Ethylmalonic aciduria associated with progressive neurological disease and partial cytochrome c oxidase deficiency

From genome to phenome-Simple inborn errors of metabolism as complex traits

Sporadically, patients with a proven defect in either mFAO or OXPHOS are described presenting with a metabolic profile and clinical phenotype expressing concurrent defects in both pathways. Biochemical linkages between both processes are tight. Therefore, it is striking that concurrent dysfunction of both systems occurs so infrequent. In this review, the linkages between OXPHOS and mFAO and the hypothesized processes responsible for concurrent problems in both systems are reviewed, both from the point of view of primary biochemical connections and secondary cellular responses, i.e. signaling pathways constituting nutrient-sensing networks. We propose that affected signaling pathways may play an important role in the phenomenon of concurrent defects. Recent data indicate that interference in the affected signaling pathways may resolve the pathological phenotype even though the primary enzyme deficiency persists. This offers new (unexpected) prospects for treatment of these inborn errors of metabolism. This article is part of a Special Issue entitled: From Genome to Function.

Disruption of mitochondrial homeostasis in organic acidurias: insights from human and animal studies

Organic acidurias or organic acidemias constitute a group of inherited disorders caused by deficient activity of specific enzymes of amino acids, carbohydrates or lipids catabolism, leading to large accumulation and excretion of one or more carboxylic (organic) acids. Affected patients usually present neurologic symptoms and abnormalities, sometimes accompanied by cardiac and skeletal muscle alterations, whose pathogenesis is poorly known. However, in recent years growing evidence has emerged indicating that mitochondrial dysfunction is directly or indirectly involved in the pathology of various organic acidemias. Mitochondrial impairment in some of these diseases are generally due to mutations in nuclear genes of the tricarboxylic acid cycle or oxidative phosphorylation, while in others it seems to result from toxic influences of the endogenous organic acids to the mitochondrion. In this minireview, we will briefly summarize the present knowledge obtained from human and animal studies showing that disruption of mitochondrial homeostasis may represent a relevant pathomechanism of tissue damage in selective organic acidemias. The discussion will focus on mitochondrial alterations found in patients affected by organic acidemias and by the deleterious effects of the accumulating organic acids on mitochondrial pathways that are crucial for ATP formation and transfer. The elucidation of the mechanisms of toxicity of these acidic compounds offers new perspectives for potential novel adjuvant therapeutic strategies in selected disorders of this group.

Antioxidant dysfunction: potential risk for neurotoxicity in ethylmalonic aciduria

Mitochondrial dysfunction and oxidative stress are central to the molecular basis of several human diseases associated with neuromuscular disabilities. We hypothesize that mitochondrial dysfunction also contributes to the neuromuscular symptoms observed in patients with ethylmalonic aciduria and homozygosity for ACADS c.625G>A-a common variant of the short-chain acyl-coenzyme A (CoA) dehydrogenase (SCAD) enzyme in the mitochondrial fatty acid oxidation pathway. This study sought to identify the specific factors that initiate cell dysfunction in these patients. We investigated fibroblast cultures from 10 patients with neuromuscular disabilities, elevated levels of ethylmalonic acid (EMA) (>50 mmol/mol creatinine), and ACADS c.625G>A homozygosity. Functional analyses, i.e., ACADS gene and protein expression as well as SCAD enzyme activity measurements, were performed together with a global nano liquid chromatography tandem mass spectroscopy (nano-LC-MS/MS)-based screening of the mitochondrial proteome in patient fibroblasts. Moreover, cell viability of patient fibroblasts exposed to menadione-induced oxidative stress was evaluated. Loss of SCAD function was detected in the patient group, most likely due to decreased ACADS gene expression and/or elimination of misfolded SCAD protein. Analysis of the mitochondrial proteome in patient fibroblasts identified a number of differentially expressed protein candidates, including reduced expression of the antioxidant superoxide dismutase 2 (SOD2). Additionally, patient fibroblasts demonstrated significantly higher sensitivity to oxidative stress than control fibroblasts. We propose that reduced mitochondrial antioxidant capacity is a potential risk factor for ACADS c.625G>A-associated ethylmalonic aciduria and that mitochondrial dysfunction contributes to the neurotoxicity observed in patients.

Respiratory complex II defect in siblings associated with a symptomatic secondary block in fatty acid oxidation

The mitochondrial oxidative phosphorylation and fatty acid oxidation pathways have traditionally been considered independent major sources of cellular energy production; however, case reports of patients with specific enzymatic defects in either pathway have suggested the potential for a complex interference between the two. This study documents a new site of interference between the two pathways, a site in respiratory complex II capable of producing clinical signs of a block in fatty acid oxidation and reduced in vitro activity of acyl-CoA dehydrogenases. The initial patient, and later her newborn sibling, had mildly dysmorphic features, lactic acidosis and a defect in mitochondrial respiratory complex II associated with many biochemical features of a block in fatty acid oxidation. Results of in vitro probing of intact fibroblasts from both patients with methyl[2H3]palmitate and L-carnitine revealed greatly increased [2H3]butyrylcarnitine; however, the ratio of dehydrogenase activity with butyryl-CoA with anti-MCAD inactivating antibody (used to reveal SCAD-specific activity) to that with octanoyl-CoA was normal, excluding a selective SCAD or MCAD deficiency. Respiratory complex II was defective in both patients, with an absent thenoyltrifluoroacetone-sensitive succinate Q reductase activity that was partially restored by supplementation with duroquinone. Although secondary, the block in fatty acid oxidation was a major management problem since attempts to provide essential fatty acids precipitated acidotic decompensations. This study reinforces the need to pursue broadly the primary genetic defect within these two pathways, making full use of increasingly available functional and molecular diagnostic tools.

Recent developments in the investigation of inherited metabolic disorders using cultured human cells

Thepurpose of this paper is to share experience with our systems and review recent "in vitro" methods using intact cells (fibroblasts, amniocytes) in which entire metabolic pathways can be probed for inherited metabolic defects reflected by elevations of intermediates determined by tandem mass spectrometry, HPLC, or gas chromatography-mass spectrometry. Currently, one can explore the integrity of mitochondrial fat oxidation, peroxisomal degradation of methyl-branched fatty acids (e.g., pristanate), and the mitochondrial degradation of the branched chain amino acids (leucine, valine, and isoleucine). For many of the diseases, the specific defect can be recognized from the acylcarnitine profile resulting from incubation of the intact cells with stable-isotope-labeled precursors to the particular pathway. This approach has also been successful in identifying new inherited metabolic disorders, biochemical correlation with clinical phenotypes of individual defects, and sequential oxidation of fatty acids by peroxisomal-mitochondrial interaction.

Syndrome of encephalopathy, petechiae, and ethylmalonic aciduria

We report a boy 20 months of age with encephalopathy, petechiae, and ethylmalonic aciduria (EPEMA). Other clinical features were severe hypotonia, orthostatic acrocyanosis, and chronic diarrhea. Magnetic resonance imaging (MRI) of the brain demonstrated bilateral lesions in the lenticular and caudate nuclei, periaqueductal region, subcortical areas, white matter, and brainstem. Short and medium chain Acyl-CoA dehydrogenase and cytochrome c oxidase (COX) activities in fibroblasts were normal. Muscle histochemistry disclosed diffuse COX deficiency, and respiratory chain activities in muscle disclosed severe COX deficiency. Twelve other patients with similar clinical features have been reported. Muscle COX activity, studied only in four, demonstrated a clear-cut defect.

Mitochondrial electron transport chain defect presenting as hypoglycemia

A profoundly deaf female infant was found to have hypoglycemia and lactic acidemia after an episode of decreased oral intake and vomiting. Electron transport chain (ETC) enzyme studies revealed a combination defect of complexes I, III, and IV in liver but not in skeletal muscle. This case highlights the fact that defects of the ETC are clinically highly heterogeneous and should be considered with hypoglycemia and lactic acidosis in the absence of a glycogen storage disorder. Moreover, ETC defects can occur with a biochemical profile suggestive of a fatty acid oxidation disorder.

Ethylmalonic aciduria is associated with an amino acid variant of short chain acyl-coenzyme A dehydrogenase

Ethylmalonic aciduria is a common biochemical finding in patients with inborn errors of short chain fatty acid beta-oxidation. The urinary excretion of ethylmalonic acid (EMA) may stem from decreased oxidation by short chain acyl-CoA dehydrogenase (SCAD) of butyryl-CoA, which is alternatively metabolized by propionyl-CoA carboxylase to EMA. We have recently detected a guanine to adenine polymorphism in the SCAD gene at position 625 in the SCAD cDNA, which changes glycine 209 to serine (G209S). The variant allele (A625) is present in homozygous and in heterozygous form in 7 and 34.8% of the general population, respectively. One hundred and thirty-five patients from Germany, Denmark, the Czech Republic, Spain, and the United States were selected for this study on the basis of abnormal EMA excretion ranging from 18 to 1185 mmol/mol of creatinine (controls < 18 mmol/mol of creatinine). Among them, we found a significant overrepresentation of the variant allele. Eighty-one patients (60%) were homozygous for the A625 allele, 40 (30%) were heterozygous, and only 14 (10%) harbored the wild-type allele (G625) in homozygous form. By overexpressing the wild-type and variant protein (G209S) in Escherichia coli and COS cells, we showed that the folding of the variant protein was slightly compromised in comparison to the wild-type and that the temperature stability of the tetrameric variant enzyme was lower than that of the wild type. Taken together, the over-representation and the biochemical studies indicate that the A625 allele confers susceptibility to the development of ethylmalonic aciduria.

Organic acidurias and related abnormalities

Organic acid analysis is a powerful technique in the diagnosis of inborn errors of metabolism. Since the development of the technique over twenty-five years ago, it has evolved into a sophisticated and powerful method and is an essential tool in the diagnosis of the organic acidurias. The chemistry and biochemistry of organic acids, as well as sample preparation, instrumentation, and many aspects of the more commonly used methods for the analysis of these compounds, are reviewed. The biochemical and clinical characteristics of each of the primary organic acidurias are described. In addition, the various noninherited causes of secondary organic acidurias that lead to the excretion of abnormal organic acids are also described, and ways of differentiating primary from secondary causes are discussed.

Ethylmalonic aciduria: an organic acidemia with CNS involvement and vasculopathy

Five infants from 3 families, one Egyptian, two Yemeni, are described with a progressive encephalopathy, four of whom have been studied in detail. All patients showed vascular lesions of the skin, characterized by waxing and waning petechiae and ecchymoses. Acrocyanosis was present in three patients. All patients showed retinal lesions characterized by tortuous veins. Protracted diarrhea was not a consistent finding, although they had metabolic crisis in association with diarrhea. They did not show failure to thrive. The neurologic symptoms were indicative of a progressive pyramidal tract disease. Three patients died following sudden emergence of severe basal ganglia, putaminal and head of caudate lesions. In one patient the CT changes in brain were suggestive of infarction. The patients who died manifested pulmonary congestion, or wet lung, and respiratory difficulties during the terminal stage of the disease. In all patients before and during the terminal event, mild-to-moderate hematuria, and in two RBC in CSF, was observed. In one patient there was mild hemoperitoneum at the terminal event. The urine organic acids indicated increased excretion of ethylmalonic, methylsuccinic, glutaric, and adipic acids. The patients invariably showed lactic acidosis, but no ketosis, during and in between the acidotic attacks of the disease. The acylcarnitine profile in blood of two patients showed a pronounced increase in C4 and C5 carnitine esters. In three patients, biopsies from petechiae indicated absence of an immune event, showing only fresh hemorrhage. An immunologic study in one patient was normal for the suppressor:cytotoxic lymphocyte ratio and concentration of interleukin-2 receptor during and in between hemorrhagic attacks. The cytochrome c oxidase activity in fibroblasts was normal. The rate of oxidation of glucose, leucine, isoleucine, valine, propionate and butyrate by fibroblasts was normal. The disease is not responsive to treatment with riboflavin, ascorbic acid, vitamin E, glycine, or carnitine. One patient remained stable on prolonged large doses of methylprednisolone. The biochemical defect leading to ethylmalonic aciduria in this disease remains unknown.

A new syndrome with ethylmalonic aciduria and normal fatty acid oxidation in fibroblasts

We describe four Italian male infants with a novel clinical phenotype characterized by orthostatic acrocyanosis, relapsing petechiae, chronic diarrhea, progressive pyramidal signs, mental retardation, and brain magnetic resonance imaging abnormalities. The first symptoms appeared after the termination of breast-feeding and introduction of formula feeding. Marked persistent 2-ethylmalonic aciduria was associated with abnormal excretion of C4-C5(n-butyryl-, isobutyryl-, isovaleryl-, and 2-methylbutyryl-)acylglycines and acylcarnitines and with intermittent lactic acidosis. Short- and branched-chain plasma acylcarnitine levels were also elevated. 2-Ethylmalonic aciduria is generally regarded as being indicative of a defect in fatty acid oxidation. Extensive studies of cultured fibroblasts failed to reveal such a defect. The observation of intermittent urinary excretion of 2-ethylhydracrylic acid pointed to involvement of the isoleucine R pathway in ethylmalonate biosynthesis. This hypothesis was tentatively corroborated by the biochemical responses to an oral isoleucine challenge in two patients. However, fibroblast studies showed normal oxidation rates of (14C)isoleucine (ul), indicating that this is not a defect of isoleucine oxidation expressed in skin fibroblasts. In one of two patients tested, cytochrome c oxidase activity was partially reduced (45%) in cultured fibroblasts. This unique clinical and biochemical phenotype identifies a new metabolic encephalopathy of yet undetermined cause.