The first described patients with pyridox(am)ine 5'-phosphate
oxidase deficiency all had neonatal onset
seizures that did not respond to treatment with
pyridoxine but responded to treatment with
pyridoxal 5'-phosphate. Our data suggest, however, that the clinical spectrum of pyridox(am)ine 5'-phosphate
oxidase deficiency is much broader than has been reported in the literature. Sequencing of the PNPO gene was undertaken for a cohort of 82 individuals who had shown a reduction in frequency and severity of
seizures in response to
pyridoxine or
pyridoxal 5'-phosphate. Novel sequence changes were studied using a new cell-free expression system and a mass spectrometry-based assay for
pyridoxamine phosphate oxidase. Three groups of patients with PNPO mutations that had reduced
enzyme activity were identified: (i) patients with neonatal onset
seizures responding to
pyridoxal 5'-phosphate (n = 6); (ii) a patient with
infantile spasms (onset 5 months) responsive to
pyridoxal 5'-phosphate (n = 1); and (iii) patients with
seizures starting under 3 months of age responding to
pyridoxine (n = 8). Data suggest that certain genotypes (R225H/C and D33V) are more likely to result in
seizures that to respond to treatment with
pyridoxine. Other mutations seem to be associated with
infertility,
miscarriage and prematurity. However, the situation is clearly complex with the same combination of mutations being seen in patients who responded and did not respond to
pyridoxine. It is possible that
pyridoxine responsiveness in
PNPO deficiency is affected by prematurity and age at the time of the therapeutic trial. Other additional factors that are likely to influence treatment response and outcome include
riboflavin status and how well the foetus has been supplied with
vitamin B6 by the mother. For some patients there was a worsening of symptoms on changing from
pyridoxine to
pyridoxal 5'-phosphate. Many of the mutations in PNPO affected residues involved in binding
flavin mononucleotide or
pyridoxal 5'-phosphate and many of them showed residual
enzyme activity. One sequence change (R116Q), predicted to affect
flavin mononucleotide binding and binding of the two PNPO dimers, and with high residual activity was found in Groups (ii) and (iii). This sequence change has been reported in the 1000 Genomes project suggesting it could be a polymorphism but alternatively it could be a common mutation, perhaps responsible for the susceptibility locus for genetic
generalized epilepsy on 17q21.32 (close to rs72823592). We believe the reduction in PNPO activity and B6-responsive
epilepsy in the patients reported here indicates that it contributes to the pathogenesis of
epilepsy.