Revised assays for the investigation of congenital lactic acidosis using 14C keto acids, eliminating problems associated with spontaneous decarboxylation

Clinical and biochemical characterization of four patients with mutations in ECHS1

BACKGROUND

Short-chain enoyl-CoA hydratase (SCEH, encoded by ECHS1) catalyzes hydration of 2-trans-enoyl-CoAs to 3(S)-hydroxy-acyl-CoAs. SCEH has a broad substrate specificity and is believed to play an important role in mitochondrial fatty acid oxidation and in the metabolism of branched-chain amino acids. Recently, the first patients with SCEH deficiency have been reported revealing only a defect in valine catabolism. We investigated the role of SCEH in fatty acid and branched-chain amino acid metabolism in four newly identified patients. In addition, because of the Leigh-like presentation, we studied enzymes involved in bioenergetics.

METHODS

Metabolite, enzymatic, protein and genetic analyses were performed in four patients, including two siblings. Palmitate loading studies in fibroblasts were performed to study mitochondrial β-oxidation. In addition, enoyl-CoA hydratase activity was measured with crotonyl-CoA, methacrylyl-CoA, tiglyl-CoA and 3-methylcrotonyl-CoA both in fibroblasts and liver to further study the role of SCEH in different metabolic pathways. Analyses of pyruvate dehydrogenase and respiratory chain complexes were performed in multiple tissues of two patients.

RESULTS

All patients were either homozygous or compound heterozygous for mutations in the ECHS1 gene, had markedly reduced SCEH enzymatic activity and protein level in fibroblasts. All patients presented with lactic acidosis. The first two patients presented with vacuolating leukoencephalopathy and basal ganglia abnormalities. The third patient showed a slow neurodegenerative condition with global brain atrophy and the fourth patient showed Leigh-like lesions with a single episode of metabolic acidosis. Clinical picture and metabolite analysis were not consistent with a mitochondrial fatty acid oxidation disorder, which was supported by the normal palmitate loading test in fibroblasts. Patient fibroblasts displayed deficient hydratase activity with different substrates tested. Pyruvate dehydrogenase activity was markedly reduced in particular in muscle from the most severely affected patients, which was caused by reduced expression of E2 protein, whereas E2 mRNA was increased.

CONCLUSIONS

Despite its activity towards substrates from different metabolic pathways, SCEH appears to be only crucial in valine metabolism, but not in isoleucine metabolism, and only of limited importance for mitochondrial fatty acid oxidation. In severely affected patients SCEH deficiency can cause a secondary pyruvate dehydrogenase deficiency contributing to the clinical presentation.

Variant non ketotic hyperglycinemia is caused by mutations in LIAS, BOLA3 and the novel gene GLRX5

Patients with nonketotic hyperglycinemia and deficient glycine cleavage enzyme activity, but without mutations in AMT, GLDC or GCSH, the genes encoding its constituent proteins, constitute a clinical group which we call 'variant nonketotic hyperglycinemia'. We hypothesize that in some patients the aetiology involves genetic mutations that result in a deficiency of the cofactor lipoate, and sequenced genes involved in lipoate synthesis and iron-sulphur cluster biogenesis. Of 11 individuals identified with variant nonketotic hyperglycinemia, we were able to determine the genetic aetiology in eight patients and delineate the clinical and biochemical phenotypes. Mutations were identified in the genes for lipoate synthase (LIAS), BolA type 3 (BOLA3), and a novel gene glutaredoxin 5 (GLRX5). Patients with GLRX5-associated variant nonketotic hyperglycinemia had normal development with childhood-onset spastic paraplegia, spinal lesion, and optic atrophy. Clinical features of BOLA3-associated variant nonketotic hyperglycinemia include severe neurodegeneration after a period of normal development. Additional features include leukodystrophy, cardiomyopathy and optic atrophy. Patients with lipoate synthase-deficient variant nonketotic hyperglycinemia varied in severity from mild static encephalopathy to Leigh disease and cortical involvement. All patients had high serum and borderline elevated cerebrospinal fluid glycine and cerebrospinal fluid:plasma glycine ratio, and deficient glycine cleavage enzyme activity. They had low pyruvate dehydrogenase enzyme activity but most did not have lactic acidosis. Patients were deficient in lipoylation of mitochondrial proteins. There were minimal and inconsistent changes in cellular iron handling, and respiratory chain activity was unaffected. Identified mutations were phylogenetically conserved, and transfection with native genes corrected the biochemical deficiency proving pathogenicity. Treatments of cells with lipoate and with mitochondrially-targeted lipoate were unsuccessful at correcting the deficiency. The recognition of variant nonketotic hyperglycinemia is important for physicians evaluating patients with abnormalities in glycine as this will affect the genetic causation and genetic counselling, and provide prognostic information on the expected phenotypic course.

A novel syndrome affecting multiple mitochondrial functions, located by microcell-mediated transfer to chromosome 2p14-2p13

We have studied cultured skin fibroblasts from three siblings and one unrelated individual, all of whom had fatal mitochondrial disease manifesting soon after birth. After incubation with 1 mM glucose, these four cell strains exhibited lactate/pyruvate ratios that were six times greater than those of controls. On further analysis, enzymatic activities of the pyruvate dehydrogenase complex, the 2-oxoglutarate dehydrogenase complex, NADH cytochrome c reductase, succinate dehydrogenase, and succinate cytochrome c reductase were severely deficient. In two of the siblings the enzymatic activity of cytochrome oxidase was mildly decreased (by approximately 50%). Metabolite analysis performed on urine samples taken from these patients revealed high levels of glycine, leucine, valine, and isoleucine, indicating abnormalities of both the glycine-cleavage system and branched-chain alpha-ketoacid dehydrogenase. In contrast, the activities of fibroblast pyruvate carboxylase, mitochondrial aconitase, and citrate synthase were normal. Immunoblot analysis of selected complex III subunits (core 1, cyt c(1), and iron-sulfur protein) and of the pyruvate dehydrogenase complex subunits revealed no visible changes in the levels of all examined proteins, decreasing the possibility that an import and/or assembly factor is involved. To elucidate the underlying molecular defect, analysis of microcell-mediated chromosome-fusion was performed between the present study's fibroblasts (recipients) and a panel of A9 mouse:human hybrids (donors) developed by Cuthbert et al. (1995). Complementation was observed between the recipient cells from both families and the mouse:human hybrid clone carrying human chromosome 2. These results indicate that the underlying defect in our patients is under the control of a nuclear gene, the locus of which is on chromosome 2. A 5-cM interval has been identified as potentially containing the critical region for the unknown gene. This interval maps to region 2p14-2p13.

A new case of multiple mitochondrial enzyme deficiencies with decreased amount of heat shock protein 60

Heat shock protein 60 (hsp60) is a mitochondrial matrix protein involved in the folding and correct assembly of polypeptides into complex mitochondrial enzymes. Its deficiency has recently been described as the most likely primary cause of congenital lactic acidaemia with multiple mitochondrial enzyme deficiencies in a female patient. We describe a new case of a girl with a substantially decreased amount of hsp60 in cultured fibroblasts. She presented from birth with hypotonia, unusual facial features, feeding difficulties and failure to thrive. Death occurred at age 4.5 years. Biochemical findings included metabolic acidosis with lactic acidaemia, hyperammonaemia and intermittent ketosis. In contrast to the previously reported case, organic acid analysis showed an altered profile throughout her life. In agreement with this profile, various mitochondrial enzyme activities were deficient in cultured fibroblasts, including enzymes of the respiratory chain and the Krebs cycle, the pyruvate dehydrogenase complex and the mitochondrial biotindependent carboxylases. Fibroblast mitochondria showed ultrastructural abnormalities, were swollen, and were mainly localized around the nucleus. The description of a second case of multiple mitochondrial enzyme deficiencies with reduced amount of hsp60 supports the idea that hsp60 deficiency might be a more common cause of mitochondrial disease. This opens new possibilities for the diagnosis and understanding of congenital lactic acidaemia.

Mitochondrial phosphoenolpyruvate carboxykinase deficiency

The sibling of a patient in whom a diagnosis of phosphoenolpyruvate carboxykinase had been made developed a similar clinical illness with liver failure. However the activity of phosphoenolpyruvate carboxykinase in leucocytes and fibroblasts was normal. Phosphoenolpyruvate carboxykinase is not the primary defect in this family.

Maple syrup urine disease in calves: a clinical, pathological and biochemical study

The clinical, pathological and biochemical findings of a study of 30 Poll Hereford, Hereford, Poll Hereford cross or Hereford cross calves affected with branched chain ketoacid dehydrogenase (BCKAD) complex deficiency or maple syrup urine disease (MSUD) are presented. In breeding studies, 6 of 21 calves from obligate heterozygote matings were affected with MSUD, suggesting the disease is inherited in an autosomal recessive manner. Calves were clinically affected from birth, but there were variations in the subsequent course of progressive deterioration of central nervous system function. Concentrations of the branched chain amino acids and keto acids were elevated in pre-suckle plasma and cerebellar water content was higher in affected calves. Activity of BCKAD complex was minimal in fibroblasts cultured from an affected calf. Spongiform encephalopathy and elevated ratios of the branched to straight chain amino acids in formalin fixed cerebral tissue were found in a stillborn foetus and a 3-month-old Hereford calf. These findings suggest the disease occurs prenatally and that a delayed form may exist.

Cell culture studies on patients with mitochondrial diseases: molecular defects in pyruvate dehydrogenase

There is a group of inborn errors of metabolism that result in the condition of chronic lacticacidemia of childhood. Nearly all of the defects that can be identified occur in mitochondrial proteins, and can be demonstrated in cultured skin fibroblasts from the patients concerned. One approach to the diagnosis of these defects involves a simple incubation of the fibroblast culture with glucose-containing medium followed by the measurement of accumulated lactate and pyruvate. The total amounts of lactate and pyruvate and the ratio between them is different in cells from patients with defects in the pyruvate dehydrogenase complex or the respiratory chain. Measurement of 1-14C-pyruvate oxidation to 14CO2 can also reveal defective oxidative metabolism. Localization of the defects can be achieved using individual assays for the enzymes concerned. The clinical sequelae of the different defects is discussed.

Immunochemical analysis of normal and mutant forms of human pyruvate dehydrogenase

Pyruvate dehydrogenase (PDH) deficiency has been described in many patients with primary lactic acidosis. However, there are very few cases in which a structural defect in the complex has been clearly demonstrated. Measurement of the activity of the PDH complex in cultured human cells has proved unreliable, and a combination of structural and functional studies are required to make a definitive diagnosis. For this reason, an immunochemical strategy has been developed to complement direct enzyme assay in the detection and further characterization of PDH deficiency. We illustrate the usefulness of this approach by describing defects in the alpha-subunit of the pyruvate decarboxylase component of the PDH complex in two patients with primary lactic acidosis. In one patient, there is no immunologically cross-reacting material corresponding to this subunit. In the second patient, there appears to be an intrinsic structural defect in the subunit which restricts dephosphorylation (and hence activation) of the inactive phosphorylated complex.

Respiratory chain defects in the mitochondria of cultured skin fibroblasts from three patients with lacticacidemia

The cultured skin fibroblasts from three patients with lacticacidemia were found to have low rates of 1-[14C]pyruvate oxidation in the face of normal pyruvate dehydrogenase activity. After incubation with 1 mM glucose, these three cell strains also exhibited lactate/pyruvate ratios which were three times greater than those of controls. In two of the patients, both ATP and oxygen consumption in fibroblast mitochondrial preparations was deficient with NAD-linked substrates but normal with succinate and ascorbate/N'N'N'N' tetramethyl phenylene diamine. In the third patient, ATP synthesis in mitochondrial preparations was deficient with all substrates tested. Measurement of Rotenone-sensitive NADH-cytochrome c reductase in mitochondrial preparations from skin fibroblasts showed that two of the patients had 14 and 18%, respectively, of control activity. In the third patient, cytochrome oxidase activity was 15% of that in controls. We conclude that respiratory chain defects can be demonstrated in cultured skin fibroblasts with consistency using a number of different techniques.

Mitochondrial phosphoenolpyruvate carboxykinase deficiency

A 3-month-old girl presented with anorexia, failure to thrive and drowsiness. She was mildly icteric with hepatomegaly and peripheral oedema. Disordered liver function tests were associated with the biopsy appearances of a giant cell hepatitis and with a Fanconi syndrome. At the age of 16 weeks she collapsed with profound hypoglycaemia. Fasting also provoked hypoglycaemia with lactic acidaemia. She became increasingly irritable and hypotonic and, although initially liver and renal function improved, she deteriorated and died of hepatocellular failure and septicaemia. A post-mortem revealed massive fatty degeneration of the liver. The activity of phosphoenolpyruvate carboxykinase in her cultured skin fibroblasts was 16% of controls. Her brother died at the age of 4 weeks of sudden infant death syndrome.

Lacticacidaemia due to pyruvate dehydrogenase deficiency, with evidence of protein polymorphism in the alpha-subunit of the enzyme

In three infants with neonatal lacticacidaemia, a deficiency in the E1 (pyruvate dehydrogenase) component of the pyruvate dehydrogenase complex was demonstrated in skin fibroblast cultures. Residual activities of the pyruvate dehydrogenase complex in the activated state were 1.6%, 3.9% and 18.8% of control values, respectively. Immunoprecipitation of extracts of cultures skin fibroblasts grown on 35S-methionine with anti-pyruvate dehydrogenase complex antibody revealed an abnormality in the E1 alpha-component of these three patients when visualised after sodium dodecyl sulphate/polyacrylamide gel electrophoresis. This component appeared to have a slightly lower molecular weight than did this protein from control cell strains. Cell strains from other patients with a deficiency of the pyruvate dehydrogenase complex did not exhibit this defect. Three patients also showed dysmorphism and developmental abnormalities of the central nervous system.

Disorders of the pyruvate dehydrogenase complex

Pyruvate dehydrogenase deficiency may be a non-specific consequence of many different neurological degenerative disorders. There are also serious methodological problems in estimating the activity of this enzyme complex.

A coupled fluorometric rate assay for pyruvate dehydrogenase in cultured human fibroblasts

A method for measuring the activity of the pyruvate dehydrogenase complex (PDC) by coupling acetyl-CoA production to acetylation of a fluorescent dye is described. Acetylation of cresyl violet acetate by pigeon liver acetyltransferase results in a shift of its fluorescence spectrum from lambda ex max = 575, lambda em max = 620 nm to lambda ex max = 475, lambda em max = 575 nm. The rate of appearance of acetylated dye was followed fluorometrically and was proportional to PDC activity in extracts of cultured human fibroblasts. The assay showed appropriate substrate and cofactor dependence and had a working range between 0.04 and 70 munits. It is 10 times more sensitive than the spectrophotometric assay on which it is based (working range 0.4-31 munits) and is equally convenient. Unactivated PDC activity in fibroblast extracts was 0.75 (0.60-0.92) munits/mg protein (mean and range for six cell lines).