A phenylalanine hydroxylase amino acid polymorphism with implications for molecular diagnostics

Mutations in the gene encoding phenylalanine hydroxylase (PAH, EC 1.14.16.1 ) are associated with various degrees of hyperphenylalaninemia, including c lassical phenylketonuria (PKU). We examined the PAM gene in a Brazilian PKU family of African origin and identified three missense variants, R252W (c. 754C --> T), K274E (c.820A --> G), and I318T (c.953T --> C), the two latter of which were transmitted in cis. Expression analyses in two different in vitro systems showed that I318T is associated with profoundly decreased enz yme activity, whereas the enzyme activity of K274E is indistinguishable fro m that of the wild-type protein. Detailed kinetic analyses of PAH expressed in E. coli showed that the K274E mutant protein has kinetic properties sim ilar to that of the wild-type protein. Population studies have suggested th at the K274E variant occurs on approximately 4% of African-American PAH all eles, whereas the neonatal screening incidence of PKU among African America ns is only 1:100,000. This is to our knowledge the first demonstration of a PAH missense variant with no apparent association to PAH deficiency. Aware ness of this common variant may be helpful to laboratories that perform mol ecular diagnosis of PAH deficiency in populations of African origin. (C) 20 01 Academic Press.