A missense variant of SMC1A causes periodic pharmaco-resistant cluster seizures similar to PCDH19-related epilepsy

Abstract

SMC1A variants causing Cornelia de Lange syndrome (CdLS) produce another phenotype characterized by moderate to severe neurological impairment and severe early-onset epilepsy without morphological characteristics of CdLS. The patients are all female and have truncation mutations in SMC1A. The epilepsy also follows a characteristic clinical course with pharmaco-resistant cluster seizures since infancy, mimicking that of PCDH19-related epilepsy. We report here that a missense variant of the SMC1A gene affecting a daughter (proband) and her mother caused similar phenotypes of early-onset (2 years and 1 month of age) and late-onset (12 years of age) epilepsy, respectively. Both patients lacked the morphological characteristics of CdLS, and had severe and moderate intellectual disability, respectively. The cluster seizures were characteristic, occurring approximately every 2–4 weeks (interval; mean ± SD: 20.2 ± 8.3 days) at the peak of the clinical course, especially in the proband. Thus, SMC1A-related encephalopathy is caused not only by truncation mutations but also by missense variants of the SMC1A gene. The periodicity of cluster seizures mimicking that of PCDH19-related epilepsy may characterize SMC1A-related encephalopathy.