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Aromatic L-amino acid decarboxylase deficiency
This condition impacts the brain's ability to produce important chemical messengers called neurotransmitters, leading to a range of neurological symptoms from infancy. It primarily affects motor development and can cause involuntary movements.
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Overview
Aromatic L-amino acid decarboxylase (AADC) deficiency is a rare genetic disorder that affects the nervous system. Individuals with this condition have a problem converting certain amino acids into vital chemical messengers, known as neurotransmitters, which are essential for proper brain function. These neurotransmitters include dopamine, serotonin, noradrenaline, and adrenaline.
The condition typically presents in infancy or early childhood. The severity of symptoms can vary significantly between affected individuals, ranging from mild to severe. Because of its rarity and the broad range of neurological symptoms, correctly diagnosing AADC deficiency can sometimes be challenging [PMID:33678099].
Symptoms & clinical features
Symptoms of AADC deficiency often become apparent during the first year of life. Common signs include low muscle tone (hypotonia), developmental delay, and problems with movement control. Infants may struggle to reach developmental milestones such as sitting, crawling, or walking.
Other symptoms can include oculogyric crises, which are episodes where the eyes involuntarily roll upwards, often lasting for minutes or hours. Involuntary movements, known as dystonia (sustained muscle contractions causing twisting or repetitive movements) or dyskinesia (uncontrolled, sudden, jerky movements), may also be present. Autonomic symptoms, affecting involuntary body functions, can include excessive sweating, nasal congestion, drooling, and unstable body temperature [PMID:31720857]. Feeding difficulties and poor growth are also commonly reported.
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Affected organs
AADC deficiency primarily affects the brain and nervous system, impacting the production and balance of key neurotransmitters. This dysregulation influences various aspects of neurological function, including motor control, autonomic regulation, and cognitive development. While the condition mainly manifests with neurological symptoms, the wider implications on autonomic functions mean other systems like the cardiovascular and digestive systems can be indirectly affected due to impaired regulation.
Risks & severity
The severity of AADC deficiency can vary. Most individuals experience significant developmental delays and motor difficulties. Without management, severe forms can lead to life-threatening complications, particularly respiratory issues and infections, due to impaired muscle control and autonomic dysfunction. The long-term outlook depends on the specific symptoms and their severity, as well as the effectiveness of management strategies [PMID:31720857]. Onset typically occurs in infancy, but there is variability in how early symptoms are recognised.
Genetic causes
AADC deficiency is caused by pathogenic variants in the DDC gene. The DDC gene provides instructions for making an enzyme called aromatic L-amino acid decarboxylase, also known as DOPA decarboxylase. This enzyme plays a crucial role in the body's production of several neurotransmitters, including dopamine and serotonin, from their precursor molecules.
When there are disease-causing changes in the DDC gene, the enzyme cannot function properly or is not produced in sufficient quantities. This leads to a deficiency of dopamine and serotonin, disrupting normal communication between nerve cells in the brain and ultimately causing the neurological symptoms seen in individuals with AADC deficiency. The specific location and type of variation in the DDC gene can influence the enzyme's residual activity and, consequently, the severity of the condition.
- DDC dopa decarboxylaseThe DDC gene encodes the aromatic L-amino acid decarboxylase (AADC) enzyme, which is vital for synthesising neurotransmitters like dopamine and serotonin in the brain and nervous system.
Inheritance pattern
AADC deficiency is inherited in an autosomal recessive pattern. This means that an individual must inherit two copies of the pathogenic DDC gene - one from each parent - to have the condition. People who have only one pathogenic copy of the gene are called carriers. Carriers typically do not show symptoms of the condition themselves but can pass the altered gene copy on to their children.
If two carriers have children together, there is a 25% (1 in 4) chance with each pregnancy that the child will inherit two altered gene copies and develop AADC deficiency. There is also a 50% (1 in 2) chance that the child will be a carrier, and a 25% chance that the child will inherit two typical gene copies and not be affected or a carrier.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosing AADC deficiency often begins with recognising characteristic clinical symptoms in an infant or young child. Initial investigations may include tests of cerebrospinal fluid (CSF) or urine to look for patterns of neurotransmitter metabolites that suggest an enzyme deficiency. High levels of 3-O-methyldopa (3-OMD) in CSF, along with low levels of homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA), are highly indicative of AADC deficiency.
A definitive diagnosis is confirmed through genetic testing, which looks for pathogenic variants in the DDC gene. This testing can be arranged through the NHS Genomic Medicine Service (GMS) after a referral from a specialist paediatrician or neurologist. The relevant R-code for genomic testing can be identified by the clinical team referring. Genetic counsellors can provide further information about testing options and implications.
Management & lifestyle
Management of AADC deficiency focuses on alleviating symptoms and improving quality of life. Treatment strategies often involve medications aimed at restoring the balance of neurotransmitters in the brain. These may include dopamine agonists, which mimic the action of dopamine, and monoamine oxidase inhibitors, which prevent the breakdown of existing neurotransmitters.
Supportive therapies are also crucial and can include occupational therapy, physical therapy, and speech and language therapy to help manage developmental delays and motor difficulties. Nutritional support may be necessary for feeding difficulties. Regular follow-up with a specialised paediatric healthcare team, including neurologists and genetic counsellors, is important to monitor the condition and adjust management plans as needed within the NHS framework.
UK care pathway
In the UK, individuals suspected of having AADC deficiency would typically be referred to a consultant paediatrician, often a paediatric neurologist, who can initiate the diagnostic process. If AADC deficiency is suspected, genomic testing through the NHS Genomic Medicine Service can be arranged. This involves a referral to a Clinical Genetics service. Patients and their families will have access to genetic counsellors who can provide comprehensive information about the condition, inheritance patterns, and support with interpretation of genetic test results.
Frequently asked questions
What does AADC stand for?
AADC stands for Aromatic L-amino acid decarboxylase. This is the name of an enzyme that is deficient or not working properly in individuals with the condition, leading to problems with neurotransmitter production.
Is AADC deficiency common?
No, AADC deficiency is considered a very rare condition. The exact prevalence is not well established, but only a small number of cases are reported worldwide, making it one of the rarer inherited metabolic disorders.
How does AADC deficiency affect brain development?
AADC deficiency impacts brain development by reducing the production of essential neurotransmitters like dopamine and serotonin. These chemicals are vital for communication between brain cells, affecting motor control, learning, and overall neurological function.
Can AADC deficiency be detected before birth?
For families with a known history of AADC deficiency, prenatal diagnosis is possible through genetic testing if the specific DDC gene variants in the family have been identified. This is typically discussed with a clinical genetics team and genetic counsellor.
Are there different types of AADC deficiency?
While it is a single condition caused by DDC gene variants, the severity and range of symptoms can vary significantly between individuals. This variability is thought to be partly influenced by the specific gene variant and the amount of residual enzyme activity.
References
- Pearl PL, Wallis DD, Gibson KM. Pediatric neurotransmitter diseases. Current neurology and neuroscience reports. 2004. PMID: 14984687
- Adam MP, Bick S, Mirzaa GM. Aromatic L-Amino Acid Decarboxylase Deficiency. 1993. PMID: 37824694
- Hwu WL, Hsu RH, Li MH. Aromatic l-amino acid decarboxylase deficiency in Taiwan. JIMD reports. 2023. PMID: 37701332
- Pearson TS, Gupta N, San Sebastian W. Gene therapy for aromatic L-amino acid decarboxylase deficiency by MR-guided direct delivery of AAV2-AADC to midbrain dopaminergic neurons. Nature communications. 2021. PMID: 34253733
- Abukhaled M, Al Muqbil M, Alghamdi MA. Aromatic L-amino acid decarboxylase deficiency in countries in the Middle East: a case series and literature review. European journal of pediatrics. 2023. PMID: 36928758
- Roubertie A, Opladen T, Brennenstuhl H. Gene therapy for aromatic L-amino acid decarboxylase deficiency: Requirements for safe application and knowledge-generating follow-up. Journal of inherited metabolic disease. 2024. PMID: 37402126
- Fusco C, Leuzzi V, Striano P. Aromatic L-amino Acid Decarboxylase (AADC) deficiency: results from an Italian modified Delphi consensus. Italian journal of pediatrics. 2021. PMID: 33478565
- Lee NC, Chien YH, Hwu WL. A review of aromatic l-amino acid decarboxylase (AADC) deficiency in Taiwan. American journal of medical genetics. Part C, Seminars in medical genetics. 2019. PMID: 30614627