Novel De Novo RALA Missense Variants Expand the Genotype Spectrum of Hiatt-Neu-Cooper Neurodevelopmental Syndrome

Abstract

Background: RALA is a small GTPase from the RAS superfamily implicated in signal transduction and cytoskeletal dynamics. Recently, de novo variants in RALA have been associated with a neurodevelopmental syndrome characterized by intellectual disability (ID), developmental delay (DD), and seizures. So far, only < 12 patients have been reported.

Methods: In this study, we report two novel patients with neurodevelopmental impairment and epilepsy carrying previously unreported RALA variants. We performed a thorough clinical investigation of these patients and performed brain MRI to detect potential abnormalities. Trio-exome sequencing and/or NGS panel testing were conducted to identify the genetic variants. Then, we reviewed previous cases reported in the literature.

Results: Affected individuals showed a complex neurodevelopmental phenotype consistent with Hiatt-Neu-Cooper neurodevelopmental syndrome. Brain MRI in both subjects showed abnormalities including megalencephaly and ventricular enlargement, previously unreported in RALA patients. Genetic testing revealed two novel de novo missense variants in RALA: c.217G>A, p.(Glu73Lys) in case #1 and c.73G>C, p.(Val25Leu) in case #2. Both variants affect highly conserved residues within the GTP/GDP-binding site of the protein. These changes are predicted to be deleterious by in silico tools, interfering with the GTPase activity of RALA.

Conclusion: Our findings expand the genotype and phenotype spectrum of Hiatt-Neu-Cooper neurodevelopmental syndrome. Our observations also support the important role of variants affecting the GTP/GDP-binding site of the RALA protein in the pathogenesis of Hiatt-Neu-Cooper neurodevelopmental syndrome.

Keywords: GTPase; RALA; brain abnormalities; epilepsy; exome sequencing; intellectual disability; neurodevelopmental syndrome.

FIGURE 1

Clinical and genetic findings. (A) Frontal and lateral view of the facial features of patient 1, showing broad forehead, triangular face, down‐slanting palpebral fissures, exotropia, mandibular hypoplasia with pointed chin, and low‐set ears. (B) Patient 2 showing dysmorphic features: Mild epicanthus, short nose with flattened nasal bridge, slightly anteverted nares, and mandibular hypoplasia with pointed chin. (C) EEG showing disorganized posterior rhythm with superimposed slow anterior activity and high‐voltage bilateral spikes, polispikes, and sharp waves, predominantly over the temporal and central regions, enhanced during sleep. (D) Linear structure of the RALA protein, showing the GTP/GDP‐binding regions (GBR), effector region (ER), effector domain (ED), and post‐translational modification regions (PTM). The GBR is essential for the protein's molecular switch activity, while the ER and ED mediate interactions with downstream effectors. PTMs regulate protein activity, localization, and interactions. Variants identified in our patients and in prior studies are highlighted to illustrate their positions within these functionally critical regions.