GNAO1-related disorder is a rare neurodevelopmental disorder associated with epilepsy, developmental delay, and involuntary movements. We encountered a patient with this condition through a research project of the Initiative on Rare and Undiagnosed Diseases (IRUD). The patient was 11 months old at the initial visit and exhibited developmental delay and hypotonia. He had been routinely examined but showed no abnormalities on brain magnetic resonance imaging (MRI), cerebrospinal fluid examination and neurophysiological tests. Truncal hypotonia gradually became evident, but he developed at a moderate pace and walked short distances. At the age of 4 years and 11 months, he received an electroencephalogram and spinal MRI, but no significant results were obtained. At this point, he could walk about 5 meters without support, eat without dysphagia, and understand easy instructions. At the age of 5 years and 4 months, he entered the IRUD with written informed consent from his parents. Chromosomal microarray testing showed no abnormalities, but whole exome sequencing revealed a known variant (NM_020988.3 (GNAO1):c.626G>A [p.Arg209His]) with de novo symptoms. GNAO1 is a causative gene of intractable epilepsy; however, epilepsy is not the most significant factor in some cases. Rather, developmental delay, hypotonia, and involuntary movements are the main clinical features. As there was no contrary evidence, we diagnosed this patient as having a GNAO1-related disorder. Therefore, we confirmed the usefulness of whole exome sequencing in undiagnosed pediatric cases.
