The evolving spectrum of vitamin B6 responsive disorders

Vitamin B6 is a micronutrient essential for normal physiological functioning in humans, serving as a cofactor for >70 human enzymes. Unlike lower organisms, humans cannot synthesise this vitamin and rely on dietary forms and those synthesised by the microbiome. These dietary forms include pyridoxine, pyridoxamine and pyridoxal and their 5’-phosphorylated derivatives. The intestine can only absorb the non-phosphorylated B6 vitamers, therefore phosphorylated forms must first be hydrolysed. Specific enzymes are then involved in the conversion of pyridoxine, pyridoxamine and pyridoxal to pyridoxal 5’-phosphate (PLP), the only active form of vitamin B6. Inborn errors of metabolism which affect the interconversion and availability of PLP, lead to a deficiency of this vitamer which manifests as an epilepsy that responds to treatment with supraphysiological doses of vitamin B6. This is not surprising given the vital role that PLP plays in neurotransmitter metabolism. Disorders include pyridox(am)ine phosphate oxidase deficiency (a disorder affecting PLP synthesis and recycling), disorders where metabolites accumulate that inactivate PLP, for example, ALDH7A1 deficiency and hyperprolinaemia type II, disorders which affect PLP import into the brain (hypophosphatasia and glycosylphosphatidylinositol anchor synthesis defects), and the recently described disorder in which mutations in an intracellular PLP-binding protein result in abnormal B6 homeostasis (PLPHP deficiency). We have shown recently however, by using next generation sequencing for the investigation of patients that have remained undiagnosed for many years, that: i) disorders of vitamin B6 deficiency do not always manifest as an epilepsy ii) supraphysiological doses of B6 can not only be used to treat the vitamin B6-dependent epilepsy disorders but also other seizure disorders that are not associated with vitamin B6 metabolism, such as the KCNQ2-related epileptic encephalopathies.

21/05/2021 Philippa Mills Great Ormond Street Institute of Child Health, University College London, London, UK.

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