Metabolic basis of sexual dimorphism in PKU mice after genome-targeted PAH gene therapy.

We have previously reported a transgene delivery system based on phiBT1 bacteriophage integrase that results in targeted insertion of transgenes into mammalian genomes, and its use in the delivery of murine phenylalanine hydroxylase (PAH) complementary DNA (cDNA) into the hepatocytes of male phenylketonuria (PKU) mice, leading to a complete and permanent correction of their hyperphenylalaninemic phenotype. In this study, we report only partial phenotypic correction in female PKU mice, even though hepatic PAH activities in both sexes after gene treatment were similar. Daily injections of tetrahydrobiopterin (BH4), an essential co-factor for phenylalanine hydroxylation, in the gene-treated females led to complete correction of their PKU phenotype. After gonadectomy, serum phenylalanine levels in the gene-treated females were reduced to normal, whereas those in the gene-treated males remained unchanged. The sterile gene-treated PKU mice were subjected to daily sex hormone injections. Whereas the estradiol-treated sterile males developed hyperphenylalaninemia, the dihydrotestosterone-treated sterile females remained normal phenylalaninemic. The results indicate that it is estrogen that suppresses the steady-state levels of BH4 in mouse hepatocytes that became limiting, which is the underlying mechanism for the observed sexual dimorphism in PKU mice after PAH gene treatment. Livers of the PAH gene-corrected PKU mice also appeared normal and without apparent pathologies.