Molecular characterization of L-CPT I deficiency in six patients: insights into function of the native enzyme

Prolonged Cholestatic Jaundice Associated with Carnitine Palmitoyltransferase IA Deficiency

Carnitine palmitoyltransferase 1A (CPT1A) deficiency is a type of fatty acid oxidation disorder in which long chain fatty acids cannot be transported into mitochondria for further processing and storage in our body. Typically, the patients present with lethargy, hypoglycemia, and raised serum transaminase levels before 2 years of age. Cholestatic jaundice as manifestation of this deficiency has been reported rarely; here, we report an adolescent male with CPT1A deficiency who developed prolonged cholestatic jaundice following a febrile illness.

Genetic Features of Lipid and Carbohydrate Metabolism in Arctic Peoples

Prolonged adaptation of ancestors of indigenous peoples of the Far North of Asia and America to extreme natural and climatic conditions of the Arctic has resulted in changes in genes controlling various metabolic processes. However, most genetic variability observed in the Eskimo and Paleoasians (the Chukchi and Koryaks) is related to adaptation to the traditional Arctic diet, which is rich in lipids and proteins but extremely poor in plant carbohydrates. The results of population genetic studies have demonstrated that specific polymorphic variants in genes related to lipid metabolism (CPT1A, FADS1, FADS2, and CYB5R2) and carbohydrate metabolism (AMY1, AMY2A, and SI) are prevalent in the Eskimo and Paleoasian peoples. When individuals deviate from their traditional dietary patterns, the aforementioned variants of genetic polymorphism can lead to the development of metabolic disorders. American Eskimo-specific variants in genes related to glucose metabolism (TBC1D and ADCY) significantly increase the risk of developing type 2 diabetes. These circumstances indicate the necessity for a large-scale genetic testing of indigenous population of the Far North and the need to study the biochemical and physiological consequences of genetically determined changes in the activity of enzymes of lipid and carbohydrate metabolism.

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.

Ketohexokinase-C regulates global protein acetylation to decrease carnitine palmitoyltransferase 1a-mediated fatty acid oxidation

BACKGROUND & AIMS

The consumption of sugar and a high-fat diet (HFD) promotes the development of obesity and metabolic dysfunction. Despite their well-known synergy, the mechanisms by which sugar worsens the outcomes associated with a HFD are largely elusive.

METHODS

Six-week-old, male, C57Bl/6 J mice were fed either chow or a HFD and were provided with regular, fructose- or glucose-sweetened water. Moreover, cultured AML12 hepatocytes were engineered to overexpress ketohexokinase-C (KHK-C) using a lentivirus vector, while CRISPR-Cas9 was used to knockdown CPT1α. The cell culture experiments were complemented with in vivo studies using mice with hepatic overexpression of KHK-C and in mice with liver-specific CPT1α knockout. We used comprehensive metabolomics, electron microscopy, mitochondrial substrate phenotyping, proteomics and acetylome analysis to investigate underlying mechanisms.

RESULTS

Fructose supplementation in mice fed normal chow and fructose or glucose supplementation in mice fed a HFD increase KHK-C, an enzyme that catalyzes the first step of fructolysis. Elevated KHK-C is associated with an increase in lipogenic proteins, such as ACLY, without affecting their mRNA expression. An increase in KHK-C also correlates with acetylation of CPT1α at K508, and lower CPT1α protein in vivo. In vitro, KHK-C overexpression lowers CPT1α and increases triglyceride accumulation. The effects of KHK-C are, in part, replicated by a knockdown of CPT1α. An increase in KHK-C correlates negatively with CPT1α protein levels in mice fed sugar and a HFD, but also in genetically obese db/db and lipodystrophic FIRKO mice. Mechanistically, overexpression of KHK-C in vitro increases global protein acetylation and decreases levels of the major cytoplasmic deacetylase, SIRT2.

CONCLUSIONS

KHK-C-induced acetylation is a novel mechanism by which dietary fructose augments lipogenesis and decreases fatty acid oxidation to promote the development of metabolic complications.

IMPACT AND IMPLICATIONS

Fructose is a highly lipogenic nutrient whose negative consequences have been largely attributed to increased de novo lipogenesis. Herein, we show that fructose upregulates ketohexokinase, which in turn modifies global protein acetylation, including acetylation of CPT1a, to decrease fatty acid oxidation. Our findings broaden the impact of dietary sugar beyond its lipogenic role and have implications on drug development aimed at reducing the harmful effects attributed to sugar metabolism.

Inhibition of carnitine palmitoyl-transferase 1 is a potential target in a mouse model of Parkinson's disease

Glucose metabolism is dysregulated in Parkinson's disease (PD) causing a shift toward the metabolism of lipids. Carnitine palmitoyl-transferase 1A (CPT1A) regulates the key step in the metabolism of long-chain fatty acids. The aim of this study is to evaluate the effect of downregulating CPT1, either genetically with a Cpt1a P479L mutation or medicinally on PD using chronic rotenone mouse models using C57Bl/6J and Park2 knockout mice. We show that Cpt1a P479L mutant mice are resistant to rotenone-induced PD, and that inhibition of CPT1 is capable of restoring neurological function, normal glucose metabolism, and alleviate markers of PD in the midbrain. Furthermore, we show that downregulation of lipid metabolism via CPT1 alleviates pathological motor and non-motor behavior, oxidative stress, and disrupted glucose homeostasis in Park2 knockout mice. Finally, we confirm that rotenone induces gut dysbiosis in C57Bl/6J and, for the first time, in Park2 knockout mice. We show that this dysbiosis is alleviated by the downregulation of the lipid metabolism via CPT1.

Mitochondrial CPT1A: Insights into structure, function, and basis for drug development

Carnitine Palmitoyl-Transferase1A (CPT1A) is the rate-limiting enzyme in the fatty acid β-oxidation, and its deficiency or abnormal regulation can result in diseases like metabolic disorders and various cancers. Therefore, CPT1A is a desirable drug target for clinical therapy. The deep comprehension of human CPT1A is crucial for developing the therapeutic inhibitors like Etomoxir. CPT1A is an appealing druggable target for cancer therapies since it is essential for the survival, proliferation, and drug resistance of cancer cells. It will help to lower the risk of cancer recurrence and metastasis, reduce mortality, and offer prospective therapy options for clinical treatment if the effects of CPT1A on the lipid metabolism of cancer cells are inhibited. Targeted inhibition of CPT1A can be developed as an effective treatment strategy for cancers from a metabolic perspective. However, the pathogenic mechanism and recent progress of CPT1A in diseases have not been systematically summarized. Here we discuss the functions of CPT1A in health and diseases, and prospective therapies targeting CPT1A. This review summarizes the current knowledge of CPT1A, hoping to prompt further understanding of it, and provide foundation for CPT1A-targeting drug development.

Novel mutation in carnitine palmitoyltransferase 1A detected through newborn screening for a presymptomatic case in China: a case report

Background

Carnitine palmitoyltransferase 1A (CPT1A) deficiency is a rare mitochondrial fatty acid oxidation (FAO) disorder that results in hypoketotic hypoglycemia and hepatic encephalopathy. It is caused by mutation in CPT1A. To date, only two symptomatic cases of CPT1A deficiency have been reported in China.

Case presentation

A newborn male, without any disease-related clinical manifestations, was diagnosed with CPT1A deficiency through newborn screening. Increased free carnitine levels and a significantly increased C0/(C16 + C18) ratio were detected by tandem mass spectrometry, and subsequently, mutations in CPT1A were found by gene sequence analysis. The patient was advised a low-fat, high-protein diet and followed up regularly. During three-years of follow-up since, the patient showed normal growth velocity and developmental milestones. Whole-exome sequence identified two mutations, c.2201 T > C (p.F734S) and c.1318G > A (p.A440T), in the patient. The c.2201 T > C mutation, which has been reported previously, was inherited from his father, while the c.1318G > A, a novel mutation, was inherited from his mother. The amino acid residues encoded by original sequences are highly conserved across different species. These mutations slightly altered the three-dimensional structure of the protein, as analyzed by molecular modeling, suggesting that they may be pathogenic.

Conclusion

This is the first case of CPT1A deficiency detected through newborn screening based on diagnostic levels of free carnitine, in China. Three years follow-up suggested that early diagnosis and diet management may improve the prognosis in CPT1A patient. In addition, we identified a novel mutation c.1318G > A in CPT1A,and a possible unique to Chinese lineage mutation c.2201 T > C. Our findings have expanded the gene spectrum of this rare condition and provided a basis for family genetic counseling and prenatal diagnosis.

Downregulating carnitine palmitoyl transferase 1 affects disease progression in the SOD1 G93A mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease characterized by death of motor neurons. The etiology and pathogenesis remains elusive despite decades of intensive research. Herein, we report that dysregulated metabolism plays a central role in the SOD1 G93A mouse model mimicking ALS. Specifically, we report that the activity of carnitine palmitoyl transferase 1 (CPT1) lipid metabolism is associated with disease progression. Downregulation of CPT1 activity by pharmacological and genetic methods results in amelioration of disease symptoms, inflammation, oxidative stress and mitochondrial function, whereas upregulation by high-fat diet or corticosterone results in a more aggressive disease progression. Finally, we show that downregulating CPT1 shifts the gut microbiota communities towards a protective phenotype in SOD1 G93A mice. These findings reveal that metabolism, and specifically CPT1 lipid metabolism plays a central role in the SOD1 G93A mouse model and shows that CPT1 might be a therapeutic target in ALS.

Association of the CPT1A p.P479L Metabolic Gene Variant With Childhood Respiratory and Other Infectious Illness in Nunavut

Objective: Infectious illness, including lower respiratory tract infection (LRTI), is a leading cause of childhood morbidity and infant mortality in Inuit children in Nunavut Canada. The carnitine palmitoyltransferase 1A (CPT1A) p.P479L variant is common in arctic Indigenous populations of Alaska, Canada, and Greenland. CPT1A is a fatty acid oxidation enzyme expressed in the liver, immunocytes and other tissues, and is needed to use fats for energy during fasting. Previous association of the variant with early childhood infectious illness and infant death has been challenged because of sample size and limited adjustment for confounders. We evaluated whether the p.P479L variant is associated with infectious illness in Inuit children of Nunavut, Canada. Methods: We conducted a retrospective clinical chart review of 2,225 Inuit children (0-5 years) for infectious illness (including otitis media, gastroenteritis, and hospital admission for LRTI), prenatal, perinatal, and socioeconomic indicators, subsequently linking to CPT1A genotype. Multivariable logistic regression adjusted for birth characteristics, breastfeeding, maternal smoking, food insecurity, and socioeconomic indicators. Results: Overall, 27% of children were hospitalized for LRTI, 86% had otitis media and 50% had gastroenteritis. The p.P479L allele frequency was 0.82. In multivariable analysis, p.P479L homozygosity was associated with LRTI admission (aOR:2.88 95%CI:1.46-5.64), otitis media (aOR:1.83, 95%CI:1.05-3.21), and gastroenteritis (aOR:1.74, 95%CI:1.09-2.77), compared to non-carriers. Conclusion: Children homozygous for the p.P479L variant were more likely to experience infectious illness than non-carriers, including hospitalization for respiratory tract infections. Given the role of CPT1A in immunocytes, our findings indicate that more study is needed to determine if there is a role of the variant in immune response. Continued Inuit involvement is essential when considering next steps.

Genetic variants for personalised management of very low carbohydrate ketogenic diets

The ketogenic diet (KD) is a low-carbohydrate, high-fat, adequate-protein diet proven to be effective for the reversal of obesity, metabolic syndrome and type 2 diabetes, and holding therapeutic potential for the prevention and treatment of other chronic diseases. Genetic and dynamic markers of KD response may help to identify individuals most likely to benefit from KD and point to individuals at higher risk for adverse health outcomes. Here, we provide a clinician-friendly review of state-of-the-art research on biomarkers of KD response for a variety of outcomes including weight loss, body composition and cognitive performance drawing data from both intervention trials and case reports of rare inborn errors of metabolism. We also present a selection of the most promising candidate genes to evaluate in future studies and discuss key aspects of study design and variant interpretation that may help accelerate the implementation of these biomarkers in clinical practice.

Genetic study of the Arctic CPT1A variant suggests that its effect on fatty acid levels is modulated by traditional Inuit diet

Several recent studies have found signs of recent selection on the carnitine palmitoyl-transferase 1A (CPT1A) gene in the ancestors of Arctic populations likely as a result of their traditional diet. CPT1A is involved in fatty acid transportation and is known to affect circulating fatty acid profiles in Inuit as does the unique traditional diet rich in marine animals. We aimed to assess which fatty acids may have driven the selection of rs80356779, a c.1436C>T (p.(Pro479Leu)) variant in CPT1A, by analyzing a potential interaction between the variant and traditional Inuit diet. We included 3005 genome-wide genotyped individuals living in Greenland, who had blood cell membrane fatty acid levels measured. Consumption of 25 traditional food items was expressed as percentage of total energy intake. We tested for CPT1A × traditional diet interaction while taking relatedness and admixture into account. Increasing intakes of traditional diet was estimated to attenuate the effect of 479L on 20:3 omega-6 levels (p = 0.000399), but increase the effect of the variant on 22:5 omega-3 levels (p = 0.000963). The 479L effect on 22:5 omega-3 more than doubled in individuals with a high intake of traditional diet (90% percentile) compared with individuals with a low intake (10% percentile). Similar results were found when assessing interactions with marine foods. Our results suggest that the association between traditional diet and blood cell fatty acid composition is affected by the CPT1A genotype, or other variants in linkage disequilibrium, and support the hypothesis that omega-3 fatty acids may have been important for adaptation to the Arctic diet.

Schizophrenic Psychosis Symptoms in a Background of Mild-To-Moderate Carnitine Palmitoyltransferase II Deficiency: A Case Report

Schizophrenia is a multifaceted mental illness characterized by cognitive and neurobehavioral abnormalities. Carnitine palmitoyltransferase II (CPT II) deficiency is a metabolic disorder resulting in impaired transport of long-chain fatty acids from the cytosol to the mitochondrial inner membrane, where fatty acid β-oxidation takes place. Here, we present an interesting clinical case of an adolescent male that presented with psychosis and a history of mild-to-moderate CPT II deficiency. To identify germline genetic variation that may contribute to the phenotypes observed, we performed whole-exome sequencing on DNA from the proband, unaffected fraternal twin, and biological parents. The proband was identified to be homozygous for the p.Val368Ile and heterozygous for the p.Met647Val variant in CPT2. Each of these variants are benign on their own; however, their combined effect is unclear. Further, variation was identified in the dopamine β-hydroxylase (DBH) gene (c.339+2T>C), which may contribute to decreased activity of DBH; however, based on the patient’s presentation, severe DBH deficiency is unlikely. In conclusion, the variants identified in this study do not clearly explain the observed patient phenotypes, indicating that the complex phenotypes are likely caused by an interplay of genetic and environmental factors that warrant further investigation.

A patient with atypical presentation of chronic hepatosteatosis harboring a novel variant in the CPT1A gene

Carnitine palmitoyltransferase 1A (CPT1A) deficiency is a rare disorder of hepatic long-chain fatty acid oxidation. Most patients with CPT1A deficiency present with hypoketotic hypoglycemia and hepatic encephalopathy. We describe an atypical case of an 8-year-old male with CPT1A deficiency presenting with chronic liver steatosis and cirrhosis. He also had a history of developmental delay, autism spectrum disorder, and mild dysmorphic features of unknown cause. His newborn screening test suggested CPT1A deficiency, but confirmatory biochemical testing was not conclusive. The patient never experienced a metabolic crisis. At age six, hepatomegaly was detected. Further investigations showed transaminitis, hepatosteatosis and cirrhosis. Repeat acylcarnitine profile and total/free carnitine were consistent with CPT1A deficiency. The CPTI enzyme activity was 18% of normal on fibroblast enzyme assay. A novel homozygous variant in the CPT1A gene, c.1394G > A (p.Gly465Glu) was identified from whole-exome sequencing. To our knowledge, the patient is the first reported individual with CPT1A deficiency and chronic liver steatosis and fibrosis. Developmental delay and autistic spectrum disorder are not typical features of CPT1A deficiency, given that the patient never experienced any metabolic decompensation.

Characterization of Exome Variants and Their Metabolic Impact in 6,716 American Indians from the Southwest US

Applying exome sequencing to populations with unique genetic architecture has the potential to reveal novel genes and variants associated with traits and diseases. We sequenced and analyzed the exomes of 6,716 individuals from a Southwestern American Indian (SWAI) population with well-characterized metabolic traits. We found that the SWAI population has distinct allelic architecture compared to populations of European and East Asian ancestry, and there were many predicted loss-of-function (pLOF) and nonsynonymous variants that were highly enriched or private in the SWAI population. We used pLOF and nonsynonymous variants in the SWAI population to evaluate gene-burden associations of candidate genes from European genome-wide association studies (GWASs) for type 2 diabetes, body mass index, and four major plasma lipids. We found 19 significant gene-burden associations for 11 genes, providing additional evidence for prioritizing candidate effector genes of GWAS signals. Interestingly, these associations were mainly driven by pLOF and nonsynonymous variants that are unique or highly enriched in the SWAI population. Particularly, we found four pLOF or nonsynonymous variants in APOB, APOE, PCSK9, and TM6SF2 that are private or enriched in the SWAI population and associated with low-density lipoprotein (LDL) cholesterol levels. Their large estimated effects on LDL cholesterol levels suggest strong impacts on protein function and potential clinical implications of these variants in cardiovascular health. In summary, our study illustrates the utility and potential of exome sequencing in genetically unique populations, such as the SWAI population, to prioritize candidate effector genes within GWAS loci and to find additional variants in known disease genes with potential clinical impact.

Neonatal hypoglycemia and the CPT1A P479L variant in term newborns: A retrospective cohort study of Inuit newborns from Kivalliq Nunavut

Introduction

Neonatal hypoglycemia (NH) in the first days of life can largely be prevented by recognizing those at risk and managing accordingly. The CPT1A P479L variant is prevalent in northern Indigenous populations and is a possible risk factor for hypoglycemia. We report on NH incidence in the Kivalliq region of Nunavut, where all Inuit newborns are screened for NH.

Methods

We reviewed clinical charts of 728 Inuit newborns from Kivalliq (January 1, 2010 to December 31, 2013) for blood glucose (BG) levels and infant/maternal characteristics, linking to CPT1A genotype; 616 newborns had BG data from 2 to 48 hours of life. NH was defined using Canadian Paediatric Society guidelines (≤2.0 mmol/L at 2 hours, <2.6 mmol/L at 2 to 48 hours).

Results

NH was documented in 21.4% overall, 24.4% of at-risk newborns and 19.5% of term newborns with no risk factors (≥37 weeks gestation, term-NRF). NH was documented in 22.0% of CPT1A P479L homozygous, 19.8% of P479L heterozygous and 4.8% of noncarrier term-NRF newborns. With multivariable logistic regression, the adjusted ORs for developing NH in term-NRF newborns was 4.97 for CPT1A P479L homozygotes (95% confidence interval [CI]:0.65-38.35, P=0.19) and 4.71 for P479L heterozygotes (95% CI:0.57-37.89, P=0.15).

Conclusion

Term-NRF newborns had a higher NH incidence than previously reported, similar to that for at-risk newborns, possibly due to the CPT1A P479L variant. Since only Inuit newborns from Kivalliq are screened for NH, further study of long-term outcomes of NH in this population and the role of the P479L variant are warranted to determine if neonatal BG screening is indicated in all Inuit newborns.

Inuit metabolism revisited: what drove the selective sweep of CPT1a L479?

This article reassesses historical studies of Inuit metabolism in light of recent developments in evolutionary genetics. It discusses the possible selective advantage of a variant of CPT1a, which encodes the rate limiting enzyme in hepatic fatty acid oxidation. The L479 variant of CPT1a underwent one of the strongest known selective sweeps in human history and is specific to Inuit and Yu'pik populations. Recent hypotheses predict that this variant may have been selected in response to possible detrimental effects of chronic ketosis in communities with very low carbohydrate consumption. Assessing these hypotheses alongside several alternative explanations of the selective sweep, this article challenges the notion that the selection of L479 is linked to predicted detrimental effects of ketosis. Bringing together for the first time data from biochemical, metabolic, and physiological studies inside and outside the Inuit sphere, it aims to provide a broader interpretative framework and a more comprehensive way to understand the selective sweep. It suggests that L479 may have provided a selective advantage in glucose conservation as part of a metabolic adaptation to very low carbohydrate and high protein consumption, but not necessarily a ketogenic state, in an extremely cold environment. A high intake of n-3 fatty acids may be linked to selection through the mitigation of a detrimental effect of the mutation that arises in the fasted state. The implications of these conclusions for our broader understanding of very low carbohydrate metabolism, and for dietary recommendations for Inuit and non-Inuit populations, are discussed.

CPT1A-mediated Fat Oxidation, Mechanisms, and Therapeutic Potential

Energy homeostasis during fasting or prolonged exercise depends on mitochondrial fatty acid oxidation (FAO). This pathway is crucial in many tissues with high energy demand and its disruption results in inborn FAO deficiencies. More than 15 FAO genetic defects have been currently described, and pathological variants described in circumpolar populations provide insights into its critical role in metabolism. The use of fatty acids as energy requires more than 2 dozen enzymes and transport proteins, which are involved in the activation and transport of fatty acids into the mitochondria. As the key rate-limiting enzyme of FAO, carnitine palmitoyltransferase I (CPT1) regulates FAO and facilitates adaptation to the environment, both in health and in disease, including cancer. The CPT1 family of proteins contains 3 isoforms: CPT1A, CPT1B, and CPT1C. This review focuses on CPT1A, the liver isoform that catalyzes the rate-limiting step of converting acyl-coenzyme As into acyl-carnitines, which can then cross membranes to get into the mitochondria. The regulation of CPT1A is complex and has several layers that involve genetic, epigenetic, physiological, and nutritional modulators. It is ubiquitously expressed in the body and associated with dire consequences linked with genetic mutations, metabolic disorders, and cancers. This makes CPT1A an attractive target for therapeutic interventions. This review discusses our current understanding of CPT1A expression, its role in heath and disease, and the potential for therapeutic opportunities targeting this enzyme.

Peroxiredoxin 6 Down-Regulation Induces Metabolic Remodeling and Cell Cycle Arrest in HepG2 Cells

Peroxiredoxin 6 (Prdx6) is the only member of 1-Cys subfamily of peroxiredoxins in human cells. It is the only Prdx acting on phospholipid hydroperoxides possessing two additional sites with phospholipase A2 (PLA2) and lysophosphatidylcholine-acyl transferase (LPCAT) activities. There are contrasting reports on the roles and mechanisms of multifunctional Prdx6 in several pathologies and on its sensitivity to, and influence on, the redox environment. We have down-regulated Prdx6 with specific siRNA in hepatoblastoma HepG2 cells to study its role in cell proliferation, redox homeostasis, and metabolic programming. Cell proliferation and cell number decreased while cell volume increased; import of glucose and nucleotide biosynthesis also diminished while polyamines, phospholipids, and most glycolipids increased. A proteomic quantitative analysis suggested changes in membrane arrangement and vesicle trafficking as well as redox changes in enzymes of carbon and glutathione metabolism, pentose-phosphate pathway, citrate cycle, fatty acid metabolism, biosynthesis of aminoacids, and Glycolysis/Gluconeogenesis. Specific redox changes in Hexokinase-2 (HK2), Prdx6, intracellular chloride ion channel-1 (CLIC1), PEP-carboxykinase-2 (PCK2), and 3-phosphoglycerate dehydrogenase (PHGDH) are compatible with the metabolic remodeling toward a predominant gluconeogenic flow from aminoacids with diversion at 3-phospohglycerate toward serine and other biosynthetic pathways thereon and with cell cycle arrest at G1/S transition.

CPT1A plays a key role in the development and treatment of multiple sclerosis and experimental autoimmune encephalomyelitis

Human mutations in carnitine palmitoyl transferase 1A (CPT1A) are correlated with a remarkably low prevalence of multiple sclerosis (MS) in Inuits (P479L) and Hutterites (G710E). To elucidate the role of CPT1A, we established a Cpt1a P479L mouse strain and evaluated its sensitivity to experimental autoimmune encephalomyelitis (EAE) induction. Since CPT1a is a key molecule in lipid metabolism, we compared the effects of a high-fat diet (HFD) and normal diet (ND) on disease progression. The disease severity increased significantly in WT mice compared to that in Cpt1 P479L mice. In addition, WT mice receiving HFD showed markedly exacerbated disease course when compared either with Cpt1a P479L mice receiving HFD or WT control group receiving ND. Induction of EAE caused a significant decrease of myelin basic protein expression in the hindbrain of disease affected WT mice in comparison to Cpt1a P479L mice. Further, WT mice showed increased expression of oxidative stress markers like Nox2 and Ho-1, whereas expression of mitochondrial antioxidants regulator Pgc1α was increased in Cpt1a P479L mice. Our results suggest that, lipids metabolism play an important role in EAE, as shown by the higher severity of disease progression in both WT EAE and WT EAF HFD-fed mice in contrast to their counterpart Cpt1a P479L mutant mice. Interestingly, mice with downregulated lipid metabolism due to the Cpt1a P479L mutation showed resistance to EAE induction. These findings support a key role for CPT1A in the development of EAE and could be a promising target in MS treatment.

Estimating carrier frequencies of newborn screening disorders using a whole-genome reference panel of 3552 Japanese individuals

Incidence rates of Mendelian diseases vary among ethnic groups, and frequencies of variant types of causative genes also vary among human populations. In this study, we examined to what extent we can predict population frequencies of recessive disorders from genomic data, and explored better strategies for variant interpretation and classification. We used a whole-genome reference panel from 3552 general Japanese individuals constructed by the Tohoku Medical Megabank Organization (ToMMo). Focusing on 32 genes for 17 congenital metabolic disorders included in newborn screening (NBS) in Japan, we identified reported and predicted pathogenic variants through variant annotation, interpretation, and multiple ways of classifications. The estimated carrier frequencies were compared with those from the Japanese NBS data based on 1,949,987 newborns from a previous study. The estimated carrier frequency based on genomic data with a recent guideline of variant interpretation for the PAH gene, in which defects cause hyperphenylalaninemia (HPA) and phenylketonuria (PKU), provided a closer estimate to that by the observed incidence than the other methods. In contrast, the estimated carrier frequencies for SLC25A13, which causes citrin deficiency, were much higher compared with the incidence rate. The results varied greatly among the 11 NBS diseases with single responsible genes; the possible reasons for departures from the carrier frequencies by reported incidence rates were discussed. Of note, (1) the number of pathogenic variants increases by including additional lines of evidence, (2) common variants with mild effects also contribute to the actual frequency of patients, and (3) penetrance of each variant remains unclear.

Genetics of metabolic traits in Greenlanders: lessons from an isolated population

In this review, we describe the extraordinary population of Greenland, which differs from large outbred populations of Europe and Asia, both in terms of population history and living conditions. Many years in isolation, small population size and an extreme environment have shaped the genetic composition of the Greenlandic population. The unique genetic background combined with the transition from a traditional Inuit lifestyle and diet, to a more Westernized lifestyle, has led to an increase in the prevalence of metabolic conditions like obesity, where the prevalence from 1993 to 2010 has increased from 16.4% to 19.4% among men, and from 13.0% to 25.4% among women, type 2 diabetes and cardiovascular diseases. The genetic susceptibility to metabolic conditions has been explored in Greenlanders, as well as other isolated populations, taking advantage of population-genetic properties of these populations. During the last 10 years, these studies have provided examples of loci showing evidence of positive selection, due to adaption to Arctic climate and Inuit diet, including TBC1D4 and FADS/CPT1A, and have facilitated the discovery of several loci associated with metabolic phenotypes. Most recently, the c.2433-1G>A loss-of-function variant in ADCY3 associated with obesity and type 2 diabetes was described. This locus has provided novel biological insights, as it has been shown that reduced ADCY3 function causes obesity through disrupted function in primary cilia. Future studies of isolated populations will likely provide further genetic as well as biological insights.

The p.P479L variant in CPT1A is associated with infectious disease in a BC First Nation

Background

The hepatic carnitine palmitoyltransferase I (CPT1A) p.P479L variant is common in Aboriginal populations across coastal British Columbia, Alaska, the Canadian North, and Greenland. While the high frequency of this variant suggests positive selection, other studies have shown an association with sudden unexpected death in infancy and infection. We utilized administrative health data to evaluate hospitalizations for a single year cohort of children of First Nations descent genotyped for the variant and, matched for location of birth. Seven years of data were reviewed for 150 children split evenly between CPT1A genotypes (homozyous, heterozygous, and noncarrier of the p.P479L variant).

Results

Children homozygous for the p.P479L allele had a higher rate of hospital admissions at 2.6 per individual as compared to noncarriers at 0.86. Heterozygous children also showed a significant increase at 1.9 per person. Length of stay per admission was increased for both p.P479L homozygotes and heterozygotes. The odds ratio (OR) for at least one hospitalization for any reason was increased for p.P479L homozygotes relative to noncarriers (OR=10.2, confidence interval [CI] 3.5 to 30.0) as were admissions for dental caries (OR=3.4, CI 1.5 to 7.8), acute lower respiratory tract infections (OR=6.0, CI 1.6 to 22.4), and otitis media (OR=13.5, CI 1.7 to 109.4).

Conclusions

The CPT1A p.P479L variant is associated with an increased rate of hospitalization for those homozygous, primarily for infectious disease causes. Heterozygotes also showed a small but significant increase in hospitalization rates suggesting some dosage effect. Functional studies will be required to identify the underlying pathological mechanism.

Identification of brain antigens recognized by autoantibodies in experimental autoimmune encephalomyelitis-induced animals treated with etomoxir or interferon-β

Multiple sclerosis (MS) is a neurodegenerative autoimmune disease, where chronic inflammation plays an essential role in its pathology. A feature of MS is the production of autoantibodies stimulated by an altered-peptide-ligand response and epitope spreading, resulting in loss of tolerance for self-proteins. The involvement of autoantibodies in MS pathogenesis has been suggested to initiate and drive progression of inflammation; however, the etiology of MS remains unknown. The effect of etomoxir and interferon-β (IFN-β) was examined in an experimental-autoimmune-encephalomyelitis (EAE) model of MS. Moreover, the impact of etomoxir and IFN-β on recognition of brain proteins in serum from EAE rats was examined with the purpose of identifying the autoantibody reactivities involved in MS. Animals treated with etomoxir on day 1 exhibited a statistically significantly lower disease score than animals treated with IFN-β (on day 1 or 5) or placebo. Etomoxir treatment on day 5 resulted in a significantly lower disease score than IFN-β treatment on day 1. After disease induction antibodies was induced to a broad pallet of antigens in the brain. Surprisingly, by blocking CPT1 and therewith lipid metabolism several alterations in the antibody response was observed suggesting that autoantibodies play a role in the EAE animal model.

Disorders of mitochondrial long-chain fatty acid oxidation and the carnitine shuttle

Mitochondrial fatty acid oxidation is an essential pathway for energy production, especially during prolonged fasting and sub-maximal exercise. Long-chain fatty acids are the most abundant fatty acids in the human diet and in body stores, and more than 15 enzymes are involved in long-chain fatty acid oxidation. Pathogenic mutations in genes encoding these enzymes result in a long-chain fatty acid oxidation disorder in which the energy homeostasis is compromised and long-chain acylcarnitines accumulate. Symptoms arise or exacerbate during catabolic situations, such as fasting, illness and (endurance) exercise. The clinical spectrum is very heterogeneous, ranging from hypoketotic hypoglycemia, liver dysfunction, rhabdomyolysis, cardiomyopathy and early demise. With the introduction of several of the long-chain fatty acid oxidation disorders (lcFAOD) in newborn screening panels, also asymptomatic individuals with a lcFAOD are identified. However, despite early diagnosis and dietary therapy, a significant number of patients still develop symptoms emphasizing the need for individualized treatment strategies. This review aims to function as a comprehensive reference for clinical and laboratory findings for clinicians who are confronted with pediatric and adult patients with a possible diagnosis of a lcFAOD.

A cell death assay for assessing the mitochondrial targeting of proteins

The mitochondrial proteome comprises 1000 to 1500 proteins, in addition to proteins for which the mitochondrial localization is uncertain. About 800 diseases have been linked with mutations in mitochondrial proteins. We devised a cell survival assay for assessing the mitochondrial localization in a high-throughput format. This protocol allows us to assess the mitochondrial localization of proteins and their mutants, and to identify drugs and nutrients that modulate the mitochondrial targeting of proteins. The assay works equally well for proteins directed to the outer mitochondrial membrane, inner mitochondrial membrane mitochondrial and mitochondrial matrix, as demonstrated by assessing the mitochondrial targeting of the following proteins: carnitine palmitoyl transferase 1 (consensus sequence and R123C mutant), acetyl-CoA carboxylase 2, uncoupling protein 1 and holocarboxylase synthetase. Our screen may be useful for linking the mitochondrial proteome with rare diseases and for devising drug- and nutrition-based strategies for altering the mitochondrial targeting of proteins.

CPT1A Missense Mutation Associated With Fatty Acid Metabolism and Reduced Height in Greenlanders

BACKGROUND

Inuit have lived for thousands of years in an extremely cold environment on a diet dominated by marine-derived fat. To investigate how this selective pressure has affected the genetic regulation of fatty acid metabolism, we assessed 233 serum metabolic phenotypes in a population-based sample of 1570 Greenlanders.

METHODS AND RESULTS

Using array-based and targeted genotyping, we found that rs80356779, a p.Pro479Leu variant in CPT1A, was strongly associated with markers of n-3 fatty acid metabolism, including degree of unsaturation (P=1.16×10^-34), levels of polyunsaturated fatty acids, n-3 fatty acids, and docosahexaoenic acid relative to total fatty acid levels (P=2.35×10^-15, P=4.02×10^-19, and P=7.92×10^-27). The derived allele (L479) occurred at a frequency of 76.2% in our sample while being absent in most other populations, and we found strong signatures of positive selection at the locus. Furthermore, we found that each copy of L479 reduced height by an average of 2.1 cm (P=1.04×10^-9). In exome sequencing data from a sister population, the Nunavik Inuit, we found no other likely causal candidate variant than rs80356779.

CONCLUSION

Our study shows that a common CPT1A missense mutation is strongly associated with a range of metabolic phenotypes and reduced height in Greenlanders. These findings are important from a public health perspective and highlight the usefulness of complex trait genetic studies in isolated populations.

Identification of Novel Genetic Determinants of Erythrocyte Membrane Fatty Acid Composition among Greenlanders

Fatty acids (FAs) are involved in cellular processes important for normal body function, and perturbation of FA balance has been linked to metabolic disturbances, including type 2 diabetes. An individual’s level of FAs is affected by diet, lifestyle, and genetic variation. We aimed to improve the understanding of the mechanisms and pathways involved in regulation of FA tissue levels, by identifying genetic loci associated with inter-individual differences in erythrocyte membrane FA levels. We assessed the levels of 22 FAs in the phospholipid fraction of erythrocyte membranes from 2,626 Greenlanders in relation to single nucleotide polymorphisms genotyped on the MetaboChip or imputed. We identified six independent association signals. Novel loci were identified on chromosomes 5 and 11 showing strongest association with oleic acid (rs76430747 in ACSL6, beta (SE): -0.386% (0.034), p = 1.8x10^-28) and docosahexaenoic acid (rs6035106 in DTD1, 0.137% (0.025), p = 6.4x10^-8), respectively. For a missense variant (rs80356779) in CPT1A, we identified a number of novel FA associations, the strongest with 11-eicosenoic acid (0.473% (0.035), p = 2.6x10^-38), and for variants in FADS2 (rs174570), LPCAT3 (rs2110073), and CERS4 (rs11881630) we replicated known FA associations. Moreover, we observed metabolic implications of the ACSL6 (rs76430747) and CPT1A (rs80356779) variants, which both were associated with altered HbA1c (0.051% (0.013), p = 5.6x10^-6 and -0.034% (0.016), p = 3.1x10^-4, respectively). The latter variant was also associated with reduced insulin resistance (HOMA-IR, -0.193 (0.050), p = 3.8x10^-6), as well as measures of smaller body size, including weight (-2.676 kg (0.523), p = 2.4x10^-7), lean mass (-1.200 kg (0.271), p = 1.7x10^-6), height (-0.966 cm (0.230), p = 2.0x10^-5), and BMI (-0.638 kg/m² (0.181), p = 2.8x10^-4). In conclusion, we have identified novel genetic determinants of FA composition in phospholipids in erythrocyte membranes, and have shown examples of links between genetic variants associated with altered FA membrane levels and changes in metabolic traits.

Disruption of fatty-acid balance has in several previous studies been linked to human health conditions, including the metabolic syndrome, type 2 diabetes, and insulin resistance. Composition of fatty acids in lipid membranes is influenced, not only by diet and lifestyle, but also by genetic variation. By identifying genes linked to changes in the level of specific fatty acids, it may be possible to identify biological mechanisms and pathways central to regulation of fatty-acid composition in lipid membranes. We therefore aimed at finding such genes by studying Greenlanders. We identified six genomic regions harboring variants, which were associated with the level of at least one of 22 assessed erythrocyte membrane fatty acids, including two novel regions not previously linked to fatty acid levels. Moreover, we showed that two of the identified variants were associated with altered levels of glycosylated hemoglobin, and one of these variants was associated with reduced insulin resistance and decreased measures of body size. These results contribute to our understanding of fatty acid metabolism, and support a link between fatty acid balance and metabolic health.

Novel Mutations in the CPT1A Gene Identified in the Patient Presenting Jaundice as the First Manifestation of Carnitine Palmitoyltransferase 1A Deficiency

Carnitine palmitoyltransferase 1A (CPT1A) is an enzyme functioning in mitochondrial fatty acid oxidation (FAO) of the liver. Patients with CPT1A deficiency have impaired mitochondrial FAO and display hypoketotic hypoglycemia and hepatic encephalopathy as typical manifestations. In this report, we present a case of CPT1A deficiency presenting jaundice as the first manifestation. A 1.9 years old boy showed jaundice and elevated levels of free and total carnitine were observed. From direct sequencing analysis of CPT1A, two novel mutations, c.1163+1G>A and c.1393G>A (p.Gly465Arg), were identified. At the age of 2.2 years, hypoglycemia, tachycardia, and altered mental status developed just after cranioplasty for craniosynostosis. High glucose infusion rate was required for recovery of his vital signs and mentality. Diet rich in high carbohydrate, low fat and inclusion of medium chain triglyceride oil resulted in improvement in cholestatic hepatitis and since then the boy has shown normal growth velocity and developmental milestones to date.

Ontogeny and kinetics of carnitine palmitoyltransferase I in hepatopancreas and skeletal muscle of grass carp (Ctenopharyngodon idella)

The ontogeny and kinetics of carnitine palmitoyltransferase I (CPT I) were investigated in hepatopancreas and muscle throughout four developmental stages (newly hatched larvae, 1-month-old juvenile, 3-month-old, and 6-month-old, respectively) of grass carp Ctenopharyngodon idella. In hepatopancreas, the maximal velocity (Vmax) significantly increased from hatching to 1-month-old grass carp and then gradually declined at 6-month-old grass carp. In muscle, CPT I activity was the highest at 1-month-old grass carp, nearly twofold higher than that at hatching (P < 0.05). The Michaelis constant (Km) value was also the highest for 1-month-old in both tested tissues. Carnitine concentrations (FC, AC and TC) were the lowest for 3-month-old grass carp and remained relatively constant in both tissues from fish under the other developmental stages. The FC concentration in hepatopancreas and muscle at four developmental stages were less than the respective Km, indicating that grass carp required supplemental carnitine in their food to ensure that CPT I activity was not constrained by carnitine availability.

Increased missense mutation burden of Fatty Acid metabolism related genes in nunavik inuit population

Background

Nunavik Inuit (northern Quebec, Canada) reside along the arctic coastline where for generations their daily energy intake has mainly been derived from animal fat. Given this particular diet it has been hypothesized that natural selection would lead to population specific allele frequency differences and unique variants in genes related to fatty acid metabolism. A group of genes, namely CPT1A, CPT1B, CPT1C, CPT2, CRAT and CROT, encode for three carnitine acyltransferases that are important for the oxidation of fatty acids, a critical step in their metabolism.

Methods

Exome sequencing and SNP array genotyping were used to examine the genetic variations in the six genes encoding for the carnitine acyltransferases in 113 Nunavik Inuit individuals.

Results

Altogether ten missense variants were found in genes CPT1A, CPT1B, CPT1C, CPT2 and CRAT, including three novel variants and one Inuit specific variant CPT1A p.P479L (rs80356779). The latter has the highest frequency (0.955) compared to other Inuit populations. We found that by comparison to Asians or Europeans, the Nunavik Inuit have an increased mutation burden in CPT1A, CPT2 and CRAT; there is also a high level of population differentiation based on carnitine acyltransferase gene variations between Nunavik Inuit and Asians.

Conclusion

The increased number and frequency of deleterious variants in these fatty acid metabolism genes in Nunavik Inuit may be the result of genetic adaptation to their diet and/or the extremely cold climate. In addition, the identification of these variants may help to understand some of the specific health risks of Nunavik Inuit.

Metabolism as a tool for understanding human brain evolution: lipid energy metabolism as an example

Genes and the environment both influence the metabolic processes that determine fitness. To illustrate the importance of metabolism for human brain evolution and health, we use the example of lipid energy metabolism, i.e. the use of fat (lipid) to produce energy and the advantages that this metabolic pathway provides for the brain during environmental energy shortage. We briefly describe some features of metabolism in ancestral organisms, which provided a molecular toolkit for later development. In modern humans, lipid energy metabolism is a regulated multi-organ pathway that links triglycerides in fat tissue to the mitochondria of many tissues including the brain. Three important control points are each suppressed by insulin. (1) Lipid reserves in adipose tissue are released by lipolysis during fasting and stress, producing fatty acids (FAs) which circulate in the blood and are taken up by cells. (2) FA oxidation. Mitochondrial entry is controlled by carnitine palmitoyl transferase 1 (CPT1). Inside the mitochondria, FAs undergo beta oxidation and energy production in the Krebs cycle and respiratory chain. (3) In liver mitochondria, the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) pathway produces ketone bodies for the brain and other organs. Unlike most tissues, the brain does not capture and metabolize circulating FAs for energy production. However, the brain can use ketone bodies for energy. We discuss two examples of genetic metabolic traits that may be advantageous under most conditions but deleterious in others. (1) A CPT1A variant prevalent in Inuit people may allow increased FA oxidation under nonfasting conditions but also predispose to hypoglycemic episodes. (2) The thrifty genotype theory, which holds that energy expenditure is efficient so as to maximize energy stores, predicts that these adaptations may enhance survival in periods of famine but predispose to obesity in modern dietary environments.

Association of a genetic variant of carnitine palmitoyltransferase 1A with infections in Alaska Native children

OBJECTIVE

To evaluate whether the arctic variant (c.1436C→T) of carnitine palmitoyltransferase type 1A (CPT1A) is associated with a higher incidence of adverse health outcomes in Alaska Native infants and children.

STUDY DESIGN

We evaluated health measures from birth certificates (n = 605) and Alaska Medicaid billing claims (n = 427) collected from birth to 2.5 years of age for a cohort of Alaska Native infants with known CPT1A genotype. To account for geographic variations in gene distribution and other variables, data also were evaluated in cohorts.

RESULTS

When analysis was restricted to residents of nonhub communities in Western and Northern Alaska, children homozygous for the arctic variant experienced more episodes of lower respiratory tract infection than did heterozygous or noncarrier children (5.5 vs 3.7, P = .067) and were more likely to have had otitis media (86% vs 69%, 95% CI 1.4-8.9). Associations were weaker for more homogeneous cohorts.

CONCLUSIONS

The association of the arctic variant of CPT1A with infectious disease outcomes in children between birth and 2.5 years of age suggests that this variant may play a role in the historically high incidence of these health outcomes among indigenous Arctic populations; further studies will need to assess if this association was confounded by other risk factors.

Comparison of the catalytic activities of three isozymes of carnitine palmitoyltransferase 1 expressed in COS7 cells

The enzyme carnitine palmitoyltransferase 1 (CPT1) catalyzes the transfer of an acyl group from acyl-CoA to carnitine to form acylcarnitine, and three isozymes of it, 1a, 1b, and 1c, have been identified. Interestingly, the 1c isozyme was reported to show no enzymatic activity, but it was not clearly demonstrated whether this inactivity was due to its dysfunction or due to its poor expression. In the present study, we (a) expressed individual CPT1 isozymes in COS7 cells, (b) evaluated quantitatively their expression levels by Western blotting using the three bacterially expressed CPT1 isozymes as standards, and (c) evaluated their catalytic activities. With these experiments, we successfully demonstrated that the absence of the enzymatic activity of the 1c isozyme was due to its dysfunction. In addition, experiments on the preparation of standard CPT1 isozymes revealed that the 1c isozyme did not show the standard relationship between migration in an SDS-PAGE gel and molecular size. We further tried to determine why the 1c isozyme was inert by preparing chimeric CPT1 between 1a and 1c, but no clear conclusion could be drawn because one of the chimeric CPT1s was not sufficiently expressed.

Investigational anti-hyperglycemic agents: the future of type 2 diabetes therapy?

As the pandemic of type 2 diabetes spreads globally, clinicians face many challenges in treating an increasingly diverse patient population varying in age, comorbidities, and socioeconomic status. Current therapies for type 2 diabetes are often unable to alter the natural course of the disease and provide durable glycemic control, and side effects in the context of individual patient characteristics often limit treatment choices. This often results in the progression to insulin use and complex regimens that are difficult to maintain. Therefore, a number of agents are being developed to better address the pathogenesis of type 2 diabetes and to overcome limitations of current therapies. The hope is to provide more options for glucose lowering and complication reduction with less risk for hypoglycemia and other adverse effects. These agents include newer incretin-based therapies and PPAR agonists, as well as new therapeutic classes such as sodium-coupled glucose cotransporter 2 inhibitors, free fatty acid receptor agonists, 11-β-hydroxysteroid dehydrogenase type 1 inhibitors, glucokinase activators, and several others that may enter clinical use over the next decade. Herein we review these agents that are advancing through clinical trials and describe the rationale behind their use, mechanisms of action, and potential for glucose lowering, as well as what is known of their limitations.

Differential effects of the chronic and acute zinc exposure on carnitine composition, kinetics of carnitine palmitoyltransferases I (CPT I) and mRNA levels of CPT I isoforms in yellow catfish Pelteobagrus fulvidraco

The present study is conducted to determine the effect of acute and chronic zinc (Zn) exposure on carnitine concentration, carnitine palmitoyltransferases I (CPT I) kinetics, and expression levels of CPT I isoforms in liver, muscle and heart of yellow catfish Pelteobagrus fulvidraco. To this end, yellow catfish are subjected to chronic waterborne Zn exposure (0.05 mg Zn L(-1), 0.35 mg Zn L(-1) and 0.86 mg Zn L(-1), respectively) for 8 weeks and acute Zn exposure (0.05 mg Zn L(-1) and 4.71 mg L(-1)Zn, respectively) for 96 h, respectively. Reduced Michaelis-Menten constants (Km) and maximal reaction rates (Vmax) values in liver and muscle are observed in fish exposed to chronic Zn concentration. In contrast, Vmax and Km values in heart increase with increasing Zn concentration. Chronic Zn exposure also significantly influences the contents of free carnitine (FC), total carnitine (TC) and acylcarnitine (AC) in liver and heart, but not in muscle. The acute Zn exposure significantly increases FC, AC, TC contents in liver and muscle, but reduces their contents in heart. The chronic and acute Zn exposure influences the mRNA levels of four CPT I isoforms (CPT Iα1b, CPT Iβ, CPT Iα2a and CPT Iα1a) in liver, muscle and heart. Furthermore, correlations are observed in the mRNA levels between CPT I isoforms and Km, and between isoforms expression and activity of CPT I. Thus, chronic and acute Zn exposure shows differential effects on carnitine content, CPT I kinetics and mRNA levels of four CPT I isoforms in yellow catfish, which provides new mechanism for Zn exposure on lipid metabolism and also novel insights into Zn toxicity in fish.

Pathophysiology of fatty acid oxidation disorders and resultant phenotypic variability

Fatty acids are a major fuel for the body and fatty acid oxidation is particularly important during fasting, sustained aerobic exercise and stress. The myocardium and resting skeletal muscle utilise long-chain fatty acids as a major source of energy. Inherited disorders affecting fatty acid oxidation seriously compromise the function of muscle and other highly energy-dependent tissues such as brain, nerve, heart, kidney and liver. Such defects encompass a wide spectrum of clinical disease, presenting in the neonatal period or infancy with recurrent hypoketotic hypoglycaemic encephalopathy, liver dysfunction, hyperammonaemia and often cardiac dysfunction. In older children, adolescence or adults there is often exercise intolerance with episodic myalgia or rhabdomyolysis in association with prolonged aerobic exercise or other exacerbating factors. Some disorders are particularly associated with toxic metabolites that may contribute to encephalopathy, polyneuropathy, axonopathy and pigmentary retinopathy. The phenotypic diversity encountered in defects of fat oxidation is partly explained by genotype/phenotype correlation and certain identifiable environmental factors but there remain many unresolved questions regarding the complex interaction of genetic, epigenetic and environmental influences that dictate phenotypic expression. It is becoming increasingly clear that the view that most inherited disorders are purely monogenic diseases is a naive concept. In the future our approach to understanding the phenotypic diversity and management of patients will be more realistically achieved from a polygenic perspective.

Molecular characterization, tissue distribution and kinetic analysis of carnitine palmitoyltransferase I in juvenile yellow catfish Pelteobagrus fulvidraco

Up to date, only limited information is available on genetically and functionally different isoforms of CPT I enzyme in fish. In the study, molecular characterization and their tissue expression profile of three CPT Iα isoforms (CPT Iα1a, CPT Iα1b and CPT Iα2a) and a CPT Iβ isoform from yellow catfish Pelteobagrus fulvidraco is determined. The activities and kinetic features of CPT I from several tissues have also been analyzed. The four CPT I isoforms in yellow catfish present distinct differences in amino acid sequences and structure. They are widely expressed in liver, heart, white muscle, spleen, intestine and mesenteric adipose tissue of yellow catfish at the mRNA level, but with the varying levels. CPT I activity and kinetics show tissue-specific differences stemming from co-expression of different isoforms, indicating more complex pathways of lipid utilization in fish than in mammals, allowing for precise control of lipid oxidation in individual tissue.

Carnitine and acylcarnitines: pharmacokinetic, pharmacological and clinical aspects

L-Carnitine (levocarnitine) is a naturally occurring compound found in all mammalian species. The most important biological function of L-carnitine is in the transport of fatty acids into the mitochondria for subsequent β-oxidation, a process which results in the esterification of L-carnitine to form acylcarnitine derivatives. As such, the endogenous carnitine pool is comprised of L-carnitine and various short-, medium- and long-chain acylcarnitines. The physiological importance of L-carnitine and its obligatory role in the mitochondrial metabolism of fatty acids has been clearly established; however, more recently, additional functions of the carnitine system have been described, including the removal of excess acyl groups from the body and the modulation of intracellular coenzyme A (CoA) homeostasis. In light of this, acylcarnitines cannot simply be considered by-products of the enzymatic carnitine transfer system, but provide indirect evidence of altered mitochondrial metabolism. Consequently, examination of the contribution of L-carnitine and acylcarnitines to the endogenous carnitine pool (i.e. carnitine pool composition) is critical in order to adequately characterize metabolic status. The concentrations of L-carnitine and its esters are maintained within relatively narrow limits for normal biological functioning in their pivotal roles in fatty acid oxidation and maintenance of free CoA availability. The homeostasis of carnitine is multifaceted with concentrations achieved and maintained by a combination of oral absorption, de novo biosynthesis, carrier-mediated distribution into tissues and extensive, but saturable, renal tubular reabsorption. Various disorders of carnitine insufficiency have been described but ultimately all result in impaired entry of fatty acids into the mitochondria and consequently disturbed lipid oxidation. Given the sensitivity of acylcarnitine concentrations and the relative carnitine pool composition in reflecting the intramitochondrial acyl-CoA to free CoA ratio (and, hence, any disturbances in mitochondrial metabolism), the relative contribution of L-carnitine and acylcarnitines within the total carnitine pool is therefore considered critical in the identification of mitochondria dysfunction. Although there is considerable research in the literature focused on disorders of carnitine insufficiency, relatively few have examined relative carnitine pool composition in these conditions; consequently, the complexity of these disorders may not be fully understood. Similarly, although important studies have been conducted establishing the pharmacokinetics of exogenous carnitine and short-chain carnitine esters in healthy volunteers, few studies have examined carnitine pharmacokinetics in patient groups. Furthermore, the impact of L-carnitine administration on the kinetics of acylcarnitines has not been established. Given the importance of L-carnitine as well as acylcarnitines in maintaining normal mitochondrial function, this review seeks to examine previous research associated with the homeostasis and pharmacokinetics of L-carnitine and its esters, and highlight potential areas of future research.

Carnitine palmitoyltransferase I and sudden unexpected infant death in British Columbia First Nations

OBJECTIVE

Infant mortality in British Columbia (BC) First Nations remains elevated relative to other residents. The p.P479L (c.1436C>T) variant of carnitine palmitoyltransferase 1 (CPT1A) is frequent in some aboriginal populations and may be associated with increased infant deaths. This work was initiated to determine the performance of acylcarnitine profiling for detecting this variant, to determine its frequency in BC, and to determine if it is associated with sudden infant deaths in this population.

METHODS

Newborn screening cards from all BC First Nations infants in 2004 and all sudden unexpected deaths in BC First Nations infants (1999-2009) were genotyped for the CPT1A p.P479L variant and linked to archival acylcarnitine data.

RESULTS

The CPT1A p.P479L variant is frequent in BC First Nations but is not evenly distributed, with higher rates in coastal regions (up to 25% homozygosity) with historically increased infant mortality. There is also an overrepresentation of p.P479L homozygotes in unexpected infant deaths from these regions, with an odds ratio of 3.92 (95% confidence interval: 1.69-9.00). Acylcarnitine profiling will identify p.P479L homozygotes with a 94% sensitivity and specificity.

CONCLUSIONS

The CPT1A p.P479L variant is common to some coastal BC First Nations, and homozygosity for this variant is associated with unexpected death in infancy. The high frequency of this variant in a wide range of coastal aboriginal communities, however, suggests a selective advantage, raising the possibility that this variant may have differing impacts on health depending on the environmental or developmental context.

A Novel Mutation in CPT1A Resulting in Hepatic CPT Deficiency

The present work presents a "from gene defect to clinics" pathogenesis study of a patient with a hitherto unreported mutation in the CPT1A gene. In early childhood, the patient developed a life-threatening episode (hypoketotic hypoglycemia, liver cytolysis, and hepatomegaly) evocative of a mitochondrial fatty acid oxidation disorder, and presented deficient fibroblast carnitine palmitoyltransferase 1 (CPT1) activity and homozygosity for the c.1783 C > T nucleotide substitution on exon 15 of CPT1A (p.R595W mutant). While confirming CPT1A deficiency, whole blood de novo acylcarnitine synthesis and the levels of carnitine and its esters formally linked intracellular free-carnitine depletion to intracellular carnitine esterification. Sequence alignment and modeling of wild-type and p.*R595W CPT1A proteins indicated that the Arg595 targeted by the mutated codon is phylogenetically well conversed. It contributes to a hydrogen bond network with neighboring residues Cys304 and Met593 but does not participate in the catalysis and carnitine pocket. Its replacement by tryptophan induces steric hindrance with the side chain of Ile480 located in α-helix 12, affecting protein architecture and function. This hindrance with Ile480 is also originally described with tryptophan 304 in the known mutant p.C304W CPT1A, suggesting that the mechanisms that invalidate CPT1A activity and underlie pathogenesis could be common in both the new (p.R595W) and previously described (p.C304W) mutants.

Impaired fasting tolerance among Alaska native children with a common carnitine palmitoyltransferase 1A sequence variant

A high prevalence of the sequence variant c.1436C→T in the CPT1A gene has been identified among Alaska Native newborns but the clinical implications of this variant are unknown. We conducted medically supervised fasts in 5 children homozygous for the c.1436C→T variant. Plasma free fatty acids increased normally in these children but their long-chain acylcarnitine and ketone production was significantly blunted. The fast was terminated early in two subjects due to symptoms of hypoglycemia. Homozygosity for the c.1436C→T sequence variant of CPT1A impairs fasting ketogenesis, and can cause hypoketotic hypoglycemia in young children. Trial registration www.clinical trials.gov NCT00653666 "Metabolic Consequences of CPT1A Deficiency"

Genetic polymorphisms in carnitine palmitoyltransferase 1A gene are associated with variation in body composition and fasting lipid traits in Yup'ik Eskimos

Variants of carnitine palmitoyltransferase 1A (CPT1A), a key hepatic lipid oxidation enzyme, may influence how fatty acid oxidation contributes to obesity and metabolic outcomes. CPT1A is regulated by diet, suggesting interactions between gene variants and diet may influence outcomes. The objective of this study was to test the association of CPT1A variants with body composition and lipids, mediated by consumption of polyunsaturated fatty acids (PUFA). Obesity phenotypes and fasting lipids were measured in a cross-sectional sample of Yup'ik Eskimo individuals (n = 1141) from the Center of Alaska Native Health Research (CANHR) study. Twenty-eight tagging CPT1A SNPs were evaluated with outcomes of interest in regression models accounting for family structure. Several CPT1A polymorphisms were associated with HDL-cholesterol and obesity phenotypes. The P479L (rs80356779) variant was associated with all obesity-related traits and fasting HDL-cholesterol. Interestingly, the association of P479L with HDL-cholesterol was still significant after correcting for body mass index (BMI), percentage body fat (PBF), or waist circumference (WC). Our findings are consistent with the hypothesis that the L479 allele of the CPT1A P479L variant confers a selective advantage that is both cardioprotective (through increased HDL-cholesterol) and associated with reduced adiposity.

Normal Levels of Plasma Free Carnitine and Acylcarnitines in Follow-Up Samples from a Presymptomatic Case of Carnitine Palmitoyl Transferase 1 (CPT1) Deficiency Detected Through Newborn Screening in Denmark

Carnitine palmitoyl transferase (CPT) 1 A deficiency is a rare disorder of hepatic long-chain fatty acid oxidation. CPT1 deficiency is included in newborn screening programs in a number of countries to allow presymptomatic detection and early treatment of affected patients.We present a case of presymptomatic CPT1A deficiency detected through newborn screening in Denmark with diagnostic levels of carnitine and acylcarnitines in the initial dried blood spot. Levels of plasma-free carnitine and acylcarnitines in follow-up samples were normal, but reverted to diagnostic levels when the patient developed clinical symptoms at the age of 8 months. At that time, a diagnosis of CPT1A deficiency was confirmed by sequence analysis of the CPT1A gene revealing homozygosity for a novel c.167C>T variation in exon 3. Enzyme activity measurements showed a relatively mild enzyme defect with a decreased residual enzyme activity of 17-25%. We conclude that CPT1A gene testing and/or enzyme assay is mandatory to confirm an abnormal newborn screen suggesting CPT1A deficiency to avoid delayed diagnoses.