What is Glycine Encephalopathy, AMT-related?
Glycine encephalopathy (GE), AMT-related, also known as nonketotic hyperglycinemia (NKH), is a disease that impairs the body's ability to breakdown glycine, an amino acid found in proteins. It is caused by harmful genetic changes (mutations) in the AMT gene, which codes for the enzyme aminomethyltransferase (AMT). AMT breaks down harmful substances, such as glycine, into smaller pieces. Over time, glycine builds up to toxic levels in the brain, organs, and other body tissues. This can lead to lack of energy (lethargy), seizures, low muscle tone, breathing difficulties, coma, and often death. Individuals have intellectual disability and seizures. The majority of individuals with encephalopathy present in the newborn period, but the age at which symptoms appear can vary. There are two forms of the disease: severe and attenuated.
Approximately 85% of those with symptoms in the newborn period and 50% of those with onset between two weeks to three months will have the severe form of GE. Individuals with the severe form will not make any developmental progress and will have seizures that are difficult to control with medications. Most affected individuals will present in the newborn period with rapidly worsening symptoms. However, individuals with the severe form can also begin to show symptoms between two weeks to three months.
Approximately 20% of all children affected with GE will have less-severe symptoms (the attenuated form). Individuals who have the attenuated form will have variable progress with their development. They may have seizures, but the seizures are typically better controlled with medications. While some individuals with the attenuated form will present in the newborn period, others may not show symptoms until after three months of age.
How common is Glycine Encephalopathy, AMT-related?
Several genes are known to cause GE and approximately 15-20% of GE is caused by mutations in the AMT gene. The incidence of Glycine Encephalopathy, AMT-related is approximately 1/275,000.
How is Glycine Encephalopathy, AMT-related treated?
There is no cure for GE. Treatment is aimed at lowering the levels of glycine in the body and controlling seizures. Glycine can be reduced through certain medications (e.g., sodium benzoate) and a low-protein diet. Anticonvulsant medications can be used to control seizures, but they may not be completely effective for all individuals. The health of individuals with GE must usually be monitored by many different specialists.
What is the prognosis for an individual with Glycine Encephalopathy, AMT-related?
Infants with the severe form of GE will not make any progress in major developmental milestones (such holding objects, sitting, crawling, walking, or talking). However, some children may learn to smile and roll. Certain skills may be lost as the child grows (such as bottle feeding and eye contact). Seizures will develop in the first year of life and are very difficult to treat with medications. Death in the first year of life is common.
Developmental progress varies in individuals with the attenuated form. Some children may learn to sit, walk, and communicate. Communication is most often non-verbal (such as by using sign language). The majority of affected individuals will have moderate to severe intellectual disability and attend special-education classes in school. These children may develop movement disorders and behavioral problems. Individuals who have seizures typically respond to treatment. Affected individuals rarely present symptoms after one year of age. Those who do typically have milder intellectual disability, and seizures are uncommon in them.