Delineating syndrome: From congenital microcephaly to hyperkinetic encephalopathy.

Objective: To provide new insights into the related clinical and imaging phenotypes and refine the phenotype-genotype correlation in syndrome.

Methods: We analyzed the clinical and imaging phenotypes of a cohort of 45 patients with a pathogenic or likely pathogenic variant and performed phenotype-genotype correlations.

Results: A total of 37 different heterozygous mutations were identified, of which 18 are novel. We described a broad spectrum of neurodevelopmental phenotypes, characterized by severe postnatal microcephaly and developmental delay accompanied by a hyperkinetic movement disorder, stereotypes and sleep disorders, and epileptic seizures. Our data highlighted 3 patterns of gyration, including frontal pachygyria in younger patients (26.7%), moderate simplified gyration (24.4%) and mildly simplified or normal gyration (48.9%), corpus callosum hypogenesis mostly in its frontal part, combined with moderate-to-severe myelination delay that improved and normalized with age. Frameshift and nonsense mutations in the N-terminus of , which are the most common mutation types, show the most severe clinical features and MRI anomalies. However, patients with recurrent frameshift mutations c.460dupG and c.256dupC had variable clinical and imaging presentations.

Conclusions: These findings have implications for genetic counseling, providing evidence that N-terminal mutations and large deletions lead to more severe syndrome, although genotype-phenotype correlations are not necessarily straightforward in recurrent mutations. Together, these analyses support the view that syndrome is a specific disorder characterized by frontal pachygyria and delayed myelination in its most severe form and hypogenetic corpus callosum in its milder form.