Test of 3-OMD levels in infants may speed AADC deficiency diagnosis
Accuracy seen with use of dried blood spots taken for newborn screening
Measuring 3-O-methyldopa (3-OMD) levels in the dried blood spots used in newborn screening programs could help in diagnosing aromatic L-amino-acid decarboxylase (AADC) deficiency early in life, a study in Taiwan suggests.
Results from 157,371 newborns showed that six of the eight babies found to have higher-than-normal 3-OMD levels carried disease-causing mutations and went on to develop symptoms of AADC deficiency.
The test “could be easily integrated into the existing newborn screening programs and facilitate the future application for early diagnosis and effective treatment,” the researchers wrote.
The study, “Streamlined determination of 3-O-methyldopa in dried blood spots: Prospective screening for aromatic l-amino-acid decarboxylase deficiency,” was published in the journal Molecular Genetics and Metabolism.
Delays common in arriving at an AADC deficiency diagnosis
Typically, AADC deficiency is caused by mutations in both copies of the DDC gene, which provides the information for producing the aromatic L-amino acid decarboxylase, an enzyme also known as AADC.
This enzyme is needed to make two major neurotransmitters: dopamine and serotonin. Neurotransmitters are chemical substances used by nerve cells to communicate with each other.
Mutations cause the enzyme to be faulty or produced at levels below what is needed. This leads to symptoms that most commonly manifest in the first six months of life, including low muscle tone and failure to reach developmental milestones.
However, an AADC deficiency diagnosis “often delayed by low disease awareness and [lack of] specific laboratory examinations,” the researchers wrote.
Widespread use of a relatively easy and quick way of diagnosing this disease also is of increasing importance, with a first disease-modifying treatment, the gene therapy Upstaza (eladocagene exuparvovec), approved in the European Union and the U.K.
Studies have shown that measuring the levels of 3-OMD in dried blood spots could help to diagnose AADC deficiency. Without a working AADC enzyme, dopamine precursors like L-dopa accumulate inside cells and are broken down into 3-OMD, so that it also is present at high levels in AADC deficiency patients.
Researchers in Taiwan previously showed that 3-OMD measures could be integrated in newborn screening programs, using a single dried blood spot collected within days of birth.
3-OMD levels helped to identify four newborns with AADC deficiency out of a total of 127,987 newborns, and these babies were able to participate in gene therapy trials, “resulting in significant motor and cognitive improvement,” the team wrote.
Analysis found high 3-OMD levels in 8 infants, 6 with disease-causing mutations
This same team now described a fast and highly specific method for screening AADC deficiency as part of routine newborn screening for inborn metabolic errors in Taiwan.
Dried blood samples collected 48 hours after birth were first analyzed for 3-OMD and other metabolites using the NeoBase 2 kit, which uses a technique called flow-injection analysis-tandem mass spectrometry to identify and quantify molecules.
Use of a kit reduces time and sample preparation, and allows researchers to run hundreds of tests a day.
Samples with 3-OMD levels outside the normal range were analyzed again with a more specific technique, called high performance liquid chromatography-tandem mass spectrometry.
If the 3-OMD level again was found to be higher than normal (500 nanograms per milliliter, or ng/mL), the newborn was referred to the researchers’ hospital for physical examination and genetic testing to confirm AADC deficiency.
Between February 2020 and December 2022, dried blood spots from 157,371 newborns were analyzed with the NeoBase 2 kit. A total of 114 samples (0.1%) were tested with the more specific technique, and eight of them showed abnormal 3-OMD levels, ranging between 839 and 5,170 ng/mL.
Genetic analyses confirmed that six of the eight cases (75%) carried mutations in both DDC gene copies, most commonly c.714+4A>T, and these infants were diagnosed with AADC deficiency. All six subsequently developed symptoms of AADC deficiency by or around the age of 6 months.
These findings translated into a disease incidence of 1 in 26,229 newborns.
One of the two remaining newborns with abnormally high 3-OMD levels was found to have a disease-causing mutation in only one DDC copy, with the other copy carrying a mutation with an unknown effect.
Given that this baby was showing normal development and 3-OMD levels dropped to nearly normal during follow-up, no further analysis or treatment for AADC deficiency were undertaken.
The other newborn had no disease-causing DDC mutations and nearly normal 3-OMD levels were reached over follow-up.
Two additional newborns with 3-OMD levels very close to the upper normal limit on the first test subsequently underwent genetic testing. They were negative for mutations in DDC and other genes that could increase 3-OMD levels, and eventually these levels fell
These last three cases were considered false positives.
“In conclusion, we have demonstrated a convenient and streamlined determination of 3-OMD in [dried blood] samples, seamlessly integrated into our expanded newborn screening program without disrupting the existing workflow,” the team wrote.
“Expanding the use of this rapid and convenient assay to other centers … can identify more patients through newborn screening or high-risk group programs, resulting in improved patient care and outcomes,” the researchers concluded.
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