Molecular genetics and impact of residual in vitro phenylalanine hydroxylase activity on tetrahydrobiopterin responsiveness in Turkish PKU population

Abstract Background The prevalence of phenylalanine hydroxylase (PAH)-deficient phenylketonuria (PKU) in Turkey is high (1 in 6500 births), but data concerning the genotype distribution and impact of the genotype on tetrahydrobiopterin (BH 4 ) therapy are scarce. Objective To characterize the phenotypic and genotypic variability in the Turkish PKU population and to correlate it with physiological response to BH 4 challenge. Methods We genotyped 588 hyperphenylalaninemic patients and performed a BH 4 loading test (20mg/kg bw) in 462 patients. Residual PAH activity of mutant proteins was calculated from available in vitro expression data. Data were tabulated in the BIOPKU database (www.biopku.org). Results Eighty-eight mutations were observed, the most common missense mutations being the splice variant c.1066-11G>A (24.6%). Twenty novel mutations were detected (11 missense, 4 splice-site, and 5 deletion/insertions). Two mutations were observed in 540/588 patients (91.8%) but in 9 patients atypical genotypes with >2 mutations were found (8 with p.R155H in cis with another variant) and in 19 patients mutations were found in BH 4 -metabolizing genes. The most common genotype was c.1066-11G>A/c.1066-11G>A (15.5%). Approximately 22% of patients responded to BH 4 challenge. A substantial in vitro residual activity (average >25% of the wild-type enzyme) was associated with response to BH 4 . In homozygous genotypes ( n =206), both severity of the phenotype ( r =0.83) and residual PAH activity ( r =0.85) correlate with BH 4 responsiveness. Conclusion Together with the BH 4 challenge, these data enable the genotype-based classification of BH 4 responsiveness and document importance of residual PAH activity. This first report of a large-scale genotype assessment in a population of Turkish PKU patients also documents a high prevalence (47%) of the severe classic phenotype.