Specific gene pairings alter the severity of AADC deficiency cases
Scientists track how certain mutation pairings help boost vital protein levels
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Different combinations of mutations in the DDC gene can have markedly different effects on the severity of aromatic l-amino acid decarboxylase (AADC) deficiency, according to a new report.
By conducting laboratory tests on four specific mutations found in two patients, scientists discovered that individual genetic changes reduce enzyme activity to varying degrees. Crucially, the researchers found that certain combinations of mutations can actually interact to partially restore enzyme function.
The study, Molecular heterogeneity in AADC deficiency: Variant-dependent effects on AADC activity,” was published in Molecular Genetics and Metabolism.
Understanding AADC deficiency and symptom variability
As the name suggests, AADC deficiency is marked by a lack of functional AADC, an enzyme that’s necessary for the production of certain brain signaling chemicals (neurotransmitters).
Low production of these neurotransmitters leads to a wide range of neurological symptoms, including developmental delays, movement problems, sleep issues, and behavioral challenges, that begin in infancy. Symptoms are typically quite severe; however, more mildly affected patients have increasingly been described.
The disease is caused by mutations in DDC, the gene responsible for producing AADC. More than a hundred mutations in the gene have been linked to the ultrarare disease.
It’s thought that symptom variability is related to the extent to which a specific disease-causing mutation affects AADC activity — meaning that certain mutations may allow more residual enzyme activity, resulting in milder symptoms.
However, some mutations associated with the disease haven’t been thoroughly analyzed to determine exactly how they affect AADC.
“Gaining insight into the pathomechanism [disease mechanisms] of AADC deficiency at the molecular level is crucial for understanding the relationship between genetic variants and clinical symptoms, as well as for developing targeted therapeutic strategies,” the researchers wrote.
In their recent report, the scientists aimed to further characterize the molecular impacts of four DDC mutations identified in two of their Polish patients. The mutations had been previously reported and classified as likely disease-causing; however, no one had yet actually functionally characterized them.
A person inherits two copies of most genes — one from each biological parent. For AADC deficiency to arise, both copies of the DDC gene must be mutated. A person can have the same mutation in both gene copies, or two different mutations in each copy. The patients in this study fell into the latter group.
The first patient was a 20-year-old man with very low AADC activity measured in his blood, who carried the p.Cys261Phe and p.Gly354Ser mutations; the second was a 15-year-old female with no detectable enzyme activity, who carried the p.Pro47Ala and p.Arg447Cys mutations. Both experienced a severe disease course starting in infancy, but eventually received gene therapy (now sold as Kebilidi) that led to some clinical improvement.
The scientists introduced the mutations into lab-grown bacterial cells, then isolated and analyzed the AADC enzyme produced from them.
Measuring individual and combined mutation effects
They found that three of the mutations — p.Pro47Ala, p.Gly354Ser, and p.Arg447Cys — were each associated with an 88% to more than 99% loss of enzyme activity toward its two major targets. These targets are the precursor molecules that AADC normally converts into neurotransmitters. The fourth mutation, p.Cys261Phe, reduced enzyme activity by 20% to 25%.
The mutations also altered AADC’s affinity for these molecules, or its likelihood of interacting with them. This was most profound for the p.Arg447Cys mutation.
Some of these differences may be related to the varying locations of the mutations on the DDC gene, according to the researchers. While some are located in areas expected to directly affect the enzyme’s active site, others are farther away and may have less profound effects.
Notably, while each mutation alone decreased enzyme function to some degree, specific combinations of mutations partially restored its function. For example, p.Gly354Ser with p.Cys261Phe — the combination seen in the male patient — restored AADC activity at one of its target molecules to near-normal.
The combination of mutations observed in the female patient resulted in 10%-15% of normal activity, although this was still an improvement.
“In summary, our findings suggest that the pathogenicity [disease-driving properties] of the examined AADC variants … is determined by several factors,” the researchers concluded, noting that the individual and combined effects of different mutations on enzyme structure and activity likely play an important role.
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