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| DiseaseID |
HGD28 |
| Genetic
Disorder |
Phenylketonuria
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| Gene
Name |
PAH gene |
| Description |
Phenylketonuria (PKU) is a genetic disorder in which the body can't process part of a protein called phenylalanine (Phe). Phe is in almost all foods. If the Phe level gets too high, it can damage the brain and cause severe mental retardation. All babies born in U.S. hospitals must now have a screening test for PKU. This makes it easier to diagnose and treat the problem early.
The best treatment for PKU is a diet of low-protein foods. There are special formulas for newborns. For older children and adults, the diet includes many fruits and vegetables. It also includes some low-protein breads, pastas and cereals. Nutritional formulas provide the vitamins and minerals they can't get from their food.
Babies who get on this special diet soon after they are born develop normally. Many have no symptoms of PKU. It is important that they stay on the diet for the rest of their lives.
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| Symptoms |
Newborns with phenylketonuria don't have any symptoms. Without treatment, though, babies usually develop signs of PKU within a few months. Phenylketonuria symptoms can be mild or severe and may include:
? Mental retardation
? Behavioral or social problems
? Seizures, tremors or jerking movements in the arms and legs
? Rocking
? Hyperactivity
? Stunted growth
? Skin rashes (eczema)
? Small head size (microcephaly)
? Vomiting
? A musty odor in the child's breath, skin or urine, caused by too much phenylalanine in the body
? Fair skin and blue eyes, because phenylalanine cannot transform into melanin — the pigment responsible for hair and skin tone
The most severe form of the disorder is known as classic PKU. Children with untreated classic PKU usually develop obvious, permanent mental retardation and behavioral problems by their first birthdays. However, even in people with exactly the same genetic defect, the severity of the disease may be different.
Less severe forms of PKU — sometimes called mild or moderate PKU — have a smaller risk of significant brain damage, but most children with these forms of the disorder still require a special diet to prevent mental retardation and other complications.
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| Causes |
A mutation in a single gene causes PKU. This gene contains the instructions for making an enzyme needed to process the amino acid called phenylalanine. Amino acids are the building blocks for protein. In a person with PKU, this gene is defective, causing a complete or near-complete deficiency of the enzyme. Without the enzyme necessary to process phenylalanine, a dangerous buildup of this amino acid can develop when a person with PKU eats foods that are high in protein, such as milk, cheese, nuts or meats. This can eventually lead to serious health problems.
For a child to inherit PKU, both the mother and father must have and pass on the defective gene. This pattern of inheritance is called autosomal recessive. A parent can have the defective gene, but not have the disease. This is called being a "carrier." Most often, PKU is passed down the family tree by parents who are carriers of the disorder but don't know it. |
| Diagnosis |
Newborn screening identifies almost all cases of PKU. All 50 states in the United States require newborns to be tested for PKU. Canada and many other countries also routinely screen infants for PKU.
If you have PKU or a family history of PKU, your doctor may recommend screening tests before pregnancy or birth. It's possible to identify PKU carriers through a blood test.
Testing your baby after birth
A PKU test is conducted a day or two after your baby's birth. In the test, a nurse or lab technician collects a few drops of blood from your baby's heel or the crook of your baby's arm using a needle or lancet. A laboratory tests the blood sample for certain metabolic disorders, including PKU. Tests for PKU are highly accurate when done between 24 hours and seven days after birth and are typically done before a new baby leaves the hospital. The test is not done before the baby is 24 hours old or before the baby has ingested some protein in the diet. If you don't deliver your baby in a hospital or are discharged soon after the birth, you may need to schedule a newborn screening with your pediatrician or family doctor.
If your baby's blood sample shows a high level of phenylalanine, he or she will need to be fed a special formula. Your doctor can help you find the right type of formula to feed your baby.
Your baby may also have additional tests to confirm the diagnosis, including more blood tests and urine tests. You and your baby may also undergo genetic testing to identify gene mutations.
Screening during pregnancy
It's also possible to detect PKU in a developing fetus using chorionic villus sampling (CVS). During this procedure, a needle is inserted through a pregnant woman's lower abdomen. Another option is to insert a catheter up through the cervix into the chorionic villi, which forms the lining of the placenta. Through the needle, your doctor removes a small sample of cells for genetic testing. Your doctor and a genetics counselor can help you decide if these tests are right for you and what to do with the results.
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| Treatment |
The main phenylketonuria treatment is a strict diet with very limited intake of phenylalanine, which is mostly found in protein-rich foods. Doctors used to believe it was OK for a person with PKU to stop the diet in adolescence, but today, doctors recommend sticking to the diet for life.
A safe amount of phenylalanine differs for each person. Your doctor will determine a safe amount through regular review of diet records, growth charts and blood levels of phenylalanine. Frequent blood tests will monitor PKU levels as they change over time, especially during childhood growth spurts and pregnancy. In general, the idea is to consume only the amount of phenylalanine that's necessary for normal growth and body processes, but no more.
Which foods to avoid
Because the amount of phenylalanine adults can eat is so low, it's crucial they avoid all high-protein foods, such as milk, ice cream, eggs, nuts, beans, chicken, steak and fish.
Children and adults should also avoid foods, including many diet sodas, and medications made with aspartame (NutraSweet, Equal). Aspartame, found in many artificial sweeteners, releases phenylalanine when digested. Because the amount of phenylalanine allowed in the PKU diet is so small, adults and children also have to limit pasta, rice, bread, cookies, and even some fruits and vegetables.
For instance, one ear of corn on the cob contains more than 90 milligrams of phenylalanine — which can be a substantial portion or all of a person's allotment for the day. Too much of a good thing can be harmful. Even if you are eating approved foods, eating too many at one time can be dangerous. Consider the total amount of phenylalanine in all the foods you eat when planning your diet.
Focus on formula
You might wonder how someone, particularly a baby, can get all of the necessary nutrition with these dietary limitations. The answer is a formula — a special nutritional drink or supplement for people with PKU. The phenylalanine-free formula provides protein and other essential nutrients in a form that is safe for people with PKU.
Because regular infant formula and breast milk contain phenylalanine, babies with PKU need to have their diets substituted with a phenylalanine-free infant formula. A dietitian will carefully calculate a sparing amount of breast milk or regular formula to be mixed with the phenylalanine-free formula. Parents introduce solid foods with low levels of phenylalanine to children with PKU on the same schedule used for other infants. These foods replace the phenylalanine a baby has been receiving from breast milk or formula and need to be precisely figured into the daily phenylalanine intake.
Older children and adults continue to drink several glasses of formula each day, as directed by a doctor or dietitian. The formula for older kids and adults is not the same as the one used for infants, but it works on the same principle. It acts as a nutritional substitute and is continued for life.
The need for a nutritional supplement and the limited food choices can make the PKU diet challenging. But, it is the only way to prevent the serious health problems people with PKU can develop. Families need to commit to this lifestyle change wholeheartedly, realizing that it may be difficult but not impossible.
Applying the PKU diet
If you or your child is "on-diet," you'll need to keep records of the amount of phenylalanine eaten every day to be sure you're sticking to the specific, individualized dietary guidelines recommended by your dietitian. To do so, use a food diary or computer program that lists the amount of phenylalanine in baby foods, solid foods, PKU formulas, and common baking and cooking ingredients. To be as accurate as possible, measure portions, using standard measuring cups and spoons and a kitchen scale that reads in grams.
Investing in some of the many low-protein products available through specialty food retailers may add variety to your diet. You'll find low-protein rice, pasta, pizza crusts, tortillas, bagels, breads, cookie dough and baking mixes, as well as egg substitutes and imitation cheeses. These products provide some dietary variety, and they allow people with PKU to eat lunches and dinners that more closely resemble what everyone else is eating. Like the PKU formulas, these products can be expensive, but you might consider splurging on a few favorites with the money you save on dairy and meat products.
Food preparation ideas
Whether you buy special low-protein products or not, you can be creative with foods you can find at your local grocery store. Here are some ideas for serving up three square meals with low amounts of phenylalanine:
? Breakfast. In the morning, dish up lower phenylalanine cereals, such as corn flakes or puffed rice, in nondairy creamer diluted with water. Or whip up a smoothie by blending fresh or frozen fruit with ice cubes, fruit juice, and almond or vanilla flavoring.
? Lunch. Pack a lunch bag with small portions of rice cakes, grapes, applesauce, lemonade and jelly beans. Or enjoy a low-protein pumpkin or vegetable soup.
? Dinner. Veggie stir-fry and a salad make a great evening meal. So do vegetable shish kebabs, soaked in a tangy vinaigrette or Asian marinade. You can also try eggplant curry, baked potatoes topped with broccoli or mushrooms, and peppers stuffed with sweet potatoes or carrots.
You can transform the same basic lower phenylalanine vegetables into a whole menu of different dishes, using a little creativity — and a lot of seasonings. Herbs and flavorings such as basil, cilantro, lemon juice, soy sauce, sesame oil, maple syrup and honey are low in phenylalanine, but they pack a flavorful punch. Just remember to measure and count every ingredient and adjust ideas to your individualized diet. Talk to your doctor or dietitian if you have any questions. If you have any other health conditions, you may need to consider those when you plan your diet too.
PKU medication
In December 2007, the Food and Drug Administration (FDA) approved the drug sapropterin dihydrochloride (Kuvan) for treatment of some people with PKU. The drug is for use in combination with a PKU diet. In approving the drug, the FDA directed that studies continue regarding the drug's efficacy |
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