Photo source

by Emily R

In class, we’ve been talking about how people’s DNA determines their traits. So I decided to write a blog post about a genetic disease called phenylketonuria, or PKU.

PKU causes mental retardation and can cause seizures. This is due to a deficiency in phenylalanine hydroxylase, an enzyme that converts the amino acid phenylalanine into another amino acid, tyrosine. Phenylalanine is an essential amino acid to the human body. Even though phenylalanine is helpful in producing proteins, too much of it is harmful to the brain. This applies to almost everything in medicine: too much of anything can be very harmful. This concept is similar to what we studied last semester about breaking down sugars. After a person eats, sugar is broken down and sent to cells that need it. If there is sugar left over, the body stores it or disposes of it. The same goes for phenylalanine. For a person who doesn’t have PKU, the excess phenylalanine would be broken down and turned into tyrosine or disposed in waste. But, people with PKU do not have phenylalanine hydroxylase to break down the phenylalanine, so it cannot be disposed of or converted. Just like too much sugar is harmful to the body, too much phenylalanine is harmful in that it leads to an underdeveloped brain.

Today, all infants are tested for PKU. In order to do so, doctors measure the amounts of phenylalanine and tyrosine in their blood. A baby is diagnosed with the disease if their blood contains 20 mg/dL or higher of phenylalanine or if it contains 1 mg/dL or less tyrosine.

Chart source

The chart above shows how autosomal recessive diseases are inherited. PKU is an autosomal recessive disease, meaning a child can have PKU only if both of their parents are carriers for the disease. Because both parents must be carriers, PKU is a rare disease. Only about 1 in 10,000 people have it. In the diagram, the white circles represent people who do not have PKU, the black circles are those who have PKU, and those that are half-shaded represent carriers for the disease. (Carriers are people who can pass the disease on to their children, but who do not have the disease themselves.) In figures 3 and 4, none of the children have PKU because one of their parents was neither a carrier nor a person with the disease. But because one of their parents was a carrier, the children can still be carriers. However, in figures 1, 2, and 5, some of the children may have the disease because both parents are either infected or are carriers. If both parents are infected, then their child/children will have the disease also. The four “children” represent percentages. For example, in figure one, if both parents are carriers for the disease, then there is a 25% chance that their child will have PKU, a 25% chance that they will not have PKU, and a 50% chance that they will be a carrier. This chart is true for all autosomal recessive diseases, not just PKU.

Fortunately, there is a very simple cure for PKU. Since people with the disease have too much phenylalanine, they should restrict it from their diet. But in order for this to be effective, they must maintain this diet from three weeks of age and older. Phenylalanine is an amino acid, so restricted foods are those that are high in protein, such as meat, eggs, and dairy products. In some cases, this diet can be discontinued after five years or more of age, because PKU is a disease which harms brain development, and at five years old, the brain is fully developed.

Are there any other genetic diseases with such a simple cure? Do you think that there are diseases now which could later be cured so easily? What are some other autosomal recessive diseases?

This entry was posted on Monday, January 21st, 2008 at 12:10 pm and is filed under Student Post, Medical Marvels. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.