Causes of AADC Deficiency

Aromatic L-amino acid decarboxylase (AADC) deficiency is a genetic condition that affects the communication between the brain and other parts of the body causing development delays.

Genetic defects

AADC deficiency is caused by mutations in the dopa decarboxylase (DDC) gene, which carries the instructions to make the aromatic L-amino acid decarboxylase enzyme.

The nervous signals from the brain are relayed to the rest of the body via the nerve cells or neurons. These neurons use chemical messengers called neurotransmitters to transmit these signals. Dopamine and serotonin are two vital neurotransmitters. Dopamine plays a vital role in coordinating movement, learning, attention, emotions, and personality traits. Serotonin regulates mood, digestion, and sleep. Problems in the production of these neurotransmitters can have an impact on their function.

The production of dopamine and serotonin is a multi-step process. Dopamine synthesis starts with an amino acid (the building block of proteins) called tyrosine, which is first converted to L-dopa. The first step of serotonin synthesis begins with the conversion of amino acid tryptophan to 5-hydroxytryptophan. Next, the AADC enzyme facilitates the final steps in the process of converting L-dopa and 5-hydroxytryptophan to dopamine and serotonin, respectively. A mutation in the DDC gene impedes AADC production and decreases its activity, which in turn reduces dopamine and serotonin levels and leads to the symptoms of AADC deficiency.

Inheritance of AADC deficiency

There are 23 pairs of chromosomes in every cell in our body. Twenty-two of these are called autosomes, and the 23rd pair is called the sex chromosomes. Each parent contributes one chromosome to the pair. Every gene in an autosome exists in two identical copies. The DDC gene is located on chromosome 7, an autosome.

AADC deficiency is inherited in an autosomal recessive manner, meaning that the disease only develops if both copies of the DDC gene are mutated. Individuals who have one copy of the mutated gene are called carriers of AADC deficiency. They usually do not exhibit any symptoms, but carry the risk of passing the disease on to their children.


Last updated: Sept. 10, 2019


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