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Aromatic L-amino Acid Decarboxylase Deficiency via the DDC Gene

  • Summary and Pricing
  • Clinical Features and Genetics
  • Citations
  • Methods
  • Ordering/Specimens
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TEST METHODS

Sequencing

Test Code Test Copy GenesIndividual Gene PriceCPT Code Copy CPT Codes
1874 DDC$870.00 81479 Add to Order
Targeted Testing

For ordering targeted known variants, please proceed to our Targeted Variants landing page.

Turnaround Time

The great majority of tests are completed within 18 days.

Clinical Sensitivity

The clinical sensitivity of this test is expected to be high for patients with a biochemical diagnosis of AADC deficiency. In a cohort of 49 patients with biochemically-diagnosed disease, two probable causative variants in the DDC gene were identified in 46/49 patients (93.9%; Brun et al. 2010). In the remaining three patients, one probable causative variant was identified in DDC

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Clinical Features

Aromatic L-amino acid decarboxylase (AADC) deficiency is a rare disorder of neurotransmitter metabolism that results in a severe deficit of serotonin and catecholamines. The majority of patients with this disorder display initial clinical symptoms during infancy or early childhood, although a few reported cases did not present until adolescence or adulthood (Brun et al. 2010). Affected individuals may exhibit severe developmental delays, oculogyric crises, hypotonia, dystonia, hypokinesia, difficulties with feeding and speech, and autonomic symptoms such as excessive sweating and/or temperature instability (Brun et al. 2010; Wassenberg et al. 2017). Less frequently, ptosis and/or insomnia have been observed. Individuals with a milder disease course have also been described (Leuzzi et al. 2015).

Genetics

Aromatic L-amino acid decarboxylase (AADC) deficiency is inherited in an autosomal recessive manner due to defects in the DDC gene. DDC encodes for a homodimeric, pyridoxal phosphate-dependent enzyme responsible for the second step in the catecholamine biosynthetic pathway that results in synthesis of the neurotransmitter dopamine. In addition, DDC can also decarboxylate 5-hydroxytryptophan into serotonin (Montioli et al. 2014). As dopamine acts as a precursor for the metabolites epinephrine and norepinephrine, patients lacking DDC activity exhibit a combined deficiency of serotonin, dopamine, epinephrine, and norepinephrine (Wassenberg et al. 2017). At least thirty missense variants, several splicing variants, and several small insertions/deletions have been reported as causative for AADC deficiency (Human Gene Mutation Database).

Due to a founder mutation that results in defective splicing (c.714+4A>T), AADC deficiency may be more prevalent in certain Asian populations, such as the Taiwanese (Lee et al. 2009). 

Testing Strategy

Full gene sequencing of DDC is performed, with bidirectional sequencing of the 13 coding exons of this gene. The full coding region of each exon plus ~20 bp of flanking non-coding DNA on either side are sequenced. We will also sequence any single exon (Test #100) or pair of exons (Test #200) in family members of patients with known mutations or to confirm research results.

Indications for Test

DDC sequencing could be considered in patients with symptoms consistent with AADC deficiency, or patients with a biochemical diagnosis of AADC deficiency. We will also sequence the DDC gene to determine carrier status. 

Gene

Official Gene Symbol OMIM ID
DDC 107930
Inheritance Abbreviation
Autosomal Dominant AD
Autosomal Recessive AR
X-Linked XL
Mitochondrial MT

Disease

Name Inheritance OMIM ID
Deficiency Of Aromatic-L-Amino-Acid Decarboxylase AR 608643

CONTACTS

Genetic Counselors
Geneticist
Citations
  • Brun L. et al. 2010. Neurology. 75:64-71. PubMed ID: 20505134
  • Human Gene Mutation Database (Bio-base).
  • Lee H.F. et al. 2009. European Journal of Paediatric Neurology. 13:135-40. PubMed ID: 18567514
  • Leuzzi V. et al. 2015. JIMD Reports. 15:39-45. PubMed ID: 24788355
  • Montioli R. et al. 2014. Human Molecular Genetics. 23:5429-40. PubMed ID: 24865461
  • Wassenberg T. et al. 2017. Orphanet Journal of Rare Diseases. 12:12. PubMed ID: 28100251
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TEST METHODS

Bi-Directional Sanger Sequencing

Test Procedure

Nomenclature for sequence variants was from the Human Genome Variation Society (http://www.hgvs.org).  As required, DNA is extracted from the patient specimen.  PCR is used to amplify the indicated exons plus additional flanking non-coding sequence.  After cleaning of the PCR products, cycle sequencing is carried out using the ABI Big Dye Terminator v.3.0 kit.  Products are resolved by electrophoresis on an ABI 3730xl capillary sequencer.  In most cases, sequencing is performed in both forward and reverse directions; in some cases, sequencing is performed twice in either the forward or reverse directions.  In nearly all cases, the full coding region of each exon as well as 20 bases of non-coding DNA flanking the exon are sequenced.

Analytical Validity

As of March 2016, we compared 17.37 Mb of Sanger DNA sequence generated at PreventionGenetics to NextGen sequence generated in other labs. We detected only 4 errors in our Sanger sequences, and these were all due to allele dropout during PCR. For Proficiency Testing, both external and internal, in the 12 years of our lab operation we have Sanger sequenced roughly 8,800 PCR amplicons. Only one error has been identified, and this was due to sequence analysis error.

Our Sanger sequencing is capable of detecting virtually all nucleotide substitutions within the PCR amplicons. Similarly, we detect essentially all heterozygous or homozygous deletions within the amplicons. Homozygous deletions which overlap one or more PCR primer annealing sites are detectable as PCR failure. Heterozygous deletions which overlap one or more PCR primer annealing sites are usually not detected (see Analytical Limitations). All heterozygous insertions within the amplicons up to about 100 nucleotides in length appear to be detectable. Larger heterozygous insertions may not be detected. All homozygous insertions within the amplicons up to about 300 nucleotides in length appear to be detectable. Larger homozygous insertions may masquerade as homozygous deletions (PCR failure).

Analytical Limitations

In exons where our sequencing did not reveal any variation between the two alleles, we cannot be certain that we were able to PCR amplify both of the patient’s alleles. Occasionally, a patient may carry an allele which does not amplify, due for example to a deletion or a large insertion. In these cases, the report contains no information about the second allele.

Similarly, our sequencing tests have almost no power to detect duplications, triplications, etc. of the gene sequences.

In most cases, only the indicated exons and roughly 20 bp of flanking non-coding sequence on each side are analyzed. Test reports contain little or no information about other portions of the gene, including many regulatory regions.

In nearly all cases, we are unable to determine the phase of sequence variants. In particular, when we find two likely causative mutations for recessive disorders, we cannot be certain that the mutations are on different alleles.

Our ability to detect minor sequence variants, due for example to somatic mosaicism is limited. Sequence variants that are present in less than 50% of the patient’s nucleated cells may not be detected.

Runs of mononucleotide repeats (eg (A)n or (T)n) with n >8 in the reference sequence are generally not analyzed because of strand slippage during PCR and cycle sequencing.

Unless otherwise indicated, the sequence data that we report are based on DNA isolated from a specific tissue (usually leukocytes). Test reports contain no information about gene sequences in other tissues.

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Ordering Options


myPrevent - Online Ordering
  • The test can be added to your online orders in the Summary and Pricing section.
  • Once the test has been added log in to myPrevent to fill out an online requisition form.
REQUISITION FORM
  • A completed requisition form must accompany all specimens.
  • Billing information along with specimen and shipping instructions are within the requisition form.
  • All testing must be ordered by a qualified healthcare provider.

SPECIMEN TYPES
WHOLE BLOOD

(Delivery accepted Monday - Saturday)

  • Collect 3 ml -5 ml (5 ml preferred) of whole blood in EDTA (purple top tube) or ACD (yellow top tube). For Test #500-DNA Banking only, collect 10 ml -20 ml of whole blood.
  • For small babies, we require a minimum of 1 ml of blood.
  • Only one blood tube is required for multiple tests.
  • Ship blood tubes at room temperature in an insulated container. Do not freeze blood.
  • During hot weather, include a frozen ice pack in the shipping container. Place a paper towel or other thin material between the ice pack and the blood tube.
  • In cold weather, include an unfrozen ice pack in the shipping container as insulation.
  • At room temperature, blood specimen is stable for up to 48 hours.
  • If refrigerated, blood specimen is stable for up to one week.
  • Label the tube with the patient name, date of birth and/or ID number.

DNA

(Delivery accepted Monday - Saturday)

  • Send in screw cap tube at least 5 µg -10 µg of purified DNA at a concentration of at least 20 µg/ml for NGS and Sanger tests and at least 5 µg of purified DNA at a concentration of at least 100 µg/ml for gene-centric aCGH, MLPA, and CMA tests, minimum 2 µg for limited specimens.
  • For requests requiring more than one test, send an additional 5 µg DNA per test ordered when possible.
  • DNA may be shipped at room temperature.
  • Label the tube with the composition of the solute, DNA concentration as well as the patient’s name, date of birth, and/or ID number.
  • We only accept genomic DNA for testing. We do NOT accept products of whole genome amplification reactions or other amplification reactions.

CELL CULTURE

(Delivery preferred Monday - Thursday)

  • PreventionGenetics should be notified in advance of arrival of a cell culture.
  • Culture and send at least two T25 flasks of confluent cells.
  • Some panels may require additional flasks (dependent on size of genes, amount of Sanger sequencing required, etc.). Multiple test requests may also require additional flasks. Please contact us for details.
  • Send specimens in insulated, shatterproof container overnight.
  • Cell cultures may be shipped at room temperature or refrigerated.
  • Label the flasks with the patient name, date of birth, and/or ID number.
  • We strongly recommend maintaining a local back-up culture. We do not culture cells.
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