Diagnosis of inborn errors of metabolism from blood spots by acylcarnitines and amino acids profiling using automated electrospray tandem mass spectrometry

Riboflavin therapy. Biochemical heterogeneity in two adult lipid storage myopathies

Two unrelated adult males, aged 36 (patient 1) and 25 (patient 2) years, presented with subacute carnitine-deficient lipid storage myopathy that was totally and partly responsive to riboflavin supplementation in the two patients, respectively. Plasma acyl-carnitine and urinary organic acid profiles indicated multiple acyl coenzyme A dehydrogenase deficiency, which was mild in patient 1 and severe in patient 2. The activities of short-chain and medium-chain acyl coenzyme A dehydrogenases in mitochondrial fractions were decreased, especially in patient 2. This was in agreement with Western blotting results. Flavin-dependent complexes I and II were studied by immunoblotting and densitometric quantification of two-dimensional electrophoresis with comparable results. Complex I was present in normal amounts in both patients, whereas complex II was decreased only in the pretherapy muscle of patient 2. Flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) concentrations in muscle and isolated mitochondria, and the activity of mitochondrial FAD pyrophosphatase, showed that patient 1 had low levels of FAD (46%) and FMN (49%) in mitochondria, with a significant increase (P < 0.01) in mitochondrial FAD pyrophosphatase (273%) compared with controls. Patient 2 had similar low levels of FAD and FMN in both total muscle (FAD and FMN 22% of controls) and mitochondria (FAD 26%; FMN 16%) and normal activity of mitochondrial FAD pyrophosphatase. All of these biochemical parameters were either totally or partly corrected after riboflavin therapy.

Automated tandem mass spectrometry for mass newborn screening for disorders in fatty acid, organic acid, and amino acid metabolism

Development of acylcarnitine and amino acid profiling using tandem mass spectrometry, and its application for use with dried blood specimens collected on filter-paper cards, has introduced an innovative new technology for detecting inborn errors of fatty acid, organic acid, and amino acid metabolism. From November 1, 1992 through June 30, 1999 we screened more than 700,000 newborns in Pennsylvania, Ohio, North Carolina, and Louisiana. We have prospectively detected 163 inborn errors of metabolism. Eighty-six patients have amino acid metabolism errors. Among them are phenylketonuria, hyperphenylalaninemia, maple syrup urine disease, and several urea cycle disorders. Thirty-two have organic acid metabolism errors, including glutaric aciduria type 1; 3-methylcrotonyl coenzyme A (CoA) carboxylase deficiency, propionic acidemia, methylmalonic acidemia, and 3-hydroxy-3-methylglutaryl-CoA lyase deficiency; and 45 have fatty acid oxidation errors, including 36 with medium-chain acyl-CoA dehydrogenase deficiency. Details of the methodology are presented and the potential of this screening technology is discussed.

Recent developments in the investigation of inherited metabolic disorders using cultured human cells

Thepurpose of this paper is to share experience with our systems and review recent "in vitro" methods using intact cells (fibroblasts, amniocytes) in which entire metabolic pathways can be probed for inherited metabolic defects reflected by elevations of intermediates determined by tandem mass spectrometry, HPLC, or gas chromatography-mass spectrometry. Currently, one can explore the integrity of mitochondrial fat oxidation, peroxisomal degradation of methyl-branched fatty acids (e.g., pristanate), and the mitochondrial degradation of the branched chain amino acids (leucine, valine, and isoleucine). For many of the diseases, the specific defect can be recognized from the acylcarnitine profile resulting from incubation of the intact cells with stable-isotope-labeled precursors to the particular pathway. This approach has also been successful in identifying new inherited metabolic disorders, biochemical correlation with clinical phenotypes of individual defects, and sequential oxidation of fatty acids by peroxisomal-mitochondrial interaction.

Modifications in electrospray tandem mass spectrometry for a neonatal-screening pilot study in Japan

In a neonatal-screening pilot study for inherited disorders in organic acid and amino acid metabolism, we analyzed butyrated acylcarnitines and amino acids in blood spots of more than 20,000 newborns by electrospray tandem mass spectrometry. In order to screen urea cycle disorders, we performed multiple scanning functions with additional stable isotope-labelled internal standards, since such reported functions as neutral loss of m/z 102 or 109 for butyrated amino acids were not sufficient. Arginine levels were measured with arginine-13C6. Hypocitrullinemia for the screening of some urea cycle disorders was detectable by measurement with synthesized citrulline-d6, although we did not find any such disorders. In the acylcarnitine analysis, we found a patient with propionic acidemia, who has been treated effectively. The increasing false positive rate due to the use of pivalic acid-containing antibiotics in the diagnosis of isovaleric acidemia was a problem in Japan.

Pilot study of gas chromatographic-mass spectrometric screening of newborn urine for inborn errors of metabolism after treatment with urease

Gas chromatographic-mass spectrometric (GC-MS) techniques for urinary organic acid profiling have been applied to high-risk screening for a wide range of diseases, mainly for inborn errors of metabolism (IEM), rather than to low-risk screening or mass screening. Using a simplified procedure with urease-pretreatment and the GC-MS technique, which allows simultaneous determination of organic acids, amino acids, sugars and sugar acids, we performed a pilot study of the application of this procedure to neonatal urine screening for 22 IEM. Out of 16,246 newborns screened, 11 cases of metabolic disorders were chemically diagnosed: two each of methylmalonic aciduria and glyceroluria, four of cystinuria, and one each of Hartnup disease, citrullinemia and alpha-aminoadipic aciduria/alpha-ketoadipic aciduria. The incidence of IEM was thus one per 1477, which was higher than the one per 3000 obtained in the USA in a study targeting amino acids and acylcarnitines in newborn blood spots by tandem mass spectrometry. Also, 227 cases were found to have transient metabolic abnormalities: 108 cases with neonatal tyrosinuria, 99 cases with neonatal galactosuria, and 20 cases with other transient metabolic disorders. Two hundred and thirty-eight cases out of 16,246 neonates (approximately 1/68) were thus diagnosed using this procedure as having either persistent or transient metabolic abnormalities.

Effect of sports activity on carnitine metabolism. Measurement of free carnitine, gamma-butyrobetaine and acylcarnitines by tandem mass spectrometry

The effects of sports activity on carnitine metabolism were studied using mass spectrometry. Serum levels of free carnitine, acylcarnitines (acetylcarnitine, propionylcarnitine, C4-, C5- and C8-acylcarnitine) and gamma-butyrobetaine, a carnitine precursor, were determined by tandem mass spectrometry in liquid secondary ion mass ionization mode. The coefficients of variation at three different concentrations were 2.8-7.9% for gamma-butyrobetaine, and 1.2 to approximately 6.7% for free carnitine. The recoveries added to serum were 109.1% for gamma-butyrobetaine, 89.3% for free carnitine. Sports activity caused increased serum levels of gamma-butyrobetaine, acetylcarnitine, C4- and C8-acylcarnitines and decreased serum levels of free carnitine. This method requires a small amount of sample volume (20 microl of serum) and short total instrumental time for the analysis (1 h for preparation, 2 min per sample for mass spectrometric analysis). Therefore, this method can be applied to study carnitine metabolism under various conditions that affect fatty acid oxidation.

Automated metabolic profiling and interpretation of GC/MS data for organic acidemia screening: a personal computer-based system

We have developed a personal computer-based system designed for automated metabolic profiling of urinary organic acids by gas chromatography-mass spectrometry (GC/MS) and data interpretation for organic acidemia screening. For the automated profiling, we compiled retention indices, two target ions and their intensity ratio for 126 urinary metabolites. Metabolites above the cut-off values were flagged as abnormal compounds. The data interpretation was based on combination of the flagged metabolites. Diagnostic or index metabolites were categorized into three groups, "AND," "OR" and "NO," and compiled for each disorder to improve the specificity of the diagnosis. Groups "AND" and "OR" comprised essential and optional compounds, respectively, which and both to reach a specific diagnosis. Group "NO" comprised metabolites that must be absent to make a definite diagnosis. We tested this system by analyzing urine specimens from 48 patients previously diagnosed as having organic acidemias. In all cases, the diagnostic metabolites were identified and each correct diagnosis could be found among the possible diseases suggested by the system. Hence, with this simplified automated system, more people will be able to participate extensively in any screening programs using GC/MS.

18Fluoro-2-deoxyglucose (18FDG) PET scan of the brain in propionic acidemia: clinical and MRI correlations

The clinical data and the imaging findings of the positron emission tomography (PET) and the magnetic resonance imaging (MRI) studies in five patients, previously diagnosed to have propionic acidemia, were retrospectively reviewed. The patients were all normal at birth. The first clinical signs, typically hypotonia and failure to thrive, appeared during the first 2 years of life. With progression of the disease, the neurological findings consisted of variable degrees of dementia and extrapyramidal symptoms, notably dystonia, choreoathetosis and rigidity of variable degrees. Initial cerebral PET and MRI studies were normal. Follow-up MRI examinations showed progressive basal ganglia degeneration, with evidence of atrophy and signal abnormalities within the caudate nuclei and the putamina. The thalamic structures were normal. The PET studies demonstrated increased uptake in the basal ganglia and thalami, followed by decreased uptake in the basal ganglia at a later stage of the disease. The structural (MRI) and the functional (PET) studies of the brain were found to be complementary in the evaluation of propionic acidemia, and were in good correlation with the clinical findings.

Clinical, biochemical, and molecular characterization of patients with glutathione synthetase deficiency

Pyroglutamic aciduria (5-oxoprolinuria) is a rare autosomal recessive disorder caused by either glutathione synthetase deficiency (GSSD) or 5-oxoprolinase deficiency. GSSD results in low glutathione levels in erythrocytes and may present with hemolytic anemia alone or together with pyroglutamic aciduria, metabolic acidosis, and CNS damage. Five patients with pyroglutamic aciduria were studied. All presented with hemolytic anemia and metabolic acidosis. Two (brothers) also had Fanconi nephropathy, which is not seen in pyroglutamic aciduria. Molecular analyses of the GSS gene was performed in 3 patients. RT-PCR and heteroduplex analysis identified a homozygous deletion in 1 patient and a homozygous mutation in 2 others (brothers with Fanconi nephropathy). Sequencing of glutathione synthetase (GSS) cDNA from the first patient showed a 141-bp deletion corresponding to the entire exon 4, whilst the corresponding genomic DNA showed a G491 --> A homozygous splice site mutation. Sequencing of GSS cDNA from the Fanconi nephropathy patients showed a C847 --> T [ARG283 --> CYS] mutation in exon 9.

Disorders of mitochondrial fatty acyl-CoA beta-oxidation

In recent years tremendous progress has been made with respect to the enzymology of the mitochondrial fatty acid beta-oxidation machinery and defects therein. Firstly, a number of new mitochondrial beta-oxidation enzymes have been identified, including very-long-chain acyl-CoA dehydrogenase (VLCAD) and mitochondrial trifunctional protein (MTP). Secondly, the introduction of tandem MS for the analysis of plasma acylcarnitines has greatly facilitated the identification of patients with a defect in fatty acid oxidation (FAO). These two developments explain why the number of defined FAO disorders has increased dramatically, making FAO disorders the most rapidly growing group of inborn errors of metabolism. In this review we describe the current state of knowledge of the enzymes involved in the mitochondrial oxidation of straight-chain, branched-chain and (poly)unsaturated fatty acyl-CoAs as well as disorders of fatty acid oxidation. The laboratory diagnosis of these disorders is described, with particular emphasis on the methods used to identify the underlying enzyme defect and the molecular mutations. In addition, a simple flowchart is presented as a guide to the identification of mitochondrial FAO-disorders. Finally, treatment strategies are discussed briefly.

Application of electrospray tandem mass spectrometry to neonatal screening

For the past 30 years, neonatal screening programs have been performed largely by using the bacterial inhibition assays developed by Dr Robert Guthrie. These programs focused on a small number of diseases such as phenylketonuria and maple syrup urine disease and involved one test for each disease. During the same period many new diseases were discovered, such as organic acidemias and fatty acid oxidation defects, and they presented a diagnostic challenge to biochemical laboratories. Different mass spectrometric approaches have been the main tools for the diagnosis; however, each has its own limitation. Recently, electrospray tandem mass spectrometry (MS/MS) has provided an alternative automated high throughput, specific, and broad-spectrum approach to screening for a relatively large number of disorders, including those covered by bacterial inhibition assays tests. By using specific scan functions, a large number of amino acids and acylcarnitines in blood spots are quantified in 2 minutes analytical time. A new scan function is described here for quantification and screening for argininosuccinic acid in blood spots, which is a key metabolite in the diagnosis of argininosuccinase deficiency. We describe the results of a 3-year tandem MS/MS-based neonatal study that was performed in our newborn population. We screened 27,624 blood spots and identified 20 cases yielding a frequency of 1:1,381. No false-negative cases were identified, but several false-positive cases were eliminated by repeat analysis by MS/MS of blood or by other means. We also used MS/MS analysis of urine or blood either for confirmation of initial positive results or for follow-up of treatment, such as in glutaric acidemia, citrullinemia, argininosuccinase deficiency, and biopterin-dependent phenylketonuria.

Clinical and brain 18fluoro-2-deoxyglucose positron emission tomographic findings in ethylmalonic aciduria, a progressive neurometabolic disease

We report a 2-year-old boy with ethylmalonic aciduria and vasculopathy syndrome evaluated by 18fluoro-2-deoxyglucose positron emission tomographic (18FDG PET) brain scan, with intense uptake of 18FDG in the caudate nucleus and putamen bilaterally but with no morphological changes on magnetic resonance imaging (MRI). A repeat 18FDG PET brain scan 1 year later showed a significant bilateral decreased uptake of glucose in the putamen and the head of the caudate nucleus as well as a decreased uptake in the frontal lobes. On MRI, there was atrophy and watershed infarcts in the basal ganglia, explaining the loss of glucose uptake. These results reflect a selective vulnerability of the basal ganglia, their functional derangement, and ultimate degeneration.

Fatty acid oxidation defects in muscle

Fatty acid oxidation defects can cause recurrent rhabdomyolysis or chronic progressive muscle weakness. Diagnosis is often possible on blood using tandem mass spectrometry or molecular genetic techniques. Riboflavin and carnitine are effective in some cases of multiple acyl-CoA dehydrogenase deficiency and primary carnitine deficiency, respectively. Controlled trials are needed to evaluate other proposed forms of treatment.

Diagnosis of isovaleric acidaemia by tandem mass spectrometry: false positive result due to pivaloylcarnitine in a newborn screening programme

Tandem mass spectrometric analysis of acylcarnitines and amino acids has been applied in newborn screening programmes for the detection of several inborn errors of metabolism. We report a false positive result for isovaleric acidaemia in a newborn screening programme using this method. The newborn screening sample showed a very prominent signal corresponding to the mass of isovalerylcarnitine. Repeat samples (age 6 days) of blood and urine showed similar results. However, urine organic acids were normal. Acylcarnitine analysis in blood, breast milk and urine of the mother also showed a prominent signal of the same mass. Gas chromatography-mass spectrometry of the methyl esters demonstrated that the signal in the patient's urine was due to the presence of pivaloylcarnitine, which is isomeric with isovalerylcarnitine. The patient's mother was receiving an antibiotic containing a derivative of pivalic acid to treat a urinary tract infection. Follow-up samples in the patient and the mother confirmed a decrease in the levels of pivaloylcarnitine, concomitant with the discontinuation of the treatment. We conclude that pivaloylcarnitine can give a false positive result for isovaleric acidaemia in newborns whose mothers are on treatment with pivoxilsulbactam-containing antibiotics.

Fluoro-2-deoxyglucose (18FDG) PET scan of the brain in glutaric aciduria type 1: clinical and MRI correlations

The clinical, PET (positron emission tomography) and MRI (magnetic resonance imaging) findings of brain studies in eight patients, previously diagnosed to have glutaric aciduria type 1, were retrospectively reviewed. The neurological findings typically consisted of variable degrees of dementia and extrapyramidal symptoms (dystonia, choreoathetosis and rigidity). Both MRI and PET showed involvement of the putamina in all the patients. The PET scan demonstrated lesions in the head of the caudate nuclei in all of the patients. Brain atrophy, and in particular the characteristically-enlarged Sylvian fissures, was better demonstrated by MRI. On the other hand, the cerebral cortex and thalamic structures were found to be normal by MRI in all patients, whereas PET scan showed decreased uptake in the cerebral cortex in seven, and in the thalami in three patients. Correlation between imaging and clinical findings was found to be good when both PET scan and MRI findings of the brain were taken into consideration. Therefore, the functional (PET) and structural (MRI) studies of the brain were complementary in the imaging evaluation of glutaric aciduria type 1.

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.

Infectious complications of propionic acidemia in Saudia Arabia

A retrospective study of 38 patients with propionic acidemia indicates a high frequency of infections; affecting 80% of such patients. The Saudi Arabian population studied is a product of consanguineous marriages, and presents with a severe phenotype. Most microorganisms implicated are unusual, which suggests an underlying immune deficiency. These frequent infections occur despite aggressive treatment with appropriate diets, carnitine and during acute episodes of the disease with metronidazole, which suggests a global effect of the disease on T and B lymphocytes as well as on the bone marrow cells. Any patient with propionic acidemia should be closely followed up for an intercurrent infection in association with acute metabolic decompensation.

Saudi experience with classic homocystinuria

BACKGROUND

Classic homocystinuria is an autosomal recessive disorder due to cystathionine ss-synthase deficiency. The clinical, radiological and neurophysiological findings of classic homocystinuria diagnosed at King Faisal Specialist Hospital and Research Centre (KFSH&RC) are presented in this report.

PATIENTS AND METHODS

Twenty-four patients (15 females and 9 males) were referred to KFSH&RC for work-up of mental retardation, seizures, thrombo-embolic episodes and dislocation of the ocular lenses.

RESULTS

The common clinical findings included ectopia lentis (20 patients), skeletal system involvement (18 patients), vascular system involvement (9 patients), and mental retardation (all patients to varying degrees). Unusual findings consisted of a patient who developed severe lower gastrointestinal bleeding, a patient with insulin-dependent diabetes mellitus, probably due to vasculopathy, and another having severe bronchiectasis, which may have been due to fibrillin disruption, and required the resection of a lobe of the lung. The parents of 21 patients were first-degree relatives, and 19 patients had one or more family members affected by the same disease. All patients had markedly elevated plasma levels of methionine. Cystathionine synthase activity in the fibroblast was measured in 25% of the patients and was deficient. Only four patients responded to pyridoxine and their methionine level decreased to almost normal range.

CONCLUSION

The aim of this study was to increase the awareness of this disease in the scientific and medical community, in particular in the general pediatrician working in Saudi Arabia who first encounters the clinical manifestations of the disease. Early detection through tandem mass spectrometry of blood spot screening and treatment are important, and may prevent the major complications of this disease.

A method for the quantitation of conjugated bile acids in dried blood spots using electrospray ionization-mass spectrometry

Bile acid concentrations are elevated in the blood of neonates with cholestatic hepatobiliary disorders providing a possible means of screening for treatable conditions including biliary atresia. A method is described for the determination of concentrations of conjugated bile acids in dried blood spots using electrospray ionization mass spectrometry. Bile acids were eluted from the blood spots using methanol containing, as internal standards, the taurine and glycine conjugates of D4-chenodeoxycholic acid and D4-cholic acid. The samples were then reconstituted in acetonitrile/water and injected by autosampler into the electrospray source operating in negative ion mode. Optimal conditions were determined for both single quadrupole and tandem mass spectrometry analysis. Blood spot bile acid profiles were studied in two groups of infants (< 1 y), a cholestatic group (conjugated bilirubin > 25 mumol/L; n = 49), and a control group (n = 96). The best discrimination between the two groups was provided by measurements of taurodihydroxycholanoates (normal < 5 mumol/L; cholestatic group 18-94 mumol/L) and glycodihydroxycholanoates (normal < 5 mumol/L; cholestatic group 11-66 mumol/L). The method can also be adapted to detect unusual bile acids which are diagnostic of inborn errors of bile acid synthesis and peroxisomal disorders. The method is fast, reliable, reproducible, and relatively cheap; however, much more work is required to determine whether it can be used for mass screening for cholestasis. It will be necessary to show that measurement of bile acid concentrations in blood spots obtained at 7-10 d can be used to detect infants who currently present with jaundice, pale stools, and dark urine during the first 6 mo of life.

Carnitine palmitoyltransferase I activity monitoring in fibroblasts and leukocytes using electrospray ionization mass spectrometry

Carnitine palmitoyltransferase I (CPT I) is one of the enzymes associated with normal mitochondrial membrane transport of certain metabolites. The importance of the enzyme in normal energy production is well illustrated during fasting conditions when a large flux of long-chain fatty acids must be transported over the mitochondrial membrane to undergo beta-oxidation. Up to now CPT I activity has been assayed in various tissues, including liver, leukocytes, platelets, and fibroblasts by the use of an isotope exchange forward assay which measures the rate of palmitoyl-l-[methyl-3H]carnitine formation from palmitoyl-CoA and l-[methyl-3H]carnitine. We have developed an electrospray ionization mass spectrometric method for detecting palmitoylcarnitine formation from palmitoyl-CoA and carnitine, thus avoiding the use of radiolabeled isotopes. In this assay, time-dependent conversion of free carnitine by CPT I to palmitoylcarnitine is measured quantitatively, relative to isotopically labelled palmitoylcarnitine, by parent ion monitoring of fragment ion m/z 85. The specific activity of CPT I in fibroblasts and leukocytes compared well with the activity determined with the isotope exchange method, however, the combination of high sensitivity and selectivity of tandem mass spectrometry along with the environment-friendly nature of the electrospray method makes it an ideal technique to measure CPT I activity.

Multivariate discrimination for phenylketonuria (PKU) and non-PKU hyperphenylalaninemia after analysis of newborns’ dried blood-spot specimens for six amino acids by ion-exchange chromatography

Ion-exchange HPLC was developed for testing dried blood-spot specimens from newborns. The method is suitable for quantitative confirmatory testing of abnormal specimens detected in the New York State Newborn Screening Program. Positive specimens were initially identified among all New York State newborns with semiquantitative bacterial inhibition assays (BIA) for aminoacidopathies, including phenylketonuria (PKU) and non-PKU hyperphenylalaninemia (HP), maple syrup urine disease, and homocystinuria. A selection of 1346 specimens from routine BIA screening, including 131 newborns with PKU or persistent HP, were tested by HPLC. Of 179 BIA results that were falsely positive, 98 (55%) were also falsely positive by HPLC in which the Phe/Tyr ratio was the discriminator and the threshold was set to attain 100% sensitivity. Investigation of three multivariate discriminatory methods revealed that linear discriminant analysis excluded all but 35 (20%) of the BIA false-positives.

HMG CoA lyase deficiency: identification of five causal point mutations in codons 41 and 42, including a frequent Saudi Arabian mutation, R41Q

The hereditary deficiency of 3-hydroxy-3-methylglutaryl (HMG) CoA lyase (HL; OMIM 246450 [http://www3.ncbi.nlm.nih. gov:80/htbin-post/Omim/dispmim?246450]) results in episodes of hypoketotic hypoglycemia and coma and is reported to be frequent and clinically severe in Saudi Arabia. We found genetic diversity among nine Saudi HL-deficient probands: six were homozygous for the missense mutation R41Q, and two were homozygous for the frameshift mutation F305fs(-2). In 32 non-Saudi HL-deficient probands, we found three R41Q alleles and also discovered four other deleterious point mutations in codons 41 and 42: R41X, D42E, D42G, and D42H. In purified mutant recombinant HL, all four missense mutations in codons 41 and 42 cause a marked decrease in HL activity. We developed a screening procedure for HL missense mutations that yields residual activity at levels comparable to those obtained using purified HL peptides. Codons 41 and 42 are important for normal HL catalysis and account for a disproportionate 21 (26%) of 82 of mutant alleles in our group of HL-deficient probands.

Rapid diagnosis of MCAD deficiency: quantitative analysis of octanoylcarnitine and other acylcarnitines in newborn blood spots by tandem mass spectrometry

We report the application of tandem mass spectrometry to prospective newborn screening for medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. MCAD deficiency is diagnosed from dried blood spots on filter paper cards from newborns on the basis of the increase of medium chain length acylcarnitines identified by isotope dilution mass spectrometry methods. A robust and accurate semiautomated method for the analysis of medium chain length acylcarnitines as their butyl esters was developed and validated. Quantitative data from the analyses of 113 randomly collected filter paper blood spots from healthy newborns showed low concentrations of medium chain length acylcarnitines such as octanoylcarnitine. The maximum concentration of octanoylcarnitine was 0.22 μmol/L, with the majority being at or below the detection limit. In all 16 blood spots from newborns with confirmed MCAD deficiency, octanoylcarnitine was highly increased [median 8.4 μmol/L (range 3.1–28.3 μmol/L)], allowing easy detection. The concentration of octanoylcarnitine was significantly higher in these 16 newborns (&lt;3 days of age) than in 16 older patients (ages 8 days to 7 years) with MCAD deficiency (median 1.57 μmol/L, range 0.33–4.4). The combined experience of prospective newborn screening in Pennsylvania and North Carolina has shown a disease frequency for MCAD deficiency of 1 in 17 706. No false-positive and no known false-negative results have been found. A validated method now exists for prospective newborn screening for MCAD deficiency.

[Study of plasma acylcarnitines using tandem mass spectrometry. Application to the diagnosis of metabolism hereditary diseases]

BACKGROUND

L-carnitine is known to transport long chain fatty acids through the mitochondrial membrane but also to export accumulated acyl-CoA's as acylcarnitine esters. Acylcarnitine identification in body fluids allows the diagnosis of mitochondrial inborn errors especially fatty oxidation defects. Tandem mass spectrometry represents a new method for isolation and identification of acylcarnitines in plasma or in blood spotted onto filter paper (Guthrie cards).

MATERIAL AND METHODS

In order to validate our method, we studied 30 plasmas from children affected with 15 different inborn errors of metabolism and five amniotic fluids from fetuses affected with several organic acidurias. Fourty-six samples from children at risk for mitochondrial fatty oxidation disorders have been analyzed. We developed a method of tandem mass spectrometry with liquid secondary ion mass spectrometry using deuterated acylcarnitines as internal standards.

RESULTS

This method is very sensitive (detection limit = 2 microM). In all affected patients specific acylcarnitine signals corresponding to the metabolic block were constantly found. This confirms the diagnosis and validates the method. Among the 46 at risk children, four defects of long chain fatty acid oxidation were identified.

CONCLUSION

This new method is of great interest especially for the long chain fatty acid oxidation defects. These defects are very difficult to diagnose with classical methods as urinary organic acid profiling. A small amount of plasma (100 microL) or blood spotted onto paper is required. The acylcarnitine profile allows a rapid diagnosis if a dedicated apparatus is available.

Screening blood spots for inborn errors of metabolism by electrospray tandem mass spectrometry with a microplate batch process and a computer algorithm for automated flagging of abnormal profiles

Metabolic profiling of amino acids and acylcarnitines from blood spots by automated electrospray tandem mass spectrometry (ESI-MS/MS) is a powerful diagnostic tool for inborn errors of metabolism. New approaches to sample preparation and data interpretation have helped establish the methodology as a robust, high-throughput neonatal screening method. We introduce an efficient 96-well-microplate batch process for blood-spot sample preparation, with which we can obtain high-quality profiles from 500-1000 samples per day per instrument. A computer-assisted metabolic profiling algorithm automatically flags abnormal profiles. We selected diagnostic parameters for the algorithm by comparing profiles from patients with known metabolic disorders and those from normal newborns. Reference range and cutoff values for the diagnostic parameters were established by measuring either metabolite concentrations or peak ratios of certain metabolite pairs. Rigorous testing of the algorithm demonstrates its outstanding clinical sensitivity in flagging abnormal profiles and its high cumulative specificity.

From gene to screen with yeast

With the complete sequence now available, the yeast genome project enters a post-sequencing phase that will concentrate on a comprehensive determination of gene function. Novel techniques have been developed to undertake genome-wide functional analysis at the levels of phenotype, transcript and protein. These include techniques for the efficient deletion of individual genes while tagging the deletants with specific oligonucleotide signatures, as well as strategies to quantify the physiological effects of such deletions by comparing growth rates and metabolite profiles under a range of conditions. Comprehensive approaches to the study of gene expression include hybridization array technology to identify and quantify transcripts, and the exploitation of mass spectometry to identify proteins resolved by two-dimensional gel electrophoresis. Yeast presents opportunities for the discovery of new human medicines both via the recognition of functional homologies between human and yeast genes and by the use of yeast to express human coding sequences specifying potential drug targets.

Evaluation of fasts for investigating hypoglycaemia or suspected metabolic disease

AIM

To assess the value and safety of fasts for investigating hypoglycaemia or suspected metabolic disease.

STUDY DESIGN

Review of all diagnostic fasts performed over a 2.5 year period.

SETTING

The neonatal intensive care unit and programmed investigation unit at a tertiary referral centre for endocrinology and metabolic disease.

RESULTS

138 diagnostic fasts were performed during the study period. Hypoglycaemia (< 2.6 mmol/l) occurred in 54 cases but in only four did the blood glucose concentration fall below 1.5 mmol/l. One patient became unwell as a result of a fast, but prompt treatment averted any sequelae. Specific endocrine or metabolic defects were identified in 30 cases, the most common being hyperinsulinism and beta-oxidation defects.

CONCLUSIONS

Fasting is safe if conducted on an experienced unit with appropriate guidelines. It continues to provide useful information for diagnosis and management, particularly in cases of hyperinsulinism. Diagnoses should, however, be established by lower risk procedures whenever possible. Thus specimens for metabolic and endocrine studies should be obtained during the presenting episode and blood acylcarnitine species should be analysed prior to fasting.

Prenatal diagnosis of organic acidemias based on amniotic fluid levels of acylcarnitines

Acylcarnitines in amniotic fluid samples were analyzed for the prenatal diagnosis of propionic acidemia, methylmalonic aciduria, isovaleric acidemia, and glutaric aciduria by electrospray tandem mass spectrometry. Although the levels of the specific acylcarnitine between affected and unaffected cases showed an overlap, the ratios of propionylcarnitine to 4-carbon acylcarnitine levels for propionic acidemia and methylmalonic aciduria, those of isovalerylcarnitine to propionylcarnitine for isovaleric acidemia, and those of glutarylcarnitine to propionylcarnitine for glutaric aciduria type I were shown to be reliable indicators in the prenatal diagnosis. In addition, it is suggested that the combination of the ratios of glutarylcarnitine, isovaleryl-carnitine, and hexanoylcarnitine to propionylcarnitine may be useful for the prenatal diagnosis of glutaric aciduria type II.

A new chemical diagnostic method for inborn errors of metabolism by mass spectrometry-rapid, practical, and simultaneous urinary metabolites analysis

In most developed countries, neonatal mass screening programs for the early diagnosis of inborn errors of metabolism (IEM) have been implemented and have been found to be effective for the prevention or significant reduction of clinical symptoms such as mental retardation. These programs rely primarily on simple bacterial inhibition assays (the "Guthrie tests"). We developed a new method for screening IEM using GC/MS, which enables accurate chemical diagnoses through urinary analyses with a simple practical procedure. The urine sample preparation for GC/MS takes one hour for one sample or three hours for a batch of 30 samples (will be fully automated shortly), and the following GC/MS measurement is completed within 15 min per sample. This method allows the simultaneous analyses of amino acids, organic acids, sugars, sugar alcohols, sugar acids, and nucleic acid bases. Therefore, a large number of metabolic disorders can be simultaneously tested by this chemical diagnostic procedure. This method is quite comprehensive and different from conventional GC/MS organic acidemia screening procedures, which are not well-suited to detect metabolic disorders except organic acidurias. Sample preparation includes urease treatment, deproteinization, and derivatization. The method has also been applied to neonate urine specimens that are absorbed into filter paper. The air-dried samples were mailed to the analytical laboratory and eluted with water. The eluate (0.1 mL) was incubated with urease, followed by deproteinization with alcohol, evaporation to dryness of the supernatant, and trimethylsilylation; the samples were applied to GC/MS. A pilot study of the application of this diagnostic procedure to the neonatal mass screening of 22 disorders was started in Japan on February 1, 1995 in cooperation with four medical institutes. This program is supported by the Japanese Society for Biomedical Mass Spectrometry and the Japanese Mass Screening Society. The initial twenty-two target metabolic diseases are: methylmalonic acidemia; propionic acidemia; isovaleric acidemia; maple syrup urine disease; β-ketothiolase deficiency; galactosemia; phenylketonuria; hyperphenylalaninemia; homocystinuria; alkaptonuria; multiple carboxylase deficiency; nonketotic hyperglycinemia; lysinuria; cystinuria; tyrosinemia; glutaric aciduria type I; β-hydroxy-β-methylglutaric acidemia; β-methylcrotonylglycinuria; α-aminoadipic-α-ketoadipic aciduria; ornitine transcarbamylase deficiency (four urea cycle disorders can be screened); glutaric aciduria type II; and neuroblastoma. Neuroblastoma is not an IEM, and is examined at ca. 6 months of age. The twenty-two target diseases will be reconsidered during the pilot study. An accurate chemical diagnosis and hence early treatment of not only organic acidemias but also amino acidemias, and sugar-, polyol-, and nucleic acid base-accumulating metabolic disorders can be made at a very early stage of life. This procedure is also applicable to metabolic profiling of other body fluids that are potentially informative for the study and characterization of a wide range of inherited and acquired metabolic disorders. © 1997 John Wiley & Sons, Inc.