Short-chain hydroxyacyl-coenzyme A dehydrogenase deficiency presenting as unexpected infant death: A family study

The Physiological and Pathological Role of Acyl-CoA Oxidation

Fatty acid metabolism, including β-oxidation (βOX), plays an important role in human physiology and pathology. βOX is an essential process in the energy metabolism of most human cells. Moreover, βOX is also the source of acetyl-CoA, the substrate for (a) ketone bodies synthesis, (b) cholesterol synthesis, (c) phase II detoxication, (d) protein acetylation, and (d) the synthesis of many other compounds, including N-acetylglutamate-an important regulator of urea synthesis. This review describes the current knowledge on the importance of the mitochondrial and peroxisomal βOX in various organs, including the liver, heart, kidney, lung, gastrointestinal tract, peripheral white blood cells, and other cells. In addition, the diseases associated with a disturbance of fatty acid oxidation (FAO) in the liver, heart, kidney, lung, alimentary tract, and other organs or cells are presented. Special attention was paid to abnormalities of FAO in cancer cells and the diseases caused by mutations in gene-encoding enzymes involved in FAO. Finally, issues related to α- and ω- fatty acid oxidation are discussed.

Personalised modelling of clinical heterogeneity between medium-chain acyl-CoA dehydrogenase patients

BACKGROUND

Monogenetic inborn errors of metabolism cause a wide phenotypic heterogeneity that may even differ between family members carrying the same genetic variant. Computational modelling of metabolic networks may identify putative sources of this inter-patient heterogeneity. Here, we mainly focus on medium-chain acyl-CoA dehydrogenase deficiency (MCADD), the most common inborn error of the mitochondrial fatty acid oxidation (mFAO). It is an enigma why some MCADD patients-if untreated-are at risk to develop severe metabolic decompensations, whereas others remain asymptomatic throughout life. We hypothesised that an ability to maintain an increased free mitochondrial CoA (CoASH) and pathway flux might distinguish asymptomatic from symptomatic patients.

RESULTS

We built and experimentally validated, for the first time, a kinetic model of the human liver mFAO. Metabolites were partitioned according to their water solubility between the bulk aqueous matrix and the inner membrane. Enzymes are also either membrane-bound or in the matrix. This metabolite partitioning is a novel model attribute and improved predictions. MCADD substantially reduced pathway flux and CoASH, the latter due to the sequestration of CoA as medium-chain acyl-CoA esters. Analysis of urine from MCADD patients obtained during a metabolic decompensation showed an accumulation of medium- and short-chain acylcarnitines, just like the acyl-CoA pool in the MCADD model. The model suggested some rescues that increased flux and CoASH, notably increasing short-chain acyl-CoA dehydrogenase (SCAD) levels. Proteome analysis of MCADD patient-derived fibroblasts indeed revealed elevated levels of SCAD in a patient with a clinically asymptomatic state. This is a rescue for MCADD that has not been explored before. Personalised models based on these proteomics data confirmed an increased pathway flux and CoASH in the model of an asymptomatic patient compared to those of symptomatic MCADD patients.

CONCLUSIONS

We present a detailed, validated kinetic model of mFAO in human liver, with solubility-dependent metabolite partitioning. Personalised modelling of individual patients provides a novel explanation for phenotypic heterogeneity among MCADD patients. Further development of personalised metabolic models is a promising direction to improve individualised risk assessment, management and monitoring for inborn errors of metabolism.

Role of short-chain hydroxyacyl CoA dehydrogenases in SCHAD deficiency

Short-chain hydroxyacyl CoA dehydrogenase deficiency is an ill-defined, severe pediatric disorder of mitochondrial fatty acid beta-oxidation of short-chain hydroxyacyl CoAs. To understand the relative contributions of the two known short-chain hydroxyacyl CoA dehydrogenases (HADH) tissue biopsies of six distinct family individuals were analyzed and kinetic parameters were compared. Steady-state kinetic constants for HADH 1 and HADH 2 suggest that type 1 is the major enzyme involved in mitochondrial beta-oxidation of short-chain hydroxyacyl-CoAs. Two patients are heterozygous carriers of a HADH 1 polymorphism, whereas no mutation is detected in the HADH 2 gene of all patients. The data suggest that protein interactions rather than HADH mutations are responsible for the disease phenotype. Pull-down experiments of recombinant HADH 1 and 2 with human mitochondrial extracts reveal two proteins interacting with HADH 1, one of which was identified as glutamate dehydrogenase. This association provides a possible link between fatty acid metabolism and the hyperinsulinism/hyperammonia syndrome.

Long-Term Stability of Amino Acids and Acylcarnitines in Dried Blood Spots

Background: Dried blood filter cards, collected for newborn screening, are often stored for long periods of time. They may be suitable for the retrospective diagnosis of inborn errors of metabolism, but no data are currently available on the long-term stability of amino acids and acylcarnitine species.

Methods: We analyzed amino acids and acylcarnitines by tandem mass spectrometry in 660 anonymous, randomly selected filter cards from 1989 through 2004. We assessed long-term stability of metabolites by linear regression and estimated annual decrease of concentration for each metabolite.

Results: Concentrations of free carnitine increased by 7.6% per year during the first 5 years of storage and decreased by 1.4% per year thereafter. Alanine, arginine, leucine, methionine, and phenylalanine decreased by 6.5%, 3.3%, 3.1%, 7.3%, and 5.7% per year, respectively. Acetylcarnitine, propionylcarnitine, citrulline, glycine, and ornithine decreased by 18.5%, 27.4%, 8.1%, 14.7%, and 16.3% per year during the first 5 years, respectively; thereafter the decline was more gradual. Tyrosine decreased by 1.7% per year during the first 5 years and 7.9% per year thereafter. We could not analyze medium- and long-chain acylcarnitine species because of low physiological concentrations.

Conclusions: Estimation of the annual decrease of metabolites may allow for the retrospective diagnosis of inborn errors of metabolism in filter cards that have been stored for long periods of time.

The value of the metabolic autopsy in the pediatric hospital setting

OBJECTIVE

To determine the utility of the metabolic autopsy in the hospitalized pediatric patient.

STUDY DESIGN

This was a retrospective review of all metabolic autopsies performed at a large pediatric hospital over a 5-year period. Premortem clinical diagnoses were correlated with autopsy findings and results of postmortem testing.

RESULTS

Of the 23 metabolic autopsies performed, a metabolic disorder was diagnosed before death in 4 and after death by extensive studies initiated before death in 2. In the remaining 17 cases, postmortem samples were inadequate for subsequent enzymatic analysis in 2, a nonmetabolic explanation for symptoms was identified in 4, and no unifying diagnosis could be defined in 8. A metabolic disorder was diagnosed by postmortem tissue study in 3 of the 17 cases; in all 3 of these cases the patient died in the neonatal period after limited premortem investigation for primary lactic acidosis. For the 8 subjects who had undergone an extensive laboratory workup before death, in each case metabolic autopsy failed to establish a diagnosis.

CONCLUSIONS

In a small but significant percentage of cases (18%), the metabolic autopsy successfully identified an undiagnosed metabolic disease. However, metabolic autopsy following an extensive nondiagnostic clinical workup is unlikely to yield a specific metabolic diagnosis.

3-Hydroxyacyl-CoA dehydrogenase and short chain 3-hydroxyacyl-CoA dehydrogenase in human health and disease

3-Hydroxyacyl-CoA dehydrogenase (HAD) functions in mitochondrial fatty acid beta-oxidation by catalyzing the oxidation of straight chain 3-hydroxyacyl-CoAs. HAD has a preference for medium chain substrates, whereas short chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) acts on a wide spectrum of substrates, including steroids, cholic acids, and fatty acids, with a preference for short chain methyl-branched acyl-CoAs. Therefore, HAD should not be referred to as SCHAD. SCHAD is not a member of the HAD family, but instead, belongs to the short chain dehydrogenase/reductase superfamily. Previously reported cases of SCHAD deficiency are due to an inherited HAD deficiency. SCHAD, also known as 17beta-hydroxysteroid dehydrogenase type 10, is important in brain development and aging. Abnormal levels of SCHAD in certain brain regions may contribute to the pathogenesis of some neural disorders. The human SCHAD gene and its protein product, SCHAD, are potential targets for intervention in conditions, such as Alzheimer's disease, Parkinson's disease, and an X-linked mental retardation, that may arise from the impaired degradation of branched chain fatty acid and isoleucine.

Familial hyperinsulinemic hypoglycemia caused by a defect in the SCHAD enzyme of mitochondrial fatty acid oxidation

Inappropriately elevated insulin secretion is the hallmark of persistent hyperinsulinemic hypoglycemia of infancy (PHHI), also denoted congenital hyperinsulinism. Causal mutations have been uncovered in genes coding for the beta-cell's ATP-sensitive potassium channel and the metabolic enzymes glucokinase and glutamate dehydrogenase. In addition, one hyperinsulinemic infant was recently found to have a mutation in the gene encoding short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD), an enzyme participating in mitochondrial fatty acid oxidation. We have studied a consanguineous family with severe neonatal hypoglycemia due to increased insulin levels and where well-established genetic causes of hyperinsulinism had been eliminated. A genome-wide, microsatellite-based screen for homozygous chromosomal segments was performed. Those regions that were inherited in accordance with the presupposed model were searched for mutations in genes encoding metabolic enzymes. A novel, homozygous deletion mutation was found in the gene coding for the SCHAD enzyme. The mutation affected RNA splicing and was predicted to lead to a protein lacking 30 amino acids. The observations at the molecular level were confirmed by demonstrating greatly reduced SCHAD activity in the patients' fibroblasts and enhanced levels of 3-hydroxybutyryl-carnitine in their blood plasma. Urine metabolite analysis showed that SCHAD deficiency resulted in specific excretion of 3-hydroxyglutaric acid. By the genetic explanation of our family's cases of severe hypoglycemia, it is now clear that recessively inherited SCHAD deficiency can result in PHHI. This finding suggests that mitochondrial fatty acid oxidation influences insulin secretion by a hitherto unknown mechanism.

Disorders of fatty acid transport and mitochondrial oxidation: challenges and dilemmas of metabolic evaluation

Inborn errors of fatty acid transport and mitochondrial oxidation (FATMO) have drawn considerable attention in recent years for the rapid pace of discovery of new defects and an ever-increasing spectrum of clinical phenotypes. Several of these disorders are not detected by conventional biochemical investigations, even when a patient is symptomatic with fasting intolerance or functional failure of fatty acid dependent tissue(s). In our view, today's major challenges are the inclusion of FATMO disorders in newborn screening programs and the investigation of the role played by individual disorders in maternal complications of pregnancy, sudden and unexpected death in early life, and pediatric acute/fulminant liver failure. Dilemmas are found in the debate over the limitations, if any, to be imposed on the expansion of newborn screening using tandem mass spectrometry, in the provision of prenatal diagnosis for otherwise treatable disorders, and in the diagnostic workup of "unclassified" cases.