Aug. 25, 2023
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Very long-chain acyl-CoA dehydrogenase and mitochondrial trifunctional protein (including long-chain L-3-hydroxyacyl-CoA dehydrogenase) deficiencies are recessively inherited inborn errors of mitochondrial fatty acid oxidation. They have a wide range of manifestations, from clinically asymptomatic to severe hypertrophic cardiomyopathy and/or acute, life-threatening episodes of hypoketotic, hypoglycemic coma induced by fasting. In infancy, the presentation can mimic sudden infant death syndrome. Milder variants may primarily affect skeletal muscle and become manifest in adolescence or early adulthood as chronic weakness, pain, recurrent rhabdomyolysis, or acute or chronic cardiomyopathies. Insights into clinical presentation, etiology, pathophysiology, diagnostic work-up, treatment, and pregnancy are presented in this article. The authors describe novel diagnostic tools and clinical trials on potential new management strategies for these fatty oxidation disorders.
• Mitochondrial fatty acid oxidation is the central metabolic pathway for ATP production, especially in liver, heart, and skeletal muscle.
• Fatty acid-oxidation disorders can present with acute, life-threatening episodes of hypoketotic, hypoglycemic coma induced by fasting.
• Milder variants become manifest in adolescence or early adulthood as chronic muscle weakness, pain, recurrent rhabdomyolysis, or acute or chronic cardiomyopathies.
• Newborn screening has significantly reduced morbidity and mortality.
Mitochondrial fatty acid oxidation plays a major role in energy production and homeostasis. Very long-chain acyl-CoA dehydrogenase and mitochondrial trifunctional protein (including long-chain L-3-hydroxyacyl-CoA dehydrogenase) deficiencies, are recessively inherited inborn errors of mitochondrial fatty acid oxidation resulting in overlapping clinical pathology.
Very long-chain acyl-CoA dehydrogenase deficiency was first identified independently by 2 groups (06; 11). All of the patients initially described with long-chain acyl-CoA dehydrogenase deficiency (51) appear in retrospect to have had defects in very long-chain acyl-CoA dehydrogenase (146). Two disorders of the mitochondrial trifunctional protein complex associated with the inner mitochondrial membrane have been described: long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency (43) and a deficiency of all 3 enzymatic components (mitochondrial trifunctional protein deficiency) (64).
The long-chain fatty acid oxidation disorders are best detected by analysis of urinary or, preferably, dried blood spots or best plasma acylcarnitine profiles utilizing tandem mass spectrometry. Distinguishing elevated metabolic markers (acylcarnitines) for very long-chain acyl-CoA dehydrogenase deficiency are C14 and C14:1 species, and for trifunctional protein deficiencies, increased hydroxy forms of C16:0H and C18:1 species. Urinary analysis may show elevated dicarboxylic acids.
Defects of the fatty acid-oxidation often present in early infancy with acute, life-threatening episodes of hypoketotic, hypoglycemic coma induced by fasting. Some patients develop cardiac arrhythmias, and many experience progressive cardiac and/or skeletal muscle disease.
Very long-chain acyl-CoA dehydrogenase deficiency. Impairment of very long-chain acyl-CoA dehydrogenation can result in severe organ dysfunction, especially of the heart, liver, and skeletal muscle. Clinical findings include hepatomegaly, hepatocellular disease, cardiomegaly, cardiomyopathy, and muscular hypotonia. Aoyama and colleagues analyzed human skin fibroblasts for very long-chain acyl-CoA dehydrogenase protein by immunoblotting in 26 patients suspected of a disorder of mitochondrial beta-oxidation (05). Seven patients had undetectable or low levels of the enzyme. All of these patients had developed cardiac disease; at least 4 of them presented with hypertrophic cardiomyopathy. Clinical onset of the disease was within 4 months of birth; 75% died within 2 months of onset, and all patients had had liver dysfunction. Baruteau and colleagues describe the liver as the main organ involved at diagnosis regardless of age at diagnosis, phenotype, or underlying enzyme deficiency in mitochondrial fatty acid beta-oxidation defects (07).
Clinical phenotypes. By now 3 distinct clinical phenotypes of very long-chain acyl-CoA dehydrogenase deficiency can be delineated: (1) a severe, early onset presentation with cardiomyopathy and hepatopathy; (2) a hepatic phenotype that usually manifests in infancy with recurrent episodes of hypoketotic hypoglycemia; and (3) a milder, later-onset myopathic form with episodic muscle weakness, myalgia, and myoglobinuria (46). In addition, extended newborn screening is identifying an increasing number of individuals who are completely asymptomatic at least until early adulthood.
In the neonatal period, patients with very long-chain acyl-CoA dehydrogenase deficiency often develop hypoglycemia, irritability, and lethargy precipitating evaluation for sepsis. Because these patients respond rapidly to glucose infusion, they can be discharged without diagnosis, and a more severe event may occur later.
Clinical studies and case reports. Aliefendioglu and colleagues report a newborn with very long-chain acyl-CoA dehydrogenase deficiency who presented with hypoglycemia, cardiomyopathy, mild hepatomegaly, and slight hypoalbuminemia. Because postmortem examination of the tissues revealed diffuse lipid accumulation in various amounts, the authors suggested that lipid accumulation had begun during intrauterine life with slight hypoalbuminemia as a silent marker of this process (01). Children with the “cardiomyopathic” form of this disease may have transient neonatal hypoglycemia before they develop hypertrophic cardiomyopathy and pericardial effusion between 2 and 5 months of age; these patients usually die. An episode of a wide complex tachycardia has been described in a 2-day-old newborn as the presenting feature (106). Mathur and colleagues studied 37 children with very long-chain acyl-CoA dehydrogenase deficiency (84). Sixty-seven percent of these individuals had severe dilated or hypertrophic cardiomyopathy at presentation with nonketotic hypoglycemia, hepatic dysfunction including hepatic steatosis, skeletal myopathy, or sudden death in infancy as additional signs. The authors conclude that infantile cardiomyopathy is a common severe clinical phenotype for very long-chain acyl-CoA dehydrogenase deficiency. Engbers and colleagues reported a patient with very long-chain acyl-CoA dehydrogenase deficiency who developed rhabdomyolysis at the age of 1 year despite normal glucose after fasting (33). Ogilvie and colleagues reported seeing a 21-year-old man with very long-chain acyl-CoA dehydrogenase deficiency who had exercise-induced myoglobinuria (93). The patient had a 5-year history of muscle pain and myoglobinuria after prolonged exercise or fasting. Rhabdomyolysis caused by very long-chain acyl-CoA dehydrogenase deficiency has been described in several adult patients (04). In addition, an 18-year-old patient with very long-chain dehydrogenase deficiency presented with acute hypercapnic respiratory failure and rhabdomyolysis after a period of prolonged fasting and exertion (126).
Summary. To summarize, the clinical presentation of very long-chain acyl-CoA dehydrogenase deficiency has a wide range of manifestations, from no symptoms to severe life-threatening episodes of hypoketotic, hypoglycemic coma induced by fasting (104), infection (158), fatal cardiomyopathy, especially the hypertrophic form (17), skeletal myopathy with myalgia (38), myoglobinuria (93), rhabdomyolysis (33; 04), and hepatocellular dysfunction (128).
Long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency. The first patient with confirmed long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency was described by Glasgow and colleagues (43). This patient was a boy who presented at the age of 9 months with an episode of fasting-induced vomiting and hypoketotic hypoglycemia resembling Reye syndrome. Further clinical symptoms were significant hypotonia, cardiomyopathy, and liver dysfunction. The boy died at the age of 19 months in cardiorespiratory arrest. At autopsy, the liver showed extensive fibrosis, massive necrosis, and steatosis.
Clinical phenotypes. Of the reported long-chain hydroxyl acyl-CoA dehydrogenase-deficient patients, the age of onset of first symptoms ranged from 1 day to 39 months (100). Most patients present with fasting-induced hypoketotic hypoglycemia, whereas some present with cardiomyopathy (usually hypertrophic) or muscle weakness. A chronic myopathy with potentially extensive rhabdomyolysis is often seen (125). In some patients, myoglobinuria (29), peripheral sensory-motor polyneuropathy (79), fulminant hepatic disease (137), or pigmentary retinopathy (10) have been observed. In addition, long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency predisposes the mother of an affected baby to develop acute fatty liver of pregnancy or HELLP syndromes (152).
Clinical studies and case reports. Tyni and colleagues reviewed the most homogeneous population of long-chain 3-hydroxyacyl-CoA dehydrogenase deficient patients (13 patients homozygous for the 1528G > C mutation) (137). Age of onset of symptoms ranged from 2 days to 21 months. Most had symptoms of hypoglycemia, hypotonia, hepatomegaly, and cardiomyopathy. Six of 11 patients had pigmentary retinopathy, and 2 patients died of hepatic failure. The early psychomotor development was normal, but the patients tended to regress following repeated episodes of metabolic decompensation. Interestingly, 6 of 10 patients had anemia and thrombocytopenia. About half of the patients with long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency died early, either in the initial metabolic deterioration or due to progressive disease resulting in cardiorespiratory failure. Fahnehjelm and colleagues correlated long-term electroretinographic findings with age, metabolic control, and clinical symptoms in 12 patients with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (36). More than 80% of the patients developed pathological or subnormal retinal function, which was related to poor clinical metabolic control and severe neonatal symptoms. The follow-up evaluation of 16 patients with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency over 13 years in Finland showed that earlier diagnosis and a stricter dietary regimen improves the survival rates and clinical course of these patients (63). Tuuli and colleagues investigated the evolution of polyneuropathy in 12 patients with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (135). The first sign of polyneuropathy was detected between the ages of 6 to 12 years, and despite early initiation of therapy with good compliance, most patients developed neuropathy.
Mitochondrial trifunctional protein deficiency. First described by Jackson and colleagues (64), is clinically similar to 3-hydroxyacyl-CoA dehydrogenase deficiency.
Clinical phenotypes. The usual clinical features are hypoketotic hypoglycemia, cardiomyopathy, or sudden infant death syndrome (32).
Clinical studies and case reports. Single reports describe recurrent rhabdomyolysis (88) or myalgia (155). Spiekerkoetter and colleagues characterized 15 patients from 13 families with beta-subunit mutations of the mitochondrial trifunctional protein (116). Again, graded clinical phenotypes became apparent: a severe neonatal presentation with cardiomyopathy, liver failure, and early death (27%); a hepatic form with recurrent hypoketotic hypoglycemia (13%); and a milder, later-onset neuromyopathic phenotype with episodic myoglobinuria (60%). Spiekerkoetter and colleagues demonstrated that trifunctional protein deficiency, as a result of either alpha- or beta-subunit mutations, presents with similar, though still heterogenous clinical phenotypes (113). They conclude that both alpha- and beta-subunit mutations result in trifunctional protein complex instability, demonstrating that the mechanism of disease is the same in alpha- or beta-mutation-derived disease, which explains the biochemical and clinical similarities. Ibdah and colleagues characterized a special phenotype of mitochondrial trifunctional protein deficiency in 2 patients with chronic progressive polyneuropathy and myopathy without hepatic or cardiac involvement (60). In adult patients, chronic sensory neuronopathy may be a major clinical feature, especially associated with c.1528G>C mutation (91). Several authors report patients with mitochondrial trifunctional protein deficiency or L-3-hydroxyacyl-CoA dehydrogenase deficiency with hypoparathyroidism (143).
Sperk and colleagues reported the clinical outcome of 6 patients with mitochondrial trifunctional protein deficiency identified by newborn screening (109). Three of 6 patients were symptomatic prior to availability of screening results. Only 2 patients remained asymptomatic during follow-up of 3 years. In combination with cardiomyopathy, a necrotizing enterocolitis and respiratory distress syndrome in 2 newborns were the first clinical symptoms of mitochondrial trifunctional protein deficiency (27). Fletcher and colleagues propose a genotype/phenotype correlation with the severity of retinopathy in patients with mitochondrial trifunctional protein deficiencies (37).
The clinical spectrum of peripheral neuropathy in long-chain L-3-hydroxyacyl-CoA dehydrogenase and mitochondrial trifunctional protein deficiency has been studied clinically and electrophysiologically in a series of 19 patients by Grünert and colleagues (48). A novel finding of this study was that acute onset of neuropathy in these patients might be partly reversible.
Very long-chain acyl-CoA dehydrogenase deficiency. As already described, this disease presents clinically with 2 phenotypes. Patients with the “cardiomyopathic” or the severe form of the disease typically present with chronic hypertrophic cardiomyopathy and pericardial effusion between 2 and 5 months of age. They usually die early. Chronic complications like hypertrophic cardiomyopathy can also develop in more mildly affected patients and improve with treatment. The mild form of very long-chain acyl-CoA has a phenotype very similar to medium-chain acyl-coenzyme A deficiency and an excellent prognosis with early diagnosis and treatment. Because fasting tolerance improves with age, the risk of episodes of coma decreases in later childhood and adulthood. Some patients identified by newborn screening may represent a group with an asymptomatic or milder disease course (102). In a 10-year longitudinal national cohort study, Bleeker and colleagues investigated the impact of newborn screening for these patients (12). Newborn screening had a clear beneficial effect on the prevention of hypoglycemic events in patients with some residual enzyme activity, but did not prevent hypoglycemia nor cardiac complications in patients with low residual enzyme activity. Acute decompensations and sudden deaths occur in these patients despite expanded newborn screening, as evaluated by Janeiro and colleagues (65).
Long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency. Long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency is a more severe fatty acid oxidation defect. Although the mortality rate among children with long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency had been reported to be 75% to 90%, Ibdah and colleagues found that 67% of the affected children in their study were alive and receiving dietary treatment at the most recent follow-up (58). Additionally, most were able to attend school.
Neuropsychological long-term outcome has been described in 8 patients (123). The mean IQ for the entire group was within the normal range. But as many as 37.5% of these patients had considerable cognitive disabilities and IQ scores in the range of intellectual disability and an autism spectrum disorder diagnosis or autistic behavior at assessment. Treatment of fatty acid oxidation defects dramatically reduces morbidity and mortality. To assess the mode of presentation, treatment, and clinical outcome in patients with long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency, den Boer and colleagues reviewed a large cohort of 50 patients (24). Twenty-two percent of the patients presented with chronic problems, consisting of failure to thrive, feeding difficulties, cholestatic liver disease, and hypotonia. Mortality was high (38%); all died before or within 3 months after diagnosis. Of the surviving patients, 94% were reported to be mostly in good clinical condition, but morbidity remained high, with recurrent metabolic crises and muscle problems. The metabolic crises were reported to be less severe than the initial acute metabolic derangement. In addition to frequent complications, like cardiomyopathy and severe liver disease, a number of long-chain L-3-hydroxyacyl-CoA dehydrogenase-deficient patients develop pigmentary retinopathy and peripheral neuropathy. These complications are not seen in any of the other mitochondrial fatty acid oxidation disorders. Another complication almost exclusively occurring in heterozygous carriers for long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency, especially the E510Q mutation, is serious liver disease during pregnancy when the mother is carrying an affected fetus.
Cardiomyopathy can be a severe complication, especially in episodes of metabolic derangement in long-chain fatty acid disorders (72).
All reported patients with trifunctional protein deficiency of the severe phenotype with cardiac involvement have died in the first weeks of life despite immediate treatment with interventions that are effective in some very long-chain acyl-CoA-deficient patients (116), including patients with cardiomyopathy who were diagnosed prenatally (23).
Summary. With the therapeutic efforts described below, the short-term evolution of patients with long-chain fatty acid oxidation defects has improved; however, the long-term prognosis still remains uncertain and morbidity alarmingly high.
The authors report a 32-year-old Polish patient who developed acute onset, severe pain in all extremities and trunk muscles during his work as a gardener. Pain treatment with diclofenac had no effect, and the muscle pain progressed. The department of neurology in the hospital was consulted.
First diagnostic tests. Routine investigations showed elevated transaminases (AST 5089 U/L, normal < 50 U/L; ALT 1456 U/L, normal < 50 U/L), a massive increase of creatine kinase (CK 225133 U/L, normal < 170 U/L), myoglobin (MG 251000 µg/L normal < 55 µg/L), lactate dehydrogenase (LDH 3100 U/L, normal < 240), and a moderate elevation of CRP (27.4 mg/L, N < 10 mg/L). Furthermore, renal function parameters were significantly elevated (creatinine 4.3 mg/dl, normal < 1.25 mg/dl; urea 144 mg/dl normal, < 55 mg/dl; potassium 5.69 mmol/L, normal 3.6-5.2 mmol/L).
Management and course. The patient was dialyzed continuously for 6 days in an intensive care unit. Because of increasing laboratory parameters (transaminases and creatine kinase) and the suspicion of autoimmune myopathy, the patient was treated with high doses of steroids. With this treatment, the symptoms and laboratory values improved. Because of a language barrier, the complete medical history of this patient could not be adequately obtained at first. The use of a professional interpreter revealed recurrent episodes of rhabdomyolysis during childhood in the patient and his 2 older sisters (34- and 35-years-old). Further metabolic investigations were performed consequently.
Metabolic investigations. Acylcarnitine profile in plasma revealed an elevation of long-chain acylcarnitines and long-chain L3-hydroxy-acylcarnitines, typical for long-chain 3-L-hydroxyacyl-CoA dehydrogenase deficiency. Enzymatic analysis in blood confirmed the diagnosis. Genetic investigations were not performed.
Management and course. High-dose steroid treatment was then interrupted. With anabolic treatment with intravenous glucose, the patient recovered rapidly. He received an emergency card and was educated in detail about the concept and the treatment of the disease. In order to avoid further metabolic crisis, prolonged fasting periods, especially in case of infections, were to be avoided. In his 2 older sisters in Poland the diagnosis of long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency was also confirmed biochemically and enzymatically, and treatment was started.
In the mitochondria the intramitochondrial beta-oxidation spiral occurs.
Very long-chain acyl-CoA dehydrogenase deficiency, mitochondrial trifunctional protein deficiency, and long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency are disorders of the mitochondrial fatty acid beta-oxidation.
Biochemistry. Very long-chain acyl-CoA dehydrogenase is the first enzyme of the beta-oxidation spiral. It is loosely bound to the inner mitochondrial membrane. It is unique among the acyl-CoA dehydrogenases in its size, structure, and intramitochondrial distribution. Whereas the other acyl-CoA dehydrogenases are homotetramers of a 43 to 45 kD subunit, very long-chain acyl-CoA dehydrogenase was shown to be a 154 kD dimer of a 70 kD subunit (05). Andresen and colleagues mapped the ACADVL gene encoding very long-chain acyl-CoA dehydrogenase, which is comprised of 20 exons, to human chromosome 17p11.13-p11.2 (02). More than 150 mutations in the ACADVL gene have been characterized (38; 100).
The 3 clinical phenotypes (see above) of human very long-chain acyl-CoA deficiency may be biochemically distinguished using acylcarnitine profiles generated by patient fibroblast cell cultures exposed to various fatty acid substrates. In severe very long-chain acyl-CoA deficiency due to null mutations with no residual enzyme activity, accumulation of longer-chain (C14-C16) acylcarnitine species has been observed after substrate loading (40). In contrast, cell lines derived from individuals with milder clinical phenotypes accumulate C12-C14 substrates. In very long-chain acyl-CoA deficiency knock-out mice that correspond to severe human very long-chain acyl-CoA deficiency, predominantly longer-chain acylcarnitines (C16-C18 species) accumulate (117). The authors hypothesize that a persistent milder elevation of long-chain acylcarnitines indicates an activated but impaired beta-oxidation and may contribute to the later-onset phenotypes.
The mitochondrial trifunctional protein is a heterooctameric (alpha4beta4) enzyme complex associated with the inner mitochondrial membrane. It has long-chain L-3-hydroxyacyl-CoA dehydrogenase, 2-enoyl-CoA hydratase, and 3-oxoacyl-CoA thiolase activities for the degradation of long-chain L-3-hydroxyacyl-CoA thioesters. The first 2 enzymatic steps (dehydrogenase and hydratase) reside in the alpha-subunit and the thiolase activity in the beta-subunit of the mitochondrial trifunctional protein.
Pathobiochemistry. Mitochondrial fatty acid oxidation is the central metabolic pathway for ATP production in the heart and skeletal muscle. Mitochondrial fatty acid oxidation disorders often present in infancy with myocardial dysfunction and arrhythmias after exposure to metabolic stress such as fasting, exercise, or intercurrent viral illness. The early onset of cardiac phenotypes in mitochondrial fatty acid oxidation disorders is caused by a switch in energy-producing substrate utilization from glucose in the fetal period to fatty acids postnatally. The perinatal cardiac substrate switch is paralleled by an increase in mitochondrial fatty acid oxidation protein expression (116).
Human patients with very long-chain acyl-CoA dehydrogenase mutations have a low ability to oxidize palmitate in fibroblasts or to dehydrogenate palmitoyl-CoA in fibroblast extracts (05), indicating a low long-chain acyl-CoA dehydrogenase activity in this type of tissue.
Genetics. For mitochondrial trifunctional protein (MTP) deficiency, some correlations between mutations and phenotypes have been established. The most common mutation, 1528G>C (E510Q) in the alpha-subunit gene (HADHA) (87%) (62) usually causes liver dysfunction with hypoketotic hypoglycemia in infancy. Yang and colleagues localized the genes for trifunctional protein (alpha-subunits and beta-subunits) to chromosome 2p23.3 (159). Ushikubo and colleagues first reported disease-causing mutations in the MTP beta-subunit gene (HADHB) (139). Four novel mutations in the alpha- and beta-subunits of the mitochondrial trifunctional protein have been reported by Choi and colleagues (19). Mutations in the beta-subunit always cause mitochondrial trifunctional protein deficiency instead of long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency, but mitochondrial trifunctional protein deficiency can also be caused by heterogeneous mutations in the alpha-subunit. In a French cohort of 52 patients with mitochondrial trifunctional protein deficiency, the majority of identified mutations generated premature termination codons resulting in nonsense mRNA-mediated decay (15). The first case of isolated long-chain 3-oxoacyl-CoA thiolase deficiency based on a mutation in the beta-subunit (F431S) of the mitochondrial trifunctional protein has been reported by Das and colleagues (22).
All inborn errors of mitochondrial fatty acid oxidation metabolism are inherited in a recessive fashion. Only in 1 case has paternal isodisomy of chromosome 2 been identified as a cause of long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (08).
Pathophysiology. To explore the onset and molecular mechanism of myocardial disease in the absence of physiological stress, Exil and colleagues generated a mouse model of very long-chain acyl-CoA deficiency (35). The study showed that cardiomyopathy, characterized by abnormal myocardial histology and increased susceptibility for arrhythmia in the absence of systolic dysfunction, occurs in mice with homozygous deletion of very long-chain acyl-CoA dehydrogenase in the absence of exogenously imposed stress. The authors postulate that the mechanisms include lipotoxicity because of lipid deposition, energy starvation manifesting in mitochondrial proliferation, and abnormal mitochondrial regulation because of the biochemical changes occurring at birth. Abnormal intracellular calcium handling has been postulated as the possible mechanism of arrhythmias in very long-chain acyl-CoA dehydrogenase knock-out mice (151). The authors speculate that this mechanism may play a role in very long-chain dehydrogenase-deficient humans. In very long-chain acyl-CoA deficient mice, prolonged QT interval and lipid alterations have been observed in cardiac muscle (39). Fatty acid oxidation in myocardium seems to be essential for maintaining normal cardiac function in very long-chain acyl-CoA dehydrogenase deficiency because fasting and cold exposure resulted in severe hypothermia, bradycardia, and markedly depressed cardiac function in a cardiac-specific very long-chain acyl-CoA dehydrogenase deficient knock-out mice (154).
Although the murine model of very long-chain acyl-CoA dehydrogenase deficiency has a less severe phenotype than in humans, essential characteristics of the human disease are replicated. Spiekerkoetter and colleagues (117) used a very long-chain acyl-CoA dehydrogenase knock-out mouse model to study changes in blood carnitine and acylcarnitines profiles under different stressors. The authors showed that very long-chain acyl-CoA dehydrogenase knock-out mice have stress-induced hypoglycemia, reduced exercise capacity, and cold insensitivity. During stress, they exhibit increased long-chain acylcarnitines (C14-C18) and decreased free carnitine in blood. Importantly, the concentrations of long-chain acylcarnitines in blood correlated with the severity of the clinical manifestations. In a further study Spiekerkoetter and colleagues demonstrate different tissue-specific, long-chain acylcarnitine profiles in response to various stressors in very long-chain acyl-CoA dehydrogenase-deficient mice (118). Furthermore, they showed that changes in blood free carnitine levels did not correlate with carnitine homeostasis in liver and skeletal muscle. Carnitine supplementation resulted only in an increase of long-chain acylcarnitine production and had no effect on carnitine concentrations in tissues of these mice (77). Another study of this group gives biochemical evidence of impaired gluconeogenesis as an underlying pathomechanism of hypoglycemia in very long-chain dehydrogenase-deficient mice (111). The authors conclude that carnitine biosynthesis in the liver seems sufficiently active to maintain liver carnitine levels during increased demand, and they suggest that carnitine supplementation in long-chain beta-oxidation defects may not be required because blood carnitine concentrations do not reflect tissue carnitine homeostasis. The same group demonstrated that carnitine supplementation in very long-chain acyl-CoA dehydrogenase knock-out mice resulted in significant accumulation of potentially toxic acylcarnitines in tissues without replenishing low free carnitine. Interestingly, long-term use of MCT oil in very long-chain dehydrogenase-deficient mice has severe cardiac adverse effects in contrast to humans and a sexually dimorphic response, which reflects the differences in long-chain fatty acid oxidation between these species (149). Altered metabolic flexibility plays a major role in pathophysiology of very-long chain acyl CoA dehydrogenase deficiency as it has been reviewed by Tucci and colleagues (129).
Long-chain acyl-CoA esters have been shown to inhibit the mitochondrial ATP/adenosine diphosphate carrier (140), the dicarboxylate carrier (53), and the pyruvate dehydrogenase complex (89). Uncoupling of oxidative phosphorylation by accumulated long-chain 3-hydroxy fatty acids has been demonstrated by Tonin and colleagues (127). In cells from individuals with very long-chain acyl-CoA dehydrogenase deficiency and long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency, a deregulation of mitochondrial fusion/fission machinery leading to changes in mitochondrial morphology and activity has been observed (50). The lack of mature cardiolipin in mitochondrial trifunctional protein (MTP) deficient cardiomyocytes results in mitochondrial abnormalities in vitro and has been postulated to be an important pathomechanism of cardiac arrhythmias in MTP-deficient patients (87).
Tyni and colleagues investigated the role of mitochondrial fatty acid beta-oxidation in the human retina and the pathogenetic mechanisms of the retinopathy in long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency (136). The findings of this study support the hypothesis that mitochondrial fatty acid beta-oxidation is involved in the metabolism of the retinal pigment epithelium, which could be the cell layer that is most severely and primarily affected.
Goetzman and colleagues investigated the differential induction of fatty acid oxidation genes in liver and brown adipose tissue regulated by peroxisome proliferator-activated receptor-alpha during fasting and cold exposure in very long-chain acyl-CoA dehydrogenase-deficient mice (45). Fasting and cold exposure led to increased expression of these genes in liver but not in brown adipose tissue. To elucidate the pathophysiological mechanisms of stress-induced death in human newborns with abnormal fat metabolism, Exil and colleagues used a mouse model of very long-chain acyl-CoA dehydrogenase deficiency (34). In the absence of stress, the knock-out mice appeared asymptomatic, similar to affected humans. Exposure to fasting and cold triggered hypoglycemia, hypothermia, liver and heart steatosis, and severe bradycardia. The administration of glucose did not rescue the mice under stress conditions, but rewarming alone led to heart rate recovery. The authors conclude that disturbed mitochondrial energy metabolism in brown adipose tissue is a critical contributing factor for the cold sensitivity in this inherited disorder.
Studies in patients with mitochondrial trifunctional protein deficiency suggested that there are 2 types of defects. Patients in group 1 have normal amounts of cross-reacting material by immunoblot and lack “only” long-chain L-3-hydroxyacyl-CoA dehydrogenase, whereas patients in group 2 have only trace amounts of cross-reacting material, with all 3 activities being deficient (139). The same author found that normal alpha- and beta-subunits and their association are important for stabilization of the trifunctional protein. Although the defect in long-chain L-3-hydroxyacyl-CoA dehydrogenase is in the mitochondrial trifunctional protein complex, most patients demonstrate isolated deficiency of the dehydrogenase activity. IJlst and colleagues showed that the E510Q mutation is directly responsible for the loss of long-chain L-3-hydroxyacyl-CoA dehydrogenase activity without changing the structure of the enzyme complex (62). Ibdah and colleagues generated and characterized a knock-out model for complete mitochondrial trifunctional protein deficiency (59). The mitochondrial trifunctional protein –/– fetuses accumulated long-chain fatty acid metabolites. The knock-out mice suffered from neonatal hypoglycemia; sudden death; severe dysfunction of the heart, liver, and diaphragm; and uniform mortality within hours postnatally. Analysis of the histopathologic changes showed rapid development of hepatic steatosis after birth and then necrosis and acute degeneration of the cardiac and diaphragmatic myocytes. The authors summarize that this mouse model is a valid model for human mitochondrial trifunctional protein deficiency and that residual mitochondrial long-chain fatty acid oxidation is essential for fetal development and for survival after birth.
Pathophysiological mechanisms underlying mutations in the HADHB gene causing mitochondrial trifunctional protein deficiency have been reviewed by Dagher and colleagues (21). The authors discuss possible pathomechanisms of sensorimotor neuropathies in these patients, such as the alteration of axons by accumulating acylcarnitines in Schwann cells.
Implications for dietary treatment. A fat-reduced and carbohydrate-enriched diet does not prevent the myopathic phenotype in very long-chain acyl-CoA dehydrogenase deficient mice (98). Because a long-term medium-chain triglyceride-based diet induced hepatic steatosis in very long-chain acyl-CoA dehydrogenase-deficient mice, Tucci and colleagues conclude that medium-chain triglycerides should be given to patients only in situations of increased energy demand (133). The authors demonstrated that the long-term supplementation of medium-chain triglycerides in very long-chain acyl-CoA dehydrogenase deficient mice results in liver damage similar to nonalcoholic steatohepatitis (132). The long-term supplementation of medium-chain triglycerides also aggravated the cardiac phenotype in these mice leading to dilated cardiomyopathy with features similar to diabetic heart disease (131). The same authors investigated the underlying pathomechanisms of hepatopathy and hepatomegaly in very long-chain acyl-CoA dehydrogenase deficient knock-out mice (134). Liver damage in fatty acid oxidation defects seemed to be attributable to fasting-induced oxidative stress as a result of fat accumulation in the liver. The supplementation of odd and even medium-chain fatty acids in very long-chain acyl-CoA dehydrogenase deficient mice resulted in de novo fatty acid synthesis and elongation of fatty acids. These data raise the question if long-term medium-chain fatty acid supplementation is an efficient treatment in humans (130).
Metabolic resistance of very long-chain acyl-CoA deficient- and long-chain acyl-CoA dehydrogenase deficient-knock-out mice against cold intolerance, a phenomenon seen on both mice models, could be increased by dietary phytoestrogens (117).
Jones and colleagues analyzed the effects of dietary treatment of long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency in an in vitro model of cultured skin fibroblasts from 2 patients with long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency and a patient with complete mitochondrial trifunctional protein deficiency (66). The results suggested that a medium-chain triglyceride preparation reduces the accumulation of potentially toxic long-chain 3-hydroxy fatty acids in long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency and that a preparation with a higher ratio of decanoate to octanoate may be the most effective.
In fibroblasts from individuals with mitochondrial trifunctional deficiency, accumulation of long-chain acylcarnitines could be partially prevented by MCT-therapy and totally corrected by Etomoxir (ETX), an inhibitor of carnitine palmitoyltransferase 1 activity. Lefort and colleagues postulate that ETX may be a new therapeutic strategy for mitochondrial trifunctional protein deficiency (75).
Gene therapy. Merritt and colleagues developed a gene construct containing the human very long-chain acyl-CoA dehydrogenase cDNA under the control of the strong CMV promoter (86). Transfected fibroblasts showed correction of the metabolic block as demonstrated by normalization of C14- and C16-acylcarnitine species in cell culture media and restoration of very long-chain acyl-CoA dehydrogenase activity in cells. Another potential therapeutic strategy is the treatment of very long-chain acyl-CoA dehydrogenase-deficient fibroblasts with S-nitroso-N-acetylcysteine, which induced an increase in very long-chain acyl-CoA dehydrogenase-specific activity and concomitant correction of acylcarnitine profile and beta-oxidation capacity (124).
Following tail vein injection of pCMV-human very long-chain acyl-CoA dehydrogenase into mice, they could demonstrate expression of this human enzyme in liver. The authors indicate the importance of these results in the development of a durable gene therapy for very long-chain dehydrogenase deficiency. Keeler and colleagues showed that the treatment of very long-chain acyl-CoA deficient mice with recombinant adeno-associated virus 9 (rAAV9) resulted in long-term biochemical correction (70). Intramuscular long-chain metabolites, especially in the heart, decreased significantly. A single injection of AAV9-VLCAD gene replacement ameliorated respiratory insufficiency induced by exercise and fasting in the VLCAD -/- mice (160).
Liver disease and pregnancy. Little is known about the mechanism of the association between mitochondrial fatty acid oxidation disorders in a fetus and liver disease in the mother during pregnancy. Oey and colleagues investigated the expression of genes involved in the mitochondrial oxidation of long-chain fatty acids during early human development (92). The results showed a strong expression of very long-chain acyl-CoA dehydrogenase and L-3-hydroxyacyl-CoA dehydrogenase mRNA and a high enzymatic activity of these enzymes in a number of tissues, such as liver and heart. Additionally, a high expression of L-3-hydroxyacyl-CoA dehydrogenase mRNA was observed in the neural retina and in the central nervous system.
Experience with newborn screening for disorders of fatty acid oxidation is becoming available from an increasing number of programs worldwide (80; 74; 68). The spectrum of disorders differs widely between ethnic groups. Incidence calculations from reports from Australia, Germany, and the United States of a total of 5,256,999 newborns give a combined incidence of approximately 1:9,300; however, it appears to be much lower in Asia. For very long-chain acyl-CoA deficiency, the incidence is 1:85,000 and for long-chain L-3-hydroxyacyl-CoA dehydrogenase/mitochondrial trifunctional protein deficiency, 1:250,000/1:750,000 newborns.
In the United States, a defect in mitochondrial trifunctional protein causing complete mitochondrial trifunctional protein deficiency or isolated long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency occurs in about 1 in 38,000 pregnancies (58). In Finland, analysis of the carrier frequency of the common G1528C mutation, causing long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency, revealed a carrier frequency of 1 out of 240, which would result in a homozygosity frequency of 1 out of 230,000. In Finland, long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency appears to be the most frequently diagnosed beta-oxidation defect (138).
Prenatal diagnosis is, at present, the only available tool to prevent the disease. Some authors postulate that women with acute fatty liver of pregnancy or HELLP syndrome, as well as their partners and children, should undergo molecular testing for the Glu474Gln mutation of the alpha subunit of the trifunctional protein (58). Verlinsky and colleagues describe preimplantation genetic diagnosis as an option for establishing an unaffected pregnancy, thereby avoiding the risk for termination of pregnancy following prenatal diagnosis in long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency (145). The method for preselection of mutation-free oocytes for long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency consists of testing the first and second polar body removed from oocytes by micromanipulation techniques and hemi-nested polymerase chain reaction.
Expanded newborn screening using tandem mass spectrometry allows the early diagnosis of very long-chain acyl-CoA deficiency and mitochondrial trifunctional protein, including long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiencies (80). This strategy of expanded newborn screening enables diagnosis and treatment before the onset of severe symptoms.
The clinical presentation of fasting-induced vomiting, lethargy, and coma with hypoketotic hypoglycemia is typical for all mitochondrial fatty acid oxidation disorders. Most also cause heart and skeletal muscle involvement (cardiomyopathy, arrhythmia, hypotonia, and rhabdomyolysis). Specific testing is required to differentiate the disorders discussed from the other fatty acid oxidation defects: plasma membrane carnitine transporter defect, carnitine palmitoyltransferase I/II deficiencies, acylcarnitine translocase deficiency, medium-chain acyl-CoA dehydrogenase deficiency, HMG synthase deficiency, HMG lyase deficiency, and electron transfer flavoprotein deficiency.
Hypoglycemia is seen in many other disorders too. It is most important to ascertain urine and serum samples at the time of hypoglycemia. One clue to mitochondrial fatty acid oxidation disorders is the finding of inappropriately low levels of urinary ketones despite high levels of free fatty acids. The duration of fasting and the age of onset are similar to ketotic hypoglycemia of hypopituitarism. The relative longer period of fasting required to induce illness, the mild degree of acidemia, and modest hepatomegaly help to distinguish long-chain acyl-CoA dehydrogenase deficiencies from glycogen storage disorders (types I, III, IX and 0) and gluconeogenic defects (pyruvate carboxylase deficiency, phosphoenolpyruvate carboxykinase deficiency, and fructose-1,6-bisphosphatase deficiency). Other inborn errors of metabolism presenting with hypoglycemia and coma include organic acidurias, which are usually associated with more severe acidemia than mitochondrial fatty acid oxidation disorders, and disorders of galactose or fructose metabolism (galactosemia or hereditary fructose intolerance). Organic acidurias can be distinguished by their distinctive urine organic acid profiles.
Hypoketotic hypoglycemia can be artificially induced by ingestion of oral hypoglycemic drugs or insulin administration. Patients with hyperinsulinism due to a mutation of glutamate dehydrogenase (hyperinsulinism and hyperammonemia syndrome) can present with similar degrees of hyperammonemia together with hypoketotic hypoglycemia (121). The hyperammonemia seen in long-chain mitochondrial fatty acid oxidation disorders due to liver dysfunction may suggest a urea cycle disorder, but elevation of blood ammonia is usually milder and attacks of illness are often provoked by prolonged fasting rather than by protein feeding.
Myoglobinuria as seen in long-chain mitochondrial fatty acid oxidation disorders is characteristic of a number of inherited metabolic myopathies: carnitine palmitoyltransferase II, phosphorylase deficiency (McArdle disease, glycogenosis type V), short-chain 3-hydroxyacyl-CoA dehydrogenase deficiency, and other genetic diseases. Much more common causes are strenuous exercise, chronic alcoholism, toxic agents, and trauma.
An acute presentation of long-chain mitochondrial fatty acid oxidation disorders with an overwhelming lactic acidosis may be mistaken for a primary respiratory chain disorder. On the contrary, primary respiratory chain disorders can mimic mitochondrial fatty acid oxidation disorders including hypoketotic hypoglycemia. A plasma acylcarnitine profile should always be included in the diagnostic workup of infantile lactic acidemia.
Long-chain mitochondrial fatty acid oxidation disorders can be diagnostically challenging and may be confused with a wide variety of other disorders including metabolic disorders, infections, and congenital malformations. In conclusion, multiorgan involvement of liver, heart, and skeletal muscle and especially hypoketotic hypoglycemia are the main clues to a defect in mitochondrial fatty acid oxidation.
The best but also most dangerous functional tests are performed by nature during an acute metabolic stress caused by an acute infection, inadvertent fasting, or consumption of a nutrient for which a metabolic intolerance exists. Initial studies in the case of suspected fatty acid oxidation disorder should include glucose, lactate, ammonia, electrolytes, blood gases, blood acylcarnitine profile analysis, and urine organic acid analysis. A typical constellation of metabolic indicators of a defect in fatty acid oxidation includes hypoglycemia, minimal acidemia, elevated blood urea nitrogen, hyperammonemia, elevated liver transaminases and creatine kinase, abnormal coagulation tests, and inappropriately low ketones in blood and urine (161). Urine organic acid analysis, which reveals both saturated and unsaturated dicarboxylic aciduria (adipic, suberic, sebacic acids), is direct evidence for insufficient mitochondrial fatty acid oxidation but less sensitive than blood acylcarnitine profiling. Dicarboxylic aciduria occurs in various amounts and patterns in different fatty acid oxidation disorders. Peroxisomal disorders and dietary medium-chain triglyceride can provoke secondary dicarboxylic aciduria in urine. During severe metabolic derangement dicarboxylic aciduria may be completely masked by excessive lactic aciduria. Secondary carnitine depletion with low plasma and tissue levels is common in fatty acid oxidation disorders, and plasma free and total carnitine can give important hints. It has been reported by Kobayashi and colleagues that urinary acylcarnitine profiles are not helpful for the characterization of long-chain fatty acid disorders but that a combination of urine and blood acylcarnitines may be useful for differential diagnosis of carnitine deficiency (73).
Very long-chain acyl-CoA deficiency is characterized by an abnormal plasma acylcarnitine profile using tandem mass spectrometry from simple blood spots collected on a Guthrie card, which is dominated by the oleate metabolite, 5-cis-tetradecanoylcarnitine (C14:1) (17; 116). Several authors report infants with long-chain fatty acid oxidation defects identified by tandem mass spectrometry (MS/MS) of newborn blood spot acylcarnitines and by molecular genetic analysis who had normal acylcarnitine profiles in confirmatory plasma samples (105). Liebig and colleagues (78) propose that a C14:1-carnitine level greater than 1 µmol/L on day 3 of life strongly suggests very long-chain acyl-CoA dehydrogenase deficiency, whereas concentrations below 1 µmol/L do not allow a clear discrimination among affected patients, carriers, and healthy individuals (103). C14:1/C2 may be a more sensitive marker than C14:1, but could raise the risk of overdiagnosis (26). Sahai and colleagues point out that newborn screening may fail to identify very long-chain acyl-CoA dehydrogenase deficiency, especially in neonates receiving therapeutic interventions (103). In certain cases, acylcarnitine profiles from healthy newborns during catabolism and VLCAD-deficient patients cannot be distinguished and may consequently result in false negative results (115). Spiekerkoetter and colleagues suggest that a reliable diagnosis for very long-chain acyl-CoA dehydrogenase deficiency is the combination of acylcarnitine analysis and enzyme analysis in lymphocytes during the first screening (111). Hoffmann and colleagues investigated the clinical relevance of higher residual enzyme activities (> 10%) in 34 individuals (56). The authors concluded that individuals with a residual enzyme activity greater than 20% present with a biochemical phenotype, but likely remain asymptomatic throughout life. Distinctive metabolomic profiles in dried blood spots of newborns with VLCAD-deficiency may allow early phenotypic prediction between mild and severe cases (72).
Implementation of newborn screening for long-chain fatty acid oxidation defects has significantly reduced morbidity and mortality and has increased its prevalence (80; 111).
These studies indicate that the timing of blood sampling for newborn screening is important and that direct enzyme analysis and/or mutation analysis should be performed as confirmatory testing to avoid false-negative diagnoses (81). Additional functional investigations are a valuable tool in order to differentiate between affected individuals and heterozygous carriers (54). Skin biopsy should be obtained for direct enzyme assay and in vitro studies with the cultured fibroblasts using deuterated or unlabeled palmitate to determine which clinical course can be anticipated (146; 94). For rapid determination of very long-chain acyl-CoA dehydrogenase activity in fibroblasts, a tandem mass spectrometry method has been established by Bouvier and colleagues (16).
In long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency plasma acylcarnitine profile shows elevation of hydroxy-C(18:1) species. In combination with an elevation of 2 other long-chain species, hydroxy-C(14)-hydroxy-C(14:1), or hydroxy-C(16:1), more than 85% of affected patients can be identified with high specificity. Blood spot acylcarnitine analysis is not as sensitive as plasma because of higher levels of long-chain species in blood samples. To confirm the diagnosis, mutation analysis for the common mutation 1528G>C (E510Q) may be performed. An accumulation of 3-hydroxy fatty acids in media from human skin fibroblast cultures has been reported to be a sensitive diagnostic tool for long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency and MTP deficiency (66). Confirmation of diagnosis is also possible by measuring long-chain L-3-hydroxyacyl-CoA dehydrogenase activity in lymphocytes, fibroblasts, muscle biopsies, or liver biopsies (148). Olpin and colleagues demonstrated that the ratio C(18)(OH)/(C(14)(OH) + C(12)(OH)) in fibroblast acylcarnitine profiling and the percentage residual activity with palmitate or the ratio of percentage activity of myristate/oleate in fibroblast fatty acid oxidation flux shows a good correlation with the clinical phenotype in patients with L-3-hydroxyacyl-CoA dehydrogenase deficiency (95). In contrast, specific enzyme assays of long-chain L-3-hydroxyacyl-CoA dehydrogenase and long-chain 3-ketothiolase activity do not correlate with the phenotype.
Deficiency of the MTP is more difficult to recognize. Again, plasma acylcarnitine profile shows elevation of long-chain acylcarnitines, with increased hydroxylated species (3-hydroxypalmitoylcarnitine (3-OH-C16), 3-OH-C18:1, and 3-OH-C18:2 acylcarnitines). Urine organic acid analysis may or may not show dicarboxylic and 3-hydroxydicarboxylic aciduria (sebacic acid). MTP deficiency should be considered whenever there is hypoglycemia associated with hypotonia with or without cardiomyopathy, especially when increased lactate levels and hydroxydicarboxylic aciduria are present, even without the 1528G>C mutation. The acylcarnitine profile may be normal. Interestingly, MTP deficiency seems to be the only fatty acid oxidation disorder in which blood lactate level is consistently elevated even when the patient is asymptomatic (100). Measurement of all 3 enzyme activities and molecular analysis of the alpha- and beta-subunits in fibroblasts sometimes needs to be performed to confirm diagnosis (113).
After the initial workup of fatty acid oxidation disorders in urine and plasma/serum, it is frequently necessary to perform confirmatory studies on cultured fibroblasts. Long-chain fatty acid oxidation can be diagnosed by quantitative acylcarnitine profiling in human skin fibroblasts by electrospray ionization-tandem mass spectrometry using labeled or unlabeled palmitic acid as substrate (94). The acylcarnitine profile of very long-chain acyl-CoA deficiency is characterized by decreased C2 and an accumulation of dodecanoylcarnitine (C12). In long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency, a massive increase of C16-OH metabolites is seen.
With the development of highly specific metabolite tests and molecular diagnostic methods, direct assay of enzyme activity or in vivo function test are rarely necessary. The fasting test has lost much of its importance and is now largely irrelevant, if not contraindicated, for the diagnosis of fatty acid oxidation defects. Another possibility to identify rare variants of long-chain fatty acid oxidation disorders is via next generation sequencing, as has been performed in a large cohort of patients with myopathy and neuropathy of unknown origin by Diebold and colleagues (25).
Diagnostic steps revealing unspecific sequelae of long-chain fatty acid oxidation disorders are a muscle biopsy showing lipid storage and the EMG, which is often myopathic. Muscle MRI may show increased T1W and STIR signal intensity as a sign of lipid accumulation and inflammation and progressive muscle damage (28). Further indicators of the myopathic phenotype of long-chain fatty acid oxidation disorders are elevated creatine kinase and myoglobinuria. In case of unexpected death, especially in infants (eg, sudden infant death syndrome), blood and urine samples should be collected to maximize the chance that a specific diagnosis can be made postmortem (161).
Guidelines in the diagnosis and management of long-chain fatty oxidation defects have been published (03; 82). Patients should be supplied with an emergency card, letter, or bracelet containing instructions for emergency measures and phone numbers. Logistics of rational therapeutic measures should be repeatedly evaluated by the specialist team with the family and the primary care physicians.
Long-term treatment. Treatment of mitochondrial fatty acid oxidation disorders consists of avoiding prolonged fasting periods, frequent carbohydrate intake, and management of intercurrent illnesses. In patients with severe forms of long-chain fatty acid oxidation disorders, continuous nocturnal intragastric feeding is necessary because even small persistent lipolysis can result in accumulation of toxic acylcarnitines. Because patients with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency seem to have an increased lipolysis in comparison to healthy subjects, the avoidance of fasting in these patients is of utmost importance (52). Small infants need continuous enteral feeding or frequent meals (every 4 hours) during daytime and continuous nocturnal enteral feeding. Preschool children continue to need frequent meals during daytime (3 meals and 3 intermeal snacks including 1 at bedtime) as well as uncooked cornstarch (1.5 to 2 g/kg) at night. It should be ensured that older children consume carbohydrates at bedtime (eg, uncooked cornstarch as a source of slowly released glucose). Adolescents and adults should never fast for more than 12 hours overnight.
In cases of severe cardiomyopathy in long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency, cardiac transplantation should be considered as a treatment option (18).
Dietary treatment and supplementals. Dietary treatment of these disorders consists of a mixture of essential long-chain fatty acids (alpha-linoleic, linoleic) and supplementation with medium-chain triglycerides to bypass the metabolic block and inhibit accumulation of toxic long-chain intermediates. This diet will lower the concentration of the long-chain species, often to a normal level. Spiekerkoetter reported that in asymptomatic very long-chain acyl-CoA dehydrogenase deficiency, a fat-reduced diet may not be necessary, whereas in later infancy and adolescence, strenuous physical exercise may require additional energy from medium-chain fat (110). Heptanoate treatment in case of physical exercise may stabilize clinical and biochemical parameters (69). A web-based nutrition management guideline for very-long chain acyl-CoA dehydrogenase summarizes nutrition management recommendations for health care providers in order to harmonize treatment of these patients (141).
Gillingham and colleagues conducted a survey of metabolic physicians who treated long-chain L-3-hydroxyacyl-CoA dehydrogenase deficient patients to determine current dietary interventions employed and the effects of these interventions (42). Survey results showed that a diet low in long-chain fatty acids supplemented with medium-chain triglyceride oil decreased the incidence of hypoketotic hypoglycemia and improved hypotonia, hepatomegaly, cardiomyopathy, and lactic acidosis. However, dietary treatment did not appear to prevent peripheral neuropathy, pigmentary retinopathy, or myoglobinuria.
In very long-chain acyl-CoA dehydrogenase deficiency, measurement of long-chain fatty acid oxidation flux has been proposed as a good predictor of clinical outcome and as a basis for individualized dietary strategy (13).
Carnitine, which is often severely decreased during intercurrent illness, may be supplemented. However, the use of carnitine supplementation in very long-chain acyl-CoA deficiency has been controversial (120), particularly because of the fear that long-chain acylcarnitines would accumulate and provoke arrhythmias (20). However, intravenous or oral carnitine has been used in individual patients in both phenotypes of very long-chain acyl-CoA dehydrogenase deficiency without any apparent cardiac side effects despite high plasma levels of long-chain acylcarnitines (100). Carnitine supplementation (50 to 100 mg/kg per day) in patients with long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency revealed no significant effect on morbidity (24). Medium-chain triglyceride replacement of long-chain fatty acids in the diet in very long-chain acyl-CoA deficiency with hypertrophic cardiomyopathy appears to be effective (17; 97; 09).
In long-chain L-3-hydroxyacyl-CoA dehydrogenase-deficient patients, supplementation of docosahexanoic acid, a fatty acid essential for proper development of the eye and the nervous system, has been discussed by several authors (66). Jones and colleagues demonstrated that a medium-chain triglyceride preparation with a higher ratio of decanoate to octanoate may be more effective in reducing the accumulation of potentially toxic long-chain 3-hydroxy fatty acids in long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency (66). The authors demonstrated that the even-numbered medium-chain fatty acids are responsible for this effect. In a further study the authors investigated if odd-numbered medium-chain fatty acids may also reduce the accumulation of potentially toxic long-chain metabolites of fatty acid oxidation. They found that odd-chain species decrease the build-up of long-chain fatty acid oxidation intermediates, but to a lesser extent than even-numbered medium-chain fatty acids (67). Gillingham and colleagues proposed a diet for long-chain L-3-hydroxyacyl-CoA dehydrogenase and MTP-deficient patients that provides an age-appropriate protein and limited long-chain fatty acids intake (10% of total energy) while providing 10% to 20% of energy as medium-chain triglycerides (41). The patients should also get a multivitamin and mineral supplement that includes all of the fat soluble vitamins. Finally, the diet should be supplemented with vegetable oil as part of the 10% total long-chain fatty acids intake to provide essential fatty acids.
The hypothesis that fibrates, acting as agonist of peroxisome proliferator-activated receptors, might stimulate fatty acid oxidation in very long-chain acyl-CoA dehydrogenase deficient cells was tested by Djouadi and colleagues (30). Addition of bezafibrate or fenofibric acid in the culture medium induced a dose-dependent increase in palmitate oxidation capacities in cells from patients with the myopathic form of the disease, but not in cells from severely affected patients. Bezafibrate diminished the production of toxic long-chain acylcarnitines by 90% in cells harboring moderate very long-chain acyl-CoA dehydrogenase deficiency. The authors summarize that fibrates may be a potential therapy for fatty acid oxidation disorders. Accordingly, the beneficial effect of bezafibrate was demonstrated in an open-label multicenter study over the long term in these patients (108). Mitochondrially enriched electron and free radical scavengers might also be viable candidate compounds for the treatment of very long-chain acyl-CoA dehydrogenase-deficient patients (107).
Interestingly not all genotypes of very long-chain acyl-CoA dehydrogenase deficiency or mitochondrial trifunctional protein deficiency may be improved in vitro with bezafibrate treatment (44; 31). In a randomized double-blind crossover study, bezafibrate treatment of patients with very long-chain acyl-CoA dehydrogenase deficiency did not improve clinical symptoms during exercise (96). In contrast, the ingestion of ketone esters prior to exercise had a beneficial effect on muscle metabolism in patients with very long-chain acyl-CoA dehydrogenase deficiency (14).
Voermans and colleagues present a patient with adult onset very long-chain acyl-CoA dehydrogenase deficiency showing a beneficial response to treatment with dantrolene sodium (147). The authors discuss the therapeutic mechanism of dantrolene sodium and its possible role as additional treatment modality for patients with very long-chain acyl-CoA dehydrogenase deficiency.
To evaluate the long-term morbidity of long-chain fatty acid oxidation and the effect of different therapeutic regimes (medium-chain triglyceride, L-carnitine, docosahexanoic acid), multicenter controlled studies are needed. Yamada and colleagues performed clinical trials evaluating these new therapeutic compounds, such as the use of triheptanoin, trioctanoin, or bezafibrate and give an overview on different management strategies in these patients (157). In a retrospective case-series of 18 patients with long-chain fatty acid oxidation disorders, early triheptanoin treatment markedly improved symptoms and quality of life (49).
Intercurrent illness and emergency treatment. During intercurrent illness, care should be taken to give small extra feedings of carbohydrates during the day and night at least every 4 to 6 hours. It is of utmost importance to intervene promptly with intravenous glucose infusion in case of gastroenteritis or vomiting, and at early signs of lethargy.
Treatment of acute episodes in fatty acid oxidation disorders is aimed at quickly reversing the catabolic state that is responsible for stimulating the pathways of lipolysis and fatty acid oxidation. Acute episodes should be treated with intravenous glucose. Glucose should be given at a rate that maintains plasma glucose levels at, or slightly above, the normal range in order to stimulate insulin secretion and suppress adipose tissue lipolysis. The oral route through a nasogastric drip can be used (if there is no vomiting or diarrhea) alone or with an intravenous infusion. Usually, specific therapy for mild hyperammonemia is not required. Because episodes of gastroenteritis are especially dangerous, early intervention with intravenous glucose is advisable.
It remains to be elucidated to what extent or even whether patients with a biochemically and clinically mild phenotype identified by extended newborn screening require treatment. At least close observation and instructions for emergency measures during periods of metabolic stress are reasonable.
Prenatal diagnosis is possible in at-risk families (eg, by measurement of very long-chain acyl-CoA activity in trophoblasts or amniocytes) (146; 76). Ibdah and colleagues performed molecular prenatal diagnosis in 9 pregnancies (8 in 6 families with long-chain L-3-hydroxyacyl-CoA-dehydrogenase deficiency and 1 in a family with complete trifunctional protein deficiency) (61). Analyses were performed on chorionic villus samples in 7 pregnancies and on amniocytes in 2 pregnancies. Molecular prenatal diagnosis successfully identified the fetal genotype in all 9 pregnancies. Two fetuses were affected, 2 had a wild-type genotype, and 5 others were heterozygous. Two pregnancies were terminated, and the other 7 resulted in healthy offspring. The author concluded that women heterozygous for trifunctional protein alpha-subunit mutations who carry fetuses with wild type or heterozygous genotypes have uncomplicated pregnancies.
Wilcken and colleagues first noted that long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency may be associated with severe maternal illness during pregnancies with affected fetuses (152). The maternal illness is characterized by acute fatty liver of pregnancy syndrome, HELLP syndrome, and hyperemesis gravidarum. The acute fatty liver of pregnancy syndrome includes anorexia, nausea, vomiting, abdominal pain, and jaundice in the third trimester. Fulminant liver failure and death may occur. Ibdah and colleagues investigated the relationship between mutations in the trifunctional protein in 24 infants and acute liver disease during pregnancy in their mothers (58). In 8 children, they identified a homozygous mutation in the alpha-subunit gene through which glutamic acid at residue 474 was changed to glutamine (Glu474Gln). Eleven other children were compound heterozygotes with this mutation. In fetuses carrying the Glu474Gln mutation, 79% of the heterozygous mothers had fatty liver of pregnancy or HELLP syndrome. Five other children presenting with neonatal dilated cardiomyopathy or progressive neuromyopathy had a complete deficiency of the trifunctional protein. Interestingly, none had the Glu474Gln mutation, and none of their mothers had liver disease during pregnancy. The authors summarized that women with acute liver disease during pregnancy may have a Glu474Gln mutation in long-chain L-3-hydroxyacyl-CoA dehydrogenase. Individual reports have also appeared of HELLP syndrome in other defects of fatty acid oxidation. On the other hand, neither maternal nor fetal Glu474Gln mutation in the alpha-subunit of the trifunctional protein is a relevant factor in pregnancies complicated by HELLP syndrome (90). Spiekerkoetter and colleagues report the intrauterine development of severe cardiomyopathy after 27 weeks’ gestation in a child with mitochondrial trifunctional protein deficiency. The mother did not have HELLP syndrome or acute fatty liver of pregnancy (114). Mutations in long-chain 3-hydroxyacyl-CoA dehydrogenase are supposed to be associated with the development of maternal floor infarction and massive perivillous fibrin deposition of the placenta (47). In a pregnant patient with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency with sinus tachycardia and abnormal biochemical parameters (elevated lactate and creatine kinase concentrations), an increase of MCT supplementation lowered heart rate and improved biochemical parameters (142).
Maternal very long-chain dehydrogenase deficiency can cause a positive newborn screening result in the healthy offspring (85). During pregnancy an unaffected placenta and fetus, or even an affected fetus with very long-chain dehydrogenase deficiency, might improve maternal β-oxidation (156).
In patients with fatty acid oxidation disorders, different precautions must be taken before anesthesia to avoid prolonged fasting (144; 55). Intravenous glucose-electrolyte infusion (8 to 12 mg/kg per min in infants) should be given throughout the perioperative fasting period to prevent activation of fatty acid oxidation. The time interval between the last meal and start of the glucose infusion should not exceed 3 hours. In addition, insulin can be used to promote anabolism, and no metabolic acidosis should develop. Ringers lactate should be avoided because of lactic acidosis. Drugs stimulating lipolysis and fatty acid oxidation, like epinephrine and other beta-agonists, theoretically might pose a hazard for patients with fatty acid oxidation disorders. Enflurane was reported to increase free fatty acids during perioperative stress caused by minor elective surgery (71). A premedication with morphine, flunitrazepam, and promethazine had no effect on the plasma concentrations of free fatty acids (57). Propofol infusion syndrome, a rare but frequently fatal complication in critically ill children given long-term propofol infusions, results from an impaired fatty acid oxidation and an inhibition of the respiratory chain at several points (153). Although Martin and colleagues report the safe use of propofol for short duration procedures in patients with long-chain fatty acid oxidation disorders, it should preferably not be used (83). Steinmann and colleagues report the perioperative management of a child with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (122). Anesthesia was performed with midazolam, fentanyl, thiopental, and remifentanil without any muscle relaxant. Welsink-Karssies and colleagues reviewed literature on perioperative management of adult patients with very long chain acyl-CoA dehydrogenase (VLCADD) and discussed that the use of some medications, such as volatile anesthetics, might be avoided (150).
Roe and colleagues reported a child with very long-chain acyl-CoA dehydrogenase deficiency whose diagnosis was unknown at the time of a dental procedure and whose subsequent death was considered to have been elicited by perioperative fasting (101). Redshaw and Stewart gave an overview on anesthetic agents in patients with very long-chain acyl-CoA dehydrogenase deficiency (99). Although these diseases are rare, they have important implications for anesthetists because perioperative fasting, infection, and even emotional stress can trigger severe metabolic decompensation.
All contributors' financial relationships have been reviewed and mitigated to ensure that this and every other article is free from commercial bias.
Georg F Hoffmann MD
Dr. Hoffmann of the University Center for Child and Adolescent Medicine in Heidelberg has no relevant financial relationships to disclose.See Profile
Marina A Morath MD
Dr. Morath of University Children's Hospital in Heidelberg, Germany, has no relevant financial relationships to disclose.See Profile
Jennifer Friedman MD
Dr. Friedman of the University of California San Diego has no relevant financial relationships to disclose.See Profile
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