Epilepsy phenotype in individuals with chromosomal duplication encompassing FGF12

Intragenic mutations in <i>FGF12</i> are associated with intractable seizures, developmental regression, intellectual disability, ataxia, hypotonia, and feeding difficulties. <i>FGF12</i> duplications are rarely reported, but it was suggested that those might have a similar gain-of-function effect and lead to a more or less comparable phenotype. A favorable response to the sodium blocker phenytoin was reported in several cases, both in patients with an intragenic mutation and in patients with a duplication of <i>FGF12</i>. We report three individuals from two families with <i>FGF12</i> duplications. The duplications are flanked and probably mediated by two long interspersed nuclear elements (LINEs). The duplication cases show phenotypic overlap with the cases with intragenic mutations. Though the onset of epilepsy might be later, after the onset of seizures both groups show developmental stagnation and regression in several cases. This illustrates and further confirms that chromosomal <i>FGF12</i> duplications and intragenic gain-of-function mutations yield overlapping phenotypes.