Oxidation of unsaturated fatty acids by human fibroblasts with very-long-chain acyl-CoA dehydrogenase deficiency: aspects of substrate specificity and correlation with clinical phenotype

Severity estimation of very-long-chain acyl-CoA dehydrogenase deficiency via 13C-fatty acid loading test

BACKGROUND

The clinical severity of very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is difficult to predict using conventional diagnostic methods.

METHODS

Peripheral blood mononuclear cells obtained from 14 VLCAD deficiency patients and 23 healthy adults were loaded with carbon-13-universally labeled (U-¹³C-) fatty acids. Differences in acylcarnitine ratios between the patients and healthy groups and correlations between acylcarnitine ratios and a newly established clinical severity score (CSS) in the patient group were statistically examined.

RESULTS

There was a significant decrease in the ¹³C-C2/¹³C-C18 and ¹³C-C12/¹³C-C14 ratios in the U-¹³C-stearic acid loading test and in the ¹³C-C2/¹³C-C18:1 and ¹³C-C12:1/¹³C-C14:1 ratios in the U-¹³C-oleic acid loading test in the patient group. The values of each ratio were significantly correlated with the CSS, suggesting that they could predict disease severity. Additionally, patients with a higher ¹³C-C16/¹³C-C18 ratio than the ¹³C-C14/¹³C-C18 ratio in the U-¹³C-stearic acid loading test had a significantly higher CSS and were presumed to have more severe disease.

CONCLUSIONS

Our data indicated that this method could be used to predict the clinical severity of VLCAD deficiency, and identify patients at a risk of severe disease.

IMPACT

We established a novel method to predict the severity of VLCAD deficiency by performing a loading test with carbon-13-labeled fatty acids on peripheral blood mononuclear cells. The U-¹³C-oleic acid loading test was useful for comparing the patient group with the control group in terms of disease severity. The U-¹³C-stearic acid loading test was useful for identifying the more severely affected patients. These methods are relatively less invasive and enable rapid evaluation of the clinical severity.

Evaluation of Metabolic Defects in Fatty Acid Oxidation Using Peripheral Blood Mononuclear Cells Loaded with Deuterium-Labeled Fatty Acids

Because tandem mass spectrometry- (MS/MS-) based newborn screening identifies many suspicious cases of fatty acid oxidation and carnitine cycle disorders, a simple, noninvasive test is required to confirm the diagnosis. We have developed a novel method to evaluate the metabolic defects in peripheral blood mononuclear cells loaded with deuterium-labeled fatty acids directly using the ratios of acylcarnitines determined by flow injection MS/MS. We have identified diagnostic indices for the disorders as follows: decreased ratios of d₂₇-C14-acylcarnitine/d₃₁-C16-acylcarnitine and d₂₃-C12-acylcarnitine/d₃₁-C16-acylcarnitine for carnitine palmitoyltransferase-II (CPT-II) deficiency, decreased ratios of d₂₃-C12-acylcarnitine/d₂₇-C14-acylcarnitine for very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency, and increased ratios of d₂₉-C16-OH-acylcarnitine/d₃₁-C16-acylcarnitine for trifunctional protein (TFP) deficiency, together with increased ratios of d₇-C4-acylcarnitine/d₃₁-C16-acylcarnitine for carnitine palmitoyltransferase-I deficiency. The decreased ratios of d₁-acetylcarnitine/d₃₁-C16-acylcarnitine could be indicative of β-oxidation ability in patients with CPT-II, VLCAD, and TFP deficiencies. Overall, our data showed that the present method was valuable for establishing a rapid diagnosis of fatty acid oxidation disorders and carnitine cycle disorders and for complementing gene analysis because our diagnostic indices may overcome the weaknesses of conventional enzyme activity measurements using fibroblasts or mononuclear cells with assumedly uncertain viability.

VLCAD enzyme activity determinations in newborns identified by screening: a valuable tool for risk assessment

Tandem mass spectrometry-based newborn screening correctly identifies individuals with very long-chain acyl-CoA dehydrogenase deficiency (VLCADD). However, a great number of healthy individuals present with identical acylcarnitine profiles during catabolism in the first three days of life. We routinely perform an enzyme activity assay as confirmation analysis in newborns identified by screening. Whereas VLCAD residual activities of less than 10% are clearly diagnostic and indicate patients at risk of clinical disease, the clinical relevance of higher residual activities is unclear. In this study we assess the molecular basis in 34 individuals with residual activities of 10-50%. We identify two pathogenic mutations in patients that result in residual activities as high as 22%, while individuals with residual activities of 25-50% either present with a heterozygous or no mutation in the VLCAD gene. In addition, confirmed heterozygous parents present with residual activities as low as 32%.In conclusion, we identify individuals with 2 pathogenic mutations and those with only one heterozygous mutation in the residual activity range of 20-30%. Whereas heterozygosity is generally regarded as clinically irrelevant, identification of 2 VLCAD mutations leads to precautions in the management of the children. Based on our data we anticipate that individuals with a residual enzyme activity >20% present with a biochemical phenotype but likely remain asymptomatic throughout life. Studies in greater patient numbers are needed to correlate residual activities >10% with the genotype and the outcome.

Effect of heat stress and bezafibrate on mitochondrial beta-oxidation: comparison between cultured cells from normal and mitochondrial fatty acid oxidation disorder children using in vitro probe acylcarnitine profiling assay

Hyperpyrexia occasionally triggers acute life-threatening encephalopathy-like illnesses, including influenza-associated encephalopathy (IAE) in childhood, and can be responsible for impaired fatty acid beta-oxidation (FAO). In this regard, patients with impaired FAO may be more susceptible to febrile episodes. The effects of heat stress and a hypolipidemic drug, bezafibrate, on mitochondrial FAO were investigated using cultured cells from children with FAO disorders and from normal controls, using an in vitro probe acylcarnitine (AC) profiling assay. Fibroblasts were incubated in medium loaded with unlabelled palmitic acid for 96 h at 37 and 41 degrees C, with or without bezafibrate. AC profiles in culture medium were analyzed by electrospray ionization tandem mass spectrometry. Heat stress, introduced by 41 degrees C, significantly increased acetylcarnitine (C2) but slightly decreased the other acylcarnitines (ACs) in controls and medium-chain acyl-CoA dehydrogenase (MCAD)-deficient cells. On the other hand, in very long-chain acyl-CoA dehydrogenase (VLCAD)-deficient cells, accumulation of long-chain ACs were enhanced at 41 degrees C, compared with that at 37 degrees C. In contrast, bezafibrate decreased long-chain ACs with significant increase of C2 in both control and VLCAD-deficient cells at 37 degrees C. These data suggest that heat stress specifically inhibits long-chain FAO, whereas bezafibrate recovers the impaired FAO. Our approach is a simple and promising strategy to evaluate the effects of heat stress or therapeutic drugs on mitochondrial FAO.

Novel mutation of early, perinatal-onset, myopathic-type very-long-chain acyl-CoA dehydrogenase deficiency

A male neonate demonstrated fetal distress, neonatal asphyxia, and transient hyper-creatine kinase-emia (8400IU/L), followed by repeated episodes of rhabdomyolysis 1-2 times/year during infancy and early childhood. At age 6 years, decreased levels of total and free carnitine in serum, and mild fiber size variation and increased fatty droplets in muscle, were confirmed. Both blood and serum fatty-acid analysis demonstrated elevated 5-tetradecenoate levels, and the acyl-CoA dehydrogenase activity of the palmitoyl-CoA/octanoyl-CoA ratio decreased in skin fibroblasts. The sequenced clone analysis of a complimentary DNA fragment revealed a compound heterozygote mutation of exon 9 (A790G) and exon 10 (997 ins T), which is a novel mutation of a myopathic-type very-long-chain acyl-CoA dehydrogenase deficiency. The patient has reached age 13 years. By treatment with an avoidance of fasting, feeding with a high-carbohydrate and low-fat diet, and intravenous drip infusion soon after every onset of rhabdomyolysis, his physical and mental development has stayed within the normal range. Patients with a perinatal onset of myopathic-type very-long-chain acyl-CoA dehydrogenase deficiency have not yet been reported. His novel mutation might be related to his clinical characterization.

Mitochondrial long chain fatty acid beta-oxidation in man and mouse

Several mouse models for mitochondrial fatty acid beta-oxidation (FAO) defects have been developed. So far, these models have contributed little to our current understanding of the pathophysiology. The objective of this study was to explore differences between murine and human FAO. Using a combination of analytical, biochemical and molecular methods, we compared fibroblasts of long chain acyl-CoA dehydrogenase knockout (LCAD(-/-)), very long chain acyl-CoA dehydrogenase knockout (VLCAD(-/-)) and wild type mice with fibroblasts of VLCAD-deficient patients and human controls. We show that in mice, LCAD and VLCAD have overlapping and distinct roles in FAO. The absence of VLCAD is apparently fully compensated, whereas LCAD deficiency is not. LCAD plays an essential role in the oxidation of unsaturated fatty acids such as oleic acid, but seems redundant in the oxidation of saturated fatty acids. In strong contrast, LCAD is neither detectable at the mRNA level nor at the protein level in men, making VLCAD indispensable in FAO. Our findings open new avenues to employ the existing mouse models to study the pathophysiology of human FAO defects.

A Delphi clinical practice protocol for the management of very long chain acyl-CoA dehydrogenase deficiency

INTRODUCTION

Very long chain acyl-CoA dehydrogenase (VLCAD) deficiency is a disorder of oxidation of long chain fat, and can present as cardiomyopathy or fasting intolerance in the first months to years of life, or as myopathy in later childhood to adulthood. Expanded newborn screening has identified a relatively high incidence of this disorder (1:31,500), but there is a dearth of evidence-based outcomes data to guide the development of clinical practice protocols. This consensus protocol is intended to assist clinicians in the diagnosis and management of screen-positive newborns for VLCAD deficiency until evidence-based guidelines are available.

METHOD

The Oxford Centre for Evidence-based Medicine system was used to grade the literature review and create recommendations graded from A (evidence level of randomized clinical trials) to D (expert opinion). Delphi was used as the consensus tool. A panel of 14 experts (including clinicians, diagnostic laboratory directors and researchers) completed three rounds of survey questions and had a face-to-face meeting.

RESULT

Panelists reviewed the initial evaluation of the screen-positive infant, diagnostic testing and management of diagnosed patients. Grade C and D consensus recommendations were made in each of these three areas. The panel did not reach consensus on all issues, particularly in the dietary management of asymptomatic infants diagnosed by newborn screening.

Choice of oils for essential fat supplements can enhance production of abnormal metabolites in fat oxidation disorders

Patients with mitochondrial long-chain fat oxidation deficiencies are usually treated with diets containing reduced fat and increased carbohydrate, at times via gastrostomy feeding. To ensure adequate intake of essential fatty acids, supplements are provided to their diets using commercially available oils. These oils contain large quantities of non-essential fats that are preferentially oxidized and produce disease-specific metabolites (acyl-CoA intermediates) due to the genetic defect. This study describes the concentrations of these intermediates as reflected by acylcarnitines as well as the % contribution from each of four fatty acids: palmitate, oleate, linoleate, and alpha-linolenate when incubated with fibroblasts from patients with VLCAD, LCHAD, and trifunctional protein (TFP) deficiencies. Palmitate and oleate produce the majority of disease-specific acylcarnitines with these defective cell lines (79-94%) whereas linoleate and linolenate produced less (6-21%). On average, the amount of acylcarnitines decreased with increasing unsaturation (C18:1>C18:2>C18:3:34%>11%>3%, respectively. This relationship may reflect the "gatekeeper" role of carnitine palmitoyltransferase I (CPT I). A diet comparison between Canola and a combination of Flax/Walnut oils revealed that the latter, containing the least amount of non-essential fats, reduced blood acylcarnitine levels by 33-36%. The etiology of the severe peripheral neuropathy of TFP deficiency may result from the unique metabolite, 3-keto-acyl-CoA, after conversion to a methylketone via spontaneous decarboxylation. Essential fatty acid supplementation with oils should consider these findings to decrease production of disease-specific acyl-CoA intermediates.

Glutaric aciduria type II: a case report

We report a case of a newborn with Glutaric aciduria type II. Pregnancy was complicated by polyhidramnios and fetal bradycardia. Cardiomegaly was detected by fetal echocardiography. The baby was admitted to the Neonatal Intensive Care Unit of Chieti with respiratory distress syndrome immediately after delivery. He showed head and neck edema, micrognathia, paucity of movement, pronounced hypotonia, bilateral cryptorchidism, micropenis, small hands, skin hyperelasticity and joint hypermobility. Serum and urine analysis showed a fatty acid beta-oxidation disorder. He died at 7 days of age for cardiac arrest and autopsy showed marked hepatic and cardiac vacuolisation, lipid storage myopathy and glial cells vacuolisation. Based upon these findings, we speculate that this infant may be suffering from inborn metabolic disease.

Human acyl-CoA dehydrogenase-9 plays a novel role in the mitochondrial beta-oxidation of unsaturated fatty acids

Unsaturated fatty acids play an important role in the prevention of human diseases such as diabetes, obesity, cancer, and neurodegeneration. However, their oxidation in vivo by acyl-CoA dehydrogenases (ACADs) that catalyze the first step of each cycle of mitochondrial fatty acid beta-oxidation is not entirely understood. Recently, a novel ACAD (ACAD-9) of unknown function that is highly homologous to human very-long-chain acyl-CoA dehydrogenase was identified by large-scale random sequencing. To characterize its enzymatic role, we have expressed ACAD-9 in Escherichia coli, purified it, and determined its pattern of substrate utilization. The N terminus of the mature form of the enzyme was identified by in vitro mitochondrial import studies of precursor protein. A 37-amino acid leader peptide was cleaved sequentially by two mitochondrial peptidases to yield a predicted molecular mass of 65 kDa for the mature subunit. Submitochondrial fractionation studies found native ACAD-9 to be associated with the mitochondrial membrane. Gel filtration analysis indicated that, like very-long-chain acyl-CoA dehydrogenase, ACAD-9 is a dimer, in contrast to the other known ACADs, which are tetramers. Purified mature ACAD-9 had maximal activity with long-chain unsaturated acyl-CoAs as substrates (C16:1-, C18:1-, C18:2-, C22:6-CoA). These results suggest a previously unrecognized role for ACAD-9 in the mitochondrial beta-oxidation of long-chain unsaturated fatty acids. Because of the substrate specificity and abundance of ACAD-9 in brain, we speculate that it may play a role in the turnover of lipid membrane unsaturated fatty acids that are essential for membrane integrity and structure.

Changes in blood carnitine and acylcarnitine profiles of very long-chain acyl-CoA dehydrogenase-deficient mice subjected to stress

BACKGROUND

In humans with deficiency of the very long-chain acyl-CoA dehydrogenase (VLCAD), C14-C18 acylcarnitines accumulate. In this paper we have used the VLCAD knockout mouse as a model to study changes in blood carnitine and acylcarnitine profiles under stress.

DESIGN

VLCAD knockout mice exhibit stress-induced hypoglycaemia and skeletal myopathy; symptoms resembling human VLCADD. To study the extent of biochemical derangement in response to different stressors, we determined blood carnitine and acylcarnitine profiles after exercise on a treadmill, fasting, or exposure to cold.

RESULTS

Even in a nonstressed, well-fed state, knockout mice presented twofold higher C14-C18 acylcarnitines and a lower free carnitine of 72% as compared to wild-type littermates. After 1 h of intense exercise, the C14-C18 acylcarnitines in blood significantly increased, but free carnitine remained unchanged. After 8 h of fasting at 4 degrees C, the long-chain acylcarnitines were elevated 5-fold in knockout mice in comparison with concentrations in unstressed wild-type mice (P < 0.05), and four out of 12 knockout mice died. Free carnitine decreased to 44% as compared with unstressed wild-type mice. An increase in C14-C18 acylcarnitines and a decrease of free carnitine were also observed in fasted heterozygous and wild-type mice.

CONCLUSIONS

Long-chain acylcarnitines in blood increase in knockout mice in response to different stressors and concentrations correlate with the clinical condition. A decrease in blood free carnitine in response to severe stress is observed in knockout mice but also in wild-type littermates. Monitoring blood acylcarnitine profiles in response to different stressors may allow systematic analysis of therapeutic interventions in VLCAD knockout mice.

Analysis of organic acids after incubation with (16-2H3)palmitic acid in fibroblasts from patients with mitochondrial beta-oxidation defects

The analysis of acylcarnitines as products of incubation of intact fibroblasts with isotope-labelled precursors, usually (16-(2)H(3))hexadecanoic acid, is an advanced in vitro method for the study of mitochondrial beta-oxidation defects. We propose a technique for the measurement of the organic acid intermediates after hydrolysis of the acylcarnitines using electron-impact gas chromatography-mass spectrometry. For some mitochondrial beta-oxidation deficiencies, the characteristic profile enables us to approach the diagnosis with clear differentiation.

Strategies for the diagnosis of mitochondrial fatty acid beta-oxidation disorders

Mitochondrial fatty acid beta-oxidation disorders (FAOD) are a group of clinically and biochemically heterogeneous inherited metabolic defects. The spectrum of phenotypes has expanded from hepatic encephalopathy to encompass myopathy, cardiomyopathy, peripheral neuropathy, sudden death and pregnancy complicated by fetal FAOD. Pre-symptomatic diagnosis is important to prevent morbidity and this is now achievable through newborn screening using tandem mass spectrometry (MS/MS). Moreover, most of the diagnosed defects are treatable and the prognosis is generally favourable. This article reviews the features of FAOD, critically evaluates methods of investigation including metabolite analyses in body fluids, in vitro oxidation rates and acylcarnitine profiling studies, enzymatic and mutational tests, and discusses genotype-phenotype correlation, treatment and monitoring options. Based on this knowledge, strategies for the biochemical investigation and differential diagnosis of patients presenting clinically, asymptomatic neonates detected by newborn screening, infants born after complications during late pregnancy, and cases of sudden death with suspected FAOD are presented. Laboratory investigation commonly begins with a search for diagnostic metabolites in physiological fluids, followed by in vitro functional studies if the initial findings are inconclusive, and confirmation by enzymology and molecular analyses. Occasionally a stress test in vivo may be required. At other times there may be no firm diagnosis achieved.

Quantitative fibroblast acylcarnitine profiles in mitochondrial fatty acid beta-oxidation defects: phenotype/metabolite correlations

Mitochondrial fatty acid beta-oxidation (FAO) disorders are clinically and biochemically heterogeneous diseases mainly associated with intolerance to catabolic stress. These disorders can now be detected pre-symptomatically by newborn screening, and thus the clinical phenotype in an individual patient may be unclear. Correlation of clinical severity with concentrations of acylcarnitine species was investigated in fibroblasts from FAO-deficient patients presenting with various phenotypes and asymptomatic neonates detected by newborn screening. Intact cells were incubated in medium containing deuterium-labelled hexadecanoic acid and L-carnitine for 72h, and the accumulated acylcarnitines in the culture medium analysed using electrospray tandem mass spectrometery. Fibroblasts from patients with long-chain FAO disorders presenting at an early age and with poor clinical outcomes accumulated higher concentrations of long-chain acylcarnitine species compared with those from patients with milder phenotypes. This suggests that the in vitro quantitative acylcarnitine profiling could perhaps predict the prognosis of some FAO defects. This would be particularly useful information for the asymptomatic/pre-symptomatic FAO-deficient infant detected by the expanded newborn screening program, in whom the risk of developing symptoms later in life is not known.

Treatment of cardiomyopathy and rhabdomyolysis in long-chain fat oxidation disorders using an anaplerotic odd-chain triglyceride

The current dietary treatment of long-chain fatty acid oxidation defects (high carbohydrate with medium-even-chain triglycerides and reduced amounts of long-chain fats) fails, in many cases, to prevent cardiomyopathy, rhabdomyolysis, and muscle weakness. We hypothesized that the apparent defect in energy production results from a depletion of the catalytic intermediates of the citric acid cycle via leakage through cell membranes (cataplerosis). We further hypothesized that replacing dietary medium-even-chain fatty acids (precursors of acetyl-CoA) by medium-odd-chain fatty acids (precursors of acetyl-CoA and anaplerotic propionyl-CoA) would restore energy production and improve cardiac and skeletal muscle function. We fed subjects with long-chain defects a controlled diet in which the fat component was switched from medium-even-chain triglycerides to triheptanoin. In three patients with very-long-chain acyl-CoA dehydrogenase deficiency, this treatment led rapidly to clinical improvement that included the permanent disappearance of chronic cardiomyopathy, rhabdomyolysis, and muscle weakness (for more than 2 years in one child), and of rhabdomyolysis and weakness in the others. There was no evidence of propionyl overload in these patients. The treatment has been well tolerated for up to 26 months and opens new avenues for the management of patients with mitochondrial fat oxidation disorders.

Inherited disorders of mitochondrial fatty acid oxidation: a new responsibility for the neonatologist

The disorders of mitochondrial fat oxidation present clinically with three major clinical phenotypes: hypoketotic hypoglycemia, cardiomyopathy, and myopathy. Although these features can present together in some of the disorders, one will be the dominant clinical problem. This review will describe these clinical phenotypes while addressing the diagnostic value of various clinical and laboratory studies which are often used for making these diagnoses. With knowledge of the clinical presentation, these diagnoses can often be made very rapidly and at relatively low cost by more specific laboratory tests. The increasing availability of expanded newborn screening by tandem mass spectrometry as well as prenatal diagnosis for these often fatal disorders now provides the opportunity for pre-symptomatic diagnosis. The neonatologist is now in the unique position of identifying these inherited disorders prior to or during severe symptom onset and has the earliest opportunity to provide successful treatment intervention.