In phenylketonuria due to phenylalanine hydroxylase (PAH) deficiency, genetic heterogeneity and the lack of functional data for PAH genotypes hamper reliable phenotype prediction and individualised treatment. This database aims to provide a comprehensive overview on genotype-specific PAH enzyme function and the frequency and distribution of PAH genotypes in Europe and the Middle East. A literature search and inclusion of data from metabolic centers revealed 690 different PAH genotypes in 3,066 phenylketonuria patients. However, the most frequent 30 genotypes cover more than 55% of the study population. The regional distribution of most common PAH genotypes is depicted in a map and we provide more detailed country-specific information on PAH genotypes. A precise knowledge of genotype-specific PAH function is essential for a comprehensive molecular understanding of the loss-of-function phenotype. But it may also serve as a helpful resource for physicians in care of PKU patients. Residual PAH function is associated with a patient’s biochemical phenotype (blood phenylalanine level) and thus relates to the course and severity of the disease. Moreover, residual PAH function correlates with the rate of response to a pharmacological treatment by tetrahydrobiopterin (BH4, Kuvan®). However, no functional testing in PKU patients is routinely available. Therefore, we established a cell-based model in order to investigate and understand PAH function in different genetic and metabolic contexts. We determined PAH enzyme function, generated activity landscapes, and assessed the optimal working range in the metabolic (phenylalanine) and therapeutic (tetrahydrobiopterin) space. Shared patterns in genotype-specific functional activity landscapes were linked to biochemical and pharmacological phenotypes, where residual activity below 3.5% was associated with classical phenylketonuria unresponsive to pharmacological treatment; lack of defined peak activity induced loss of response to tetrahydrobiopterin; a higher cofactor need was linked to inconsistent clinical phenotypes and low rates of tetrahydrobiopterin response; and residual activity above 5%, a defined peak of activity, and a normal cofactor need were associated with pharmacologically treatable mild phenotypes.
Map and Genotypes
A map displays all countries included in the study (highlighted in blue). Information on the five most frequent genotypes (top 5 in %) in each country can be accessed by mouse-over. The map links to more detailed country-specific information on genotypes. (A work-up for tablet computer will follow.)
Residual PAH function was assessed in a cell-based model of in vivo PAH function in the context of the metabolic state and potential therapeutic intervention. Activity Landscapes are color coded maps of PAH enzyme activity at varying phenylalanine and BH4 concentrations. The PAH enzyme activity of full genotypes was tested by overexpression of wild-type PAH or PAH derived from one mutant allele in case of homozyogous genotypes or by co-overexpression of PAH derived from two mutant alleles in case of compound heterozyogous genotypes. A table summarizes enzyme kinetic parameters in order to provide information on the overall residual activity (in percent of the wild-type), the position of maxium activity with respect to the phenylalanine and the BH4 concentration, as well as the range of maximum activity with respect to the phenylalanine and the BH4 concentration. In addition, bar diagrams depict the association of the respective genotype with clinical phenotypes according to blood phenylalanine levels and to response to BH4 treatment.