Translational reading frame predicts the pathogenicity of C-terminal frameshift deletions in MeCP2.

Mutations in the MECP2 gene cause the severe neurological disorder Rett syndrome. A cluster of frameshift-causing C-terminal deletions (CTDs) removes ~100 amino acids and accounts for approximately 10% of RTT-causing mutations. Their pathogenicity is unexpected because this C-terminal domain is dispensable in mice. Analysis of pathogenic and benign human MECP2 variants reveals that some individuals with apparently typical CTDs do not develop Rett syndrome, confirming that C-terminal truncations are not intrinsically pathogenic. Using human sequence data and mouse models we show that pathogenicity results from a marked reduction in MeCP2 levels and depends on the presence of a proline proline stop motif (-PPX) generated by a shift to the +2 reading frame. CTDs that shift to the +1 frame avoid this motif and are benign. Replacing the stop codon of the PPX motif with tryptophan restores MeCP2 expression and rescues RTT-like phenotypes in a CTD mouse model. An adenine base editor efficiently introduces this substitution in cultured cells. These findings define a reliable prognostic distinction between benign and pathogenic CTDs and establish a potential editing strategy for correcting disease-causing CTD mutations.