Causes of PKU

PKU or Phenylketonuria is a genetically inherited condition.

PKU occurs due to the lack of an enzyme called phenylalanine hydroxylase. This enzyme is a protein. Consequently, it is synthesized based on the genetic code that lies in the DNA.

Genetic causes of PKU

There are 23 pairs of chromosomes. Each of them carries the DNA that is the blue print for the synthesis of various proteins in the body.

The segment that contains the blue print coding for the enzyme phenylalanine hydroxylase lies in the 12th chromosome.

When a parent carries a faulty copy of the gene they are named carriers. Since the parent also has a normal copy of the gene, they do not have signs or symptoms of PKU.

Autosomal recessive pattern of inheritance occurs when the disease is passed to the baby as a result of both parents being a carrier.

In addition, if both parents carry a faulty copy of the gene without manifesting the disease their biological child has a 25% risk of having PKU at birth. (1, 2)

Biochemistry of PKU

The gene that determines the formation of the enzyme phenylalanine hydroxylase is termed PAH gene.

When there is a fault or mutation in this gene the activity of phenylalanine hydroxylase is reduced.

This causes build up to toxic levels of phenylalanine in blood and brain leading to brain damage and other complications.

For the past few decades all babies are screened for PKU at birth. This means avoidance of severe consequences of the disease with early detection of the condition. (1, 2)

PKU: a multifactorial disorder?

Now it is also speculated that deficiency of phenylalanine hydroxylase is a multifactorial disorder.

This means that both environment; i.e. dietary intake of phenylalanine, as well as genetic inheritance is responsible for the condition.

This is proven by the fact that some siblings who have the same genetic makeup may not show the signs and symptoms of PKU.

The actual reason for this is not known.

Some studies show that the body’s handling of dietary phenylalanine might be different in individuals who function normally in spite of a faulty gene. This is called pharmacokinetic difference in amino acid phenylalanine metabolism.

Although environmental components are speculated, defects in the PAH gene is still thought to be the primary risk factor that determines the chances of acquiring the condition at birth. (3)

Further Reading

Last Updated: May 31, 2019

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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