A profile of cerebral and hepatic carnitine, ammonia, and energy metabolism in a model of organic aciduria: BALB/cByJ mouse with short-chain acyl-CoA dehydrogenase deficiency

Short-chain acyl-CoA dehydrogenase deficiency: from gene to cell pathology and possible disease mechanisms

Short-chain acyl-CoA dehydrogenase deficiency (SCADD) is an inherited disorder of mitochondrial fatty acid oxidation that is characterized by the presence of increased butyrylcarnitine and ethylmalonic acid (EMA) concentrations in plasma and urine. Individuals with symptomatic SCADD may show relatively severe phenotype, while the majority of those who are diagnosed through newborn screening by tandem mass spectrometry may remain asymptomatic. As such, the associated clinical symptoms are very diverse, ranging from severe metabolic or neuromuscular disabilities to asymptomatic. Molecular analysis of affected individuals has identified rare gene variants along with two common gene variants, c.511C > T and c.625G > A. In vitro studies have demonstrated that the common variants as well as the great majority of rare variants, which are missense variants, impair folding, that may lead to toxic accumulation of the encoded protein, and/or metabolites, and initiate excessive production of ROS and chronic oxidative stress. It has been suggested that this cell toxicity in combination with yet unknown factors can trigger disease development. This association and the full implications of SCADD are not commonly appreciated. Accordingly, there is a worldwide discussion of the relationship of clinical manifestation to SCADD, and whether SCAD gene variants are disease associated at all. Therefore, SCADD is not part of the newborn screening programs in most countries, and consequently many patients with SCAD gene variants do not get a diagnosis and the possibilities to be followed up during development.

Studies on the chemopreventive effect of carnitine on tumorigenesis in vivo, using two experimental murine models of colon cancer

Carnitine is known for its essential role in intermediary metabolism. In vitro studies suggest that its antioxidant and anti-inflammatory properties are potentially beneficial toward cancer prevention. This study tested effects of carnitine on the development of colon cancer in vivo using 2 murine models: azoxymethane (AOM) treatment as a model of carcinogen-induced colon cancer and a genetically induced model using Apc (Min/+) mice. AOM and Apc (Min/+) mice divided into dietary groups varying in lipid content, with or without carnitine supplementation (0.08%). AOM-exposed mice on a high butterfat diet had significantly increased aberrant crypts (ACF) (9.3 ± 0.88 vs. 6.3 ± 0.65), and macroscopic tumors (3.8 ± 0.95 vs. 2.0 ± 0.25) compared to mice on a control diet. In AOM mice fed the high butterfat diet, carnitine supplementation inhibited ACF (4.9 ± 0.7 vs. 9.3 ± 0.88, P < 0.001), crypt multiciplicity (1.6 ± 0.08 vs. 1.92 ± 0.1, P < 0.01) and tumors (1.5 ± 0.38 vs. 3.8 ± 0.95, P < 0.001). Carnitine supplementation resulted in significantly increased tissue carnitine and acylcarnitine levels. Carnitine inhibited the development of precancerous lesions and macroscopic colonic tumors in AOM-treated mice. However, carnitine did not exert protective effects on intestinal tumors in Apc (Min/+) mice.

Brain transcriptional responses to high-fat diet in Acads-deficient mice reveal energy sensing pathways

Background

How signals from fatty acid metabolism are translated into changes in food intake remains unclear. Previously we reported that mice with a genetic inactivation of Acads (acyl-coenzyme A dehydrogenase, short-chain), the enzyme responsible for mitochondrial beta-oxidation of C4–C6 short-chain fatty acids (SCFAs), shift consumption away from fat and toward carbohydrate when offered a choice between diets. In the current study, we sought to indentify candidate genes and pathways underlying the effects of SCFA oxidation deficiency on food intake in Acads−/− mice.

Methodology/Principal Findings

We performed a transcriptional analysis of gene expression in brain tissue of Acads−/− and Acads+/+ mice fed either a high-fat (HF) or low-fat (LF) diet for 2 d. Ingenuity Pathway Analysis revealed three top-scoring pathways significantly modified by genotype or diet: oxidative phosphorylation, mitochondrial dysfunction, and CREB signaling in neurons. A comparison of statistically significant responses in HF Acads−/− vs. HF Acads+/+ (3917) and Acads+/+ HF vs. LF Acads+/+ (3879) revealed 2551 genes or approximately 65% in common between the two experimental comparisons. All but one of these genes were expressed in opposite direction with similar magnitude, demonstrating that HF-fed Acads-deficient mice display transcriptional responses that strongly resemble those of Acads+/+ mice fed LF diet. Intriguingly, genes involved in both AMP-kinase regulation and the neural control of food intake followed this pattern. Quantitative RT-PCR in hypothalamus confirmed the dysregulation of genes in these pathways. Western blotting showed an increase in hypothalamic AMP-kinase in Acads−/− mice and HF diet increased, a key protein in an energy-sensing cascade that responds to depletion of ATP.

Conclusions

Our results suggest that the decreased beta-oxidation of short-chain fatty acids in Acads-deficient mice fed HF diet produces a state of energy deficiency in the brain and that AMP-kinase may be the cellular energy-sensing mechanism linking fatty acid oxidation to feeding behavior in this model.

Short-chain acyl-coenzyme A dehydrogenase deficiency

Short-chain acyl-CoA dehydrogenase deficiency (SCADD) is a disorder of mitochondrial fatty acid oxidation that leads to the accumulation of butyrylcarnitine and ethylmalonic acid in blood and urine. Originally described with a relatively severe phenotype, most patients are now diagnosed through newborn screening by tandem mass spectrometry and remain asymptomatic. Molecular analysis of affected individuals has identified a preponderance of private inactivating point mutations and one common one present in high frequency in individuals of Ashkenazi Jewish ancestry. In addition, two polymorphic variants have been identified that have little affect on enzyme kinetics but impair folding and stability. Individuals homozygous for one of these variants or compound heterozygous for one of each often show an increased level of ethylmalonic acid excretion that appears not to be clinically significant. The combination of asymptomatic affected newborns and the frequent variants can cause much confusion in evaluating and treating individuals with SCADD. The long-term consequences and the need for chronic therapy remain current topics of contention and investigation.

Short-chain acyl-CoA dehydrogenase gene mutation (c.319C>T) presents with clinical heterogeneity and is candidate founder mutation in individuals of Ashkenazi Jewish origin

We report 10 children (7 male, 3 female), 3 homozygous for c.319C>T mutation and 7 heterozygous for c.319C>T on one allele and c.625G>A variant on the other in the short-chain acyl-CoA dehydrogenase (SCAD) gene (ACADS). All were of Ashkenazi Jewish origin in which group we found a c.319C>T heterozygote frequency of 1:15 suggesting the presence of a founder mutation or selective advantage. Phenotype was variable with onset from birth to early childhood. Features included hypotonia (8/10), developmental delay (8/10), myopathy (4/10) with multicore changes in two and lipid storage in one, facial weakness (3/10), lethargy (5/10), feeding difficulties (4/10) and congenital abnormalities (3/7). One female with multiminicore myopathy had progressive external ophthalmoplegia, ptosis and cardiomyopathy with pneumonia and respiratory failure. Two brothers presented with psychosis, pyramidal signs, and multifocal white matter abnormalities on MRI brain suggesting additional genetic factors. Two other infants also had white matter changes. Elevated butyrylcarnitine (4/8), ethylmalonic aciduria (9/9), methylsuccinic aciduria (6/7), decreased butyrate oxidation in lymphoblasts (2/4) and decreased SCAD activity in fibroblasts or muscle (3/3) were shown. Expression studies of c.319C>T in mouse liver mitochondria showed it to be inactivating. c.625G>A is a common variant in ACADS that may confer disease susceptibility. Five healthy parents were heterozygous for c.319C>T and c.625G>A, suggesting reduced penetrance or broad clinical spectrum. We conclude that the c.319C>T mutation can lead to wide clinical and biochemical phenotypic variability, suggesting a complex multifactorial/polygenic condition. This should be screened for in individuals with multicore myopathy, particularly among the Ashkenazim.

Mice bearing Acads mutation display altered postingestive but not 5-s orosensory response to dietary fat

A previous survey of mouse inbred strains revealed a wide range in self-selected fat intake, from 26 to 83% of energy. The BALB/cByJ strain selected a lower percentage of fat intake (36%) than all other strains tested except for the CAST/Ei. BALB/cByJ mice are deficient in the short-chain acyl-CoA dehydrogenase (SCAD) enzyme due to a spontaneous mutation in Acads. We hypothesized that this deficiency would alter fat appetite and used three behavioral test paradigms to compare the response of BALB/cByKz. Acads -/- and BALB/cByKz. Acads +/+ mice to fat stimuli. First, during 10-day exposure to a macronutrient self-selection diet, Acads -/- mice consumed proportionately less fat and more carbohydrate than Acads +/+ mice, yet total energy intake was similar between strains. Next, in 48-h two-bottle preference tests, Acads +/+ mice displayed a preference for 50% corn oil, but Acads -/- mice did not. Finally, in brief-access taste tests employing successive 5-s presentations of corn oil in an ascending concentration series ending with 50%, there were no effects of strain on total licks, indicating that Acads does not alter acute orosensory response to this fat stimulus. With 15-s presentations, however, the Acads +/+ mice licked more of the 50% oil than Acads -/-, suggesting orosensory effects related to the increased exposure time. In contrast to corn oil, there were no strain differences in licking response to sucrose solution in either the two-bottle or brief-access taste tests. The observation that SCAD-deficient mice display altered postingestive responses to dietary fat provides further evidence for the metabolic control of feeding.

Control of mitochondrial beta-oxidation flux

The control of mitochondrial beta-oxidation, including the delivery of acyl moieties from the plasma membrane to the mitochondrion, is reviewed. Control of beta-oxidation flux appears to be largely at the level of entry of acyl groups to mitochondria, but is also dependent on substrate supply. CPTI has much of the control of hepatic beta-oxidation flux, and probably exerts high control in intact muscle because of the high concentration of malonyl-CoA in vivo. beta-Oxidation flux can also be controlled by the redox state of NAD/NADH and ETF/ETFH(2). Control by [acetyl-CoA]/[CoASH] may also be significant, but it is probably via export of acyl groups by carnitine acylcarnitine translocase and CPT II rather than via accumulation of 3-ketoacyl-CoA esters. The sharing of control between CPTI and other enzymes allows for flexible regulation of metabolism and the ability to rapidly adapt beta-oxidation flux to differing requirements in different tissues.

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.

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.