The use of phenylpropionic acid as a loading test for medium-chain acyl-CoA dehydrogenase deficiency

Screening for medium chain acyl-CoA dehydrogenase deficiency using electrospray ionisation tandem mass spectrometry

OBJECTIVE

To establish criteria for the diagnosis of medium chain acyl-CoA dehydrogenase (MCAD) deficiency in the UK population using a method in which carnitine species eluted from blood spots are butylated and analysed by electrospray ionisation tandem mass spectrometry (ESI-MS/MS).

DESIGN

Four groups were studied: (1) 35 children, aged 4 days to 16.2 years, with proven MCAD deficiency (mostly homozygous for the A985G mutation, none receiving carnitine supplements); (2) 2168 control children; (3) 482 neonates; and (4) 15 MCAD heterozygotes.

RESULTS

All patients with MCAD deficiency had an octanoylcarnitine concentration ([C8-Cn]) > 0.38 microM and no accumulation of carnitine species > C10 or < C6. Among the patients with MCAD deficiency, the [C8-Cn] was significantly lower in children > 10 weeks old and in children with carnitine depletion (free carnitine < 20 microM). Neonatal blood spots from patients with MCAD deficiency had a [C8-Cn] > 1.5 microM, whereas in heterozygotes and other normal neonates the [C8-Cn] was < 1.0 microM. In contrast, the blood spot [C8-Cn] in eight of 27 patients with MCAD deficiency > 10 weeks old fell within the same range as five of 15 MCAD heterozygotes (0.38-1.0 microM). However, the free carnitine concentrations were reduced (< 20 microM) in the patients with MCAD deficiency but normal in the heterozygotes.

CONCLUSIONS

Criteria for the diagnosis of MCAD deficiency using ESI-MS/MS must take account of age and carnitine depletion. If screening is undertaken at 7-10 days, the number of false positive and negative results should be negligible. Because there have been no instances of death or neurological damage following diagnosis of MCAD deficiency in our patient group, a strong case can be made for neonatal screening for MCAD deficiency in the UK.

Simple high-performance liquid chromatographic method for the detection of phenylpropionylglycine in urine as a diagnostic tool in inherited medium-chain acyl-coenzyme A dehydrogenase deficiency

Deficiency of medium-chain acyl-CoA dehydrogenase is a frequent and treatable metabolic defect, which can be diagnosed by detection of phenylpropionylglycine in urine after an oral load of phenylpropionic acid. We studied the determination of phenylpropionylglycine in urine by isocratic ion-exclusion chromatography on a cation-exchange column using water-sulphuric acid (pH values between 2 and 4) as mobile phase. Phenylpropionylglycine, phenylpropionic acid and hippuric acid exhibited high retention factors with only a slight decline at increasing solvent pH. This resulted in a good separation from interfering substances after direct injection of urine. We hypothesize that pi-pi interactions between the aromatic carbonic acids and the ion-exchange resin are responsible for the strong retention on the stationary phase. We conclude that, even in asymptomatic patients, determination of phenylpropionylglycine in urine after a phenylpropionic acid load by ion-exclusion chromatography is a rapid and reliable diagnostic tool for the detection of medium-chain acyl-CoA dehydrogenase deficiency.

Morbidity and mortality in medium chain acyl coenzyme A dehydrogenase deficiency

Medium chain acyl coenzyme A dehydrogenase (MCAD) deficiency presents with episodic fasting, hypoketotic hypoglycaemia, and coma. It is known to be potentially lethal, but the outlook for survivors is thought to be good. We reassessed all patients with MCAD deficiency diagnosed in New South Wales (population six million) to explore long term morbidity and mortality. There were 16 probands and two siblings were confirmed and two presumed to be affected. Assuming an incidence of 1:20,000 births, these represented about 22% of the total number of expected cases. Five (25%) of the 20 patients died aged 3 days-30 months, all during the first episode of illness. Seven others had only one episode and one affected sibling was asymptomatic. Eight had had significant neonatal symptoms. Only two had a significant, serious life threatening episode after diagnosis. Of 15 survivors, one has severe handicap after a single severe episode, and four, aged 9-17 years, have mild intellectual handicap. Eight (including six aged less than 7 years), have apparently normal development. Two are lost to follow up. Our study of unselected patients with MCAD deficiency from a defined population shows not only a substantial risk of death, but also of long term morbidity.

Group tests for selective screening of inborn errors of metabolism

Selective screening for inherited metabolic disorders can be performed efficiently by chromatographic techniques. Each technique is suited for a well-defined group of substances present in urine, plasma and CSF. A comprehensive screening programme may involve the analysis of amino acids, organic acids, imidazoles, purines and pyrimidines, oligosaccharides and mucopolysaccharides in urine as well as very long-chain fatty acids in plasma. The experienced laboratory, in close co-operation with a specialized paediatrician, will make a positive diagnosis in 6% of the referred samples, provided a careful selection of the patients is made. Our experience of 10 years of screening revealed 100 different defects; 30% of these were so-called amino acid disorders, 50% organic acidurias and the remaining 20% miscellaneous defects. Chromatographic methods are well suited for the discovery of novel defects. In this respect group screening tests will remain of major importance for the study of inborn errors of metabolism.

Neonatal symptoms in medium chain acyl coenzyme A dehydrogenase deficiency

Medium chain acyl coenzyme A dehydrogenase (MCAD) deficiency has not been thought to be associated with significant neonatal symptoms. To determine the validity of this, all known MCAD cases from New South Wales were reassessed. A total of 16 confirmed and three presumed cases has been identified in New South Wales, from 15 families. The casenotes of patients were reviewed, and where possible the mothers interviewed, either directly or by telephone, to obtain information about neonatal events. Six of the 16 confirmed cases had significant neonatal symptoms, with onset from 17 hours to 3 days of age. All required intravenous dextrose and four of the six needed other interventions, including hospital transfer. One baby died. All six were breast fed, but so were five of the eight asymptomatic neonates for whom information was available. Four of the six symptomatic neonates were homozygous for the common MCAD mutation, an A to G transition at position 985, and one was heterozygous. It is concluded that serious neonatal symptoms are common in MCAD. Newborn siblings of MCAD cases must have careful monitoring and support during the first few days of life.

Novel glycine conjugates in medium-chain acyl-CoA dehydrogenase deficiency

The glycine conjugates of isocaproic, 4-methylhexanoic, 7-hydroxyoctanoic and 8-hydroxyoctanoic acids have been identified in the urine of children with medium-chain acyl-CoA dehydrogenase (MCAD) deficiency using gas chromatography-mass spectrometry of the trimethylsilyl derivatives. A quantitative study showed that the glycine conjugates of isocaproic and 4-methylhexanoics acids were excreted during acute episodes and in smaller amounts when subjects were asymptomatic. The glycine conjugates of 7-hydroxyoctanoic and 8-hydroxyoctanoic acids were detectable during acute episodes. None of the conjugates was detected in controls or controls receiving a diet containing medium-chain triglycerides. It is suggested that the glycine conjugates of isocaproic acid and 4-methylhexanoic acid are metabolites of branched-chain fatty acids and that they are specific for MCAD deficiency.

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

Medium chain acyl-CoA dehydrogenase (MCAD) and long chain acyl-CoA dehydrogenase (LCAD) deficiency are defects of mitochondrial beta-oxidation. The method of choice to measure specifically acyl-CoA dehydrogenase activity in human tissues uses purified electron transfer flavoprotein (ETF). We describe a simple and optimized method of purification allowing isolation of ETF with a degree of purity never reported so far. An assay for acyl-CoA dehydrogenase activity in cultured skin fibroblasts was developed using microquantities of electron transfer flavoprotein and substrate. MCAD deficiency was demonstrated in fibroblasts from nine patients and LCAD deficiency in fibroblasts from two patients.

Alpha-oxidation of fatty acids in fasted or diabetic rats

Induction of alpha-oxidation, a possible gluconeogenic process, which should produce odd-chain fatty acids from even-chain fatty acids, was studied in rats fasted or made diabetic with streptozotocin. When a omega-phenylated even-chain fatty acid, phenylbutyric acid (1.2 mmol/kg), was administered to rats under these conditions, a significant increase in the urinary excretion of benzoic acid, the metabolic end-product of omega-phenylated odd-chain fatty acids, was observed in fasted (3.54 +/- 0.46 mumol/day) and diabetic (6.73 +/- 2.10) rats (control, 0.58 +/- 0.43; P less than 0.001). Phenylated longer chain fatty acids, phenylhexanoic and phenyldecanoic acid, did not produce significantly more benzoic acid than did phenylbutyric acid. Although the rate of alpha-oxidation was very low compared to that of beta-oxidation, these results suggested that alpha-oxidation of fatty acids was induced under fasting or diabetic conditions, and that alpha-oxidation might take place at the butyric acid stage.

Medium chain acyl-CoA dehydrogenase deficiency

From 65 reported cases of medium chain acyl-CoA dehydrogenase deficiency, we found an average presenting age of 13.5 months and a mean age at death of 18.5 months. One quarter of patients died of a Reye-like syndrome and/or sudden infant death. In half the cases there had been at least one sibling death. Asymptomatic cases were not uncommon (12% of cases). The crises were generally induced by a prolonged fast and after a viral prodromal phase in three quarters of cases. The crises consisted of somnolence progressing to lethargy which could lead to coma. Vomiting was frequent (60% of cases). Seizures, which were found in 29% of cases, represented a bad prognosis. The physical examinations revealed frequently a variable and regressive anicteric hepatomegaly. Blood and urine analysis revealed in most instances hypoglycaemia (96% of cases) with hypoketonuria and sometimes metabolic acidosis. Hepatic and muscular cytolytic enzymes were frequently raised, as were plasma ammonia, urea, and uric acid. Plasma total or free carnitine concentrations, especially non-fasting, were diminished in most cases. Plasma saturated medium chain fatty acids and particularly unsaturated cis-4-decenoate were on the other hand raised during the crises or during fasting. Urinary organic acid analysis revealed a characteristic profile of medium chain aciduria: C6-C10 dicarboxylic acids, hydroxy acids, glycine conjugates, and carnitine conjugates. Oral loading tests with carnitine or phenylpropionate allow a precise diagnosis. The diagnosis is confirmed by specific assays in various tissues. Avoidance of prolonged fasting seems to be the mainstay of treatment.

Specific diagnosis of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency in dried blood spots by a polymerase chain reaction (PCR) assay detecting a point-mutation (G985) in the MCAD gene

The discovery of a point-mutation, adenine-to-guanine, at position 985 in the gene coding for MCAD (G985), gave the basis for an easy and specific polymerase chain reaction test. We tested the specificity of such a PCR based assay and detected correctly G985 and A985 in sequence verified cDNA clones. We showed that the G985 mutation is present in genomic DNA from 48 of 50 patients with confirmed MCAD deficiency, originating from various European countries, Australia and the USA. On the basis of this high frequency of the G985 mutation among patients, we improved and optimized the assay with respect to reliability and convenience for routine diagnostic and screening purposes. As little as 2 microliters blood from filter-paper blood-spots (Guthrie spots) is sufficient for the test.

Determination of medium chain acyl-CoA dehydrogenase activity in cultured skin fibroblasts using mass spectrometry

Medium chain acyl-CoA dehydrogenase deficiency, a defect of mitochondrial beta-oxidation, is one of the most frequently occurring among inborn errors of metabolism. We describe a rapid and sensitive gas chromatographic/mass spectrometric method allowing reliable assessment of medium chain acyl-CoA dehydrogenase activity in cultured skin fibroblasts. We investigated MCAD activity in three presumed medium chain acyl-CoA dehydrogenase deficient (MCADD) patients and 10 control subjects. The medium chain acyl-CoA dehydrogenase activity determined in three patients was 1.0 +/- 0.4 nmol.min-1.mg-1 protein (mean +/- SD; range: 0.6-1.4) and in controls it was 2.8 +/- 1.0 nmol.min-1.mg-1 protein (mean +/- SD; range: 1.6-4.4).

Possible deleterious effect of L-carnitine supplementation in a patient with mild multiple acyl-CoA dehydrogenation deficiency (ethylmalonic-adipic aciduria)

A patient with riboflavin-responsive mild multiple acyl-CoA dehydrogenation deficiency of the ethylmalonic--adipic aciduria type experienced a recurrence of spontaneous hypoglycaemic episodes whilst being given supplementary L-carnitine. This phenomenon is explicable in terms of the known biochemical features of this condition and suggests caution in the carnitine supplementation of patients with defective oxidation of medium- or short-chain fatty acyl-CoA esters. This patient excreted excessive phenylpropionylglycine after an oral phenylpropionic acid load. Thus the phenylpropionic acid loading test is not completely specific for primary medium-chain acyl-CoA dehydrogenase deficiency as has been supposed.

Measurement of urinary medium chain acyl glycines by gas chromatography--negative ion chemical ionization mass spectrometry

Medium chain acyl-CoA dehydrogenase (MCAD) deficiency is an inborn error of fatty acid metabolism, which is difficult to diagnose, partly because of its unpredictable clinical presentation. A specific diagnostic marker is an increased excretion of certain medium chain acyl glycines. A sensitive and specific method has been developed for the extraction, derivatization, identification and quantitation of urinary medium chain acyl glycines by gas chromatography-negative ion chemical ionization mass spectrometry (GC-NICIMS). The following series of standard acyl glycines has been synthesized and characterized: hexanoyl, octanoyl, 3-phenylpropionyl and suberyl and their respective isotopomers (using 13C2-glycine; for use as internal standards). The range of excretion of these compounds in normal subjects has been established using this method and increased excretion of acyl glycines, particularly hexanoyl, 3-phenylpropionyl and suberyl was successfully demonstrated in three MCAD deficient subjects from one family.

In vivo enzyme activity in inborn errors of metabolism

Low-dose continuous infusions of [2H5]phenylalanine, [1-13C]propionate, and [1-13C]leucine were used to quantitate phenylalanine hydroxylation in phenylketonuria (PKU, four subjects), propionate oxidation in methylmalonic acidaemia (MMA, four subjects), and propionic acidaemia (PA, four subjects) and leucine oxidation in maple syrup urine disease (MSUD, four subjects). In vivo enzyme activity in PKU, MMA, and PA subjects was similar to or in excess of that in adult controls (range of phenylalanine hydroxylation in PKU, 3.7 to 6.5 mumol/kg/h, control 3.2 to 7.9, n = 7; propionate oxidation in MMA, 15.2 to 64.8 mumol/kg/h, and in PA, 11.1 to 36.0, control 5.1 to 19.0, n = 5). By contrast, in vivo leucine oxidation was undetectable in three of the four MSUD subjects (less than 0.5 mumol/kg/h) and negligible in the remaining subject (2 mumol/kg/h, control 10.4 to 15.7, n = 6). These results suggest that significant substrate removal can be achieved in some inborn metabolic errors either through stimulation of residual enzyme activity in defective enzyme systems or by activation of alternate metabolic pathways. Both possibilities almost certainly depend on gross elevation of substrate concentrations. By contrast, only minimal in vivo oxidation of leucine appears possible in MSUD.

Analysis of abnormal urinary metabolites in the newborn period in medium-chain acyl-CoA dehydrogenase deficiency

In order to determine which are useful early diagnostic markers for medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, we have analysed urine from an asymptomatic neonate. Profiling of urinary organic acids followed by peak confirmation by electron impact mass spectrometry revealed a high suberate/adipate ratio (greater than 1.0) and the presence of n-hexanoylglycine (HG). Acylcarnitine analysis by fast atom bombardment mass spectrometry (FAB-MS) was inconclusive, but FAB-MS/MS (tandem mass spectrometry) revealed diagnostic amounts of octanoylcarnitine and hexanoylcarnitine. Quantitative analysis of acylglycines by stable isotope dilution and chemical ionization mass spectrometry revealed a 30-fold increase in HG and increased suberylglycine, but no increase in 3-phenylpropionylglycine.

Stable isotope dilution analysis of n-hexanoylglycine, 3-phenylpropionylglycine and suberylglycine in human urine using chemical ionization gas chromatography/mass spectrometry selected ion monitoring

We describe a gas chromatographic/mass spectrometric method for the accurate determination of n-hexanoylglycine, 3-phenylpropionylglycine and suberylglycine in urine for the diagnosis of hereditary medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. These acylglycines had previously been detected in urine from patients with MCAD deficiency, but their diagnostic values were unknown because of a lack of appropriate analytical methods. n-Hexanoyl(1,2-13C)glycine, 3-phenylpropionyl(2-13C,15N)glycine and suberyl(2-13C,15N)glycine were synthesized and used as internal standards. Ammonia chemical ionization was utilized to generate intense [M + H]+ ions for selected-ion monitoring quantification. The whole procedure is fast and can be performed by a low-resolution gas chromatographic/mass spectrometric system, giving accurate results over a range of three orders of magnitude (0.0167-16.7 micrograms/ml). The results from the analyses of 54 urine samples from 21 MCAD-deficient patients and various control samples using this method established that n-hexanoyglycine and 3-phenylpropionylglycine were highly diagnostic for this disease, while suberylglycine was found less specific.

Disorders of mitochondrial beta-oxidation: prenatal and early postnatal diagnosis and their relevance to Reye's syndrome and sudden infant death

There are still many problems with the diagnosis and classification of inherited disorders of mitochondrial beta-oxidation. At present only the acyl-CoA dehydrogenase step of the beta-oxidation spiral has been explored in any detail and a large number of patients have disorders that cannot be properly characterized. beta-Oxidation defects may present in a wide variety of ways, the most dramatic being acute encephalopathy with hepatic involvement (atypical Reye's syndrome) or 'sudden' death. Investigations may include urinary and plasma organic acids, metabolic stress tests and assays of overall metabolic pathways or of specific enzymes in cultured fibroblasts, lymphocytes, or other material. Early postnatal diagnosis presents particular difficulties but in medium-chain acyl-CoA dehydrogenase deficiency the diagnosis may be apparent from careful examination of urine. There is as yet little general experience in prenatal diagnosis of this group of disorders except for glutaric aciduria type II. Single prenatal diagnoses of medium-chain acyl-CoA dehydrogenase deficiency and of an incompletely characterized defect of medium-chain fatty acid oxidation have been performed.