Mild phenylketonuria

Other Names: Mild PKU; mPKU; Variant phenylketonuria; Variant PKU Mild phenylketonuria is a rare form of phenylketouria (PKU variant), an inborn error of amino acid metabolism, characterized by symptoms of PKU of mild to moderate severity. Patients with blood phenylalanine concentrations of 600-1,200 micromol/L are considered to have mild PKU.

Blood is taken from a two-week-old baby to test for phenylketonuria

Cause

It is caused by certain mutations in the PAH gene which result in slightly higher activity of the phenylalanine hydroxylase compared with the classic phenylketonuria where there is a complete or near-complete deficiency of phenylalanine hydroxylase activity. The PAH gene provides instructions for making an enzyme called phenylalanine hydroxylase. This enzyme converts the amino acid phenylalanine to other important compounds in the body. If gene mutations reduce the activity of phenylalanine hydroxylase, phenylalanine from the diet is not processed effectively. As a result, this amino acid can build up to toxic levels in the blood and other tissues. Because nerve cells in the brain are particularly sensitive to phenylalanine levels, excessive amounts of this substance can cause brain damage.

Inheritance

Autosomal recessive inheritance, a 25% chance

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Signs

Clinical signs include reduced cognitive function and behavioral and developmental disorders.

Diagnosis

PKU is commonly included in the newborn screening panel of many countries, with varied detection techniques. Most babies in developed countries are screened for PKU soon after birth. Screening for PKU is done with bacterial inhibition assay (Guthrie test), immunoassays using fluorometric or photometric detection, or amino acid measurement using tandem mass spectrometry (MS/MS). Measurements done using MS/MS determine the concentration of Phe and the ratio of Phe to tyrosine, the ratio will be elevated in PKU.

Treatment

PKU is not curable. However, if PKU is diagnosed early enough, an affected newborn can grow up with normal brain development by managing and controlling phenylalanine ("Phe") levels through diet, or a combination of diet and medication. The medication(s) listed below have been approved by the Food and Drug Administration (FDA) as orphan products for treatment of this condition.

• Sapropterin (Brand name: Kuvan) approved to reduce blood phenylalanine (Phe) levels in patients with hyperphenylalaninemia (HPA) due to tetrahydrobiopterin- (BH4-) responsive Phenylketonuria (PKU). Kuvan is to be used in conjunction with a Phe-restricted diet.
• Pegvaliase (Brand name: Palynziq) approved to reduce blood phenylalanine concentrations in adult patients with phenylketonuria (PKU) who have uncontrolled blood phenylalanine concentrations greater than 600 micromol/L on existing management.

NIH genetic and rare disease info

• NIH info on Mild phenylketonuria

Mild phenylketonuria is a rare disease.