NM_000463.3(UGT1A1):c.1124C>T (p.Ser375Phe)

AAV-mediated liver-directed gene therapy delivering a functional UGT1A1 transgene represents the most clinically advanced approach for Crigler-Najjar syndrome type I. Recent preclinical work has demonstrated that repeated AAV dosing in juvenile animal models can achieve durable bilirubin reduction, addressing the critical limitation of transgene dilution during hepatocyte proliferation in growing pediatric patients. For carriers of pathogenic variants like c.1124C>T (p.Ser375Phe), which abolishes UGT1A1 enzymatic function, exogenous gene delivery is a mechanistically sound strategy since even partial restoration of UGT1A1 activity (>5% of normal) can reduce bilirubin to sub-toxic levels.

Shi et al. (Mol Ther Methods Clin Dev 2024; PMID: 39618425) demonstrated successful repeated dosing of AAV vectors in juvenile rat and mouse models of Crigler-Najjar type I, showing that redosing can overcome the transgene dilution problem inherent in treating growing livers. This is a critical advance since Crigler-Najjar typically presents in neonates who undergo rapid hepatocyte proliferation. A comprehensive scoping review by Sambati et al. (Int J Mol Sci 2024; PMID: 39456788) cataloged all therapeutic options for Crigler-Najjar, positioning AAV gene therapy as the leading curative approach alongside emerging CRISPR strategies. Bortolussi et al. (Mol Ther Methods Clin Dev 2023; PMID: 38094199) showed that CRISPR-Cas9 somatic correction of Ugt1a mutations can ameliorate hyperbilirubinemia in mice, providing an alternative gene editing approach. Aronson et al. (2019; PMID: 31502485) characterized the prevalence of pre-existing anti-AAV antibodies relevant to patient eligibility, and developed quantitative potency assays for AAV-UGT1A1 vectors (2020; PMID: 32637454). The p.Ser375Phe variant is located in the UDP-glucuronosyltransferase domain of UGT1A1, and functional studies indicate it abolishes glucuronidation activity, consistent with the severe CN-I phenotype.

No completed human clinical trials for AAV-UGT1A1 gene therapy have been published to date, though GNT-0003 (Genethon) has been in clinical development. Pre-existing anti-AAV neutralizing antibodies exclude approximately 30-40% of potential patients depending on serotype and geographic population. Immune responses to the AAV capsid and transgene product remain concerns, particularly with repeated dosing. Liver-directed AAV therapies carry dose-dependent hepatotoxicity risk as seen in other programs (e.g., high-dose AAV in SMA). The durability of UGT1A1 expression in human liver after a single dose is unknown and may be insufficient in pediatric patients. Cost projections for AAV gene therapies suggest pricing above $1 million, raising access concerns for a rare disease affecting approximately 1 in 1,000,000 live births.

Base Editing (ABE8e) via LNP delivery is a rationale-driven therapeutic strategy for Crigler-Najjar syndrome type I targeting the UGT1A1 c.1124C>T (p.Ser375Phe) variant (Pathogenic, missense variant). The editing system (ABE8e-nSpCas9 (adenine base editor)) converts the pathogenic A back to G on the target strand, restoring the wild-type codon. Target tissue: Liver. Therapeutic goal: Correct loss-of-function UGT1A1 mutations at the hepatic locus to restore bilirubin conjugation and prevent kernicterus in Crigler-Najjar syndrome type I. Risk profile: off-target Medium (bystander bases in editing window), delivery complexity Low, immunogenicity Low.

1. Molecular basis: UGT1A1 NM_000463.3(UGT1A1):c.1124C>T (p.Ser375Phe) is classified as Pathogenic (ClinVar variation ID 12267). Molecular consequence: missense variant. Protein change: S376F, S107F, S372F, S374F, S375F. 2. Epidemiology: Exceptionally rare autosomal recessive disorder with incidence ~0.6-1 per 1,000,000 newborns worldwide; presents in neonates with severe unconjugated hyperbilirubinemia and high risk of kernicterus. 3. Standard of care: CN1 requires intensive, often daily, phototherapy and sometimes plasmapheresis to control bilirubin; liver transplantation is currently the only curative treatment and is recommended before irreversible neurological damage. 4. Pipeline: Multiple liver-directed UGT1A1 gene therapies are in early to pivotal clinical development. An AAV8-based UGT1A1 gene therapy (e.g., AlphaCN/GT-UGT1A1-AAV8-02, GNT-0003) is in Phase I/II with EMA PRIME designation and transitioning to pivotal trials; preclinical LNP mRNA therapies are also being mod 5. ABE clinical validation: ABE8e (Richter et al. 2020, Nat Biotechnol) achieves ~1.7x higher editing efficiency than ABE7.10. VERVE-101 demonstrated first-in-human LNP-ABE liver editing with 55-66% PCSK9 reduction (Raal et al. 2025, NEJM). Beam Therapeutics is advancing multiple ABE programs.

1. No published data specifically correcting UGT1A1 c.1124C>T (p.Ser375Phe) with Base Editing (ABE8e); strategy is based on general principles and must be validated preclinically. 2. PAM availability and bystander base analysis for the specific genomic context have not been performed. If no canonical NGG PAM positions the target within the editing window, PAM-flexible variants (SpRY) may be needed. 3. Long-term durability, off-target genome-wide effects, and immunogenicity in the target patient population require thorough preclinical and clinical evaluation.