A Novel Type of Autosomal Dominant Episodic Nystagmus Segregating with a Variant in the FRMD5 Gene

To describe the phenotype of a novel form of autosomal dominant episodic nystagmus and to identify the potential genetic aetiology. We identified several individuals in a large Swedish family affected by episodic nystagmus. In total, 39 family members from five generations were invited to participate in the study, of which 17 were included (12 affected and 5 unaffected). The phenotype of the nystagmus was described based on data collected from family members through questionnaires, interviews, clinical examinations and from video recordings of ongoing episodes of nystagmus. Whole genome sequencing (WGS) and further Sanger sequencing for segregation of the identified candidate variants was performed in eight participants (six affected and two unaffected). The 12 affected participants showed a phenotype with episodic nystagmus of early onset. A vertical jerk nystagmus with variable amplitude and frequency was characterized in the analysed video material. No other eye pathology or other disease that could explain the episodic nystagmus was identified among the family participants. Genetic analysis identified a missense variant (p.Ser375Phe) in the gene FRMD5, which segregated with the disease in the eight individuals analysed, from three generations. We describe a novel autosomal dominant form of early onset episodic nystagmus and suggest the FRMD5 gene as a strong candidate gene for this disorder.