Jos P.N. Ruiter

Common missense mutation G1528C in long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency. Characterization and expression of the mutant protein, mutation analysis on genomic DNA and chromosomal localization of the mitochondrial trifunctional protein alpha subunit gene

Mitochondrial trifunctional protein (MTP) is a recently identified enzyme involved in mitochondrial beta-oxidation, harboring long-chain enoyl-CoA hydratase, long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) and long-chain 3-ketothiolase activity. A deficiency of this protein is associated with impaired oxidation of long-chain fatty acids which can lead to sudden infant death. Furthermore, it is clear that this inborn error of fatty acid oxidation is very frequent, second to medium chain acyl-CoA dehydrogenase deficiency. In most patients only the LCHAD activity of MTP is deficient with near normal activity of the two other enzyme activities of the complex. We recently described the occurrence of a frequent G1528C mutation in the cDNA coding for the a subunit of MTP. Using S. cerevisiae for expression of wild type and mutant protein we show that the G1528C mutation is directly responsible for the loss of LCHAD activity. Furthermore, we describe a newly developed method allowing identification of the G1528C mutation in genomic DNA. The finding of an 87% allele frequency of the G1528C mutation in 34 LCHAD deficient patients makes this a valuable test for prenatal diagnosis. Finally, we show that the gene encoding the alpha subunit of MTP is located on chromosome 2p24.1-23.3.

The enzymology of mitochondrial fatty acid beta-oxidation and its application to follow-up analysis of positive neonatal screening results

Oxidation of fatty acids in mitochondria is a key physiological process in higher eukaryotes including humans. The importance of the mitochondrial beta-oxidation system in humans is exemplified by the existence of a group of genetic diseases in man caused by an impairment in the mitochondrial oxidation of fatty acids. Identification of patients with a defect in mitochondrial beta-oxidation has long remained notoriously difficult, but the introduction of tandem-mass spectrometry in laboratories for genetic metabolic diseases has revolutionalized the field by allowing the rapid and sensitive analysis of acylcarnitines. Equally important is that much progress has been made with respect to the development of specific enzyme assays to identify the enzyme defect in patients subsequently followed by genetic analysis. In this review, we will describe the current state of knowledge in the field of fatty acid oxidation enzymology and its application to the follow-up analysis of positive neonatal screening results.

Progressive Infantile Neurodegeneration Caused by 2-Methyl-3-Hydroxybutyryl-CoA Dehydrogenase Deficiency: A Novel Inborn Error of Branched-Chain Fatty Acid and Isoleucine Metabolism

We report a novel inborn error of metabolism identified in a child with an unusual neurodegenerative disease. The male patient was born at term and recovered well from a postnatal episode of metabolic decompensation and lactic acidosis. Psychomotor development in the first year of life was only moderately delayed. After 14 mo of age, there was progressive loss of mental and motor skills; at 2 years of age, he was severely retarded with marked restlessness, choreoathetoid movements, absence of directed hand movements, marked hypotonia and little reaction to external stimuli. Notable laboratory findings included marked elevations of urinary 2-methyl-3-hydroxybutyrate and tiglylglycine without elevation of 2-methylacetoacetate, mild elevations of lactate in CSF and blood, and a slightly abnormal acylcarnitine profile. These abnormalities became more apparent after isoleucine challenge. Enzyme studies showed absent activity of 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) in the mitochondrial oxidation of 2-methyl branched-chain fatty acids and isoleucine. Under dietary isoleucine restriction, neurologic symptoms stabilized over the next 7 months.

Quantitative acylcarnitine profiling in fibroblasts using [U-13C] palmitic acid: an improved tool for the diagnosis of fatty acid oxidation defects.

A method was developed for the investigation of mitochondrial fatty acid beta-oxidation in cultured fibroblasts. Monolayer cultures were incubated without foetal calf serum with commercially available [U-13C] palmitic acid and L-carnitine for 96 h. The acylcarnitines produced by the cells were extracted from the cell suspension and analysed either by quantitative stable isotope dilution gas chromatography chemical ionization mass spectrometry, or by fast atom bombardment mass spectrometry. Characteristic acylcarnitine profiles were obtained for all the different enzyme deficiencies investigated, with the exception of carnitine palmitoyltransferase II deficiency and carnitine/acylcarnitine carrier deficiency which showed similar patterns. Comparison between this method and the 3H-myristate and 3H-palmitate tritium release assays revealed that the method described here is superior, allowing unequivocal identification of patients.

A phytol-enriched diet induces changes in fatty acid metabolism in mice both via PPARα-dependent and -independent pathways

Branched-chain fatty acids (such as phytanic and pristanic acid) are ligands for the nuclear hormone receptor peroxisome proliferator-activated receptor α (PPARα) in vitro. To investigate the effects of these physiological compounds in vivo, wild-type and PPARα-deficient (PPARα−/−) mice were fed a phytol-enriched diet. This resulted in increased plasma and liver levels of the phytol metabolites phytanic and pristanic acid. In wild-type mice, plasma fatty acid levels decreased after phytol feeding, whereas in PPARα−/− mice, the already elevated fatty acid levels increased. In addition, PPARα−/− mice were found to be carnitine deficient in both plasma and liver. Dietary phytol increased liver free carnitine in wild-type animals but not in PPARα−/− mice. Investigation of carnitine biosynthesis revealed that PPARα is likely involved in the regulation of carnitine homeostasis. Furthermore, phytol feeding resulted in a PPARα-dependent induction of various peroxisomal and mitochondrial β-oxidation enzymes. In addition, a PPARα-independent induction of catalase, phytanoyl-CoA hydroxylase, carnitine octanoyltransferase, peroxisomal 3-ketoacyl-CoA thiolase, and straight-chain acyl-CoA oxidase was observed. In conclusion, branched-chain fatty acids are physiologically relevant ligands of PPARα in mice. These findings are especially relevant for disorders in which branched-chain fatty acids accumulate, such as Refsum disease and peroxisome biogenesis disorders.

Molecular basis of hepatic carnitine palmitoyltransferase I deficiency.

Mitochondrial fatty acid beta-oxidation is important for energy production, which is stressed by the different defects found in this pathway. Most of the enzyme deficiencies causing these defects are well characterized at both the protein and genomic levels. One exception is carnitine palmitoyltransferase I (CPT I) deficiency, of which until now no mutations have been reported although the defect is enzymatically well characterized. CPT I is the key enzyme in the carnitine-dependent transport across the mitochondrial inner membrane and its deficiency results in a decreased rate of fatty acid beta-oxidation. Here we report the first delineation of the molecular basis of hepatic CPT I deficiency in a new case. cDNA analysis revealed that this patient was homozygous for a missense mutation (D454G). The effect of the identified mutation was investigated by heterologous expression in yeast. The expressed mutant CPT IA displayed only 2% of the activity of the expressed wild-type CPT IA, indicating that the D454G mutation is the disease-causing mutation. Furthermore, in patients fibroblasts the CPT IA protein was markedly reduced on immunoblot, suggesting that the mutation renders the protein unstable.

Mitochondrial long chain fatty acid β-oxidation in man and mouse

Several mouse models for mitochondrial fatty acid beta-oxidation (FAO) defects have been developed. So far, these models have contributed little to our current understanding of the pathophysiology. The objective of this study was to explore differences between murine and human FAO. Using a combination of analytical, biochemical and molecular methods, we compared fibroblasts of long chain acyl-CoA dehydrogenase knockout (LCAD(-/-)), very long chain acyl-CoA dehydrogenase knockout (VLCAD(-/-)) and wild type mice with fibroblasts of VLCAD-deficient patients and human controls. We show that in mice, LCAD and VLCAD have overlapping and distinct roles in FAO. The absence of VLCAD is apparently fully compensated, whereas LCAD deficiency is not. LCAD plays an essential role in the oxidation of unsaturated fatty acids such as oleic acid, but seems redundant in the oxidation of saturated fatty acids. In strong contrast, LCAD is neither detectable at the mRNA level nor at the protein level in men, making VLCAD indispensable in FAO. Our findings open new avenues to employ the existing mouse models to study the pathophysiology of human FAO defects.

Long-Chain Fatty Acid Oxidation during Early Human Development

Patients with very long-chain acyl-CoA dehydrogenase (VLCAD) and long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD)/mitochondrial trifunctional protein (MTP) deficiency, disorders of the mitochondrial long-chain fatty acid oxidation, can present with hypoketotic hypoglycemia, rhabdomyolysis, and cardiomyopathy. In addition, patients with LCHAD/MTP deficiency may suffer from retinopathy and peripheral neuropathy. Until recently, there was no indication of intrauterine morbidity in these disorders. This observation was in line with the widely accepted view that fatty acid oxidation (FAO) does not play a significant role during fetal life. However, the high incidence of the gestational complications acute fatty liver of pregnancy and hemolysis, elevated liver enzymes, and low platelets syndrome observed in mothers carrying a LCHAD/MTP–deficient child and the recent reports of fetal hydrops due to cardiomyopathy in MTP deficiency, as well as the high incidence of intrauterine growth retardation in children with LCHAD/MTP deficiency, suggest that FAO may play an important role during fetal development. In this study, using in situ hybridization of the VLCAD and the LCHAD mRNA, we report on the expression of genes involved in the mitochondrial oxidation of long-chain fatty acids during early human development. Furthermore, we measured the enzymatic activity of the VLCAD, LCHAD, and carnitine palmitoyl-CoA transferase 2 (CPT2) enzymes in different human fetal tissues. Human embryos (at d 35 and 49 of development) and separate tissues (5–20 wk of development) were used. The results show a strong expression of VLCAD and LCHAD mRNA and a high enzymatic activity of VLCAD, LCHAD, and CPT2 in a number of tissues, such as liver and heart. In addition, high expression of LCHAD mRNA was observed in the neural retina and CNS. The observed pattern of expression during early human development is well in line with the spectrum of clinical signs and symptoms reported in patients with VLCAD or LCHAD/MTP deficiency.

Studies on mitochondrial oxidative phosphorylation in permeabilized human skin fibroblasts: application to mitochondrial encephalomyopathies

In this report we describe that the functional capacity of the mitochondrial oxidative phosphorylation system can be studied in cultured skin fibroblasts permeabilized with a limited amount of digitonin. By using a variety of different oxidizable substrates, information can be obtained on the functional activity of complex I, for instance, which is important since different diseases in man have recently been identified in which complex I is deficient. The method described may contribute to the biochemical characterization of patients suffering from one of a variety of encephalomyopathies.

Differentiation and proliferation of respiration-deficient human myoblasts

Replication and transcription of mitochondrial DNA were impaired in dividing human myoblasts exposed to ethidium bromide. MtDNA content decreased linearly per cell division and mitochondrial transcript levels declined rapidly, resulting in respiration-deficiency of the myoblasts. Despite the absence of functional mitochondria the cells remained able to proliferate when grown under specific culture conditions. However, the formation of myotubes was severely impaired in respiration-deficient myoblasts. We conclude that differentiation of myoblasts into myotubes is more dependent on mitochondrial function than proliferation of myoblasts.