Purification of electron transfer flavoprotein from pig liver mitochondria and its application to the diagnosis of deficiencies of acyl-CoA dehydrogenases in human fibroblasts

Development of a Tandem Mass Spectrometry Method for Rapid Measurement of Medium- and Very-Long-Chain Acyl-CoA Dehydrogenase Activity in Fibroblasts

Mitochondrial fatty acid oxidation is a vital biochemical process for energy metabolism. Among the known fatty-acid metabolism disorders, very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency and medium-chain acyl-CoA dehydrogenase (MCAD) deficiency count among the most frequent. Both are potentially very serious diseases as they carry a risk of severe neurological post-crisis sequelae, and even sudden death. Diagnosis relies on plasma acylcarnitine profile analysis and urine organic acid analysis, followed by genetic testing to confirm diagnosis. However, in some cases, it is crucial to run a specific diagnostic assay for enzyme activity, which is generally performed in leukocytes or fibroblasts. The aim of this study was to address this need, first by developing a MCAD and VLCAD enzyme activity-specific diagnostic assay in fibroblasts (by measuring the reaction products, i.e. enoyl-CoA) via a rapid LC-MS/MS-based technique, and then by testing MCAD-deficient patients (n = 6), VLCAD-deficient patients (n = 10), and control patients (n = 12). MCAD activity was significantly different in the MCAD-deficiency (MCADD) group (mean = 0.07 nmol C8:1 formed/min/mg protein) compared to the control group (mean = 0.36 nmol C8:1 formed/min/mg protein). All MCADD patients showed less than 35% residual MCAD activity. VLCAD activity was significantly decreased in the VLCADD group (mean = 0.06 nmol C16:1 formed/min/mg protein) compared to the control group (mean = 0.86 nmol C16:1 formed/min/mg protein, respectively). All VLCADD patients showed less than 35% residual VLCAD activity. This technique allowed also to confirm that a novel ACADVL gene mutation (c.1400T>C) is responsible for a defective VLCAD activity (residual activity at 10%).

Multiple acyl-CoA dehydrogenase deficiency (MADD) as a cause of late-onset treatable metabolic disease

INTRODUCTION

Late-onset multiple acyl-CoA dehydrogenase deficiency (MADD) is a rare, treatable, beta-oxidation disorder responsible for neuromuscular symptoms in adults. This case series describes the clinical and biochemical features of 13 French patients with late-onset MADD.

METHODS AND RESULTS

Thirteen ambulant patients (eight women, five men), with a median age at onset of 27 years, initially experienced exercise intolerance (n=9), isolated muscle weakness (n=1) and a multisystemic pattern with either central nervous system or hepatic dysfunction (n=3). During the worsening period, moderate rhabdomyolysis (n=5), a pseudomyasthenic pattern (n=5) and acute respiratory failure (n=1) have been observed. Weakness typically affected the proximal limbs and axial muscles, and there was sometimes facial asymmetry (n=3). Moderate respiratory insufficiency was noted in one case. Median baseline creatine kinase was 190IU/L. Lactacidemia was sometimes moderately increased at rest (3/10) and after exercise (1/3). The acylcarnitine profile was characteristic, with increases in all chain-length acylcarnitine species. Electromyography revealed a myogenic pattern, while muscle biopsy showed lipidosis, sometimes with COX-negative fibers (n=2). The mitochondrial respiratory chain was impaired in five cases, with coenzyme Q10 decreased in two cases. All patients harbored mutations in the ETFDH gene (four homozygous, seven compound heterozygous, two single heterozygous), with nine previously unidentified mutations. All patients were good responders to medical treatment, but exercise intolerance and/or muscular weakness persisted in 11 of them.

CONCLUSION

Late-onset forms of MADD may present as atypical beta-oxidation disorders. Acylcarnitine profiling and muscle biopsy remain the most decisive investigations for assessing the diagnosis. These tests should thus probably be performed more widely, particularly in unexplained cases of neuromuscular and multisystemic disorders.

A novel mutation of the ACADM gene (c.145C>G) associated with the common c.985A>G mutation on the other ACADM allele causes mild MCAD deficiency: a case report

A female patient, with normal familial history, developed at the age of 30 months an episode of diarrhoea, vomiting and lethargy which resolved spontaneously. At the age of 3 years, the patient re-iterated vomiting, was sub-febrile and hypoglycemic, fell into coma, developed seizures and sequels involving right hemi-body. Urinary excretion of hexanoylglycine and suberylglycine was low during this metabolic decompensation. A study of pre- and post-prandial blood glucose and ketones over a period of 24 hours showed a normal glycaemic cycle but a failure to form ketones after 12 hours fasting, suggesting a mitochondrial β-oxidation defect. Total blood carnitine was lowered with unesterified carnitine being half of the lowest control value. A diagnosis of mild MCAD deficiency (MCADD) was based on rates of 1-¹⁴C-octanoate and 9, 10-³H-myristate oxidation and of octanoyl-CoA dehydrogenase being reduced to 25% of control values. Other mitochondrial fatty acid oxidation proteins were functionally normal. De novo acylcarnitine synthesis in whole blood samples incubated with deuterated palmitate was also typical of MCADD. Genetic studies showed that the patient was compound heterozygous with a sequence variation in both of the two ACADM alleles; one had the common c.985A>G mutation and the other had a novel c.145C>G mutation. This is the first report for the ACADM gene c.145C>G mutation: it is located in exon 3 and causes a replacement of glutamine to glutamate at position 24 of the mature protein (Q24E). Associated with heterozygosity for c.985A>G mutation, this mutation is responsible for a mild MCADD phenotype along with a clinical story corroborating the emerging literature view that patients with genotypes representing mild MCADD (high residual enzyme activity and low urinary levels of glycine conjugates), similar to some of the mild MCADDs detected by MS/MS newborn screening, may be at risk for disease presentation.

Electron transfer flavoprotein deficiency: functional and molecular aspects

Multiple acyl-CoA dehydrogenase deficiency (MADD) is a recessively inherited metabolic disorder that can be due to a deficiency of electron transfer flavoprotein (ETF) or its dehydrogenase (ETF-ubiquinone oxidoreductase). ETF is a mitochondrial matrix protein consisting of alpha- (30kDa) and beta- (28kDa) subunits encoded by the ETFA and ETFB genes, respectively. In the present study, we have analysed tissue samples from 16 unrelated patients with ETF deficiency, and we report the results of ETF activity, Western blot analysis and mutation analysis. The ETF assay provides a reliable diagnostic tool to confirm ETF deficiency in patients suspected to suffer from MADD. Activity ranged from less than 1 to 16% of controls with the most severely affected patients disclosing the lowest activity values. The majority of patients had mutations in the ETFA gene while only two of them harboured mutations in the ETFB gene. Nine novel disease-causing ETF mutations are reported.

Recurrent vomiting and ethylmalonic aciduria associated with rare mutations of the short-chain acyl-CoA dehydrogenase gene

We report identification of short-chain acyl-CoA dehydrogenase (SCAD) deficiency in a 12-year-old boy who suffered from recurrent attacks of vomiting once or twice a year from infancy. Growth and development were normal and there were no muscular symptoms. Metabolic screening was performed during a hospitalization at 8 years of age and revealed an increased excretion of ethylmalonic acid (EMA; 45-80 mmol/mol creatinine, normal 0.2-6.6), suggesting a degradation defect of short-chain fatty acids. An increased n-butyrylcarnitine was found in freshly collected serum (0.9 micromol/L; normal <0.4) but not in dry blood spots. Neither of the frequent SCAD gene variants 625G>A and 511C>T was present, but direct sequencing of the promoter and coding regions of the SCAD gene revealed that the patient had mutations on both alleles: 417G>C (Trpl15Cys) and 1095G>T (Gln341His). Neither mutation has been described before in compound heterozygosity or homozygosity. Enzymatic investigations subsequently confirmed a defect of SCAD in both fibroblasts and muscle extracts. Furthermore, expression studies of both mutations demonstrated impaired enzyme function or structure. To our knowledge, this case is the first description of a patient with proven SCAD deficiency presenting with recurrent emesis but without other symptoms, and emphasizes the wide clinical phenotype of this disorder.

Respiratory complex II defect in siblings associated with a symptomatic secondary block in fatty acid oxidation

The mitochondrial oxidative phosphorylation and fatty acid oxidation pathways have traditionally been considered independent major sources of cellular energy production; however, case reports of patients with specific enzymatic defects in either pathway have suggested the potential for a complex interference between the two. This study documents a new site of interference between the two pathways, a site in respiratory complex II capable of producing clinical signs of a block in fatty acid oxidation and reduced in vitro activity of acyl-CoA dehydrogenases. The initial patient, and later her newborn sibling, had mildly dysmorphic features, lactic acidosis and a defect in mitochondrial respiratory complex II associated with many biochemical features of a block in fatty acid oxidation. Results of in vitro probing of intact fibroblasts from both patients with methyl[2H3]palmitate and L-carnitine revealed greatly increased [2H3]butyrylcarnitine; however, the ratio of dehydrogenase activity with butyryl-CoA with anti-MCAD inactivating antibody (used to reveal SCAD-specific activity) to that with octanoyl-CoA was normal, excluding a selective SCAD or MCAD deficiency. Respiratory complex II was defective in both patients, with an absent thenoyltrifluoroacetone-sensitive succinate Q reductase activity that was partially restored by supplementation with duroquinone. Although secondary, the block in fatty acid oxidation was a major management problem since attempts to provide essential fatty acids precipitated acidotic decompensations. This study reinforces the need to pursue broadly the primary genetic defect within these two pathways, making full use of increasingly available functional and molecular diagnostic tools.

Purification and characterization of a mammalian homolog of Escherichia coli MutY mismatch repair protein from calf liver mitochondria

A protein homologous to the Escherichia coli MutY glycosylase, referred to as mtMYH, has been purified from calf liver mitochondria. SDS-polyacrylamide gel electrophoresis, western blot analysis as well as gel filtration chromatography predicted the molecular mass of the purified calf mtMYH to be 35-40 kDa. Gel mobility shift analysis showed that the purified mtMYH formed specific binding complexes with A/8-oxoG, G/8-oxoG and T/8-oxoG, weakly with C/8-oxoG, but not with A/G and A/C mismatches. The purified mtMYH exhibited DNA glycosylase activity removing adenine mispaired with G, C or 8-oxoG and weakly removing guanine mispaired with 8-oxoG. The mtMYH glycosylase activity was insensitive to high concentrations of NaCl and EDTA. The purified mtMYH cross-reacted with antibodies against both intact MutY and a peptide of human MutY homolog (hMYH). DNA glycosylase activity of mtMYH was inhibited by anti-MutY antibodies but not by anti-hMYH peptide antibodies. Together with the previously described mitochondrial MutT homolog (MTH1) and 8-oxoG glycosylase (OGG1, a functional MutM homolog), mtMYH can protect mitochondrial DNA from the mutagenic effects of 8-oxoG.

Mitochondrial very-long-chain acyl-coenzyme A dehydrogenase deficiency: clinical characteristics and diagnostic considerations in 30 patients

Very-long-chain acyl-CoA dehydrogenase (VLCAD) is an enzyme catalyzing the dehydrogenation of long-chain fatty acids in the first step of mitochondrial fatty acid oxidation. Using an ETF (electron transfer flavoprotein, the physiological electron acceptor of VLCAD) reduction assay, we identified VLCAD deficiency in cultured skin fibroblasts or liver tissue from 30 patients in 27 families. They clinically presented two phenotypes: a 'severe' presentation characterized by an early onset of symptoms, with hypertrophic cardiomyopathy and a high incidence of death, and a 'mild' form with hypoketotic hypoglycaemia, resembling MCAD (medium-chain acyl-CoA dehydrogenase) deficiency. Cells isolated from patients who develop cardiomyopathy characteristically accumulate longer-chain length acylcarnitines (hexadecanoylcarnitine and tetradecanoylcarnitine) when incubated with palmitate. However, cells from patients with the hypoglycaemic presentation produced relatively shorter-chain-length intermediates (mainly dodecanoylcarnitine). Inhibition of carnitine palmitoyl transferase I, in vitro, eliminated these intermediates with cells from both phenotypes indicating their intramitochondrial origin. Although the explanation for these distinct biochemical findings is not obvious, the correlation with the two phenotypes provides an opportunity for accurate prognosis and early implementation of appropriate treatment. Prenatal diagnosis of this life-threatening disorder was successfully performed in seven pregnancies in six of those families by assay of trophoblasts or amniocytes. In an at risk family, diagnosis of an affected fetus by measurement of VLCAD activity in noncultured chorionic villi allowed termination of the pregnancy before 13 weeks of gestation.

Prenatal diagnosis of mitochondrial fatty acid oxidation defects

Amniocytes isolated from two pregnancies at risk for fatty acid oxidation defects were incubated with stable isotopically labelled palmitate, in the presence of L-carnitine, to probe that pathway. The labelled acylcarnitines were then quantitated using tandem mass spectrometry. Amniocytes from a pregnancy at risk for medium-chain acyl-CoA dehydrogenase (MCAD) deficiency produced a characteristic acylcarnitine profile with increased levels of octanoylcarnitine and decanoylcarnitine, indicative of MCAD deficiency. DNA analysis confirmed that the fetus was homozygous for the MCAD A985G mutation. Acylcarnitine and DNA analysis of the infant's blood obtained post-partum confirmed MCAD deficiency. Amniocytes from a pregnancy at risk for an unspecified fat oxidation defect produced increased levels of long-chain acylcarnitines consistent with a deficiency in very-long-chain acyl-CoA dehydrogenase (VLCAD). Measurements of the enzymatic activity confirmed VLCAD deficiency in amniocytes. Acylcarnitine profiles of the infant's blood obtained post-partum in addition to enzyme activities measured in fibroblasts confirmed VLCAD deficiency. The successful prenatal diagnosis of VLCAD and MCAD deficiencies using in vitro probes of fatty acid oxidation in fibroblasts suggests that this approach can potentially recognize many mitochondrial fatty acid oxidation defects even if no prior diagnosis is determined in the family at risk.

Carnitine palmitoyl transferase I deficiency presenting as a Reye-like syndrome without hypoglycaemia

An apparently healthy girl aged 2 years 9 months developed a coma with hepatomegaly within 24 h after an influenza-like infection. Plasma glucose and urinary organic acid profile were normal but plasma and urinary carnitine concentrations were increased. Despite symptomatic therapy, she died 11 days later. Oxidation of [1-14C] palmitic acid in the patient's fibroblasts was severely decreased (13% of controls). Further investigations revealed a deficiency of carnitine palmitoyl transferase I (CPT I) in the patient's fibroblasts (15% of controls) whereas CPT II activity was normal. Only four patients with CPT I deficiency have been reported so far. The subtle clinical and biochemical presentation of this disorder, which may account for the small number of cases diagnosed, is discussed.

Very long chain acyl-CoA dehydrogenase deficiency: identification of a new inborn error of mitochondrial fatty acid oxidation in fibroblasts

A patient highly suspected of long chain fatty acid oxidation defect was investigated for membrane-bound palmitoyl-CoA dehydrogenation in a membrane extract from skin fibroblasts, using 1% sodium cholate as detergent. The profoundly decreased activity observed is consistent with a deficiency of the newly identified mitochondrial 'very long chain acyl-CoA dehydrogenase'.