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The Native American Pima population has the highest incidence of insulin resistance (IR) and type 2 diabetes mellitus (T2DM) of any reported population, but the pathophysiologic mechanism is unknown. Genetic studies in Pima Indians have linked acyl-CoA dehydrogenase 10 (ACAD10) gene polymorphisms, among others, to this predisposition. The gene codes for a protein with a C-terminus region that is structurally similar to members of a family of flavoenzymes-the acyl-CoA dehydrogenases (ACADs)-that catalyze α,β-dehydrogenation reactions, including the first step in mitochondrial FAO (FAO), and intermediary reactions in amino acids catabolism. Dysregulation of FAO and an increase in plasma acylcarnitines are recognized as important in the pathophysiology of IR and T2DM. To investigate the deficiency of ACAD10 as a monogenic risk factor for T2DM in human, an Acad-deficient mouse was generated and characterized. The deficient mice exhibit an abnormal glucose tolerance test and elevated insulin levels. Blood acylcarnitine analysis shows an increase in long-chain species in the older mice. Nonspecific variable pattern of elevated short-terminal branch-chain acylcarnitines in a variety of tissues was also observed. Acad10 mice accumulate excess abdominal adipose tissue, develop an early inflammatory liver process, exhibit fasting rhabdomyolysis, and have abnormal skeletal muscle mitochondria. Our results identify Acad10 as a genetic determinant of T2DM in mice and provide a model to further investigate genetic determinants for insulin resistance in humans.

Children with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHAD) have a defect in the degradation of long-chain fatty acids and are at risk of hypoketotic hypoglycemia and insufficient energy production as well as accumulation of toxic fatty acid intermediates. Knowledge on substrate metabolism in children with LCHAD deficiency during fasting is limited. Treatment guidelines differ between centers, both as far as length of fasting periods and need for night feeds are concerned. To increase the understanding of fasting intolerance and improve treatment recommendations, children with LCHAD deficiency were investigated with stable isotope technique, microdialysis, and indirect calometry, in order to assess lipolysis and glucose production during 6 h of fasting. We found an early and increased lipolysis and accumulation of long chain acylcarnitines after 4 h of fasting, albeit no patients developed hypoglycemia. The rate of glycerol production, reflecting lipolysis, averaged 7.7 ± 1.6 µmol/kg/min, which is higher compared to that of peers. The rate of glucose production was normal for age; 19.6 ± 3.4 µmol/kg/min (3.5 ± 0.6 mg/kg/min). Resting energy expenditure was also normal, even though the respiratory quotient was increased indicating mainly glucose oxidation. The results show that lipolysis and accumulation of long chain acylcarnitines occurs before hypoglycemia in fasting children with LCHAD, which may indicate more limited fasting tolerance than previously suggested.

Lipoate is a covalently bound cofactor essential for five redox reactions in humans: in four 2-oxoacid dehydrogenases and the glycine cleavage system (GCS). Two enzymes are from the energy metabolism, α-ketoglutarate dehydrogenase and pyruvate dehydrogenase; and three are from the amino acid metabolism, branched-chain ketoacid dehydrogenase, 2-oxoadipate dehydrogenase, and the GCS. All these enzymes consist of multiple subunits and share a similar architecture. Lipoate synthesis in mitochondria involves mitochondrial fatty acid synthesis up to octanoyl-acyl-carrier protein; and three lipoate-specific steps, including octanoic acid transfer to glycine cleavage H protein by lipoyl(octanoyl) transferase 2 (putative) (LIPT2), lipoate synthesis by lipoic acid synthetase (LIAS), and lipoate transfer by lipoyltransferase 1 (LIPT1), which is necessary to lipoylate the E2 subunits of the 2-oxoacid dehydrogenases. The reduced form dihydrolipoate is reactivated by dihydrolipoyl dehydrogenase (DLD). Mutations in LIAS have been identified that result in a variant form of nonketotic hyperglycinemia with early-onset convulsions combined with a defect in mitochondrial energy metabolism with encephalopathy and cardiomyopathy. LIPT1 deficiency spares the GCS, and resulted in a combined 2-oxoacid dehydrogenase deficiency and early death in one patient and in a less severely affected individual with a Leigh-like phenotype. As LIAS is an iron-sulphur-cluster-dependent enzyme, a number of recently identified defects in mitochondrial iron-sulphur cluster synthesis, including NFU1, BOLA3, IBA57, GLRX5 presented with deficiency of LIAS and a LIAS-like phenotype. As in DLD deficiency, a broader clinical spectrum can be anticipated for lipoate synthesis defects depending on which of the affected enzymes is most rate limiting.

Tandem mass spectrometry-based newborn screening correctly identifies individuals with very long-chain acyl-CoA dehydrogenase deficiency (VLCADD). However, a great number of healthy individuals present with identical acylcarnitine profiles during catabolism in the first three days of life. We routinely perform an enzyme activity assay as confirmation analysis in newborns identified by screening. Whereas VLCAD residual activities of less than 10% are clearly diagnostic and indicate patients at risk of clinical disease, the clinical relevance of higher residual activities is unclear. In this study we assess the molecular basis in 34 individuals with residual activities of 10-50%. We identify two pathogenic mutations in patients that result in residual activities as high as 22%, while individuals with residual activities of 25-50% either present with a heterozygous or no mutation in the VLCAD gene. In addition, confirmed heterozygous parents present with residual activities as low as 32%.In conclusion, we identify individuals with 2 pathogenic mutations and those with only one heterozygous mutation in the residual activity range of 20-30%. Whereas heterozygosity is generally regarded as clinically irrelevant, identification of 2 VLCAD mutations leads to precautions in the management of the children. Based on our data we anticipate that individuals with a residual enzyme activity >20% present with a biochemical phenotype but likely remain asymptomatic throughout life. Studies in greater patient numbers are needed to correlate residual activities >10% with the genotype and the outcome.

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD), caused by mutated ACADM gene, is a potentially fatal fatty acid oxidation defect. Detection of MCADD is now part of tandem mass spectrometry (MS-MS)-based newborn screening programs worldwide. To date, more than 67 mutations have been reported to cause MCADD with a single allele, c.985A>G, being the most common in patients of northwestern European descent. In Saudi Arabia, the Newborn Screening Program, officially launched in 2005, screens for 16 disorders including MCADD. Over a period of 3 years, 237,812 newborns were screened; 13 were identified to have MCADD giving an incidence of 1:18,293. Since the introduction of MS-MS to our institution, however, a total of 30 patients were detected to have MCADD. These cases were either newborns, at high-risk family members, or clinically suspected. The C8-carnitine levels (median 3.31, range 0.81-16.33 µM) were clearly diagnostic in all analyzed samples. Sequencing ACADM in 20 DBS revealed two novel mutations: c.362C>T (p.T121I) and c.347G>A (p.C116Y) substitutions, neither of which were detected in 300 chromosomes from controls. Eighteen (90%) patients were homozygous for the T121I mutation and two (10%) were compound heterozygous (T121I/C116Y). Our molecular data lend further support to MS-MS biochemical screening for MCADD and provide evidence for the relatively high incidence of MCADD in the Arab population. The identification of a founder mutation for MCADD has important implications for the preventive screening programs not only in Saudi Arabia but potentially also in other countries in the region.

Isolated long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) is associated with c.1528G>C substitution in the HADHA gene, since most patients have the prevalent mutation on at least one allele. As it is known that the disease is relatively frequent in Europe, especially around the Baltic Sea, and that the majority of Polish LCHADD patients originate from the coastal Pomeranian province, partly inhabited by an ancient ethnic group, the Kashubians, we aimed to determine the carrier frequency of the prevalent HADHA mutation in various districts of Poland with special focus on the Kashubian district. A total of 6,854 neonatal dried blood samples from the entire country, including 2,976 Pomeranian neonates of Kashubian origin, were c.1528G>C genotyped. Fifty-nine heterozygous carriers for the prevalent c.1528G>C substitution (41 Pomeranian children) were detected in the studied group. Our data reveal a geographically skewed distribution of the c.1528C allele in the Polish population; in the northern Pomeranian province the carrier frequency is 1:73, which is the highest frequency ever reported, whereas in the remaining regions it is 1:217. Hence, the incidence of LCHADD in Poland is predicted to be 1:118,336 versus 1:16,900 in the Pomeranian district. Despite the relative rarity of the disease, screening for LCHADD in neonates born in the northern part of Poland, especially those of Kashubian origin, is justified. Our data allow us to suggest a probable Kashubian origin of the prevalent c.1528G>C mutation.

Very-long-chain acyl-coenzyme A (CoA) dehydrogenase deficiency (VLCADD) is an autosomal recessive disorder of fatty acid oxidation. The phenotype of VLCADD is heterogeneous, and patients are typically classified into three categories based upon onset of symptoms and clinical findings. As a result of early diagnosis and treatment, many patients with VLCADD have remained asymptomatic. A general genotype-phenotype correlation has been elicited. A genotype that is associated with residual enzyme activity is more likely to present with an attenuated phenotype. One prevailing mutation, the c.848T>C (p.V283A), has been associated with residual enzyme activity and has been identified in many asymptomatic individuals diagnosed through either newborn or family screening. We present a patient who died as a result of fatal hypoglycemia at 38 h of life before diagnosis of VLCADD could be established by newborn screening. Despite the early onset of the disease, the patient was found to have a missense mutation within the ACADVL gene with a c.848T>C, c.342+1G>C genotype. Genotype alone remains limited in its predictive ability to determine which affected individuals are at risk for fatal complications.

α-Methyl-acyl-CoA-racemase (AMACR) deficiency (OMIM 604489) is a rare peroxisomal disorder with a variable age of onset from infancy to late adulthood. We describe a 45-year-old male with a history of seizures who presented with relapsing encephalopathy. Laboratory studies revealed an elevated serum pristanic acid concentration, an elevated pristanic/phytanic acid ratio, as well as the previously described homozygous mutation in the AMACR gene, c.154T>C, consistent with AMACR deficiency. This homozygous mutation is associated with a variable phenotype ranging from neonatal cholestasis to late-onset sensorimotor neuropathy. Dietary pristanic acid restriction was attempted to improve clinical status and the patient has remained in remission for more than 16 months.

The different long-chain fatty acid oxidation defects present with similar heterogeneous clinical phenotypes of different severity. Organs mainly affected comprise the heart, liver, and skeletal muscles. All symptoms are reversible with sufficient energy supply. In some long-chain fatty acid oxidation defects, disease-specific symptoms occur. Only in disorders of the mitochondrial trifunctional protein (TFP) complex, including long-chain 3-hydroxyacyl-coenzyme A (CoA) dehydrogenase (LCHAD) deficiency, neuropathy and retinopathy develop that are progressive and irreversible despite current treatment measures. In most long-chain fatty acid oxidation defects, no clear genotype-phenotype correlation exists due to molecular heterogeneity. However, some isolated mutations have been identified to be associated with only mild phenotypes, e.g., the V243A mutation in very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency. LCHAD deficiency is due to the prevalent homozygous 1528G>C mutation and presents with heterogeneous clinical phenotypes, suggesting the importance of other environmental and genetic factors. For some disorders, it was shown that residual enzyme activity measured in fibroblasts or lymphocytes correlated with severity of clinical phenotype. Implementation of newborn screening has significantly reduced morbidity and mortality of long-chain fatty acid oxidation defects. However, the severest forms of TFP deficiency are still highly associated with neonatal death. Newborn screening also identifies a great number of mildly affected patients who may never develop clinical symptoms throughout life. However, later-onset exercise-induced myopathic symptoms remain characteristic clinical features of long-chain fatty acid oxidation defects. Disease prevalence has increased with newborn screening.

Oxidation of fatty acids in mitochondria is a key physiological process in higher eukaryotes including humans. The importance of the mitochondrial beta-oxidation system in humans is exemplified by the existence of a group of genetic diseases in man caused by an impairment in the mitochondrial oxidation of fatty acids. Identification of patients with a defect in mitochondrial beta-oxidation has long remained notoriously difficult, but the introduction of tandem-mass spectrometry in laboratories for genetic metabolic diseases has revolutionalized the field by allowing the rapid and sensitive analysis of acylcarnitines. Equally important is that much progress has been made with respect to the development of specific enzyme assays to identify the enzyme defect in patients subsequently followed by genetic analysis. In this review, we will describe the current state of knowledge in the field of fatty acid oxidation enzymology and its application to the follow-up analysis of positive neonatal screening results.

Evidence-based guidelines for monitoring patients with disorders in fatty acid oxidation (FAO) are lacking, and most protocols are based on expert statements. Here, we describe our protocol for Danish patients. Clinical monitoring is the most important measure and has the main aims of checking growth, development and diet and of bringing families to the clinic regularly to remind them of their child's risk and review how they cope and adjust, e.g. to an acute intercurrent illness. Most of these measures are simple and can be carried out during a routine out-patient visit; we seldom do more complicated assessments by a neuropsychologist, speech therapist, or physical and occupational therapists. Paraclinical measurements are not used for short-chain and medium-chain disorders; electrocardiography (including 24 h monitoring) and echocardiography are done for most patients with long-chain and carnitine transporter deficiencies. Eye examination is done in all, and liver ultrasonography in some patients with long-chain 3-hydroxyacyl-coenzyme A dehydrogenase/tri-functional protein (LCHAD/TFP) deficiencies. Biochemical follow-up includes determination of free carnitine and acylcarnitines. Free carnitine is measured to monitor carnitine supplementation in patients with multiple acyl-coenzyme A dehydrogenase deficiency (MADD) and carnitine transporter deficiency (CTD) and to follow metabolic control and disclose deficiency states in other FAO disorders. We are evaluating long-chain acylcarnitines in patients with long-chain disorders; so far there does not seem to be any clear-cut benefit in following these levels. An erythrocyte fatty acid profile is done in patients with long-chain disorders to test for essential fatty acid and docosahexanoic acid (DHA) deficiencies. The measurement of creatine kinase is helpful in long-chain disorders. Ongoing follow-up and education of the patient is important throughout life to prevent disease morbidity or death from metabolic crises.

Over the years, the mitochondrial fatty acid β-oxidation (FAO) pathway has been characterised at the biochemical level as well as the molecular biological level. FAO plays a pivotal role in energy homoeostasis, but it competes with glucose as the primary oxidative substrate. The mechanisms behind this so-called glucose-fatty acid cycle operate at the hormonal, transcriptional and biochemical levels. Inherited defects for most of the FAO enzymes have been identified and characterised and are currently included in neonatal screening programmes. Symptoms range from hypoketotic hypoglycaemia to skeletal and cardiac myopathies. The pathophysiology of these diseases is still not completely understood, hampering optimal treatment. Studies of patients and mouse models will contribute to our understanding of the pathogenesis and will ultimately lead to better treatment.

Experience with new-born screening (NBS) for disorders of fatty-acid oxidation (FAOD) is now becoming available from an increasing number of programs worldwide. The spectrum of FAOD differs widely between ethnic groups. Incidence calculations from reports from Australia, Germany, and the USA of a total of 5,256,999 newborns give a combined incidence of all FAOD of approximately 1:9,300. However, it appears to be much lower in Asians. Consequently, a significant prevalence and evidence for a clear benefit of NBS is proven for medium-chain acyl-CoA dehydrogenase deficiency (MCAD) only in countries with a high percentage of Caucasians, with very-long-chain acyl-CoA dehydrogenase deficiency (VLCAD) and long-chain 3-hydroxy acyl-CoA dehydrogenase deficiency (LCHAD) being additional candidates. The long-term benefit for many disorders has still to be evaluated and will require international collaboration, especially for the rarest disorders. Short-chain acyl-CoA dehydrogenase deficiency (SCAD) [as well as Systemic carnitine transporter deficiency (CTD) and dienoyl-CoA reductase deficiency (DE-RED)] are conditions of uncertain clinical significance, but most FAOD have a spectrum of clinical presentations (healthy-death). Confirmatory diagnostic procedures should be agreed upon to ensure international comparability of results and evidence-based modifications. The case of short-chain acyl-CoA dehydrogenase deficiency (SCAD) deficiency shows that even inclusion of conditions without a clearly known natural course may prove useful with respect to gain of knowledge and consecutive exclusion of a biochemical abnormality without clinical significance, although this line of argument implies the existence of structured follow-up programs and bears ethical controversies. As a final conclusion, the accumulated evidence suggests all FAOD should to be included into tandem mass spectrometry (MS/MS)-based NBS programs provided sufficient laboratory performance is guaranteed.

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is the most common fatty acid oxidation disorder. Typically, undiagnosed individuals are asymptomatic until an episode of increased energy demand and fasting occurs, resulting in metabolic derangement. Phenotypic heterogeneity has been increasingly realized, with reports of both neonates and adults manifesting with life-threatening symptoms including encephalopathy, rhabdomyolysis, and cardiac failure. If diagnosed presymptomatically, outcome is favorable basically by avoidance of fasting. Early detection by newborn screening (NBS) has significantly reduced the incidence of severe adverse events including deaths. In this manuscript we focus on the natural course of the disease in both children and adults. Although NBS for MCADD has been successfully established, continuing efforts need to be made to avoid acute crises and deterioration of outcome in screened patients entering adolescence and adulthood.

Mitochondrial fatty acid oxidation represents an important pathway for energy generation during periods of increased energy demand such as fasting, febrile illness and muscular exertion. In liver, the primary end products of the pathway are ketone bodies, which are released into the circulation and provide energy to tissues that are not able to oxidize fatty acids such as brain. Other tissues, such as cardiac and skeletal muscle are capable of direct utilization of the fatty acids as sources of energy. This article provides an overview of the pathogenesis of fatty acid oxidation disorders. It describes the different tissue involvement with the disease processes and correlates disease phenotype with the nature of the genetic defect for the known disorders of the pathway.

Mouse models have been designed for a number of fatty acid oxidation defects. Studies in these mouse models have demonstrated that different pathogenetic mechanisms play a role in the pathophysiology of defects of fatty acid oxidation. Supplementation with L-carnitine does not prevent low tissue carnitine levels and induces acylcarnitine production having potentially toxic effects, as presented in very-long-chain acyl-CoA dehydrogenase (VLCAD)-deficient mice. Energy deficiency appears to be an important mechanism in the development of cardiomyopathy and skeletal myopathy in fatty acid oxidation defects and is also the underlying mechanism of cold intolerance. Hypoglycemia as one major clinical sign in all fatty acid oxidation defects occurs due to a reduced hepatic glucose output and an enhanced peripheral glucose uptake rather than to transcriptional changes that are also observed simultaneously, as presented in medium-chain acyl-CoA dehydrogenase (MCAD)-deficient mice. There are reports that an impaired fatty acid oxidation also plays a role in intrauterine life. The embryonic loss demonstrated for some enzyme defects in the mouse supports this hypothesis. However, the exact mechanisms are unknown. This observation correlates to maternal hemolysis, elevated liver enzymes, low platelets (HELLP) syndrome, as observed in pregnancies carrying a long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD)-deficient fetus. Synergistic heterozygosity has been shown in isolated patients and in mouse models to be associated with clinical phenotypes common to fatty acid oxidation disorders. Synergistic mutations may also modulate severity of the clinical phenotype and explain in part clinical heterogeneity of fatty acid oxidation defects. In summary, knowledge about the different pathogenetic mechanisms and the resulting pathophysiology allows the development of specific new therapies.

In fatty acid oxidation defects, the majority of gene variations are of the missense type and, therefore, prone to inducing misfolding in the resulting mutant protein. The fate of the mutant protein depends on the nature of the gene variation and other genetic factors as well as cellular and environmental factors. Since it has been shown that certain fatty acid oxidation enzyme proteins, exemplified by mutant medium-chain and short-chain acyl-CoA dehydrogenases as well as electron transfer flavoprotein and electron transfer flavoprotein dehydrogenase, may accumulate during cellular stress, e.g. elevated temperature, there is speculation about how such proteins may disturb the integrity of the putative fatty acid oxidation metabolone, in which the two flavoproteins link the matrix-located acyl-CoA dehydrogenases to the respiratory chain in the mitochondrial inner membrane. However, since studies so far have not been able to define the fatty acid oxidation metabolone, it is concluded that new concepts and refined techniques are required to answer these questions and thereby contribute to the elucidation of the cellular pathophysiology and the genotype-phenotype relationship in fatty acid oxidation defects.

Assessing the outcome of fatty acid oxidation disorders is difficult, as most are rare. For diagnosis by newborn screening, the situation is compounded: far more cases are diagnosed by screening than by clinical presentation, representing a somewhat different cohort. The literature on outcome was reviewed. For disorders other than medium-chain acyl-coenzyme A (CoA) dehydrogenase (MCAD) deficiency there was insufficient evidence to make many firm statements. In MCAD deficiency, risk of death in the first 72 h is around 4%, with a further approximately 5-7% fatality rate in the first 6 years but very low subsequent risk in previously undiagnosed patients. The risk of death after diagnosis is very low at any age, with good management. The long-term outcome is good nowadays. Very-long-chain acyl-CoA dehydrogenase deficiency poses a risk of death in early infancy, but the condition is generally treatable, with a good outcome after diagnosis. Approximately 10-20% of patients diagnosed by newborn screening and treated nevertheless suffer episodic rhabdomyolysis. Some patients never become symptomatic. Isolated long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency is treatable, but most patients suffer episodic hypoketotic hypoglycaemia and rhabdomyolysis. Generalised mitochondrial tri-functional protein deficiency has high early mortality rate. A more insidious presentation also occurs, with symptoms sometimes confined to progressive axonal neuropathy. Among carnitine cycle disorders, carnitine transporter deficiency, potentially lethal, is uniformly successfully treated orally with carnitine. Carnitine-acylcarnitine translocase and early-onset carnitine palmitoyl transferase type II (CPT II) deficiencies have an extremely high neonatal mortality rate. Late-onset CPT II is characterised only by episodic rhabdomyolysis on severe exercise. CPT type IA deficiency may often be benign, although early presentation with hypoketotic hypoglycaemia certainly occurs.

Treatment recommendations in mitochondrial fatty acid oxidation (FAO) defects are diverse. With implementation of newborn screening and identification of asymptomatic patients, it is necessary to define whom to treat and how strictly. We here discuss critical questions that are currently under debate. For some asymptomatic long-chain defects, long-chain fat restriction plays a minor role, and a normal diet may be introduced. For patients presenting only with myopathic symptoms, e.g., during exercise, treatment may be adapted to energy demand. As a consequence, patients with exercise-induced myopathy may be able to return to normal activity when provided with medium-chain triglycerides (MCT) prior to exercise. There is no need to limit participation in sports. Progression of retinopathy in disorders of the mitochondrial trifunctional protein complex is closely associated with hydroxyacylcarnitine accumulation. A strict low-fat diet with MCT supplementation is recommended to slow or prevent progression of chorioretinopathy. Additional docosahexanoic acid does not prevent the decline in retinal function but does promote nonspecific improvement in visual acuity and is recommended. There is no evidence that L-carnitine supplementation is beneficial. Thus, supplementation with L-carnitine in a newborn identified by screening with either a medium-chain or long-chain defect is not supported. With respect to the use of the odd-chain medium-chain triglyceride triheptanoin in myopathic phenotypes, randomized trials are needed to establish whether triheptanoin is more effective than even-chain MCT. With increasing pathophysiological knowledge, new treatment options have been identified and are being clinically evaluated. These include the use of bezafibrates in myopathic long-chain defects.

Short-chain acyl-CoA dehydrogenase deficiency (SCADD) is an autosomal recessive inborn error of mitochondrial fatty acid oxidation. SCADD is biochemically characterized by increased C4-carnitine in plasma and ethylmalonic acid in urine. The diagnosis of SCADD is confirmed by DNA analysis showing SCAD gene mutations and/or variants. SCAD gene variants are present in homozygous form in approximately 6% of the general population and considered to confer susceptibility to development of clinical disease. Clinically, SCADD generally appears to present early in life and to be most frequently associated with developmental delay, hypotonia, epilepsy, behavioral disorders, and hypoglycemia. However, these symptoms often ameliorate and even disappear spontaneously during follow-up and were found to be unrelated to the SCAD genotype. In addition, in some cases, symptoms initially attributed to SCADD could later be explained by other causes. Finally, SCADD relatives of SCADD patients as well as almost all SCADD individuals diagnosed by neonatal screening remained asymptomatic during follow-up. This potential lack of clinical consequences of SCADD has several implications. First, the diagnosis of SCADD should never preclude extension of the diagnostic workup for other potential causes of the observed symptoms. Second, patients and parents should be clearly informed about the potential lack of relevance of the disorder to avoid unfounded anxiety. Furthermore, to date, SCADD is not an optimal candidate for inclusion in newborn screening programs. More studies are needed to fully establish the relevance of SCADD and solve the question as to whether SCADD is involved in a multifactorial disease or represents a nondisease.

Mevalonic aciduria is a rare disease that is a consequence of a deficiency of mevalonate kinase, an inborn error in the biosynthesis of cholesterol. Approximately 30 cases have been reported. We present our data on two siblings with mevalonic aciduria as a contribution to the recognition of this subject. Both were born after uneventful pregnancies. Their parents were healthy and not consanguineous. They had normal somatic and psychomotor development until they were around 2 years old. After the second year of life they developed mental retardation, ataxia and hypotonia. MRI showed cerebellar atrophy of both hemispheres and vermis. One sibling, from the age of 10 years onwards, suffered from complex partial seizures that were controlled with levetiracetam and lamotrigine. At 11 and 12 years of age, respectively, they were able to walk without help, but their gait was broad and ataxic. Their speech was dysarthric, fine motor skills were impaired as result of cerebellar ataxia, and they had moderate mental retardation. Diagnosis of mevalonic aciduria was made at this age through urinary organic acid analysis by gas chromatography-mass spectroscopy, which revealed high urinary excretion of mevalonic acid. They are currently 18 and 17 years old, respectively, show mental retardation and are able to walk but with difficulty. In our patients, ataxia due to cerebellar atrophy and mental retardation have been the predominant clinical manifestations. In mildly affected patients who survive infancy, these seem to be the predominant findings.

Very long-chain acyl-CoA dehydrogenase (VLCAD) catalyses the initial step of mitochondrial beta-oxidation of long-chain fatty acids with a chain length of 14 to 20 carbons. Deficiency of VLCAD activity has been associated with a range of phenotypes, including a severe lethal form presenting in the infantile period and a milder variant with onset in childhood. Varying rates of residual enzyme activity partly explain the heterogeneity in presentations. Here we report the course of disease in a pair of monozygotic twin sisters who were diagnosed in their late forties during an evaluation for rhabdomyolysis and fatigue. Interestingly, the patients' complaints were most severe during puberty and declined significantly after the menopause. The basis for this observation is uncertain, but may be related to hormonally-mediated changes in lipid metabolism that may occur at these times. As metabolic decompensation can be associated with significant morbidity, timely diagnosis of VLCAD deficiency is important. The introduction of appropriate dietary measures (i.e. avoidance of fasting, long-chain fat restriction and supplementation with medium-chain triglycerides) greatly reduces the likelihood of complications.

As judged by tandem mass spectrometry blood spot screening, the incidence of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is 1:14 600 (CI 95%: 1:13 500-1:15 900) in almost 8.2 million newborns worldwide and is 2- to-3 fold higher than that identified in the same populations after clinical presentation. In mass-screened newborn populations, the 985A>G (K329E) mutation accounts for 54-90% of disease alleles, with homozygotes representing about 47-80% of MCAD deficiency cases. Worldwide, octanoylcarnitine levels are an effective primary screen for MCAD deficiency in newborns. Newborns homozygous for the 985A < G mutation have higher octanoylcarnitine levels than do those compound heterozygous for 985A < G and those with other genotypes. Time of sampling after birth also significantly affects octanoylcarnitine levels in MCAD-deficient newborns. Tandem mass spectrometry newborn blood spot screening for MCAD deficiency is accurate and effective, reduces morbidity and mortality, and merits expansion to other populations worldwide.

Carnitine palmitoyltransferase type II (CPT II) deficiency has three basic phenotypes, late-onset muscular (mild), infantile/juvenile hepatic (intermediate) and severe neonatal. We have measured fatty acid oxidation and CPT II activity and performed mutation studies in 24 symptomatic patients representing the full clinical spectrum of disease. Severe and intermediate phenotypes show a clear correlation with biochemical indices and genetic analysis revealed causative mutations in most patients. Studies of mild phenotypes suggest a more complex interaction, with higher residual fatty acid oxidation, a wider range of CPT II activity (10-60%) but little evidence of genotype-phenotype correlation. Residual CPT II mutant protein from myopathic patients shows thermal instability at 41 degrees C. The common 'polymorphisms' V3681 and M647V are strikingly overrepresented in the myopathic patients, the implication being that they may significantly influence the manifestation of clinical disease and could therefore potentially be considered as a susceptibility variants. Among myopathic individuals, males comprised 88% of patients, suggesting increased susceptibility to clinical disease. A small number of symptomatic patients appear to have significant residual CPT II activity (42-60%) The synergistic interaction of partial deficiencies of CPT II, muscle adenosine monophosphate deaminase and possibly other enzymes of muscle energy metabolism in the aetiology of episodic myopathy deserves wider consideration.

A neonate subsequently diagnosed with carnitine palmitoyltransferase I deficiency died at 34 h of untreatable bradycardia. There was fatty infiltration of the liver and increased free carnitine and reduced acylcarnitines in the blood.

In recent years tremendous progress has been made with respect to the enzymology of the mitochondrial fatty acid beta-oxidation machinery and defects therein. Firstly, a number of new mitochondrial beta-oxidation enzymes have been identified, including very-long-chain acyl-CoA dehydrogenase (VLCAD) and mitochondrial trifunctional protein (MTP). Secondly, the introduction of tandem MS for the analysis of plasma acylcarnitines has greatly facilitated the identification of patients with a defect in fatty acid oxidation (FAO). These two developments explain why the number of defined FAO disorders has increased dramatically, making FAO disorders the most rapidly growing group of inborn errors of metabolism. In this review we describe the current state of knowledge of the enzymes involved in the mitochondrial oxidation of straight-chain, branched-chain and (poly)unsaturated fatty acyl-CoAs as well as disorders of fatty acid oxidation. The laboratory diagnosis of these disorders is described, with particular emphasis on the methods used to identify the underlying enzyme defect and the molecular mutations. In addition, a simple flowchart is presented as a guide to the identification of mitochondrial FAO-disorders. Finally, treatment strategies are discussed briefly.

Amniocytes isolated from two pregnancies at risk for fatty acid oxidation defects were incubated with stable isotopically labelled palmitate, in the presence of L-carnitine, to probe that pathway. The labelled acylcarnitines were then quantitated using tandem mass spectrometry. Amniocytes from a pregnancy at risk for medium-chain acyl-CoA dehydrogenase (MCAD) deficiency produced a characteristic acylcarnitine profile with increased levels of octanoylcarnitine and decanoylcarnitine, indicative of MCAD deficiency. DNA analysis confirmed that the fetus was homozygous for the MCAD A985G mutation. Acylcarnitine and DNA analysis of the infant's blood obtained post-partum confirmed MCAD deficiency. Amniocytes from a pregnancy at risk for an unspecified fat oxidation defect produced increased levels of long-chain acylcarnitines consistent with a deficiency in very-long-chain acyl-CoA dehydrogenase (VLCAD). Measurements of the enzymatic activity confirmed VLCAD deficiency in amniocytes. Acylcarnitine profiles of the infant's blood obtained post-partum in addition to enzyme activities measured in fibroblasts confirmed VLCAD deficiency. The successful prenatal diagnosis of VLCAD and MCAD deficiencies using in vitro probes of fatty acid oxidation in fibroblasts suggests that this approach can potentially recognize many mitochondrial fatty acid oxidation defects even if no prior diagnosis is determined in the family at risk.

Two enzymes, lipoprotein lipase and hepatic triglyceride lipase, are involved in the hydrolysis of triglycerides from chylomicrons and very low density lipoprotein (VLDL). Lipoprotein lipase has an absolute requirement for apolipoprotein CII for activity. Three inborn errors of metabolism which give rise to hypertriglyceridaemia have been described. The biochemical and clinical aspects of these disorders, lipoprotein lipase deficiency (familial type I hyperlipoproteinaemia), hepatic triglyceride lipase deficiency and apo-CII deficiency are discussed.

Medium-chain acyl-CoA dehydrogenase deficiency is a recently described inborn error of metabolism characterized by episodes of coma and hypoketotic hypoglycaemia in response to prolonged fasting. Secondary carnitine deficiency has been documented in these patients as well as the excretion in the urine of medium-chain-length acyl carnitine esters, such as octanoylcarnitine. Based on the potential toxicity of medium-chain fatty acid metabolites and the beneficial responses of patients with other inborn errors of metabolism and secondary carnitine deficiency, oral carnitine has been proposed as treatment for children with medium-chain acyl-CoA dehydrogenase deficiency. We report the results of carefully monitored fasting challenges of an infant with this deficiency both before and after 3 months of oral carnitine therapy. Carnitine supplementation failed to prevent lethargy, vomiting, hypoglycaemia and accumulation of free fatty acids in response to fasting despite normalization of plasma carnitine levels and a marked increase in urinary excretion of acyl-carnitine esters. Potentially toxic medium-chain fatty acids accumulated in the plasma in spite of therapy. Based on this study of one patient, we stress that avoidance of fasting and prompt institution of glucose supplementation in situations when oral intake is interrupted remain the mainstays of therapy for medium-chain acyl-CoA dehydrogenase deficient patients.