Hepatocyte-directed AAV gene therapy for PAH deficiency: variant-agnostic restoration of phenylalanine hydroxylase

AAV-mediated PAH gene replacement in hepatocytes is the leading curative strategy for classical PKU, including variants like p.His170Gln that severely impair enzyme activity. His170 is located in the catalytic domain and coordinates with the iron cofactor essential for phenylalanine hydroxylation; its substitution to glutamine abolishes enzymatic function. Because PKU pathology stems entirely from hepatic PAH deficiency, liver-targeted gene therapy can in principle normalize systemic phenylalanine levels regardless of the specific underlying mutation. Multiple AAV-PAH programs have advanced to clinical trials, including BMN 307 (BioMarin, AAV5-PAH) which demonstrated dose-dependent Phe reductions in Phase 1/2.

BioMarin reported Phase 1/2 results for BMN 307 (AAV5 carrying human PAH cDNA under a liver-specific promoter) showing dose-dependent blood phenylalanine reductions in adult PKU patients, though the magnitude and durability of Phe lowering were variable across dose cohorts. Preclinical studies in Pah-enu2 mice (the standard PKU model carrying a missense mutation at the equivalent catalytic domain position) have consistently shown that AAV-mediated hepatic PAH expression normalizes blood Phe and reverses hypopigmentation. The PAH gene (2.5 kb coding sequence) fits comfortably within AAV packaging limits. Structural studies confirm His170 participates in the active-site iron coordination sphere (PDB: 1PAH), and H170Q disrupts the catalytic geometry required for tetrahydrobiopterin-dependent hydroxylation.

AAV-mediated liver gene therapy faces several unresolved challenges for PKU. Pre-existing anti-AAV5 neutralizing antibodies exclude a substantial fraction of adult patients. Transgene expression from episomal AAV genomes can decline over years as hepatocytes turn over, potentially requiring re-dosing — but anti-capsid immunity precludes re-administration with the same serotype. BioMarin paused BMN 307 development in 2023 citing the evolving competitive landscape rather than safety failure, creating uncertainty about the commercial viability of this approach. Immunosuppression regimens needed to manage capsid-directed T-cell responses add complexity, especially for a non-life-threatening condition where the risk-benefit calculus differs from lethal diseases. mRNA-based approaches (e.g., lipid nanoparticle-delivered PAH mRNA) are emerging as alternatives that allow repeat dosing but require chronic administration.