Phenylketonuria

Characteristics/Phenotype

Phenylketonuria is a rare metabolic condition in which a baby is born without the ability to properly break down an amino acid called phenylalanine. If not treated shortly after birth, PKU can be destructive to the nervous system, causing mental retardation.

Phenylketonuria (commonly known as PKU) is an inherited disorder that increases the levels of a substance called phenylalanine in the blood. Phenylalanine is a building block of proteins (an amino acid) that is obtained through the diet. It is found in all proteins and in some artificial sweeteners.

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.

If PKU is not treated, phenylalanine can build up to harmful levels in the body, causing intellectual disability and other serious health problems. The signs and symptoms of PKU vary from mild to severe. The most severe form of this disorder is known as classic PKU. Infants with classic PKU appear normal until they are a few months old. 

Babies born with PKU usually have no symptoms at first. But if the disease is left untreated, babies experience severe brain damage. This damage can cause epilepsy, behavioral problems, and stunt the growth of the baby. Other symptoms include: eczema (skin rash), a musty body odor (from too much phenylalanine), a small head (microcephaly), and fair skin (because phenylalanine is necessary for skin pigmentation). Children with classic PKU tend to have lighter skin and hair than unaffected family members.

Less severe forms of this condition, sometimes called variant PKU and non-PKU hyperphenylalaninemia, have a smaller risk of brain damage. People with very mild cases may not require treatment with a low-phenylalanine diet.

Frequency

The occurrence of PKU varies among ethnic groups and geographic regions worldwide. In the United States, PKU occurs in 1 in 10,000 to 15,000 newborns. 

Diagnosis

Because PKU must be treated early, babies are routinely tested for the disease. Diagnosis is made through Newborn Screening. A small blood sample is taken from the baby's heel or arm and checked in a laboratory for high levels of phenylalanine.

Cause/s

PKU is caused by a mutation in a gene on chromosome 12. The gene codes for a protein called PAH (phenylalanine hydroxylase), an enzyme in the liver. This enzyme breaks down the amino acid phenylalanine into other products the body needs. When this gene is mutated, the shape of the PAH enzyme changes and it is unable to properly break down phenylalanine. Phenylalanine builds up in the blood and poisons nerve cells (neurons) in the brain.

Treatments/ Recommendations

People who have PKU must eat a protein-free diet, because nearly all proteins contain phenylalanine. Infants are given a special formula without phenylalanine. Older children and adults have to avoid protein-rich foods such as meat, eggs, cheese, and nuts. They must also avoid artificial sweeteners with aspertame, which contains phenylalanine.

References

http://www.medicinenet.com/phenylketonuria/article.htm

http://learn.genetics.utah.edu/content/disorders/whataregd/pku/

http://ghr.nlm.nih.gov/condition/phenylketonuria

http://health.nytimes.com/health/guides/disease/phenylketonuria/overview.html

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