Liver-directed UGT1A1 gene replacement rationale for UGT1A1 c.222C>A (p.Tyr74Ter)

For UGT1A1 c.222C>A (p.Tyr74Ter), liver-directed gene replacement is a mechanistically strong therapeutic direction because this early nonsense variant is expected to behave as a loss-of-function allele, and current AAV-UGT1A1 programs are designed to restore enzymatic activity without needing codon-specific rescue.

ClinVar classifies UGT1A1 c.222C>A (p.Tyr74Ter) as pathogenic. Preclinical work with translationally optimized AAV-UGT1A1 vectors showed durable bilirubin correction in Crigler-Najjar models (PMID:27722180), and additional mouse data supported phenotypic rescue with liver-directed AAV8 delivery (PMID:29448836). In patients with Crigler-Najjar syndrome, the phase 1/2 GNT0003 study reported clinically meaningful bilirubin reduction and major phototherapy reduction after systemic AAV-UGT1A1 delivery (PMID:37585628; NCT:NCT03466463). Because p.Tyr74Ter is an upstream truncating variant, supplementation with a functional UGT1A1 coding sequence is more directly supported than speculative variant-specific editing.

The human evidence is disease-level rather than specific to p.Tyr74Ter, so this should be read as a mechanism-based interpretation, not proof of allele-specific response. Benefit may still depend on liver status, vector dose, immune responses, and the practical risk-benefit balance of systemic AAV exposure. Re-dosing remains a general challenge for AAV platforms, and longer-term durability in Crigler-Najjar still needs continued follow-up.