New Method May Help Diagnose AADC Deficiency, Other Genetic Disorders of Metabolism

Measuring multiple metabolites at once in a single drop of cerebrospinal fluid (CSF) may hasten the diagnosis of many rare genetic disorders of metabolism that cause neurological problems, including aromatic l-amino acid decarboxylase (AADC) deficiency, research suggests.

The new approach, which identified a new biomarker of AADC deficiency, also is likely to be less invasive for patients, who often undergo multiple lumbar punctures before receiving a final diagnosis.

The study, “Targeted cerebrospinal fluid analysis for inborn errors of metabolism on an LC‐MS/MS analysis platform,” was published in the Journal of Inherited Metabolic Disease.

Diseases in which metabolic genetic mutations cause neurological symptoms require a rapid and effective diagnosis to prevent irreversible brain damage. But their rarity and nonspecific clinical symptoms often mean that patients undergo multiple tests before receiving an accurate diagnosis.

In some of these diseases, biomarkers that aid in diagnosis can be measured only in the cerebrospinal fluid (CSF, the fluid surrounding the brain and spinal cord); it must be collected via a spinal tap. Multiple rounds of testing mean that patients, who mostly are infants and toddlers, undergo several lumbar punctures to collect CSF samples.

In addition to the burden and risk it poses to patients, these tests require a wide range of specialized instruments, each examining specific metabolites, and qualified laboratory staff.

To overcome that, researchers at the University Hospital Heidelberg in Germany and the Center for Biomedical Research on Rare Diseases in Spain set out to develop an approach that measures multiple molecules of metabolism — selected to aid in the diagnosis of an array of rare metabolic conditions — in a single CSF sample.

The approach consisted of liquid chromatography, a chemistry technique that divides a sample into its constituent molecules based on their mass and charge, followed by a specific detection method called mass spectrometry. The general idea is to enrich the compounds of interest, then measure them.

Researchers tested two approaches using these techniques, one that measured metabolites of various chemical natures in a single run, and a second one that measured metabolites of the same chemical classes. Together, they could measure the levels of 38 specific metabolites.

After validating the two methods, the team established the appropriate cut-offs for a diagnosis, compared them with the cut-off values used in other studies, and tested CSF samples from patients with five metabolic genetic disorders to make sure their approach offered an accurate diagnosis.

Overall, both approaches had reference values in agreement with those reported in previous research. For some metabolites, however, this was the first time a cut-off value was established for CSF samples, researchers noted.

The team examined samples from one patient with guanidinoacetate methyltransferase (GAMT) deficiency, one with ornithine aminotransferase (OAT) deficiency, another with cerebral folate deficiency (CFD), and one with methylenetetrahydrofolate reductase (MTHFR) deficiency. They also tested CSF samples from four AADC patients.

Results showed that the approach was able to detect alterations in specific disease biomarkers for all patients. In people with AADC deficiency it also detected high levels of vanillactic acid — a proposed urine biomarker of disease that had not been detected yet in CSF samples of these patients.

“For the first time, we showed that vanillactic acid is also increased in CSF in AADC-deficiency, in line with its elevation in urine, and may be used as an additional biomarker for this disease,” researchers wrote.

Overall, the team concluded that its platform “allowed for reliable clinical diagnosis in the examined disorders and, for the first time, the establishment of CSF reference concentrations of several biomarkers,” they wrote.

“By measurement on a single analytical platform, less sample volume is required …, diagnostic results are obtained faster, and pre-analytical issues are reduced,” they added.

While the approach is designed to diagnose a wide range of diseases, investigators believe it may be further expanded to analyze even more CSF metabolites, increasing its diagnostic accuracy.