Effect of dietary lysine restriction and arginine supplementation in two patients with pyridoxine-dependent epilepsy

Highlights

• Our results support the use of dietary therapies for the treatment of Pyridoxine Dependent Epilepsy (PDE).

• PDE biomarkers directly correlated with Lysine and Threonine in plasma.

• Threonine levels could represent a novel biomarker of pyridoxine availability.

Abstract

Pyridoxine-Dependent Epilepsy (PDE) is a recessive disorder caused by deficiency of α-aminoadipic semialdehyde dehydrogenase in the catabolic pathway of lysine. It is characterized by intractable seizures controlled by the administration of pharmacological doses of vitamin B6. Despite seizure control with pyridoxine, intellectual disability and developmental delays are still observed in some patients with PDE, likely due to the accumulation of toxic intermediates in the lysine catabolic pathway: alpha-aminoadipic semialdehyde (AASA), delta-1-piperideine-6-carboxylate (P6C), and pipecolic acid. Here we evaluate biochemical and clinical parameters in two PDE patients treated with a lysine-restricted diet and arginine supplementation (100–150 mg/kg), aimed at reducing the levels of PDE biomarkers. Lysine restriction resulted in decreased accumulation of PDE biomarkers and improved development. Plasma lysine but not plasma arginine, directly correlated with plasma levels of AASA-P6C (p < 0.001, r² = 0.640) and pipecolic acid (p < 0.01, r² = 0.484). In addition, plasma threonine strongly correlated with the levels of AASA-P6C (p < 0.0001, r² = 0.732) and pipecolic acid (p < 0.005, r² = 0.527), suggesting extreme sensitivity of threonine catabolism to pyridoxine availability. Our results further support the use of dietary therapies in combination with pyridoxine for the treatment of PDE.

Introduction

Pyridoxine-Dependent Epilepsy (PDE) is a recessive disorder of lysine catabolism caused by impaired function of α-aminoadipic semialdehyde dehydrogenase (AASADH), the enzyme responsible for the oxidation of α-aminoadipic semialdehyde to α-aminoadipic acid [1]. AASADH deficiency leads to accumulation of α-aminoadipic semialdehyde (AASA), its cyclic form Δ1-piperideine-6-carboxylate (P6C), and pipecolic acid in plasma, urine and CSF. The sequestration of the active form of vitamin B6 (pyridoxal 5′-phosphate) by excess P6C is believed to be an underlying cause of intractable seizures in PDE [2], [3] and therefore these patients are treated with high doses of Vitamin B6 (pyridoxine) [2]. Neuronal migration defects and multiple brain malformations, such as hydrocephalus and abnormalities of the corpus callosum, white matter and posterior fossa, have also been reported in association with PDE, suggesting a role of ALDH7A1 gene, encoding AASADH, in neuronal development [4]. Despite seizure control, the majority of PDE patients on pyridoxine monotherapy have intellectual disability and developmental delays [5], [6], possibly due to the accumulation of neurotoxic intermediates in lysine catabolism such as AASA, P6C and pipecolic acid [7], [8], [9]. In fact, lysine restriction significantly decreases the levels of these PDE biomarkers in plasma, urine and CSF, and is associated with noticeable developmental progress in the majority of PDE patients [8]. Improvements in biochemical and neurocognitive outcomes were also achieved in one patient on l-arginine supplementation, possibly through the competitive inhibition of lysine uptake into the central nervous system [10]. Overall, the combination of a lysine-restricted diet with pyridoxine and arginine supplements, known as triple therapy, reduced AASA, P6C and pipecolic acid levels and improved cognitive and motor deficits in > 50% of reported patients with PDE [11]. However, the benefits of dietary therapies were not clear for some PDE patients [11], [12] suggesting that treatment efficiency may depend on genetic and phenotypic variability as well as the degree of developmental structural brain abnormalities prior to therapy [4], [13]. Due to the rarity of this disorder the number of patients on a lysine-restricted therapy and/or arginine supplementation reported in the literature is limited (n = 15). Therefore, additional data are necessary for better evaluation of treatment efficiencies and to correlate dietary interventions with outcome.

Here we report the treatment of two PDE patients with a lysine-restricted diet and arginine supplements in addition to pyridoxine, and show a direct correlation between PDE biomarkers and amino acids in plasma of these patients.