Genotype-phenotype characterization and functional reconstitution of pathogenic β-catenin variants from CTNNB1 syndrome patients.

Germline variants in the CTNNB1 gene, encoding β-catenin protein, cause severe neurodevelopmental alterations manifested early in the infancy, and define the CTNNB1 syndrome. Patients with CTNNB1 syndrome display heterogeneous clinical manifestations, and most of them carry CTNNB1 pathogenic nonsense or frameshift variants that generate premature termination codons (PTC). We have previously described the neuropsychological manifestations of a group of CTNNB1 syndrome patients harboring novel β-catenin variants. Here, we have analysed the molecular and functional characterization of these β-catenin variants, performed genotype-phenotype analyses, and tested for β-catenin functional reconstitution. We describe a complex variety of N-terminal and C-terminal truncated β-catenin proteoforms generated by PTC. Protein stability of truncated proteoforms was variable, as indicated by their expression levels and biophysical analysis, and high protein stability correlated with better patient performance in visuospatial tests. Transcriptional activity was abrogated in most of the β-catenin variants, although some specific truncations, as well as a three-residues in-frame deletion variant, retained partial transcriptional activity. Reconstitution of full-length β-catenin expression and function was achieved in specific β-catenin PTC variants by induction of translational readthrough with aminoglycosides and protein synthesis stimulators. Inhibition of β-catenin degradation by MG-132 proteasome inhibitor also resulted in partial rescue of β-catenin transcriptional activity. Our results suggest the existence of intricate patterns of truncated β-catenin proteoforms in CTNNB1 syndrome patients, which may correlate with clinical manifestations, and provide insights to increase the function of β-catenin in patients carrying CTNNB1 pathogenic variants.