Functional correction of short-chain acyl-CoA dehydrogenase deficiency in transgenic mice: implications for gene therapy of human mitochondrial enzyme deficiencies

A review of fatty acid oxidation disorder mouse models

Fatty acid oxidation disorders (FAODs) are a family of rare, genetic disorders that affect any part of the fatty acid oxidation pathway. Patients present with severe phenotypes, such as hypoketotic hypoglycemia, cardiomyopathy, and rhabdomyolysis, and currently manage these symptoms by the avoidance of fasting and maintaining a low-fat, high-carbohydrate diet. Because knowledge about FAODs is limited due to the small number of patients, rodent models have been crucial in learning more about these disorders, particularly in studying the molecular mechanisms involved in different phenotypes and in evaluating treatments for patients. The purpose of this review is to present the different FAOD mouse models and highlight the benefits and limitations of using these models. Specifically, we discuss the phenotypes of the available FAOD mouse models, the potential molecular causes of prominent FAOD phenotypes that have been studied using FAOD mouse models, and how FAOD mouse models have been used to evaluate treatments for patients.

Adeno-associated virus-mediated gene therapy for metabolic myopathy

Metabolic myopathies are a diverse group of rare diseases in which impaired breakdown of stored energy leads to profound muscle dysfunction ranging from exercise intolerance to severe muscle wasting. Metabolic myopathies are largely caused by functional deficiency of a single gene and are generally subcategorized into three major types of metabolic disease: mitochondrial, lipid, or glycogen. Treatment varies greatly depending on the biochemical nature of the disease, and unfortunately no definitive treatments exist for metabolic myopathy. Since this group of diseases is inherited, gene therapy is being explored as an approach to personalized medical treatment. Adeno-associated virus-based vectors in particular have shown to be promising in the treatment of several forms of metabolic myopathy. This review will discuss the most recent advances in gene therapy efforts for the treatment of metabolic myopathies.

Cell and gene therapy for genetic diseases: inherited disorders affecting the lung and those mimicking sudden infant death syndrome

Some of the first human gene therapy trials targeted diseases of the lung and provided important information that will continue to help shape future trials. Here we describe both cell and gene therapies for lung diseases such as cystic fibrosis and alpha-1 antitrypsin disorder as well as fatty acid oxidation disorders that mimic sudden infant death syndrome (SIDS). Human clinical gene therapy trials for cystic fibrosis and alpha-1 antitrypsin have been performed using a variety of vectors including adenovirus, adeno-associated virus, and nonviral vectors. No human clinical gene therapy trials have been performed for disorders of fatty acid oxidation; however, important proof-of-principle studies have been completed for multiple fatty acid oxidation disorders. Important achievements have been made and have yet to come for cell and gene therapies for disorders of the lung and those mimicking SIDS.

Pantothenate kinase 1 is required to support the metabolic transition from the fed to the fasted state

Coenzyme A (CoA) biosynthesis is regulated by the pantothenate kinases (PanK), of which there are four active isoforms. The PanK1 isoform is selectively expressed in liver and accounted for 40% of the total PanK activity in this organ. CoA synthesis was limited using a Pank1(-/-) knockout mouse model to determine whether the regulation of CoA levels was critical to liver function. The elimination of PanK1 reduced hepatic CoA levels, and fasting triggered a substantial increase in total hepatic CoA in both Pank1(-/-) and wild-type mice. The increase in hepatic CoA during fasting was blunted in the Pank1(-/-) mouse, and resulted in reduced fatty acid oxidation as evidenced by abnormally high accumulation of long-chain acyl-CoAs, acyl-carnitines, and triglycerides in the form of lipid droplets. The Pank1(-/-) mice became hypoglycemic during a fast due to impaired gluconeogenesis, although ketogenesis was normal. These data illustrate the importance of PanK1 and elevated liver CoA levels during fasting to support the metabolic transition from glucose utilization and fatty acid synthesis to gluconeogenesis and fatty acid oxidation. The findings also suggest that PanK1 may be a suitable target for therapeutic intervention in metabolic disorders that feature hyperglycemia and hypertriglyceridemia.

Metabolism as a complex genetic trait, a systems biology approach: implications for inborn errors of metabolism and clinical diseases

Multifactorial and polygenic inheritance is commonly recognized for many genetic conditions including physical anomalies, complex congenital malformation syndromes, and even common disorders such as adult-onset diabetes mellitus. It has only recently been suggested as a mechanism for inheritance in inborn errors of metabolism. This article reviews the phenomenon of multiple partial enzyme deficiencies leading to clinical relevant biochemical derangements (synergistic heterozygosity) and its implications for other more common disorders such as diabetes and obesity.

Biochemical correction of short-chain acyl-coenzyme A dehydrogenase deficiency after portal vein injection of rAAV8-SCAD

Recombinant adeno-associated viral vectors pseudotyped with serotype 5 and 8 capsids (AAV5 and AAV8) have been shown to be efficient gene transfer reagents for the liver. We have produced AAV5 and AAV8 vectors that express mouse short-chain acyl-CoA dehydrogenase (mSCAD) cDNA under the transcriptional control of the cytomegalovirus-chicken beta-actin hybrid promoter. We hypothesized that these vectors would produce sufficient hepatocyte transduction (after administration via the portal vein) and thus sufficient SCAD enzyme to correct the phenotype observed in the SCAD-deficient (BALB/cByJ) mouse, which includes elevated blood butyrylcarnitine and hepatic steatosis. Ten weeks after portal vein injection into 8-week-old mice, AAV8-treated livers contained acyl-CoA dehydrogenase activity (14.3 mU/mg) toward butyryl-CoA, compared with 7.6 mU/mg in mice that received phosphate-buffered saline. Immunohistochemistry showed expression of mSCAD within rAAV8-mSCAD-transduced hepatocytes, as seen by light microscopy. A significant reduction of circulating butyrylcarnitine was seen in AAV5-mSCAD- and AAV8-mSCAD-injected mice. Magnetic resonance spectroscopy of fasted mice demonstrated a significant reduction in relative lipid content within the livers of AAV8-mSCAD-treated mice. These results demonstrate biochemical correction of SCAD deficiency after AAV8-mediated SCAD gene delivery.

Handling of human short-chain acyl-CoA dehydrogenase (SCAD) variant proteins in transgenic mice

To investigate the in vivo handling of human short-chain acyl-CoA dehydrogenase (SCAD) variant proteins, three transgenic mouse lines were produced by pronuclear injection of cDNA encoding the wild-type, hSCAD-wt, and two disease causing folding variants hSCAD-319C>T and hSCAD-625G>A. The transgenic mice were mated with an SCAD-deficient mouse strain (BALB/cByJ) and, in the second generation, three mouse lines were obtained without endogenous SCAD expression but harboring hSCAD-wt, hSCAD-319C>T, and hSCAD-625G>A transgenes, respectively. All three lines had expression of the transgene at the RNA level in liver, muscle or brain tissues. Expression at the protein level was detected only in the brain tissue of hSCAD-wt mice, but here it was significantly higher than the level of endogenous SCAD protein in control mouse brains--in correlation with expression at the RNA level. The results may indicate that the two hSCAD folding variants are degraded by the mouse mitochondrial protein quality control system. Indeed, pulse-chase studies with isolated mitochondria revealed that soluble variant hSCAD protein was rapidly eliminated. This is in agreement with the fact that no disease phenotype developed for any of the lines transgenic for the hSCAD folding variants. The indicated remarkable efficiency of the mouse protein quality control system in the degradation of SCAD folding variants should be further substantiated and investigated, since it might indicate ways to prevent disease-causing effects.

Systemic correction of a fatty acid oxidation defect by intramuscular injection of a recombinant adeno-associated virus vector

Mitochondrial beta-oxidation of fatty acids is required to meet physiologic energy requirements during illness and periods of fasting or physiologic stress, and is most active in liver and striated muscle. Acyl-CoA dehydrogenases of varying chain-length specificities represent the first step in the mitochondria for each round of beta-oxidation, each of which removes two-carbon units as acetyl-CoA for entry into the tricarboxylic acid cycle. We have used recombinant adeno-associated virus (rAAV) vectors expressing short-chain acyl-CoA dehydrogenase (SCAD) to correct the accumulation of fatty acyl-CoA intermediates in deficient cell lines. The rAAV-SCAD vector was then packaged into either rAAV serotype 1 or 2 capsids and injected intramuscularly into SCAD-deficient mice. A systemic effect was observed as judged by restoration of circulating butyryl- carnitine levels to normal. Total lipid content at the injection site was also decreased as demonstrated by noninvasive magnetic resonance spectroscopy (MRS). SCAD enzyme activity in the injected muscle was found at necropsy to be above the normal control mouse level. This study is the first to demonstrate the systemic correction of a fatty acid oxidation disorder with rAAV and the utility of MRS as a noninvasive method to monitor SCAD correction after in vivo gene therapy.

In vitro characterization and in vivo expression of human very-long chain acyl-CoA dehydrogenase

Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is a disorder of fatty acid beta-oxidation that can present at any age with cardiomyopathy, rhabdomyolysis, hepatic dysfunction, and/or nonketotic hypoglycemia. Through the expansion of newborn screening programs an increasing number of individuals with VLCAD deficiency are being identified prior to the onset of symptoms allowing early initiation of therapy. The development of a safe, durable, and effective VLCAD gene delivery system for use at the time of diagnosis could result in a significant improvement in the quality and duration of life for patients with VLCAD deficiency. To this end, we developed a construct containing the human VLCAD cDNA under the control of the strong CMV promoter (pCMV-hVLCAD). A novel rabbit polyclonal anti-VLCAD antibody was prepared using a 24 amino-acid peptide unique to the human VLCAD protein to study human VLCAD expression in immune competent mice. Antibody specificity was demonstrated in Western blots of human VLCAD deficient fibroblasts and in pCMV-hVLCAD transiently transfected VLCAD deficient fibroblasts. Transfected fibroblasts showed correction of the metabolic block as demonstrated by normalization of C14- and C16-acylcarnitine species in cell culture media and restoration of VLCAD activity in cells. Following tail vein injection of pCMV-hVLCAD into mice, we demonstrated expression of hVLCAD in liver. Altogether, these steps are important in the development of a durable gene therapy for VLCAD deficiency.

In vitro correction of medium chain acyl CoA dehydrogenase deficiency with a recombinant adenoviral vector

Defects of mitochondrial beta-oxidation are a growing group of disorders with variable clinical presentations ranging from mild hypotonia to sudden infant death. Current therapy involves avoidance of fasting, dietary restrictions, and cofactor supplementation. Unfortunately, times of acute illness and noncompliance can interfere with these therapies and result in a rapid clinical decline. The development of a safe, durable, and effective gene delivery system remains an attractive alternative therapy for individuals with these disorders. To this end, a recombinant first-generation adenovirus vector (Ad/cmv-hMCAD) has been prepared that constitutively expresses the human medium chain acyl CoA dehydrogenase (MCAD) protein under the control of the CMV promoter and bovine polyadenylation signal. Characterization of human fibroblasts deficient in MCAD infected with Ad/cmv-hMCAD including Western analysis, immunohistological staining visualized with confocal microscopy, electron transfer protein (ETF) reduction assay, and palmitate loading studies was performed. Infection of MCAD deficient fibroblast with Ad/cmv-hmcad resulted in the production of a 55kDa protein that co-localized in cells with a mitochondrial marker. Extracts prepared from Ad/cmv-hMCAD infected deficient fibroblasts demonstrated correction of the block seen in the MCAD catalyzed reduction of ETF in the presence of octanoyl CoA. Finally, MCAD deficient fibroblasts infected with increasing amounts of Ad/cmv-hMCAD showed a stepwise improvement of the abnormal acylcarnitine profile exhibited by the deficient cells. Together these studies demonstrate our ability to express and monitor the expression of MCAD in treated cells and support further in vivo murine studies to assess toxicity and duration of correction with this and other MCAD recombinant vectors.

Fatty acid oxidation disorders

Genetic disorders of mitochondrial fatty acid beta-oxidation have been recognized within the last 20 years as important causes of morbidity and mortality, highlighting the physiological significance of fatty acids as an energy source. Although the mammalian mitochondrial fatty acid-oxidizing system was recognized at the beginning of the last century, our understanding of its exact nature remains incomplete, and new components are being identified frequently. Originally described as a four-step enzymatic process located exclusively in the mitochondrial matrix, we now recognize that long-chain-specific enzymes are bound to the inner mitochondrial membrane, and some enzymes are expressed in a tissue-specific manner. Much of our new knowledge of fatty acid metabolism has come from the study of patients who were diagnosed with single-gene autosomal recessive defects, a situation that seems to be further evolving with the emergence of phenotypes determined by combinations of multiple genetic and environmental factors. This review addresses the normal process of mitochondrial fatty acid beta-oxidation and discusses the clinical, metabolic, and molecular aspects of more than 20 known inherited diseases of this pathway that have been described to date.

Mouse models for disorders of mitochondrial fatty acid beta-oxidation

Mitochondrial beta-oxidation of fatty acids is vital for energy production in periods of fasting and other metabolic stress. Human patients have been identified with inherited disorders of mitochondrial beta-oxidation of fatty acids with enzyme deficiencies identified at many of the steps in this pathway. Although these patients exhibit a range of disease processes, Reye-like illness (hypoketotic-hypoglycemia, hyperammonemia and fatty liver) and cardiomyopathy are common findings. There have been several mouse models developed to aid in the study of these disease conditions. The characterized mouse models include inherited deficiencies of very long-chain acyl-CoA dehydrogenase, long-chain acyl-CoA dehydrogenase, short-chain acyl-CoA dehydrogenase, mitochondrial trifunctional protein-alpha, and medium-/short-chain hydroxyacyl-CoA dehydrogenase. Mouse mutants developed, but presently incompletely characterized as models, include carnitine palmitoyltransferase-1a and medium-chain acyl-CoA dehydrogenase deficiencies. In general, the mouse models of disorders of mitochondrial fatty acid beta-oxidation have shown clinical signs that include Reye-like syndrome and cardiomyopathy, and many are cold intolerant. It is expected that these mouse models will provide vital contributions in understanding the mechanisms of disease pathogenesis of fatty acid oxidation disorders and the development of appropriate treatments and supportive care.

Characterization of altered myocardial fatty acid metabolism in patients with inherited cardiomyopathy

Inherited defects in myocardial long-chain fatty acid metabolism are increasingly recognized as a cause of cardiomyopathy and sudden death in children. To evaluate whether the phenotypic expression of these genetic diseases could be delineated using positron emission tomography (PET), 11 patients with inherited defects in fatty acid metabolism were evaluated and results were compared with those of 6 nonaffected siblings. Myocardial perfusion, myocardial oxygen consumption (MVO2), and long-chain fatty acid metabolism were determined noninvasively with PET using quantitative mathematical models. There were no differences in haemodynamics, perfusion, MVO2 or plasma substrate levels between groups. Patients with defects in enzymes of fatty acid beta-oxidation (acyl-CoA dehydrogenase and 3-hydroxyacyl-CoA dehydrogenase deficiencies) (n = 5) had diminished myocardial palmitate oxidation compared with healthy siblings (3.2 +/- 3.0 vs. 13.0 +/- 5.6 nmol/g per min, p < 0.03) and a decrease in the percentage of MVO2 accounted for by palmitate (2% +/- 3% vs. 9% +/- 5%, p < 0.04). In these patients, extracted palmitate was shunted into a slow-turnover compartment (predominantly reflecting esterification to triglycerides) with expansion of palmitate in that pool (185 +/- 246 compared with 27 +/- 67 nmol/g in healthy siblings,p < 0.02). In contrast, myocardium of patients with carnitine deficiency (n = 6) (all on oral carnitine therapy) had normal palmitate extraction but expansion of the interstitial/cytosolic fatty acid pool (617 +/- 399 vs. 261 +/- 73 nmol/g in healthy siblings, p < 0.04), suggesting different mechanisms for handling upstream fatty acyl intermediates. Thus, PET can be used to noninvasively assess abnormal myocardial handling of fatty acids in patients with inherited defects of metabolism. This approach should be useful in the assessment of altered myocardial fatty acid metabolism associated with cardiomyopathy as well as for evaluating the efficacy of therapeutic interventions in affected patients.

Lessons learned from the mouse model of short-chain acyl-CoA dehydrogenase deficiency

The SCAD deficient mouse model has been useful to investigate mechanisms of deficient fatty acid oxidation disease in human patients. This mouse model has been thoroughly characterized and is readily available from the Jackson Laboratory. Using the new technologies of gene-knockout mouse modeling, we envisage developing additional members of the acyl-CoA dehydrogenase family of enzyme deficiencies in mice and furthering our understanding of fatty acid metabolism in health and disease.

Formation of propionate after short-term ethanol treatment and its interaction with the carnitine pool in rat

Organic acidurias are genetic disorders of mitochondrial metabolism that lead to the accumulation in tissues and biological fluids of organic acids. It has been demonstrated that interaction of carnitine with the cellular CoA pool, through the production of acyl-carnitines, is potentially critical for maintaining normal cellular metabolism under conditions of impaired acyl-CoA use and that exposure of humans and other mammals to ethanol effects leads to impairment of mitochondrial function. The aim of the present study was to evaluate the role of ethanol on urinary excretion of short-chain organic acids and endogenous carnitines in rats. The data reported show that ethanol significantly increases urinary excretion of propionate, methylmalonate, as well as free acetate, butyrate, pyruvate, lactate, and beta-hydroxybutyrate. Furthermore, the increased formation of propionate and methylmalonate was dependent on the dose of ethanol; did not require the metabolism of ethanol, as was shown in experiments with pyrazole treatment of ethanol rats; and appears to be mediated by beta-adrenergic mechanisms because propranolol almost completely suppresses propionate accumulation. Alcohol administration also increased excretion of specific acyl-carnitines, corresponding to the accumulating acyl groups, whereas excretion of free carnitine was significantly reduced, with respect to control values. The data presented indicate that the short-term ethanol administration is associated with increased excretion of selected organic acids. This study suggests that endogenous carnitine pool might play a role against the deleterious effects of accumulating short-chain organic acids.

Defects in mitochondrial beta-oxidation of fatty acids

Mitochondrial beta-oxidation of fatty acids generates energy by direct electron transfer at the dehydrogenase steps along with the ultimate product of acetyl-coenzyme A that can be further oxidized for ATP synthesis, or conversion to ketone bodies. This review describes the human inborn errors of this pathway and recent results concerning the development and use of mouse models of these inherited enzyme deficiencies.