De novo dominant variants affecting the motor domain of KIF1A are a cause of PEHO syndrome

PEHO syndrome (OMIM no. 260565) is characterized by myoclonic jerking and infantile spasms, profound psychomotor retardation with the absence of motor milestones and speech, absence or early loss of visual fixation with atrophy of optic discs by 2 years of age and progressive brain atrophy on neuroimaging. We describe the results of a genomic study of a girl with PEHO syndrome and review the literature on cases with a disease-causing variant in the same gene. Exome sequencing of the index and unaffected parents followed by Sanger confirmation identified nine candidate genes harboring nonsynonymous rare variants identified by trio whole-exome sequencing. The de novo variant, a missense variant (c.296C>T, p.(T99M)), affecting the motor domain of KIF1A was considered the pathogenic mutation. The literature review revealed 24 cases with disease-causing variants in the motor domain of KIF1A, of which three met all the criteria for PEHO syndrome and an additional patient with incomplete clinical data met four of the five criteria. If the criteria were modified to include cases with any convulsive disorder and less profound intellectual disability, a total of six patients met all five of the criteria, three patients met four of the criteria and six met three of the criteria. Our results indicate that the molecular basis for PEHO syndrome, in at least a subset of patients, is a dominant KIF1A variant affecting the motor domain of the protein. Variable expressivity is seen with recurrent variants causing the full phenotype of PEHO syndrome in some patients and in other patients, a partial or milder PEHO phenotype.