Phenylketonuria (PKU) is an example of a disorder caused by a gene-environment interaction. As a result of the defect, the affected individual is unable to convert phenylalanine into tyrosine. Phenylalanine in the body accumulates in the blood and can reach toxic levels. (See Figure 5.4.) This toxicity may impair brain and nerve development and result in mental retardation, organ damage, unusual posture, and, when it occurs during pregnancy, may jeopardize the health and viability of the unborn child.
Originally, PKU was considered simply an autosomal recessive inherited error of metabolism that occurred when an individual received two defective copies, caused by mutations in both alleles of the phenylalanine hydroxylase gene found on chromosome 12. (See Figure 5.18.) The environmental trigger—dietary phenylalanine—was not identified at first because phenylalanine is so prevalent in the diet, occurring in common foods such as milk and eggs and in the artificial sweetener aspartame. Recognition of dietary phenylalanine as a critical environmental trigger has enabled children born with PKU to lead normal lives when they are placed on low-phenylalanine diets, and mothers with the disease can bear healthy children.
To identify people at risk of PKU, all newborns in the United States and many other countries are screened at birth for high levels of phenylalanine in the blood. After additional screening of those with elevated blood levels, approximately one in 10,000 infants is diagnosed with PKU and, with proper diet, is likely to lead a healthy, normal life.
Chromosome 12 and the gene for phenylalanine hydroxylase
SOURCE: "The PAH Gene Maps to Chromosome 12," in "Phenylketonuria (PKU)," Science: The Human Gene Maps 7, National Institutes of Health, National Center for Biotechnology Information, 2004, http://www.ncbi.nlm.nih.gov/SCIENCE96/gene.cgi?PAH (accessed February 8, 2005)