Ring Chromosome 20

Ring Chromosome 20 (RC20) was first reported in 1972 and there are now over 120 patients described in the literature, with more females than males affected (6:4 ratio). There is a wide range of phenotypic severity which relates in part to whether there is deletion of coding genetic material. As well, the majority of reported patients have been mosaic for RC20 and degree of mosaicism also influences the severity of presentation. Patients usually have intellectual disability and epilepsy. Seizures most commonly have focal impaired awareness semiology, involving some or all of behavioral arrest, staring, frightened expression, and automatisms. Some patients have developmental regression at the time of seizure onset or during periods of increased seizure frequency. In addition to intellectual disability, patients may have other neuropsychiatric features including Attention Deficit Hyperactivity Disorder (ADHD), learning difficulties, impulsivity, and oppositional behavior. The underlying pathogenesis of RC20 remains poorly understood. While there are several genes associated with epilepsy in the peri-telomeric regions of chromosome 20, there is no evidence that these genes are deleted or have altered expression in RC20 patients.