NM_000132.4(F8):c.5291A>G (p.Gln1764Arg)

Base Editing (BE4max) via LNP delivery is a rationale-driven therapeutic strategy for Hemophilia A targeting the F8 c.5291A>G (p.Gln1764Arg) variant (Pathogenic, missense variant). The editing system (BE4max (cytosine base editor)) converts the pathogenic C to T (or G to A on the target strand), restoring the wild-type codon. Target tissue: Liver. Therapeutic goal: Correct loss-of-function F8 mutations in hepatocyte/endothelial lineages at Xq28 to restore >5-10% FVIII activity and convert severe hemophilia A to a mild or asymptomatic phenotype.. Risk profile: off-target Medium (bystander bases in editing window), delivery complexity Low, immunogenicity Low.

1. Molecular basis: F8 NM_000132.4(F8):c.5291A>G (p.Gln1764Arg) is classified as Pathogenic (ClinVar variation ID 1685789). Molecular consequence: missense variant. Protein change: Q1764R. 2. Epidemiology: Hemophilia A is an X-linked congenital bleeding disorder caused by deficiency or dysfunction of coagulation factor VIII and accounts for ~80% of hemophilia cases. Birth prevalence in males is ~24.6 per 100,000, and overall prevalence is ~1 in 5,000 live male births (WEB-01, WEB-11). Severe disease ( 3. Standard of care: Standard care is lifelong replacement or mimetic prophylaxis. Severe patients receive regular intravenous or subcutaneous prophylaxis with plasma-derived or recombinant factor VIII, extended half-life FVIII products, or non-factor agents such as the FVIII-mimetic bispecific antibody emicizumab and r 4. Pipeline: Multiple AAV-F8 gene replacement therapies have reached late-stage development. Giroctocogene fitelparvovec (AAV6-BDD F8) is in Phase 3 (Alta Phase 1/2, AFFINE Phase 3) with RMAT/Fast Track/Orphan designations (WEB-04). Additional AAV-F8 products have achieved regulatory approval in major markets (E 5. CBE clinical validation: BE4max (Koblan et al. 2018) is the gold-standard cytosine base editor. Multiple CBE programs are in clinical development for liver and hematologic targets.

1. No published data specifically correcting F8 c.5291A>G (p.Gln1764Arg) with Base Editing (BE4max); 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.

Base Editing (BE4max) via LNP delivery is a rationale-driven therapeutic strategy for Hemophilia A targeting the F8 c.5291A>G (p.Gln1764Arg) variant (Pathogenic, missense variant). The editing system (BE4max (cytosine base editor)) converts the pathogenic C to T (or G to A on the target strand), restoring the wild-type codon. Target tissue: Liver. Therapeutic goal: Correct loss-of-function F8 mutations in hepatocyte/endothelial lineages at Xq28 to restore >5-10% FVIII activity and convert severe hemophilia A to a mild or asymptomatic phenotype.. Risk profile: off-target Medium (bystander bases in editing window), delivery complexity Low, immunogenicity Low.

1. Molecular basis: F8 NM_000132.4(F8):c.5291A>G (p.Gln1764Arg) is classified as Pathogenic (ClinVar variation ID 1685789). Molecular consequence: missense variant. Protein change: Q1764R. 2. Epidemiology: Hemophilia A is an X-linked congenital bleeding disorder caused by deficiency or dysfunction of coagulation factor VIII and accounts for ~80% of hemophilia cases. Birth prevalence in males is ~24.6 per 100,000, and overall prevalence is ~1 in 5,000 live male births (WEB-01, WEB-11). Severe disease ( 3. Standard of care: Standard care is lifelong replacement or mimetic prophylaxis. Severe patients receive regular intravenous or subcutaneous prophylaxis with plasma-derived or recombinant factor VIII, extended half-life FVIII products, or non-factor agents such as the FVIII-mimetic bispecific antibody emicizumab and r 4. Pipeline: Multiple AAV-F8 gene replacement therapies have reached late-stage development. Giroctocogene fitelparvovec (AAV6-BDD F8) is in Phase 3 (Alta Phase 1/2, AFFINE Phase 3) with RMAT/Fast Track/Orphan designations (WEB-04). Additional AAV-F8 products have achieved regulatory approval in major markets (E 5. CBE clinical validation: BE4max (Koblan et al. 2018) is the gold-standard cytosine base editor. Multiple CBE programs are in clinical development for liver and hematologic targets.

1. No published data specifically correcting F8 c.5291A>G (p.Gln1764Arg) with Base Editing (BE4max); 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.

AAV5-mediated liver-directed F8 gene replacement (valoctocogene roxaparvovec / Roctavian, BioMarin) received conditional EMA approval in 2022, representing the first approved gene therapy for hemophilia A. The c.5291A>G (p.Gln1764Arg) missense variant in the A3 domain may produce a dysfunctional Factor VIII protein with impaired cofactor activity, making it amenable to gene replacement with a functional B-domain-deleted (BDD) F8 transgene.

Factor VIII is a large glycoprotein (2,332 amino acids) that functions as a cofactor for Factor IXa in the intrinsic coagulation pathway. The full-length F8 cDNA (~7 kb) exceeds AAV capacity, but the B domain (residues ~741-1648) is dispensable for procoagulant function, allowing a B-domain-deleted construct (~4.4 kb) to fit within AAV packaging limits. Valoctocogene roxaparvovec (Roctavian) uses AAV5 with a liver-specific promoter driving BDD-F8 expression. The Phase 3 GENEr8-1 trial (NCT03370913) demonstrated mean Factor VIII activity of 41.9 IU/dL at year 1, substantially above the 1 IU/dL severe hemophilia threshold, with significant reduction in annualized bleed rate and factor replacement use. However, Factor VIII levels declined over time — by year 3-4, mean levels dropped to approximately 5-15 IU/dL in many patients, raising durability concerns. The p.Gln1764Arg variant is located in the A3 domain (residues ~1649-2019), which mediates binding to Factor IXa and phospholipid surfaces. Substitution of glutamine with arginine at this position may alter local charge distribution and impair protein folding or cofactor interactions.

Durability of transgene expression is the primary concern: Factor VIII levels declined substantially after year 1 in most GENEr8-1 participants, and it remains unclear whether clinically meaningful expression will persist beyond 5-10 years. Pre-existing anti-AAV5 neutralizing antibodies exclude approximately 30-40% of patients. Re-dosing with the same AAV serotype is not currently feasible due to anti-capsid immunity. Inhibitor development (anti-Factor VIII antibodies) is a theoretical risk with any novel F8 protein expression, though this has not been a significant finding in trials to date. The high cost of Roctavian (initially priced >$2.5M per dose) raises access concerns. The specific c.5291A>G variant has not been individually characterized in gene therapy trial cohorts, and the residual activity of the p.Gln1764Arg mutant protein is not established, which could affect baseline severity classification.