Galactosemia

Definition

Galactosemia is an inherited disease where the transformation of galactose to glucose is blocked, allowing galactose to increase to toxic levels in the body. If galactosemia is untreated, high levels of galactose cause vomiting, diarrhea, lethargy, low blood sugar, brain damage, jaundice, liver enlargement, cataracts, susceptibility to infection, and death.

Description

Galactosemia is a rare but potentially tragic disease that kills very young babies. However, thanks to an understanding of the root of the problem, infant death from galactosemia can be prevented by giving simple tests to newborns.

Galactosemia is an inborn error of metabolism. "Metabolism" refers to all the chemical reactions that take place in living organisms. A metabolic pathway is a series of reactions where the product of each step in the series is the starting material for the next step. Because of energy barriers, essentially none of the reactions in organisms occur at any measurable rate unless a catalyst (a compound that affects the rate of a chemical reaction) is present. Most catalysts in organisms, including those required for the transformation of galactose to glucose in humans, are enzymes (large protein molecules). Their ability to function depends on their structure, and their structure is determined by the deoxyribonucleic acid (DNA) sequence of the genes that encode them. Inborn errors of metabolism are caused by defective genes.

Sugars are sometimes called "the energy molecules," and galactose and glucose are both sugars. For galactose to be utilized for energy, it must be transformed into something that can enter the metabolic pathway that converts glucose into energy (plus water and carbon dioxide). This is important for infants because they typically get most of their nutrient energy from milk, which contains a high level of galactose. Each molecule of lactose, the major sugar constituent of milk, is made up of a molecule of galactose and a molecule of glucose, and so galactose makes up 20% of the energy source of a typical infant's diet.

Three enzymes are required to convert galactose into glucose-1-phosphate (a phosphorylated glucose that can enter the metabolic pathway that turns glucose into energy). Each of these three enzymes is encoded by a separate gene. If any of these enzymes fail to function, galactose build-up and galactosemia result. Thus, there are three types of galactosemia with a different gene responsible for each.

Each of the forms of galactosemia is inherited as a recessive trait, which means that galactosemia is only present in individuals with two defective copies of one of the three genes. This also means that carriers, with one copy of a defective recessive gene, will not be aware that they are carrying a defective gene (unless they have had a genetic test), as it is masked by the normal gene they also carry and they have no symptoms of the disease. If two carriers of the same defective gene have children, the chance of any of their children getting galactosemia (the chance of a child getting two copies of the defective gene) is 25% for each pregnancy.

Every cell nucleus in a person's body has two copies of each gene (with some exceptions that are not relevant to galactosemia). For each step in the conversion of galactose to glucose, if only one of the two copies of the gene controlling that step is normal, enough functional enzyme is made so that the pathway is not blocked at that step. Thus, if a person has galactosemia, both copies of the gene coding for one of the enzymes required to convert glucose to galactose are defective.

Causes & symptoms

Galactosemia I

Galactosemia I (also called classic galactosemia), the first form to be discovered, is caused by defects in both copies of the gene that codes for an enzyme called galactose-1-phosphate uridyl transferase (GALT). The frequency of occurrence of this form of galactosemia in the United States is about one in every 50,000-70,000 births. There are 30 known different mutations in this gene that cause GALT to malfunction.

Newborns with galactosemia I appear normal when first born, but after they are given milk for the first time, symptoms appear. They include vomiting, diarrhea, lethargy (sluggishness or fatigue), low blood glucose, jaundice (a yellowing of the skin and eyes), enlarged liver, protein and amino acids in the urine, and susceptibility to infection, especially from gram negative bacteria. Cataracts (a grayish white film on the eye lens) can appear within a few days after birth. People with galactosemia frequently have symptoms as they grow older even though they have been given a galactose-free diet. These symptoms include speech disorders, cataracts, ovarian atrophy and infertility in females, learning disabilities, and behavioral problems.

Galactosemia II

Galactosemia II is caused by defects in both copies of the gene that codes for an enzyme called galactokinase (GALK). The frequency of occurrence of galactosemia II is about one in 100,000-155,000 births.

Galactosemia II is less harmful than galactosemia I. Babies born with galactosemia II will develop cataracts at an early age unless they are given a galactose-free diet. They do not generally suffer from liver damage or neurologic disturbances.

Galactosemia III

Galactosemia III is caused by defects in the gene that codes for an enzyme called uridyl diphosphogalactose-4-epimerase (GALE). This form of galactosemia is very rare.

There are two forms of galactosemia III, a severe form, which is exceedingly rare, and a benign form. The benign form has no symptoms and requires no special diet. However, newborns with galactosemia III, including the benign form, have high levels of galactose-1-phosphate that show up on the initial screenings for elevated galactose and galactose-1-phosphate. This situation illustrates one aspect of the importance of follow-up enzyme function tests. Tests showing normal levels of GALT and GALK allow people affected by the benign form of galactosemia III to enjoy a normal diet.

The severe form has symptoms similar to those of galactosemia I, but with more severe neurological problems, including seizures. Only two cases of this rare form had been reported as of 1997.

Diagnosis

The diagnostic test for galactosemia is quick and straightforward; almost all states require that all newborns be tested. Blood from a baby who is two to three days old is usually first screened for high levels of galactose and galactose-1-phosphate. If either of these compounds is elevated, further tests are performed to find out which enzymes (GALT, GALK, or GALE) are present or missing.

Naturally, if there is a strong suspicion that a baby will have galactosemia, galactose is removed from their diet right away. In this case, an initial screen for galactose or galactose-1-phosphate will be meaningless. In the absence of galactose in the diet, this test will be negative whether the baby has galactosemia or not. In this case, tests to measure enzyme levels must be given to find out if the suspected baby is indeed galactosemic.

In addition, galactosemic babies who are refusing milk or vomiting will not have elevated levels of galactose or galactose phosphate, and their condition will not be detected by the initial screen. Any baby with symptoms of galactosemia (for example, vomiting) should be given enzyme tests.

Treatment

Galactosemia I and II are treated by removing galactose from the diet. Since galactose is a break-down product of lactose, the primary sugar constituent of milk, this means all milk and foods containing milk products must be totally eliminated. Other foods like legumes, organ meats, and processed meats also contain considerable galactose and must be avoided. Pills that use lactose as a filler must also be avoided. Soy-based and casein hydrolysate-based formulas are recommended for infants with galactosemia.

Treatment of the severe form of galactosemia III with a galactose-restricted diet has been tried, but this disorder is so rare that the long-term effects of this treatment are uncertain.

Prognosis

It is critically important that all newborn babies be tested for galactosemia because the prognosis for any baby with galactosemia I who doesn't start a galactose-free diet within the first five days of life is tragic. About 75% of the untreated babies die within their first two weeks. On the other hand, with treatment, a significant proportion of people with galactosemia I can lead nearly normal lives, although speech defects, learning disabilities, and behavioral problems are common. In addition, cataracts due to galactosemia II can be completely prevented by a galactose-free diet.

Prevention

Since galactosemia is a genetic disease, it cannot be prevented. Prospective parents can undergo genetic testing to determine if they are carriers of the defective genes causing the disease and use that information to conduct family planning. Children born with galactocemia should be put on a special diet right away, to reduce the symptoms and complications of the disease.

Key Terms

Casein hydrolysate

A preparation made from the milk protein casein, which is hydrolyzed to break it down into its constituent amino acids. Amino acids are the building blocks of proteins.

Catalyst

A substance that changes the rate of a chemical reaction, but doesn't itself get changed by the reaction.

Enzyme

A protein catalyst; one of the two kinds of biological catalysts, which are exceedingly specific. Each different enzyme only catalyzes one or two specific reactions.

Galactose

A sugar that contains six carbons in its chemical structure. It is similar to glucose and is toxic in high levels.

Glucose

A sugar with six carbon molecules. It is of central importance in energy metabolism and is a part of many other biological molecules, for example lactose, sucrose (table sugar), cellulose, starch, and dextran.

Lactose

A sugar made up of of glucose and galactose. It is the primary sugar in milk.

Metabolic pathway

A sequence of chemical reactions that lead from some precursor to a product, where the product of each step in the series is the starting material for the next step.

Metabolism

The sum of all the chemical reactions that take place in living organisms (whether they use energy or not; whether they build large molecules from small ones or break large molecules into smaller ones).

Recessive trait

An inherited trait or characteristic that is outwardly obvious only when two copies of the gene for that trait are present, as opposed to a dominant trait where one copy of the gene for the dominant trait is necessary to display the trait.

Further Reading

For Your Information

Books

• Ng, Won G., Thomas F. Roe, and George N. Donnell. "Carbohydrate Metabolism." In Emery and Rimoin's Principles and Practice of Medical Genetics, 3rd. ed., edited by David L. Rimoin, J. Michael Connor, and Reed E. Pyeritz. New York: Churchill Livingstone, 1998.

  Organizations

• Association for Neuro-Metabolic Disorders. 5223 Brookfield Lane, Sylvania, OH 43560. (419) 885-1497.
• Metabolic Information Network. P.O. Box 670847, Dallas, TX 75367-0847. (214) 696-2188 or (800) 945-2188.
• Parents of Galactosemic Children, Inc. 2148 Bryton Dr., Powell OH 43065. http://www.galactosemia.org/index.htm.

  Other

• "GeneCards: Human Genes, Proteins and Diseases." http://bioinfo.weizmann.ac.il/cards/.
• "Vermont Newborn Screening Program: Galactosemia." http://www.vtmednet.org/~m145037/vhgi_mem/nbsman/galacto.htm.

Gale Encyclopedia of Medicine. Gale Research, 1999.