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

Autistic Siblings with Novel Mutations in Two Different Genes: Insight for Genetic Workups of Autistic Siblings and Connection to Mitochondrial Dysfunction

The prevalence of autism spectrum disorder (ASD) is high, yet the etiology of this disorder is still uncertain. Advancements in genetic analysis have provided the ability to identify potential genetic changes that may contribute to ASD. Interestingly, several genetic syndromes have been linked to metabolic dysfunction, suggesting an avenue for treatment. In this case study, we report siblings with ASD who had similar initial phenotypic presentations. Whole exome sequencing (WES) revealed a novel c.795delT mutation in the WDR45 gene affecting the girl, which was consistent with her eventual progression to a Rett-like syndrome phenotype including seizures along with a stereotypical cyclic breathing pattern. Interestingly, WES identified that the brother harbored a novel heterozygous Y1546H variant in the DEP domain-containing protein 5 (DEPDC5) gene, consistent with his presentation. Both siblings underwent a metabolic workup that demonstrated different patterns of mitochondrial dysfunction. The girl demonstrated statistically significant elevations in mitochondrial activity of complex I + III in both muscle and fibroblasts and increased respiration in peripheral blood mononuclear cells (PBMCs) on Seahorse Extracellular Flux analysis. The boy demonstrates a statistically significant decrease in complex IV activity in buccal epithelium and decreased respiration in PBMCs. These cases highlight the differences in genetic abnormalities even in siblings with ASD phenotypes as well as highlights the individual role of novel mutations in the WDR45 and DEPDC5 genes. These cases demonstrate the importance of advanced genetic testing combined with metabolic evaluations in the workup of children with ASD.

Gastrointestinal dysfunction in autism spectrum disorder: the role of the mitochondria and the enteric microbiome

Autism spectrum disorder (ASD) affects a significant number of individuals worldwide with the prevalence continuing to grow. It is becoming clear that a large subgroup of individuals with ASD demonstrate abnormalities in mitochondrial function as well as gastrointestinal (GI) symptoms. Interestingly, GI disturbances are common in individuals with mitochondrial disorders and have been reported to be highly prevalent in individuals with co-occurring ASD and mitochondrial disease. The majority of individuals with ASD and mitochondrial disorders do not manifest a primary genetic mutation, raising the possibility that their mitochondrial disorder is acquired or, at least, results from a combination of genetic susceptibility interacting with a wide range of environmental triggers. Mitochondria are very sensitive to both endogenous and exogenous environmental stressors such as toxicants, iatrogenic medications, immune activation, and metabolic disturbances. Many of these same environmental stressors have been associated with ASD, suggesting that the mitochondria could be the biological link between environmental stressors and neurometabolic abnormalities associated with ASD. This paper reviews the possible links between GI abnormalities, mitochondria, and ASD. First, we review the link between GI symptoms and abnormalities in mitochondrial function. Second, we review the evidence supporting the notion that environmental stressors linked to ASD can also adversely affect both mitochondria and GI function. Third, we review the evidence that enteric bacteria that are overrepresented in children with ASD, particularly Clostridia spp., produce short-chain fatty acid metabolites that are potentially toxic to the mitochondria. We provide an example of this gut–brain connection by highlighting the propionic acid rodent model of ASD and the clinical evidence that supports this animal model. Lastly, we discuss the potential therapeutic approaches that could be helpful for GI symptoms in ASD and mitochondrial disorders. To this end, this review aims to help better understand the underlying pathophysiology associated with ASD that may be related to concurrent mitochondrial and GI dysfunction.

Redox metabolism abnormalities in autistic children associated with mitochondrial disease

Research studies have uncovered several metabolic abnormalities associated with autism spectrum disorder (ASD), including mitochondrial disease (MD) and abnormal redox metabolism. Despite the close connection between mitochondrial dysfunction and oxidative stress, the relation between MD and oxidative stress in children with ASD has not been studied. Plasma markers of oxidative stress and measures of cognitive and language development and ASD behavior were obtained from 18 children diagnosed with ASD who met criteria for probable or definite MD per the Morava et al. criteria (ASD/MD) and 18 age and gender-matched ASD children without any biological markers or symptoms of MD (ASD/NoMD). Plasma measures of redox metabolism included reduced free glutathione (fGSH), oxidized glutathione (GSSG), the fGSH/GSSG ratio and 3-nitrotyrosine (3NT). In addition, a plasma measure of chronic immune activation, 3-chlorotyrosine (3CT), was also measured. Language was measured using the preschool language scale or the expressive one-word vocabulary test (depending on the age), adaptive behaviour was measured using the Vineland Adaptive Behavior Scale (VABS) and core autism symptoms were measured using the Autism Symptoms Questionnaire and the Social Responsiveness Scale. Children with ASD/MD were found to have lower scores on the communication and daily living skill subscales of the VABS despite having similar language and ASD symptoms. Children with ASD/MD demonstrated significantly higher levels of fGSH/GSSG and lower levels of GSSG as compared with children with ASD/NoMD, suggesting an overall more favourable glutathione redox status in the ASD/MD group. However, compare with controls, both ASD groups demonstrated lower fGSH and fGSH/GSSG, demonstrating that both groups suffer from redox abnormalities. Younger ASD/MD children had higher levels of 3CT than younger ASD/NoMD children because of an age-related effect in the ASD/MD group. Both ASD groups demonstrated significantly higher 3CT levels than control subjects, suggesting that chronic inflammation was present in both groups of children with ASD. Interestingly, 3NT was found to correlate positively with several measures of cognitive function, development and behavior for the ASD/MD group, but not the ASD/NoMD group, such that higher 3NT concentrations were associated with more favourable adaptive behaviour, language and ASD-related behavior. To determine whether difference in receiving medications and/or supplements could account for the differences in redox and inflammatory biomarkers across ASD groups, we examined differences in medication and supplements across groups and their effect of redox and inflammatory biomarkers. Overall, significantly more participants in the ASD/MD group were receiving folate, vitamin B12, carnitine, co-enzyme Q10, B vitamins and antioxidants. We then determined whether folate, carnitine, co-enzyme Q10, B vitamins and/or antioxidants influenced redox or inflammatory biomarkers. Antioxidant supplementation was associated with a significantly lower GSSG, whereas antioxidants, co-enzyme Q10 and B vitamins were associated with a higher fGSH/GSSG ratio. There was no relation between folate, carnitine, co-enzyme Q10, B vitamins and antioxidants with 3NT, 3CT or fGSH. Overall, our findings suggest that ASD/MD children with a more chronic oxidized microenvironment have better development. We interpret this finding in light of the fact that more active mitochondrial can create a greater oxidized microenvironment especially when dysfunctional. Thus, compensatory upregulation of mitochondria which are dysfunctional may both increase activity and function at the expense of a more oxidized microenvironment. Although more ASD/MD children were receiving certain supplements, the use of such supplements were not found to be related to the redox biomarkers that were related to cognitive development or behavior in the ASD/MD group but could possibly account for the difference in glutathione metabolism noted between groups. This study suggests that different subgroups of children with ASD have different redox abnormalities, which may arise from different sources. A better understanding of the relationship between mitochondrial dysfunction in ASD and oxidative stress, along with other factors that may contribute to oxidative stress, will be critical to understanding how to guide treatment and management of ASD children. This study also suggests that it is important to identify ASD/MD children as they may respond differently to specific treatments because of their specific metabolic profile.

Unique acyl-carnitine profiles are potential biomarkers for acquired mitochondrial disease in autism spectrum disorder

Autism spectrum disorder (ASD) has been associated with mitochondrial disease (MD). Interestingly, most individuals with ASD and MD do not have a specific genetic mutation to explain the MD, raising the possibility of that MD may be acquired, at least in a subgroup of children with ASD. Acquired MD has been demonstrated in a rodent ASD model in which propionic acid (PPA), an enteric bacterial fermentation product of ASD-associated gut bacteria, is infused intracerebroventricularly. This animal model shows validity as it demonstrates many behavioral, metabolic, neuropathologic and neurophysiologic abnormalities associated with ASD. This animal model also demonstrates a unique pattern of elevations in short-chain and long-chain acyl-carnitines suggesting abnormalities in fatty-acid metabolism. To determine if the same pattern of biomarkers of abnormal fatty-acid metabolism are present in children with ASD, the laboratory results from a large cohort of children with ASD (n=213) who underwent screening for metabolic disorders, including mitochondrial and fatty-acid oxidation disorders, in a medically based autism clinic were reviewed. Acyl-carnitine panels were determined to be abnormal if three or more individual acyl-carnitine species were abnormal in the panel and these abnormalities were verified by repeated testing. Overall, 17% of individuals with ASD demonstrated consistently abnormal acyl-carnitine panels. Next, it was determined if specific acyl-carnitine species were consistently elevated across the individuals with consistently abnormal acyl-carnitine panels. Significant elevations in short-chain and long-chain, but not medium-chain, acyl-carnitines were found in the ASD individuals with consistently abnormal acyl-carnitine panels-a pattern consistent with the PPA rodent ASD model. Examination of electron transport chain function in muscle and fibroblast culture, histological and electron microscopy examination of muscle and other biomarkers of mitochondrial metabolism revealed a pattern consistent with the notion that PPA could be interfering with mitochondrial metabolism at the level of the tricarboxylic-acid cycle (TCAC). The function of the fatty-acid oxidation pathway in fibroblast cultures and biomarkers for abnormalities in non-mitochondrial fatty-acid metabolism were not consistently abnormal across the subgroup of ASD children, consistent with the notion that the abnormalities in fatty-acid metabolism found in this subgroup of children with ASD were secondary to TCAC abnormalities. Glutathione metabolism was abnormal in the subset of ASD individuals with consistent acyl-carnitine panel abnormalities in a pattern similar to glutathione abnormalities found in the PPA rodent model of ASD. These data suggest that there are similar pathological processes between a subset of ASD children and an animal model of ASD with acquired mitochondrial dysfunction. Future studies need to identify additional parallels between the PPA rodent model of ASD and this subset of ASD individuals with this unique pattern of acyl-carnitine abnormalities. A better understanding of this animal model and subset of children with ASD should lead to better insight in mechanisms behind environmentally induced ASD pathophysiology and should provide guidance for developing preventive and symptomatic treatments.

Mitochondrial disease in 22q13 duplication syndrome

Both copy number changes in the 22q13 region and mitochondrial disease have been associated with autism spectrum disorder. In this report, for the first time, a girl with autism spectrum disorder is described who exhibits both mitochondrial disease and a 22q13.1-33 duplication. This child demonstrated hypotonia, developmental delays, growth deficiency, microcephaly, dysmorphic facial features, and white matter abnormalities, consistent with previous cases of 22q13 region duplication. The patient also demonstrated a unique pattern of electron transport chain abnormalities with marked decreases in complex II and II/III in fibroblasts and complex I/III and II/III in muscle tissue. The 22q13.1-33 region contains 6 genes associated with mitochondrial function. Thus, disruption of this chromosomal region could cause many of the clinical findings in this child through disruption of mitochondrial function. Therefore, a mitochondrial workup should be considered in individuals with copy number changes within the 22q13 region, such as those with Phelan-McDermid syndrome.

Deuterated palmitate-driven acylcarnitine formation by whole-blood samples for a rapid diagnostic exploration of mitochondrial fatty acid oxidation disorders

BACKGROUND

The biochemical diagnosis of mitochondrial fatty acid oxidation defects (FAOD) currently rests on enzyme assays. A dynamic ex vivo exploration consisting of incubations of whole-blood samples with stable-labeled palmitate and determining leukocyte capacities to produce deuterated acylcarnitines was developed on healthy controls (n=52) and patients with very-long- (VLCADD) (n=2), medium- (MCADD) (n=6), or short- (SCADD) (n=1) chain acyl-CoA dehydrogenase deficiencies.

METHODS

Incubations were optimized with L-carnitine and [16-(2)H(3), 15-(2)H(2)]-palmitate at 37 degrees C for various time periods on MCADD and control whole-blood samples. Labeled acylcarnitines were quantified by electrospray-ionization tandem mass spectrometry after thawing, extraction and derivatization to their butyl esters and the method was applied to patients with defects mentioned above.

RESULTS

The production of acylcarnitines was linear until 6 h of incubation and optimal on 50 to 200 nmol deuterated substrate. A good discrimination between MCADD patient and control data was found, with median C8/C4 acylcarnitine production rate ratios of 81.0 (5th-95th percentile range: 16.6-209.9) and 0.21 (5th-95th percentile range: 0.06-0.79), respectively. The method also discriminated from controls the VLCADD and SCADD patients. Preliminary studies on a healthy control indicated that the storage at 4 degrees C does little or not alter capacities of whole-blood samples to generate labeled acylcarnitines over a period of 48 h.

CONCLUSION

The rapid management afforded by the method, its abilities to characterize patients and to work on whole-blood samples after a stay of 24-48 h at 4 degrees C make it promising for the diagnostic exploration of FAOD.

Essential fatty acid profiling for routine nutritional assessment unmasks adrenoleukodystrophy in an infant with isovaleric acidaemia

We report a 16-month-old asymptomatic male with enzyme confirmed isovaleric acidaemia (IVA; isovaleryl-CoA dehydrogenase deficiency; OMIM 243500) who, upon routine nutritional follow-up, presented evidence of peroxisomal dysfunction. The newborn screen (2 days of life) revealed elevated C(5)-carnitine (2.95 μmol/L; cutoff <0.09 μmol/L) and IVA was subsequently confirmed by metabolic profiling and in vitro enzymology. Plasma essential fatty acid (EFA) analysis, assessed to evaluate nutritional status during protein restriction and L: -carnitine supplementation, revealed elevated C(26:0) (5.0 μmol/L; normal <1.3). Subsequently, metabolic profiling and molecular genetic analysis confirmed X-linked adrenoleukodystrophy (XALD). Identification of co-inherited XALD with IVA in this currently asymptomatic patient holds significant treatment ramifications for the proband prior to the onset of neurological sequelae, and critically important counselling implications for this family.

Carnitine palmitoyltransferase II deficiency: successful anaplerotic diet therapy

BACKGROUND

Carnitine palmitoyltransferase II (CPT II) deficiency is an important cause of recurrent rhabdomyolysis in children and adults. Current treatment includes dietary fat restriction, with increased carbohydrate intake and exercise restriction to avoid muscle pain and rhabdomyolysis.

METHODS

CPT II enzyme assay, DNA mutation analysis, quantitative analysis of acylcarnitines in blood and cultured fibroblasts, urinary organic acids, the standardized 36-item Short-Form Health Status survey (SF-36) version 2, and bioelectric impedance for body fat composition. Diet treatment with triheptanoin at 30% to 35% of total daily caloric intake was used for all patients.

RESULTS

Seven patients with CPT II deficiency were studied from 7 to 61 months on the triheptanoin (anaplerotic) diet. Five had previous episodes of rhabdomyolysis requiring hospitalizations and muscle pain on exertion prior to the diet (two younger patients had not had rhabdomyolysis). While on the diet, only two patients experienced mild muscle pain with exercise. During short periods of noncompliance, two patients experienced rhabdomyolysis with exercise. None experienced rhabdomyolysis or hospitalizations while on the diet. All patients returned to normal physical activities including strenuous sports. Exercise restriction was eliminated. Previously abnormal SF-36 physical composite scores returned to normal levels that persisted for the duration of the therapy in all five symptomatic patients.

CONCLUSIONS

The triheptanoin diet seems to be an effective therapy for adult-onset carnitine palmitoyltransferase II deficiency.

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

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

Early energy deficit in Huntington disease: identification of a plasma biomarker traceable during disease progression

Huntington disease (HD) is a fatal neurodegenerative disorder, with no effective treatment. The pathogenic mechanisms underlying HD have not been elucidated, but weight loss, associated with chorea and cognitive decline, is a characteristic feature of the disease that is accessible to investigation. We, therefore, performed a multiparametric study exploring body weight and the mechanisms of its loss in 32 presymptomatic carriers and HD patients in the early stages of the disease, compared to 21 controls. We combined this study with a multivariate statistical analysis of plasma components quantified by proton nuclear magnetic resonance (¹H NMR) spectroscopy. We report evidence of an early hypermetabolic state in HD. Weight loss was observed in the HD group even in presymptomatic carriers, although their caloric intake was higher than that of controls. Inflammatory processes and primary hormonal dysfunction were excluded. ¹H NMR spectroscopy on plasma did, however, distinguish HD patients at different stages of the disease and presymptomatic carriers from controls. This distinction was attributable to low levels of the branched chain amino acids (BCAA), valine, leucine and isoleucine. BCAA levels were correlated with weight loss and, importantly, with disease progression and abnormal triplet repeat expansion size in the HD1 gene. Levels of IGF1, which is regulated by BCAA, were also significantly lower in the HD group. Therefore, early weight loss in HD is associated with a systemic metabolic defect, and BCAA levels may be used as a biomarker, indicative of disease onset and early progression. The decreased plasma levels of BCAA may correspond to a critical need for Krebs cycle energy substrates in the brain that increased metabolism in the periphery is trying to provide.

Determination of 1-methylhistidine and 3-methylhistidine by capillary and chip electrophoresis with contactless conductivity detection

CE with capacitively coupled contactless detection (C4D) was used to determine 3-methylhistidine (3-MH) and 1-methylhistidine (1-MH). The C4D response to 3-MH was studied in a BGE consisting of 500 mM acetic acid and ammonia at varying concentration and the results were compared with the theory. Complete separation of a model mixture of 3-MH, 1-MH, and histidine (His) was attained in two optimized BGEs, one containing 500 mM HAc, 20 mM NH4OH, and 0.1 % m/v hydroxyethylcellulose (HEC), pH 3.4 (I) and the other consisting of 100 mM morpholinoethanesulfonic acid (MES), 25 mM LiOH, and 0.1 % m/v HEC, pH 5.5 (II). These optimized BGEs were tested in CE/C4D analyses of urine. Promising results were obtained for separation and determination of 3-MH, 1-MH, and His on a silicon microchip, using aluminum strips as the C4D electrodes; the three analytes were baseline-separated within less than 30 s with a separation channel effective length of 38 mm. The LOD were satisfactory and amounted to 26.4 microM for 3-MH and 18.3 microM for 1-MH.

A novel functional assay for simultaneous determination of total fatty acid beta-oxidation flux and acylcarnitine profiling in human skin fibroblasts using (2)H(31)-palmitate by isotope ratio mass spectrometry and electrospray tandem mass spectrometry

BACKGROUND

Two separate and complementary assays, total mitochondrial fatty acid beta-oxidation (FAO) flux rate and acylcarnitine profiling, have been used to establish a definitive diagnosis of FAO defects (FAOD) in cultured cells. We developed a novel functional assay for total FAO rate assay by measurement of deuterated water enrichment and to combine it with the conventional acylcarnitine profiling method into a single tracer incubation experiment.

METHODS

Skin fibroblasts were incubated in a medium containing universal deuterium-labeled palmitate ((2)H(31)-palmitate) and l-carnitine without glucose supplementation for 96 h. The culture medium was assayed for deuterated water enrichment using isotope ratio mass spectrometry (IRMS) and acylcarnitine profiling by electrospray-ionization tandem mass spectrometry (ESI/MS/MS).

RESULTS

The medians of (2)H(2)O enrichment after 96 h of incubation of (2)H(31)-palmitate of the control, other inherited metabolic diseases and FAOD cell lines were 109.9, 102 and 23.1 ppm/mg protein/96 h, respectively. All fibroblasts with FAOD except carnitine uptake defective, multiple acyl-CoA dehydrogenase and short-chain 3-hydroxyacyl-CoA dehydrogenase deficient cells were well separated from the control (<60% control median, p<0.05) and could be identified by IRMS assay. Accumulations of disease-specific acylcarnitines due to blockage in the carnitine cycle and FAO spiral were also demonstrated by acylcarnitine profiling.

CONCLUSIONS

This novel functional assay is less time consuming and relatively simple by comparison to other published methods and can be used to investigate patients suspected to have FAO defects.

Determination of free amino acids and related compounds in amniotic fluid by capillary electrophoresis with contactless conductivity detection

A capillary electrophoretic (CE) method with contactless conductivity detection (CCD) has been developed for the determination of free amino acids (AAs) in the amniotic fluid. Apart from 20 proteinogenic AAs, 12 other biogenic compounds have been identified including ethanolamine, choline, beta-alanine, 2-aminobutyric acid, 4-aminobutyric acid, creatinine, ornithine, carnitine, citrulline, 4-hydroxyproline, 1-methylhistidine and 3-methylhistidine. The running electrolyte consisted of 1.7 M acetic acid and 0.1% hydroxyethyl-cellulose (pH 2.15). An addition of acetonitrile to the sample improved the separation of AAs significantly and permitted an increase in the amount of the sample injected. As a result, the sensitivity of the determination increased and the limit of detection (LOD) decreased by a factor of ca. 4, as compared with our previous study. The LOD values were between 1.5 microM (arginine) and 6.7 microM (aspartic acid). The CE/CCD method has then been applied to clinical analyses of the amniotic fluid collected from 20 pregnant women aged over 35 years and 24 pregnant women with whom abnormal foetus development was suspected. The latter group of women was found to exhibit systematically enhanced amniotic levels of most of the AAs studied.

Differentiation of long-chain fatty acid oxidation disorders using alternative precursors and acylcarnitine profiling in fibroblasts

The differentiation of carnitine-acylcarnitine translocase deficiency (CACT) from carnitine palmitoyltransferase type II deficiency (CPT-II) and long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency from mitochondrial trifunctional protein deficiency (MTP) continues to be ambiguous using current acylcarnitine profiling techniques either from plasma or blood spots, or in the intact cell system (fibroblasts/amniocytes). Currently, enzyme assays are required to unequivocally differentiate CACT from CPT-II, and LCHAD from MTP. Over the years we have studied the responses of numerous FOD deficient cell lines to both even and odd numbered fatty acids of various chain lengths as well as branched-chain amino acids. In doing so, we discovered diagnostic elevations of unlabeled butyrylcarnitine detected only in CACT deficient cell lines when incubated with a shorter chain fatty acid, [7-2H3]heptanoate plus l-carnitine compared to the routinely used long-chain fatty acid, [16-2H3]palmitate. In monitoring the unlabeled C4/C5 acylcarnitine ratio, further differentiation from ETF/ETF-DH is also achieved. Similarly, incubating LCHAD and MTP deficient cell lines with the long-chain branched fatty acid, pristanic acid, and monitoring the C11/C9 acylcarnitine ratio has allowed differentiation between these disorders. These methods may be considered useful alternatives to specific enzyme assays for differentiation between these long-chain fatty acid oxidation disorders, as well as provide insight into new treatment strategies.

Bezafibrate increases very-long-chain acyl-CoA dehydrogenase protein and mRNA expression in deficient fibroblasts and is a potential therapy for fatty acid oxidation disorders

Inherited defect in very-long-chain acyl-CoA dehydrogenase (VLCAD), a mitochondrial enzyme catalyzing the initial step of long-chain fatty acid beta-oxidation (FAO), is one of the most frequent FAO enzyme defects. VLCAD deficiency is associated with clinical manifestations varying in severity, tissue involvement and age of onset. The molecular basis of VLCAD deficiency has been elucidated but therapeutic approaches are quite limited. In this study, we tested the hypothesis that fibrates, acting as agonist of peroxisome proliferator-activated receptors (PPARs), might stimulate FAO in VLCAD-deficient cells. We demonstrate that addition of bezafibrate or fenofibric acid in the culture medium induced a dose-dependent (up to 3-fold) increase in palmitate oxidation capacities in cells from patients with the myopathic form of VLCAD deficiency, but not in cells from severely affected patients. Complete normalization of cell FAO capacities could be achieved after exposure to 500 microm bezafibrate for 48 h. Cell therapy of VLCAD deficiency was related to drug-induced increases in VLCAD mRNA (+44 to +150%; P<0.001), protein (1.5-2-fold) and residual enzyme activity (up to 7.7-fold) in patient cells. Bezafibrate also diminished the production of toxic long-chain acylcarnitines by 90% in cells harboring moderate VLCAD deficiency. Finally, real-time PCR studies indicated that bezafibrate potentially stimulated gene expression of other enzymes in the beta-oxidation pathway. These data highlight the potential of fibrates in the correction of inborn FAO defects, as most mutations associated with these defects are compatible with the synthesis of a mutant protein with variable levels of residual enzyme activity.

Pyruvate carboxylase deficiency: clinical and biochemical response to anaplerotic diet therapy

A six-day-old girl was referred for severe hepatic failure, dehydratation, axial hypotonia, and both lactic acidosis and ketoacidosis. Biotin-unresponsive pyruvate carboxylase deficiency type B was diagnosed. Triheptanoin, an odd-carbon triglyceride, was administrated as a source for acetyl-CoA and anaplerotic propionyl-CoA. Although this patient succumbed to a severe infection, during the six months interval of her anaplerotic and biochemical management, the following important observations were documented: (1) the immediate reversal (less than 48 h) of major hepatic failure with full correction of all biochemical abnormalities, (2) on citrate supplementation, the enhanced export from the liver of triheptanoin's metabolites, namely 5 carbon ketone bodies, increasing the availability of these anaplerotic substrates for peripheral organs, (3) the demonstration of the transport of C5 ketone bodies-representing alternative energetic fuel for the brain-across the blood-brain barrier, associated to increased levels of glutamine and free gamma-aminobutyric acid (f-GABA) in the cerebrospinal fluid. Considering that pyruvate carboxylase is a key enzyme for anaplerosis, besides the new perspectives brought by anaplerotic therapies in those rare pyruvate carboxylase deficiencies, this therapeutic trial also emphasizes the possible extended indications of triheptanoin in various diseases where the citric acid cycle is impaired.

A common mutation is associated with a mild, potentially asymptomatic phenotype in patients with isovaleric acidemia diagnosed by newborn screening

Isovaleric acidemia (IVA) is an inborn error of leucine metabolism that can cause significant morbidity and mortality. Since the implementation, in many states and countries, of newborn screening (NBS) by tandem mass spectrometry, IVA can now be diagnosed presymptomatically. Molecular genetic analysis of the IVD gene for 19 subjects whose condition was detected through NBS led to the identification of one recurring mutation, 932C-->T (A282V), in 47% of mutant alleles. Surprisingly, family studies identified six healthy older siblings with identical genotype and biochemical evidence of IVA. Our findings indicate the frequent occurrence of a novel mild and potentially asymptomatic phenotype of IVA. This has significant consequences for patient management and counseling.

Disease-related metabolites in culture medium of fibroblasts from patients with D-2-hydroxyglutaric aciduria, L-2-hydroxyglutaric aciduria, and combined D/L-2-hydroxyglutaric aciduria

BACKGROUND

D-2-Hydroxyglutaric aciduria (D-2-HGA), L-2-hydroxyglutaric aciduria (L-2-HGA), and the combined D/L-2-hydroxyglutaric aciduria (D/L-2-HGA) are poorly understood organic acidurias. To investigate the usefulness of cultured human skin fibroblasts for both diagnostic and research purposes, we measured disease-related metabolites in the cell culture medium.

METHODS

We measured D-2-hydroxyglutarate (D-2-HG), L-2-hydroxyglutarate (L-2-HG), succinate, 2-ketoglutarate, and citrate in fibroblast cell medium by stable-isotope-dilution gas chromatography-mass spectrometry and glutamine, glutamic acid, and lysine with an amino acid analyzer. We used six cell lines from patients with D-2-HGA, two from patients with L-2-HGA, three from patients with D/L-2-HGA, and seven control cell lines. Culture medium was analyzed after a 96-h incubation period.

RESULTS

Culture media from cell lines from D-2-HGA patients contained D-2-HG at concentrations 5- to 30-fold higher than media from controls, whereas the concentration of L-2-HG in media was not increased. Media from L-2-HGA cell lines showed a fivefold increase in L-2-HG compared with controls. Media containing fibroblasts from D/L-2-HGA patients contained moderately increased amounts of both D-2-HG and L-2-HG. For all cell lines, succinate concentrations in the blank medium were higher than after 96 h of incubation with the exception of two of three D/L-2-HGA cell lines. Media of D-2-HGA cell lines had 2-ketoglutarate concentrations that were 40% of that for controls. Glutamic acid concentrations in media of these cell lines were 60% lower than in controls.

CONCLUSIONS

Cell culture media from fibroblasts from patients with D-2-HGA, L-2-HGA, or D/L-2-HGA contain increased amounts the corresponding 2-HGs, demonstrating the suitability of fibroblasts for both diagnosis of and research concerning 2-HGAs.

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

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

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

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

The application of tandem mass spectrometry to neonatal screening for inherited disorders of intermediary metabolism

This review is intended to serve as a practical guide for geneticists to current applications of tandem mass spectrometry to newborn screening. By making dried-blood spot analysis more sensitive, specific, reliable, and inclusive, tandem mass spectrometry has improved the newborn detection of inborn errors of metabolism. Its innate ability to detect and quantify multiple analytes from one prepared blood specimen in a single analysis permits broad recognition of amino acid, fatty acid, and organic acid disorders. An increasing number of newborn screening programs are either utilizing or conducting pilot studies with tandem mass spectrometry. It is therefore imperative that the genetics community be familiar with tandem mass spectrometric newborn screening.

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

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

Application of tandem mass spectrometry to biochemical genetics and newborn screening

Tandem mass spectrometry (MS/MS) has become a key technology in the fields of biochemical genetics and newborn screening. The development of electrospray ionisation (ESI) and associated automation of sample handling and data manipulation have allowed the introduction of expanded newborn screening for disorders which feature accumulation of acylcarnitines and certain amino acids in a number of programs worldwide. In addition, the technique has proven valuable in several areas of biochemical genetics including quantification of carnitine and acylcarnitines, in vitro studies of metabolic pathways (in particular beta-oxidation), and diagnosis of peroxisomal and lysosomal disorders. This review covers some of the basic theory of MS/MS and focuses on the practical application of the technique in these two interrelated areas.

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

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

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

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

Long-chain L-3-hydroxyacyl-coenzyme a dehydrogenase deficiency: a molecular and biochemical review

Since the first report of long-chain L-3-hydroxyacyl-coenzyme A dehydrogenase deficiency a little more than a decade ago, its phenotypic and genotypic heterogeneity in individuals homozygous for the enzyme defect has become more and more evident. Even more interesting is its association with pregnancy-specific disorders, including preeclampsia, HELLP syndrome (hemolysis, elevated liver enzymes, low platelets), hyperemesis gravidarum, acute fatty liver of pregnancy, and maternal floor infarct of the placenta. In this review we discuss the biochemical and molecular basis, clinical features, diagnosis, and management of long-chain L-3-hydroxyacyl-coenzyme A dehydrogenase deficiency.

Analysis of mitochondrial fatty acid oxidation intermediates by tandem mass spectrometry from intact mitochondria prepared from homogenates of cultured fibroblasts, skeletal muscle cells, and fresh muscle

Defects of mitochondrial fatty acid beta-oxidation are an important group of inherited metabolic disorders in children. Despite improved screening opportunities, diagnosis of these disorders is not often straightforward and requires enzyme analyses. Because therapy is effective in many of these disorders, rapid diagnosis is essential. We report a technique that allows analysis of fatty acid oxidation not only in cultured cells (fibroblasts, myoblasts, and myotubes) but also in fresh muscle homogenate. Fatty acid oxidation analysis was performed by incubating fresh muscle homogenate or harvested cultured cells with stable isotopically labeled palmitate. The intermediates generated were analyzed by tandem mass spectrometry. Results of patients with seven different beta-oxidation disorders were compared with controls. Acylcarnitine intermediates in patient samples could be easily differentiated from the control samples. The acylcarnitine profile of each beta-oxidation defect was compatible with localization of the enzyme defect. Both in patient and control samples, the same pattern of intermediates could be detected in fibroblasts, muscle cells, and fresh muscle homogenate. The procedure described allowed correct diagnosis of all the beta-oxidation defects studied. Utilization of fresh muscle samples reduces the delay in diagnosis related to tissue culture and is useful in diagnostic of patients with neuromuscular phenotype. Measurement of fatty acid oxidation intermediates from myoblasts or myotubes is an additional tool in investigating pathogenetic mechanisms of myopathy in beta-oxidation defects.

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

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

Characterization of plasma acylcarnitines in patients under valproate monotherapy using ESI-MS/MS

OBJECTIVES

The effect of administration of the antiepileptic drug valproate (VPA), on the composition of the plasma acylcarnitine profile (including free carnitine) was investigated.

DESIGN AND METHODS

Plasma samples were obtained from 18 individuals (13 males:5 females; 15-65 y) on long-term treatment with VPA (resulting in plasma levels of 14.6-135.0 mg/L; therapeutic conc.: 40-100 mg/L). Acylcarnitines (AC) in plasma were quantified by electrospray tandem mass spectrometry (ESI-MS/MS).

RESULTS

VPA was found to increase the levels (mean +/- SD, microM) of 3-hydroxy-isovalerylcarnitine (0.10 +/- 0.04; controls: 0.02-0.06), C14:2 acylcarnitine (0.11 +/- 0.05; controls: 0.02-0.08), propylglutarylcarnitine (0.06 +/- 0.05; controls: 0.00-0.04), and C18-0H-acylcarnitine (0.09 +/- 0.05; controls: 0.00-0.04). The free carnitine (C) (42.2 +/- 9.0; controls: 22.3-54.9) and the total carnitine (52.3 +/- 10.1; controls: 26.5-73.6) were not significantly altered by VPA. Other AC (C2-C18, monounsaturated and hydroxylated) were all within the control range and especially no increase of C8 (valproyl) carnitine was observed. A positive correlation was found between the ratios [AC] / [C] (p < 0.05) or [long-chain AC (C10-C18)] / [C] (p < 0.09) with the plasma VPA concentration.

CONCLUSIONS

The unequivocal increase in 3-hydroxy-isovalerylcarnitine is consistent with the increase of 3-hydroxy-isovaleric acid observed in urine of VPA treated patients. This finding suggests an interaction mechanism of VPA with specific enzymes, namely involved in leucine metabolism. Adult patients under VPA monotherapy do not suffer from carnitine deficiency; the effect of the accumulating acylcarnitines is ill-defined.

Prenatal diagnosis of multiple acyl-CoA dehydrogenase deficiency: association with elevated alpha-fetoprotein and cystic renal changes

We report the occurrence of multiple acyl-CoA dehydrogenase deficiency (MADD) in two consecutive pregnancies in a young, Caucasian, non-consanguineous couple. In the first pregnancy, the maternal serum alpha-fetoprotein was elevated. A sonogram showed growth delay, cystic renal disease, and oligohydramnios; the parents decided to terminate the pregnancy. Postmortem examination confirmed the cystic renal disease and showed hepatic steatosis, raising the suspicion of a metabolic disorder. The diagnosis of MADD was made by immunoblot studies on cultured fibroblasts. In the subsequent pregnancy, a sonogram at 15 weeks' gestation showed an early growth delay but normal kidneys. The maternal serum and amniotic fluid concentrations of alpha-fetoprotein were elevated, and the amniotic fluid acylcarnitine profile was consistent with MADD. In vitro metabolic studies on cultured amniocytes confirmed the diagnosis. A follow-up sonogram showed cystic renal changes. These cases provide additional information regarding the evolution of renal changes in affected fetuses and show a relationship with elevated alpha-fetoprotein, which may be useful in counseling the couple at risk. MADD should be considered in the differential diagnosis of elevated alpha-fetoprotein and cystic renal disease. Early growth delay may be an additional feature.

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

The degradation of unsaturated fatty acids was examined in fibroblasts from 16 patients with very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency. Analysis of acylcarnitine intermediates following incubation of intact human cells with these compounds revealed that the milder clinical phenotypes could be distinguished from the severe cardiomyopathic phenotype. These findings may reflect more effective contributions of alternate pathways in the milder forms of the disease. Incubation of VLCAD-deficient cells with cis-9 or trans-9 unsaturated fatty acids indicate that VLCAD is largely responsible for the 2,3-dehydrogenation of cis-5 or trans-5 intermediates in fibroblasts. The first two cycles of beta-oxidation with oleic and linoleic acids occur in the absence of VLCAD activity suggesting the presence of an additional acyl-CoA dehydrogenase or alternate pathway for the oxidation of these unsaturated fatty acids. These observations have clinical relevance for determining diagnosis, prognosis and strategies for dietary treatment of these patients.

Differential effect of valproate and its Delta2- and Delta4-unsaturated metabolites, on the beta-oxidation rate of long-chain and medium-chain fatty acids

Overall fatty acid oxidation rates were investigated in rat hepatocytes using [9,10-3H]-palmitic, [9,10-3H]-oleic, [9,10-3H]-myristic and [2,3-3H]-phenylpropionic acids. The effect of both valproate (VPA) (0-10 mM) and two of its unsaturated metabolites, Delta2(E)-VPA and Delta4-VPA (0-10 mM), on the overall 3H2O production rate was studied. The results give evidence of a general inhibitory effect of VPA on the beta-oxidation rate of all the tested substrates. Similar effects were observed with both VPA metabolites but these effects appeared to be dependent on the chain length of the substrate. When the effect on the oxidation of the medium-chain fatty acid 3-phenylpropionate (PPA) was studied, Delta2(E)-VPA at 0.5 mM caused a 94% inhibition of the overall beta-oxidation rate. However, with long-chain substrates, 0.5 mM Delta(4)-VPA was a more potent inhibitor (20-30% of control activity) than 0.5 mM Delta(2E)-VPA (60-80% of control activity). Our results suggest that VPA and/or its metabolites inhibit fatty acyl-CoA metabolism within the mitochondrion by two different mechanisms. The first mechanism involves CoASH sequestration, which affects the oxidation rate of all fatty acids with different chain length. The second mechanism is more specific in nature and involves selective inhibition of particular enzymes implicated in fatty acid beta-oxidation.

Valproate induces in vitro accumulation of long-chain fatty acylcarnitines

To elucidate the interference mechanisms of valproate (VPA) with mitochondrial fatty acid beta-oxidation (FAO), the profile of acylcarnitine formation was studied in vitro. Human fibroblasts were incubated with 0.2 mmol/L [U-(13)C]palmitate, 0.4 mmol/L l-carnitine, +/- VPA (2 mmol/L) (96 h at 37 degrees C). Acylcarnitines (AC) were analyzed by GC-CI-MS. VPA induced an impaired production of acetylcarnitine (C2) and an increase on long-chain AC (C10 to C16) both in control and in FAO-deficient cell lines (VLCAD, LCHAD, MTP).

Clinical applications of tandem mass spectrometry: ten years of diagnosis and screening for inherited metabolic diseases

This paper reviews the clinical applications of tandem mass spectrometry (MS-MS) in diagnosis and screening for inherited metabolic diseases in the last 10 years. The broad-spectrum of diseases covered, specificity, ease of sample preparation, and high throughput provided by the MS-MS technology has led to the development of multi-disorder newborn screening programs in many countries for amino acid disorders, organic acidemias, and fatty acid oxidation defects. Issues related to sample acquisition, sample preparation, quantification of metabolites, and validation are discussed. Our current experience with the technique in screening is presented. The application of MS-MS in selective screening has revolutionized the field and made a major impact on the detection of certain disease classes such as the fatty acid oxidation defects. New specific and rapid MS-MS and LC-MS-MS methods for highly polar small molecules are supplementing or replacing some of the classical GC-MS methods for a multitude of metabolites and disorders. New exciting applications are appearing in fields of prenatal, postnatal, and even postmortem diagnosis. Examples for pitfalls in the technique are also presented.

Accumulation of Free 3-Hydroxy Fatty Acids in the Culture Media of Fibroblasts from Patients Deficient in Long-Chain l-3-Hydroxyacyl-CoA Dehydrogenase: A Useful Diagnostic Aid

Background: The diagnosis of long-chain l-3-hydroxy-acyl-coenzyme A dehydrogenase (LCHAD) deficiency frequently requires the study of cultured fibroblasts. We developed such a test that does not require disruption and loss of the cells.

Methods: We measured free 3-hydroxy fatty acids (3-OHFAs) in media of skin fibroblasts cultures from 11 patients with a genetic deficiency of LCHAD and the associated disorder of mitochondrial trifunctional protein (MTFP). Fibroblasts were cultured for 24 h with 100 μmol/L nonisotopic palmitate added. 3-OHFAs were measured by selected-ion monitoring, stable-isotope dilution gas chromatography-mass spectrometry with [13C]-labeled internal standards.

Results: 3-OH-hexadecanoic and 3-OH-tetradecanoic FAs were increased 14- and 11-fold, respectively, in all patients with LCHAD or MTFP deficiency when compared with control fibroblast cell lines after overnight incubation with palmitate. 3-OH-dodecanoic FA demonstrated a modest, fivefold increase in LCHAD-deficient cells. The concentrations of all 3-OHFAs were similar whether or not the medium samples were hydrolyzed to release conjugated species such as acylcarnitines, suggesting that 3-OHFAs accumulate in the media as free FAs.

Conclusions: Measurement of 3-OHFA excretion from LCHAD- or MTFP-deficient cell lines can be used as a diagnostic tool. Free FAs are the predominant form of these abnormal metabolic intermediates in culture media.

Carnitine/acylcarnitine translocase deficiency (neonatal phenotype): successful prenatal and postmortem diagnosis associated with a novel mutation in a single family

The neonatal phenotype of carnitine-acylcarnitine translocase (CACT) deficiency is one of the most severe and usually lethal mitochondrial fat oxidation disorders characterized by hypoketotic hypoglycemia, hyperammonemia, cardiac abnormalities, and early death. In this study, the proband was the daughter of consanguineous Hispanic parents. At 36 h of life, she had bradycardia and died at 4 days of age without a specific diagnosis. In a subsequent pregnancy, prenatal counseling and amniocentesis were provided. Incubation of the amniocytes from this pregnancy and fibroblasts (from the dead proband) with [16-(2)H(3)]palmitic acid and analysis by tandem mass spectrometry revealed an increasedconcentration of [16-(2)H(3)]palmitoylcarnitine, suggesting the diagnoses of either CACT or carnitine palmitoyltransferase II (CPT-II) deficiency. CACT enzyme activity was absent in both cell lines. Molecular investigation of cDNA from the dead proband and her affected sibling revealed aberrant CACT cDNA species, including exon 3 skipping, both exon 3 and 4 skipping, and a 13-bp insertion at cDNA position 388. Investigation of these cell lines for mutations affecting CACT RNA processing by analysis of CACT gene sequences, including intron and exon boundaries, revealed a single nucleotide G deletion at the donor site in intron 3 which resulted in exon skipping and a 13-bp insertion. The proband and her affected sibling were homozygous for this deletion.

Applications of mass spectrometry in the study of inborn errors of metabolism

During the twentieth century, and particularly in its last decade, there have been major advances in mass spectrometry (MS). As a result, MS remains one of the most powerful tools for the investigation of genetic metabolic disease. Analysis of organic acids by gas chromatography-mass spectrometry (GC-MS) and analysis of acylcarnitines by tandem mass spectrometry are still leading to the discovery of new disorders. Tandem mass spectrometry is increasingly being used for neonatal screening. New methods for lipid analysis have opened up the fields of inborn errors of cholesterol synthesis, of bile acid synthesis and ofleukotriene synthesis. The latest developments in MS allow it to be used for determination of the amino acid sequence and posttranslational modifications of proteins. There are still some major hurdles to be overcome, but soon it should be possible to detect mutant proteins directly rather than by cDNA or genomic DNA analysis. Measurement of which proteins are overexpressed and underexpressed ('proteomics') should provide further information on the pathogenesis of complications of inborn errors, e.g. hepatic cirrhosis. The use of stable isotopes in conjunction with MS allows us to probe metabolic pathways. As an example, evidence is presented to support the contention that vitamin E and its oxidation product are catabolized by peroxisomal beta-oxidation. Mass spectrometry also has a major role in monitoring new forms of treatment for inborn errors.

Carnitine palmitoyltransferase I deficiency in neonate identified by dried blood spot free carnitine and acylcarnitine profile

A neonate at risk for hepatic carnitine palmitoyltransferase I (L-CPT I) deficiency was investigated from birth. The free carnitine and acylcarnitine profile in dried whole blood filter paper samples collected at ages 1 and 4 days showed a markedly elevated concentration of free carnitine (141 and 142 micromol/L, respectively), normal concentrations of acetyl- and propionylcarnitine, with the near absence of all other species. The diagnosis was confirmed by in vitro fatty acid oxidation screening assays and enzyme assay in cultured skin fibroblasts. Retrospective study of the newborn whole blood sample of the index case showed a similar profile (free carnitine 181 micromol/L). The newborn population distribution of free carnitine (n = 143,981) showed that only three samples had free carnitine > 140 micromol/L (>99.9th centile), two were from L-CPT I-deficient neonates and one from a baby with sepsis. While there are other conditions that can cause elevated concentrations of free carnitine, an isolated elevation of free carnitine only in an apparently healthy term neonate warrants further investigation to exclude L-CPT I deficiency.

Evidence for a short-chain carnitine-acylcarnitine translocase in mitochondria specifically related to the metabolism of branched-chain amino acids

Carnitine-acylcarnitine translocase (CATR) deficiency is a severe defect in fatty acid oxidation which presents early in life most frequently with hypoglycemia, hyperammonemia, and severe cardiac abnormalities. CATR exchanges acylcarnitines of various chain lengths for free carnitine across the mitochondrial membrane. In vitro studies in intact fibroblasts from patients with documented deficiency of CATR were probed with stable-isotope-labeled precursors and the resulting acylcarnitines were analyzed by tandem mass spectrometry. After a 72-h incubation with l-[(2)H(3)]carnitine the translocase-deficient cells produced acylcarnitines in which the deuterium was incorporated into short-chain acylcarnitines, C2-C5. Experiments with simultaneous incubation of l-[(2)H(3)]carnitine and l-[(13)C(6)]isoleucine produced [(13)C(5)]2-methylbutyryl-[(2)H(3)]carnitine and [(13)C(3)]propionyl-[(2)H(3)]carnitine indicating exchange of labeled acylcarnitine from inside the mitochondrial matrix with labeled free carnitine. These studies support the possible existence of a "branched-chain" carnitine-acylcarnitine translocator in mitochondria.