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Clinical features: Classic PKU is caused by a complete or near-complete deficiency of phenylalanine hydroxylase (PAH) enzyme activity which leads to hyperphenylalaninaemia (HPA). This enzyme deficiency is diagnosed by newborn screening. Dietary restriction of phenylalanine is the treatment and should be started immediately. Without dietary restriction, most children with PKU develop profound and irreversible mental retardation, but this is now rarely seen. PAH deficiency can also cause non-PKU HPA, which is associated with a much lower risk of impaired cognitive development in the absence of treatment, and variant PKU, which is intermediate between PKU and non-PKU HPA.

Incidence: The prevalence of PAH deficiency is approximately 1 in 10,000 in Caucasian populations, with a carrier frequency of 1 in 50. Since the introduction of universal newborn screening, symptomatic classic PKU is rarely seen.

Inheritance : Autosomal recessive.

Gene: Phenylalanine hydroxylase gene (PAH) on chromosome 12. Several hundred mutations have been identified, 5 of which account for approximately 60% of European mutations.

Prenatal diagnosis: Usually by DNA testing. If a family is considering prenatal diagnosis, referral should be made to the local clinical genetics service prior to a pregnancy. This ensures family members are offered appropriate advice and investigations, to confirm whether or not prenatal diagnosis is possible.

Patient support groups

NSPKU (National Society For Phenylketonuria)
P O Box 26642
N14 4TD
Tel: 08456 039136

National Information & Advice Centre for Metabolic Disease
Climb Building
176 Nantwich Road
Tel: (Freephone):0800 652 3181
Tel: 0870 7700 326
Fax: 0870 7700 327

If you would like to access specialist information on phenylketonuria please go to:-
Genetests - Phenylketonuria