Clinical manifestations

Presentation and course

Metabolic decompensation occurs during periods of catabolism when fatty acid oxidation provides crucial energy. Most frequently, the first episode of metabolic decompensation occurs within the first hours to days of life because this is a period of metabolic adaptation when lipids are an essential metabolic fuel and can recur during later episodes of fasting or infection. The organs most affected are those that depend strongly on energy from fat oxidation, particularly heart, muscle, and liver (19). Classic features include hypoketotic hypoglycemia, cardiomyopathy, cardiac dysrhythmias, hyperammonemia, liver dysfunction, and skeletal muscle damage. Less specific features commonly include variable degrees of encephalopathy ranging from lethargy to coma, poor feeding, hypotonia, seizures, hypothermia, pallor, respiratory distress, and cardiorespiratory collapse. This presentation is strongly suggestive of a disorder of long-chain fatty acid oxidation and is not unique to carnitine-acylcarnitine translocase deficiency, but carnitine-acylcarnitine translocase deficiency and carnitine palmitoyltransferase type 2 deficiency in particular should be suspected when onset occurs in the first hours of life (24). There may be history of a sibling who died suddenly in the neonatal period or in early infancy. Metabolic testing typically reveals low levels of free carnitine and elevated long-chain acylcarnitine esters. Variable and transient dicarboxylic aciduria may also be present.

Prognosis and complications

After the initial acute episode of metabolic decompensation, patients are placed on a low-fat/high-carbohydrate diet supplemented with medium chain triglycerides (which do not require carnitine to enter the mitochondria), frequent meals, and avoidance of fasting. Although the patients who survive the initial decompensation can improve, they are highly vulnerable to fasting or illness that can lead to acute metabolic decompensation with the subsequent risk of a permanent neurologic impairment or sudden death.

Despite treatment, the outcome may be poor, and patients continue to have recurrent episodes of deterioration precipitated by fasting or by intercurrent febrile illnesses (13). In between episodes, the infants are often normal, though they may show muscle hypotonia, cardiac hypertrophy/cardiomyopathy, chronic liver dysfunction, and hyperammonemia. Approximately 65% of the patients reported died within the first year of life. The main cause of death is cardiomyopathy or lethal arrhythmia, often occurring in the setting of an intercurrent febrile illness. Sudden death may also occur.

It is generally accepted that the accumulating experience in diagnosis and management of these disorders, the timelier diagnosis, and the institution of better acute and chronic medical treatment has contributed to the increasing number of patients with milder phenotypes and longer-term survival. In a review, outcome seemed to correlate better with the absence of cardiac disease and higher long-chain fatty acid oxidation rate in cultured fibroblasts than with residual enzyme activity (22).

Clinical heterogeneity is also becoming more evident. An exceptional case with a hepatocerebral form resembling mitochondrial depletion syndrome has been described (24), another patient with dysplastic brain lesions (07), and another with retinal changes have been reported (22).

Clinical vignette

A male newborn appeared well at delivery. He had an acute life-threatening event at 36 hours of age with a brief seizure and cardiorespiratory collapse. The collapse was thought to be triggered by fasting stress associated with the initiation of breast-feeding. The infant had hypoglycemia and hyperammonemia up to 400 µmol/L. Following resuscitation with intravenous fluids and glucose, the infant had premature ventricular contractions and ventricular tachycardia for several days. Physical examination was normal, although moderate hepatomegaly was noted with later episodes. Plasma carnitine determinations showed reduced total carnitine concentrations with nearly 100% as acylcarnitines and unusually elevated long-chain acylcarnitine levels.

During the 3 years that the child survived, he had recurrent episodes of hypoketotic hypoglycemic coma when fasting for greater than 10 to 12 hours. Plasma ammonia remained slightly elevated. Mental development appeared to be normal, but he had chronic generalized muscle weakness that limited walking to a few steps. When otherwise well, echocardiography revealed no evidence of cardiomyopathy. He was managed with continuous nasogastric tube feedings in order to limit exposure to fasting and because he had gastroesophageal reflux. Ultimately, the child died of progressive liver failure combined with increasing weakness and respiratory insufficiency (18).

Biological basis

Etiology and pathogenesis

The basis for this disease is deficient activity of the mitochondrial inner membrane carrier protein, carnitine-acylcarnitine translocase (CACT). Studies have provided more detail on the in silico membrane dynamics (15).

Carnitine-acylcarnitine translocase is a component of the carnitine shuttle system. Mitochondrial fatty acid oxidation provides crucial energy during fasting, and there are separate enzymes for metabolism of short chain (4 to 5 carbon), medium chain (6 to 10 carbon) and long chain (12 to 18 carbon) fats. Although short and medium chain length fatty acids can penetrate the mitochondria directly, long chain fatty acids required a shuttle system to enter the mitochondria.

The carnitine shuttle system consists of a series of four steps: 1) the long chain fatty acid is “activated” by esterification to coenzyme A, creating a long chain fatty acylCoA; 2) the enzyme carnitine palmitoyl transferase 1 (CPT1) exchanges the CoA for carnitine, creating an acylcarnitine, which can cross the outer mitochondrial membrane; 3) the enzyme carnitine-acylcarnitine translocase (CACT) transports (translocates) the acylcarnitine across the inner mitochondrial membrane (and discharges a free carnitine back into the cytoplasm); and 4) the enzyme carnitine palmitoyl transferase 2 (CPT2) exchanges the carnitine for a coenzyme A, at which point oxidation of the long chain fat can commence (19). For a detailed description of the carnitine shuttle system, more information can be found in the article published by Stanley and colleagues (19).

Mitochondrial oxidation of fatty acids provides an essential source of energy for the heart, for exercising skeletal muscle, and during long-term fasting (longer than 15 to 20 hours in infants; longer than 24 to 36 hours in adults) (17). Fatty acids are the preferred substrate for the heart. They provide more than 60% of the energy consumed by muscle working aerobically. In late stages of fasting, fatty acid oxidation accounts for over 80% of oxygen consumption. Thus, cardiomyopathy, arrhythmias, and cardiovascular collapse are common in infants with severe carnitine-acylcarnitine translocase deficiency, often within the first days of life. The end products of hepatic fatty acid oxidation are the ketones, beta-hydroxybutyrate and acetoacetate, which are exported to be used by the brain and other organs. Because the long-chain fatty acids do not cross the blood-brain barrier, hepatic ketogenesis serves an essential function in permitting the brain to indirectly use body fat stores and, thus, spare glucose consumption.

The acute clinical manifestations in patients with the carnitine-acylcarnitine translocase deficiency reflect the disruption of fasting adaptation caused by a very severe block in the fatty acid oxidation pathway (18). Unfortunately, failure of fatty acid oxidation has secondary effects on other metabolic processes. In particular, fatty acid oxidation provides the energy needed to initiate gluconeogenesis at a time when glucose utilization is increased, thus, hypoketotic hypoglycemia ensues, worsening the energy crisis. The presentation can mimic Reye syndrome, with acute disruption of liver function including elevated serum transaminases, hyperuricemia, and impaired synthesis of clotting factors; there is also a direct inhibition of the urea cycle leading to hyperammonemia (19). The failure of fuel supply for the brain leads to lethargy, coma, or seizures and may result in permanent brain injury. The illness induced by fasting can be associated with acute cardiac failure or dysrhythmias and acute evidence of skeletal muscle weakness or rhabdomyolysis. It is likely that the accumulation of long-chain acylcarnitines contributes to poor myocardial function and inhibits ATP formation (19).

Chronic effects of impaired fatty acid oxidation are more prominent in the reported cases of carnitine-acylcarnitine translocase deficiency compared to most of the other inborn errors of mitochondrial fatty acid oxidation. As in the acute presentations, the organs most affected tend to be those most dependent on fatty acid oxidation, mostly heart, skeletal muscle, and liver (19). Skeletal weakness, persistent liver dysfunction with hyperammonemia and progressive liver failure, and cardiomyopathies and dysrhythmias have been noted (18).

The physical examination is typically nonspecific. Patients with genetic defects in fatty acid oxidation may show increased neutral fat deposits in liver or skeletal muscle, and transient hepatomegaly can be seen. In several cases of carnitine-acylcarnitine translocase deficiency examined postmortem, increased fat deposits were detected in liver, heart, and kidneys, and minimal or no fatty changes were noted in skeletal muscle (03); iron deposition has been reported in the liver and heart (02).

Although milder phenotypes have been associated with higher residual enzyme activities, no clear correlation has been found between the clinical phenotype and the enzyme defect (19). Generally, the residual activity ranges from less than 1% to 7%. However, some correlation has been observed between the clinical phenotype and the impairment of fatty acid oxidation. Although severely affected patients have residual palmitate oxidation of less than 5%, milder patients have 27% (09).

Common to all of the genetic defects in mitochondrial fatty acid oxidation identified to date, carnitine-acylcarnitine translocase deficiency is inherited in an autosomal recessive fashion. Intermediate levels of translocase activity can be demonstrated in cultured fibroblasts from parents, but heterozygotes appear to be asymptomatic (18).

The cDNA encoding rat carnitine-acylcarnitine translocase deficiency was cloned in 1997 (10), and the human homolog was cloned shortly thereafter (06). The carnitine-acylcarnitine translocase deficiency gene is composed of nine exons and spans about 16.5 kb along chromosome 3p21 (08). Sequencing of the gene encoding carnitine-acylcarnitine translocase (SCL25A20) in patients with carnitine-acylcarnitine translocase deficiency has shown a heterogenous range of mutations (13). The c.199-10T>G splice site variant appears to be most common (24), whereas the c.82G>T variant common in those of Pakistani heritage appears to confer an attenuated phenotype. In Asian patients, a common mutation (IVS2-10T> G) has been detected that is thought to be a founder mutation (21; 25).

Epidemiology

Carnitine-acylcarnitine translocase deficiency is a rare pan-ethnic disorder. Incidence has been reported to be about 1:750000 to 1:2000000 and increases to 1:60000 in Hong Kong (25). It accounts for 7% of the fatty acid oxidation disorders in the French population (01) and 33% in the Asian population (05).

Prevention

Early recognition and treatment are crucial in this disease. In this respect, patients will greatly benefit from newborn screening programs that include fatty acid oxidation disorders.

Differential diagnosis

Associated or underlying disorders

The clinical manifestations of carnitine-acylcarnitine translocase deficiency are similar to those seen in other acute, life-threatening events that may occur in newborn infants. Thus, the disorder in affected infants may be confused with a wide variety of other disorders including other metabolic errors, infections, congenital malformations, and other severe perinatal insults. Hyperammonemia may lead to confusion with urea cycle defects, and cardiac failure may lead to confusion with congenital cardiomyopathy or dysrhythmia. Evidence of multiorgan involvement of liver, heart, and skeletal muscle, and, especially, hypoketotic hypoglycemia, are important clues pointing to a block in the fatty acid oxidation pathway. However, in newborns the absence of ketosis even during hypoglycemia is not a specific finding, as the high metabolic rate of the newborn tends to utilize ketones quickly such that absence of ketosis is typical, and significant ketosis in newborns is suspicious for other inborn errors of metabolism.

Diagnostic workup

The presence of carnitine-acylcarnitine translocase deficiency or other defects in fatty acid oxidation is most easily suspected by abnormalities found during an acute attack of illness, as abnormal metabolites associated with metabolic stress are less prominent in the convalescent state (17; 20). Suspicious findings include pervasive hypoglycemia (particularly with hypoketosis), cardiovascular collapse, or multisystemic dysfunction involving liver, heart, and muscle. Extra samples of serum or plasma and urine should be collected during the acute episode for special testing as described below.

Once a fatty acid oxidation defect is suspected, several avenues need to be pursued to determine which pathway is involved and to identify the specific site of defect. Standard metabolic testing includes plasma/urine carnitine, plasma acylcarnitine analysis (by tandem mass spectrometry), urine organic acids, plasma amino acids, ammonia, liver transaminases and other functional testing, plasma/urine ketones, blood gases, anion gap, bicarbonate, and others as indicated. The most specific findings for carnitine-acylcarnitine deficiency are most likely to be found on assays of plasma or tissue carnitine concentrations and acylcarnitine profiles, urinary organic acid profile, and the use of cultured fibroblasts or lymphoblasts to test overall fatty acid oxidation capacity and for assays of specific steps in the pathway.

Carnitine. Measurement of plasma carnitine is useful in suspected fatty acid oxidation defects because nearly all are associated with abnormal levels (18). In carnitine-acylcarnitine translocase deficiency, plasma total carnitine levels are reduced to 5 to 30 µmol/L (10% to 50% of normal, but can be within the low normal range), whereas esterified carnitine levels esters are elevated to as much as 90% of the total (normal is less than 10%), thus, the ratio of esterified to free carnitine is high (18). Before carnitine supplementation, in carnitine uptake disorders plasma carnitine will be low but urine carnitine will be high, differentiating these disorders from carnitine-acylcarnitine translocase deficiency.

Plasma acylcarnitine. Fast atom bombardment-mass spectrometry (tandem mass spectrometry) can quantify the acylcarnitines that have built up in fatty acid oxidation disorders (as well as in some other inborn errors of metabolism). In carnitine-acylcarnitine translocase deficiency (and CPT2 deficiency), the analysis usually shows increased long-chain acylcarnitines (C16, C16:1, C18:1, and C18:2). When mitochondrial beta oxidation is inadequate to meet energy needs, omega oxidation will be augmented, resulting in dicarboxylic acids, which are also detectable on acylcarnitine analysis.

Urinary organic acids. Urine organic acid analysis is generally less sensitive and specific than acylcarnitine profiles but can help to identify metabolites associated with fatty acid oxidation disorders, particularly in the acute state. It is also helpful in working through the differential diagnosis by identifying metabolites of other metabolic disorders. The limited information on carnitine-acylcarnitine translocase deficiency suggests that, at times of acute illness, the urinary organic acid profile shows only nonspecific evidence of impaired fatty acid oxidation, such as low ketones and increased medium-chain dicarboxylic acid levels (most commonly adipic, suberic and sebacic acids).

Plasma amino acid analysis. Analysis of amino acids is not helpful in the diagnosis of carnitine-acylcarnitine translocase deficiency but can be useful to rule out other disorders (for example, a urea cycle defect causing hyperammonemia).

Carnitine-acylcarnitine deficiency and carnitine palmitoyl transferase type 2 deficiency are subsequent steps in the carnitine cycle, and both result in accumulation of long-chain acylcarnitines with similar metabolic profiles. Thus, additional testing is required to confirm the diagnosis. This can include molecular analysis of the SLC25A20 or specific assay of carnitine-acylcarnitine translocase activity using cultured skin fibroblasts or lymphoblasts (19). To date, at least 36 mutations have now been identified. More information can be found on the following site:www.hgmd.org.

Prenatal diagnosis has been reported on several occasions: by in vitro beta-oxidation analysis in cultured amniocytes and in chorionic villi by assaying carnitine-acylcarnitine translocase deficiency activity in chorionic villi and by mutation analysis in amniocytes and chorionic villi.

Management

Management of carnitine-acylcarnitine translocase deficiency is aimed at providing sufficient glucose to minimize the demand for fatty acid oxidation (19). During acute episodes of illness, intravenous glucose (plus insulin, if needed) should be given at rates sufficient to suppress lipolysis and subsequent fatty acid oxidation, typically at 8 to 10 mg/kg/min for infants and children and 4 to 5 mg/kg/min for adults. Glucose infusion at these rates should be administered regardless of whether hypoglycemia is present or not, as some cases lethal metabolic crisis can occur without apparent hypoglycemia. Whenever possible, alternate energy source with medium chain triglyceride should be provided. Triheptanoin (a 7-carbon medium-chain fat) has been reported to be helpful in acute or chronic therapy (23; 12; 14; 16). The hyperammonemia is not associated with elevated glutamine levels, and response to traditional ammonia scavengers has been disappointing (16). At present, specific crisis management protocols have not been written for this rare defect, but protocols appropriate for other long chain fatty acid oxidation disorders are generally appropriate (for example, https://www.newenglandconsortium.org/vlcadd).

Long-term treatment is aimed at limiting fasting stress with frequent meals and often continuous overnight feeding. The diet, as in other long-chain fatty acid oxidation disorders, consists of a low fat/high carbohydrate diet. Long-chain fatty acids are restricted, and medium chain triglycerides and essential fatty acids are provided. Infants can be fed a formula high in medium chain triglycerides. Skimmed breast milk has been reported to be successful in the treatment of an infant with carnitine-acylcarnitine translocase deficiency (11).

Carnitine supplementation for carnitine-acylcarnitine translocase deficiency and other disorders of long-chain fatty acid oxidation is controversial. Because of evidence that long-chain acylcarnitines can have adverse effects on cardiac arrhythmias, there is reason for concern that carnitine treatment might have adverse consequences. However, carnitine supplementation has been found to increase free carnitine and acetylcarnitine formation and to decrease long-chain acylcarnitines (07). Several patients with carnitine-acylcarnitine translocase deficiency have been supplemented with carnitine and have shown a favorable course, in some instances as high as up to 200 to 300 mg/kg per day (07; 03).

Special considerations

Pregnancy

A case of preeclampsia and recurrent early miscarriages in a carrier mother was reported. The authors hypothesized that lack of energy supply in the placenta and fetus with a defect in the beta-oxidation may be the underlying pathophysiological mechanism of these complications during pregnancy (04).