CTBP1 In Brain Development: A Novel Variant c.107G>C,p.(R36P) Leads to a Distinct Neurodevelopmental Disorder

CTBP1 (C-terminal-binding protein 1) is a multifunctional protein that acts as a transcriptional co-repressor in the nucleus and a regulator of membrane fission in the cytoplasm. Variants in CTBP1 have been associated with neurodevelopmental disorder termed HADDTS (Hypotonia, ataxia, developmental delay, and tooth enamel defect syndrome; OMIM#617915). However, the pathophysiological mechanism of this genetic disorder remains unclear. Whole exome sequencing was performed on a 20-year-old male patient with severe mental retardation, atrial septal defect, ataxia, and dysmorphic features. The patient was found to have a de novo missense variant, c.107G>C,p.(R36P), within the PLDLS (Pro-Leu-Asp-Leu-Ser) binding cleft of CTBP1. However, the patient did not fulfill the diagnostic criteria for HADDTS. Therefore, the pathophysiological significance of this variant was investigated in vitro and in vivo, comparing it with p.R342W, a recurrent pathogenic variant in HADDTS. Transient expression of the p.R36P and p.R342W variants reduced the number and total length of dendrites in primary cultured hippocampal neurons. In vivo acute expression of them caused a migration delay of excitatory neurons and disrupted both dendritic arborization and spine formation during corticogenesis. Subsequent electrophysiological analyses suggested that these variants reduced excitatory synaptic transmission. Additionally, the p.R36P variant, but not p.R342W, reduced the excitability of layer II/III pyramidal neurons. We also report two new cases with the p.R342W variant that meet the diagnostic criteria for HADDTS. Our results show that CTBP1 plays an essential role in brain development and that the novel variant may cause a new developmental disorder distinct from HADDTS.