Mutations in PROSC and vitamin B6 dependent epilepsy – a functional study in zebrafish

Abstract

Background: Vitamin B6 dependent epilepsies are a group of rare, recessive genetic diseases that causes seizures refractory to standard epilepsy treatment but that respond rapidly to vitamin B6. Several disease-causing mutations have already been described in detail, but the mechanism behind one of them, PROSC (proline synthetase co-transcribed homolog), remains poorly understood. Although the seizures can be treated effectively, the syndrome is accompanied by features such as brain defects and developmental delay that are not improved by todays treatment regime, which motivates further research. Aim: To investigate whether absence of PROSC causes vitamin B6 deficiency and epilepsy in a zebrafish model. Methods: Morphological characteristics, survival rate, and behavior were assessed in mutant fish with and without vitamin B6 treatment. Epileptic zebrafish typically show a hyperactive swimming pattern that can be quantified objectively using automated video-tracking equipment and software. Results: Mutant fish cannot be distinguished from wildtype based on physical appearance. No signs of epileptic activity have been observed, instead mutant fish are significantly less active than their wildtype siblings. Activity levels were normalized in mutant fish treated with vitamin B6. All mutants died between day ten and fourteen, however a significant reduction in mortality rate was observed in B6 treated fish. Discussion: This study has shown that homozygous prosc mutant fish are vitamin B6 deficient. Although no signs of seizure-like epileptic activity were found in the present study this does not contradict the findings in humans. It is quite possible that the fish were epileptic only for a short time and not observed frequently enough to detect it or that the hypo activity observed may be due to an abnormal electroencephalograph (EEG) response. Vitamin B6 deficiency causes both brain malformations and a wide range of systemic disturbances that might have affected the behavior of the fish to such an extent that the hyperactivity that otherwise would have arisen due to a lowered seizure threshold, was masked.