Pyridoxine Responsive Seizures: Beyond Aldehyde Dehydrogenase 7A1

Integrative physiology of lysine metabolites

Lysine is an essential amino acid that serves as a building block in protein synthesis. Beside this, the metabolic activity of lysine has only recently been unraveled. Lysine metabolism is tissue specific and is linked to several renal, cardiovascular, and endocrinological diseases through human metabolomics datasets. As a free molecule, lysine takes part in the antioxidant response and engages in protein modifications, and its chemistry shapes both proteome and metabolome. In the proteome, it is an acceptor for a plethora of posttranslational modifications. In the metabolome, it can be modified, conjugated, and degraded. Here, we provide an update on integrative physiology of mammalian lysine metabolites such as α-aminoadipic acid, saccharopine, pipecolic acid, and lysine conjugates such as acetyl-lysine, and sugar-lysine conjugates such as advanced glycation end products. We also comment on their emerging associative and mechanistic links to renal disease, hypertension, diabetes, and cancer.

Infantile Spasms without Hypsarrhythmia and Paroxysmal Eye-Head Movements in an Infant with a Pyridoxine-Dependent Epilepsy due to PLPBP/PLPHP Deficiency

To describe a new phenotype and the diagnostic workup of a vitamin-B6-dependent epilepsy due to pyridoxal 5'-phosphate-binding protein (PLPBP) deficiency in an infant with early-onset epilepsy at the age of 5 years 6 months. Following immediate and impressive clinical response to treatment with pyridoxine, metabolic screening for vitamin-B6-dependent epilepsies and targeted next-generation sequencing (NGS)-based gene panel analysis were performed. Potentially pathogenic variants were confirmed by Sanger sequencing in the patient, and variants were analyzed in both parents to confirm biallelic inheritance. The clinical phenotype and course of disease were compared to the 44 cases reported in the literature, harboring variants in pyridoxal phosphate homeostasis protein (PLPHP) and with cases of vitamin-B6-dependent epilepsy due to other known causative genes. Levels of alpha-aminoadipic semialdehyde in urine and amino acids were normal. Two inherited pathogenic variations in PLPHP were found in compound heterozygosity, including one novel deletion. We here describe a previously unreported individual harboring biallelic pathogenic PLPHP variants presenting with paroxysmal eye-head movements followed by epileptic spasms and an almost normal interictal electroencephalogram, thus expanding the clinical spectrum of PLPBP deficiency. This warrants consideration of vitamin-B6-dependent epilepsies in patients with early-onset epilepsy, including epileptic spasms, and eye movement disorders also beyond the neonatal period even when metabolic screening for vitamin-B6-dependent epilepsies is negative. PLPHP should be included systematically in NGS epilepsy gene panels.

Maintenance of cellular vitamin B6 levels and mitochondrial oxidative function depend on pyridoxal 5'-phosphate homeostasis protein

Recently, biallelic variants in PLPBP coding for pyridoxal 5'-phosphate homeostasis protein (PLPHP) were identified as a novel cause of early-onset vitamin B6-dependent epilepsy. The molecular function and precise role of PLPHP in vitamin B6 metabolism are not well understood. To address these questions, we used PLPHP-deficient patient skin fibroblasts and HEK293 cells and YBL036C (PLPHP ortholog)-deficient yeast. We showed that independent of extracellular B6 vitamer type (pyridoxine, pyridoxamine, or pyridoxal), intracellular pyridoxal 5'-phosphate (PLP) was lower in PLPHP-deficient fibroblasts and HEK293 cells than controls. Culturing cells with pyridoxine or pyridoxamine led to the concentration-dependent accumulation of pyridoxine 5'-phosphate and pyridoxamine 5'-phosphate (PMP), respectively, suggesting insufficient pyridox(am)ine 5'-phosphate oxidase activity. Experiments utilizing 13C4-pyridoxine confirmed lower pyridox(am)ine 5'-phosphate oxidase activity and revealed increased fractional turnovers of PLP and pyridoxal, indicating increased PLP hydrolysis to pyridoxal in PLPHP-deficient cells. This effect could be partly counteracted by inactivation of pyridoxal phosphatase. PLPHP deficiency had a distinct effect on mitochondrial PLP and PMP, suggesting impaired activity of mitochondrial transaminases. Moreover, in YBL036C-deficient yeast, PLP was depleted and PMP accumulated only with carbon sources requiring mitochondrial metabolism. Lactate and pyruvate accumulation along with the decrease of tricarboxylic acid cycle intermediates downstream of α-ketoglutarate suggested impaired mitochondrial oxidative metabolism in PLPHP-deficient HEK293 cells. We hypothesize that impaired activity of mitochondrial transaminases may contribute to this depletion. Taken together, our study provides new insights into the pathomechanisms of PLPBP deficiency and reinforces the link between PLPHP function, vitamin B6 metabolism, and mitochondrial oxidative metabolism.

Characteristics, Genetic Testing, and Diagnoses of Infants with Neonatal Encephalopathy Not Due to Hypoxic Ischemic Encephalopathy: A Cohort Study

OBJECTIVE

To characterize the presentation and evaluation of infants with neonatal encephalopathy (NE) not due to hypoxic-ischemic encephalopathy (non-HIE NE) and to describe the genetic abnormalities identified.

STUDY DESIGN

Retrospective cohort study of 193 non-HIE NE neonates admitted to a level IV NICU from 2015 through 2019. For changes in testing over time, Cochrane-Armitage test for trend was used with a Bonferroni-corrected P-value, and comparison between groups was performed using Fisher exact test.

RESULT

The most common symptom of non-HIE NE was abnormal tone in 47% (90/193). Ten percent (19/193) died prior to discharge, and 48% of survivors (83/174) required medical equipment at discharge. Forty percent (77/193) underwent genetic testing as an inpatient. Of 52 chromosomal studies, 54 targeted tests, and 16 exome sequences, 10%, 41%, and 69% were diagnostic, respectively, with no difference in diagnostic rates between infants with and without an associated congenital anomaly and/or dysmorphic feature. Twenty-eight genetic diagnoses were identified.

CONCLUSIONS

Neonates with non-HIE NE have high rates of morbidity and mortality and may benefit from early genetic testing, even in the absence of other exam findings. This study broadens our knowledge of genetic conditions underlying non-HIE NE, which may enable families and care teams to anticipate the needs of the individual, allow early initiation of targeted therapies, and facilitate decisions surrounding goals of care.

Case report: PLPHP deficiency, a rare but important cause of B6-responsive disorders: A report of three novel individuals and review of 51 cases

PLPHP (pyridoxal-phosphate homeostasis protein) deficiency is caused by biallelic pathogenic variants in PLPBP and is a rare cause of pyridoxine-responsive disorders. We describe three French-Canadian individuals with PLPHP deficiency, including one with unusual paroxysmal episodes lacking EEG correlation with a suspicious movement disorder, rarely reported in B6RDs. In addition, we review the clinical features and treatment responses of all 51 previously published individuals with PLPHP deficiency. Our case series underlines the importance of considering PLPBP mutations in individuals with partially B6-responsive seizures and highlights the presence of a founder effect in the French-Canadian population.

Early-onset vitamin B6-dependent epilepsy due to pathogenic PLPBP variants in a premature infant: A case report and review of the literature

Vitamin B6-dependent epilepsies are a heterogeneous group of disorders characterized by decreased availability of the active cofactor pyridoxal-5'-phosphate (PLP). While pathogenic variants in ALDH7A1 or PNPO genes account for most cases of these disorders, biallelic pathogenic variants in PLPBP have been shown to cause a form of early onset vitamin B6-dependent epilepsy (EPVB6D). PLPBP is thought to play a role in the homeostatic regulation of vitamin B6, by supplying PLP to apoenzymes while limiting side-reaction toxicity related to excess unbound PLP. Neonatal-onset intractable seizures that respond to pyridoxine and/or PLP are a predominant feature of EPVB6D in humans. Unlike other causes of vitamin B6-dependent epilepsies; however, a specific biomarker for this disorder has yet to be identified. Here we present data from a premature infant found to have pathogenic variants in PLPBP and propose that prematurity may provide an additional clue for early consideration of this diagnosis. We discuss these findings in context of previously published genotypic, phenotypic, and metabolic data from similarly affected patients.