PLPHP deficiency: clinical, genetic, biochemical, and mechanistic insights

Abstract

Biallelic pathogenic variants in PLPBP (formerly called PROSC) have recently been shown to cause a novel form of vitamin B6-dependent epilepsy, the pathophysiological basis of which is poorly understood. When left untreated, the disease can progress to status epilepticus and death in infancy. Here we present 12 previously undescribed patients and six novel pathogenic variants in PLPBP. Suspected clinical diagnoses prior to identification of PLPBP variants included mitochondrial encephalopathy (two patients), folinic acid-responsive epilepsy (one patient) and a movement disorder compatible with AADC deficiency (one patient). The encoded protein, PLPHP is believed to be crucial for B6 homeostasis. We modelled the pathogenicity of the variants and developed a clinical severity scoring system. The most severe phenotypes were associated with variants leading to loss of function of PLPBP or significantly affecting protein stability/PLP-binding. To explore the pathophysiology of this disease further, we developed the first zebrafish model of PLPHP deficiency using CRISPR/Cas9. Our model recapitulates the disease, with plpbp À/À larvae showing behavioural, biochemical, and electrophysiological signs of seizure activity by 10 days post-fertilization and early death by 16 days post-fertilization. Treatment with pyridoxine significantly improved the epileptic phenotype and extended lifespan in plpbp À/À animals. Larvae had disruptions in amino acid metabolism as well as GABA and catecholamine biosynthesis, indicating impairment of PLP-dependent enzymatic activities. Using mass spectrometry, we observed significant B6 vitamer level changes in plpbp À/À zebrafish, patient fibroblasts and PLPHP-deficient HEK293 cells. Additional studies in human cells and yeast provide the first empirical evidence that PLPHP is localized in mitochondria and may play a role in mitochondrial metabolism. These models provide new insights into disease mechanisms and can serve as a platform for drug discovery.

Diagnostic pitfalls in vitamin B6‐dependent epilepsy caused by mutations in the PLPBP gene

“…Its gene product, termed pyridoxal phosphate homeostasis protein (PLPHP), is believed to be involved in PLP homeostasis. PLPHP is likely to be a protein with dual localization, in both cytoplasm and mitochondria . It has a PLP‐binding domain, but no enzymatic activity .…”

“…Darin et al suggested that PLPHP acts as a PLP‐carrier that prevents PLP from reacting with other molecules, supplies it to apo‐enzymes, and protects it from intracellular phosphatases . PLPHP is also implicated in mitochondrial metabolism …”

“…So far, 28 cases of vitamin B6‐dependent epilepsy caused by PLPBP variants have been described . Neonatal onset seizures are the dominating feature in this patient group.…”

“…Neonatal onset seizures are the dominating feature in this patient group. Single cases with more deviating clinical or biochemical presentations with predominant movement disorder, a picture of mitochondrial encephalopathy, and hyperglycinemia have been described …”

PDXK mutations cause polyneuropathy responsive to pyridoxal 5′‐phosphate supplementation

“…These include PNPO (Mendelian Inheritance in Man [MIM]: 603287), 48 ALDH7A1 (MIM: 266100), 49,50 and PROSC deficiency 51,52 where the lack of PLP in the brain leads to early onset, vitamin B 6 -dependent epilepsy refractory to anticonvulsants. We have shown previously that several inherited disorders affecting B 6 vitamer metabolism, or resulting in inactivation of PLP, are implicated in neurological conditions.…”