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

Establishment of Age Specific Reference Interval for Aminoacids and Acylcarnitine in Dried Blood Spot by Tandem Mass Spectrometry

The Extended Screening for Inborn Errors of Metabolism is done for aminoacidopathies, fatty acid oxidation disorders and organic acid disorders. In a single dried blood spot, the tandem mass spectrometry is capable of measuring multiple analytes like amino acids, acylcarnitines, nucleosides, succinylacetone and lysophosphatidylcholines. This study was proposed to establish age specific reference internal for aminoacids and acylcartinitine in dried blood spot by tandem mass spectrometry. A total of 480 apparently healthy children were enrolled for the study and sub classified into four groups as follows: Group A: 0-1 month, Group B: 1 month-1 year, Group C: 1-5 year and Group D: 5-12 years each having 120 participants. Sample size were calculated as per CLSI approved guidelines. Tables 1 and 2 presents the age-specific percentile distribution of aminoacids and acylcarnitines established from healthy subjects as per rank-based method recommended by the IFCC and CLSI. Tables 3, 4 and 5 presents the cut-off values of primary and secondary marker/ratios for screening of aminoacidopathies, fatty acid oxidation disorders and organic acid disorders respectively. As a general principle, the interpretation of extended newborn screening results should be based on age specific cut-off established by the laboratory for primary analyte concentration and secondary analyte concentration/ ratios. This study was useful in establishing age specific cut-off values for various amino acids and acylcarnitines in South Indian population. [Table: see text] [Table: see text] [Table: see text] [Table: see text] [Table: see text].

Neonatal screening for isovaleric aciduria: Reducing the increasingly high false-positive rate in Germany

Newborn screening (NBS) for isovaleric acidemia (IVA) is performed by flow injection tandem mass spectrometry quantifying C5 carnitines (C5). Isovalerylcarnitine, however, is isomeric with pivaloylcarnitine which can be present in blood due to maternal use of pivaloylester-containing antibiotics, available in Germany since late 2016. During a 36-month period (January 19-December 21), all newborns screened in Hamburg with a C5 above cutoff (NeoGram®: 0.50 μmol/L or Neobase®2: 0.45 μmol/L) were included in the study. As a second-tier test, a simple ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was developed to differentiate the C5 isomers pivaloyl-, 2-methylbutyryl-, isovaleryl-, and valerylcarnitine. Out of 156 772 newborns tested, one turned out to have genetically proven IVA while 99 were false positive (C5: 0.5-8.2 μmol/L) due to the presence of pivaloylcarnitine. These cases have increased year by year and show local clusters. Retrospective analysis of another 39 cases from 287 206 neonates tested at the NBS center in Heidelberg with C5 elevation (0.9-10.6 μmol/L) but clinical and biochemical exclusion of IVA yielded evidence of pivaloylcarnitine in all cases. Inclusion of a second-tier test into NBS significantly reduces the high and increasing false-positive rate of IVA screening. This avoids further diagnostic steps, prevents unnecessary stress and anxiety of parents in a remarkably high number of cases. If Hamburg data of 2021 are extrapolated to all of Germany, one can assume around 800 (1‰) false-positive cases in comparison to an average of two classic IVA cases per year. Unless licensing of pivaloylester-containing drugs for use during pregnancy is reconsidered, a second-tier test for C5 determination is indispensable.

Combining data acquisition modes in liquid-chromatography-tandem mass spectrometry for comprehensive determination of acylcarnitines in human serum

Acylcarnitines (ACs) are metabolites involved in fatty acid β-oxidation and organic acid metabolism. Metabolic disorders associated to these two processes can be evaluated by determining the complete profile of ACs. In this research, we present an overall strategy for identification, confirmation, and quantitative determination of acylcarnitines in human serum. By this strategy we identified the presence of 47 ACs from C2 to C24 with detection of the unsaturation degree by application of a data-independent acquisition (DIA) liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. Complementary, quantitative determination of ACs is based on a high-throughput and fully automated method consisting of solid-phase extraction on-line coupled to LC-MS/MS in data-dependent acquisition (DDA) to improve analytical features avoiding the errors associated to sample processing. Quantitation limits were at pg mL^-1 level, the intra-day and between-day variability were below 15-20%, respectively; and the accuracy, expressed as bias, was always within ± 25%. The proposed method was tested with 40 human volunteers to determine the relative concentration of ACs in serum and identify predominant forms. Significant differences were detected by comparing the ACs profile of obese versus non-obese individuals.

A Simple Flow Injection Analysis-Tandem Mass Spectrometry Method to Reduce False Positives of C5-Acylcarnitines Due to Pivaloylcarnitine Using Reference Ions

Flow injection analysis−tandem mass spectrometry (FIA-TMS) has been applied in a first-tier test of newborn screening (NBS). Although isovalerylcarnitine (i-C5), which is a diagnostic indicator of isovaleric acidemia (IVA), is isobaric with pivaloylcarnitine (p-C5), 2-methylbutyrylcarnitine, and n-valerylcarnitine, these isomers cannot be distinguished by the FIA-TMS. There are many reports of false positives derived from p-C5 due to the use of pivalate-conjugated antibiotics. In this study, we developed a new FIA-TMS method to distinguish i-C5 and p-C5. We found that the intensity ratio of product ions for i-C5 and p-C5 was different in a certain range even under the same analytical conditions. The product ions with the most distinct differences in ionic intensity between the isomers and the collision energies that produce them were determined to be m/z 246.2 > 187.1 and −15 V, respectively. In addition to the quantification ion, a reference ion was defined, and the similarity of the i-C5 and p-C5 reference ion ratios (i-C5 score and p-C5 score, respectively) were used to estimate which isomer (i-C5 and p-C5) was responsible for elevated C5 acylcarnitine in dried blood spots (DBSs). As a result of analyses of 11 DBS samples derived from pivalate-conjugated antibiotics and four DBS samples from IVA patients using our method, it was found that our method was able to correctly determine the type of C5-acylcarnitine (i-C5 or p-C5) in the DBS samples. Implementation of this new FIA-TMS method into the current NBS protocol will allow for a reduction in false positives in IVA.

Mode of delivery was associated with transient changes in the metabolomic profile of neonates

AIMS

To estimate potential differences in neonatal metabolomic profiles at birth and at the time of newborn screening by delivery mode.

METHODS

A prospective study at Women's Clinic at Landspitali-The National University Hospital of Iceland. Women having normal vaginal birth or elective caesarean section from November 2013 to April 2014 were offered participation. Blood samples from mothers before birth and umbilical cord at birth were collected and amino acids and acylcarnitines measured by tandem mass spectrometry. Results from the Newborn screening programme in Iceland were collected. Amino acids and acylcarnitines from different samples were compared by delivery mode.

RESULTS

Eighty three normal vaginal births and 32 elective caesarean sections were included. Mean differences at birth were higher for numerous amino acids, and some acylcarnitines in neonates born vaginally compared to elective caesarean section. Maternal blood samples and newborn screening results showed small differences that lost significance after correction for multiple testing. Many amino acids and some acylcarnitines were numerically higher in cord blood compared to maternal. Many amino acids and most acylcarnitines were numerically higher in newborn screening results compared to cord blood.

CONCLUSION

We observed transient yet distinct differences in metabolomic profiles between neonates by delivery mode.

Laboratory analysis of acylcarnitines, 2020 update: a technical standard of the American College of Medical Genetics and Genomics (ACMG)

Acylcarnitine analysis is a useful test for identifying patients with inborn errors of mitochondrial fatty acid β-oxidation and certain organic acidemias. Plasma is routinely used in the diagnostic workup of symptomatic patients. Urine analysis of targeted acylcarnitine species may be helpful in the diagnosis of glutaric acidemia type I and other disorders in which polar acylcarnitine species accumulate. For newborn screening applications, dried blood spot acylcarnitine analysis can be performed as a multiplex assay with other analytes, including amino acids, succinylacetone, guanidinoacetate, creatine, and lysophosphatidylcholines. Tandem mass spectrometric methodology, established more than 30 years ago, remains a valid approach for acylcarnitine analysis. The method involves flow-injection analysis of esterified or underivatized acylcarnitines species and detection using a precursor-ion scan. Alternative methods utilize liquid chromatographic separation of isomeric and isobaric species and/or detection by selected reaction monitoring. These technical standards were developed as a resource for diagnostic laboratory practices in acylcarnitine analysis, interpretation, and reporting.

Mass Spectral Library of Acylcarnitines Derived from Human Urine

We describe the creation of a mass spectral library of acylcarnitines and conjugated acylcarnitines from the LC-MS/MS analysis of six NIST urine reference materials. To recognize acylcarnitines, we conducted in-depth analyses of fragmentation patterns of acylcarnitines and developed a set of rules, derived from spectra in the NIST17 Tandem MS Library and those identified in urine, using the newly developed hybrid search method. Acylcarnitine tandem spectra were annotated with fragments from carnitine and acyl moieties as well as neutral loss peaks from precursors. Consensus spectra were derived from spectra having similar retention time, fragmentation pattern, and the same precursor m/z and collision energy. The library contains 157 different precursor masses, 586 unique acylcarnitines, and 4 332 acylcarnitine consensus spectra. Furthermore, from spectra that partially satisfied the fragmentation rules of acylcarnitines, we identified 125 conjugated acylcarnitines represented by 987 consensus spectra, which appear to originate from Phase II biotransformation reactions. To our knowledge, this is the first report of conjugated acylcarnitines. The mass spectra provided by this work may be useful for clinical screening of acylcarnitines as well as for studying relationships among fragmentation patterns, collision energies, structures, and retention times of acylcarnitines. Further, these methods are extensible to other classes of metabolites.

Carnitine Precursors and Short-Chain Acylcarnitines in Water Buffalo Milk

Ruminants' milk contains δ-valerobetaine originating from rumen through the transformation of dietary N^ε-trimethyllysine. Among ruminant's milk, the occurrence of δ-valerobetaine, along with carnitine precursors and metabolites, has not been investigated in buffalo milk, the second most worldwide consumed milk, well-known for its nutritional value. HPLC-ESI-MS/MS analyses of bulk milk revealed that the Italian Mediterranean buffalo milk contains δ-valerobetaine at levels higher than those in bovine milk. Importantly, we detected also γ-butyrobetaine, the l-carnitine precursor, never described so far in any milk. Of interest, buffalo milk shows higher levels of acetylcarnitine, propionylcarnitine, butyrylcarnitine, isobutyrylcarnitine, and 3-methylbutyrylcarnitine (isovalerylcarnitine) than cow milk. Moreover, buffalo milk shows isobutyrylcarnitine and butyrylcarnitine at a 1-to-1 molar ratio, while in cow's milk this ratio is 5 to 1. Results indicate a peculiar short-chain acylcarnitine profile characterizing buffalo milk, widening the current knowledge about its composition and nutritional value.

Aspects of Newborn Screening in Isovaleric Acidemia

Isovaleric acidemia (IVA), an inborn error of leucine catabolism, is caused by mutations in the isovaleryl-CoA dehydrogenase (IVD) gene, resulting in the accumulation of derivatives of isovaleryl-CoA including isovaleryl (C5)-carnitine, the marker metabolite used for newborn screening (NBS). The inclusion of IVA in NBS programs in many countries has broadened knowledge of the variability of the condition, whereas prior to NBS, two distinct clinical phenotypes were known, an "acute neonatal" and a "chronic intermittent" form. An additional biochemically mild and potentially asymptomatic form of IVA and its association with a common missense mutation, c.932C>T (p.A282V), was discovered in subjects identified through NBS. Deficiency of short/branched chain specific acyl-CoA dehydrogenase (2-methylbutyryl-CoA dehydrogenase), a defect of isoleucine degradation whose clinical significance remains unclear, also results in elevated C5-carnitine, and may therefore be detected by NBS for IVA. Treatment strategies for the long-term management of symptomatic IVA comprise the prevention of catabolism, dietary restriction of natural protein or leucine intake, and supplementation with l-carnitine and/or l-glycine. Recommendations on how to counsel and manage individuals with the mild phenotype detected by NBS are required.

Raising Awareness of False Positive Newborn Screening Results Arising from Pivalate-Containing Creams and Antibiotics in Europe When Screening for Isovaleric Acidaemia

While the early and asymptomatic recognition of treatable conditions offered by newborn screening confers clear health benefits for the affected child, the clinical referral of patients with screen positive results can cause significant harm for some families. The use of pivalate-containing antibiotics and more recently the inclusion of neopentanoate as a component within moisturising creams used as nipple balms by nursing mothers can result in a significant number of false positive results when screening for isovaleric acidaemia (IVA) by measuring C5 acylcarnitine. A recent survey conducted within centres from nine countries indicated that this form of contamination had been or was a significant confounding factor in the detection of IVA in seven of the nine who responded. In three of these seven the prominent cause was believed to derive from the use of moisturising creams and in another three from antibiotics containing pivalate; one country reported that the cause was mixed. As a result, four of these seven centres routinely perform second tier testing to resolve C5 isobars when an initial C5 result is elevated, and a fifth is considering making this change within their national programme. The use of creams containing neopentanoate by nursing mothers and evolving patterns in the prescription of pivalate-containing antibiotics during pregnancy require those involved in the design and operation of newborn screening programmes used to detect IVA and the doctors who receive clinical referrals from these programmes to maintain an awareness of the potential impact of this form of interference on patient results.

Selective and accurate C5 acylcarnitine quantitation by UHPLC-MS/MS: Distinguishing true isovaleric acidemia from pivalate derived interference

Tandem MS acylcarnitine "profiles" are extremely valuable. Although used appropriately in newborn screening programs to identify patients with possible diseases, their inadequate quantitative accuracy and lack of selectivity is problematic for confirmatory testing. In this report, we show the application of our validated, selective, accurate, precise, and robust UHPLC-MS/MS method for quantitation of acylcarnitines, specifically to C5 acylcarnitines: pivaloyl-, 2-methylbutyryl-, isovaleryl-, and valerylcarnitine. Standardized calibrants were used to generate 13-point, 200-fold concentration range calibration curves. Samples were isolated by solid-phase extraction and derivatized with pentafluorophenacyl trifluoromethanesulfonate. Acylcarnitine pentafluorophenacyl esters were eluted in 14min chromatograms. Data demonstrating quantitative stability and method robustness over a five year time period are shown and these results validate the method's accuracy and robustness. Urine from patients with isovaleric acidemia (with the disease marker isovalerylcarnitine) and with pivaloylcarnitine present are shown. These results demonstrate the method's ability to distinguish true isovaleric acidemia from pivalate derived interference. Our method for acylcarnitine quantitation is shown to be accurate, precise, and robust for selective quantitation of isovalerylcarnitine, and thus is recommended for confirmatory testing of suspected isovaleric acidemia patients.

Introduction of a Simple Second Tier Screening Test for C5 Isobars in Dried Blood Spots: Reducing the False Positive Rate for Isovaleric Acidaemia in Expanded Newborn Screening

In 2015 the English Newborn Screening programme expanded to include Isovaleric Acidaemia (IVA). Screening is performed by flow injection analysis tandem mass spectrometry of isovalerylcarnitine. Isovalerylcarnitine is isobaric with pivaloylcarnitine which can be present in blood due to the use of pivalic ester pro-drugs or pivalic acid derivatives used as emollients in some nipple creams; the potential for false positives (FP) is well documented. A pilot study in England screened 438,164 babies, 18 had presumptive positive results but only 4 were confirmed as true positives (TP). We developed a simple test to separate the isobaric compounds and investigate these samples further.We studied newborn screening blood spots from 122 randomised controls and 34 infants with an initial raised C5 result. Dried blood spots were eluted with 30% acetonitrile (150 μL) and injected into a Waters Acquity UPLC coupled to a Waters Premier XE tandem mass spectrometer operating in positive ion mode. Isocratic separation of isovalerylcarnitine, pivaloylcarnitine, valerylcarnitine and 2-methylbutyrylcarnitine was achieved within 8 min. Assay performance characteristics were acceptable and non-parametric reference ranges (n = 122) were determined for each analyte.If this method had been used as a second tier test for the 34 presumptive positive samples, the number of FP's would have reduced from 24 to 8 and the positive predictive value of the screening test would have increased from 29 to 56%. Introduction of this test into the screening protocol has the potential to significantly reduce FP results for IVA and prevent unnecessary anxiety.

Advances in Clinical Mass Spectrometry

Although mass spectrometry has been used clinically for decades, the advent of immunoassay technology moved the clinical laboratory to more labor saving automated platforms requiring little if any sample preparation. It became clear, however, that immunoassays lacked sufficient sensitivity and specificity necessary for measurement of certain analytes or for measurement of analytes in specific patient populations. This limitation prompted clinical laboratories to revisit mass spectrometry which could additionally be used to develop assays for which there was no commercial source. In this chapter, the clinical applications of mass spectrometry in therapeutic drug monitoring, toxicology, and steroid hormone analysis will be reviewed. Technologic advances and new clinical applications will also be discussed.

The use of mass spectrometry to analyze dried blood spots

Dried blood spots (DBS) typically consist in the deposition of small volumes of capillary blood onto dedicated paper cards. Comparatively to whole blood or plasma samples, their benefits rely in the fact that sample collection is easier and that logistic aspects related to sample storage and shipment can be relatively limited, respectively, without the need of a refrigerator or dry ice. Originally, this approach has been developed in the sixties to support the analysis of phenylalanine for the detection of phenylketonuria in newborns using bacterial inhibition test. In the nineties tandem mass spectrometry was established as the detection technique for phenylalanine and tyrosine. DBS became rapidly recognized for their clinical value: they were widely implemented in pediatric settings with mass spectrometric detection, and were closely associated to the debut of newborn screening (NBS) programs, as a part of public health policies. Since then, sample collection on paper cards has been explored with various analytical techniques in other areas more or less successfully regarding large-scale applications. Moreover, in the last 5 years a regain of interest for DBS was observed and originated from the bioanalytical community to support drug development (e.g., PK studies) or therapeutic drug monitoring mainly. Those recent applications were essentially driven by improved sensitivity of triple quadrupole mass spectrometers. This review presents an overall view of all instrumental and methodological developments for DBS analysis with mass spectrometric detection, with and without separation techniques. A general introduction to DBS will describe their advantages and historical aspects of their emergence. A second section will focus on blood collection, with a strong emphasis on specific parameters that can impact quantitative analysis, including chromatographic effects, hematocrit effects, blood effects, and analyte stability. A third part of the review is dedicated to sample preparation and will consider off-line and on-line extractions; in particular, instrumental designs that have been developed so far for DBS extraction will be detailed. Flow injection analysis and applications will be discussed in section IV. The application of surface analysis mass spectrometry (DESI, paper spray, DART, APTDCI, MALDI, LDTD-APCI, and ICP) to DBS is described in section V, while applications based on separation techniques (e.g., liquid or gas chromatography) are presented in section VI. To conclude this review, the current status of DBS analysis is summarized, and future perspectives are provided.

Quantitative acylcarnitine determination by UHPLC-MS/MS--Going beyond tandem MS acylcarnitine "profiles"

Tandem MS "profiling" of acylcarnitines and amino acids was conceived as a first-tier screening method, and its application to expanded newborn screening has been enormously successful. However, unlike amino acid screening (which uses amino acid analysis as its second-tier validation of screening results), acylcarnitine "profiling" also assumed the role of second-tier validation, due to the lack of a generally accepted second-tier acylcarnitine determination method. In this report, we present results from the application of our validated UHPLC-MS/MS second-tier method for the quantification of total carnitine, free carnitine, butyrobetaine, and acylcarnitines to patient samples with known diagnoses: malonic acidemia, short-chain acyl-CoA dehydrogenase deficiency (SCADD) or isobutyryl-CoA dehydrogenase deficiency (IBD), 3-methyl-crotonyl carboxylase deficiency (3-MCC) or ß-ketothiolase deficiency (BKT), and methylmalonic acidemia (MMA). We demonstrate the assay's ability to separate constitutional isomers and diastereomeric acylcarnitines and generate values with a high level of accuracy and precision. These capabilities are unavailable when using tandem MS "profiles". We also show examples of research interest, where separation of acylcarnitine species and accurate and precise acylcarnitine quantification is necessary.

Elevation of pivaloylcarnitine by sivelestat sodium in two children

BACKGROUND

Sivelestat sodium (sivelestat), a neutrophil elastase inhibitor, is used to treat acute respiratory distress syndrome (ARDS). We report two cases that developed elevated C5-acylcarnitine (C5-AC) levels following treatment with sivelestat. Case 1 was a 14-day-old female infant born at 25 weeks and 1 day of gestation who was treated with sivelestat for the prophylaxis of Wilson-Mikity syndrome soon after birth. Isovaleric acidemia (IVA) was suspected based on a newborn screening using tandem mass spectrometry (MS/MS). Her C5-AC level was elevated to 4.49 μM (cut-off, <1.0) after treatment with sivelestat. Case 2 was a 4-year-old female with pneumocystis pneumonia that developed during chemotherapy for disseminated medulloblastoma. Sivelestat was given for the complication of ARDS. Her C5-AC level increased (1.09 μM) after eight days of treatment with sivelestat.

RESULTS

In both cases, IVA was ruled out because isovalerylglycine was not observed in the urinary organic acid analysis. Case 1 was associated with carnitine deficiency (C0 9.16 μM; reference value, 10-60). Liquid chromatography-MS/MS confirmed elevated pivaloylcarnitine (PVC) in both cases.

DISCUSSION

Similar to antibiotics containing pivalic acid (PVA), sivelestat contains PVA, which has the potential to cause secondary carnitine deficiency. In addition, elevated PVC can lead to false positive findings of IVA in newborns screened using MS/MS.

Screening newborns for metabolic disorders based on targeted metabolomics using tandem mass spectrometry

The main purpose of newborn screening is to diagnose genetic, metabolic, and other inherited disorders, at their earliest to start treatment before the clinical manifestations become evident. Understanding and tracing the biochemical data obtained from tandem mass spectrometry is vital for early diagnosis of metabolic diseases associated with such disorders. Accordingly, it is important to focus on the entire diagnostic process, including differential and confirmatory diagnostic options, and the major factors that influence the results of biochemical analysis. Compared to regular biochemical testing, this is a complex process carried out by a medical physician specialist. It is comprised of an integrated program requiring multidisciplinary approach such as, pediatric specialist, expert scientist, clinical laboratory technician, and nutritionist. Tandem mass spectrometry is a powerful tool to improve screening of newborns for diverse metabolic diseases. It is likely to be used to analyze other treatable disorders or significantly improve existing newborn tests to allow broad scale and precise testing. This new era of various screening programs, new treatments, and the availability of detection technology will prove to be beneficial for the future generations.

Validated method for the quantification of free and total carnitine, butyrobetaine, and acylcarnitines in biological samples

A validated quantitative method for the determination of free and total carnitine, butyrobetaine, and acylcarnitines is presented. The versatile method has four components: (1) isolation using strong cation-exchange solid-phase extraction, (2) derivatization with pentafluorophenacyl trifluoromethanesulfonate, (3) sequential ion-exchange/reversed-phase (ultra) high-performance liquid chromatography [(U)HPLC] using a strong cation-exchange trap in series with a fused-core HPLC column, and (4) detection with electrospray ionization multiple reaction monitoring (MRM) mass spectrometry (MS). Standardized carnitine along with 65 synthesized, standardized acylcarnitines (including short-chain, medium-chain, long-chain, dicarboxylic, hydroxylated, and unsaturated acyl moieties) were used to construct multiple-point calibration curves, resulting in accurate and precise quantification. Separation of the 65 acylcarnitines was accomplished in a single chromatogram in as little as 14 min. Validation studies were performed showing a high level of accuracy, precision, and reproducibility. The method provides capabilities unavailable by tandem MS procedures, making it an ideal approach for confirmation of newborn screening results and for clinical and basic research projects, including treatment protocol studies, acylcarnitine biomarker studies, and metabolite studies using plasma, urine, tissue, or other sample matrixes.

An LC-MS/MS method to quantify acylcarnitine species including isomeric and odd-numbered forms in plasma and tissues

Acylcarnitines are intermediates of fatty acid and amino acid oxidation found in tissues and body fluids. They are important diagnostic markers for inherited diseases of peroxisomal and mitochondrial oxidation processes and were recently described as biomarkers of complex diseases like the metabolic syndrome. Quantification of acylcarnitine species can become challenging because various species occur as isomers and/or have very low concentrations. Here we describe a new LC-MS/MS method for quantification of 56 acylcarnitine species with acyl-chain lengths from C2 to C18. Our method includes amino acid-derived positional isomers, like methacrylyl-carnitine (2-M-C3:1-CN) and crotonyl-carnitine (C4:1-CN), and odd-numbered carbon species, like pentadecanoyl-carnitine (C15:0-CN) and heptadecanoyl-carnitine (C17:0-CN), occurring at very low concentrations in plasma and tissues. Method validation in plasma and liver samples showed high sensitivity and excellent accuracy and precision. In an application to samples from streptozotocin-treated diabetic mice, we identified significantly increased concentrations of acylcarnitines derived from branched-chain amino acid degradation and of odd-numbered straight-chain species, recently proposed as potential biomarkers for the metabolic syndrome. In conclusion, the LC-MS/MS method presented here allows robust quantification of isomeric acylcarnitine species and extends the palette of acylcarnitines with diagnostic potential derived from fatty acid and amino acid metabolism.

Clinical and genetical heterogeneity of late-onset multiple acyl-coenzyme A dehydrogenase deficiency

BACKGROUND

Multiple acyl-CoA dehydrogenase deficiency (MADD) is an autosomal recessive disorder caused by deficiency of electron transfer flavoprotein or electron transfer flavoprotein dehydrogenase. The clinical picture of late-onset forms is highly variable with symptoms ranging from acute metabolic decompensations to chronic, mainly muscular problems or even asymptomatic cases.

METHODS

All 350 cases of late-onset MADD reported in the literature to date have been analyzed and evaluated with respect to age at presentation, diagnostic delay, biochemical features and diagnostic parameters as well as response to treatment.

RESULTS

Mean age at onset was 19.2 years. The mean delay between onset of symptoms and diagnosis was 3.9 years. Chronic muscular symptoms were more than twice as common as acute metabolic decompensations (85% versus 33% of patients, respectively). 20% had both acute and chronic symptoms. 5% of patients had died at a mean age of 5.8 years, while 3% of patients have remained asymptomatic until a maximum age of 14 years. Diagnosis may be difficult as a relevant number of patients do not display typical biochemical patterns of urine organic acids and blood acylcarnitines during times of wellbeing. The vast majority of patients carry mutations in the ETFDH gene (93%), while mutations in the ETFA (5%) and ETFB (2%) genes are the exceptions. Almost all patients with late-onset MADD (98%) are clearly responsive to riboflavin.

CONCLUSIONS

Late-onset MADD is probably an underdiagnosed disease and should be considered in all patients with acute or chronic muscular symptoms or acute metabolic decompensation with hypoglycemia, acidosis, encephalopathy and hepatopathy. This may not only prevent patients from invasive diagnostic procedures such as muscle biopsies, but also help to avoid fatal metabolic decompensations.

Combination of UHPLC/Q-TOF-MS, NMR spectroscopy, and ECD calculation for screening and identification of reactive metabolites of gentiopicroside in humans

The metabolic investigation of natural products is a great challenge because of unpredictable metabolic pathways, little knowledge on metabolic effects, and lack of recommended analytical methodology. Herein, a combined strategy based on ultrahigh-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UHPLC/Q-TOF-MS), nuclear magnetic resonance (NMR) spectroscopy, and electronic circular dichroism (ECD) calculation was developed and employed for the human metabolism study of gentiopicroside (GPS), a naturally hepato-protective iridoid glycoside. The whole metabolic study consisted of three major procedures. First, an improved UHPLC/Q-TOF-MS method was used to separate and detect a total of 15 GPS metabolites that were obtained from urine samples (0 to 72 h) of 12 healthy male participants after a single 50-mg oral dose of GPS. Second, a developed "MS-NMR-MS" method was applied to accurately identify molecular structures of the observed metabolites. Finally, given that the associated stereochemistry may be a crucial factor of the metabolic activation, the absolute configuration of the reactive metabolites was revealed through chemical calculations. Based on the combined use, a pair of diastereoisomers (G05 and G06) were experimentally addressed as the bioreactive metabolites of GPS, and the stereochemical determination was completed. Whereas several novel metabolic transformations, occurring via oxidation, N-heterocyclization and glucuronidation after deglycosylation, were also observed. The results indicated that GPS has to undergo in vivo metabolism-based activation to generate reactive molecules capable of processing its hepato-protective activity.

Surprising causes of C5-carnitine false positive results in newborn screening

During an 18-month period, we noticed an alarming increase of newborn screening false positivity rate in identifying isovaleric acidemia. In 50 of 50 newborns presenting elevated C5-carnitine, we confirmed the presence of pivaloylcarnitine. Exogenous pivalate administration had been previously identified as the causal agent of this concern. No pivalic-ester prodrug is commercially available in Belgium, but pivalic derivates are also used in the cosmetic industry as emollient under the term "neopentanoate". We have identified neopentanoate-esters in a nipple-fissure unguent that was provided to young mothers. Ceasing distribution of this product hugely reduced the C5-carnitine false positivity rate.

Application of a second-tier newborn screening assay for c5 isoforms

We report a case of false-positive metabolic screening for isovaleric acidemia in a newborn due to treatment of the mother with pivalic acid containing antibiotics before delivery. By using a recently established second-tier test based on the tandem-MS technique, we could identify pivalic acid in a dried blood sample taken during routine neonatal screening. Before this second-tier test was initiated, diverse analytical procedures were performed in the baby to rule out isovaleric acidemia and carnitine supplementation was started. This caused additional psychological burden to the family. The direct use of the second-tier test would have avoided these negative consequences of a false-positive screening result.

Separation and identification of underivatized plasma acylcarnitine isomers using liquid chromatography-tandem mass spectrometry for the differential diagnosis of organic acidemias and fatty acid oxidation defects

A simple HPLC-MS/MS method has been established to separate and identify underivatized acylcarnitine isomers. Human plasma samples were deproteinized and concentrated. Acylcarnitines were separated on a reverse phase column and detected with triple quadrupole linear ion trap mass spectrometry. Deuterium-labeled internal standards were used for quantitation. To identify acylcarnitines without pure standards, information-dependent acquisition linking to enhanced product ion scan mode was used. 112 acylcarnitines, including stereoisomers, were found in samples of patients. Dicarboxylic acylcarnitines, such as methylmalonylcarnitine and glutarylcarnitine, were detected with high sensitivity. Three stereoisomers of (R,S)2-methyl-3-hydroxy butyrylcarnitine were detected in samples of patients with β-ketothiolase deficiency. Validation results revealed excellent precision and accuracy of the method. In general the within- and between-run coefficients of variation (CV%) were less than 15%, and recoveries were in the range of 92.7-117.5%. In addition, the reference intervals of acylcarnitines for children aged 3-day to13-year old were established. Using the new method and reference intervals, we have correctly diagnosed 49 patients with fatty acid oxidation defects or organic acidemias in 176 high-risk patients.

Measurement of free carnitine and acylcarnitines in plasma by HILIC-ESI-MS/MS without derivatization

Measurement of carnitine and acylcarnitines in plasma is important in diagnosis of fatty acid β-oxidation disorders and organic acidemia. The usual method uses flow injection tandem mass spectrometry (FIA-MS/MS), which has limitations. A rapid and more accurate method was developed to be used for high-risk screening and diagnosis. Carnitine and acylcarnitines were separated by hydrophilic interaction liquid chromatography (HILIC) without derivatization and detected with a QTRAP MS/MS System. Total analysis time was 9.0min. The imprecision of within- and between-run were less than 6% and 17%, respectively. Recoveries were in the range of 85-110% at three concentrations. Some acylcarnitine isomers could be separated, such as dicarboxylic and hydroxyl acylcarnitines. The method could also separate interferent to avoid false positive results. 216 normal samples and 116 patient samples were detected with the validated method, and 49 patients were identified with fatty acid oxidation disorders or organic acidemias.

Liquid chromatography-mass spectrometric determination of losartan and its active metabolite on dried blood spots

A simple and rapid quantitative bioanalytical liquid chromatography-tandem mass spectrometric (LC-MS/MS) method for simultaneous determination of losartan and its active metabolite, losartan carboxylic acid on rat dried blood spots was developed and validated as per regulatory guidelines. Losartan and its metabolite were extracted from dried blood spots using 50% aqueous methanol and separated on Waters XTerra(®) RP18 (250 mm × 4.6 mm, 5 μm) column using mobile phase composed of 40% acetonitrile and 60% aqueous ammonium acetate (10mM). The eluents were monitored using ESI tandem mass spectrometric detection with negative polarity in MRM mode using ion transitions m/z 421.2→179.0, m/z 435.3→157.0 and m/z 427.3→193.0 for losartan, losartan carboxylic acid and Irbesartan (internal standard), respectively. The method was validated over the linear range of 1-200 ng/mL and 5-1000 ng/mL with lower limits of quantification of 1.0 ng/mL and 5.0 ng/mL for losartan and losartan carboxylic acid, respectively. Inter and intra-day precision and accuracy (Bias) were below 5.96% and between -2.8 and 1.5%, respectively. The mean recoveries of the analytes from dried blood spots were between 89% and 97%. No significant carry over and matrix effects were observed. The stability of stock solution, whole blood, dried blood spot and processed samples were tested under different conditions and the results were found to be well within the acceptable limits. Additional validation parameters such as influence of hematocrit and spot volume were also evaluated and found to be well within the acceptable limits.

Newborn screening for isovaleric acidemia using tandem mass spectrometry: data from 1.6 million newborns

BACKGROUND

Electrospray ionization-tandem mass spectrometry (ESI-MS/MS) has been used in the Bavarian newborn screening (NBS) program since 1999. The use of ESI-MS/MS has led to the inclusion of isovaleric acidemia (IVA) into NBS. We retrospectively evaluated data on more than 1.6 million newborns screened during 9.5 years.

METHODS

Acylcarnitines from whole blood spotted on filter paper were converted to their corresponding butyl esters, and the samples were analyzed by use of ESI-MS/MS with stable isotope labeled internal standards.

RESULTS

A total of 24 individuals with IVA were detected by use of a multiparametric threshold criteria panel including isovalerylcarnitine (C5) and the ratios of C5 to octanoyl-, butyryl-, and propionylcarnitine. A cutoff set at the 99.99th percentile for isolated C5 or at the 99th percentile for C5 plus at least 2 ratios resulted in a positive predictive value for IVA screening of 7.0% and an overall recall rate of 0.024%. Adjusted reference ranges for age and birth weight were applied, and the incidence of IVA in the study population was calculated to be 1 in 67,000. Missed cases were not brought to our attention. IVA was also detectable in cord blood and early postnatal blood samples.

CONCLUSIONS

IVA can be reliably detected in NBS through acylcarnitine analysis in dried blood spots by using multiparametric threshold criteria. Further improvement (positive predictive value 13.0%, recall rate 0.01%) can be achieved by using more stringent recall criteria. In view of the potentially life-threatening natural course of IVA in early life, presymptomatic diagnosis may thus prevent mortality and morbidity.

Rapid determination of C4-acylcarnitine and C5-acylcarnitine isomers in plasma and dried blood spots by UPLC-MS/MS as a second tier test following flow-injection MS/MS acylcarnitine profile analysis

BACKGROUND

Flow-injection MS/MS methods for elevated acylcarnitines are routinely performed in most newborn screening and biochemical genetics laboratories; however this technique cannot distinguish between isobaric compounds; therefore, chromatographic separation is required to quantitate isomers for differential diagnosis of some inborn errors of metabolism.

METHODS

A UPLC-MS/MS method has been developed for the simultaneous quantitation of isobutyrylcarnitine and butyrylcarnitine, and a second UPLC-MS/MS method for the quantitation of isovalerylcarnitine, (S) and (R) 2-methylbutyrylcarnitine, pivaloylcarnitine and valerylcarnitine. Plasma and dried blood spots samples are extracted with methanol and derivatized with butanolic HCl. Deuterium labeled internal standards are used for quantitation. Separation is obtained using a methanol/water gradient with a C18 BEH, 1x100mm, 1.7microm UPLC column, at 60 degrees C; run time is less than 10min. The isomers are detected with a Quattro Premier triple quadrupole, with electrospray ionization in positive ion mode.

RESULTS

Intra-day precision in plasma and dried blood spots ranged from 1.4% to 14% and accuracy from 88% to 114% respectively for butyrylcarnitine and isobutyrylcarnitine. Precision for the isomers of C5-acylcarnitine ranged from 1.3% to 15% and accuracy 87% to 119%, respectively in plasma or dried blood spots. Inter-day precision was within 20% at each concentration of isobutyrylcarnitine and butyrylcarnitine. Precision for 2-methylbutyrylcarnitine and isovalerylcarnitine at concentrations above the normal range was within 24%.

CONCLUSIONS

Two diagnostic tests based on the separation of C4-acylcarnitine and C5-acylcarnitine isomers by UPLC-MS/MS provide fast differential diagnosis of SCAD deficiency versus IBCD deficiency and IVA versus 2-MBCD deficiency. The separation of C5-acylcarnitines can reveal false elevation due to pivalic acid-containing antibiotics. Abnormal newborn screen results due to pivalate-generating prodrug antibiotics of maternal origin were confirmed. This separation of isomers can resolve multiple diagnostic challenges in both newborn screening and in cases with ambiguous metabolic test results.

Biochemical characterization of L-carnitine dehydrogenases from Rhizobium sp. and Xanthomonas translucens

Recently, we obtained two L-carnitine dehydrogenases (CDHs) from soil isolates, Rhizobium sp. (Rs-CDH) and Xanthomonas translucens (Xt-CDH). The respective molecular masses of Rs-CDH and Xt-CDH were approximately 50 kDa and 37 kDa. In this study, the genes encoding both enzymes were cloned. Their primary structures exhibited high identities with those of 3-hydroxyacyl-CoA dehydrogenases. In addition, Rs-CDH had a 180-residue long extra sequence in its C-terminal region. Except for the initial 20 residues, the extra sequence exhibited similarity to thioesterase. The activity of Rs-CDH was affected only slightly by deletion of thioesterase domain, but it was eliminated by the deletion of the whole C-terminal extra sequence. A further deletion experiment indicated that the region of Ala330-Pro335 of Rs-CDH has important functions in catalytic activity. Moreover, based on the deletion experiment on Xt-CDH, the five-residue tail is considered to have a function similar to Ala330-Pro335 of Rs-CDH.

Integration of metabolomics and transcriptomics data to aid biomarker discovery in type 2 diabetes

Type 2 diabetes (T2D), one of the most common diseases in the western world, is characterized by insulin resistance and impaired beta-cell function but currently it is difficult to determine the precise pathophysiology in individual T2D patients. Non-targeted metabolomics technologies have the potential for providing novel biomarkers of disease and drug efficacy, and are increasingly being incorporated into biomarker exploration studies. Contextualization of metabolomics results is enhanced by integration of study data from other platforms, such as transcriptomics, thus linking known metabolites and genes to relevant biochemical pathways. In the current study, urinary NMR-based metabolomic and liver, adipose, and muscle transcriptomic results from the db/db diabetic mouse model are described. To assist with cross-platform integration, integrative pathway analysis was used. Sixty-six metabolites were identified in urine that discriminate between the diabetic db/db and control db/+ mice. The combined analysis of metabolite and gene expression changes revealed 24 distinct pathways that were altered in the diabetic model. Several of these pathways are related to expected diabetes-related changes including changes in lipid metabolism, gluconeogenesis, mitochondrial dysfunction and oxidative stress, as well as protein and amino acid metabolism. Novel findings were also observed, particularly related to the metabolism of branched chain amino acids (BCAAs), nicotinamide metabolites, and pantothenic acid. In particular, the observed decrease in urinary BCAA catabolites provides direct corroboration of previous reports that have inferred that elevated BCAAs in diabetic patients are caused, in part, by reduced catabolism. In summary, the integration of metabolomics and transcriptomics data via integrative pathway mapping has facilitated the identification and contextualization of biomarkers that, presuming further analytical and biological validation, may be useful in future T2D clinical studies by identifying patient populations that share common disease pathophysiology and therefore may identify those patients that may respond better to a particular class of anti-diabetic drugs.

Acylcarnitine analysis by tandem mass spectrometry

Carnitine plays an essential role in fatty acid metabolism, as well as modulation of intracellular concentrations of free coenzyme A by esterification of acyl residues. Acylcarnitine analysis of various biological fluids is a sensitive method to detect >20 inborn errors of metabolism that result in abnormal accumulation of acylcarnitine species due to several organic acidemias and most fatty acid beta-oxidation disorders. In addition, acylcarnitine analysis may aid in monitoring treatment of known patients affected with these inborn errors of metabolism. This unit describes protocols that can be used to measure acylcarnitine species of various carbon chain lengths in several biological specimen types including plasma, dried blood and bile spots, and urine, by derivatization to butylesters and flow-injection electrospray ionization tandem mass spectrometry (ESI-MS/MS).

International cooperation in the expansion of a newborn screening programme in Lebanon: a possible model for other programmes

Tandem mass spectrometry (MS/MS) is rapidly gaining support, even in less-developed nations, as the method of choice for the newborn screening of metabolic disorders, although difficulties in acquiring this technology may at times be major obstacles in several Middle East and North Africa (MENA) countries. In Lebanon, international cooperation allowed this acquisition at the Newborn Screening Laboratory (NSL) of the Saint Joseph University (USJ) in the capital city of Beirut. NSL is currently screening up to 20% of all newborns in Lebanon. The expansion was made possible through initial collaboration with the Metabolic Laboratory at the Hamburg University Medical Center (HUMC) and subsequently with other centres. During phase I of the expansion (2006-2007), blood spots were shipped to HUMC with rapid couriers twice a week and electronic reports were sent back generally within 4 days after shipment. Positive cases were recalled to NSL and new specimens were sent back for confirmation at HUMC. During that first phase, the Beirut staff received training at the HUMC and in other centres. Phase II was a transitory period of 4 months during which machines were installed in Beirut and working procedures were adopted and documented. The activity has now entered a consolidation phase (Phase III) in which all measurements are exclusively performed in Beirut while HUMC acts as a backup centre. International cooperation remains crucial for periodic quality assurance procedures, and for supporting the transformation of the USJ-NSL into a training centre able to transfer MS/MS technology to the MENA region.

Quantification of carnitine and acylcarnitines in biological matrices by HPLC electrospray ionization-mass spectrometry

BACKGROUND

Analysis of carnitine and acylcarnitines by tandem mass spectrometry (MS/MS) has limitations. First, preparation of butyl esters partially hydrolyzes acylcarnitines. Second, isobaric nonacylcarnitine compounds yield false-positive results in acylcarnitine tests. Third, acylcarnitine constitutional isomers cannot be distinguished.

METHODS

Carnitine and acylcarnitines were isolated by ion-exchange solid-phase extraction, derivatized with pentafluorophenacyl trifluoromethanesulfonate, separated by HPLC, and detected with an ion trap mass spectrometer. Carnitine was quantified with d(3)-carnitine as the internal standard. Acylcarnitines were quantified with 42 synthesized calibrators. The internal standards used were d(6)-acetyl-, d(3)-propionyl-, undecanoyl-, undecanedioyl-, and heptadecanoylcarnitine.

RESULTS

Example recoveries [mean (SD)] were 69.4% (3.9%) for total carnitine, 83.1% (5.9%) for free carnitine, 102.2% (9.8%) for acetylcarnitine, and 107.2% (8.9%) for palmitoylcarnitine. Example imprecision results [mean (SD)] within runs (n = 6) and between runs (n = 18) were, respectively: total carnitine, 58.0 (0.9) and 57.4 (1.7) micromol/L; free carnitine, 44.6 (1.5) and 44.3 (1.2) micromol/L; acetylcarnitine, 7.74 (0.51) and 7.85 (0.69) micromol/L; and palmitoylcarnitine, 0.12 (0.01) and 0.11 (0.02) micromol/L. Standard-addition slopes and linear regression coefficients were 1.00 and 0.9998, respectively, for total carnitine added to plasma, 0.99 and 0.9997 for free carnitine added to plasma, 1.04 and 0.9972 for octanoylcarnitine added to skeletal muscle, and 1.05 and 0.9913 for palmitoylcarnitine added to skeletal muscle. Reference intervals for plasma, urine, and skeletal muscle are provided.

CONCLUSIONS

This method for analysis of carnitine and acylcarnitines overcomes the observed limitations of MS/MS methods.

Acylcarnitine profile analysis

These Technical Standards and Guidelines were developed primarily as an educational resource for clinical laboratory geneticists to help them provide quality clinical laboratory genetic services. Adherence to these standards and guidelines is voluntary and does not necessarily assure a successful medical outcome. These Standards and Guidelines should not be considered inclusive of all proper procedures and tests or exclusive of other procedures and tests that are reasonably directed to obtaining the same results. In determining the propriety of any specific procedure or test, the clinical laboratory geneticist should apply his or her own professional judgment to the specific circumstances presented by the individual patient or specimen. Clinical laboratory geneticists are encouraged to document in the patient's record the rationale for the use of a particular procedure or test, whether or not it is in conformance with these Standards and Guidelines. They also are advised to take notice of the date any particular standard or guidelines was adopted, and to consider other relevant medical and scientific information that becomes available after that date.

Stable-isotope dilution measurement of isovalerylglycine by tandem mass spectrometry in newborn screening for isovaleric acidemia

BACKGROUND

Recent neonatal screening for isovaleric acidemia by tandem mass spectrometry based on dried blood-spot levels of C5-acylcarnitines, including isovalerylcarnitine and its isomer, pivaloylcarnitine, which is derived from pivalate-generating antibiotics, has caused many false-positive results. We have developed a method to overcome this interference.

METHODS

The amounts of isovalerylglycine were determined by a stable-isotope dilution electrospray tandem mass spectrometric analysis, using multiple-reaction monitoring with product ions of m/z 132, which were generated predominantly from quasi-molecular ions of isovalerylglycine butylester but apparently not from those of pivaloylglycine butylester.

RESULTS

Isovalerylglycine concentrations in dried blood spots of control newborns were 0.17+/-0.03 nmol/ml, and those of patients with isovaleric acidemia ranged from 1.3 to 80.0 nmol/ml. Those of the newborns treated with antibiotics, which caused high C5-acylcarnitine levels (1.9+/-1.7 nmol/ml) in dried blood spots, were 0.22+/-0.05 nmol/ml.

CONCLUSIONS

Our data showed that the present method is useful in eliminating the false-positive results due to antibiotics use in newborn screening for isovaleric acidemia.

Reduction of the false-positive rate in newborn screening by implementation of MS/MS-based second-tier tests: the Mayo Clinic experience (2004-2007)

The continued expansion of newborn screening programmes to include additional conditions increases the responsibility of newborn screening laboratories to provide testing with the highest sensitivity and specificity to allow for identification of affected patients while minimizing the false-positive rate. Some assays and analytes are particularly problematic. Over recent years, our laboratory tried to improve this situation by developing second-tier tests to reduce false-positive results in the screening for congenital adrenal hyperplasia (CAH), tyrosinaemia type I, methylmalonic acidaemias, homocystinuria, and maple syrup urine disease (MSUD). Beginning in 2004, this approach was applied to Mayo's newborn screening programme and resulted in a false-positive rate of 0.09%, a positive predictive value of 41%, and a positive detection rate of 1 affected case in 1672 babies screened.

ESI–MS/MS study of acylcarnitine profiles in urine from patients with organic acidemias and fatty acid oxidation disorders

Acylcarnitines in urine from 45 patients with organic acidemias and fatty acid oxidation disorders were evaluated using ESI-MS/MS. The urinary acylcarnitine profiles in organic acidemias, SCAD deficiency and MCAD deficiency were compatible with blood acylcarnitine profiles, and abnormalities in urinary acylcarnitine profiles in these conditions were enhanced following carnitine loading. Urinary acylcarnitine profiles were not helpful for characterization of long-chain fatty acid disorders, but a combination of urine and blood acylcarnitine analysis was useful for differential diagnosis of carnitine deficit.

A sensitive liquid chromatographic method for the analysis of isovaleric and valeric acids in urine as fluorescent derivatives

A simple and sensitive isocratic liquid chromatographic method was developed for the analysis of isovaleric and valeric acids in human urine as biomarkers in metabolic acidosis. The method is based on the derivatization of isovaleric and valeric acids with a fluorescent reagent 2-(2-naphthoxy)ethyl-2-(piperidino)ethanesulfonate for labeling the analytes with the naphthoxy fluorophore. The resulting fluorescent derivatives of isovaleric and valeric acids were separated on a phenyl-hexyl column, using a mixed solvent of methanol-water-tetrahydrofuran (55:31:14, v/v) as the mobile phase. The separated derivatives were monitored with a fluorimetric detector (excitation at 225 nm and emission at 360 nm). The linear range of the method for the determination of isovaleric acid or valeric acid derivative was over 0.2 approximately 8.0 microM. The detection limit (signal to noise ratio=3 with 10 microl injected) of isovaleric acid or valeric acid was about 0.04 microM. Application of the method to the analysis of isovaleric acid in the urine of a patient with isovaleric acidemia proved feasible.

Genetic mutation profile of isovaleric acidemia patients in Taiwan

Isovaleric acidemia (IVA), a rare recessive autosomal disorder, is caused by isovaleryl-CoA dehydrogenase (IVD) deficiency. IVA may present with symptoms during the acute stage of severe metabolic acidosis, ketosis, vomiting, and altered mental status. With the help of newborn screening (NBS) by tandem mass spectrometry (MS/MS), IVA can now be diagnosed presymptomatically. According to statistic data, the incidence of IVA in Taiwan was about 1/365,000. In this study, six IVA patients from five families were investigated and followed-up clinically. As for the timing, two patients were found before MS technique introduced to Taiwan, the others were identified after MS/MS applied to NBS. The blood level of C5-carnitine in our patients was 7.43-18.96 microM (with upper limit in our laboratory <0.51 microM) and all of their urines contained raised amounts of 3-hydroxyisovaleric acid and isovalerylglycine. Molecular analysis of their IVD gene revealed six mutation profiles, among which the 149G-->A (Arg21His) and 1174 C-->T (Arg363Cys) mutations have been reported previously, while the other four mutations, 386A-->G (His100Arg), 347C-->T (Ser87Phe), 1007G-->A (Cys307Tyr) and 1199A-->G (Tyr371Cys), were first reported. Specially, we found 1199A-->G (Tyr371Cys) mutated was a common recurring missense mutation in our population (4 in 10 mutant alleles).

Isovaleric acidemia: new aspects of genetic and phenotypic heterogeneity

Isovaleric acidemia (IVA) is an autosomal recessive inborn error of leucine metabolism caused by a deficiency of the mitochondrial enzyme isovaleryl-CoA dehydrogenase (IVD) resulting in the accumulation of derivatives of isovaleryl-CoA. It was the first organic acidemia recognized in humans and can cause significant morbidity and mortality. Early diagnosis and treatment with a protein restricted diet and supplementation with carnitine and glycine are effective in promoting normal development in severely affected individuals. Both intra- and interfamilial variability have been recognized. Initially, two phenotypes with either an acute neonatal or a chronic intermittent presentation were described. More recently, a third group of individuals with mild biochemical abnormalities who can be asymptomatic have been identified through newborn screening of blood spots by tandem mass spectrometry. IVD is a flavoenzyme that catalyzes the conversion of isovaleryl-CoA to 3-methylcrotonyl-CoA and transfers electrons to the electron transfer flavoprotein. Human IVD has been purified from tissue and recombinant sources and its biochemical and physical properties have been extensively studied. Molecular analysis of the IVD gene from patients with IVA has allowed characterization of different types of mutations in this gene. One missense mutation, 932C>T (A282V), is particularly common in patients identified through newborn screening with mild metabolite elevations and who have remained asymptomatic to date. This mutation leads to a partially active enzyme with altered catalytic properties; however, its effects on clinical outcome and the necessity of therapy are still unknown. A better understanding of the heterogeneity of this disease and the relevance of genotype/phenotype correlations to clinical management of patients are among the challenges remaining in the study of this disorder in the coming years.

Expanded newborn screening of inherited metabolic disorders by tandem mass spectrometry: Clinical and laboratory aspects

Newborn screening started in the 1960s for the purpose of identifying phenylketonuric patients to begin early intervention and to prevent mental retardation in these patients. Soon thereafter, screening programs expanded to include additional genetic disorders added individually one at a time. In the 1980s, tandem mass spectrometry (MS/MS) was introduced in clinical laboratories, and in the 1990s, the technique was used for newborn screening. Unlike measuring one analyte at a time, MS/MS allows measurement of >40 analytes, in a few minutes with the use of a single assay. Currently, MS/MS is being used for the identification of several amino acid, organic acid and fatty acid disorders. Several states in the United States and many other countries are using MS/MS in newborn screening. However, there is a significant disparity among different newborn screening programs for disorders being screened by MS/MS and many other challenges are faced by the expanded newborn screening. It is anticipated that in the future the use of MS/MS in newborn screening will expand both at the analyte and geographic levels. Clinicians and laboratory scientists should become familiar with MS/MS, disorders being screened in their patients' population and the future of this emerging technology.

Determination of total and free plasma carnitine concentrations on the Dade Behring Dimension RxL: Integrated chemistry system

BACKGROUND

L-carnitine is a naturally occurring quaternary ammonium compound present in all mammalian species. Its major function is to facilitate the passage of long-chain fatty acids through the mitochondrial membrane for subsequent beta-oxidation and ketone synthesis. Clinical interest in carnitine disorders relates particularly to possible deficiency states that may result in a phenotypic spectrum that includes cardiomyopathy, skeletal myopathy, hypoglycemia and hyperammonemia. The objective of this study was to develop a method on the Dade Behring Dimension RxL analyzer for measuring free and total carnitine levels in plasma.

METHODS

Plasma samples were deproteinized by ultrafiltration to remove interference by endogenous thiols. Filtrates were measured directly on the RxL for free carnitine or after alkaline hydrolysis for total carnitine by an endpoint enzymatic assay that uses carnitine acetyltransferase.

RESULTS

Within-run imprecision was <5% at high and low levels for both free and total carnitine while between-day imprecision was <15%. Recovery of free carnitine from spiked plasma >90%. The method was linear between 5.0 and 150.0 micromol/l and the limit of quantification was 5.0 micromol/l. Comparison of our method with another automated procedure developed on the Hitachi 917 system using Deming regression analysis resulted in the following equations: Dimension=1.034(Hitachi)-7.44 for total carnitine (r=0.955) and Dimension=0.805(Hitachi)+1.96 for free carnitine (r=0.951), respectively.

CONCLUSIONS

Our method is suitable for analyzer platforms where the level of imprecision is lower and the throughput is higher than manual methods. It also avoids the use of radioisotopes and is appropriate in labs where access to reference methods such as tandem mass spectrometry and HPLC is limited or unavailable.

Rapid measurement of plasma acylcarnitines by liquid chromatography–tandem mass spectrometry without derivatization

BACKGROUND

Tandem mass spectrometry (MS/MS) is being increasingly used to identify and measure acylcarnitines in blood and urine of children suspected of having fatty oxidation disorders and other inborn errors of metabolism. Rapid MS/MS analysis requires simple and efficient sample preparation. We developed a LC-MS/MS method for the online extraction of acylcarnitines in plasma without derivatization that requires only precipitation of proteins by acetonitrile followed by centrifugation, thus increasing efficiency.

METHODS

An API-3000 tandem mass spectrometer (SCIEX, Toronto, Canada) equipped with electrospray ionization (ESI), TurboIon Spray source, three Shimadzu LC10AD micropumps and autosampler (Shimadzu Scientific Instruments, Columbia, MD) was used to perform the analysis. Within-day and between-day imprecision was evaluated for 10 analytes in the MRM mode using 3 levels of controls. Accuracy was determined by comparing the method with another MS/MS procedure and by recovery experiments. Sensitivity and specificity were evaluated by identifying patient samples under a wide variety of clinical conditions.

RESULTS

Within-day CVs was <10% for all analytes tested and between-day CVs ranged from 4.4% to 14.2%. The method was linear in the range between 1.0 and 100 micromol/l for C2 and 0.1 and 10 micromol/l for the other acylcarnitines. The results of the comparison study yielded r values ranging between 0.948 and 0.999. Recovery ranged from 84% to 112%. The method correctly identified patients with a variety of fatty acid oxidation disorders and organic acidemias.

CONCLUSIONS

Our method is a simple procedure for the analysis of acylcarnitines in plasma with minimal sample preparation. It is thus ideal in a routine clinical setting where efficient processing of clinical samples is necessary to reduce turnaround time under conditions of high-throughput.

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.