Glycine Encephalopathy

A number sign (#) is used with this entry because mutations in several genes in the mitochondrial glycine cleavage system have been found to cause glycine encephalopathy (GCE), also known as nonketotic hyperglycinemia (NKH). These include the genes encoding P protein (GLDC; 238300), T protein (AMT; 238310), and, in 1 patient, the H protein (GCSH; 238330). Most patients with GCE have a defect in the GLDC gene.

Clinical Features

Classic Neonatal Form

Most patients with GCE have the neonatal phenotype, presenting in the first few days of life with lethargy, hypotonia, and myoclonic jerks, and progressing to apnea, and often to death. Those who regain spontaneous respiration develop intractable seizures and profound mental retardation. In the infantile form of GCE, patients present with seizures and have various degrees of mental retardation after a symptom-free interval and seemingly normal development for up to 6 months. In the mild-episodic form, patients present in childhood with mild mental retardation and episodes of delirium, chorea, and vertical gaze palsy during febrile illness. In the late-onset form, patients present in childhood with progressive spastic diplegia and optic atrophy, but intellectual function is preserved and seizures have not been reported (Hamosh and Johnston, 2001). See review by Tada and Hayasaka (1987).

Unlike glycinemia with ketoacidosis and leukopenia, also known as propionic acidemia (606054), episodic ketoacidosis with vomiting, neutropenia, and thrombocytopenia does not occur in nonketotic hyperglycinemia. Glycine is the only amino acid elevated in serum and urine and the only amino acid harmful to these patients. Some have died in the newborn period after a course characterized by lethargy, weak cry, generalized hypotonia, absent reflexes, and periodic myoclonic jerks (Balfe et al., 1965). The few who attain an older age show severe mental retardation (Mabry and Karam, 1963; Gerritsen et al., 1965).

Hayasaka et al. (1983) studied the glycine cleavage system in the liver and brain obtained at autopsy in 2 male infants with the typical form of nonketotic hyperglycinemia. In one a defect in the P protein was found; in the second, T protein was defective. The infant with the P protein defect was born of unrelated parents, was lethargic with a poor suck from birth, developed marked hypotonia, intermittent apnea, and poor responsiveness to stimuli, had mildly elevated blood ammonia and markedly elevated glycine in blood and cerebrospinal fluid, and died at age 12 days. Immunochemical analysis indicated absence of the enzyme P protein itself. The second infant appeared well at birth and nursed well the first day. He was hospitalized on the third day with 'lethargy, bordering on coma.' Despite ventilatory support, 7 exchange transfusions to lower blood glycine, and treatment with sodium benzoate and strychnine, he died on the twentieth day. T protein was undetectable in the brain and extremely low in liver. Autopsy in the first case, with P protein deficiency, showed absence of the corpus callosum and spinal cord hydromelia. The authors stated that they had seen a similar structural defect with deficiency of the pyruvate dehydrogenase complex (see 312170).

Schutgens et al. (1986) reported a case with T protein deficiency.

Cataltepe et al. (2000) reported 4 patients with nonketotic hyperglycinemia who developed pulmonary hypertension. Two patients had classic neonatal GCE and developed pulmonary hypertension in the newborn period; both died from pulmonary hypertension. The other 2 patients were sibs from Bangladesh with atypical GCE, the first of whom presented with pulmonary hypertension at the age of 6. His brother had documented pulmonary at the age of 4 years, which resolved spontaneously and then recurred in association with thiamine deficiency when he was 21 years old.

Van Hove et al. (2000) reported 4 patients with typical neonatal-onset NKH who developed hydrocephalus requiring shunting in early infancy. Brain imaging revealed acute hydrocephalus, a megacisterna magna or posterior fossa cyst, pronounced atrophy of the white matter, and an extremely thin corpus callosum in all. The 3 older patients had profound developmental disabilities. Van Hove et al. (2000) concluded that the development of hydrocephalus in NKH is an additional poor prognostic sign.

In cells derived from a deceased boy, born of unrelated Serbian parents, with GCE, Swanson et al. (2017) identified a homozygous missense mutation in the AMT gene (S117L; 238310.0009). In vitro functional expression studies showed that the mutant AMT protein was unstable and had only 9% residual enzymatic activity compared to controls. The patient was unusual because he had originally been reported as having D-glyceric aciduria (220120) caused by a homozygous frameshift mutation in the GLYCTK gene (610516.0001) (Brandt et al., 1974; Sass et al., 2010). Increased glycine in the patient had been thought to be secondary to the GLYCTK defect; however, the molecular findings confirmed that the patient had the unusual cooccurrence of 2 inborn errors of metabolism. Swanson et al. (2017) concluded that D-glyceric aciduria does not cause deficient glycine cleavage enzyme activity or nonketotic hyperglycinemia.

Atypical Mild Form

Unlike the classic neonatal form of the disorder, atypical or mild glycine encephalopathy is phenotypically heterogeneous and nonspecific, making diagnosis difficult (Flusser et al., 2005).

Cole and Meek (1985) emphasized the occurrence of an expressive speech deficit and neurologic abnormalities during intercurrent infections as striking features of the milder form of the disease. The cases of Ando et al. (1978), Frazier et al. (1978), and Flannery et al. (1983) also fall into this category. Hayasaka et al. (1987) cited one patient with atypical GCE and features of progressive degeneration of the central nervous system.

Dinopoulos et al. (2005) reported 3 unrelated adults with a mild form of glycine encephalopathy confirmed by genetic analysis (238300.0008; 238300.0009). All 3 patients showed hypotonia as infants and had developmental delay. One patient showed appendicular ataxia and choreoathetoid movements at age 4 years. Between ages 5 and 12 years, he had frequent outbursts of aggressiveness. He attended special education classes and graduated from high school. The second patient was hypotonic at birth and developed seizures during the first week of life. Aggressive behavior was noted at age 12 years; he was fully dependent on his family. The third patient developed hypotonia at age 6 months. He was diagnosed with attention deficit-hyperactivity disorder (ADHD) and had outbursts of aggression and impulsivity. Treatment with dextromethorphan was ineffective. He graduated from school in special education classes. Biochemical analysis showed residual GLDC activity ranging from 6 to 8%, which Dinopoulos et al. (2005) suggested may explain the milder clinical phenotype. The authors emphasized the clinical heterogeneity of the mild form of GCE.

Flusser et al. (2005) reported a large consanguineous Israeli Bedouin kindred in which 9 members had atypical GCE confirmed by genetic analysis (238300.0010). Most patients presented during the first months of life with abnormal movements, including mild to moderate generalized hypotonia, lateral head nodding, choreoathetoid hand movements, and pill rolling. Seven patients had seizures with generalized spike and slow wave abnormalities in EEG; 2 had infantile spasms with hypsarrhythmia. All had delayed motor development, moderate mental retardation, and limited expressive language. The patients also showed irritability and restlessness as infants and later showed aggressive and destructive behavior. Treatment was ineffective.

Yu et al. (2013) described 3 children from a consanguineous family who had autism spectrum disorder and who carried a homozygous mutation in the AMT gene. While individually nondiagnostic, the 3 affected children exhibited a range of neurologic symptoms that in aggregate were strongly suggestive of NKH. The eldest child was 12 years of age and had, in addition to a diagnosis of autism spectrum disorder, a history of severe epilepsy, with first seizures presenting at 10 months of age. The second child was 9 years of age and suffered from autism and epilepsy, but her seizures were milder. The third child was 2 years of age and had language and motor delays and carried a diagnosis of pervasive developmental disorder (PDD), but had had only 1 febrile seizure. Direct biochemical analysis of the mutation (ile308 to phe, I308F) demonstrated reduced activity. When compared to classical NKH-associated values, glycine cleavage activity of the mutated allele was at the mild end of the range of reported values, suggesting that the affected children in this family suffered from undiagnosed atypical NKH presenting as autism spectrum disorder and seizures. Plasma amino acid screening was normal in the 2 older children, a result that is typically seen in milder forms of NKH.

Transient Neonatal Hyperglycinemia

Transient neonatal hyperglycinemia (TNH) is characterized by elevated plasma and CSF glycine levels at birth that are normalized within 2 to 8 weeks. TNH is clinically and biochemically indistinguishable from typical nonketotic hyperglycinemia at onset. Applegarth and Toone (2001) reviewed 7 cases of transient NKH.

Korman et al. (2004) reported 3 sibs from a consanguineous Muslim Palestinian family who had an unusual NKH phenotype. All 3 sibs were diagnosed with NKH within the first 3 days of life with characteristic elevated CSF and plasma glycine levels and elevated CSF-to-plasma glycine ratios. However, none of them developed neurologic symptoms, and all showed appropriate development, including good school performance in the 2 children of school age. The 2 older children showed persistent hyperglycinemia. A patient from a second unrelated family diagnosed with NKH had mild neurologic sequelae. In all 4 patients, Korman et al. (2004) identified a homozygous mutation in the GLDC gene (238300.0006), which was shown to retain 32% residual enzyme activity in vitro. The authors suggested that these patients exhibited a new phenotype of NKH.

Inheritance

Nonketotic hyperglycinemia is inherited as an autosomal recessive trait.

Biochemical Features

Gerritsen et al. (1965) described abnormally low oxalate excretion in the urine and postulated a defect in glycine oxidase. Ando et al. (1968) located the defect to glycine formiminotransferase. Tada et al. (1969) concluded that the primary lesion in hyperglycinemia of the nonketotic variety is in the glycine cleavage reaction. Baumgartner et al. (1969) showed that the nonketotic variety can have a fulminant early onset. The defect concerns the enzyme involved in the conversion of glycine to CO2, NH3 and hydroxymethyltetrahydrofolic acid. De Groot et al. (1970) described 2 affected sisters with consanguineous parents and presented evidence indicating that the defect lies in glycine decarboxylase, rather than in glycine oxidase.

Pathogenesis

Toone et al. (2003) performed a retrospective analysis of a group of NKH patients and found that greater than 50% had T protein (238310) mutations. The patients studied had 1 or more unusual biochemical findings: residual glycine cleavage system activity in liver, residual glycine cleavage system activity in lymphoblasts, and/or increased amniotic fluid glycine-to-serine ratio with a normal amniotic fluid glycine level in prenatal diagnosis. The selected patients had a much higher incidence of T-protein defects than expected in the general NKH patient population. Toone et al. (2003) reported 3 novel mutations and 5 polymorphisms of the T protein gene, PCR/restriction enzyme methods for 1 mutation and 2 polymorphisms, and an estimation of their frequency in normal controls.

Population Genetics

A high frequency of glycine encephalopathy has been found in Finland; the incidence has been estimated to be 1 in 55,000 newborns overall, and 1 in 12,000 in northern Finland (von Wendt and Simila, 1980; Boneh et al., 2005). High incidences have also been reported in British Columbia and in small Arab villages in Israel (Boneh et al., 2005).

Diagnosis

Applegarth and Toone (2001) reviewed the laboratory diagnosis of glycine encephalopathy and confirmed 9 mutations in the T protein and 8 mutations in the P protein.

Tan et al. (2007) reported that they screened 733,527 babies over 8 years as part of the New South Wales Newborn Screening Program and subsequently diagnosed 9 babies with nonketotic hyperglycinemia. Two had newborn glycine levels above their cutoff and presented within 72 hours. The remaining patients could not have been diagnosed by newborn screening without an unacceptably high recall rate. Tan et al. (2007) concluded that babies with nonketotic hyperglycinemia were not usually identifiable by newborn screening strategies available at that time.

Prenatal Diagnosis

Hayasaka et al. (1990) described prenatal diagnosis of nonketotic hyperglycinemia by enzymatic analysis of the glycine cleavage system in chorionic villi. Toone et al. (1994) described their experience with direct assay of glycine cleavage enzyme in chorionic villus samples in 50 at-risk pregnancies.

Applegarth et al. (2000) reported 3 false-negative prenatal diagnostic results using direct measurement of glycine cleavage enzyme activity in uncultured chorionic villus tissue from 290 pregnancies at risk for glycine encephalopathy. Because of these false negatives, Applegarth et al. (2000) counseled that there is a gray zone of uninterpretable activity where affected and normal enzyme values overlap, and suggested that there is an approximately 1% chance of a pregnancy with a normal chorionic villus sample activity resulting in an affected child.

Kure et al. (1999) performed prenatal diagnosis for NKH by enzymatic analysis of chorionic villus samples in 28 families and by DNA analysis in 2 families. In 26 families, enzymatic analysis of the glycine cleavage system (GCS) yielded an unambiguous diagnosis; inconclusive results in 2 families were due to borderline GCS activity. A second chorionic sample was analyzed in both these families. In one case, GCS activity was normal in the second specimen and the baby did not have NKH. In the other case, Kure et al. (1999) again found extremely low GCS activity in a second specimen, but a healthy baby was born. The cause of this false-positive result was unknown. Kure et al. (1999) also reported the ability to obtain unambiguous prenatal diagnosis in both Finnish and Israeli Arab families due to prevalent mutations in those populations. The H42R mutation in the T protein (238310.0003) may lead to ambiguous enzymatic activity, suggesting an advantage for DNA analysis.

Clinical Management

Hamosh et al. (1992) reported clinical and electrophysiologic improvement in a child with GCE who was treated with dextromethorphan and sodium benzoate beginning with the twelfth day of life. Dextromethorphan is a noncompetitive antagonist of the NMDA type of glutamate receptor, which can be stimulated by glycine. Zammarchi et al. (1994) reported only transient improvement on the same regimen when the treatment was instituted at 65 hours of life. The child died at 5 months and 7 days of age in spite of increasing doses of dextromethorphan as high as 40 mg per kilogram per day. The enzymatic basis for the GCE in either the successfully or unsuccessfully treated infant was not specified. The authors speculated that the different responses may reflect genetic heterogeneity.

Treatment of patients with GCE with high doses of benzoate can result in decreased CSF glycine levels and will improve seizure control and wakefulness (Hamosh et al., 1992), thus improving the quality of life in surviving infants, but even when started early, may not prevent the development of mental retardation (Zammarchi et al., 1994). Episodes of lethargy, coma, and increased seizures can be caused both by hyperglycinemia from underdosing benzoate, or by toxicity due to overdosing. Van Hove et al. (1995) found plasma carnitine deficiency in 3 of 4 patients with GCE treated with sodium benzoate, and benzoylcarnitine was identified in plasma, urine, and CSF. Treatment with L-carnitine normalized plasma-free carnitine. Close monitoring of glycine, benzoate and carnitine levels is advisable in patients receiving benzoate.

Neuberger et al. (2000) reported a 6-month-old girl who presented with hypotonia and mild psychomotor retardation who was subsequently found to have NKH confirmed by decreased glycine cleavage system activity in the liver. After the patient developed hypsarrhythmia and had a single seizure, treatment with both sodium benzoate and dextromethorphan was started. During the following year, the girl was free of seizures with improvement of EEG activity and showed retarded but continuously progressing psychomotor development. At the age of 20 months she began to walk freely but had generalized muscular hypotonia and moderate mental retardation. Discontinuation of dextromethorphan after one year did not change the clinical or EEG status. However, after cessation of sodium benzoate, epileptic activity in the EEG and behavioral changes occurred. These changes disappeared promptly after sodium benzoate therapy was reinstituted.

Korman et al. (2006) reported a patient with NKH caused by a homozygous mutation in the GLDC gene. He was born of first-cousin Palestinian Arabs. The patient was diagnosed prenatally and treated from birth with oral sodium benzoate and the NMDA receptor antagonist ketamine. Although neonatal hypotonia and apnea did not occur, the long-term outcome at age 11 months was poor, with intractable seizures and severe psychomotor retardation. Korman et al. (2006) noted that the plasma glycine level in this child was normal at birth, presumably reflecting placental clearance, whereas the CSF glycine level was markedly elevated, suggesting that the developing brain had been exposed prenatally to the potential toxicity of glycine.

Nomenclature

Nonketotic hyperglycinemia was originally named to distinguish it from ketotic hyperglycinemia, which is now known to be propionic acidemia (606054). Since the distinction is no longer required and clinical confusion between hyperglycinemia and hyperglycemia occurs, a more appropriate name for this disorder is glycine encephalopathy (Hamosh, 2001).