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Phenylketonuria (PKU)

By: Ben Bernarding & Mary Grace Antonich

What is the genotype of a person with this disorder?

How can this disorder be treated?

People with PKU need to follow a diet that limits foods with phenylalanine. People with PKU need to avoid various high-protein foods, including: Milk and cheese, Eggs, Nuts, Soybeans, Beans, all meats, Peas, and Beer. This diet would slow down or stop a person’s mental decline. They should also drink a phenylalanine-free formula to still get the protein they’d need.

The PAH gene and its PKU mutations are found on chromosome 12 in the human genome, being autosomal recessive (pp).

What is the prognosis for a person with this disorder?

What is happening with the DNA and proteins produced to cause these phenotypic effects?

It is when there’s a defect in phenylalanine hydroxylase. It will fail to break down the amino acid phenylalanine to turn it into tyrosine.Therefore, there is too much phenylalanine in the bloodstream and brain. This disrupts brain development by coating nerve cells with a protective myelin sheath, leaving those nerve cells to become ineffective, blocking the brain from fully developing.

It really all depends on how quickly the disorder is found. If the disorder isn’t found quick enough, a young infant could already be afflicted with severe mental retardation that’ll stick with them for the rest of their lives. However if given attention to right away, severe mental retardation can be prevented if the person sticks to a restrictive diet that limits their intake of phenylalanine during their brain development stage of life. After the brain is fully developed, people can stick to the diet and be successful in life. If they stray from the diet, it won’t necessarily cause mental retardation but will cause the brain to slow down and limit intellectual capacity. Those who didn’t stick to the diet will also suffer many mental and physical side effects. Adult women with the disorder can still have children but the likelihood that the child will obtain the disorder as well is severely high and can be lowered only if the mother maintains the restrictive diet before pregnancy up until she gives birth. The disorder will not kill the people who have it but will make them struggle throughout the rest of their lives. They won’t be able to freely indulge on different foods or be independent enough to lead their own lives.

How is this disorder inherited?

Both parents have to pass on an autosomal recessive gene affecting the enzyme phenylalanine hydroxylase.

What are the phenotypic effects of this disorder?

Are there carrier forms and/or lethal forms of this disorder?

A person can be a heterozygous carrier for this disorder. They won’t feel the effects of the disorder but they could pass it on to their children. There are no lethal forms.

Growth failure, poor skin pigmentation, eczema, microcephaly, seizures, global developmental delay and severe intellectual impairment, asthma, behavioral problems, hyperactivity, and lethargy.

How many people are affected by this disorder? Does it affect any gender/population/ethnicity/race more than others?

What types of tests are used to diagnose the disorder?

The incidence of PKU in Caucasian populations is between 1 in 10,000 and 1 in 15,000 people. The table shows the variability in incidence in various countries and regions. It has been suggested that the high incidence of PKU in Turkey is due to the high prevalence of consanguinity and the low incidence seen in Finland and Japan is due to a pronounced negative founder effect in Finland and genetic drift in the founding of the Japanese island population.

Region / Country Incidence of PKU

Asian Populations China 1 : 17,000

Japan 1 : 125,000

Turkey 1 : 2,600

Yemenite Jews (in Israel) 1 : 5,300

Scotland 1 : 5,300

Czechoslovakia 1 : 7,000

Hungary 1 : 11,000

Denmark 1 : 12,000

European Populations

France 1 : 13,500

Norway 1 : 14,500

United Kingdom 1 : 14,300

Italy 1 : 17,000

Canada 1 : 22,000

Finland 1 : 200,000

Arabic Populations Up to 1 : 6,000

Oceania

Australia 1: 10,000

Urine tests; the excess of phenylalanine builds up so much that it’ll excrete phenylpyruvic acid in a person’s urine.

The Guthrie test (Guthrie bacterial inhibition assay); where a doctor could take a sample of blood and put it on a disk of filter paper with bacteria that needs more phenylalanine to grow. Therefore if the blood came from a PKU child then the bacteria would rapidly grow.

An overall blood screening for many genetic diseases

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