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Investigating vitamin B6-dependent epileptic encephalopathies in human patients and a mouse model Al Shekaili, Hilal Hamed

Abstract

PLPHP deficiency is a recently discovered form of vitamin B6-dependent epilepsies (B6Es) that is caused by recessive mutations in PLPBP. PLPHP is involved in pyridoxal 5’-phosphate (PLP) homeostatic regulation. However, the mechanism by which PLPHP dysfunction disrupts PLP homeostasis and leads to the observed encephalopathy in patients was still elusive. We characterized the clinical, genomic and biochemical abnormalities in a new series of 12 PLPHP deficiency patients. Our results identified previously undescribed clinical features of PLPHP deficiency, including non-epileptic movement disorder, fatal mitochondrial encephalopathy and folinic acid-responsive seizures. We characterized the pathogenicity of patients’ PLPBP variants using in silico tools and 3D modelling of PLPHP and developed a system of clinical severity score. We generated and characterized PLPBP knockout models in HEK293 cells, yeast and zebrafish. Our plpbp-KO zebrafish model replicated the clinical phenotype of PLPHP-deficient patients by showing vitamin B6-dependent seizures and death in untreated KO larvae. Consistent with the biochemical picture in patients, Plphp-deficient fish displayed decreased systemic levels of PLP. In the future this model can be utilized as a tool for investigating the disease pathophysiology, drug screening and identifying diagnostic biomarkers. Pyridoxine-dependent epilepsy (PDE-ALDH7A1) is another form of B6Es that is caused by mutations in ALDH7A1, a gene which encodes an enzyme within the lysine catabolism pathway. We have successfully generated and characterized transgenic mouse strain with constitutive genetic ablation of Aldh7a1. Results showed that KO mice accumulated high concentrations of upstream lysine metabolites including ∆¹-piperideine-6-carboxylic acid (P6C), α-aminoadipic semialdehyde (α-AASA) and pipecolic acid (PIP), similar to the biochemical picture in ALDH7A1-defiecint patients. KO mice fed the regular diet (0.9% lysine) did not exhibit seizures based on EEG analysis. When KO mice are switched to a diet containing higher amount of lysine (4.7%), they developed severe recurrent seizures which led to their quick death. In analogy to the patients’ picture also, treating KO mice under high lysine diet with pyridoxine injections prevented seizures and prolonged their survival. This study provides a proof-of-concept for the utility of the model to study PDE-ALDH7A1 biochemistry and to test new therapeutics.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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