Clinical manifestations

Presentation and course

Patients with CPT1A deficiency present with fasting-induced episodes of life-threatening hypoketotic hypoglycemia and hepatic encephalopathy similar to that seen in Reye syndrome. The presentation is indistinguishable from that of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, a much more common defect of fatty acid oxidation (31; 21; 36; 28; 34), and the disorders of ketone body formation, 3-hydroxy-3 methylglutaryl (HMG)-CoA synthetase and HMG-CoA lyase deficiencies. The initial presentation is typically after the age of 3 to 6 month, when infants are starting to sleep through the night and are being fed less frequently. Presentation in the newborn period is also possible when breast feeding is insufficiently established and the flow of milk is not providing enough calories. Typically, there is a precipitating factor, such as a gastrointestinal illness with vomiting. In the situation where the disease has not already been identified through newborn screening or prior presentation, this prodromal illness is not typically regarded as an urgent medical condition. However, following the prodromal period, the progression of signs and symptoms due to CPT1A deficiency can be rapid, including lethargy and feeding difficulty that can lead precipitously to hypoglycemic seizures, coma, and death if untreated.

As children become older, fasting tolerance improves. But there remains a risk of fasting-induced catabolic crisis throughout life, including adulthood. Consistent with the tissue distribution of CPT1A, affected individuals can have acute renal tubular acidosis, encephalopathy, and hypoglycemia (15; 05; 10). Unlike other disorders of long-chain fatty acid oxidation, there are no progressive cardiac or skeletal muscle manifestations in CPT1A deficiency.

Physical examination during periods of metabolic decompensation may reveal hepatomegaly due to the accumulation of lipid in the liver. Laboratory investigation will reveal severe hypoglycemia (glucose may be essentially undetectable). There can also be evidence of hepatic dysfunction, including mild hyperammonemia (blood ammonium levels up to several hundreds of µmol/L) and moderately elevated AST and ALT levels (3 to 10 times normal). With a prolonged acute illness, a coagulopathy reflecting loss of hepatic synthetic function may develop.

Measurement of blood ketones will demonstrate hypoketonemia, and urinalysis will demonstrate either lack of ketones or inappropriately mild ketonuria. Subsequent testing should include analysis of urine organic acids, an acylcarnitine profile, and total and free blood carnitine levels. These tests will allow the investigator to distinguish CPT1A deficiency from MCAD and HMG-CoA lyase deficiency. MCAD and HMG-CoA lyase deficiencies are characterized by unique acylcarnitine profiles and the presence of pathognomic biomarkers on organic acid analysis. CPT1A deficiency does not have a specific metabolic biomarker and is often a presumed diagnosis after exclusion of MCAD and HMG-CoA lyase deficiencies. Similarly, HMG-CoA synthase deficiency is not readily distinguished because there are no specific biomarkers for this rare disorder.

A liver biopsy taken at the time of decompensation will demonstrate lipid accumulation in hepatocytes, which on microscopic analysis reveals both micro- and macrovesicular steatosis. This pathological feature is reversed on therapy with intravenous glucose as the hypoglycemia is corrected and fatty acid oxidative flux is switched off. A muscle biopsy has shown the prominent muscle fat accumulation during the period of decompensation (02).

Prognosis and complications

Patients with CPT1A deficiency may suffer permanent brain injury following prolonged attacks of acute illness. Prompt recognition and avoidance of potentially profound hypoglycemia can result in better outcome. Furthermore, 2 patients with CPT1A deficiency have been described with proximal renal tubular acidosis requiring chronic bicarbonate therapy (15; 05). With avoidance of fasting during an intercurrent illness, the long-term prognosis in CPT1A deficiency is very good.

Clinical vignette

A 10-month-old boy presented to the emergency department with a history of new-onset seizures. Prior to this illness, the child had been healthy and was growing and developing normally. The patient had been well until 2 days prior to presentation, when he had a low-grade fever of 101°F associated with fussy feeding and reduced oral intake. On the day prior to admission, he went to sleep at his normal time in the evening. When his mother went to wake him the next morning, she found him unresponsive, sweaty, and pale. He was rushed to the ER, where he was found to have an undetectably low glucose by dextrose stick. He was immediately given intravenous glucose, which resulted in increased responsiveness.

Physical examination was significant for an enlarged liver. Laboratory evaluation demonstrated an ammonium level of 100 µmol/L (normal less than 50), AST of 450 U/L, PT of 45.6 seconds, and PTT of 76.2 seconds. A trace of ketones was detected on urinalysis. Analysis of acylcarnitines revealed no abnormal acylcarnitine species, but the laboratory commented on the low acylated fraction. The total acylcarnitine level was elevated to 75 µmol/L (reference interval 25-55) with a free carnitine level of 73 µmol/L. The child eventually made a full recovery.

The diagnosis of CPT1A deficiency was subsequently confirmed by enzyme activity measurement in skin fibroblasts and molecular analysis of the CPT1A gene. He has done well on a normal, unrestricted diet. The family was instructed to avoid fasting by providing regular feedings and to notice the onset of fasting signs and symptoms. They were instructed to bring him directly to the emergency room if these signs and symptoms occurred.

Biological basis

Etiology and pathogenesis

This rare defect of mitochondrial fatty acid oxidation and ketone body synthesis has conclusively been shown to arise from genetic mutations in the CPT1A gene on chromosome 11q13.1. The disorder is inherited in an autosomal recessive mode, and carriers appear to be unaffected. The first mutations in the CPT1A gene were reported in 1998 (22). A number of disease-causing mutations have now been described, mostly in symptomatic patients (09; 03; 25; 16; 29; 04; 10; 02; 13).

CPT1A encodes the CPT1A protein, an outer mitochondrial membrane bound protein that is the rate-limiting step for transport of fatty acids into the mitochondria for eventual oxidation and, in liver, for ketone body formation. During the latter stages of fasting after glycogen reserves are depleted, fatty acids are released from lipid stores and enter the circulation as nonesterified fatty acids. They are taken up by certain peripheral tissues and transported to the mitochondria. CPT1A regulates transport of these fatty acids into the mitochondrial matrix. In the absence of CPT1A this process cannot take place in the liver or kidneys. In the liver, the process of ketogenesis is impaired. Because ketone bodies are not generated, an alternative energy source for tissues without fatty acid oxidative capability, including the brain, is lost. Thus, although there is normal mobilization of fatty acids, there is no production of ketone bodies. Any remaining available glucose is rapidly depleted, and this results in profound hypoglycemia with concurrent hypoketosis. Failure to generate acetyl-CoA, the end product of fatty acid oxidation, impairs gluconeogenesis, further depleting available glucose. As mobilized lipid accumulates in tissues, it is esterified as triglyceride and causes steatosis. Energy supplies to the kidney are also diminished in CPT1A deficiency, and this leads to renal tubular acidosis.

Biochemistry of CPT1A. The mitochondrial beta-oxidation of long-chain fatty acids provides an important source of energy, primarily during prolonged fasts of greater than 12 to 15 hours' duration (31). In this late stage of fasting adaptation, glycogen reserves have been depleted, and gluconeogenesis occurs through breakdown of tissue protein. Yet irrespective of fasting status, fatty acids account for 80% or more of oxygen consumption in high-energy tissues, such as cardiac and skeletal muscle. During lipolysis, stored triglycerides are hydrolyzed by intracellular lipases and released into the circulation bound to albumin as nonesterified fatty acids. They are taken up and utilized by peripheral tissues such as liver, kidney, heart, and muscle. Because long-chain fatty acids do not cross the blood-brain barrier, the brain is unable to utilize fatty acids directly. However, the brain readily uses the ketones beta-hydroxybutyrate and acetoacetate, the end-products of hepatic fatty acid oxidation and ketogenesis. Thus, hepatic ketone synthesis provides an important mechanism to reduce glucose consumption by the brain during prolonged fasting and limits the need for prolonged tissue protein breakdown.

The pathway of mitochondrial long-chain fatty acid oxidation requires over a dozen enzymatic steps and components of the respiratory chain. These steps can be divided into (1) the carnitine cycle, which transports the fatty acids across the barrier of the mitochondrial membrane and activates them to the acyl-CoAs; (2) the beta-oxidation cycle, which sequentially cleaves 2-carbon acetyl-CoA units from the long-chain acyl-CoAs until only acetyl-CoA remains; (3) electron transfer flavoproteins, which transfer energy equivalents from the reduced acyl-CoA to coenzyme Q, and a mechanism to transfer reduced NADH through complex I of the respiratory chain; and (4) a specific 3-hydroxy-3-methylglutaryl-CoA pathway in the liver, which converts acetyl-CoA to acetoacetate and 3-hydroxybutyrate.

Medium-chain fatty acids, in the form of medium-chain triglycerides, can enter mitochondria independently of the carnitine cycle and can be oxidized directly by medium-chain specific beta-oxidation enzymes. This pathway provides a useful mechanism for providing fatty acid energy in long-chain fatty acid oxidation defects. It is a potentially useful treatment for CPT1A deficiency, although most patients appear to do well by simply preventing prolonged fasting with regular and frequent carbohydrate supplementation.

CPT1 is the major site of regulation of mitochondrial long-chain fatty acid oxidation. The enzyme activity is inhibited through specific binding by malonyl-CoA, a metabolite that increases inside cells following a meal (14). When CPT1 is inhibited by high levels of malonyl-CoA, the malonyl-CoA serves as the starting substrate for fatty acid biosynthesis. When fasting, the intracellular level of malonyl-CoA falls, inhibition of CPT1 is removed, and fatty acids are transported into the mitochondrion. The kinetics of CPT1 inhibition by malonyl-CoA are different for hepatic CPT1A isoenzyme compared to the heart and muscle CPT1B isoenzymes. For example, CPT1B remains activated when the cellular levels of malonyl-CoA are equivalent to the fed state. Consequently, the enzyme is not activated by fasting, but it is activated by increased skeletal muscular energy demand and normal cardiac demand even in the fed state when there are ample carbohydrate energy supplies available.

In response to prolonged periods of fasting or increased energy demand, long-chain fatty acid oxidation is activated. Hepatic CPT1A represents the rate-limiting step in this response to fasting. Under normal circumstances in a well-fed state or in the early phase of fasting, malonyl-CoA levels are high, CPT1A is inhibited, the flux through the pathway is very low, and the production of ketone bodies is unmeasurable. Under these conditions, individuals with CPT1A deficiency do not manifest any signs or symptoms. However, signs and symptoms can present during episodes of fasting stress, including intercurrent gastrointestinal disease, where calories are lost through vomiting or diarrhea, and febrile illnesses, where ingested calories are inadequate due to refusal to eat or increased metabolic rate from fever.

In these scenarios glycogen reserves are depleted, and there is a switch to a fasting metabolism. After approximately 12 hours of fasting in an infant or young child, lipids are mobilized from fat stores and are targeted to the liver for the production of ketone bodies to provide energy for essential non-fatty acid oxidizing tissue, such as the brain. In CPT1A deficiency, the lipids that are targeted to the liver cannot be transported into the mitochondria for oxidation, and there is a complete loss of this essential pathway. Long-chain acyl-CoA species accumulate in hepatocytes; they are converted to triglycerides, subsequently accumulate in the liver, and cause steatosis and hepatic dysfunction. Lack of ketone body production results in insufficient nutrient supply for brain metabolism. The brain rapidly consumes any remaining glucose. If untreated, insufficient energy supply for the brain results in coma and potential death of the patient. If the patient survives, recovery from the profound hypoglycemia is likely to result in long-term neurologic damage.

CPT1A is also expressed in the kidney where fatty acids are a direct fuel source. The loss of renal energy production may be associated with renal tubular acidosis. Finally, there may also be toxic effects of the high levels of circulating fatty acids released during the lipolytic process.

Epidemiology

CPT1A deficiency is 1 of the least common disorders of long-chain fatty acid oxidation. Many of the cases have been found to be homozygous for mutations in the CPT1A gene. This suggests a high frequency of consanguinity. All of the early cases were diagnosed as a result of fasting-induced catabolic events. Cases have been identified through newborn screening, and many of them have remained asymptomatic as a result of the early diagnosis and prompt management (30; 24). Data collected prospectively from newborn screening should eventually give us better prevalence data.

The significance of the Inuit CPT1A variant. Brown and colleagues described an unusual variant in their series of patients with CPT1A deficiency (09). This variant was identified in a native North American who was subsequently shown to be homozygous for a P479L missense mutation. This mutation caused relatively high residual CPT1 activity (20% of normal), but it ablated the malonyl-CoA inhibition. Subsequently, programs in Alaska, Canada, and Greenland have identified this mutation to be the predominant form of CPT1A in individuals of Inuit origin and in some First Nations tribes (20; Rajakumar et al 2009; 18). The physiological effect of this mutation is that an affected individual has a low level of fatty acid oxidation functioning at all times, without being switched off postprandially. This raises a number of important questions, including whether it is an important adaptive lifestyle variant for the ancient Inuit lifestyle and lacks pathophysiologic effect for individuals continuing this lifestyle, or whether it is merely a founder effect in a small population and could have clinical significance. An untargeted whole-genome high coverage sequencing study of a Siberian population has identified P479L as being a deleterious mutation likely to be responsible for the high infant mortality in the population (11). Data have also been presented to show that children with this variant do have fasting intolerance compared to children with the major variant and that this may also indicate an association with infant mortality (17; 19). Cross sectional population studies of medical outcomes, such as associated infant death rates, should provide greater insight into the significance of this variant.

Prevention

CPT1A deficiency is an autosomal recessive condition, and parents should be counseled about the 25% recurrence risk once an affected child is identified. Recurrent episodes of hypoglycemia and potential brain damage can be avoided or reduced by prevention of fasting through frequent feeding or the use of uncooked cornstarch. Prenatal diagnosis by molecular testing is available, but the recent experience of improved outcomes with the initiation of treatment following early diagnosis indicates that this is not necessary.

Differential diagnosis

Because attacks of illness in patients with CPT1A deficiency can occur as acute life-threatening events, the initial differential diagnosis can include a wide range of possibilities, including congenital metabolic or endocrine emergencies, sepsis, and seizures. A history of a preceding interval of fasting or febrile illness is helpful in directing attention to the underlying defect in long-term fasting adaptation. It is particularly important to demonstrate the combination of hypoglycemia with inappropriately low plasma or urinary ketones. This pattern of "hypoketotic hypoglycemia" is typical of the entire group of long-chain fatty acid oxidation defects.

Other disorders associated with hypoketotic hypoglycemia in the neonatal period include normal newborns excessively fasted in the first day after birth, infants of diabetic mothers, congenital hyperinsulinism, and congenital pituitary deficiency. Beyond the neonatal period, hypoketotic hypoglycemia can occur with congenital or acquired pancreatic hyperinsulinism, from administration of insulin and oral hypoglycemics, or from other drug ingestions. Most of these can be distinguished based on the patient's history, physical examination (such as large birth weight in hyperinsulinism and microphallus or midline facial malformations in hypopituitarism), the duration of the fast (relatively brief in hyperinsulinism or congenital hypopituitarism), or evidence of increased glucose consumption in hyperinsulinism or hypopituitarism. The presence of markedly abnormal liver enzymes, mild to moderate hyperammonemia, and moderate hepatomegaly during acute metabolic decompensations is common in fatty acid oxidation defects. CPT1A deficiency is distinguished from other long-chain fatty acid oxidation defects in that it does not have muscle or cardiac involvement. The only other fatty acid oxidation defect to have a similar presentation is medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. Defects of ketone body formation, including 3-hydroxy-3-methylglutaryl-(HMG) CoA synthase and 3-hydroxy-3-methylglutaryl-(HMG) CoA lyase deficiencies, can also have an identical presentation.

Diagnostic workup

The overall strategy in the diagnosis of CPT1A deficiency is to demonstrate that hepatic fatty acid oxidation and ketone synthesis are impaired and then to distinguish CPT1A deficiency from the other potential enzymatic defects in the fatty acid oxidation and ketogenesis pathway. Infants with CPT1A deficiency are now diagnosed through newborn screening when the program includes acylcarnitine analysis by tandem mass spectrometry. Using this approach there is not a specific acylcarnitine species that is diagnostic for CPT1A deficiency, but the ratio of free carnitine (C0) to the total C16 plus C18 acylcarnitines will typically be very high. This immediately distinguishes CPT1A deficiency from other defects of the pathway. The confirmatory workup is likely to require measurement of total and free carnitine where the total carnitine value may be elevated and the acyl fraction will be low. However, the diagnosis can be missed if analysis is performed on plasma alone. This is because long-chain acylcarnitines are absorbed on the surface of red blood cells. Therefore, dried, blood-soaked, filter paper spots are the preferred samples for these determinations (13). Molecular analysis will identify mutations in the CPT1A gene, and direct measurement of CPT1 activity in skin fibroblasts may be required to confirm the functional significance of any identified mutations. There has been a single report of a patient diagnosed in the newborn period in whom subsequent confirmatory testing was normal, indicating that follow-up testing may be problematic (07).

When newborn screening is not available or when a diagnosis is missed due to lack of sensitivity of the screening process, diagnosis of an index case is likely to be through symptomatic presentation. The most useful information on biomarkers indicating the integrity of the fatty acid oxidation pathway and on possible sites of the biochemical defect is obtained at the time of presentation. Therefore, efforts should be made to obtain blood and urine specimens at the time of the acute illness, before therapy is begun. These have been termed the "critical samples," both because they are collected during the critical stage of illness and because they are critical to making the diagnosis. In addition to routine laboratory tests, portions of the "critical" plasma and urine should be saved for later special analyses including acylcarnitine and organic acid analysis.

At the time of acute illness, the presence of a defect in fatty acid oxidation or ketogenesis is suggested by profound hypoglycemia with inappropriately low ketones. Typical findings include plasma glucose lower than 50 mg/dL (2.7 mmol/L); beta-hydroxybutyrate lower than 1.0 mmol/L (an appropriate fasting level is higher than 2.0 mmol/L); and free fatty acids higher than 2.0 mmol/L (an appropriate fasting level is 1.5 to 2.5 mmol/L) (31). Hyperlipidemia may also be observed during acute presentation (37). The urine dipstick test for ketones may be "negative" or "trace," but may be as high as "moderate" if the urine is concentrated. The serum bicarbonate may be as high as 16 to 18 mmol/L because of the relative absence of ketoacids. Plasma AST level is usually elevated 3- to 10-fold, but may reach values of 1000 to 10,000 U/L in extreme catabolic events with severe hepatic encephalopathy. Uric acid, urea, and ammonium levels are frequently elevated, probably reflecting increased protein catabolism.

When the diagnosis is not evident from molecular and enzyme testing, in vivo testing of hepatic fatty acid oxidation can be done by a study of fasting adaptation, a process that requires close monitoring of plasma levels of glucose, free fatty acids, and ketones. This provocative test should be done cautiously because it involves fasting stress that can induce severe illness in patients with CPT1A deficiency.

Plasma free and total carnitine concentrations in the nonfasting state provide a useful clue to the presence of CPT1A deficiency. Many of the reported cases have had normal or increased plasma levels of total carnitine in the free form as a consequence of an elevated renal threshold for carnitine (32; 05; 10; 13). This can help distinguish CPT1A deficiency from HMG-CoA synthase deficiency, which has a normal or low total and acylcarnitine pattern. An explanation for the elevated renal carnitine threshold in CPT1A deficiency is provided by the observation that long-chain acylcarnitines potently inhibit the plasma membrane carnitine transporter protein (32).

Urinary organic acid profile by gas chromatography or mass spectrometry should be obtained at a time of fasting stress to aid in demonstrating impaired ketone production and to help define the site of the defect. In unaffected individuals, the fasting urine shows increased concentrations of 3-hydroxybutyrate and acetoacetate with modest increases in medium-chain dicarboxylic acids (C6-adipic, C8-suberic, and C10-sebacic). In MCAD and HMG-CoA lyase deficiency, there are characteristic organic acid profiles that lead to a diagnosis. In CPT1A deficiency, there may be a non-specific non- or hypoketotic medium-chain dicarboxylic aciduria during metabolic crisis that is not present when the individual is well. In a case report of 3 patients, elevations in dodecanedioic (C12 dicarboxylic) acid were demonstrated (25). The acylcarnitine profile provides additional useful diagnostic information as there is a relative lack of acylcarnitine species of all chain lengths when compared to a high free carnitine level.

Management

The essential therapy for patients with CPT1A deficiency is dietary management to avoid the stress of prolonged fasting. Fasts of longer than 12 hours should be avoided. Usually this can be accomplished with a normal age-appropriate diet emphasizing the need for a bedtime feeding and for not missing breakfast. Nocturnal uncooked cornstarch has been used as well (33). Patients may benefit from diets that are high in carbohydrates, have a reduced amount of saturated long-chain fatty acids, and are supplemented with medium-chain triglycerides and essential fatty acids (01).

During intercurrent illness, extra care to provide carbohydrates is essential. Patients should be instructed to seek emergency room care in the event of illnesses associated with nausea and vomiting. Prompt treatment with intravenous glucose solutions containing 10% dextrose should be given and maintained until normal feedings can be resumed.

Special considerations

Pregnancy

There is no experience in homozygous affected women during pregnancy. However, pregnancy can be predicted to pose extra risk for metabolic decompensation during a fasting illness. Unaffected women are known to have decreased fasting tolerance during pregnancy because of the increased metabolic rate associated with the placenta and fetus. There are a few reports of pregnant women with other disorders of fatty acid oxidation in whom severe attacks of fasting illness occurred during the latter part of pregnancy.

Several of the fatty acid oxidation disorders have been reported to cause complications during pregnancy, including acute fatty liver of pregnancy (AFLP) and the hemolysis, elevated liver enzymes, and low platelets (HELLP) syndrome. Pregnancies associated with defects of the mitochondrial trifunctional protein have also been strongly associated with these obstetric complications. Two case reports describe AFLP and obstetric complications in pregnancies carrying a fetus subsequently shown to have CPT1A deficiency (23; 38). Most pregnancies appear to be normal, but caution needs to be advised, and monitoring maternal liver enzymes during an at-risk pregnancy is recommended.

Affected newborns are at risk for life-threatening illness if exposed to fasting stress. This might occur in association with any form of perinatal problems or with attempted breastfeeding if maternal milk production is slow.

Anesthesia

Anesthesia and surgery probably do not pose extra hazards as long as precautions are taken to avoid fasting stress. Thus, patients should fast for no more than 8 to 10 hours. They should be prophylactically placed on 10% dextrose intravenous infusions for any period of fasting beyond this time. In a reported case in which the diagnosis of CPT1A deficiency was not known at the time of surgery, the postoperative course was complicated by drowsiness and hypoxia that progressed to coma (27).