Disorders of carnitine transport and the carnitine cycle

Division of Medical Genetics, Department of Pediatrics, University of Utah, 2C412 SOM, 50 North Medical Drive, Salt Lake City, UT, USA.
American Journal of Medical Genetics Part C Seminars in Medical Genetics (Impact Factor: 3.91). 05/2006; 142C(2):77-85. DOI: 10.1002/ajmg.c.30087
Source: PubMed


Carnitine plays an essential role in the transfer of long-chain fatty acids across the inner mitochondrial membrane. This transfer requires enzymes and transporters that accumulate carnitine within the cell (OCTN2 carnitine transporter), conjugate it with long chain fatty acids (carnitine palmitoyl transferase 1, CPT1), transfer the acylcarnitine across the inner plasma membrane (carnitine-acylcarnitine translocase, CACT), and conjugate the fatty acid back to Coenzyme A for subsequent beta oxidation (carnitine palmitoyl transferase 2, CPT2). Deficiency of the OCTN2 carnitine transporter causes primary carnitine deficiency, characterized by increased losses of carnitine in the urine and decreased carnitine accumulation in tissues. Patients can present with hypoketotic hypoglycemia and hepatic encephalopathy, or with skeletal and cardiac myopathy. This disease responds to carnitine supplementation. Defects in the liver isoform of CPT1 present with recurrent attacks of fasting hypoketotic hypoglycemia. The heart and the muscle, which express a genetically distinct form of CPT1, are usually unaffected. These patients can have elevated levels of plasma carnitine. CACT deficiency presents in most cases in the neonatal period with hypoglycemia, hyperammonemia, and cardiomyopathy with arrhythmia leading to cardiac arrest. Plasma carnitine levels are extremely low. Deficiency of CPT2 present more frequently in adults with rhabdomyolysis triggered by prolonged exercise. More severe variants of CPT2 deficiency present in the neonatal period similarly to CACT deficiency associated or not with multiple congenital anomalies. Treatment for deficiency of CPT1, CPT2, and CACT consists in a low-fat diet supplemented with medium chain triglycerides that can be metabolized by mitochondria independently from carnitine, carnitine supplements, and avoidance of fasting and sustained exercise.

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    • "The dysfunction leads to high renal loss of carnitine and lower concentrations of carnitine in blood and tissues. Thus, lower concentrations of carnitine will be available for the transfer of long chain fatty acids [2] [3]. SPCD has many variations in its presentation, from asymptomatic over metabolic crises at young age (sudden infant death syndrome, episodes of hypoglycemia, and Reye-like syndrome) to progressive cardiomyopathy [4]. "
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    ABSTRACT: We present two cases of carnitine deficiency in pregnancy. In our first case, systematic screening revealed L-carnitine deficiency in the first born of an asymptomatic mother. In the course of her second pregnancy, maternal carnitine levels showed a deficiency as well. In a second case, a mother known with carnitine deficiency under supplementation was followed throughout her pregnancy. Both pregnancies had an uneventful outcome. Because carnitine deficiency can have serious complications, supplementation with carnitine is advised. This supplementation should be continued throughout pregnancy according to plasma concentrations.
    06/2015; 2015:1-4. DOI:10.1155/2015/101468
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    • "OCT1/SLC22A1, OCT2/SLC22A2, and OCT3/SLC22A3) and pH-dependent novel transporters, namely OCTN1/SLC22A4 and OCTN2/SLC22A5 [3]. Although OCTNs may mediate the transport of cationic chemicals, they are most notably known for their ability to mediate the influx of carnitine, and several mutations in the SLC22A5 gene have been identified in patients with primary carnitine deficiency [4]. OCTs are involved in the bidirectional translocation of small (b500 Da) organic cations across the cell membrane. "
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    ABSTRACT: Organic cation transporters (OCT1-3) mediate the transport of organic cations including inhaled drugs across the cell membrane, although their role in lung epithelium hasn't been well understood yet. We address here the expression and functional activity of OCT1-3 in human airway epithelial cells A549, Calu-3 and NCl-H441. Kinetic and inhibition analyses, employing [(3)H]1-methyl-4-phenylpyridinium (MPP+) as substrate, and the compounds quinidine, prostaglandine E2 (PGE2) and corticosterone as preferential inhibitors of OCT1, OCT2, OCT3, respectively, have been performed. A549 cells present a robust MPP+ uptake mediated by one high-affinity component (Km ~ 50 μM) which is identifiable with OCT3. Corticosterone, indeed, completely inhibits MPP+ transport, while quinidine and PGE2 are inactive and SLC22A3/OCT3 silencing with siRNA markedly lowers MPP+ uptake. Conversely, Calu-3 exhibit both an high (Km < 20 μM) and a low affinity (Km > 0.6 mM) transport components, referable to OCT3 and OCT1, respectively, as demonstrated by the inhibition analysis performed at proper substrate concentrations and confirmed by the use of specific siRNA. These transporters are active also when cells are grown under air-liquid interface (ALI) conditions. Only a very modest saturable MPP+ uptake is measurable in NCl-H441 cells and the inhibitory effect of quinidine points to OCT1 as the subtype functionally involved in this model. Finally, the characterization of MPP+ transport in human bronchial BEAS-2B cells suggests that OCT1 and OCT3 are operative. These findings could help to identify in vitro models to be employed for studies concerning the specific involvement of each transporter in drug transportation. Copyright © 2015. Published by Elsevier B.V.
    Biochimica et Biophysica Acta 04/2015; 1848(7). DOI:10.1016/j.bbamem.2015.04.001 · 4.66 Impact Factor
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    • "Analysis of data from the screening programme revealed that the prevalence of PCD in the Faroe Islands, a small island community in the North Atlantic Ocean with a population of approximately 50,000 inhabitants, was by far the highest reported in the world (1:300) [2] [3] [4]. PCD is an autosomal recessive disorder of fatty acid β-oxidation caused by a lack of functional organic cation transporter 2 (OCTN2) transporters, which transport carnitine from the extracellular to the intracellular space and also prevent excretion of carnitine in urine [3] [5]. Carnitine is involved in the transfer of long chain fatty acids across the inner mitochondrial membrane for β-oxidation and also participates in several other important cellular processes [6] [7] [8]. "
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    ABSTRACT: Background The prevalence of primary carnitine deficiency (PCD) in the Faroe Islands is the highest reported in the world (1:300). Serious symptoms related to PCD, e.g. sudden death, have previously only been associated to the c.95A > G/c.95A > G genotype in the Faroe Islands. We report and characterize novel mutations associated with PCD in the Faroese population and report and compare free carnitine levels and OCTN2 transport activities measured in fibroblasts from PCD patients with different genotypes. Methods Genetic analyses were used to identify novel mutations, and carnitine uptake analyses in cultured skin fibroblasts from selected patients were used to examine residual OCTN2 transporter activities of the various genotypes. Results Four different mutations, including the unpublished c.131C > T (p.A44V), the novel splice mutation c.825-52G > A and a novel risk-haplotype (RH) were identified in the Faroese population. The two most prevalent genotypes were c.95A > G/RH (1:600) and c.95A > G/c.95A > G (1:1300). Patients homozygous for the c.95A > G mutation had both the significantly (p < 0.01) lowest mean free carnitine level at 2.03 (SD 0.66) μmol/L and lowest residual OCTN2 transporter activity (4% of normal). There was a significant positive correlation between free carnitine levels and residual OCTN2 transporter activities in PCD patients (R2 = 0.430, p < 0.01). Conclusion There was a significant positive correlation between carnitine levels and OCTN2 transporter activities. The c.95A > G/c.95A > G genotype had the significantly lowest mean free carnitine level and residual OCTN2 transporter activity.
    Molecular Genetics and Metabolism Reports 12/2014; 1(1):241–248. DOI:10.1016/j.ymgmr.2014.04.008
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