Glycine encephalopathy (GE), also known as non-ketotic hyperglycinemia or simply NKH, is a rare, inherited, autosomal recessive disorder of glycine metabolism. The condition arises from defects in the glycine cleavage system, an essential enzyme complex for glycine breakdown. This results in toxic accumulation, particularly in the brain, causing seizures, lethargy, muscle weakness, respiratory problems, and a high risk of early mortality. The condition typically manifests within the first couple months of life presenting as progressive lethargy and muscle weakness that can quickly lead to respiratory failure. The diagnosis of GE or NKH is often suspected based on abnormally high levels of the amino acid, glycine, in bodily fluids and tissues, especially the cerebrospinal fluid. Severity of symptoms and outcomes correlate with age at onset, with those presenting symptoms in the first few days having worse outcomes than those in the first few weeks to months. Among individuals that survive, they often have mild to severe intellectual disabilities, motor difficulties, and intractable seizures. Some individuals develop cortical blindness, scoliosis, and hip dysplasia.
NKH is primarily caused by mutations in the GLDC gene and, less frequently the AMT gene. These mutations result in defective P-protein and T-protein subunits of the glycine cleavage system, which are key components of glycine metabolism in mitochondria. Excess glycine results in disrupted glycine signaling in the brain especially NMDA receptors. Additionally, without glycine metabolism the body and brain are depleted of glycine-derived one carbon donors which are important for early brain development.
There is currently no cure for NKH; treatment focuses on reducing plasma glycine levels and symptom management. Sodium benzoate is used to reduce plasma glycine levels and NMDA receptor antagonists, like dextromethorphan or ketamine, are used to decrease NMDA receptor stimulation which can improve seizures, though many individuals require multiple anti-seizure drugs. Ongoing research continues to explore replacing the faulty gene through gene therapy, evaluating alternative glycine lowering agents especially that may do so in the brain, and providing alternative one carbon units including both dietary interventions and drugs. NKH is a life-limiting disorder and while life-expectancy is reduced advancements in care and treatment have improved outcomes for those with the disorder, especially the attenuated form.
NKH is an ultra-rare disease with an estimated 1 in every 76,000 globally or around 500 people worldwide. After phenylketonuria, glycine encephalopathy is the second most common disorder of amino acid metabolism. NKH was first described in 1965 and referred to as 'a new type of idiopathic hyperglycinemia' and later 'non-ketotic hyperglycinemia', a reference to the biochemical findings seen in patients with the disorder, and to distinguish it from the disorders that cause "ketotic hyperglycinemia" (as seen in propionic acidemia and several other inherited metabolic disorders). To avoid confusion, the term "glycine encephalopathy" is sometimes used, to better describe the clinical underpinnings of the disorder.
Signs and symptoms The first signs and symptoms usually appear in the neonatal period (first hours or days of life) and to a lesser extent the infantile period (2 weeks to 3 months). For onset in the neonatal period, the hallmark sign is progressive lethargy often leading to coma and marked muscle weakness (hypotonia). If artificial ventilation is not provided, death often occurs. Most later regain spontaneous respiration and see improvement in alertness in the first month of life including oral feeding (bottle drinking). Additionally, pronounced hiccups or myoclonic jerks may be present, which are often a sign of epilepsy. If onset occurs in the infantile period, the primary symptom is prolonged hypotonia combined with developmental delays and seizures. Although late presentation (beyond 3 months) is possible, it is uncommon and typically features a more subtle presentation of symptoms, such as developmental delays and mild seizures. Infants with this presentation have frequently been associated with the attenuated form of the disease.
In infants who survive and are managed with sodium benzoate, a range of symptoms may manifest, dependent upon the severity of the disease. The most prominent symptom is seizures which are present in a majority of cases. Other common symptoms include continued hypotonia and lethargy. In the attenuated forms, these symptoms are often milder than the severe form. Seizures are readily controlled and individuals can learn to walk, reach, grasp, and may learn to speak or use sign language. Lethargy is often occasional and linked to infections. Additionally, hyperactivity is common and often severe and treatment resistant. In the severe form, however, seizures are difficult to treat and may become progressively worse. Children do not learn to sit or grasp and often have limited ability to interact with their environment. They also often develop cortical blindness, scoliosis, and hip dysplasia. There is typically no hyperactivity, but spasticity is common. Some have cleft palate or clubfeet and secondary microencephaly has also been noted.
Causes
Glycine encephalopathy is an autosomal-recessive disorder caused by a defect in the glycine cleavage system (GCS). In an autosomal recessive inheritance pattern, two defective copies of the gene (one inherited from each parent) are required for a child to be born with the disorder. Individuals with only one copy of the defective gene are considered carriers for the disorder and do not show signs or symptoms of the disorder.
The GCS is a large enzyme complex made up of four protein subunits, each of these four subunits is encoded by a separate gene. Defects in three of these four genes have been linked to glycine encephalopathy, which include GLDC (chromosome 9), AMT (chromosome 3), and GCSH (chromosome 16). There is a fourth subunit in the GCS, Dihydrolipoamide dehydrogenase or L-protein, encoded by the DLD gene (chromosome 7). However, defects in the L-protein are associated with a distinct disorder known as Dihydrolipoamide Dehydrogenase Deficiency, likely owing to its roles in multiple enzymes complexes beside the GCS.
Gene Protein Name Percent GLDC Encodes for the P-protein or glycine dehydrogenase subunit, also called glycine decarboxylase of the glycine cleavage system. About 80% of cases of glycine encephalopathy result from mutations in the GLDC gene. GCST or AMT Encodes for the T-protein or the aminomethyltransferase subunit of the glycine cleavage system. About 20% of cases are caused by mutations in the AMT gene. GCSH Encodes for the H-protein subunit of the glycine cleavage system. Mutations in the GCSH gene account for less than 1% of cases.
Variant NKH A small subset of affected individuals do not have detectable mutations in any of the three genes (listed above) that are typically associated with the disease. However, they still show impaired glycine metabolism and toxic accumulation. This is called variant NKH, where individuals often have mutations in the genes encoding one of the cofactors associated with the GCS complex. These cases are often linked to lipoate deficiency disorder, commonly associated with mutations in the LIAS, BOLA3, and GLRX5 genes, as well as pyridoxal phosphate deficiency disorder, which is associated with the ALDH7A1 gene. Sometimes this is referred to as atypical NKH.
Pathophysiology Defects in the subunits of the GCS typically cause glycine encephalopathy, although some causes of the disease are due to defects in cofactors of the GCS. While the GCS shows its highest enzymatic activity in kidney, liver, and brain it is considered that the buildup of glycine in the brain is responsible for the majority of symptoms. Individuals with NKH have a thin and shortened corpus callosum and atropy is often present in older individuals with severe, but not attenuated NKH. Some develop hydrocephalus.
Glycine is the simplest amino acid, having no stereoisomers. In the cortical neuroepithelium, glycine levels double during embryogenesis, peak at birth, and then gradually decline to approximately 60% within the first two weeks after birth, a period during which the GSC enzyme is expressed at high levels. Glycine plays an important role in central nervous system development.
In adulthood, glycine acts primarily as an inhibitory neurotransmitter in the spinal cord, brain stem, and retina and is associated with motor control, respiration, pain signals, vision, and hearing. It also modulates excitation by co-agonizing NMDA receptors which regulate neuronal excitability and are important for brain development and learning and memory.
Glycine and NMDA receptor overactivation Elevated glycine levels cause overactivation of NMDA receptors through excessive stimulation at the glycinergic binding site, resulting in excitotoxicity. This process contributes to neuronal and axonal injury, manifesting as encephalopathy, seizures, and other neurological features. Overstimulation of inhibitory glycine receptors in the brainstem and spinal cord further contributes to symptoms such as hypotonia, apnea, and hiccups. Outside the brain glycine receptors are found in the retina, liver, kidney, and immune cells.
Glycine and one-carbon metabolism Glycine is metabolized in the body's cells to end products of carbon dioxide and ammonia via the glycine cleavage system, in the mitochondria This multi-enzyme complex not only catabolizes glycine but also transfers a one-carbon unit to tetrahydrofolate, generating 5,10-methylenetetrahydrofolate. This intermediate supports one-carbon metabolism, which is essential for processes such as purine synthesis, thymidylate production, and methionine regeneration. Deficient glycine cleavage activity impairs the supply of one-carbon units, potentially contributing to disrupted folate-dependent pathways and associated developmental impacts.
Diagnosis Newborns and infants, typically within the first three months of life, commonly present with a combination of several symptoms upon physical examination. These can include low muscle tone (hypotonia), decreased alertness (lethargy), breathing difficulties (apnea), seizures, poor feeding, and developmental delays of unknown etiology. Confirmation of a diagnosis requires laboratory tests and/or brain imaging evaluations.
Elevated levels of glycine in both plasma and cerebrospinal fluid (CSF), particularly a high CSF-to-plasma glycine ratio, are strong indicators of Non-Ketotic Hyperglycinemia (NKH). Additionally, a ¹³C-glycine breath test may demonstrate reduced exhalation of carbon dioxide in affected individuals. Genetic testing revealing a pathogenic variant in the glycine cleavage system (GCS) serves as definitive confirmation of NKH. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) can also assist in the diagnosis by identifying specific brain abnormalities and assessing brain glycine signals, respectively.
Differential Diagnosis Other inherited metabolic conditions that may mimic the clinical presentation of NKH include:
Organic acidurias Pyridoxine-dependent epilepsy Pyridox(am)ine 5'-Phosphate Oxidase deficiency (PNPO) deficiency Pyridoxal 5'-Phosphate-Binding Protein Deficiency (PLPBP) deficiency Disorders of intracellular cobalamin metabolism Lipoate deficiency GLYT1 encephalopathy