PKU is a rare inherited metabolic disorder in which the body is unable to break down the amino acid phenylalanine due to a defect in the metabolic enzyme phenylalanine hydroxylase. Untreated, affected individuals rapidly accumulate toxic levels of phenylalanine that result in irreversible brain damage and severe learning disability.
Fortunately, countries such as the US and the UK with newborn screening programmes are able to detect the condition soon after birth; intervention in the form of a special low-protein diet prevents the neurological damage. However, there is a need for on-going clinical management because milder neurodevelopmental symptoms can still occur in some individuals if their condition is not properly controlled, and women also require particular care during pregnancy to control phenylalanine levels and prevent harm to the developing foetus.
The ACMG has reviewed the available evidence for PKU treatment and state that treatment must be lifelong, with the goal of maintaining blood phenylalanine levels within a specific range. They state that the main element of management is likely to remain dietary control ‘for the immediate future’ but note that one phenylalanine-lowering therapeutic has already been approved (Kuvan®, or sapropterin dihydrochloride) and is appropriate for use in certain forms of the disease and that more may become available in the future. Ultimately they suggest that personalised clinical management will be needed with both dietary and pharmaceutical interventions tailored to best meet the needs of each individual.
The guidance also recommends that the name of the condition is changed from PKU to phenylalanine hydroxylase deficiency (PAH deficiency) in order to reflect ‘the continuous spectrum of disease severity’.