Mosaic Synapses in Epilepsy

Mismatch of synaptic cadherins perturbs hippocampal circuitry The inherited X-linked early-onset childhood epilepsy, called EFMR (epilepsy and mental retardation limited to females), has baffled clinicians and geneticists for more than 50 years. In contrast to other X-linked disorders in which the hemizygous (hemi) male, but not the heterozygous (het) female is affected, it is only the het females that exhibit seizures and intellectual disability (1, 2). Clues to the origin of this enigmatic disorder came when deleterious mutations in the X chromosome gene protocadherin-19 (PCDH19), which encodes a cell adhesion molecule, were identified (3). On page 255 of this issue, Hoshina et al. (4) provide answers to two pieces of the EFMR puzzle: They reveal that hippocampal synaptic transmission is compromised in Pcdh19 het female mice but not in hemi males, and they provide a molecular explanation for how the retention of one wild-type (WT) allele, but not the loss of both alleles, disrupts neuronal connectivity.