Myopathic form of very-long chain acyl-coa dehydrogenase deficiency: evidence for temperature-sensitive mild mutations in both mutant alleles in a Japanese girl

Specifications of the ACMG/AMP guidelines for ACADVL variant interpretation

Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency (VLCADD) is a relatively common inborn error of metabolism, but due to difficulty in accurately predicting affected status through newborn screening, molecular confirmation of the causative variants by sequencing of the ACADVL gene is necessary. Although the ACMG/AMP guidelines have helped standardize variant classification, ACADVL variant classification remains disparate due to a phenotype that can be nonspecific, the possibility of variants that produce late-onset disease, and relatively high carrier frequency, amongst other challenges. Therefore, an ACADVL-specific variant curation expert panel (VCEP) was created to facilitate the specification of the ACMG/AMP guidelines for VLCADD. We expect these guidelines to help streamline, increase concordance, and expedite the classification of ACADVL variants.

[A case of very long chain acyl-CoA dehydrogenase deficiency diagnosed due to a trigger of hyperemesis gravidarum during pregnancy]

A 25-year-old Japanese woman with a history of repeated episodes of rhabdomyolysis since the age of 12 presented with rhabdomyolysis caused by hyperemesis gravidarum. Blood tests showed an elevated serum CK level (11,755 ‍IU/l; normal: 30-180 ‍IU/l). Carnitine fractionation analysis revealed low levels of total carnitine (18.3 ‍μmol/l; normal: 45-91 ‍μmol/l), free carnitine (13.1 ‍μmol/l; normal: 36-74 ‍μmol/l), and acylcarnitine (5.2 ‍μmol/l; normal: 6-23 ‍μmol/l). Tandem mass spectrometry showed high levels of C14:1 acylcarnitine (0.84 ‍nmol/ml: normal: <0.4 ‍nmol/ml) and a high C14:1/C2 ratio of 0.253 (normal: <0.013), indicating a potential diagnosis of very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency. Enzyme activity measurement in the patient's peripheral blood lymphocytes confirmed the diagnosis of VLCAD deficiency, with low palmitoyl-CoA dehydrogenase levels (6.5% of normal control value). With the patient's informed consent, acyl-CoA dehydrogenase very long-chain (ACADVL) gene analysis revealed compound heterozygous mutations of c.1332G>A in exon 13 and c.1349G>A (p.R450H) in exon 14. In Japan, neonatal mass screening is performed to detect congenital metabolic diseases. With the introduction of tandem mass screening in 2014, fatty acid metabolism disorders, including VLCAD deficiency, are being detected before the onset of symptoms. However, it is important to note that mass screening cannot detect all cases of this disease. For patients with recurrent rhabdomyolysis, it is essential to consider congenital diseases, including fatty acid metabolism disorders, as a potential diagnosis.

The frequencies of very long-chain acyl-CoA dehydrogenase deficiency genetic variants in Japan have changed since the implementation of expanded newborn screening

Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency has been a target of expanded newborn screening (ENBS) using tandem mass spectrometry in Japan. Since the implementation of ENBS, a number of novel ACADVL variants responsible for VLCAD deficiency have been identified. In this study, genotypic differences in Japanese patients with VLCAD deficiency were investigated before and after ENBS. The ACADVL variants in 61 subjects identified through ENBS (ENBS group) and in 40 patients who subsequently developed clinical symptoms without undergoing ENBS (pre-ENBS group) were compared. Subjects in the ENBS group underwent genetic testing and/or VLCAD enzyme activity measurements. Patients in the pre-ENBS group were stratified into three clinical phenotypes and underwent genetic testing. This study revealed that the variants p.K264E, p.K382Q and c.996dupT were found in both groups, but their frequencies were lower in the ENBS group (5.2%, 3.1% and 4.2%, respectively) than in the pre-ENBS group (16.5%, 12.7% and 10.1%, respectively). In addition, p.C607S, p.T409M, p.M478I, p.G289R, p.C237R, p.T260M, and p.R229* were exclusively identified in the ENBS group. Among these variants, p.C607S exhibited the highest frequency (18.8%). The patients who were heterozygous for p.C607S demonstrated 7-42% of control enzyme activity. p.C607S is suspected to be unique to Japanese individuals. According to a comparison of enzyme activity, patients with the p.C607S variant may exhibit higher enzyme activity than those with the p.A416T, p.A180T, p.R450H, and p.K264E variants, which are responsible for the myopathic form of the disease. The VLCAD deficiency genotypes have changed since the initiation of ENBS in Japan.

Adult-onset Repeat Rhabdomyolysis with a Very Long-chain Acyl-CoA Dehydrogenase Deficiency Due to Compound Heterozygous ACADVL Mutations

Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is a genetic disorder of fatty acid beta oxidation that is caused by a defect in ACADVL, which encodes VLCAD. The clinical presentation of VLCAD deficiency is heterogeneous, and either a delayed diagnosis or a misdiagnosis may sometimes occur. We herein describe a difficult-to-diagnose case of the muscle form of adult-onset VLCAD deficiency with compound heterozygous ACADVL mutations including c.790A>G (p.K264E) and c.1246G>A (p.A416T).

Anesthesia management in a patient with very long-chain acyl-Coenzyme A dehydrogenase deficiency

Background

In a patient with very long-chain acyl-Coenzyme A dehydrogenase (VLCAD) deficiency, metabolism of fatty acids is impaired and a supply of alternative energy is limited when glucose level is insufficient on starvation.

Case presentation

A 37-year-old woman with VLCAD deficiency was diagnosed with an ovarian cyst and was scheduled for laparoscopic ovarian cystectomy. Glucose was administered intravenously with the start of fasting. Anesthesia was induced with remifentanil, midazolam, and thiamylal, maintained with desflurane and remifentanil. Body temperature was maintained at 36.2–36.7 °C. During anesthesia, hypoglycemia did not occur, creatine kinase levels were in the normal range, and myoglobinuria was not detected. No shivering was observed after extubation.

Conclusions

Glucose was administered to avoid perioperative hypoglycemia. Body temperature was controlled to avoid shivering, which would otherwise increase skeletal muscle energy needs. Blood creatine kinase did not increase, and myoglobinuria was not detected; thus, rhabdomyolysis was unlikely to develop.

One potential hotspot ACADVL mutation in Chinese patients with very-long-chain acyl-coenzyme A dehydrogenase deficiency

Very long-chain acyl-coenzyme A dehydrogenase deficiency (VLCAD deficiency), a rare autosomal recessive disorder, is characterized by hypoketotic hypoglycemia, cardiomyopathy, liver damage, and myopathy. VLCAD deficiency is caused by defects of ACADVL gene, which encodes VLCAD protein. The aim of this study was to determine the clinical, biochemical, prognosis and mutation spectrum of patients with VLCAD deficiency in mainland China. A total of Six families visited us, four patients (2 boys and 2 girls) were admitted in hospital due to liver dysfunction, hypoglycemia, and positive newborn screen result. The parents of the other two patients (2 girls) visited us for genetic consultation after their children's death. All the six patients had elevated level of serum tetradecenoylcarnitine (C14:1-carnitine), four of them showed decreased free carnitine (C0) level, and three had dicarboxylic aciduria. Eight types of mutations of the ACADVL gene were detected, three of them are novel, including c.563G > A (p.G188D) c.1387G > A (p.G463R) and c.1582_1586del (p.L529Sfs*31). The p.R450H mutation accounts for 9/52 alleles (5/40 in previous study of 20 unrelated patients, and 4/12 in this study) of genetically diagnosed Chinese VLCAD deficiency cases. The four alive patients (Patient 1-4) responded well to diet prevention and drug therapy with stable hepatic dysfunction condition. In conclusion, we describe three novel mutations of the ACADVL gene among six unrelated families with VLCAD deficiency. Moreover, we suggest that the p.R450H may be a potential hotspot mutation in the Chinese population.

Defective fatty acid oxidation in mice with muscle-specific acyl-CoA synthetase 1 deficiency increases amino acid use and impairs muscle function

Loss of long-chain acyl-CoA synthetase isoform-1 (ACSL1) in mouse skeletal muscle (Acsl1^{M -/-}) severely reduces acyl-CoA synthetase activity and fatty acid oxidation. However, the effects of decreased fatty acid oxidation on skeletal muscle function, histology, use of alternative fuels, and mitochondrial function and morphology are unclear. We observed that Acsl1^{M -/-} mice have impaired voluntary running capacity and muscle grip strength and that their gastrocnemius muscle contains myocytes with central nuclei, indicating muscle regeneration. We also found that plasma creatine kinase and aspartate aminotransferase levels in Acsl1^{M -/-} mice are 3.4- and 1.5-fold greater, respectively, than in control mice (Acsl1^flox/flox ), indicating muscle damage, even without exercise, in the Acsl1^{M -/-} mice. Moreover, caspase-3 protein expression exclusively in Acsl1^{M -/-} skeletal muscle and the presence of cleaved caspase-3 suggested myocyte apoptosis. Mitochondria in Acsl1^{M -/-} skeletal muscle were swollen with abnormal cristae, and mitochondrial biogenesis was increased. Glucose uptake did not increase in Acsl1^{M -/-} skeletal muscle, and pyruvate oxidation was similar in gastrocnemius homogenates from Acsl1^{M -/-} and control mice. The rate of protein synthesis in Acsl1^{M -/-} glycolytic muscle was 2.1-fold greater 30 min after exercise than in the controls, suggesting resynthesis of proteins catabolized for fuel during the exercise. At this time, mTOR complex 1 was activated, and autophagy was blocked. These results suggest that fatty acid oxidation is critical for normal skeletal muscle homeostasis during both rest and exercise. We conclude that ACSL1 deficiency produces an overall defect in muscle fuel metabolism that increases protein catabolism, resulting in exercise intolerance, muscle weakness, and myocyte apoptosis.

A Nonsense Variant in the ACADVL Gene in German Hunting Terriers with Exercise Induced Metabolic Myopathy

Several enzymes are involved in fatty acid oxidation, which is a key process in mitochondrial energy production. Inherited defects affecting any step of fatty acid oxidation can result in clinical disease. We present here an extended family of German Hunting Terriers with 10 dogs affected by clinical signs of exercise induced weakness, muscle pain, and suspected rhabdomyolysis. The combination of clinical signs, muscle histopathology and acylcarnitine analysis with an elevated tetradecenoylcarnitine (C14:1) peak suggested a possible diagnosis of acyl-CoA dehydrogenase very long chain deficiency (ACADVLD). Whole genome sequence analysis of one affected dog and 191 controls revealed a nonsense variant in the ACADVL gene encoding acyl-CoA dehydrogenase very long chain, c.1728C>A or p.(Tyr576*). The variant showed perfect association with the phenotype in the 10 affected and more than 500 control dogs of various breeds. Pathogenic variants in the ACADVL gene have been reported in humans with similar myopathic phenotypes. We therefore considered the detected variant to be the most likely candidate causative variant for the observed exercise induced myopathy. To our knowledge, this is the first description of this disease in dogs, which we propose to name exercise induced metabolic myopathy (EIMM), and the identification of the first canine pathogenic ACADVL variant. Our findings provide a large animal model for a known human disease and will enable genetic testing to avoid the unintentional breeding of affected offspring.

Clinical, Biochemical, and Molecular Features in 37 Saudi Patients with Very Long Chain Acyl CoA Dehydrogenase Deficiency

BACKGROUND

Very long chain acyl CoA dehydrogenase (VLCAD) deficiency (OMIM#201475) is an autosomal recessive disorder of fatty acid beta oxidation caused by defect in the ACADVL. The aim of this study was to analyze the clinical, biochemical, and molecular features of VLCAD deficiency in Saudi Arabia, including the treatment and outcome.

METHODS

We carried out a retrospective chart review analysis of 37 VLCAD deficiency patients from two tertiary centers in Saudi Arabia, over a 14-year period (2002-2016). Twenty-three patients were managed at King Abdul-Aziz Medical City and fourteen patients at King Fahad Medical City.

RESULTS

Severe early onset VLCAD deficiency is the most frequent phenotype in our patients, caused by four different mutations in ACADVL; 31 patients (83.7%) had a homozygous nonsense mutation in exon 2 of ACADVL c.65C>A;p. Ser22X. Twenty-three patients died before the age of 2 years, despite early detection by newborn screening and implementation of treatment, including supplementation with medium chain triglycerides.

CONCLUSION

This study reports the clinical, biochemical, molecular findings, treatment, and outcome of patients with VLCAD deficiency over the last 14 years. We identified the most common variant and one new variant in ACADVL. Despite early diagnosis and treatment, the outcome of VLCAD deficiency in this Saudi Arabian population remains poor. Preventive measures, such as prenatal diagnosis, could be implemented.

Clinical features and mutations in seven Chinese patients with very long chain acyl-CoA dehydrogenase deficiency

BACKGROUND

Very long chain acyl-CoA dehydrogenase deficiency (VLCADD) is an inherited metabolic disease caused by deleterious mutations in the ACADVL gene that encodes very long chain acyl-CoA dehydrogenase (VLCAD), and which can present as cardiomyopathy in neonates, as hypoketotic hypoglycemia in infancy, and as myopathy in late-onset patients. Although many ACADVL mutations have been described, no prevalent mutations in the ACADVL gene have been associated with VLCADD. Herein, we report the clinical course of the disease and explore the genetic mutation spectrum in seven Chinese patients with VLCADD.

METHODS

Seven Chinese patients, from newborn to 17 years old, were included in this study. Tandem mass spectrometry was performed to screen for VLCAD deficiency. All exons and flanking introns of the ACADVL gene were analyzed using polymerase chain reaction and direct sequencing. Online analysis tools were used to predict the impact of novel mutations.

RESULTS

All cases had elevated serum levels of tetradecanoylcarnitine (C14:1) which is the characteristic biomarker for VLCADD. The phenotype of VLCADD is heterogeneous. Two patients were hospitalized for hypoactivity and hypoglycemia shortly after birth. Three patients showed hepatomegaly and hypoglycemia in infancy. The other two adolescent patients showed initial manifestations of exercise intolerance or rhabdomyolysis. Three of the patients died at the age of 6-8 months. Eleven different mutations in the ACADVL gene in the 7 patients were identified, including seven reported mutations (p.S22X, p.W427X, p.A213T, p.G222R, p.R450H, c.296-297delCA, c.1605+1G>T) and four novel mutations (p.S72F, p.Q100X, p.M437T, p.D466Y). The p.R450H and p.D466Y (14.28%, 2/14 alleles) mutations were identified in two alleles respectively.

CONCLUSIONS

The clinical manifestations were heterog-eneous and ACADVL gene mutations were heterozygous in the seven VLCADD Chinese patients. R450H may be a relatively common mutation in Asian populations. The genotype and phenotype had a certain correlation in our patients.

Compared effects of missense mutations in Very-Long-Chain Acyl-CoA Dehydrogenase deficiency: Combined analysis by structural, functional and pharmacological approaches

Very-Long-Chain Acyl-CoA Dehydrogenase deficiency (VLCADD) is an autosomal recessive disorder considered as one of the more common ss-oxidation defects, possibly associated with neonatal cardiomyopathy, infantile hepatic coma, or adult-onset myopathy. Numerous gene missense mutations have been described in these VLCADD phenotypes, but only few of them have been structurally and functionally analyzed, and the molecular basis of disease variability is still poorly understood. To address this question, we first analyzed fourteen disease-causing amino acid changes using the recently described crystal structure of VLCAD. The predicted effects varied from the replacement of amino acid residues lining the substrate binding cavity, involved in holoenzyme-FAD interactions or in enzyme dimerisation, predicted to have severe functional consequences, up to amino acid substitutions outside key enzyme domains or lying on near enzyme surface, with predicted milder consequences. These data were combined with functional analysis of residual fatty acid oxidation (FAO) and VLCAD protein levels in patient cells harboring these mutations, before and after pharmacological stimulation by bezafibrate. Mutations identified as detrimental to the protein structure in the 3-D model were generally associated to profound FAO and VLCAD protein deficiencies in the patient cells, however, some mutations affecting FAD binding or monomer-monomer interactions allowed a partial response to bezafibrate. On the other hand, bezafibrate restored near-normal FAO rates in some mutations predicted to have milder consequences on enzyme structure. Overall, combination of structural, biochemical, and pharmacological analysis allowed assessment of the relative severity of individual mutations, with possible applications for disease management and therapeutic approach.

Clinical and molecular aspects of Japanese patients with mitochondrial trifunctional protein deficiency

Mitochondrial trifunctional protein (MTP) deficiency is a rare inherited metabolic disorder of mitochondrial fatty acid oxidation. We newly characterized three novel mutations in 2 Japanese patients with MTP deficiency, and investigated the clinical and molecular aspects of 5 Japanese patients including 3 previously reported cases. Herein, we describe the characterization of four missense mutations, R214C, H346R, R411K, and V422G, in the HADHB gene, which have been identified in Japanese patients, employing a newly developed, sensitive transient expression analysis. Co-transfection of wild-type HADHA and HADHB cDNAs in SV40-transfected fibroblasts from a MTP-deficient patient yielded sufficient enzyme activity to evaluate low-level residual enzyme activity, using two incubation temperatures of 30 degrees C and 37 degrees C. At 30 degrees C, residual enzyme activity was higher than that at 37 degrees C in V422G, R214C, and R411K. However, H346R, which was seen in the most severe case, showed no enzyme activity at both temperatures. Our results demonstrate that a defect of HADHB in MTP deficiency is rather common in Japanese patients, and the mutational spectrum is heterogeneous. The present findings showed that all missense mutations in this study were disease-causing. Although the number of patients is still limited, it is suggested that the phenotype is correlated with the genotype and a combination of two mutant alleles of the HADHB gene in MTP deficiency.

Genetic basis for correction of very-long-chain acyl-coenzyme A dehydrogenase deficiency by bezafibrate in patient fibroblasts: toward a genotype-based therapy

Very-long-chain acyl-coenzyme A dehydrogenase (VLCAD) deficiency is an inborn mitochondrial fatty-acid beta-oxidation (FAO) defect associated with a broad mutational spectrum, with phenotypes ranging from fatal cardiopathy in infancy to adolescent-onset myopathy, and for which there is no established treatment. Recent data suggest that bezafibrate could improve the FAO capacities in beta-oxidation-deficient cells, by enhancing the residual level of mutant enzyme activity via gene-expression stimulation. Since VLCAD-deficient patients frequently harbor missense mutations with unpredictable effects on enzyme activity, we investigated the response to bezafibrate as a function of genotype in 33 VLCAD-deficient fibroblasts representing 45 different mutations. Treatment with bezafibrate (400 microM for 48 h) resulted in a marked increase in FAO capacities, often leading to restoration of normal values, for 21 genotypes that mainly corresponded to patients with the myopathic phenotype. In contrast, bezafibrate induced no changes in FAO for 11 genotypes corresponding to severe neonatal or infantile phenotypes. This pattern of response was not due to differential inductions of VLCAD messenger RNA, as shown by quantitative real-time polymerase chain reaction, but reflected variable increases in measured VLCAD residual enzyme activity in response to bezafibrate. Genotype cross-analysis allowed the identification of alleles carrying missense mutations, which could account for these different pharmacological profiles and, on this basis, led to the characterization of 9 mild and 11 severe missense mutations. Altogether, the responses to bezafibrate reflected the severity of the metabolic blockage in various genotypes, which appeared to be correlated with the phenotype, thus providing a new approach for analysis of genetic heterogeneity. Finally, this study emphasizes the potential of bezafibrate, a widely prescribed hypolipidemic drug, for the correction of VLCAD deficiency and exemplifies the integration of molecular information in a therapeutic strategy.

VLCAD deficiency: pitfalls in newborn screening and confirmation of diagnosis by mutation analysis

We diagnosed six newborn babies with very long-chain acyl-CoA dehydrogenase deficiency (VLCADD) through newborn screening in three years in Victoria (prevalence rate: 1:31,500). We identified seven known and two new mutations in our patients (2/6 homozygotes; 4/6 compound heterozygotes). Blood samples taken at age 48-72 h were diagnostic whereas repeat samples at an older age were normal in 4/6 babies. Urine analysis was normal in 5/5. We conclude that the timing of blood sampling for newborn screening is important and that it is important to perform mutation analysis to avoid false-negative diagnoses of VLCADD in asymptomatic newborn babies. In view of the emerging genotype-phenotype correlation in this disorder, the information derived from mutational analysis can be helpful in designing the appropriate follow-up and therapeutic regime for these patients.

Inhibition of very long chain acyl-CoA dehydrogenase during cardiac ischemia

The heart utilizes primarily fatty acids for energy production. During ischemia, however, diminished oxygen supply necessitates a switch from beta-oxidation of fatty acids to glucose utilization and glycolysis. Molecular mechanisms responsible for these alterations in metabolism are not fully understood. Mitochondrial acyl-CoA dehydrogenase catalyzes the first committed step in the beta-oxidation of fatty acids. In the current study, an in vivo rat model of myocardial ischemia was utilized to determine whether specific acyl-CoA dehydrogenases exhibit ischemia-induced alterations in activity, identify mechanisms responsible for changes in enzyme function, and assess the effects on mitochondrial respiration. Very long chain acyl-CoA dehydrogenase (VLCAD) activity declined 34% during 30 min of ischemia. Loss in activity appeared specific to VLCAD as medium chain acyl-CoA dehydrogenase activity remained constant. Loss in VLCAD activity during ischemia was not due to loss in protein content. In addition, activity was restored in the presence of the detergent Triton X-100, suggesting that changes in the interaction between the protein and inner mitochondrial membrane are responsible for ischemia-induced loss in activity. Palmitoyl-carnitine supported ADP-dependent state 3 respiration declined as a result of ischemia. When octanoyl-carnitine was utilized state 3 respiration remained unchanged. State 4 respiration increased during ischemia, an increase that appears specific to fatty acid utilization. Thus, VLCAD represents a likely site for the modulation of substrate utilization during myocardial ischemia. However, the dramatic increase in mitochondrial state 4 respiration would be predicted to accentuate the imbalance between energy production and utilization.

MS/MS-based newborn and family screening detects asymptomatic patients with very-long-chain acyl-CoA dehydrogenase deficiency

OBJECTIVES

To determine whether asymptomatic persons with biochemical evidence of very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency identified through expanded newborn screening with tandem mass spectometry have confirmed disease.

STUDY DESIGN

We characterized 8 asymptomatic VLCAD-deficient individuals by enzyme and/or mutational analysis and compared them with clinically diagnosed, symptomatic patients with regard to mutations, enzyme activity, phenotype, and age of disease onset.

RESULTS

VLCAD molecular analyses in 6 unrelated patients revealed the previously reported V243A mutation, associated with hepatic or myopathic phenotypes, on 7/12 alleles. All other mutations were also missense mutations. Residual VLCAD activities of 6% to 11% of normal were consistent with milder phenotypes. In these identified individuals treated prospectively with dietary modification as preventive measures, clinical symptoms did not develop during follow-up.

CONCLUSIONS

MS/MS-based newborn screening correctly identifies VLCAD-deficient individuals. Based on mutational and enzymatic findings, these infants probably are at risk of future disease. Because life-threatening metabolic derangement can occur even in otherwise mild phenotypes, we advocate universal newborn screening programs for VLCAD deficiency to detect affected patients and prevent development of metabolic crises. Longer-term follow-up is essential to define outcomes, the definite risk of future disease, and appropriate treatment recommendations.

Metabolic and drug-induced muscle disorders

PURPOSE OF REVIEW

The inherited disorders of muscle metabolism affect both substrate utilization and the final intramitochondrial oxidation through the Krebs cycle and the respiratory chain. Almost every step of these complex biochemical pathways can be affected by inborn errors, whose expression depends on peculiar tissue-specific or systemic gene expression. This review updates current knowledge in this broad field.

RECENT FINDINGS

New inherited defects are still being discovered, such as the beta-enolase deficiency in glycogenosis type XIII and mutations in the gene encoding an esterase/lipase/thioesterase protein in Chanarin-Dorfman syndrome, a multisystem triglyceride storage disease.

SUMMARY

Therapeutic approaches to the metabolic myopathies are still lagging behind, although remarkable observations have been made on the rare coenzyme Q10 deficiency syndrome. However, transgenic animal models may offer the opportunity both to investigate muscle pathogenesis and explore therapeutic targets. Finally, human myotoxicity may provide novel paradigms for naturally occurring muscle disorders.

Identification and characterization of temperature-sensitive mild mutations in three Japanese patients with nonsevere forms of very-long-chain acyl-CoA dehydrogenase deficiency

Very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is clinically classified into severe, intermediate, and myopathic forms. We identified mutations in three unrelated Japanese patients with VLCAD deficiency: two with the myopathic form and one with the intermediate form, all compound heterozygotes of K264E/M437V, A416T/1798delA, and P89S/IVS16-3delAA, respectively. We characterized four missense mutations, K264E, M437V, A416T, and P89S, by transisent expression analysis, using SV40-transformed fibroblasts derived from a VLCAD-null patient, as recipient cells. In transient expression of the wild-type VLCAD cDNA, VLCAD activity at 30 degrees C was higher than at 37 degrees C. Moreover, this temperature-sensitive character is more evident in all the mutant proteins tested than in wild type. Based on characterization of the five missense mutations identified in four Japanese patients, including data on one patient with the myopathic form previously reported, patients with the nonsevere forms (intermediate or myopathic forms) have missense mutations with residual activities in at least one allele. Expression analysis at 30 degrees C may be more useful for evaluating these missense mutations, compared with that at 37 degrees C.