NM_000463.3(UGT1A1):c.1006C>T (p.Arg336Trp)

Adenine base editing (ABE) via LNP delivery to hepatocytes is a highly plausible one-time curative strategy for Crigler-Najjar syndrome type I caused by the UGT1A1 c.1006C>T (p.Arg336Trp) variant. The coding-strand C>T transition corresponds to a G>A change on the opposite strand, making it a canonical ABE8e target (A-to-G correction). The liver is an ideal target organ given UGT1A1 hepatocyte-restricted expression and proven LNP hepatotropism in clinical programs (VERVE-101/PCSK9, NTLA-2001/TTR). Critically, Crigler-Najjar I has a low therapeutic threshold: restoring only 5-10% of normal UGT1A1 activity is sufficient to prevent kernicterus and potentially eliminate the need for lifelong 10-16 hour daily phototherapy. Beam Therapeutics BEAM-301 program, targeting UGT1A1 point mutations with LNP-ABE, has received FDA Rare Pediatric Disease Designation, providing direct clinical validation of this approach.

1. Molecular basis: UGT1A1 c.1006C>T (p.Arg336Trp, ClinVar VCV000437450) is classified Pathogenic by multiple submitters. The R336W substitution replaces a charged arginine with a hydrophobic tryptophan in the UDP-glucuronosyltransferase catalytic domain, reducing bilirubin conjugation activity to approximately 0.4% of wild-type (Sneitz et al. 2010, PMID:19830808; Ciotti et al. 1998, PMID:9639672). Genomic location: chr2:233,767,858 (GRCh38), NM_000463.3. 2. ABE applicability: The C>T transition on the coding strand (NM_000463.3:c.1006C>T) corresponds to a G>A on the antisense strand. ABE8e (TadA-8e deaminase, Richter et al. 2020, Nat Biotechnol, PMID:32433547) can convert this A back to G, restoring the wild-type arginine codon. ABE8e achieves approximately 1.7x higher editing efficiency than ABE7.10 with an editing window at protospacer positions 4-8. 3. LNP liver delivery: Clinical proof-of-concept for LNP-delivered base editing in human hepatocytes exists: VERVE-101 achieved 55-66% PCSK9 protein reduction via single-dose LNP-ABE infusion (Raal et al. 2025, NEJM). NTLA-2001 demonstrated durable >90% TTR knockdown over 2+ years via LNP-Cas9 (Gillmore et al. 2021, NEJM, PMID:34215024). 4. Preclinical validation: Villiger et al. (2021, Nat Biomed Eng, PMID:33398131) demonstrated in vivo cytidine base editing in the Gunn rat CN1 model, correcting UGT1A1 and durably reducing bilirubin without detectable off-target mutations. Beam Therapeutics BEAM-301 preclinical data (ASGCT 2023-2024) showed bilirubin normalization and UGT1A1 activity restoration in mouse models using LNP-ABE. 5. Therapeutic threshold: CN2 patients with 5-10% residual UGT1A1 activity generally avoid severe neurological sequelae, establishing a low correction threshold that is achievable with current LNP-ABE technology. 6. Clinical precedent: Genethon GNT0003 AAV8-UGT1A1 Phase I/II trial (NCT03466463) demonstrated proof-of-concept for genetic correction in CN1, with bilirubin reductions and phototherapy discontinuation in some patients.

1. Bystander editing: ABE8e has an expanded editing window (positions 3-9), and bystander adenine residues near the target may undergo unintended A-to-G conversion. The local sequence context around c.1006 (GGTCCTGTGG[C/T]GGTACACTGG) must be analyzed for bystander bases when a specific sgRNA is designed; high-precision ABE variants (e.g., ABE8e-V106W) may be needed if bystanders are clinically significant. 2. PAM availability: Whether a canonical NGG PAM (SpCas9) positions the target adenine within the ABE editing window (positions 4-8) has not been verified in this analysis. If no suitable PAM exists, PAM-flexible Cas variants (SpCas9-NG, SpRY) or alternative Cas scaffolds may be required, potentially reducing editing efficiency. 3. No peer-reviewed ABE data for this specific variant: BEAM-301 data remain at conference-abstract stage; no published study has specifically corrected UGT1A1 c.1006C>T with ABE. The Villiger 2021 study used CBE (not ABE) in the Gunn rat, which carries a different mutation (frameshift, not point mutation). 4. Delivery limitations: LNP hepatocyte editing efficiency in humans has been modest at tolerated doses (~50-60% protein reduction for PCSK9). Achieving sufficient hepatocyte correction to cross the 5-10% UGT1A1 activity threshold requires further dose-optimization. Transient ALT elevations were observed in VERVE-101, indicating hepatotoxicity risk. 5. Durability uncertainty: While base editing creates permanent genomic changes, edited hepatocytes must persist long-term. Hepatocyte turnover in pediatric patients (the primary CN1 population) may dilute therapeutic benefit over years. 6. This variant (R336W) retains 0.4% residual activity; compound heterozygous genotype must be considered — the second allele may also need correction for full therapeutic benefit.