Glycine encephalopathy (non-ketotic hyperglycinemia) | |
---|---|
Other names | Non-ketotic hyperglycinemia or NKH |
Glycine | |
Specialty | Neurology, medical genetics, endocrinology |
Glycine encephalopathy is a rare autosomal recessive disorder of glycine metabolism. After phenylketonuria, glycine encephalopathy is the second most common disorder of amino acid metabolism. The disease is caused by defects in the glycine cleavage system, an enzyme responsible for glycine catabolism. There are several forms of the disease, with varying severity of symptoms and time of onset. The symptoms are exclusively neurological in nature, and clinically this disorder is characterized by abnormally high levels of the amino acid glycine in bodily fluids and tissues, especially the cerebrospinal fluid.
Glycine encephalopathy is sometimes referred to as "nonketotic hyperglycinemia" (NKH), as a reference to the biochemical findings seen in patients with the disorder, and to distinguish it from the disorders that cause "ketotic hyperglycinemia" (seen in propionic acidemia and several other inherited metabolic disorders). To avoid confusion, the term "glycine encephalopathy" is often used, as this term more accurately describes the clinical symptoms of the disorder.
It typically presents as a severe encephalopathy with myoclonic seizures, is rapidly progressive and eventually results in respiratory arrest. Standard evaluation for inborn errors of metabolism and other causes of this presentation does not reveal any abnormality (no acidosis, no hypoglycaemia, or hyperammonaemia and no other organ affected). Pronounced and sustained hiccups in an encephalopathic infant have been described as a typical observation in non-ketotic hyperglycinaemia.[ citation needed ]
Glycine encephalopathy has an estimated incidence of 1 in 60,000, making it the second most common disorder of amino acid metabolism, after phenylketonuria. It is caused by a defect in the glycine cleavage system (GCS), which is 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. [1] : 790
Gene | Name | Percent |
---|---|---|
GLDC | encodes the "glycine dehydrogenase" subunit, also called "glycine decarboxylase" | About 70-75% of cases of glycine encephalopathy result from mutations in the GLDC gene. |
GCST or AMT | encoding the "aminomethyltransferase" subunit | About 20% of cases are caused by mutations in the AMT gene. |
GCSH | encoding the subunit "glycine cleavage system protein H" | Mutations in the GCSH gene account for less than 1% of cases. |
There is a fourth unit in the GCS: dihydrolipoamide dehydrogenase or GCSL. However, to date there have been no mutations in GCSL found to be associated with glycine encephalopathy.[ citation needed ]
A small percentage 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 defective glycine-cleavage enzymatic activity. It is thought that these patients may have mutations in the genes encoding one of the cofactors associated with the GCS complex.[ citation needed ]
Defects in the GCS proteins can prevent the complex from functioning properly or can prevent the GCS complex from forming entirely. When the complex is unable to metabolize glycine properly, this causes excess glycine to build up to toxic levels in the body's organs and tissues. Damage caused by elevated levels of glycine in the brain and cerebrospinal fluid is responsible for the characteristic seizures, breathing difficulties, movement disorders, and intellectual disability.[ citation needed ]
This disorder is inherited in an autosomal recessive pattern. The term "autosomal" signifies that the gene associated with the disorder is located on an autosome. In an autosomal recessive inheritance pattern, two defective copies of the gene (one inherited from each parent) are required in order for a child to be born with the disorder. Therefore, each parent of an individual with an autosomal recessive disorder has at least one defective copy of the gene. With autosomal recessive disorders, individuals with only one copy of a defective gene (heterozygotes) are considered "carriers" for the disorder. Carriers usually do not show signs or symptoms of the disorder. [2]
Glycine is the simplest amino acid, having no stereoisomers. It can act as a neurotransmitter in the brain, act as an inhibitor in the spinal cord and brain stem, while having excitatory effects in the cortex of the brain. Glycine is metabolized to final end products of ammonia and carbon dioxide through the glycine cleavage system (GCS), an enzyme complex made up of four protein subunits. Defects in these subunits can cause glycine encephalopathy, although some causes of the disease are still unknown. Normally, GCS shows its highest enzymatic activity in liver, brain and placental tissue. One of its main functions is to maintain normal glycine levels in the brain. Defects in GCS cause an increase of glycine concentration in blood plasma and cerebrospinal fluid. [1] The exact pathophysiology of the disorder is not known, but it is considered likely that buildup of glycine in the brain is responsible for the symptoms. [3]
All forms of glycine encephalopathy show elevated levels of glycine in the plasma, as well as in cerebral spinal fluid (CSF). [1] : 793 Glycine concentrations in the CSF of affected patients are typically more markedly elevated than in plasma, leading to a corresponding elevation in the ratio of glycine concentrations in the cerebral spinal fluid to that in the plasma. This ratio can also be slightly elevated in patients receiving valproic acid. [1] : 811
Glycine encephalopathy (nonketotic hyperglycinemia, or NKH) should not be confused with other metabolic disorders that can produce elevated glycine levels. For example, certain inherited 'organic acidurias' (aka 'organic acidemias') can produce elevated glycine in plasma and urine, although these disorders are not caused by defects in the glycine cleavage system, and they are not typically accompanied by corresponding elevations of glycine in cerebrospinal fluid (CSF). [4] Glycine encephalopathy is unique in the fact that levels of glycine are disproportionately elevated in CSF (in addition to elevations in blood and urine), whereas CSF glycine levels are normal or near-normal in patients with inherited organic acidurias.[ citation needed ]
Glycine is metabolized in the body's cells to end products of carbon dioxide and ammonia. The glycine cleavage system, which is responsible for glycine metabolism in the mitochondria is made up of four protein subunits, the P-protein, H-protein, T-protein and L-protein. [1] : 793
There are several different forms of glycine encephalopathy, which can be distinguished by the age of onset, as well as the types and severity of symptoms. All forms of glycine encephalopathy present with only neurological symptoms, including intellectual disability (IQ scores below 20 are common [5] ), hypotonia, apneic seizures, and brain malformations. [1] : 811
With the classical, or neonatal presentation of glycine encephalopathy, the infant is born after an unremarkable pregnancy, but presents with lethargy, hypotonia, apneic seizures and myoclonic jerks, which can progress to apnea requiring artificial ventilation, and often death. Apneic patients can regain spontaneous respiration in their second to third week of life. After recovery from the initial episode, patients have intractable seizures and profound intellectual disability, remaining developmentally delayed. Some mothers comment retrospectively that they noticed fetal rhythmic "hiccuping" episodes during pregnancy, most likely reflecting seizure episodes in utero. [3] Patients with the infantile form of glycine encephalopathy do not show lethargy and coma in the neonatal period, but often have a history of hypotonia. They often have seizures, which can range in severity and responsiveness to treatment, and they are typically developmentally delayed. [6] Glycine encephalopathy can also present as a milder form with episodic seizures, ataxia, movement disorders, and gaze palsy during febrile illness. These patients are also developmentally delayed, to varying degrees. In the later onset form, patients typically have normal intellectual function, but present with spastic diplegia and optic atrophy. [6] The mild form of the disorder corresponds to greatly reduced but not fully absent GCS activity. [7]
Transient neonatal hyperglycinemia has been described in a very small number of cases. Initially, these patients present with the same symptoms and laboratory results that are seen in the classical presentation. However, levels of glycine in plasma and cerebrospinal fluid typically normalize within eight weeks, and in five of six cases there were no neurological issues detected at follow-up times up to thirteen years. A single patient was severely retarded at nine months. The suspected cause of transient neonatal hyperglicinemia is attributed to low activity of the glycine cleavage system in the immature brain and liver of the neonate. [3]
A treatment of sodium benzoate, which binds to glycine and forms hippurate, and dextromethorphan, which weakly inhibits the N-methyl-D-aspartate receptors that glycine acts on has been shown to improve outcomes in select cases where the disorder is present in attenuated form. [7]
The prognosis is very poor. Two studies reported typical age of deaths in infancy or early childhood, with the first reporting a median age of death of 2.6 for boys and less than 1 month for girls. [8] [5]
Response to treatment is variable and the long-term and functional outcome is unknown. To provide a basis for improving the understanding of the epidemiology, genotype/phenotype correlation and outcome of these diseases and their impact on the quality of life of patients, and for evaluating diagnostic and therapeutic strategies a patient registry was established by the noncommercial International Working Group on Neurotransmitter Related Disorders (iNTD). [9]
NKH is a rare disease but maintains a very active community presence for educating parents with newly diagnosed children and fundraising for genetic therapy. Leaders in the space include the Jack Richard Urban Foundation and the NKH Crusaders.
Encephalopathy means any disorder or disease of the brain, especially chronic degenerative conditions. In modern usage, encephalopathy does not refer to a single disease, but rather to a syndrome of overall brain dysfunction; this syndrome has many possible organic and inorganic causes.
Propionic acidemia, also known as propionic aciduria or propionyl-CoA carboxylase deficiency, is a rare autosomal recessive metabolic disorder, classified as a branched-chain organic acidemia.
Isovaleric acidemia is a rare autosomal recessive metabolic disorder which disrupts or prevents normal metabolism of the branched-chain amino acid leucine. It is a classical type of organic acidemia.
Maple syrup urine disease (MSUD) is a rare, inherited metabolic disorder that affects the body’s ability to metabolize amino acids due to a deficiency in the activity of the branched-chain alpha-ketoacid dehydrogenase (BCKAD) complex. It particularly affects the metabolism of amino acids- leucine, isoleucine, and valine. With MSUD, the body is not able to properly break down these amino acids, therefore leading to the amino acids to build up in urine and become toxic. The condition gets its name from the distinctive sweet odor of affected infants' urine and earwax due to the buildup of these amino acids.
Glutaric acidemia type 2 is an autosomal recessive metabolic disorder that is characterised by defects in the ability of the body to use proteins and fats for energy. Incompletely processed proteins and fats can build up, leading to a dangerous chemical imbalance called acidosis. It is a metabolic myopathy, categorized under fatty acid metabolism disorder as that is the bioenergetic system that it affects the most. It also affects choline metabolism.
Adenylosuccinate lyase deficiency is a rare autosomal recessive metabolic disorder characterized by the appearance of succinylaminoimidazolecarboxamide riboside and succinyladenosine (S-Ado) in cerebrospinal fluid, urine. These two succinylpurines are the dephosphorylated derivatives of SAICA ribotide (SAICAR) and adenylosuccinate (S-AMP), the two substrates of adenylosuccinate lyase (ADSL), which catalyzes an important reaction in the de novo pathway of purine biosynthesis. ADSL catalyzes two distinct reactions in the synthesis of purine nucleotides, both of which involve the β-elimination of fumarate to produce aminoimidazole carboxamide ribotide (AICAR) from SAICAR or adenosine monophosphate (AMP) from S-AMP.
Hyperekplexia is a neurological disorder characterized by a pronounced startle response to tactile or acoustic stimuli and an ensuing period of hypertonia. The hypertonia may be predominantly truncal, attenuated during sleep, or less prominent after one year of age.
Histidinemia is a rare autosomal recessive metabolic disorder caused by a deficiency of the enzyme histidase. Histidase is needed for the metabolism of the amino acid histidine. Although originally thought to be linked to multiple developmental disorders histidinemia is now accepted as a relatively benign disorder, leading to a reduction in the prevalence of neonatal screening procedures.
Glucose transporter 1, also known as solute carrier family 2, facilitated glucose transporter member 1 (SLC2A1), is a uniporter protein that in humans is encoded by the SLC2A1 gene. GLUT1 facilitates the transport of glucose across the plasma membranes of mammalian cells. This gene encodes a facilitative glucose transporter that is highly expressed in erythrocytes and endothelial cells, including cells of the blood–brain barrier. The encoded protein is found primarily in the cell membrane and on the cell surface, where it can also function as a receptor for human T-cell leukemia virus (HTLV) I and II. GLUT1 accounts for 2 percent of the protein in the plasma membrane of erythrocytes.
Ethylmalonic encephalopathy (EE) is a rare autosomal recessive inborn error of metabolism. Patients affected with EE are typically identified shortly after birth, with symptoms including diarrhea, petechiae and seizures. The genetic defect in EE is thought to involve an impairment in the degradation of sulfide intermediates in the body. Hydrogen sulfide then builds up to toxic levels. EE was initially described in 1994. Most cases of EE have been described in individuals of Mediterranean or Arabic origin.
Aminomethyltransferase is an enzyme that catabolizes the creation of methylenetetrahydrofolate. It is part of the glycine decarboxylase complex.
Hyperprolinemia is a condition which occurs when the amino acid proline is not broken down properly by the enzymes proline oxidase or pyrroline-5-carboxylate dehydrogenase, causing a buildup of proline in the body.
Glycine decarboxylase also known as glycine cleavage system P protein or glycine dehydrogenase is an enzyme that in humans is encoded by the GLDC gene.
Glycine cleavage system H protein, mitochondrial is a protein that in humans is encoded by the GCSH gene. Degradation of glycine is brought about by the glycine cleavage system (GCS), which is composed of 4 protein components: P protein, H protein, T protein, and L protein. The H protein shuttles the methylamine group of glycine from the P protein to the T protein. The protein encoded by GCSH gene is the H protein, which transfers the methylamine group of glycine from the P protein to the T protein. Defects in this gene are a cause of nonketotic hyperglycinemia (NKH). Two transcript variants, one protein-coding and the other probably not protein-coding, have been found for this gene. Also, several transcribed and non-transcribed pseudogenes of this gene exist throughout the genome.
Carnosinemia is a rare autosomal recessive metabolic disorder caused by a deficiency of carnosinase, a dipeptidase.
The glycine cleavage system (GCS) is also known as the glycine decarboxylase complex or GDC. The system is a series of enzymes that are triggered in response to high concentrations of the amino acid glycine. The same set of enzymes is sometimes referred to as glycine synthase when it runs in the reverse direction to form glycine. The glycine cleavage system is composed of four proteins: the T-protein, P-protein, L-protein, and H-protein. They do not form a stable complex, so it is more appropriate to call it a "system" instead of a "complex". The H-protein is responsible for interacting with the three other proteins and acts as a shuttle for some of the intermediate products in glycine decarboxylation. In both animals and plants, the glycine cleavage system is loosely attached to the inner membrane of the mitochondria. Mutations in this enzymatic system are linked with glycine encephalopathy.
Megalencephalic leukoencephalopathy with subcortical cysts is a form of hereditary CNS demyelinating disease. It belongs to a group of disorders called leukodystrophies. It is characterized by early-onset enlargement of the head (macrocephaly) as well as delayed-onset neurological deterioration to include spasticity, epilepsy, and lack of muscular coordination. MLC does not appear to be a disease that is fatal at birth or early in life despite its symptoms, although the number of patients throughout history known to have the disease is fairly limited.
Sepiapterin reductase deficiency is an inherited pediatric disorder characterized by movement problems, and most commonly displayed as a pattern of involuntary sustained muscle contractions known as dystonia. Symptoms are usually present within the first year of age, but diagnosis is delayed due to physicians lack of awareness and the specialized diagnostic procedures. Individuals with this disorder also have delayed motor skills development including sitting, crawling, and need assistance when walking. Additional symptoms of this disorder include intellectual disability, excessive sleeping, mood swings, and an abnormally small head size. SR deficiency is a very rare condition. The first case was diagnosed in 2001, and since then there have been approximately 30 reported cases. At this time, the condition seems to be treatable, but the lack of overall awareness and the need for a series of atypical procedures used to diagnose this condition pose a dilemma.
Aicardi–Goutières syndrome (AGS), which is completely distinct from the similarly named Aicardi syndrome, is a rare, usually early onset childhood, inflammatory disorder most typically affecting the brain and the skin. The majority of affected individuals experience significant intellectual and physical problems, although this is not always the case. The clinical features of AGS can mimic those of in utero acquired infection, and some characteristics of the condition also overlap with the autoimmune disease systemic lupus erythematosus (SLE). Following an original description of eight cases in 1984, the condition was first referred to as 'Aicardi–Goutières syndrome' (AGS) in 1992, and the first international meeting on AGS was held in Pavia, Italy, in 2001.
SLC13A5 citrate transporter disorder, or SLC13A5 Epilepsy, is a rare genetic spectrum disorder that presents with neurological symptoms. Symptoms include severe seizures, ataxia, dystonia, teeth hypoplasia, poor communication skills, difficulty standing or walking, as well as developmental delay. Other names associated with SLC13A5 Epilepsy include SLC13A5 Citrate Transporter Disorder, Citrate Transporter Disorder, SLC13A5 Deficiency, Early Infantile Epilepsy Encephalopathy 25 (EIEE25), Developmental Epilepsy Encephalopathy 25 (DEE25), and Kohlschutter-Tonz Syndrome (non-ROGDI).