Individuals with mutations in both copies of the DDC gene show severe neurological problems because of a deficiency in the aromatic amino acid decarboxylase (AADC) enzyme. This enzyme is necessary for the production of neurotransmitters or cell signaling molecules in the brain, such as dopamine and serotonin.
Genetic testing is one of the diagnostic tests to confirm AADC deficiency and can determine the exact molecular changes in the DDC gene that cause the disease.
How does genetic testing work?
Genetic testing can confirm a diagnosis of AADC deficiency and help doctors decide on treatments to manage the symptoms.
In genetic testing, a sample of cells, mostly blood cells, is used to isolate DNA, which contains the DDC gene. Then, the DDC gene is sequenced using routine technologies such as Sanger or next-generation sequencing. After aligning the nucleotide sequence of a patient’s DDC gene, its sequence is matched against the sequence of the normal DDC gene to identify any deviation or variation from the normal sequence.
How do DDC gene mutations cause AADC deficiency?
Nearly 50 different mutations in the DDC gene have been reported in the literature, and cause AADC deficiency. The types of mutations include:
- Missense mutations – one or more nucleotide in the gene is changed, which alters the amino acid sequence of the AADC enzyme. (Nucleotides are “letters” in the DNA code and amino acids are building blocks of proteins).
- Nonsense mutations – a single nucleotide change introduces a premature stop signal to the DNA sequence and results in a truncated AADC enzyme with reduced or no function.
- Deletions – either a single nucleotide or large section of the DDC gene is missing, resulting in the production of a truncated AADC enzyme with reduced or no function.
- Insertions – a single nucleotide is inserted into the coding sequence of the DDC gene, thereby altering the coding sequence and producing a dysfunctional AADC enzyme.
Genetic testing by sequencing the DDC gene can identify the exact mutation or mistake that is responsible for AADC deficiency. Knowing the genetic mutation in a particular individual can also help define the treatment, including possible gene therapy, in an attempt to fix the defect.
Last updated: Sept. 14, 2019
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