NM_000277.3(PAH):c.1259G>T (p.Arg420Met)

Two distinct nucleic acid therapy strategies are being developed for PKU: (1) LNP-encapsulated mRNA encoding PAH (ARCT-810, Arcturus Therapeutics) for repeated hepatic PAH restoration, and (2) AAV5-mediated PAH gene replacement (BMN 307, BioMarin) for durable one-time correction. The c.1259G>T (p.Arg420Met) missense variant in the catalytic domain likely disrupts tetrahydrobiopterin (BH4) binding or catalytic geometry, resulting in reduced but potentially not absent enzymatic activity. Both strategies could benefit this variant, though the residual activity profile may also make it a candidate for BH4-responsive therapy (sapropterin).

PAH (phenylalanine hydroxylase) is a hepatic enzyme that converts L-phenylalanine to L-tyrosine using BH4 as cofactor. The p.Arg420Met substitution occurs in the catalytic domain (residues ~118-427), where Arg420 participates in positioning the active-site iron and BH4 cofactor. Arcturus Therapeutics developed ARCT-810 using their LUNAR lipid nanoparticle platform to deliver codon-optimized PAH mRNA to hepatocytes. In the Pah^enu2 mouse model, a single IV dose of LNP-PAH mRNA reduced blood phenylalanine levels substantially within 24 hours. A Phase 1 clinical trial (NCT04442347) was initiated. BioMarin BMN 307 uses AAV5 with a liver-specific promoter to drive durable PAH expression; however, preclinical studies raised safety concerns when AAV vector integration was associated with hepatocellular carcinoma in mouse long-term studies, resulting in an FDA clinical hold. Separately, enzyme substitution (pegvaliase/Palynziq, FDA-approved 2018) provides a non-genetic therapeutic comparator.

For mRNA therapy (ARCT-810): requires repeated IV dosing (likely every 2-4 weeks), potential for anti-LNP immune responses limiting re-dosing efficacy, and transient rather than durable correction. For AAV gene therapy (BMN 307): the FDA clinical hold due to insertional mutagenesis concerns is a significant safety signal, and the long-term oncogenic risk of liver-directed AAV in humans remains under investigation. The p.Arg420Met variant may retain partial catalytic activity — genotype-phenotype correlation in PKU is complex, and some missense variants respond to BH4 supplementation (sapropterin), potentially making gene/RNA therapy unnecessary for this specific variant if BH4-responsiveness is confirmed. Neither ARCT-810 nor BMN 307 has reported variant-stratified efficacy data.