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About ketotic glycinemia

What is ketotic glycinemia?

Propionic Acidemia is a rare metabolic disorder characterized by deficiency of propionyl CoA carboxylase, an enzyme involved in the breakdown (catabolism) of the chemical "building blocks" (amino acids) of certain proteins. Symptoms most commonly become apparent during the first weeks of life and may include abnormally diminished muscle tone (hypotonia), poor feeding, vomiting, listlessness (lethargy), excessive loss of fluids from bodily tissues (dehydration), and episodes of uncontrolled electrical activity in the brain (seizures). Without appropriate treatment, coma and potentially life-threatening complications may result. In rare cases, the condition may become apparent later during infancy and may be associated with less severe symptoms and findings. Propionic Acidemia is inherited as an autosomal recessive trait.

What are the symptoms for ketotic glycinemia?

Listlessness symptom was found in the ketotic glycinemia condition

The severe classic form of NKH typically presents in the first week of life with low muscle tone, lethargy, seizures, coma, and apnea requiring ventilator support. The ventilator is typically needed for a period of 10-20 days before the apnea resolves. A portion of individuals with severe classical NKH die during the neonatal period, often due to withdrawal of intensive care supports. All children with severe classical NKH who survive the neonatal period have severe developmental delay. Most individuals do not reach milestones past those reached by the typical 6-week-old infant. Seizures gradually worsen and can be difficult to control. Feeding difficulties and orthopedic problems can occur. Airway maintenance becomes poor over time due to low muscle tone, and is often the cause of death.

Individuals with attenuated classic NKH can present in the neonatal period or later in infancy. Presentation in the neonatal period resembles that of severe classic NKH. Those who present in infancy can have low muscle tone, lethargy, and seizures. Individuals with attenuated classic NKH have variable developmental progress. Developmental delays can range from mild to profound. They can often walk and achieve various motor skills. They often have hyperactivity and behavioral problems.

The clinical picture of individuals with variant NKH is rapidly evolving. Presentation varies depending upon what gene is mutated and the specific mutation itself. Particular symptoms can include: problems with spasticity or balance, problems with the nerve of the eye (optic neuropathy), problems with the white matter of the brain, heart weakness, increased resistance to blood flow in the lungs, accumulation of acid in the blood, loss of skills that the child had achieved, or seizures. Most children have only some of these problems.

What are the causes for ketotic glycinemia?

Classic NKH is caused by genetic variants (mutations) in the genes that encode the components of the glycine cleavage enzyme system. This enzyme system is responsible for breaking down the amino acid glycine in the body. When it is not working properly, glycine accumulates in the body, resulting in the symptoms associated with NKH.

The glycine cleavage enzyme system is composed of 4 proteins, the P-protein encoded by the GLDC gene, the H-protein encoded by the GCSH gene, the T-protein encoded by the AMT gene, and the L-protein. Mutations in GLDC or AMT cause classic NKH. The majority of individuals with classic NKH have mutations within the GLDC gene. No mutations have been identified in the GCSH gene.

Individuals with deficient enzyme activity, but no mutation in GLDC or AMT, have variant NKH. Many genes have been described in variant NKH including LIAS, BOLA3, GLRX5, NFU1, ISCA2, IBA56, LIPT1 and LIPT2.

NKH is inherited in an autosomal recessive inheritance pattern, meaning that an individual must have pathogenic variants in both copies of the causative gene in order to be affected. Individuals with a pathogenic variant in only one copy of the gene are carriers for the disorder, and are not affected themselves, but could potentially have an affected child if their partner is also a carrier. If both parents are carriers for NKH, then there is a 1 in 4 chance, with each pregnancy, of the child being affected with NKH.

What are the treatments for ketotic glycinemia?

There is no curative treatment for NKH. However, there are treatments that can improve outcomes.

Seizure management in individuals with severe classic NKH is difficult and usually requires multiple anticonvulsants. Valproate is not recommended for patients with NKH as it inhibits the residual glycine cleavage enzyme activity. Vigabatrin should rarely be used as many children with NKH have had adverse reactions to it.

What are the risk factors for ketotic glycinemia?

The incidence of NKH is predicted to be approximately 1:76,000. NKH can occur in individuals of any ancestry.

Is there a cure/medications for ketotic glycinemia?

Sodium benzoate is used to reduce serum glycine levels. Benzoate binds to glycine in the body to form hippurate, which is excreted in the urine. This treatment reduces seizures and improves alertness. Plasma glycine levels must be monitored closely to ensure sodium benzoate is at an effective and non-toxic level.

Dextromethorphan is commonly used to reduce seizures and improve alertness. Dextromethorphan binds to NMDA receptors in the brain. These receptors are over-stimulated in individuals with NKH due to increased glycine levels in the brain. Glutamate is the neurotransmitter that predominately binds to these receptors. Dextromethorphan binds to the NMDA receptors, blocking glutamate from binding to the receptor. Ketamine is another NMDA receptor blocker that is also used. In patients with attenuated NKH, use of dextromethorphan can help with attention and chorea, and if treated early together with benzoate, can improve development and seizures.

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