Compound heterozygosity in GPR56 with bilateral frontoparietal polymicrogyria
Polymicrogyria is caused by a diverse etiology, one of which is gene mutation. At present, only one gene (GPR56) is known to cause polymicrogyria, which leads to a distinctive phenotype termed bilateral frontoparietal polymicrogyria (BFPP). BFPP is an autosomal recessive inherited human brain malformation with abnormal cortical lamination. Here, we identified compound heterozygous GPR56 mutations in a patient with BFPP. The proband was a Japanese female born from non-consanguineous parents. She presented with mental retardation, developmental motor delay, epilepsy exhibiting the feature of Lennox-Gastaut syndrome, exotropia, bilateral polymicrogyria with a relatively spared perisylvian region, bilateral patchy-white-matter MRI signal changes, and hypoplastic pontine basis. GPR56 sequence analysis revealed a c.107G>A substitution leading to a p.S36N, and a c.113G>A leading to a p.R38Q. Although affected individuals with compound heterozygosity in GPR56 have not been previously described, we presume that compound heterozygosity of these two mutations in a ligand binding domain within the extracellular N-terminus of protein could result in BFPP. In addition, we observed unusually less involvement of perisylvian cortex for polymicrogyria, and Lennox-Gastaut syndrome for epilepsy, which are likely common features in patients with BFPP caused by GPR56 mutations.