Phenylketonuria (PKU) and related forms of hyperphenylalaninemia (HPA) result from deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH). Although PAH deficiency is the major determinant of the cognitive and metabolic phenotype, with epistasis and background, the observed phenotype in vivo may not correlate absolutely with the predicted phenotype by genotypic and in vitro expression analysis. Genotypic classification is also complicated by extensive allelic heterogeneity.

We present a novel, rapid (80 minute) non-invasive method to estimate whole body phenylalanine oxidation. Cumulative labelled metabolite recovered at isotopic plateau (60 minutes) was measured in individuals with classical PKU(diet phe tolerance < 500 mg/d), variant PKU (phe. tolerance higher than in classical PKU), and non-PKU hyperphenylalaninemia, and in obligate heterozygotes and controls. In vivo data were correlated with the genotype(PAH mutations analyzed by PCR, DGGE and DNA sequencing) and other metabolic data including a density function noon plasma phenylalanine level. The parameter measured at 60 minutes post dose at this stage of an ongoing study differed in subjects with PAH deficiency (n=8) (mean: 0.44 ± 0.56) from heterozygotes (mean: 4.56 ± 2.2), and controls (mean: 7.95± 1.43) (p < 0.001). This method distinguishes the differing types of hyperphenylalaninemia, can define the heterozygote and allows for analysis of in vivo genotype-phenotype correlations. [This work was funded in part by MRC (Canada) Canadian Genetic Diseases Network (NCE program) and Martek Biosciences Corp].