Progressive Myoclonus Epilepsy and Beyond: A Systematic Review of SEMA6B-related Disorders

Progressive myoclonus epilepsy (PME) is a rare, clinically and genetically heterogeneous epilepsy syndrome, and pathogenic variants in the semaphorin 6B (SEMA6B) gene have recently been reported to be among the causes of PME. Cases with pathogenic variants in the SEMA6B gene are extremely rare, only a limited number of cases have been reported in the literature. In this systematic review, we aimed to present a summary of a PME case in which a heterozygous nonsense variant of c.2086C > T p.(Gln696*) in the SEMA6B gene was detected in the etiology and other cases with SEMA6B pathogenic variant in the literature. Except for our case, 35 cases from 12 studies were included. The main clinical findings in these patients were cognitive problems, seizures, gait and speech disturbances, and cognitive and/or motor regression, and they had a wide spectrum of severity. Response to antiseizure medications was also highly variable, almost half of the patients had pharmacoresistant seizures. Patients were divided into four different phenotypic groups according to their clinical presentations: PME (18/36), developmental and epileptic encephalopathy (13/36), neurodevelopmental disorder (4/36), and epilepsy (1/36), respectively. In conclusion, although SEMA6B has been associated with PME, it may actually cause a much broader phenotypic spectrum. Due to their extreme rarity, our knowledge of SEMA6B-related disorders is limited. As with all other rare diseases, each new SEMA6B-related disorder case could contribute to a better understanding of the disease. A better understanding of the disease may allow the development of specific treatment options in the future.