Phenylketonuria (PKU), one of the commonest inherited metabolic disorders, is caused by deficiency of the enzyme phenylalanine hydroxylase (PAH). To date, over 460 different mutations in the PAH gene have been described. PAH deficiency results in failure of hydroxylation of phenylalanine to tyrosine. Raised body levels of phenylalanine interfere with normal brain development in infancy and childhood. For over 30 years, there has been a newborn screening programme for PKU. Early detection combined with an effective dietary treatment results in normal development for affected individuals. Although relaxation of diet after childhood has little or no detrimental effect upon intelligence, the current recommendation in the UK is for such treatment to be continued into adulthood. Women with PKU must be under strict dietary control at the time of conception and during pregnancy in order to prevent damage to the fetus. Although new therapies are being investigated, they are unlikely to replace the need for dietary treatment in the foreseeable future.
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