A novel mutation in STXBP1 causing epileptic encephalopathy (late onset infantile spasms) with partial respiratory chain complex IV deficiency.

STXBP1 (MUNC18.1), encoding syntaxin binding protein 1, has been reported in Ohtahara syndrome, a rare epileptic encephalopathy with suppression burst pattern on EEG, in patients with infantile spasms and in a few patients with nonsyndromic mental retardation without epilepsy. We report a patient who presented late onset infantile spasms. Epilepsy was controlled but the patient developed severe mental delay. A first diagnosis of mitochondrial disease was based on clinical presentation and on a partial deficit of respiratory chain complex IV, but molecular screening for mitochondrial genes was negative. The sequencing of STXBP1 gene found a de novo nonsense mutation (c.585C>G/p.Tyr195X). This observation widens the clinical spectrum linked to STXBP1 mutations with the description of a patient with late onset infantile spasms. It raises the question of the value of epilepsy genes screening in patients with uncertain, partial or unconfirmed mitochondrial dysfunction.