Table 3.

Rationale for CSF analysis

CSF Metabolite	Rationale/expected insights
AASA and P6C and pipecolic acid	Metabolites reflecting lysine metabolism in the brain/surrogate parameters of toxicity
	We expect reduction in these metabolites upon lysine-restricted diet
Pyridoxal-5-phosphate (PLP)	Metabolite reflecting the biochemical interaction of accumulating AASA/P6C with PLP
	We expect increase of this metabolite upon lysine-restricted diet
Monoaminergic neurotransmitter metabolites (HVA, 5-HIAA)	Synthesis of Dopamin and Serotonin requires pyridoxal phosphate as cofactor
	We expect that these neurotransmitter metabolite levels increase with increasing availability of pyridoxal phosphate upon the lysine-restricted diet
Amino acids	We are primarily interested in CSF lysine, which has been observed in the low normal range in patients on the lysine-restricted diet.7 Other amino acids of interest are (but not restricted to) tryptophan (due to inadequate tryptophan content of most lysine-free amino acid formulas), glycine, serine, and branched chain amino acids (given that many reactions the metabolism of these amino acids requires PLP as a cofactor)
Cells count, protein, and glucose	CSF specimen that is bloody, or has an increased protein level have to be excluded from analysis)
1 mL	To store for possible additional research studies such as metabolomics. AASA as a semialdehyde is a reactive substance and its accumulation might result in changes in numerous other metabolites. Metabolomic metabolite patterns might reveal chemical compounds that might contribute to the pathophysiology of ATQ deficiency prior and during treatment with a lysine-restricted diet