Metabolite Identification Using Infrared Ion Spectroscopy─Novel Biomarkers for Pyridoxine-Dependent Epilepsy

Untargeted LC-MS based metabolomics strategies are being increasingly applied in metabolite screening for a wide variety of medical conditions. The long-standing “grand challenge” in the utilization of this approach is metabolite identification – confidently determining the chemical structures of m/z-detected unknowns. Here, we use a novel workflow based on the detection of molecular features of interest by high-throughput untargeted LC-MS analysis of patient body fluids combined with targeted molecular identification of those features using infrared ion spectroscopy (IRIS), effectively providing diagnostic IR fingerprints for mass-isolated targets. A significant advantage of this approach is that in silico predicted IR spectra of candidate chemical structures can be used to suggest the molecular structure of unknown features, thus mitigating the need for the synthesis of a broad range of physical reference standards. Pyridoxine dependent epilepsy (PDE-ALDH7A1) is an inborn error of lysine metabolism, resulting from a mutation in the ALDH7A1 gene that leads to an accumulation of toxic levels of α-aminoadipic semialdehyde (α-AASA), piperideine-6-carboxylate (P6C), and pipecolic acid in body fluids. While α-AASA and P6C are known biomarkers for PDE in urine, their instability makes them poor candidates for diagnostic analysis from blood, which would be required for application in newborn screening protocols. Here, we use combined untargeted metabolomics-IRIS to identify several new biomarkers for PDE-ALDH7A1 that can be used for diagnostic analysis in urine, plasma, and cerebrospinal fluids, and are compatible with analysis in dried blood spots for newborn screening. The identification of these novel metabolites has directly rendered novel insights in the pathophysiology of PDE-ALDH7A1.

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Identification of novel biomarkers for pyridoxine-dependent epilepsy using untargeted metabolomics and infrared ion spectroscopy - biochemical insights and clinical implications

10.1101/2021.01.22.20248925 ◽

2021 ◽

Author(s):

Udo F.H. Engelke ◽

Rianne E. van Outersterp ◽

Jona Merx ◽

Fred A.M.G. van Geenen ◽

Arno van Rooij ◽

...

Keyword(s):

Carboxylic Acid ◽

Newborn Screening ◽

Vitamin B6 ◽

Untargeted Metabolomics ◽

Zebrafish Model ◽

Knock Out ◽

Ion Spectroscopy ◽

Semialdehyde Dehydrogenase ◽

Novel Biomarkers ◽

Knock Out Mice

AbstractPyridoxine-dependent epilepsy (PDE-ALDH7A1), also known as antiquitin deficiency, is an inborn error of lysine metabolism that presents with refractory epilepsy in newborns. Bi-allelic ALDH7A1 variants lead to deficiency of α-aminoadipic semialdehyde dehydrogenase, resulting in accumulation of piperideine-6-carboxylate (P6C), and secondary deficiency of the important co-factor pyridoxal-5’-phosphate (PLP, active vitamin B6) through its complexation with P6C. Vitamin B6 supplementation resolves epilepsy in patients, but despite this treatment, intellectual disability may occur. Early diagnosis and treatment, preferably based on newborn screening, potentially optimize long-term clinical outcome. However, the currently known diagnostic PDE-ALDH7A1 biomarkers are incompatible with newborn screening procedures. Using a combination of the innovative analytical methods untargeted metabolomics and infrared ion spectroscopy, we have been able to discover novel biomarkers for PDE-ALDH7A1: 2S,6S-and 2S,6R-oxopropylpiperidine-2-carboxylic acid (2-OPP) and 6-oxopiperidine-2-carboxylic acid (6-oxoPIP). We demonstrate the applicability of 2-OPP as a PDE-ALDH7A1 biomarker in newborn screening. Additionally, we show that 2-OPP accumulates in brain tissue of patients and Aldh7a1 knock-out mice, and induces epilepsy-like behavior in a zebrafish model system. We speculate that 2-OPP may contribute to ongoing neurotoxicity, also in treated PDE-ALDH7A1 patients. As 2-OPP formation appears to increase upon ketosis, we emphasize the importance of avoiding catabolism in PDE-ALDH7A1 patients.

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