Establishment of a practical enzymatic assay method for determination of isovaleryl-CoA dehydrogenase activity using high-performance liquid chromatography

Mitochondrial FAD shortage in SLC25A32 deficiency affects folate-mediated one-carbon metabolism

The SLC25A32 dysfunction is associated with neural tube defects (NTDs) and exercise intolerance, but very little is known about disease-specific mechanisms due to a paucity of animal models. Here, we generated homozygous (Slc25a32Y174C/Y174C and Slc25a32K235R/K235R) and compound heterozygous (Slc25a32Y174C/K235R) knock-in mice by mimicking the missense mutations identified from our patient. A homozygous knock-out (Slc25a32-/-) mouse was also generated. The Slc25a32K235R/K235R and Slc25a32Y174C/K235R mice presented with mild motor impairment and recapitulated the biochemical disturbances of the patient. While Slc25a32-/- mice die in utero with NTDs. None of the Slc25a32 mutations hindered the mitochondrial uptake of folate. Instead, the mitochondrial uptake of flavin adenine dinucleotide (FAD) was specifically blocked by Slc25a32Y174C/K235R, Slc25a32K235R/K235R, and Slc25a32-/- mutations. A positive correlation between SLC25A32 dysfunction and flavoenzyme deficiency was observed. Besides the flavoenzymes involved in fatty acid β-oxidation and amino acid metabolism being impaired, Slc25a32-/- embryos also had a subunit of glycine cleavage system-dihydrolipoamide dehydrogenase damaged, resulting in glycine accumulation and glycine derived-formate reduction, which further disturbed folate-mediated one-carbon metabolism, leading to 5-methyltetrahydrofolate shortage and other folate intermediates accumulation. Maternal formate supplementation increased the 5-methyltetrahydrofolate levels and ameliorated the NTDs in Slc25a32-/- embryos. The Slc25a32K235R/K235R and Slc25a32Y174C/K235R mice had no glycine accumulation, but had another formate donor-dimethylglycine accumulated and formate deficiency. Meanwhile, they suffered from the absence of all folate intermediates in mitochondria. Formate supplementation increased the folate amounts, but this effect was not restricted to the Slc25a32 mutant mice only. In summary, we established novel animal models, which enabled us to understand the function of SLC25A32 better and to elucidate the role of SLC25A32 dysfunction in human disease development and progression.

Measuring propionyl-CoA carboxylase activity in phytohemagglutinin stimulated lymphocytes using high performance liquid chromatography

BACKGROUND

Propionyl-CoA carboxylase (PCC) is a mitochondrial enzyme involved in the catabolism of several essential amino acids and odd chain fatty acids. Previous PCC assays have involved either a radiometric assay or have required mitochondria isolation and/or enzyme purification.

METHODS

We developed an enzymatic method to analyze PCC activity in phytohemagglutinin (PHA) stimulated lymphocytes that involves high performance liquid chromatography.

RESULTS

The method shows good linearity and sensitivity. PCC activity was unaffected even when lymphocytes were isolated and PHA stimulated after a whole blood sample had been stored at 4°C for 5days. This indicates that this method is suitable for analyzing samples from distant medical centers. The PCC activity of patients with propionic acidemia was found to be much lower than that of normal individuals and carriers. However, this PCC assay is significantly affected by the red blood cell contamination. In conclusion, this is a reliable method for performing PCC assays and only requires 0.5 to 1.0ml of whole blood from newborns.

CONCLUSIONS

The PCC assay established in this study is useful for the confirmation of PA in individuals, and prenatal diagnosis and genetic counseling for the affected families.

Chronic intermittent form of isovaleric aciduria in a 2-year-old boy

Isovaleric aciduria (IVA) is caused by an autosomal recessive deficiency of isovaleryl-CoA dehydrogenase (IVD). IVA presents either in the neonatal period as an acute episode of fulminant metabolic acidosis, which may lead to coma or death, or later as a "chronic intermittent form" that is associated with developmental delays, with or without recurrent acidotic episodes during periods of stress, such as infections. Here, we report the case of a 2-year old boy with IVA who presented with the chronic intermittent form. He was admitted to Asan Medical Center Children's Hospital with recurrent vomiting. Metabolic acidosis, hyperammonemia, elevated serum lactate and isovalerylcarnitine levels, and markedly increased urine isovalerylglycine concentration were noted. Sequence analysis of the IVD gene in the patient revealed the novel compound mutations-a missense mutation, c.986T>C (p.Met329Thr) and a frameshift mutation, c.1083del (p.Ile361fs*11). Following stabilization during the acute phase, the patient has remained in a stable condition on a low-leucine diet.

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.

Development of a new enzymatic diagnosis method for very-long-chain Acyl-CoA dehydrogenase deficiency by detecting 2-hexadecenoyl-CoA production and its application in tandem mass spectrometry-based selective screening and newborn screening in Japan

The introduction of tandem mass spectrometry (MS/MS) has made it possible to screen for very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency. To confirm the diagnosis in cases with an abnormal profile of blood acylcarnitines, we developed a new enzymatic assay method for determining dehydrogenase activity toward palmitoyl-CoA (C16:0) in lymphocytes. Using this method, the production of 2-hexadecenoyl-CoA (C16:1) by crude cell lysates can be directly quantified using high performance liquid chromatography (HPLC). We applied the assay to 7 myopathic patients, 7 hypoglycemic patients, and 2 presymptomatic newborns with elevated levels of tetradecenoylcarnitine (C14:1 AC) in blood, and found impaired VLCAD activity in all of the 7 myopathic patients and both of the 2 newborns. All of the 7 hypoglycemic patients had normal level of the enzyme activity. Results of the ACADVL gene analysis were in consistent with the enzymatic diagnosis. These results suggest that MS/MS-based screening for VLCAD deficiency using blood C14:1 AC as the indicator may show a considerably high false-positive rate in selective screening of symptomatic patients. Our practical enzymatic assay can be a useful test for the accurate diagnosis of VLCAD deficiency cases screened by MS/MS.

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.

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.