A phenylalanine hydroxylase amino acid polymorphism with implications for molecular diagnostics.

[classical phenylketonuria]

Mutations in the gene encoding phenylalanine hydroxylase (PAH , EC 1 .14 .16 .1 ) are associated with various degrees of hyperphenylalaninemia , including classical phenylketonuria (PKU ). We examined the PAH gene in a Brazilian PKU family of African origin and identified three missense variants , R 252 W (c .754 C --> T ), K 274 E (c .820 A --> G ), and I 318 T (c .953 T --> C ), the two latter of which were transmitted in cis . Expression analyses in two different in vitro systems showed that I 318 T is associated with profoundly decreased enzyme activity , whereas the enzyme activity of K 274 E is indistinguishable from that of the wild-type protein . Detailed kinetic analyses of PAH expressed in E . coli showed that the K 274 E mutant protein has kinetic properties similar to that of the wild-type protein . Population studies have suggested that the K 274 E variant occurs on approximately 4 % of African-American PAH alleles , whereas the neonatal screening incidence of PKU among African Americans 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 . Awareness of this common variant may be helpful to laboratories that perform molecular diagnosis of PAH deficiency in populations of African origin .