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

Acylglycine Analysis by Ultra-Performance Liquid Chromatography-Tandem Mass Spectrometry (UPLC-MS/MS)

Quantitative analysis of urine acylglycines has shown to be a highly sensitive and specific method with proven clinical utility for the diagnosis of several inherited metabolic disorders including: medium chain acyl-CoA dehydrogenase deficiency, multiple acyl-CoA dehydrogenase deficiency, short chain acyl-CoA dehydrogenase deficiency, 3-methylcrotonyl-CoA carboxylase deficiency, 2-methylbutyryl-CoA dehydrogenase deficiency, isovaleric acidemia, propionic academia, and isobutyryl-CoA dehydrogenase deficiency. Here, a method that is currently performed using ultra-performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS) is described. © 2023 Wiley Periodicals LLC. Basic Protocol: Urinary acylglycine analysis by UPLC-MS/MS Support Protocol 1: Quality control preparation Support Protocol 2: Internal standard (ISTD) preparation Support Protocol 3: Standard (STD)/calibrator preparation.

Acylcarnitines: Nomenclature, Biomarkers, Therapeutic Potential, Drug Targets, and Clinical Trials

Acylcarnitines are fatty acid metabolites that play important roles in many cellular energy metabolism pathways. They have historically been used as important diagnostic markers for inborn errors of fatty acid oxidation and are being intensively studied as markers of energy metabolism, deficits in mitochondrial and peroxisomal β -oxidation activity, insulin resistance, and physical activity. Acylcarnitines are increasingly being identified as important indicators in metabolic studies of many diseases, including metabolic disorders, cardiovascular diseases, diabetes, depression, neurologic disorders, and certain cancers. The US Food and Drug Administration-approved drug L-carnitine, along with short-chain acylcarnitines (acetylcarnitine and propionylcarnitine), is now widely used as a dietary supplement. In light of their growing importance, we have undertaken an extensive review of acylcarnitines and provided a detailed description of their identity, nomenclature, classification, biochemistry, pathophysiology, supplementary use, potential drug targets, and clinical trials. We also summarize these updates in the Human Metabolome Database, which now includes information on the structures, chemical formulae, chemical/spectral properties, descriptions, and pathways for 1240 acylcarnitines. This work lays a solid foundation for identifying, characterizing, and understanding acylcarnitines in human biosamples. We also discuss the emerging opportunities for using acylcarnitines as biomarkers and as dietary interventions or supplements for many wide-ranging indications. The opportunity to identify new drug targets involved in controlling acylcarnitine levels is also discussed. SIGNIFICANCE STATEMENT: This review provides a comprehensive overview of acylcarnitines, including their nomenclature, structure and biochemistry, and use as disease biomarkers and pharmaceutical agents. We present updated information contained in the Human Metabolome Database website as well as substantial mapping of the known biochemical pathways associated with acylcarnitines, thereby providing a strong foundation for further clarification of their physiological roles.

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.

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.

Newborn screening

Newborn screening is a major aspect of public health success. Babies in every state are tested for a recommended uniform screening panel of conditions not otherwise immediately evident in the first days of life. With the goal of reducing morbidity and mortality, conditions should be added to newborn screening panels using a scientific, evidence-based process. Newborn screening is a system involving partners at many levels; neonatologists have a special role in ensuring that their vulnerable patients also receive this life-saving test. Careful attention to the social, legal, and ethical aspects will help increase the scope of newborn screening.

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.

Quantitation of heat-shock proteins in clinical samples using mass spectrometry

Mass spectrometry (MS) is a powerful analytical tool for proteomics research and drug and biomarker discovery. MS enables identification and quantification of known and unknown compounds by revealing their structural and chemical properties. Proper sample preparation for MS-based analysis is a critical step in the proteomics workflow because the quality and reproducibility of sample extraction and preparation for downstream analysis significantly impact the separation and identification capabilities of mass spectrometers. The highly expressed proteins represent potential biomarkers that could aid in diagnosis, therapy, or drug development. Because the proteome is so complex, there is no one standard method for preparing protein samples for MS analysis. Protocols differ depending on the type of sample, source, experiment, and method of analysis. Molecular chaperones play significant roles in almost all biological functions due to their capacity for detecting intracellular denatured/unfolded proteins, initiating refolding or denaturation of such malfolded protein sequences and more recently for their role in the extracellular milieu as chaperokines. In this chapter, we describe the latest techniques for quantitating the expression of molecular chaperones in human clinical samples.

Analysis of plasma ghrelin in patients with medium-chain acyl-CoA dehydrogenase deficiency and glutaric aciduria type II

OBJECTIVE

Ghrelin requires a fatty acid modification for binding to the GH secretagogue receptor. Acylation of the Ser3 residue of ghrelin is essential for its biological activities. We hypothesized that acyl-CoA is the fatty acid substrate for ghrelin acylation. Because serum octanoyl-CoA levels are altered by fatty acid oxidation disorders, we examined circulating ghrelin levels in affected patients.

MATERIALS AND METHODS

Blood levels of acyl (A) and des-acyl (D) forms of ghrelin and acylcarnitine of patients with medium-chain acyl-CoA dehydrogenase (MCAD) deficiency and glutaric aciduria type II (GA2) were measured.

RESULTS

Plasma acyl ghrelin levels and A/D ratios increased in patients with MCAD deficiency or GA2 when compared with normal subjects. Reverse-phase HPLC confirmed that n-octanoylated ghrelin levels were elevated in these patients.

CONCLUSION

Changing serum medium-chain acylcarnitine levels may affect circulating acyl ghrelin levels, suggesting that acyl-CoA is the substrate for ghrelin acylation.

N-carbamylglutamate treatment for acute neonatal hyperammonemia in isovaleric acidemia

Hyperammonemia occurs mainly in patients with branched-chain organic acidemias such as propionic, methylmalonic, and isovaleric acidemias. Its pathophysiological process is mainly via the competitive inhibition of N-acetylglutamate synthetase. Oral carglumic acid (N-carbamylglutamate) administration can correct hyperammonemia in neonates with propionic and methylmalonic acidemias, thus avoiding dialysis therapy. Isovaleric acidemia is an autosomal recessive disease of leucine metabolism due to deficiency of isovaleryl-CoA dehydrogenase. For the first time, we report a neonate with isovaleric acidemia, whose plasma ammonia concentration dropped dramatically after one oral load of carglumic acid. This experience suggests that carglumic acid could be considered for acute hyperammonemia resulting from isovaleric acidemia. However, trials with more patients are needed.

A method for comprehensive analysis of urinary acylglycines by using ultra-performance liquid chromatography quadrupole linear ion trap mass spectrometry

Acylglycines are an important class of metabolites that have been used in the diagnosis of several inborn errors of metabolism (IEM). However, current analytical methods detect only a few acylglycines. There is a need to profile these metabolites in a comprehensive manner for studying their functions and improving their diagnostic values for different IEM and potentially other diseases. We describe a sensitive method that combines the chromatographic resolving power of ultra-performance liquid chromatography (UPLC) to separate closely related metabolites including isomers with tandem mass spectrometry (MS/MS). Acylglycines were extracted from urine using an anion exchange solid-phase extraction (SPE) cartridge. After UPLC separation, the acylglycines were detected on a hybrid triple quadrupole linear ion trap mass spectrometer. A set of standards were used for the development of an optimal MS acquisition method. Several acquisition modes using information derived from collision-induced dissociation breakdown curves were used to detect acylglycines. Using this method, 18 acylglycines were detected in the urine of healthy individuals and confirmed using standards, while 47 additional acylglycines were detected and tentatively identified, based on their retention and fragmentation pattern. Among the 65 acylglycines detected, only 18 of them have been previously reported in biofluids of healthy individuals. These results will be deposited in a public human metabolome database. This example illustrates that by developing a method tailored to the analysis of a class of metabolites sharing similar structural moieties, we can potentially identify many more new metabolites, thereby expanding the overall metabolome coverage.

Useful second-tier tests in expanded newborn screening of isovaleric acidemia and methylmalonic aciduria

Common use of pivalate-generating antibiotics in newborns in Japan and low cutoff value of C5-acylcarnitine (C5) to detect mild forms of isovaleric acidemia (IVA) led to 1,065 positive results from IVA screening among 146,000 newborns tested by tandem mass spectrometry over the last 3 years. Using our method to determine isovalerylglycine (IVG) levels in dried blood spots (DBS) as a second-tier test with IVG cutoff value of 0.5 nmol/ml in DBS, one patient with severe IVA was identified, and no recall of the second DBS was needed. Retrospective analysis revealed that most patients with moderate to severe forms of IVA have decreased free-carnitine levels shortly after birth and higher levels of IVG than those of C5, which suggests that this method is useful in evaluating the severity of IVA. Another second-tier test, to measure methylmalonic acid (MMA) levels in DBS by gas chromatography/mass spectrometry (GC/MS), has been developed to overcome difficulties in screening methylmalonic aciduria (MMAU) and propionic acidemia. Methanol extract from DBS was dried and derivatized using N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide. GC/MS was performed using splitless injection, electron-impact ionization, and selected ion monitoring for data recording. MMAU patients had much higher DBS concentrations of MMA (24.2-321.9 nmol/ml) than control newborns (0.34 ± 0.11 nmol/ml). MMA measurement in DBS was thought to provide useful information about the severity of MMAU, as MMAU patients with high levels of MMA had decreased levels of free carnitine and mildly increased levels of propionylcarnitine.

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.