IN-VIVO ASSESSMENT OF MUTATIONS IN THE PHENYLALANINE-HYDROXYLASE GENEBY PHENYLALANINE LOADING - CHARACTERIZATION OF 7 COMMON MUTATIONS

Mutations in the gene encoding phenylalanine hydroxylase (PAH) cause p ersistent hyperphenylalaninaemia. To date, more than 200 point mutatio ns and microdeletions have been characterized. Each mutation has a par ticular quantitative effect on enzyme activity and recessive expressio n of different mutant alleles results in a marked interindividual hete rogeneity of metabolic and clinical phenotypes. In this paper we demon strate how a simple clinical test can be used to evaluate the correlat ion between mutation genotype and phenylalanine metabolism. In hyper p henylalaninaemic patients with known PAH mutation genotype, we have in vestigated phenylalanine turnover in vivo by measuring the ability to eliminate a test dose of L-phenylalanine. All patients could be consid ered functionally hemizygous for one of their mutant alleles by carryi ng on the other allele a mutation that is known to completely abolish PAH activity and encode a peptide with no immunoreactivity. Seven muta tions (R408W, IVS-12nt1, R261Q, G46S, Y414C, A104D, and D415N) were ch aracterized by oral phenylalanine loading, each mutation being represe nted by at least three patients. The elimination profile determined fo r a 3-day period provides a measure to compare residual activity of th e mutant proteins and to assign each mutation to a particular metaboli c phenotype. The established relation between genotype and phenotype m ay enable prediction of the severity of the disease by genotype determ ination in the newborn period. This will aid in the management of hype rphenylalaninaemia and may improve prognosis. Conclusion The possibili ty of predicting the residual enzyme activity by DNA analysis performe d already in the newborn period allows the prompt implementation of a diet that is adjusted to the degree of PAH deficiency. This may improv e management and prognosis of hyperpenylalaninaemia.