What is Phenylketonuria?

Phenylketonuria (PKU) is an autosomal recessive genetic disorder that is inherited due to a contribution of the genetic disorder gene from each parent. Phenylketonuria is due to a variety of mutations to the gene of the liver enzyme phenylalanine hydroxylase (PAH) (Vockley 2014). Mutations of this enzyme result in its dysfunction and deficiency in the liver. There are over 300 possible mutations of the gene that makes this enzyme (Nowacki 1998). Each mutation has been associated with varying severity of the phenylalanine hydroxylase’s dysfunction, which results in the varying severity of physical symptoms of PKU (Pey 2003). Dysfunctional PAH cannot do its job of converting the amino acid phenylalanine to another amino acid tyrosine. Amino acids are the building blocks of all proteins in cells and are essential for life, but too much of certain amino acids can result in many toxic effects to the body. One of the PAH’s essential roles in the liver is to maintain non-toxic phenylalanine levels by converting phenylalanine to tyrosine. In phenylketonuria, phenylalanine hydroxylase is not able to convert excess phenylalanine to tyrosine which results in high concentrations of the amino acid phenylalanine in the blood. The high concentrations of phenylalanine in the blood result in the production of these compounds called phenylketones, which is where the disorder gets its name (Vockley 2014). High levels of phenylalanine in the blood, if left at high concentrations without treatment, have been shown to result in severe physical and mental disability (Embury 2007) (Moraes 2013).

How PKU and normal individuals differ with regards to conversion of phenylalanine
How PKU and normal individuals differ with regards to conversion of phenylalanine

Screening for the presence of the phenylketone compounds has led to early detection of newborns with phenylketonuria, resulting in a decrease in levels of mental and physical disability with early treatment (Vockley 2014). The most common treatment suggested by doctors is the life-long implementation of a phenylalanine-free diet to keep phenylalanine levels low in the blood (Vockley 2014).

Phenylalanine-free diet suggested foods
Phenylalanine-free diet suggested foods

Phenylalanine is present in many of the foods we eat. A lifelong phenylalanine-free diet has resulted in PKU patients experiencing normal cognitive function (Vockley 2014). A newly approved drug on the market called sapropterin has improved phenylalanine tolerance in the body (Cunningham 2012). This drug coupled with the lifelong diet has demonstrated success with many PKU patients, thus allowing these individuals to maintain normal cognitive function and carry out normal lives (Simon 2008).

Simplified PKU pathway resulting in the production of phenylketones
Simplified PKU pathway resulting in the production of phenylketones

4 Replies to “What is Phenylketonuria?”

  1. I think this page is very accessible– the images are especially helpful! I would consider re-ordering the sentences in the second half of your first paragraph– you bounce back and forth among the ideas of PAH’s normal function, the danger of amino acid accumulation, and the molecular consequences PAH deficiency which to me makes it seem little redundant and confusing. Can you tell me more about what sapropterin does to improve phenylalanine tolerance? Also, is the mechanism by which phenylketone accumulation leads to mental/physical dsiability known?

    1. Thanks Becca for comment. I will work on wording in the second paragraph. Sapropterin actually acts as a chaperone for phenylalanine hydroxylase in order to improve folding and quaternary protein structure towards its active homotetramer, helping increase catalytic activity. Your second question is interesting because the field at the current moment does not know why elevated phenylalanine and phenylketone concentrations lead to the neurological pathologies associated with the disease. There has been demonstrated an indirect correlation between elevated phenylalanine concentration in the brain and hypomyelination.

  2. The second half regarding treatment is really interesting and informative, and perhaps could even be expanded upon. The first half I would consider rearranging. It took me until the end to know what the disease actually was, so I would start off with an immediate definition, and then expand into the genetics of it. Very interesting though!

    1. Thank you Mary for your comment. I will work on the organization in order to clarify the disease. I will also expand on treatments such as what sapropterin does to alleviate the symptoms of PKU and other forms of treatment that have been established to bolster that section.

Comments are closed.