AAV-mediated F8 gene replacement for hemophilia A: valoctocogene roxaparvovec and implications for F8 c.5291A>G (p.Gln1764Arg)

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.