Phenytoin neurotoxicity in a child carrying new STXBP1 and CYP2C9 gene mutations.

We report the clinical history of a boy, affected by early-onset epileptic encephalopathy with an episode of phenytoin neurotoxicity, found by whole exome sequencing to carry a previously undescribed mutation of the STXBP1 gene, combined with two heterozygous missense mutations in the same exon of the CYP2C9 gene. Highlights: - a new mutation in STXBP1 gene in a patient with a clinical history of Ohtahara syndrome and a severe adverse reaction to phenytoin, co-occurring with compound heterozygous mutations in CYP2C9 gene. - in patients with genetic epileptic encephalopthies refractory to therapy, whole exome sequencing may help disclose co-mutations in genes coding for drug metabolizing enzymes.