagnosis for at-risk pregnancies can be performed either by molecular genetic testing of the causative genes or by GCS enzyme analysis of chorionic villi sample.

Glycine encephalopathy is inherited in an autosomal recessive manner.

The following tests should be used to guide treatment: brain MRI, EEG, and developmental and neurological assessment. There are only supportive and symptomatic measures for GE including antiepileptics for seizure control, placement of a gastrostomy tube for swallowing disorders, and treatment for gastroesophageal reflux. Sodium benzoate is used to reduce plasma glycine levels. NMDA receptor antagonists may ameliorate neurological symptoms although it remains to be established whether they improve long term outcome.

Prognosis depends on disease severity. Most patients with neonatal or infantile forms have a severe outcome. In the neonatal form, early death sometimes occurs due to apnea. Prognosis in atypical cases is variable.

Expert reviewer(s)

• Shigeo KURE