Carnitine palmitoyltransferase deficiency

views updated

Carnitine palmitoyltransferase deficiency

Definition

Carnitine palmitoyltransferase (CPT) deficiency refers to two separate, hereditary diseases of lipid metabolism, CPT-I deficiency and CPT-II deficiency. CPT-I deficiency affects lipid metabolism in the liver, with serious physical symptoms including coma and seizures. Two types of CPT-II deficiency are similar in age of onset and type of symptoms to CPT-I deficiency. The third, most common type of CPT-II deficiency involves intermittent muscle disease in adults, with a potential for myoglobinuria, a serious complication affecting the kidneys. Preventive measures and treatments are available for CPT-I deficiency, and the muscle form of CPT-II deficiency.

Description

Carnitine palmitoyltransferase (CPT) is an important enzyme required by the body to use (metabolize) lipids (fats). CPT speeds up the transport of long-chain fatty acids across the inner mitochondria membrane. This transport also depends on carnitine, also called vitamin B₇.

Until the 1990s, discussion centered on whether defects in a single CPT enzyme were responsible for all the conditions resulting from CPT deficiency. Careful chemical and genetic analysis eventually pointed to two different enzymes: CPT-I and CPT-II. Both CPT-I and CPT-II were shown to play an important role in the metabolism of lipids. CPT deficiency of any type affects the muscles, so these disorders are considered to be metabolic myopathies (muscle diseases), or more specifically, mitochondrial myopathies, meaning myopathies that result from abnormal changes occurring in the mitochondria of the cells as a result of excessive lipid build-up.

Understanding the symptoms of CPT requires some familiarity with the basics of lipid metabolism in muscle cells. Fatty acids (FA) are the major component of lipids. FAs contain a chain of carbon atoms of varying length. Long-chain fatty acids (LCFAs) are the most abundant type, and have at least 12 carbon atoms. Lipids and glucose (sugar) are the primary sources of energy for the body. Both are converted into energy (oxidized) inside mitochondria, structures within each cell where numerous energy-producing chemical reactions take place. Each cell contains many mitochondria.

A single mitochondrion is enclosed by a doublelayer membrane. LCFAs are unable to pass through the inner portion of this membrane without first being bound to carnitine, a type of amino acid. CPT-I chemically binds carnitine to LCFAs, allowing transfer through the inner membrane. However, LCFAs cannot be oxidized inside the mitochondrion while still attached to carnitine, so CPT-II reverses the action of CPT-I and removes carnitine. Once accomplished, LCFAs can proceed to be metabolized. Therefore, deficiency of either CPT-I or CPT-II results in defective transfer and utilization of LCFAs in the mitochondria.

CPT-I is involved in lipid metabolism in several tissues, most importantly the liver. There, LCFAs are broken down and ketone bodies are produced. Like lipids and glucose, ketone bodies are used by the body as fuel, especially in the brain and muscles. Deficiency of CPT-I in the liver results in decreased levels of ketone bodies (hypoketosis), as well as low blood-sugar levels (hypoglycemia). Hypoketosis combined with hypoglycemia in a child can lead to weakness, seizures, and coma. Symptoms can be reversed by glucose infusions, as well as supplementation with medium-chain fatty acids, which do not require CPT-I to produce energy.

As noted, glucose and fatty acids are important energy sources for the body. During exercise, the muscles initially use glucose as their primary fuel. After some time, however, glucose is depleted and the muscles switch to using fatty acids by a chemical process called oxidation. CPT-II deficiency results in a decrease in LCFAs that can be used by the mitochondria, and the muscles eventually exhaust their energy supply. This explains why prolonged exercise may cause an attack of muscle fatigue, stiffness, and pain in people with CPT-II deficiency. The ability to exercise for short periods is not affected. Infections, stress, muscle trauma, and exposure to cold also put extra demands on the muscles and can trigger an attack. Fasting, or a diet high in fats and low in carbohydrates (complex sugars), deplete glucose reserves in the muscles and are risk factors as well.

In some cases, CPT deficiency results in the breakdown of muscle tissue, a process called rhabdomyolysis, and it causes some components of muscle cells to "leak" into the bloodstream. Myoglobin, the muscle-cell equivalent of hemoglobin in the blood, is one of these components. Myoglobin is filtered from the blood by the kidneys and deposited in the urine, causing myoglobinuria. Dark-colored urine is the typical sign of myoglobinuria. Severe and/or repeated episodes of rhabdomyolysis and myoglobinuria can cause serious kidney damage.

Genetic profile

CPT-I deficiency is caused by defects in the CPT1 gene located on chromosome 11. CPT-II deficiency results from mutations in the CPT2 gene on chromosome 1.

Both CPT-I and CPT-II deficiency are considered autosomal recessive conditions. This means that both parents of an affected person carry one defective CPT gene, but also have a normal gene of that pair. Carriers of a single recessive gene typically do not express the deficiency because the second normal functioning gene, is able to compensate. A person with two mutated genes has no normal gene to make up for the deficiency, and thus expresses the disease. Parents who are both carriers for the same autosomal recessive condition face a 25% chance in each pregnancy that they will both pass on the defective gene and have an affected child.

Several individuals proven to be carriers of CPT-II deficiency have had mild symptoms of the disorder. Measurement of CPT-II enzyme levels (the protein coded for by CPT2) in most of the carriers tested show lower levels, as would be expected when one gene is mutated and the other is not. It is not yet clear why some carriers show mild symptoms, but this phenomenon occasionally occurs in other autosomal recessive conditions.

Demographics

CPT-I deficiency is rare, with fewer than 15 cases having been reported. CPT-II deficiency is more common, but its true occurrence is unknown. Muscle CPT-II deficiency makes up the majority of cases that have been reported; liver and multiorgan CPT-II deficiency are both quite rare. There seems to be no geographic area or ethnic group that is at greater risk for either type of CPT deficiency.

Approximately equal numbers of males and females with CPT-I deficiency have been seen, which is typical of autosomal recessive inheritance . However, about 80% of those individuals diagnosed with CPT-II deficiency are male. Males and females do have an equal likelihood of inheriting a defective CPT2 gene from a parent, but effects of the gene in each sex can be different. Hormonal differences between males and females may have some effect—a clue being the tendency of an affected woman to have more symptoms while pregnant.

Signs and symptoms

CPT-I deficiency

The CPT-I enzyme has two forms, coded for by different genes. CPT-IA is the form present in liver, skin, kidney, and heart cells, while CPT-IB functions in skeletal muscle, heart, fat, and testis cells. CPT-I deficiency refers to the CPT-IA form since a defective CPT-IB enzyme has not yet been described in humans. CPT-I deficiency has always been diagnosed in infants or children.

The brain and muscles use ketone bodies as a source of energy. The brain especially, relies heavily on ketone bodies for energy during times of stress, such as after fasting when low sugar levels (hypoglycemia) occur. In fact, children with CPT-I deficiency are usually first diagnosed after they have fasted due to an illness or diarrhea. Hypoketosis and hypoglycemia in CPT-I deficiency can become severe, and result in lethargy (lack of physical energy), seizures, and coma.

CPT-II deficiency

CPT-II deficiency is divided into three subtypes. "Muscle CPT deficiency" is the most common form of the condition. Onset of symptoms is usually in adolescence or adulthood, but varies. "Hepatic CPT-II deficiency" is rare and is diagnosed in childhood. The remaining cases are classified as "Multiorgan CPT-II deficiency," and have been diagnosed in infants. Differences in the severity of symptoms between the groups, as well as within each group, are due in part to different mutations in the CPT2 gene. Environmental factors may assist the triggering of attacks and thus may contribute to the variety of observed symptoms.

MUSCLE CPT DEFICIENCY Muscle fatigue, pain, and stiffness are typically caused by prolonged exercise or exertion. Other possible triggers include fasting, infection, muscle injury, exposure to cold, and even emotional stress. Cases of adverse reactions to certain types of general anesthesia have also been reported.

These muscle "attacks" after a triggering event are the classic physical signs of muscle CPT-II deficiency. When an attack is associated with the breakdown of muscle tissue (rhabdomyolysis), myoglobinuria is the other classic sign. Unlike other metabolic myopathies, there are no obvious signs of an impending attack, and resting will not stop the symptoms once they have begun. Muscle symptoms may begin during or up to several hours after prolonged exercise or other triggering events. A specific muscle group may be affected, or generalized symptoms may occur. Muscle weakness between attacks is not a problem, unlike some other metabolic myopathies. In addition, muscle cells examined under the microscope typically appear normal. Some people with muscle CPT deficiency have only had a few attacks in their lifetime, while others may experience several attacks per week. Renal failure due to repeated episodes of myoglobinuria occurs in about 25% of individuals with muscle CPT deficiency.

HEPATIC CPT-II DEFICIENCY Symptoms and age of onset in hepatic CPT-II deficiency are similar to CPT-I deficiency, primarily, coma and seizures associated with hypoketotic hypoglycemia. However, unlike CPT-I deficiency, most infants with liver CPT-II deficiency have had heart problems and have died.

MULTIORGAN CPT-II DEFICIENCY This type of CPTII deficiency has only been reported a few times and involves the liver, skeletal muscles and heart. Infants with this type have all died.

Diagnosis

The symptoms of CPT-I deficiency can be dramatic, but the rare nature of the disease means that some time may elapse while other more common diseases are ruled out. Definitive diagnosis of CPT-I deficiency is made by measuring the activity of the CPT enzyme in fibroblasts, leukocytes, or muscle tissue. Abnormal results on several blood tests are also typical of CPT-I deficiency, but the most important finding is hypoketotic hypoglycemia. Analysis of the CPT1 gene on chromosome 11 may be possible, but is not yet considered a diagnostic test.

CPT-II deficiency is somewhat more common than CPT-I deficiency. However, the milder symptoms of muscle CPT deficiency and their similarity to other diseases often leads to a wrong diagnosis (misdiagnosis). For example, the symptoms of CPT-II deficiency are sometimes initially diagnosed as fibromyalgia or chronic fatigue syndrome. Misdiagnosis is a special concern for people with muscle CPT-II deficiency, since the use of available preventive measures and treatment are then delayed.

Analysis of the CPT-II enzyme levels can confirm the diagnosis, but must be done carefully if performed on any tissue other than a muscle specimen. Direct testing of the CPT2 gene is available and is probably the easiest method (simple blood sample) of making the diagnosis. If genetic testing shows two mutated CPT2 genes, the diagnosis is confirmed. However, not all disease-causing mutations in the gene have been discovered, so demonstration of only one mutated CPT2 gene, or a completely negative test, does not exclude the diagnosis. In those individuals in whom genetic testing is not definitive, the combination of clinical symptoms and a laboratory finding of low levels of CPT-II enzyme activity should be enough to confirm the diagnosis.

Treatment and management

While CPT-I and CPT-II deficiency differ in their typical age of onset and in the severity of the symptoms, treatment of both conditions is similar. Attacks may be prevented by avoiding those situations that lead to them, as noted above. Someone undergoing surgery should discuss the possibility of alternative anesthetics with their doctor. Most people with CPT deficiency find it necessary to carry or wear some type of identifying information about their condition such as a Medic-Alert bracelet.

Those who find that they cannot avoid a situation known to be a trigger for them should try to supplement their diet with carbohydrates. Since medium-chain fatty acids to not require carnitine to enter the mitochondrion, use of a dietary supplement containing them results in significant improvement in people with CPT-I deficiency and also helps prevent attacks in most people with CPTII deficiency. The use of carnitine supplements (vitamin B₇) is also helpful for some individuals diagnosed with the deficiency.

Anyone diagnosed with CPT deficiency, or anyone concerned about a family history of CPT deficiency, should be offered genetic counseling to discuss the most up-to-date treatment and testing options available to them.

Prognosis

Children with CPT-I deficiency improve significantly with treatment. So far, however, all have had some lasting neurological problems, possibly caused by damage to the brain during their first attack. The outlook at this point for infants and children with liver and multiorgan CPT-II deficiency is still poor.

Once a person with muscle CPT-II deficiency is correctly diagnosed, the prognosis is good. While it is impossible for many patients to completely avoid attacks, most people with the condition eventually find the right mix of preventive measures and treatments. CPT-II deficiency then has much less of a harmful impact on their lives. A number of excellent sources of information are available for families affected by CPT deficiency. Any new treatments in the future would likely attempt to directly address the enzyme deficiency, so that normal metabolism of lipids might occur.

Resources

ORGANIZATIONS

Fatty Oxidation Disorders (FOD) Family Support Group. Deb Lee Gould, MEd, Director, FOD Family Support Group, MCAD Parent and Grief Consultant, 805 Montrose Dr., Greensboro, NC 24710. (336) 547-8682. <http://www.fodsupport.org>.

Genetic Alliance. 4301 Connecticut Ave. NW, #404, Washington, DC 20008-2304. (800) 336-GENE (Helpline) or (202) 966-5557. Fax: (888) 394-3937 info @geneticalliance. <http://www.geneticalliance.org>.

March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. (888) 663-4637. [email protected]. <http://www.modimes.org>.

National Organization for Rare Disorders (NORD). PO Box 8923, New Fairfield, CT 06812-8923. (203) 746-6518 or (800) 999-6673. Fax: (203) 746-6481. <http://www.rarediseases.org>.

National Society of Genetic Counselors. 233 Canterbury Dr., Wallingford, PA 19086-6617. (610) 872-1192. <http://www.nsgc.org/GeneticCounselingYou.asp>.

United Mitochondrial Disease Foundation. PO Box 1151, Monroeville, PA 15146-1151. (412) 793-8077. Fax: (412) 793-6477. <http://www.umdf.org>.