Phenylketonuria (PKU) is a rare, inherited metabolic disorder that is characterized by the inability of the body to utilize the essential amino acid, phenylalanine (Phe). Amino acids, usually obtained from the food we eat, are the building blocks for body proteins. PKU is caused by a deficiency of the liver produced enzyme phenylalanine hydroxylase (PAH).
This enzyme normally converts Phe to another amino acid, tyrosine. Without this enzyme, Phe accumulates in the blood and body tissues. Excess Phe is toxic to the central nervous system and causes the severe problems normally associated with PKU. When left untreated, PKU patients who consume too much Phe are at risk of severe neurological complications, including IQ loss, memory loss, concentration problems, mood disorders, and in some cases, severe mental retardation.
Damage done is irreversible so early detection is crucial. When treatment is begun early (within the first few weeks of life) and rigorously adhered to, affected children can expect normal development and a normal life span. PKU can be treated by a diet low in phenylalanine and high in tyrosine. While there is no cure, in recent years a few drug products have become available that can be used in limited cases to mitigate the effects of the disorder. Other therapies currently under investigation include an injectable form of PAH and gene therapy.
PKU CAN BE CLASSIFIED INTO THREE GROUPS
- Classical PKU (PKU), where the dietary tolerance to Phe is less than 350-400 mg per day and then residual activity of PAH is less than 5%. This is the most common form.
- Moderate or Mild PKU (PKU), where the dietary tolerance to Phe is less than 350-600 mg per day and then residual activity of PAH is less than 10%.
- Benign PKU (PKU), where there are no dietary restrictions of Phe but the residual activity of PAH is less than 5%.
CAUSE AND INCIDENCE OF PKU
The gene defect for PKU is an autosomal recessive genetic defect and is unknowingly passed down from generation to generation. This means an affected person inherited two genetic defects for the disorder (one from each parent). A person with one genetic defect for the disorder, is called a 'carrier' for PKU. Carriers do not have symptoms of the disorder. When two carriers conceive a child, there is a one in four (or 25%) chance for each pregnancy that the baby will have PKU. The incidence of carriers in the general population is approximately one in fifty people, but the chance that two carriers will mate is only one in 2500. Carrier tests are available only through PKU treatment programs.
Classical PKU affects between 1 in 10,000 and 1 in 20,000 depending on the country of origin. The incidence varies widely in different human populations from 1 in 4,500 births among the population of Ireland to fewer than one in 100,000 births among the population of Finland. Even higher levels have been reported in the Eastern Mediterranean region (1 in 4,000 in Turkey and 1 in 3,627 in the Islamic Republic of Iran). Poland and the Czech Republic also reportedly have high incidence rates. In the US the incidence rate appears to be between 1 in 10,000 to 1 in 20,000 for Caucasians and Asians. In the USA there are roughly 9000 persons with PKU since Newborn Screening (NBS) initiated (est. 210 live births per year X 43.5 years ) Approx. 5500 more persons with PKU who predated NBS are in dependent living homes/institutions (avg live span 75 years-31.5 years pre-1965 X est. 185 live births per year). Total: 14,500 persons in the US living with PKU. The incidence among African Americans is much less. There is no difference in frequency of occurrence between males and females .
SYMPTOMS AND DIAGNOSIS
Newborns affected by PKU usually do not show any signs of the disease at birth. But within the first few weeks of life they begin to show neurologic disturbances such as epilepsy. Children suffering from undiagnosed PKU also may have a "musty or mousy" odor of skin, hair, sweat and urine (due to phenylacetate accumulation). It has been shown that almost 90% of affected people have light coloration such as blond hair and blue eyes. Signs also include skeletal changes such as a small head, short stature, and flat feet. PKU sufferers may also exhibit skin disorder ( eczema). tremors, jerking movements of the arms or legs, and unusual hand posturing. Hyperactivity, EEG abnormalities, seizures, and severe learning disabilities are major clinical problems later in life. If left untreated, the child is likely to experience behavior problems and developmental delays. Severe brain problems may occur such as mental retardation.
The initiation of screening in the mid-1960’s throughout the US of all newborns has resulted in the early identification and treatment of PKU affected children with very successful results. The several hundred babies diagnosed each year and placed on diet are growing up normally. They are attending college and becoming productive adults as doctors, lawyers, teachers and engineers because of early diagnosis and strict treatment.
Usually children are tested at least 12 hours and generally 24–28 hours after birth, using a blood sample drawn from the heel of the foot.