Functional analysis of mutations in the OCTN2 transporter causing primary carnitine deficiency: lack of genotype-phenotype correlation

Inflammation and Organic Cation Transporters Novel (OCTNs)

Inflammation is a physiological condition characterized by a complex interplay between different cells handled by metabolites and specific inflammatory-related molecules. In some pathological situations, inflammation persists underlying and worsening the pathological state. Over the years, two membrane transporters namely OCTN1 (SLC22A4) and OCTN2 (SLC22A5) have been shown to play specific roles in inflammation. These transporters form the OCTN subfamily within the larger SLC22 family. The link between these proteins and inflammation has been proposed based on their link to some chronic inflammatory diseases such as asthma, Crohn's disease (CD), and rheumatoid arthritis (RA). Moreover, the two transporters show the ability to mediate the transport of several compounds including carnitine, carnitine derivatives, acetylcholine, ergothioneine, and gut microbiota by-products, which have been specifically associated with inflammation for their anti- or proinflammatory action. Therefore, the absorption and distribution of these molecules rely on the presence of OCTN1 and OCTN2, whose expression is modulated by inflammatory cytokines and transcription factors typically activated by inflammation. In the present review, we wish to provide a state of the art on OCTN1 and OCTN2 transport function and regulation in relationships with inflammation and inflammatory diseases focusing on the metabolic signature collected in different body districts and gene polymorphisms related to inflammatory diseases.

Role of carnitine in regulation of blood pressure (MAP/SBP) and gene expression of cardiac hypertrophy markers (α/β-MHC) during insulin-induced hypoglycaemia: Role of oxidative stress

Cardiovascular disease is a leading cause of death in diabetic patients. Hyperglycaemia and iatrogenic hypoglycaemia exacerbate several pathogenic mechanisms underlying hypertension and heart diseases. Carnitine is a potent endogenous antioxidant and cellular fatty acid transporter for antioxidative stress and energy production in the cardiovascular system. The current study aimed to find the role of carnitine in the regulation of hypoglycaemia-induced hypertension and cardiac hypertrophy. Male rats received insulin glargine (InG) to induce hypoglycaemia followed by D-carnitine or acetyl-L-carnitine for carnitine depletion or carnitine supplementation, respectively. The obtained results showed that carnitine deficiency provoked hypoglycaemia-induced hypertension. Mean arterial pressure was elevated from 78.16 ± 11.4 to 100 ± 5.11 mm Hg in InG treated group, and from 78.2 ± 8.5 to 123.4 ± 28.2 mm Hg in InG + D-carnitine treated group. Acetyl-L-carnitine resisted the elevation in blood pressure in all hypoglycaemic animals and kept it within the normal values (68.33 ± 6.7 mm Hg). Acetyl-L-carnitine increased myocardial carnitine content leading to the attenuation of hypoglycaemia-induced oxidative stress, which was evaluated through measurement of the oxidative stress biomarkers such as inducible nitric oxide synthase, NAD(P)H quinone dehydrogenase-1, heme oxygenase-I, and glutathione S-transferase. Moreover, acetyl-L-carnitine prevented induction of gene expression of cardiac hypertrophy markers during hypoglycaemic conditions, which was assessed via the evaluation of mRNA expression of α-myosin heavy chain and β-myosin heavy chain. These findings demonstrate that carnitine might play an essential role in prevention of hypoglycaemia-induced hypertension and cardiac hypertrophy through providing energy and antioxidants to the cardiovascular system.

A pilot study of expanded newborn screening for 573 genes related to severe inherited disorders in China: results from 1,127 newborns

Background

Newborn screening (NBS) in China is mainly aimed at detecting biochemical levels of metabolites in the blood, which may generate false-positive/negative results. Current biochemical NBS includes tandem mass spectrometry (MS/MS) screening for metabolites as well as phenylalanine (Phe), thyroid-stimulating hormone (TSH), 17-α-hydroxyprogesterone (17-OHP), and glucose-6-phosphate dehydrogenase (G6PD) test. This study intended to explore whether next-generation sequencing (NGS) for dried blood spots combining with biochemical screening could improve the current screening efficiency and to investigate the carrier frequencies of mutations in causative genes related to amino acid metabolism, organic acid metabolism, and fatty acid oxidation in this cohort.

Methods

We designed a panel of 573 genes related to severe inherited disorders and performed NGS in 1,127 individuals who had undergone biochemical NBS. The NGS screening results of neonates were used to compare with the biochemical results.

Results

NGS screening results revealed that all the four newborns with abnormal G6PD values carried hemizygous G6PD mutations, which were consistent with the decreased G6PD enzymatic activity. The NGS results revealed an individual with compound heterozygous mutations of SLC22A5, who was biochemically negative in 2016. The MS/MS screening results in 2019 showed free carnitine deficiency, which was consistent with the genetic findings. The top five genes with the highest carrier frequencies of mutations in these newborns were PAH (1:56, 1.79%), ETFDH (1:81, 1.23%), MMACHC (1:87, 1.15%), SLC25A13 (1:102, 0.98%), and GCDH (1:125, 0.80%).

Conclusions

Our study highlighted that combining NGS screening with biochemical screening could improve the current NBS efficiency. This is the first study to investigate carrier frequencies of mutations in 77 genes causing inherited metabolic diseases (IMDs) in China.

First Case Report of Primary Carnitine Deficiency Manifested as Intellectual Disability and Autism Spectrum Disorder

Systemic primary carnitine deficiency (PCD) is a genetic disorder caused by decreased or absent organic cation transporter type 2 (OCTN2) carnitine transporter activity, resulting in low serum carnitine levels and decreased carnitine accumulation inside cells. In early life, PCD is usually diagnosed as a metabolic decompensation, presenting as hypoketotic hypoglycemia, Reye syndrome, or sudden infant death; in childhood, PCD presents with skeletal or cardiac myopathy. However, the clinical presentation of PCD characterized by autism spectrum disorder (ASD) with intellectual disability (ID) has seldom been reported in the literature. In this report, we describe the clinical features of a seven-year-old girl diagnosed with PCD who presented atypical features of the disease, including a developmental delay involving language skills, concentration, and attention span, as well as autistic features and brain alterations apparent in magnetic resonance imaging. We aim to highlight the difficulties related to the diagnostic and therapeutic approaches used to diagnose such patients. The case reported here presented typical signs of PCD, including frequent episodes of hypoglycemia, generalized muscle weakness, decreased muscle mass, and physical growth deficits. A molecular genetic study confirmed the definitive diagnosis of the disease (c.1345T>G (p.Y449D)) in gene SLC22A5, located in exon 8. PCD can be accompanied by less common clinical signs, which may delay its diagnosis because the resulting global clinical picture can closely resemble other metabolic disorders. In this case, the patient was prescribed a carnitine-enriched diet, as well as oral carnitine at a dose of 100 mg/kg/day. PCD has a better prognosis if it is diagnosed and treated early; however, a high level of clinical suspicion is required for its timely and accurate diagnosis.

Functional and molecular studies in primary carnitine deficiency

Primary carnitine deficiency is caused by a defect in the OCTN2 carnitine transporter encoded by the SLC22A5 gene. It can cause hypoketotic hypoglycemia or cardiomyopathy in children, and sudden death in children and adults. Fibroblasts from affected patients have reduced carnitine transport. We evaluated carnitine transport in fibroblasts from 358 subjects referred for possible carnitine deficiency. Carnitine transport was reduced to 20% or less of normal in fibroblasts of 140 out of 358 subjects. Sequencing of the 10 exons and flanking regions of the SLC22A5 gene in 95 out of 140 subjects identified causative variants in 84% of the alleles. The missense variants identified in our patients and others previously reported (n = 92) were expressed in CHO cells. Carnitine transport was impaired by 73 out of 92 variants expressed. Prediction algorithms (Polyphen-2, SIFT) correctly predicted the functional effects of expressed variants in about 80% of cases. These results indicate that mutations in the coding region of the SLC22A5 gene cannot be identified in about 16% of the alleles causing primary carnitine deficiency. Prediction algorithms failed to determine the functional effects of amino acid substitutions in this transmembrane protein in about 20% of cases. Therefore, functional studies in fibroblasts remain the best strategy to confirm or exclude a diagnosis of primary carnitine deficiency.

Systemic primary carnitine deficiency with hypoglycemic encephalopathy

Acute hypoglycemia in children is not an uncommon disease that can be encountered in the Emergency Department. Most cases of childhood hypoglycemia are caused by ketotic hypoglycemia due to missed meals. Often, hypoketotic hypoglycemia can also occur, which suggests hyperinsulinemia or a defect in fatty acid oxidation. Carnitine is essential for long chain fatty acids transfer into mitochondria for oxidation. We present a case of systemic primary carnitine deficiency who presented with seizures due to hypoketotic hypoglycemia.

Carnitine transport and fatty acid oxidation

Carnitine is essential for the transfer of long-chain fatty acids across the inner mitochondrial membrane for subsequent β-oxidation. It can be synthesized by the body or assumed with the diet from meat and dairy products. Defects in carnitine biosynthesis do not routinely result in low plasma carnitine levels. Carnitine is accumulated by the cells and retained by kidneys using OCTN2, a high affinity organic cation transporter specific for carnitine. Defects in the OCTN2 carnitine transporter results in autosomal recessive primary carnitine deficiency characterized by decreased intracellular carnitine accumulation, increased losses of carnitine in the urine, and low serum carnitine levels. Patients can present early in life with hypoketotic hypoglycemia and hepatic encephalopathy, or later in life with skeletal and cardiac myopathy or sudden death from cardiac arrhythmia, usually triggered by fasting or catabolic state. This disease responds to oral carnitine that, in pharmacological doses, enters cells using the amino acid transporter B(0,+). Primary carnitine deficiency can be suspected from the clinical presentation or identified by low levels of free carnitine (C0) in the newborn screening. Some adult patients have been diagnosed following the birth of an unaffected child with very low carnitine levels in the newborn screening. The diagnosis is confirmed by measuring low carnitine uptake in the patients' fibroblasts or by DNA sequencing of the SLC22A5 gene encoding the OCTN2 carnitine transporter. Some mutations are specific for certain ethnic backgrounds, but the majority are private and identified only in individual families. Although the genotype usually does not correlate with metabolic or cardiac involvement in primary carnitine deficiency, patients presenting as adults tend to have at least one missense mutation retaining residual activity. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Analysis of genetic mutations in Chinese patients with systemic primary carnitine deficiency

Systemic primary carnitine deficiency (CDSP) is caused by mutations in SLC22A5 gene, which encodes organic cation transporter 2(OCTN2). CDSP leads to skeletal or cardiac myopathy and hepatic encephalopathy. The present study aimed to identify SLC22A5 gene mutations and analyze the potential relationship between genotype and clinical symptoms in 20 Chinese patients with CDSP. The complete coding region of the SLC22A5 gene including intron-exon boundaries were amplified and sequenced in all patients. Eighteen different mutations were found; of which, nine were novel. The mutations clustering in exons 1 and 4 accounted for 66.7% of all mutant alleles (26/39). The c.760C>T (p. R254X) was the most frequent mutation (25.6%, 10/39), suggesting it as an ethnic founder mutation. The relationship between genotype and phenotype was investigated in patients carrying the R254X mutation. Homozygous patients with R254X were late-onset cases who presented with dilated cardiomyopathy and muscle weakness after 1 year of age. Compound heterozygous patients carrying R254X, combined with other missense mutations occurred in very specific positions, dramatically altered OCTN2 protein function. Based on the analysis of case studies, a clear relationship between free carnitine (C0) level in plasma and OCTN2 genotype was not found in the present work, however, the low plasma C0 level could not indicate disease severity or genotype. Further functional studies with a large sample size are required to understand the relationship between R254X mutation and CDSP.

Primary carnitine deficiency and cardiomyopathy

Carnitine is essential for the transfer of long-chain fatty acids from the cytosol into mitochondria for subsequent β-oxidation. A lack of carnitine results in impaired energy production from long-chain fatty acids, especially during periods of fasting or stress. Primary carnitine deficiency (PCD) is an autosomal recessive disorder of mitochondrial β-oxidation resulting from defective carnitine transport and is one of the rare treatable etiologies of metabolic cardiomyopathies. Patients affected with the disease may present with acute metabolic decompensation during infancy or with severe cardiomyopathy in childhood. Early recognition of the disease and treatment with L-carnitine may be life-saving. In this review article, the pathophysiology, clinical presentation, diagnosis, treatment and prognosis of PCD are discussed, with a focus on cardiac involvements.

Residual OCTN2 transporter activity, carnitine levels and symptoms correlate in patients with primary carnitine deficiency

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.

The human OCTN1 (SLC22A4) reconstituted in liposomes catalyzes acetylcholine transport which is defective in the mutant L503F associated to the Crohn's disease

The organic cation transporter (OCTN1) plays key roles in transport of selected organic cations, but understanding of its biological functions remains limited by restricted knowledge of its substrate targets. Here we show capacity of human OCTN1-reconstituted proteoliposomes to mediate uptake and efflux of [(3)H]acetylcholine, the Km of transport being 1.0mM with V(max) of 160nmol⋅mg(-1)protein⋅min(-1). OCTN1-mediated transport of this neurotransmitter was time-dependent and was stimulated by intraliposomal ATP. The transporter operates as uniporter but translocates acetylcholine in both directions. [(3)H]acetylcholine uptake was competitively inhibited by tetraethylammonium, γ-butyrobetaine and acetylcarnitine, and was also inhibited by various polyamines. Decreasing intraliposomal ATP concentrations increased OCTN Km for acetylcholine, but V(max) was unaffected. Evaluation of the acetylcholine transporter properties of a variant form of OCTN1, the Crohn's disease-associated 503F variant, revealed time course, Km and V(max) for acetylcholine uptake to be comparable to that of wild-type OCTN1. Km for acetylcholine efflux was also comparable for both OCTN1 species, but V(max) of OCTN1 503F-mediated acetylcholine efflux (1.9nmol⋅mg(-1)protein⋅min(-1)) was significantly lower than that of wild-type OCTN1 (14nmol⋅mg(-1)protein⋅min(-1)). These data identify a new transport role for OCTN1 and raise the possibility that its involvement in the non-neuronal acetylcholine system may be relevant to the pathogenesis of Crohn's disease.

Primary carnitine deficiency and sudden death: in vivo evidence of myocardial lipid peroxidation and sulfonylation of sarcoendoplasmic reticulum calcium ATPase 2

OBJECTIVES

Primary carnitine deficiency is an autosomal recessive disorder caused by mutations in the SLC22A5 gene which results in impaired carnitine transport, cytosolic fatty acid accumulation and impaired beta oxidation. The disease is associated with cardiomyopathy and arrhythmias, but the mechanism is unknown. We hypothesized that carnitine deficiency results in increased myocardial oxidative stress.

METHODS

We evaluated a 22-year-old woman with primary carnitine deficiency and ventricular fibrillation, as well as her first-degree relatives.

RESULTS

Sequencing of SLC22A5 identified two deleterious mutations (A142S and R488H) and a novel mutation predicted to be a splice variant. Histology demonstrated increased myocardial lipid deposition and swollen mitochondria. Immunohistochemistry demonstrated accumulation of the reactive aldehyde 4-hydroxy-2-nonenal, indicative of increased lipid peroxidation, and sulfonylation of sarcoendoplasmic reticulum calcium ATPase 2 at cysteine 674.

CONCLUSIONS

These findings suggest that increased oxidant stress may contribute to myocardial dysfunction and arrhythmogenesis in this disorder.

Genotype-phenotype correlation in primary carnitine deficiency

Primary carnitine deficiency is caused by defective OCTN2 carnitine transporters encoded by the SLC22A5 gene. Lack of carnitine impairs fatty acid oxidation resulting in hypoketotic hypoglycemia, hepatic encephalopathy, skeletal and cardiac myopathy. Recently, asymptomatic mothers with primary carnitine deficiency were identified by low carnitine levels in their infant by newborn screening. Here, we evaluate mutations in the SLC22A5 gene and carnitine transport in fibroblasts from symptomatic patients and asymptomatic women. Carnitine transport was significantly reduced in fibroblasts obtained from all patients with primary carnitine deficiency, but was significantly higher in the asymptomatic women's than in the symptomatic patients' fibroblasts (P < 0.01). By contrast, ergothioneine transport (a selective substrate of the OCTN1 transporter, tested here as a control) was similar in cells from controls and patients with carnitine deficiency. DNA sequencing indicated an increased frequency of nonsense mutations in symptomatic patients (P < 0.001). Expression of the missense mutations in Chinese hamster ovary (CHO) cells indicated that many mutations retained residual carnitine transport activity, with no difference in the average activity of missense mutations identified in symptomatic versus asymptomatic patients. These results indicate that cells from asymptomatic women have on average higher levels of residual carnitine transport activity as compared to that of symptomatic patients due to the presence of at least one missense mutation.

Identification of mutations and evaluation of cardiomyopathy in Turkish patients with primary carnitine deficiency

Primary systemic carnitine deficiency (SCD) is an autosomal recessive disorder caused by defective cellular carnitine transport. Patients usually present with predominant metabolic or cardiac manifestations. SCD is caused by mutations in the organic cation/carnitine transporter OCTN2 (SLC22A5) gene. Mutation analysis of SLC22A5 gene was carried out in eight Turkish patients from six families. Six patients presented with signs and symptoms of heart failure, cardiomyopathy, and low plasma carnitine levels, five of them with concurrent anemia. A patient with dilated cardiomyopathy had also facial dysmorphia, microcephaly, and developmental delay. Tandem MS analyses in siblings of the patients revealed two more cases with low plasma carnitine levels. SCD diagnosis was confirmed in these two cases by mutation screening. These two cases were asymptomatic but echocardiography revealed left ventricular dilatation in one of them. Carnitine treatment was started before the systemic signs and symptoms developed in these patients. Mean value of serum carnitine levels of the patients was 2.63±1.92μmol/L at the time of diagnosis. After 1year of treatment, carnitine values increased to 16.62±5.11 (p<0.001) and all responded to carnitine supplementation clinically. Mutation screening of the OCTN2 gene study in the patients revealed two novel (p.G411V, p.G152R), and four previously identified mutations (p.R254X, p.R282X, p.R289X, p.T337Pfs12X). Early recognition and carnitine supplementation can be lifesaving in this inborn error of fatty acid oxidation.

Enzymology of the carnitine biosynthesis pathway

The water-soluble zwitterion carnitine is an essential metabolite in eukaryotes required for fatty acid oxidation as it functions as a carrier during transfer of activated acyl and acetyl groups across intracellular membranes. Most eukaryotes are able to synthesize carnitine endogenously, besides their capacity to take up carnitine from the diet or extracellular medium through plasma membrane transporters. This review discusses the current knowledge on carnitine homeostasis with special emphasis on the enzymology of the four steps of the carnitine biosynthesis pathway.

Diagnoses of newborns and mothers with carnitine uptake defects through newborn screening

Carnitine uptake defect (CUD) is an autosomal recessive fatty acid oxidation defect caused by a deficiency of the high-affinity carnitine transporter OCTN2. CUD patients may present with hypoketotic hypoglycemia, hepatic encephalopathy or dilated cardiomyopathy. Tandem mass spectrometry screening of newborns can detect CUD, although transplacental transport of free carnitine from the mother may cause a higher free carnitine level and cause false negatives during newborn screening. From Jan 2001 to July 2009, newborns were screened for low free carnitine levels at the National Taiwan University Hospital screening center. Confirmation tests included dried blood spot free acylcarnitine levels and mutation analyses for both babies and their mothers. Sixteen newborns had confirmation tests for persistent low free carnitine levels; four had CUD, six had mothers with CUD, and six cases were false positives. All babies born to mothers with CUD had transient carnitine deficiency. The six mothers with CUD were put on carnitine supplementation (50-100mg/kg/day). One mother had dilated cardiomyopathy at diagnosis and her cardiac function improved after treatment. Analysis of the SLC22A5 gene revealed that p.S467C was the most common mutation in mothers with CUD, while p.R254X was the most common mutation in newborns and children with CUD. Newborn screening allows for the detection of CUD both in newborns and mothers, with an incidence in newborns of one in 67,000 (95% CI: one in 31,600-512,000) and a prevalence in mothers of one in 33,000 (95% CI: one in 18,700-169,000). Detection of CUD in mothers may prevent them from developing dilated cardiomyopathy.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Interplay of commensal and pathogenic bacteria, genetic mutations, and immunoregulatory defects in the pathogenesis of inflammatory bowel diseases

Enteric microbiota can contribute to Crohn's disease and ulcerative colitis in several ways. Pathogenic or functionally altered commensal bacteria with increased mucosal adherence, invasion and intracellular persistence can activate pathogenic T cells and chronic intestinal inflammation. Compositional changes in intestinal microbiota can lead to decreased protective and increased aggressive species. Genetic polymorphisms resulting in increased mucosal permeability, decreased microbial killing, ineffective clearance of bacteria, biased TH1 and TH17 immune responses and loss of immunological tolerance are probably key contributors to IBD. Future therapies for these heterogeneous diseases should be individualized based on the patient-specific subset.

Cardiomyopathy and carnitine deficiency

Carnitine is essential for the transfer of long-chain fatty acids across the mitochondrial membrane for subsequent beta-oxidation. A defect in the high-affinity carnitine transporter OCTN2 causes autosomal recessive primary carnitine deficiency that can present with hypoketotic hypoglycemia, mainly in infancy or cardiomyopathy. Heterozygotes for primary carnitine deficiency can have mildly reduced plasma carnitine levels and can develop benign cardiac hypertrophy. In animal models, heterozygotes for this disease have a higher incidence of cardiomyopathy with aging. This study tested whether heterozygosity for primary carnitine deficiency was associated with cardiomyopathy. The frequency of mutations in the SLC22A5 gene encoding the OCTN2 carnitine transporter was determined in 324 patients with cardiomyopathy and compared to that described in the normal population. Missense variations identified in normal controls and patients with cardiomyopathy were expressed in Chinese Hamster Ovary cells to confirm a functional effect. Exons 2-10 of the SLC22A5 gene were amplified by PCR in the presence of LCGreen I and analyzed by dye-binding/high-resolution thermal denaturation. Exon 1 of the gene was sequenced in all patients. Heterozygosity for a few variants (L144F, T264M, I312V, E317K, and R488H) was found in 6/324 patients with cardiomyopathy. Expression of these variants in CHO cells indicated that T264M decreased, E317K increased, while L144F, I312V, and R488H did not significantly affect carnitine transport. Expression in CHO cells of all the variants identified in a normal population indicated that only two had a functional effect (L17F and Y449D), while L144F, V481I, V481F, M530V, and P549S did not change significantly carnitine transport. The frequency of variants affecting carnitine transport was 2/324 patients with cardiomyopathy (0.61%) not significantly different from frequency of 3/270 (1.11%) in the general population. These results indicate that heterozygosity for primary carnitine deficiency is not more frequent in patients with unselected types of cardiomyopathy and is unlikely to be an important cause of cardiomyopathy in humans.

Comparative gene expression profiles of intestinal transporters in mice, rats and humans

We have studied gene expression profiles of intestinal transporters in model animals and humans. Total RNA was isolated from duodenum and the mRNA expression was measured using Affymetrix GeneChip oligonucleotide arrays. Detected genes from the intestine of mice, rats, and humans were about 60% of 22,690 sequences, 40% of 8739, and 47% of 12,559, respectively. A total of 86 genes involving transporters expressed in mice, 50 genes in rats, and 61 genes in humans were detected. Mice exhibited abundant mRNA expressions for peptide transporter HPT1, amino acid transporters CSNU3, CT1 and ASC1, nucleoside transporter CNT2, organic cation transporter SFXN1, organic anion transporter NBC3, glucose transporter SGLT1, and fatty acid transporters FABP1 and FABP2. Rats showed high expression profiles of peptide transporter PEPT1, amino acid transporters CSNU1 and 4F2HC, nucleoside transporter CNT2, organic cation transporter OCT5, organic anion transporter SDCT1, glucose transporter GLUT2 and GLUT5, and folate carrier FOLT. In humans, the highly expressed genes were peptide transporter HPT1, amino acid transporters LAT3, 4F2HC and PROT, nucleoside transporter CNT2, organic cation transporter OCTN2, organic anion transporters NADC1, NBC1 and SBC2, glucose transporters SGLT1 and GLUT5, multidrug resistance-associated protein RHO12, fatty acid transporters FABP1 and FABP2, and phosphate carrier PHC. Overall these data reveal diverse transcriptomic profiles for intestinal transporters among these species. Therefore, this transcriptional data may lead to more effective use of the laboratory animals as a model for oral drug development.

Polyspecific organic cation transporters: structure, function, physiological roles, and biopharmaceutical implications

The body is equipped with broad-specificity transporters for the excretion and distribution of endogeneous organic cations and for the uptake, elimination and distribution of cationic drugs, toxins and environmental waste products. This group of transporters consists of the electrogenic cation transporters OCT1-3 (SLC22A1-3), the cation and carnitine transporters OCTN1 (SLC22A4), OCTN2 (SLC22A5) and OCT6 (SLC22A16), and the proton/cation antiporters MATE1, MATE2-K and MATE2-B. The transporters show broadly overlapping sites of expression in many tissues such as small intestine, liver, kidney, heart, skeletal muscle, placenta, lung, brain, cells of the immune system, and tumors. In epithelial cells they may be located in the basolateral or luminal membranes. Transcellular cation movement in small intestine, kidney and liver is mediated by the combined action of electrogenic OCT-type uptake systems and MATE-type efflux transporters that operate as cation/proton antiporters. Recent data showed that OCT-type transporters participate in the regulation of extracellular concentrations of neurotransmitters in brain, mediate the release of acetylcholine in non-neuronal cholinergic reactions, and are critically involved in the regulation of histamine release from basophils. The recent identification of polymorphisms in human OCTs and OCTNs allows the identification of patients with an increased risk for adverse drug reactions. Transport studies with expressed OCTs will help to optimize pharmacokinetics during development of new drugs.

Polyspecific organic cation transporters: structure, function, physiological roles, and biopharmaceutical implications

The body is equipped with broad-specificity transporters for the excretion and distribution of endogeneous organic cations and for the uptake, elimination and distribution of cationic drugs, toxins and environmental waste products. This group of transporters consists of the electrogenic cation transporters OCT1-3 (SLC22A1-3), the cation and carnitine transporters OCTN1 (SLC22A4), OCTN2 (SLC22A5) and OCT6 (SLC22A16), and the proton/cation antiporters MATE1, MATE2-K and MATE2-B. The transporters show broadly overlapping sites of expression in many tissues such as small intestine, liver, kidney, heart, skeletal muscle, placenta, lung, brain, cells of the immune system, and tumors. In epithelial cells they may be located in the basolateral or luminal membranes. Transcellular cation movement in small intestine, kidney and liver is mediated by the combined action of electrogenic OCT-type uptake systems and MATE-type efflux transporters that operate as cation/proton antiporters. Recent data showed that OCT-type transporters participate in the regulation of extracellular concentrations of neurotransmitters in brain, mediate the release of acetylcholine in non-neuronal cholinergic reactions, and are critically involved in the regulation of histamine release from basophils. The recent identification of polymorphisms in human OCTs and OCTNs allows the identification of patients with an increased risk for adverse drug reactions. Transport studies with expressed OCTs will help to optimize pharmacokinetics during development of new drugs.

Task force guidelines handbook: EFNS guidelines on diagnosis and management of fatty acid mitochondrial disorders

Guidelines in the diagnosis and current dietary treatment of long-chain fatty acid (LCFA) defects have been collected according to evidence-based medicine. Since the identification of carnitine and carnitine palmitoyltransferase deficiency more than 25 years ago, nearly every enzymatic step required for beta-oxidation has been associated with an inherited metabolic disorder. These disorders effectively preclude the use of body fat as an energy source. Clinical consequences can range from no symptoms to severe manifestations including cardiomyopathy, hypoglycaemia, peripheral neuropathy and sudden death. A diet high in carbohydrates, diet with medium-chain triglycerides and reduced amount of LCFA has a beneficial effect (class IV evidence) and in appropriate deficiency states carnitine and riboflavin are used (good practice points).

Pharmacological rescue of carnitine transport in primary carnitine deficiency

Primary carnitine deficiency is a recessive disorder caused by heterogeneous mutations in the SLC22A5 gene encoding the OCTN2 carnitine transporter. Here we extend mutational analysis to eight new families with this disorder. To determine the mechanism by which missense mutations impaired carnitine transport, the OCTN2 transporter was tagged with the green fluorescent protein and expressed in CHO cells. Analysis by confocal microscopy indicated that several missense mutants (M1I, R169W, T232 M, G242 V, S280F, R282Q, W283R, A301D, W351R, R399Q, T440 M, E452 K, and T468R) matured normally to the plasma membrane. By contrast, other mutations (including R19P, DeltaF22, R83L, S280F, P398L, Y447C, and A142S/R488 H) caused significant retention of the mutant OCTN2 transporter in the cytoplasm. Failed maturation to the plasma membrane is a common mechanism in disorders affecting membrane transporters/ion channels, including cystic fibrosis. To correct this defect, we tested whether drugs reducing the efficiency of protein degradation in the endoplasmic reticulum (ER) (phenylbutyrate, curcumin) or capable of binding the OCTN2 carnitine transporter (verapamil, quinidine) could improve carnitine transport. Prolonged incubation with phenylbutyrate, quinidine, and verapamil partially stimulated carnitine transport, while curcumin was ineffective. These results indicate that OCTN2 mutations can affect carnitine transport by impairing maturation of transporters to the plasma membrane. Pharmacological therapy can be effective in partially restoring activity of mutant transporters.

Disorders of carnitine transport and the carnitine cycle

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.

Validation of dye-binding/high-resolution thermal denaturation for the identification of mutations in the SLC22A5 gene

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation resulting from defective carnitine transport. This disease is caused by mutations in the OCTN2 carnitine transporter encoded by the SLC22A5 gene. Here we validate dye-binding/high-resolution thermal denaturation as a screening procedure to identify novel mutations in this gene. This procedure is based on the amplification of DNA by PCR in capillaries with the dsDNA binding dye LCGreen I. The PCR reaction is then analyzed in the same capillary by high-resolution thermal denaturation. Samples with abnormal melting profiles are sequenced. This technique correctly identified all known patients who were compound heterozygotes for different mutations in the carnitine transporter gene and about 30% of homozygous patients. The remaining 70% of homozygous patients were identified by a second amplification, in which the patient's DNA was mixed with the DNA of a normal control. This screening system correctly identified eight novel mutations and both abnormal alleles in six new families with primary carnitine deficiency. The causative role of the missense mutations identified (c.3G>T/p.M1I, c.695C>T/p.T232M, and c.1403 C>G/p.T468R) was confirmed by expression in Chinese hamster ovary (CHO) cells. These results expand the mutational spectrum in primary carnitine deficiency and indicate dye-binding/high-resolution thermal denaturation as an ideal system to screen for mutations in diseases with no prevalent molecular alteration.

Carnitine deficiency disorders in children

Mitochondrial oxidation of long-chain fatty acids provides an important source of energy for the heart as well as for skeletal muscle during prolonged aerobic work and for hepatic ketogenesis during long-term fasting. The carnitine shuttle is responsible for transferring long-chain fatty acids across the barrier of the inner mitochondrial membrane to gain access to the enzymes of beta-oxidation. The shuttle consists of three enzymes (carnitine palmitoyltransferase 1, carnitine acylcarnitine translocase, carnitine palmitoyl-transferase 2) and a small, soluble molecule, carnitine, to transport fatty acids as their long-chain fatty acylcarnitine esters. Carnitine is provided in the diet (animal protein) and also synthesized at low rates from trimethyl-lysine residues generated during protein catabolism. Carnitine turnover rates (300-500 micromol/day) are <1% of body stores; 98% of carnitine stores are intracellular (total carnitine levels are 40-50 microM in plasma vs. 2-3 mM in tissue). Carnitine is removed by urinary excretion after reabsorption of 98% of the filtered load; the renal carnitine threshold determines plasma concentrations and total body carnitine stores. Because of its key role in fatty acid oxidation, there has long been interest in the possibility that carnitine might be of benefit in genetic or acquired disorders of energy production to improve fatty acid oxidation, to remove accumulated toxic fatty acyl-CoA metabolites, or to restore the balance between free and acyl-CoA. Two disorders have been described in children where the supply of carnitine becomes limiting for fatty acid oxidation: (1) A recessive defect of the muscle/kidney sodium-dependent, plasma membrane carnitine symporter, which presents in infancy with cardiomyopathy or hypoketotic hypoglycemia; treatment with oral carnitine is required for survival. (2) Chronic administration of pivalate-conjugated antibiotics in which excretion of pivaloyl-carnitine can lead to carnitine depletion; tissue levels may become low enough to limit fatty acid oxidation, although no cases of illness due to carnitine deficiency have been described. There is speculation that carnitine supplements might be beneficial in other settings (such as genetic acyl-CoA oxidation defects--"secondary carnitine deficiency", chronic ischemia, hyperalimentation, nutritional carnitine deficiency), but efficacy has not been documented. The formation of abnormal acylcarnitines has been helpful in expanded newborn screening programs using tandem mass-spectrometry of blood spot acylcarnitine profiles to detect genetic fatty acid oxidation defects in neonates. Carnitine-deficient diets (vegetarian) do not have much effect on carnitine pools in adults. A modest 50% reduction in carnitine levels is associated with hyperalimentation in newborn infants, but is of doubtful significance. The above considerations indicate that carnitine does not become rate-limiting unless extremely low; testing the benefits of nutritional supplements may require invasive endurance studies of fasting ketogenesis or muscle and cardiovascular work.

Phenotypic manifestations of the OCTN2 V295X mutation: Sudden infant death and carnitine-responsive cardiomyopathy in Roma families

In two non-consanguineous Hungarian Roma (Gypsy) children who presented with cardiomyopathy and decreased plasma carnitine levels, we identified homozygous deletion of 17081C of the SLC22A5 gene that results in a frameshift at R282D and leads ultimately to a premature stop codon (V295X) in the OCTN2 carnitine transporter. Carnitine treatment resulted in dramatic improvement of the cardiac symptoms, echocardiographic, and EKG findings in both cases. Family investigations revealed four sudden deaths, two of them corresponded to the classic SIDS phenotype. In postmortem tissue specimens available from three of them we could verify the homozygous mutation. In liver tissue reserved from two patients lipid droplet vacuolization could be observed; the lipid vacuoles were located mainly in the peripherolobular regions of the acini. In the heart tissue signs of generalized hypertrophy and lipid vacuoles were seen predominantly in the subendocardial areas in both cases; some aggregates of smaller lipid vacuoles were separated, apparently by membranes. Review of all OCTN2 deficiency cases reported so far revealed that this is the first presentation of histopathology in classic familial sudden infant death syndrome (SIDS) with an established SLC22A5 mutation. In addition to the two affected homozygous cardiomyopathic children and three homozygous sudden death patients, the genetic analysis in 25 relatives showed 14 carriers. The mutant gene derived from five non-consanguineous grandparents, each of them having 6-14 brothers and sisters. This alone suggests a wide ancestral spread of the mutation in certain Roma subpopulations.

Organic cation transporters

Over the last 15 years, a number of transporters that translocate organic cations were characterized functionally and also identified on the molecular level. Organic cations include endogenous compounds such as monoamine neurotransmitters, choline, and coenzymes, but also numerous drugs and xenobiotics. Some of the cloned organic cation transporters accept one main substrate or structurally similar compounds (oligospecific transporters), while others translocate a variety of structurally diverse organic cations (polyspecific transporters). This review provides a survey of cloned organic cation transporters and tentative models that illustrate how different types of organic cation transporters, expressed at specific subcellular sites in hepatocytes and renal proximal tubular cells, are assembled into an integrated functional framework. We briefly describe oligospecific Na(+)- and Cl(-)-dependent monoamine neurotransmitter transporters ( SLC6-family), high-affinity choline transporters ( SLC5-family), and high-affinity thiamine transporters ( SLC19-family), as well as polyspecific transporters that translocate some organic cations next to their preferred, noncationic substrates. The polyspecific cation transporters of the SLC22 family including the subtypes OCT1-3 and OCTN1-2 are presented in detail, covering the current knowledge about distribution, substrate specificity, and recent data on their electrical properties and regulation. Moreover, we discuss artificial and spontaneous mutations of transporters of the SLC22 family that provide novel insight as to the function of specific protein domains. Finally, we discuss the clinical potential of the increasing knowledge about polymorphisms and mutations in polyspecific organic cation transporters.

Genetic defects in fatty acid beta-oxidation and acyl-CoA dehydrogenases. Molecular pathogenesis and genotype-phenotype relationships

Mitochondrial fatty acid oxidation deficiencies are due to genetic defects in enzymes of fatty acid beta-oxidation and transport proteins. Genetic defects have been identified in most of the genes where nearly all types of sequence variations (mutation types) have been associated with disease. In this paper, we will discuss the effects of the various types of sequence variations encountered and review current knowledge regarding the genotype-phenotype relationship, especially in patients with acyl-CoA dehydrogenase deficiencies where sufficient material exists for a meaningful discussion. Because mis-sense sequence variations are prevalent in these diseases, we will discuss the implications of these types of sequence variations on the processing and folding of mis-sense variant proteins. As the prevalent mis-sense variant K304E MCAD protein has been studied intensively, the investigations on biogenesis, stability and kinetic properties for this variant enzyme will be discussed in detail and used as a paradigm for the study of other mis-sense variant proteins. We conclude that the total effect of mis-sense sequence variations may comprise an invariable--sequence variation specific--effect on the catalytic parameters and a conditional effect, which is dependent on cellular, physiological and genetic factors other than the sequence variation itself.

Carnitine transporter defect due to a novel mutation in the SLC22A5 gene presenting with peripheral neuropathy

The carnitine transporter defect (McKusick 212140) is an autosomal recessive disorder caused by mutations in the SLC22A5 gene, which encodes the high-affinity carnitine transporter OCTN2 (Wang et al 2001). Diagnosis is suspected when plasma carnitine levels are extremely low and secondary causes of carnitine loss are excluded. The disease can present with recurrent Reye-like episodes of hypoketotic hypoglycaemia or with cardiomyopathy associated with myopathy (Stanley et al 1991). Here we report novel clinical findings in a 3-year-old with primary carnitine deficiency.

Tyrosine residues affecting sodium stimulation of carnitine transport in the OCTN2 carnitine/organic cation transporter

Primary carnitine deficiency is a disorder of fatty acid oxidation caused by mutations in the Na+-dependent carnitine/organic cation transporter OCTN2. Studies with tyrosyl group-modifying reagents support the involvement of tyrosine residues in Na+ binding by sodium-coupled transporters. Here we report two new patients with carnitine deficiency caused by mutations affecting tyrosyl residues (Y447C and Y449D) close to a residue (Glu-452) previously shown to affect sodium stimulation of carnitine transport. Kinetic analysis indicated that the Y449D substitution, when expressed in Chinese hamster ovary cells, increased the concentration of sodium required to half-maximally stimulate carnitine transport from 14.8 +/- 1.8 to 34.9 +/- 5.8 mM (p<0.05), whereas Y447C completely abolished carnitine transport. Substitution of these tyrosine residues with phenylalanine restored normal carnitine transport in Y449F but resulted in markedly impaired carnitine transport by Y447F. This was associated with an increase in the concentration of sodium required to half-maximally stimulate carnitine transport to 57.8 +/- 7.4 mM (p<0.01 versus normal OCTN2). The Y447F and Y449D mutant transporters retained their ability to transport the organic cation tetraethylammonium indicating that their effect on carnitine transport was specific and likely associated with the impaired sodium stimulation of carnitine transport. By contrast, the Y447C natural mutation abolished the transport of organic cations in addition to carnitine. Confocal microscopy of OCTN2 transporters tagged with green fluorescent protein indicated that the Y447C mutant transporters failed to reach the plasma membrane, whereas Y447F, Y449D, and Y449F had normal membrane localization. These natural mutations identify tyrosine residues possibly involved in coupling the sodium electrochemical gradient to transmembrane solute transfer in the sodium-dependent co-transporter OCTN2.

Functional domains in the carnitine transporter OCTN2, defective in primary carnitine deficiency

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation characterized by hypoketotic hypoglycemia and skeletal and cardiac myopathy. It is caused by mutations in the Na+-dependent organic cation transporter, OCTN2. To define the domains involved in carnitine recognition, we evaluated chimeric transporters created by swapping homologous domains between OCTN1, which does not transport carnitine, and OCTN2. Substitution of the C terminus of OCTN2 (amino acid residues 342-557) with the corresponding residues of OCTN1 completely abolished carnitine transport. The progressive substitution of the N terminus of OCTN2 with OCTN1 resulted in a decrease in carnitine transport associated with a progressive increase in the Km toward carnitine from 3.9 +/- 0.5 to 141 +/- 19 microM. The largest drop in carnitine transport (and increase in Km toward carnitine) was observed with the substitution of residues 341-454 of OCTN2. An additional chimeric transporter (CHIM-9) in which only residues 341-454 of OCTN2 were substituted by OCTN1 had markedly reduced carnitine transport, with an elevated Km toward carnitine (63 +/- 5 microM). Site-directed mutagenesis and introduction of residues nonconserved between OCTN1 and OCTN2 in the OCTN2 cDNA indicated that the R341A, L409W, L424Y, and T429I substitutions significantly decreased carnitine transport. Single substitutions did not increase the Km toward carnitine. By contrast, the combination of three of these substitutions (R341W + L409W + T429I) greatly decreased carnitine transport and increased the Km toward carnitine (20.2 +/- 4.5 microm). The Arg-341, Leu-409, and Thr-429 residues are all located in predicted transmembrane domains. Involvement of these residues in carnitine transport was further supported by the partial restoration of carnitine transport by the introduction of these OCTN2 residues in the OCTN1 portion of CHIM-9. These studies indicate that multiple domains of the OCTN2 transporter are required for carnitine transport and identify transmembrane residues important for carnitine recognition.

Carnitine transport: pathophysiology and metabolism of known molecular defects

Early-onset dilatative and/or hypertrophic cardiomyopathy with episodic hypoglycaemic coma and very low serum and tissue concentrations of carnitine should alert the clinician to the probability of the plasmalemmal high-affinity carnitine transporter defect. The diagnosis can be established by demonstration of impaired carnitine uptake in cultured skin fibroblasts or lymphoblasts and confirmed by mutation analysis of the human OCTN2 gene in the affected child and obligate heterozygote parents. The institution of high-dose oral carnitine supplementation reverses the pathology in this otherwise lethal autosomal recessive disease of childhood, and carnitine therapy from birth in prospectively screened siblings may altogether prevent the development of the clinical phenotype. Heterozygotes may be at risk for cardiomyopathy in later adult life, particularly in the presence of additional risk factors such as hypertension and competitive pharmacological agents. OCTN2 belongs to a family of organic cation/carnitine transporters that function primarily in the elimination of cationic drugs and other xenobiotics in kidney, intestine, liver and placenta. The high- and low-affinity human carnitine transporters, OCTN2 and OCTN1, are multifunctional polyspecific organic cation transporters; therefore, defects in these transporters may have widespread implications for the absorption and/or elimination of a number of key pharmacological agents such as cephalosporins, verapamil, quinidine and valproic acid. A third organic/cation carnitine transporter with high specificity for carnitine, Octn3, has been cloned in mice. The juvenile visceral steatosis (jvs) mouse serves as an excellent clinical, biochemical and molecular model for the high-affinity carnitine transporter OCTN2 defect and is due to a spontaneous point mutation in the murine Octn2 gene on mouse chromosome 11, which is syntenic to the human locus at 5q31 that harbours the human OCTN2 gene.

Silent and symptomatic primary carnitine deficiency within the same family due to identical mutations in the organic cation/carnitine transporter OCTN2

A family of Turkish origin with primary systemic carnitine deficiency in the father and his two sons is described. In all three individuals, the same homozygous mutation in the OCTN2 gene (R471H) was present and carnitine uptake in fibroblasts was deficient. Whereas one boy became symptomatic with a Reye-syndrome-like picture of hepatopathy and encephalopathy in infancy, the other affected family members remained asymptomatic up to their current ages of 28 and 5 years, respectively.

Studies on functional sites of organic cation/carnitine transporter OCTN2 (SLC22A5) using a Ser467Cys mutant protein

The organic cation/carnitine transporter OCTN2 mediates transport of carnitine and organic cations in Na(+)-dependent and Na(+)-independent manners, respectively. However, the mechanism of molecular recognition of different substrates has not been clarified yet. We previously found a single amino acid change in OCTN2, Ser467Cys (S467C), in the Japanese population and observed a decreased carnitine transport but unchanged organic cation transport compared with wild type. Therefore, we conducted detailed kinetic and functional analyses of the substrate recognition sites of wild-type and S467C-mutant OCTN2. The K(m) value for carnitine of S467C-mutant was increased about 15-fold over that of the wild type. Mutual inhibition kinetics of carnitine and tetraethylammonium (TEA) were not completely competitive, suggesting that the binding sites are very close to each other, but not identical. Several organic anions such as valproate, as well as organic cations, significantly inhibited carnitine and TEA uptake by OCTN2, and valproate showed Na(+)-dependent inhibition of OCTN2-mediated TEA uptake. The Na(+)-activation kinetics of the S467C mutant was similar to that of the wild type. Furthermore, a significant decrease of the TEA uptake-inhibitory potency of valproate was observed in S467C-mutant OCTN2. These observations suggest that the decrease in affinity of S467C-mutant OCTN2 for carnitine was caused by functional alteration of the anion (carboxyl moiety of carnitine) recognition site located in trans-membrane domain 11, which is closely related to the Na(+)-binding site, on OCTN2 protein. These results demonstrate that OCTN2 has functional sites for carnitine and Na(+) and that the carnitine-binding site is involved, in part, in the recognition of organic cations.

Strategies for the diagnosis of mitochondrial fatty acid beta-oxidation disorders

Mitochondrial fatty acid beta-oxidation disorders (FAOD) are a group of clinically and biochemically heterogeneous inherited metabolic defects. The spectrum of phenotypes has expanded from hepatic encephalopathy to encompass myopathy, cardiomyopathy, peripheral neuropathy, sudden death and pregnancy complicated by fetal FAOD. Pre-symptomatic diagnosis is important to prevent morbidity and this is now achievable through newborn screening using tandem mass spectrometry (MS/MS). Moreover, most of the diagnosed defects are treatable and the prognosis is generally favourable. This article reviews the features of FAOD, critically evaluates methods of investigation including metabolite analyses in body fluids, in vitro oxidation rates and acylcarnitine profiling studies, enzymatic and mutational tests, and discusses genotype-phenotype correlation, treatment and monitoring options. Based on this knowledge, strategies for the biochemical investigation and differential diagnosis of patients presenting clinically, asymptomatic neonates detected by newborn screening, infants born after complications during late pregnancy, and cases of sudden death with suspected FAOD are presented. Laboratory investigation commonly begins with a search for diagnostic metabolites in physiological fluids, followed by in vitro functional studies if the initial findings are inconclusive, and confirmation by enzymology and molecular analyses. Occasionally a stress test in vivo may be required. At other times there may be no firm diagnosis achieved.

Novel OCTN2 mutations: no genotype-phenotype correlations: early carnitine therapy prevents cardiomyopathy

Primary systemic carnitine deficiency or carnitine uptake defect (OMIM 212140) is a potentially lethal, autosomal recessive disorder characterized by progressive infantile-onset cardiomyopathy, weakness, and recurrent hypoglycemic hypoketotic encephalopathy, which is highly responsive to L-carnitine therapy. Molecular analysis of the SLC22A5 (OCTN2) gene, encoding the high-affinity carnitine transporter, was done in 11 affected individuals by direct nucleotide sequencing of polymerase chain reaction products from all 10 exons. Carnitine uptake (at Km of 5 microM) in cultured skin fibroblasts ranged from 1% to 20% of normal controls. Eleven mutations (delF23, N32S, and one 11-bp duplication in exon 1; R169W in exon 3; a donor splice mutation [IVS3+1 G > A] in intron 3; frameshift mutations in exons 5 and 6; Y401X in exon 7; T440M, T468R and S470F in exon 8) are described. There was no correlation between residual uptake and severity of clinical presentation, suggesting that the wide phenotypic variability is likely related to exogenous stressors exacerbating carnitine deficiency. Most importantly, strict compliance with carnitine from birth appears to prevent the phenotype.

Phenotype and genotype variation in primary carnitine deficiency

PURPOSE

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation resulting from defective carnitine transport. This disease is caused by mutations in the carnitine transporter gene SLC22A5. The objective of this study was to extend mutational analysis to four additional families with this disorder and determine whether recurrent mutations could be found.

METHODS

The SLC22A5 gene encoding the OCTN2 carnitine transporter was sequenced, and the missense mutations identified were expressed in Chinese hamster ovary (CHO) cells.

RESULTS

DNA sequencing revealed four novel mutations (Y4X; dup 254-264, 133X; R19P; R399Q). Alleles introducing premature STOP codons reduced the levels of OCTN2 mRNA. Carnitine transport in CHO cells expressing the R19P and R399Q mutations was reduced to < 5% of normal. The 133X mutation was found in two unrelated European families. Two patients within the same family, both homozygous for the same mutation (R399Q) had completely different clinical presentation.

CONCLUSIONS

Heterogeneous mutations in the SLC22A5 gene cause primary carnitine deficiency. Different presentations are observed even in children with identical mutations.

Carnitine transport by organic cation transporters and systemic carnitine deficiency

The intracellular homeostasis is controlled by different membrane transporters. Organic cation transporters function primarily in the elimination of cationic drugs, endogenous amines, and other xenobiotics in tissues such as the kidney, intestine, and liver. Among these molecules, carnitine is an endogenous amine which is an essential cofactor for mitochondrial beta-oxidation. Recently, a new family of transporters, named OCT (organic cation transporters) has been described. In this minireview, we present the recent knowledge about OCT and focus on carnitine transport, more particularly by the OCTN2. The importance of this sodium-dependent carnitine cotransporter, OCTN2, comes from various recently reported mutations in the gene which give rise to the primary systemic carnitine deficiency (SCD; OMIM 212140). The SCD is an autosomal recessive disorder of fatty acid oxidation characterized by skeletal myopathy, progressive cardiomyopathy, hypoglycemia and hyperammonemia. Most of the OCTN2 mutations identified in humans with SCD result in loss of carnitine transport function. Identifying these mutations will allow an easy targeting of the SCD syndrome. The characteristics of the juvenile visceral steatosis (jvs) mouse, an animal model of SCD showing similar symptoms as humans having this genetic disorder, are also described. These mice have a mutation in the gene encoding the mouse carnitine transporter octn2. Although various OCTN carnitine transporters have been identified and functionally characterized, their membrane localization and regulation are still unknown and must be investigated. This knowledge will also help in designing new drugs that regulate carnitine transport activity.