BACKGROUND AND PURPOSE: Zhu-Tokita-Takenouchi-Kim syndrome is a severe multisystem malformation disorder characterized by developmental delay and a diverse array of congenital abnormalities. However, these currently identified phenotypic components provide limited guidance in diagnostic situations, due to both the nonspecificity and variability of these features. Here we report a case series of 7 individuals with a molecular diagnosis of Zhu-Tokita-Takenouchi-Kim syndrome, 5 ascertained by their presentation with the neuronal migration disorder, periventricular nodular heterotopia.MATERIALS AND METHODS: Individuals with a molecular diagnosis of Zhu-Tokita-Takenouchi-Kim syndrome were recruited from 2 sources, a high-throughput sequencing study of individuals with periventricular nodular heterotopia or from clinical diagnostic sequencing studies. We analyzed available brain MR images of recruited individuals to characterize periventricular nodular heterotopia distribution and to identify the presence of any additional brain abnormalities.RESULTS: Pathogenic variants in SON, causative of Zhu-Tokita-Takenouchi-Kim syndrome, were identified in 7 individuals. Brain MR images from these individuals were re-analyzed. A characteristic set of imaging anomalies in addition to periventricular nodular heterotopia was identified, including the elongation of the pituitary stalk, cerebellar enlargement with an abnormally shaped posterior fossa, rounding of the caudate nuclei, hippocampal malformations, and cortical anomalies including polymicrogyria or dysgyria.
CONCLUSIONS:The recurrent neuroradiologic changes identified here represent an opportunity to guide diagnostic formulation of Zhu-Tokita-Takenouchi-Kim syndrome on the basis of brain MR imaging evaluation.ABBREVIATIONS: PVNH ¼ periventricular nodular heterotopia; VUS ¼ variant of uncertain significance; WES ¼ whole-exome sequencing; WGS ¼ wholegenome sequencing; ZTTK syndrome ¼ Zhu-Tokita-Takenouchi-Kim syndrome U nderstanding of the distinctive phenotypic features of a rare syndromic disorder facilitates earlier diagnosis. This is particularly evident when genomic sequencing studies yield ambiguous findings, such as variants of uncertain significance (VUSs) in syndromic genes. 1 Moreover, characteristic phenotypic features, or "phenotypic handles," often guide focused investigations. Rare phenotypically heterogeneous disorders present a considerable challenge in this respect due to prior ascertainment biases and limited cohort sizes.Zhu-Tokita-Takenouchi-Kim (ZTTK) syndrome (OMIM #617140) is a rare phenotypically heterogeneous disorder characterized by developmental delay and a wide array of congenital multisystem anomalies. Haploinsufficiency of SON, mediated by either truncating variants or whole-gene deletions, has been identified as the underlying cause of ZTTK syndrome. 2-5 SON encodes the RNA-binding protein SON and is constitutively expressed, though expression is enriched during early brain