Commentary: What degree of hyperphenylalaninaemia requires treatment?

Despite some 50 years' experience in the treatment of phenylketonuria and numerous scientific publications on the subject there is no clear consensus as to what degree of hyperphenylalaninaemia will result in intellectual impairment. Studies of three main types, on untreated cases of moderate hyperphenylalaninaemia, on treated cases of phenylketonuria, and on the effects of current blood phenylalanine concentration on executive function, have lead to different conclusions. Overall, there appears to be a fairly strong case for limiting dietary treatment to individuals whose blood phenylalanine levels would otherwise exceed 600 μmol/L. This is now policy in some European countries but a formal large-scale study of long-term outcomes to validate the approach is urgently required.

Newborn blood spot screening: new opportunities, old problems

Newborn screening is evolving very rapidly. Geographical coverage is expanding, particularly for common disorders such as congenital hypothyroidism. New technologies, particularly tandem mass spectrometry and high throughput mutation analysis, have increased greatly the range of disorders which could be covered. However, these new possibilities are being exploiting at very different rates in different countries. This is due in part to the different ways in which generally-accepted screening criteria, based on the ten principles of Wilson and Jungner, are being interpreted and applied to policy. The appropriate management of some of the conditions newly-detectable by screening also remains controversial and there is a pressing need to align screening policy and clinical practice. Critical analysis and careful collection of data on an international basis are required to resolve these issues.

Qualitative urinary organic acid analysis: methodological approaches and performance

A programme for proficiency testing of biochemical genetics laboratories undertaking urinary qualitative organic acid analysis and its results for 50 samples examined for factors contributing to poor performance are described. Urine samples from patients in whom inherited metabolic disorders have been confirmed as well as control urines were circulated to participants and the results from 94 laboratories were evaluated. Laboratories showed variability both in terms of their individual performance and on a disease-specific basis. In general, conditions including methylmalonic aciduria, propionic aciduria, isovaleric aciduria, mevalonic aciduria, Canavan disease and 3-methylcrotonyl-CoA carboxylase were readily identified. Detection was poorer for other diseases such as glutaric aciduria type II, glyceric aciduria and, in one sample, 3-methylcrotonyl-CoA carboxylase deficiency. To identify the factors that allow some laboratories to perform well on a consistent basis while others perform badly, we devised a questionnaire and compared the responses with the results for performance in the scheme. A trend towards better performance could be demonstrated for those laboratories that regularly use internal quality control (QC) samples in their sample preparation (p = 0.079) and those that participate in further external quality assurance (EQA) schemes (p = 0,040). Clinicians who depend upon these diagnostic services to identify patients with these defects and the laboratories that provide them should be aware of the potential for missed diagnoses and the factors that may lead to improved performance.

Introducing new screens: why are we all doing different things?

The disease panels covered by newborn blood spot screening vary greatly from country to country. There are different interpretations of the Wilson and Jungner principles and of underlying data in the scientific literature, and great disparities between the value judgements applied in screening and in routine clinical practice.

Population quantile-quantile plots for monitoring assay performance in newborn screening

Immunoreactive trypsinogen (IRT) used in screening for cystic fibrosis is heterogeneous, poorly characterized, and displays marked matrix effects when incorporated into blood spots, making long-term control of assay calibration difficult. The cut-off required to select a fixed proportion of samples (0.5% for the UK protocol) for second-tier mutation analysis varies over time, partly owing to slight differences in calibration of individual lots of assay kit. To investigate this and possible inter-laboratory differences in analytical performance, we developed a monitoring system based on the distribution of measured IRT concentrations in the screened samples. Results were collected fortnightly from five UK screening laboratories in the form of numbers of samples in histogram 'bins' of IRT concentration. These data were converted to cumulative percentages and the IRT concentrations at fixed centiles then approximated by triangulation. The quantile-quantile plot of any subset (by laboratory or by kit lot) of these data using pooled results (all-laboratories all-kit-lots, approximately 270,000 samples) as the reference distribution is analogous to a calibration curve and gives a measure of bias in terms of sensitivity (slope) and baseline (y-intercept). This allows a revised 99.5th centile cut-off for each subset to be calculated directly. A similar approach has allowed inter-laboratory comparison of tandem-mass spectrometric assays for free carnitine (with emphasis on low values) and phenylalanine and has demonstrated that apparently trivial differences in instrumentation and procedures have resulted in marked variation in resultant assay performance.

The formation of 1-dimethylaminonaphthalene-5-sulphonamide during the preparation of 1-dimethylaminonaphthalene-5-sulphonylamino acids

1. The amount of 1-dimethylaminonaphthalene-5-sulphonamide formed during the reaction of an amino acid with 1-dimethylaminonaphthalene-5-sulphonyl chloride depends on the structure of the amino acid and on the conditions used. 2. The reaction probably involves attack of a further molecule of 1-dimethylaminonaphthalene-5-sulphonyl chloride on the 1-dimethylaminonaphthalene-5-sulphonyl-amino acid and also gives the aldehyde (or ketone) with one carbon atom less than the parent amino acid.

International perspectives on newborn screening

The development of electrospray tandem mass spectrometry (MS-MS) has greatly increased the number of diseases that can be detected by newborn blood-spot screening. Different countries are introducing the technology at different rates and for different disease panels. Current policies in the United Kingdom, Germany and the United States are taken as examples. In the United Kingdom, many laboratories are using MS-MS for routine screening for phenylketonuria but, except for those participating in a two-year pilot study of screening for medium-chain acyl-CoA dehydrogenase deficiency, are forbidden use MS-MS to screen for other disorders. In Germany there has been considerable experience of MS-MS screening for a wide range of diseases, but recently the Federal Ministry for Health and Social Security prescribed a much more restricted disease panel, with the instruction that any other diagnostic results are to be suppressed and not reported. By contrast, a recent report from the American College of Medical Genetics, still being debated, recommends screening procedures that will detect an extremely broad range of disorders, including some that are very rare or of unproven clinical significance. The lack of even broad concordance at the level of national policy is extremely disturbing. Though all discussion is nominally founded on the ten principles laid down by Wilson and Jungner in 1968, there seems no generally accepted way of using these principles, or derived criteria, as objective decision tools. Alternative, less categorical, approaches are needed: the disorders concerned are not homogeneous entities and there may be advantages to screening other than reducing morbidity or mortality.

Mutation and biochemical analysis in carnitine palmitoyltransferase type II (CPT II) deficiency

Carnitine palmitoyltransferase type II (CPT II) deficiency has three basic phenotypes, late-onset muscular (mild), infantile/juvenile hepatic (intermediate) and severe neonatal. We have measured fatty acid oxidation and CPT II activity and performed mutation studies in 24 symptomatic patients representing the full clinical spectrum of disease. Severe and intermediate phenotypes show a clear correlation with biochemical indices and genetic analysis revealed causative mutations in most patients. Studies of mild phenotypes suggest a more complex interaction, with higher residual fatty acid oxidation, a wider range of CPT II activity (10-60%) but little evidence of genotype-phenotype correlation. Residual CPT II mutant protein from myopathic patients shows thermal instability at 41 degrees C. The common 'polymorphisms' V3681 and M647V are strikingly overrepresented in the myopathic patients, the implication being that they may significantly influence the manifestation of clinical disease and could therefore potentially be considered as a susceptibility variants. Among myopathic individuals, males comprised 88% of patients, suggesting increased susceptibility to clinical disease. A small number of symptomatic patients appear to have significant residual CPT II activity (42-60%) The synergistic interaction of partial deficiencies of CPT II, muscle adenosine monophosphate deaminase and possibly other enzymes of muscle energy metabolism in the aetiology of episodic myopathy deserves wider consideration.

2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency in a 23-year-old man

2-Methyl-3-hydroxybutyryl-CoA dehydrogenase (EC 1.1.1.178) deficiency is a recently described defect of isoleucine catabolism. The disorder is characterized by normal early development followed by a progressive loss of mental and motor skills. Deterioration may be rapid or may follow a slower decline with a possible stabilization of the disorder on a low-protein diet and appropriate medication. We report a 23-year-old man with 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency with a very mild clinical course. He had apparently normal early development and remained relatively well until the age of 6 years, when he contracted measles. Following this illness, his motor skills and school progress deteriorated. At 15 years he had significant dysarthria, and generalized rigidity with some dystonic and unusual posturing. He was then treated with a low-protein high-carbohydrate diet with a good response in terms of balance and gait. At 18 years he was given benzhexol (Artane), increased slowly from 2 mg to 6 mg daily, resulting in improvement in tremor and dystonia. At 23 years he can dress himself and works in sheltered employment but remains severely dysarthric.

Newborn mass screening versus selective investigation: benefits and costs

Cost-benefit analysis of newborn screening has an unimpressive record and yet it is still regarded as an important decision tool. This workshop surveyed ongoing research into the costs and benefits of systematic whole-population screening, as opposed to selective investigation of symptomatic patients, for inherited metabolic disease. Much current interest is focused on newborn screening by tandem mass spectrometry, which can replace current methods for detecting phenylketonuria and cover a much wider range of diseases. Two observational studies are comparing cost-effectiveness of tandem mass spectrometry screening versus symptomatic diagnosis in either concurrent or historical control populations. A number of other studies are assessing screening performance against predetermined criteria but without any formal control group. Medium-chain acyl-CoA dehydrogenase deficiency is the most common of the additional diseases being detected and it seems that octanoylcarnitine in blood is a particularly sensitive indicator: some of the cases detected by screening have genotypes suggesting a relatively low risk of serious metabolic decompensation. Ongoing studies should provide further quantitative and qualitative data but will not in themselves define the optimum balance between screening sensitivity and specificity.

Features of carnitine palmitoyltransferase type I deficiency

Carnitine palmitoyltransferase type I (CPT I) is unique among long-chain fatty acid oxidation enzymes in that there are two tissue-specific isoforms, 'hepatic' and 'muscle', which are encoded by two separate genes. The 'hepatic' isoform is expressed in liver, kidney and fibroblasts and at low levels in the heart, while the other isoform occurs in skeletal muscle and is the predominant form in heart. Reported patients with CPT I deficiency lack activity of the hepatic isoform and present before 30 months of age with hypoketotic hypoglycaemia, hepatomegaly with raised transaminases, seizures and coma. We discuss four new cases in three families showing, variously, renal tubular acidosis, transient hyperlipidaemia and, paradoxically, myopathy with elevated creatinine kinase or cardiac involvement in the neonatal period as additional features that deserve wider recognition.

Molecular characterization of methylmalonate semialdehyde dehydrogenase deficiency

Three patients have been reported with (putative) methylmalonic semialdehyde dehydrogenase (MMSDH) deficiency. The urine metabolic pattern was strikingly different in all, including beta-alanine, 3-hydroxypropionic acid, both isomers of 3-amino- and 3-hydroxyisobutyric acids in one and 3-hydroxyisobutyric and lactic acids in a second, and mild methylmalonic aciduria in a third patient. In an effort to clarify these disparate metabolite patterns, we completed the cDNA structure, and characterized the genomic structure of human MMSDH gene in order to undertake molecular analysis. Only the first patient had alterations in the MMSDH coding region, revealing homozygosity for a 1336G > A transversion, which leads to substitution of arginine for highly conserved glycine at amino acid 446. No abnormalities of the MMSDH cDNA were detected in the other patients. These data provide the first molecular characterization of an inborn error of metabolism specific to the L-valine catabolic pathway.

Principles and performance: assessing the evidence

Despite the apparent rigour of the systematic review process there are a number of reasons why it may not accurately reflect the true state of current knowledge. For inborn errors of intermediary metabolism in general, the formal quality of literature on treatment is poor, partly because the rarity of the conditions makes systematic study difficult, partly because of the self-evident evidence paradox with treatments that are so obviously life-saving, and partly because of the widespread use of biochemical normalisation as a proxy outcome. In the field of newborn screening there is a paucity of formal trials so that assessment must be either by the 'causal pathway' approach or by reference to screening "criteria". The causal pathway approach may be misleading in that there are discontinuities, for example in projecting from known incidence (clinically diagnosed cases) to the incidence expected on screening. The criteria-based approach suffers from qualitative descriptors, such as "important" and "adequate", with no clear end-points, whilst in the context of neonatal screening some of the Wilson and Jungner criteria conflict with current evidence-based views. These problems are illustrated by reference to two reviews recently produced as part of the UK Health Technology Assessment Programme.

Tandem mass spectrometry screening: proving effectiveness

Despite the considerable literature now emerging on screening by tandem mass spectrometry there are many points of uncertainty about test performance and much work to do in formalizing and validating follow-up protocols. The relative sensitivities of tandem mass spectrometry and previously-used diagnostic methods may differ for some diseases but not others, with the possibility of some metabolites being over-sensitive and diagnosing mild variants or non-disease. Cut-offs should be set using clinical rather than analytical criteria and at this early stage it is important that presumptive positive cases diagnosed through acylcarnitine profiling are compared as thoroughly as possible with symptomatic cases diagnosed using other techniques. Caution should be used in accepting mutation analysis as the ultimate diagnostic criterion. Consideration of incidence and careful examination of family history will all contribute to evidence of the equivalence or otherwise of screen-detected and symptomatic disease. Finally, there should be registration and long-term surveillance of all cases detected.

Population newborn screening for inherited metabolic disease: current UK perspectives

Some of the generally accepted criteria for screening programmes are inappropriate for newborn metabolic screening as they ignore the family dimension and the importance of timely genetic information. Uncritical application of such criteria creates special difficulties for screening by tandem mass spectrometry, which can detect a range diseases with widely different natural histories and responsiveness to treatment. Further difficulties arise from increasing demands for direct proof of the effects of screening on long-term morbidity and mortality. The randomized controlled trial is held to be the gold standard, but for ethical and practical reasons it will be impossible to achieve for such relatively rare diseases. This approach also oversimplifies the complex matrix of costs and benefits of newborn metabolic screening. A more workable approach could involve Bayesian synthesis, combining quantitative performance data from carefully designed prospective pilot studies of screening with existing experience of the natural history, diagnosis, and management of the individual disorders concerned.

Prospective surveillance study of medium chain acyl-CoA dehydrogenase deficiency in the UK

BACKGROUND

Medium chain acyl-CoA dehydrogenase (MCAD) deficiency is a common disorder of fatty acid oxidation in north west Europe. It is very variable in its clinical consequences and is believed to be considerably underdiagnosed.

OBJECTIVE

To investigate the diagnosis and outcome of MCAD deficiency in the UK.

METHOD

A prospective surveillance study through the British Paediatric Surveillance Unit.

RESULTS

Of 62 affected individuals identified, 57 were from England, giving an incidence of 4.5 cases/100,000 births. Forty six cases presented with an acute illness (10 of whom died), 13 cases were identified because of family history, and three for other reasons. Six of the survivors were neurologically impaired.

CONCLUSIONS

Despite increased clinical awareness, the mortality and morbidity from MCAD deficiency remain high. The frequency and severity of the disease support the case for the introduction of universal neonatal screening in England and Scotland.

Molecular analysis and prenatal diagnosis of human fumarase deficiency

Fumarase deficiency is a rare autosomal recessive disorder of the citric acid cycle causing severe neurological impairment. The cDNA for both the rat and human enzymes has been cloned previously and shown to encode a coding region of 1.46 kb. To scan for mutations in fumarase-deficient patients we amplified the coding region of fumarase from fibroblast/lymphoblast cDNA employing the oligonucleotide primers designed from the published human and rat cDNA sequence. We then directly sequenced the polymerase chain reaction product. In seven unrelated patients, we detected four missense mutations (A265T, D383V, F269C, K187R), a nonsense mutation (W458X), a 3-bp AAA insertion that introduces an additional lysine residue at codon 435, and a spontaneous new mutation resulting in a 74-bp deletion (66del74). Seven at-risk pregnancies were monitored with one prenatal diagnosis of fumarase deficiency by molecular analysis and favorable outcome of the other pregnancies as predicted by enzyme assay of cultured fetal cells or molecular analysis.

Neonatal screening for inborn errors of metabolism: cost, yield and outcome

OBJECTIVES. To systematically review the literature on inborn errors of metabolism, neonatal screening technology and screening programmes in order to analyse the costs and benefits of introducing screening based on tandem mass-spectrometry (tandem MS) for a wide range of disorders of amino acid and organic acid metabolism in the UK. To evaluate screening for cystic fibrosis, Duchenne muscular dystrophy and other disorders which are tested on an individual basis. HOW THE RESEARCH WAS CONDUCTED. Systematic searches were carried out of the literature on inborn errors of metabolism, neonatal screening programmes, tandem MS-based neonatal screening technology, economic evaluations of neonatal screening programmes and psychological aspects of neonatal screening. Background material on the biology of inherited metabolic disease, the basic philosophy, and the history and current status of the UK screening programme was also collected. Relevant papers in the grey literature and recent publications were identified by hand-searching. Each paper was graded. For each disease an aggregate grade for the state of knowledge in six key areas was awarded. Additional data were prospectively collected on activity and costs in UK neonatal screening laboratories, and expert clinical opinion on current treatment modalities and outcomes. These data were used to construct a decision-analysis model of neonatal screening technologies, comparing tandem MS with the existing phenylketonuria screening methods. This model determined the cost per additional case identified and, for each disease, the additional treatment costs per case, and the cost per life-year saved. All costs and benefits were discounted at 6% per annum. One-way sensitivity analysis was performed showing the effect of varying the discount rate, the incidence rate of each disorder, the number of neonates screened and the cost of tandem MS, on the cost per life-year gained. RESEARCH FINDINGS. The UK screening programmes for phenylketonuria and congenital hypothyroidism have largely achieved the expected objectives and are cost-effective. Current concerns are the difficulty of maintaining adequate coverage, perceived organisational weaknesses, and a lack of overview. For many of the organic acid disorders it was necessary to rely on data obtained from clinically-diagnosed cases. Many of these diseases can be treated very effectively and a sensitive screening test was available for most of the diseases. Except for cystic fibrosis, there have been no randomised controlled trials of the overall effectiveness of neonatal screening. Despite the anxiety generated by the screening process, there is strong parental support for screening. The effects of diagnosis through screening on subsequent reproductive behaviour is less clear. Conflicts exist between current concepts and the traditional principles of screening. The availability of effective treatment is not an absolute prerequisite: early diagnosis is of value to the family concerned and, to the extent that is leads to increased use of prenatal diagnosis, may help to reduce the overall burden of disease. Neonatal screening is also of value in diseases which present early but with non-specific symptoms. Indeed, almost all of the diseases considered could merit neonatal screening. The majority of economic evaluations failed to incorporate the health benefits from screening, and therefore failed to address the value of the information which the screening programmes provided to parents. The marginal cost of changing from present technology to tandem MS would be approximately 0.60 pounds per baby at a workload of 100,000 samples a year, and 0.87 pounds at 50,000 samples per year. The ability to screen for a wider range of diseases would lead to the identification of some 20 additional cases per 100,000 infants screened, giving a laboratory cost per additional diagnosis of 3000 pounds at an annual workload of 100,000 babies per year.(ABSTRACT TRUNCATED)

Population screening for the common G985 mutation causing medium-chain acyl-CoA dehydrogenase deficiency with Eu-labeled oligonucleotides and the DELFIA system

We have screened 10171 neonatal blood spots from the Trent and West Midlands regions of the UK for the common G985 mutation to more accurately establish the incidence of medium-chain acyl coenzyme (Co)A dehydrogenase (MCAD) deficiency. We have used a technique involving PCR and Eu-labeled allele-specific oligonucleotides detected by using time-resolved fluorometry on the dissociation-enhanced fluorescence immunoassay (DELFIA) system for the detection of the G985 mutation. We have also evaluated the feasibility of neonatal screening with this technique. We identified 158 G985 heterozygotes and no G985 homozygotes. The calculated incidence of MCAD deficiency in the population studied (all mutations, assuming 90% of MCAD mutations are G985) is 1 in 13426 (95% confidence limits 1 in 10070-1 in 18791). At the optimum cutoff criteria, the technique has a sensitivity of 97.5%, specificity of 99.6%, and positive predictive value of 80.2%. We conclude that this study confirms that MCAD deficiency is a common inherited metabolic disease and is a candidate for neonatal screening. The methodology used is robust and suitable for large-scale population studies such as this. The technique is also potentially suitable for screening.

Neonatal screening for cystic fibrosis in the Trent region (UK): two-stage immunoreactive trypsin screening compared with a three-stage protocol with DNA analysis as an intermediate step

OBJECTIVES

To assess neonatal screening for cystic fibrosis using immunoreactive trypsin, either alone or in conjunction with DNA analysis for the delta F508 mutation. A novel three-stage screening protocol was compared with the previously introduced two-stage immunoreactive trypsin-DNA protocol.

DESIGN

(a) Collection of data from a 4 1/2 year period (phase 1) of two-stage immunoreactive trypsin screening. The initial dried blood samples were obtained at 6 days of age and repeat samples at 27 days of age from babies with results above the 99.5th centile. Babies with persistent hypertrypsinaemia were referred for a diagnostic sweat test. (b) Retrospective DNA analysis: patients with cystic fibrosis diagnosed in phase 1 were genotyped and most samples from babies with increased initial immunoreactive trypsin but normal results in the second sample were analysed for the delta F508 mutation. (c) Phase 2, a prospective study of a three-stage neonatal screening protocol, in which only babies heterozygous for the delta F508 cystic fibrosis mutation progressed to the second immunoreactive trypsin test.

SETTING

The Trent neonatal screening programme.

SUBJECTS

437 859 babies born between August 1989 and March 1996.

MAIN OUTCOME MEASURES

Proportions of unaffected babies requiring a second blood sample or a sweat test. Overall sensitivity for the detection of cystic fibrosis.

RESULTS

The two-stage screen failed to identify six out of 94 cases of cystic fibrosis (without meconium ileus). The introduction of the DNA analysis step would have resulted in one additional case being missed. With the three-stage screen there was a 92% reduction in babies requiring a second blood sample and an 80% reduction in negative sweat tests, results close to the predictions of the retrospective study.

CONCLUSIONS

The three-stage screening protocol is a marked improvement on the two-stage immunoreactive trypsin strategy and on the two-stage immunoreactive trypsin-DNA strategy recently introduced in some other screening programmes.

Endocardial fibroelastosis and primary carnitine deficiency due to a defect in the plasma membrane carnitine transporter

Endocardial fibroelastosis (EFE) has previously been shown to be associated with tissue carnitine deficiency, although the basis for the carnitine deficiency has not been documented. A patient with the classical features of EFE and marked deficiency of carnitine in heart muscle, skeletal muscle, and liver is presented in this report. Cultured skin fibroblasts from both parents demonstrated levels of carnitine uptake at 50% of the normal rate. This is consistent with heterozygosity for the plasma membrane carnitine transporter defect, indicating likely homozygosity for this recently recognized inborn error in the index patient.

Trichothiodystrophy with sideroblastic anaemia and developmental delay

A patient with sideroblastic anaemia, development delay, and trichothiodystrophy is presented. Trichothiodystrophy is a feature of several autosomal recessive diseases. Photosensitivity, failure to thrive, and developmental delay are commonly observed in affected cases. X linked inheritance accounts for the bulk of cases with sideroblastic anaemia. This case highlights the importance of routine hair microscopy in cases of atypical ectodermal dysplasia.

Disorders of mitochondrial long-chain fatty acid oxidation

The oxidation of long-chain fatty acids requires a series of enzymes which are located in or on the mitochondrial membranes. These include carnitine palmitoyltransferases I and II, a carnitine-acylcarnitine translocase and, newly discovered, very long-chain acyl-CoA dehydrogenase and the mitochondrial trifunctional protein. These last two chain-shorten acyl-CoA esters to the point where they can be transferred to the more soluble medium- and short-chain-specific enzymes within the mitochondrial matrix. The disorders of long-chain fatty acid oxidation show a rather similar range of clinical and biochemical features, though with different emphasis in the different conditions. Patients with severe defects usually present early with acute attacks of hypoketotic hypoglycaemia and impaired liver function, or with cardiomyopathy or cardiac arrhythmia. In milder variants, skeletal myopathy with intermittent myoglobinuria develops later in life. 3-Hydroxyacyl-CoA dehydrogenase deficiency is unusual in producing peripheral neuropathy and retinitis pigmentosa. Treatment is based on the avoidance of fasting and replacement of normal dietary fat by medium-chain triglyceride, the medium-chain fatty acids entering the mitochondria in a carnitine-independent manner and bypassing the long-chain part of the spiral. Diagnosis must ultimately be based on direct assay of the enzyme involved, but preliminary indicators may come from determination of carnitine and intermediate metabolites in plasma, urinary organic acid profiling, and radioisotopic screening assays with lymphocytes or cultured fibroblasts.

Prenatal diagnosis of glutathione synthase deficiency

Prenatal diagnosis for glutathione synthase (EC 6.3.2.3) deficiency in two pregnancies of an at-risk couple was performed on amniotic fluid taken at 16 weeks' gestation. 5-Oxoproline (pyroglutamic acid) levels were 970 and 790 mumol/l compared with the normal mean value of 29 mumol/l (range 13-51 mumol/l). The pregnancies were terminated and the diagnosis in one case was subsequently confirmed by assay of glutathione synthase in cultured fetal fibroblasts. In the other, post-mortem tissue samples failed to grow.

Quality assessment of urinary organic acid analysis

The number of known inherited metabolic disorders resulting in an organic aciduria has increased steadily over the past two decades. Prompt and reliable detection is both clinically and technically demanding but is essential if appropriate treatment is to be undertaken. This is the first study of laboratory performance in the detection of these disorders to be undertaken in the UK. Some conditions were accurately identified by most laboratories: for example for maple syrup urine disease, 12 of 14 laboratories provided an appropriate response and medium chain acyl-CoA dehydrogenase deficiency was correctly identified by 15 of 17 laboratories. However, accuracy of detection was poorer for other conditions: for example, only eight of 17 laboratories detected tyrosinaemia type 1 and nine of 18 laboratories detected 4-hydroxybutyric aciduria. The strongest correlation with good performance was obtained by comparison with the extent of peak identification: r = 0.62, P = 0.002. The need for regular attendance at scientific symposia was also supported by a weaker positive correlation with the average score achieved, P = 0.08. Evidence also suggested that some of the laboratories with a low workload performed less well. No significant difference in performance could be demonstrated between the 17 laboratories who used gas chromatography-mass spectrometry and the six participants who used gas chromatography alone.

6-Methyluracil excretion in 2-methylacetoacetyl-CoA thiolase deficiency and in two children with an unexplained recurrent ketoacidaemia

6-Methyluracil (6MU) has been identified in urine collected during acute illness in two children with beta-ketothiolase deficiency (approximately 1 mmol/L) and in two children with recurrent infection-related ketoacidaemia of unknown aetiology (levels of 6.3 and 7.1 mmol/mmol creatinine). Significant amounts of 6MU were not detected in children with fasting ketosis in whom a metabolic disorder was excluded (normal levels less than 25 mumol/mmol creatinine). We propose that the production of 6MU may be related to the accumulation of acetoacetyl-CoA and thus be a marker for disorders where this occurs.

Syndromes associated with trichothiodystrophy

Sparse, brittle, sulphur-deficient hair is an excellent marker for several autosomal recessive neurocutaneous syndromes. The term 'trichothiodystrophy' is commonly used in publications on such syndromes and the best characterized trichothiodystrophy syndrome is associated with skin photosensitivity and intellectual impairment. Patients with these three cardinal signs usually have an underlying DNA repair defect. Here we describe clinical and laboratory findings in two patients with trichothiodystrophy and defective DNA repair alongside findings in three other cases who have different trichothiodystrophy syndromes without defective DNA repair. These patients' features are discussed in the light of a practical classification scheme which is based upon a check-list of clinical abnormalities associated with trichothiodystrophy syndromes (Van Neste, 1991).

Familial hypercholesterolaemia: pilot study to identify children at risk

AIMS

To evaluate a more effective method of identifying children with familial hypercholesterolaemia by screening a population at high risk.

METHODS

Domiciliary measurement of random cholesterol concentration was made in 200 children who were first or second degree relatives of subjects with premature onset coronary artery disease. Measurements were taken by a health visitor using a portable analyser.

RESULTS

Twelve new cases of familial hypercholesterolaemia were identified during the first nine months of the study. Random cholesterol concentrations were within the normal range (< 5.2 mmol/l) in 70.5% of samples tested. Forty two (21%) of patients tested had a borderline cholesterol (5.2-5.9 mmol/l) but 50% of these fell within the normal range when fasting capillary samples were analysed. Children with significant hypercholesterolaemia on random testing (concentrations of > 5.9 mmol/l) (8.5%) also had fasting venous blood assayed for high density lipoprotein (HDL) cholesterol and tri-glyceride in the laboratory. Results indicated that 6.5% of patients screened were at high risk of cardiovascular disease (ratio of total: HDL cholesterol of > 4.5), and 1% had a moderately increased risk (ratio 3.5-4.5).

CONCLUSIONS

Children with familial hypercholesterolaemia can be identified from a selected "high risk" population by measuring random capillary cholesterol concentration.

Long-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency--diagnosis, plasma carnitine fractions and management in a further patient

Long-chain 3-hydroxyacyl-coenzyme A dehydrogenase (LCHAD), the third enzyme of the mitochondrial beta-oxidation pathway, carries out the dehydrogenation of 3-hydroxyacyl-CoA compounds of 12-18 carbon length. To date only nine cases of LCHAD deficiency have been documented. We report a further patient who as a neonate developed non-specific gastrointestinal symptoms and at 5 months of age cardiomyopathy, recurrent hypoketotic hypoglycaemia and gross alterations of plasma carnitine fractions. Dietary management with medium chain triglycerides led rapidly to clinical improvement. There was a close correlation between the clinical condition, plasma carnitine fractions and cardiac function. At 2 years of age she is developing normally.

Identification of urinary acylcarnitines using gas chromatography-mass spectrometry: preliminary clinical applications

Many disorders of organic acid metabolism are associated with abnormalities in the levels of acylcarnitines excreted in urine. Profiling of urinary acylcarnitines allows diagnosis and characterisation of many acidurias and acidemias, monitoring dietary treatment of such patients, and elucidation of the metabolism of some exogenous acidic compounds. Urine (ca. 0.5 ml) was subjected to a simple work-up by ion-exchange chromatography, and the isolated acylcarnitines were derivatized by cyclization in 35 min to give volatile lactones that are compatible with gas chromatography-mass spectrometry using electron or chemical ionization. The feasibility of this new and affordable procedure has been confirmed by identifying urinary acylcarnitines in cases of medium-chain acyl-coenzyme A dehydrogenase deficiency, propionic acidemia and isovaleric acidemia.

Human liver long-chain 3-hydroxyacyl-coenzyme A dehydrogenase is a multifunctional membrane-bound beta-oxidation enzyme of mitochondria

We have purified to homogeneity the long-chain specific 3-hydroxyacyl-CoA dehydrogenase from mitochondrial membranes of human infant liver. The enzyme is composed of non-identical subunits of 71 kDa and 47 kDa within a native structure of 230 kDa. The pure enzyme is active with 3-ketohexanoyl-CoA and gives maximum activity with 3-ketoacyl-CoA substrates of C10 to C16 acyl-chain length but is inactive with acetoacetyl-CoA. In addition to 3-hydroxyacyl-CoA dehydrogenase activity, the enzyme possesses 2-enoyl-CoA hydratase and 3-ketoacyl-CoA thiolase activities which cannot be separated from the dehydrogenase. None of these enzymes show activity with C4 substrates but all are active with C6 and longer acyl-chain length substrates. They are thus distinct from any described previously. This human liver mitochondrial membrane-bound enzyme catalyses the conversion of medium- and long-chain 2-enoyl-CoA compounds to: 1) 3-ketoacyl-CoA in the presence of NAD alone and 2) to acetyl-CoA (plus the corresponding acyl-CoA derivatives) in the presence of NAD and CoASH. It is therefore a multifunctional enzyme, resembling the beta-oxidation enzyme of E. coli, but unique in its membrane location and substrate specificity. We propose that its existence explains the repeated failure to detect any intermediates of mitochondrial beta-oxidation.

Differential diagnosis of hydroxydicarboxylic aciduria based on release of 3H2O from [9,10-3H]myristic and [9,10-3H]palmitic acids by intact cultured fibroblasts

Intact cultured fibroblasts from patients with deficiency of long-chain 3-hydroxyacyl-CoA dehydrogenase release 3H2O from [9,10-3H]myristic acid and [9,10-3H]palmitic acid more slowly than normal. The ratio of activity (palmitate/myristate) is also low and the expression (rate with palmitate2/(rate with myristate) gives good differentiation between affected and unaffected cells. In some patients who have shown hydroxydicarboxylic aciduria when unwell there is reduced 3H2O production from [9,10-3H]myristic and [9,10-3H]palmitic acids by intact cultured fibroblasts but normal 3-hydroxyacyl-CoA dehydrogenase activities in disrupted cells. The palmitate/myristate ratio is higher than in long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency. The basic defect in these patients is still unknown but it is suggested that caution be used over the administration of medium-chain triglyceride.

Long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency

We describe the clinical features and biochemical findings of two patients with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency. Both children presented with an acute metabolic crisis. Both had hypoglycemia and excreted even-chain unsubstituted dicarboxylic and 3-hydroxy-dicarboxylic acids in the urine. Measurement of the enzymes of fatty acid oxidation in cultured skin fibroblasts showed low activity of long-chain 3-hydroxyacyl-CoA dehydrogenase, but normal activity of short-chain 3-hydroxyacyl-CoA dehydrogenase. The defect was further characterized by immunoprecipitating the short-chain enzyme using monospecific antibodies. It is probably inherited as an autosomal recessive trait, inasmuch as intermediate enzyme activity was found in the fibroblasts from the parents of one child.

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.