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Sir, We read with interest the Letter to the Editor by Mignot et al.(2016), which reports that mutations in the chromatin-remodelling gene ARID1B are a major genetic cause of isolated agenesis of the corpus callosum in patients with intellectual disability. Agenesis of the corpus callosum (ACC) is among the most common brain malformations (Guillem et al., 2003; Wang et al., 2004; Glass et al., 2008), and can be associated with no or variable levels of intellectual disability (Paul et al., 2007). The term ‘isolated ACC’is used to distinguish ACC that is not associated with additional cerebral or extra-cerebral malformations, from syndromic ACC. This distinction is prognostically useful, because whilst individuals with isolated ACC are still likely to have learning difficulties, intellectual disability (ID) is more common in syndromic ACC (Moutard et al., 2012; Sotiriadis and Makrydimas, 2012). The genetics underlying ACC …