Mitochondrial trifunctional protein deficiency: a severe fatty acid oxidation disorder with cardiac and neurologic involvement

Neurological outcome in long-chain hydroxy fatty acid oxidation disorders

OBJECTIVE

This study aims to elucidate the long-term benefit of newborn screening (NBS) for individuals with long-chain 3-hydroxy-acyl-CoA dehydrogenase (LCHAD) and mitochondrial trifunctional protein (MTP) deficiency, inherited metabolic diseases included in NBS programs worldwide.

METHODS

German national multicenter study of individuals with confirmed LCHAD/MTP deficiency identified by NBS between 1999 and 2020 or selective metabolic screening. Analyses focused on NBS results, confirmatory diagnostics, and long-term clinical outcomes.

RESULTS

Sixty-seven individuals with LCHAD/MTP deficiency were included in the study, thereof 54 identified by NBS. All screened individuals with LCHAD deficiency survived, but four with MTP deficiency (14.8%) died during the study period. Despite NBS and early treatment neonatal decompensations (28%), symptomatic disease course (94%), later metabolic decompensations (80%), cardiomyopathy (28%), myopathy (82%), hepatopathy (32%), retinopathy (17%), and/or neuropathy (22%) occurred. Hospitalization rates were high (up to a mean of 2.4 times/year). Disease courses in screened individuals with LCHAD and MTP deficiency were similar except for neuropathy, occurring earlier in individuals with MTP deficiency (median 3.9 vs. 11.4 years; p = 0.0447). Achievement of dietary goals decreased with age, from 75% in the first year of life to 12% at age 10, and consensus group recommendations on dietary management were often not achieved.

INTERPRETATION

While NBS and early treatment result in improved (neonatal) survival, they cannot reliably prevent long-term morbidity in screened individuals with LCHAD/MTP deficiency, highlighting the urgent need of better therapeutic strategies and the development of disease course-altering treatment.

A tail of their own: regulation of cardiolipin and phosphatidylinositol fatty acyl profile by the acyltransferase LCLAT1

Cardiolipin and phosphatidylinositol along with the latter's phosphorylated derivative phosphoinositides, control a wide range of cellular functions from signal transduction, membrane traffic, mitochondrial function, cytoskeletal dynamics, and cell metabolism. An emerging dimension to these lipids is the specificity of their fatty acyl chains that is remarkably distinct from that of other glycerophospholipids. Cardiolipin and phosphatidylinositol undergo acyl remodeling involving the sequential actions of phospholipase A to hydrolyze acyl chains and key acyltransferases that re-acylate with specific acyl groups. LCLAT1 (also known as LYCAT, AGPAT8, LPLAT6, or ALCAT1) is an acyltransferase that contributes to specific acyl profiles for phosphatidylinositol, phosphoinositides, and cardiolipin. As such, perturbations of LCLAT1 lead to alterations in cardiolipin-dependent phenomena such as mitochondrial respiration and dynamics and phosphoinositide-dependent processes such as endocytic membrane traffic and receptor signaling. Here we examine the biochemical and cellular actions of LCLAT1, as well as the contribution of this acyltransferase to the development and specific diseases.

Newborn Screening Conditions: Early Intervention and Probability of Developmental Delay

OBJECTIVES

The purpose of this study is to explore which newborn screening (NBS) conditions are automatically eligible for early intervention (EI) across states and to determine the extent to which each disorder should automatically qualify for EI because of a high probability of developmental delay.

METHODS

We examined each state's EI eligibility policy and reviewed the literature documenting developmental outcomes for each NBS condition. Using a novel matrix, we assessed the risk of developmental delay, medical complexity, and risk of episodic decompensation, revising the matrix iteratively until reaching consensus. Three NBS conditions (biotinidase deficiency, severe combined immunodeficiency, and propionic acidemia) are presented in detail as examples.

RESULTS

Most states (88%) had Established Conditions lists to autoqualify children to EI. The average number of NBS conditions listed was 7.8 (range 0-34). Each condition appeared on average in 11.7 Established Conditions lists (range 2-29). After the literature review and consensus process, 29 conditions were likely to meet national criteria for an Established Condition.

CONCLUSION

Despite benefiting from NBS and timely treatment, many children diagnosed with NBS conditions are at risk for developmental delays and significant medical complexity. The results demonstrate a need for more clarity and guidance regarding which children should qualify for EI. We suggest that most NBS conditions should automatically qualify based on the probability of resulting in a developmental delay. These findings suggest a future opportunity for collaboration between NBS and EI programs to create a consistent set of Established Conditions, potentially expediate referrals of eligible children, and streamline children's access to EI services.

Genetic, biochemical, and clinical spectrum of patients with mitochondrial trifunctional protein deficiency identified after the introduction of newborn screening in the Netherlands

Long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) is included in many newborn screening (NBS) programs. Acylcarnitine-based NBS for LCHADD not only identifies LCHADD, but also the other deficiencies of the mitochondrial trifunctional protein (MTP), a multi-enzyme complex involved in long-chain fatty acid β-oxidation. Besides LCHAD, MTP harbors two additional enzyme activities: long-chain enoyl-CoA hydratase (LCEH) and long-chain ketoacyl-CoA thiolase (LCKAT). Deficiency of one or more MTP activities causes generalized MTP deficiency (MTPD), LCHADD, LCEH deficiency (not yet reported), or LCKAT deficiency (LCKATD). To gain insight in the outcomes of MTP-deficient patients diagnosed after the introduction of NBS for LCHADD in the Netherlands, a retrospective evaluation of genetic, biochemical, and clinical characteristics of MTP-deficient patients, identified since 2007, was carried out. Thirteen patients were identified: seven with LCHADD, five with MTPD, and one with LCKATD. All LCHADD patients (one missed by NBS, clinical diagnosis) and one MTPD patient (clinical diagnosis) were alive. Four MTPD patients and one LCKATD patient developed cardiomyopathy and died within 1 month and 13 months of life, respectively. Surviving patients did not develop symptomatic hypoglycemia, but experienced reversible cardiomyopathy and rhabdomyolysis. Five LCHADD patients developed subclinical neuropathy and/or retinopathy. In conclusion, patient outcomes were highly variable, stressing the need for accurate classification of and discrimination between the MTP deficiencies to improve insight in the yield of NBS for LCHADD. NBS allowed the prevention of symptomatic hypoglycemia, but current treatment options failed to treat cardiomyopathy and prevent long-term complications. Moreover, milder patients, who might benefit from NBS, were missed due to normal acylcarnitine profiles.

Neonatal Long-Chain 3-Ketoacyl-CoA Thiolase deficiency: Clinical-biochemical phenotype, sodium-D,L-3-hydroxybutyrate treatment experience and cardiac evaluation using speckle echocardiography

Isolated long-chain 3-keto-acyl CoA thiolase (LCKAT) deficiency is a rare long-chain fatty acid oxidation disorder caused by mutations in HADHB. LCKAT is part of a multi-enzyme complex called the mitochondrial trifunctional protein (MTP) which catalyzes the last three steps in the long-chain fatty acid oxidation. Until now, only three cases of isolated LCKAT deficiency have been described. All patients developed a severe cardiomyopathy and died before the age of 7 weeks. Here, we describe a newborn with isolated LCKAT deficiency, presenting with neonatal-onset cardiomyopathy, rhabdomyolysis, hypoglycemia and lactic acidosis. Bi-allelic 185G > A (p.Arg62His) and c1292T > C (p.Phe431Ser) mutations were found in HADHB. Enzymatic analysis in both lymphocytes and cultured fibroblasts revealed LCKAT deficiency with a normal long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD, also part of MTP) enzyme activity. Clinically, the patient showed recurrent cardiomyopathy, which was monitored by speckle tracking echocardiography. Subsequent treatment with special low-fat formula, low in long chain triglycerides (LCT) and supplemented with medium chain triglycerides (MCT) and ketone body therapy in (sodium-D,L-3-hydroxybutyrate) was well tolerated and resulted in improved carnitine profiles and cardiac function. Resveratrol, a natural polyphenol that has been shown to increase fatty acid oxidation, was also considered as a potential treatment option but showed no in vitro benefits in the patient's fibroblasts. Even though our patient deceased at the age of 13 months, early diagnosis and prompt initiation of dietary management with addition of sodium-D,L-3-hydroxybutyrate may have contributed to improved cardiac function and a much longer survival when compared to the previously reported cases of isolated LCKAT-deficiency.

Reversible sensory neuropathy in mitochondrial trifunctional protein deficiency

Axonal peripheral neuropathy is a common complication of mitochondrial trifunctional protein (MTP) deficiency and long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency that is usually considered progressive. Current treatment strategies are not able to fully prevent neuropathic symptoms in the majority of patients. We herein report three sisters with genetically proven MTP deficiency who were untreated until adolescence, when electrophysiological studies first revealed isolated axonal sensory neuropathy. Apart from mild exercise intolerance and missing deep tendon reflexes of the lower extremities, all three girls were clinically asymptomatic. A fat-reduced and fat-modified diet together with a reduction of the nocturnal fasting time resulted in complete normalisation of the electrophysiological studies after 1 year of dietary treatment. Our findings suggest that neuropathy might be responsive to dietary interventions in MTP patients at a very early stage of disease.

Muscular and Molecular Pathology Associated with SPATA5 Deficiency in a Child with EHLMRS

Mutations in the SPATA5 gene are associated with epilepsy, hearing loss and mental retardation syndrome (EHLMRS). While SPATA5 is ubiquitously expressed and is attributed a role within mitochondrial morphogenesis during spermatogenesis, there is only limited knowledge about the associated muscular and molecular pathology. This study reports on a comprehensive workup of muscular pathology, including proteomic profiling and microscopic studies, performed on an 8-year-old girl with typical clinical presentation of EHLMRS, where exome analysis revealed two clinically relevant, compound-heterozygous variants in SPATA5. Proteomic profiling of a quadriceps biopsy showed the dysregulation of 82 proteins, out of which 15 were localized in the mitochondrion, while 19 were associated with diseases presenting with phenotypical overlap to EHLMRS. Histological staining of our patient’s muscle biopsy hints towards mitochondrial pathology, while the identification of dysregulated proteins attested to the vulnerability of the cell beyond the mitochondria. Through our study we provide insights into the molecular etiology of EHLMRS and provide further evidence for a muscle pathology associated with SPATA5 deficiency, including a pathological histochemical pattern accompanied by dysregulated protein expression.

The spectrum of peripheral neuropathy in disorders of the mitochondrial trifunctional protein

Peripheral neuropathy is a known irreversible long-term complication of long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) and mitochondrial trifunctional protein deficiency (MTPD), two inherited disorders of mitochondrial long-chain fatty acid oxidation. The underlying pathophysiology of neuropathy is still not fully understood. We report electrophysiological studies and neurological findings in a series of 8 LCHAD-deficient and 11 MTP-deficient patients. The median age at time of the study was 8.0 years (0.5-25 years). The overall prevalence of neuropathy was 58% with neuropathic symptoms being slightly more common in MTPD compared to LCHADD (70% vs 50%, respectively). Onset of neuropathy was significantly earlier in MTPD patients compared to LCHADD patients (median age at onset 4.7 vs 15.3 years, respectively, P = .047). In four patients, isolated peripheral neuropathy was the first and only presenting symptom, and in all four the diagnosis was missed by newborn screening. About half of the patients (45.5%) had a sensorimotor neuropathy, while 27.3% showed a pure motor form and another 27.3% an isolated sensory form. Despite early diagnosis by newborn screening and early initiation of therapy, peripheral neuropathy cannot be prevented in all patients with LCHADD/MTPD and has severe impact on the life of affected patients. Electrophysiology classifies LCHADD/MTPD neuropathy as axonal with secondary demyelination. A novel observation is that in patients with acute, fulminant onset of neuropathy, symptoms can be partly reversible. Further studies are needed to elucidate the underlying pathophysiology of axonal damage and possible therapeutic targets.

MTP deficiency caused by HADHB mutations: Pathophysiology and clinical manifestations

Mutations in the HADHB gene lead to Mitochondrial Trifunctional Protein (MTP) deficiency. MTP deficiency is a rare autosomal recessive disorder affecting long-chain fatty acid oxidation. Patients affected by MTP deficiency are unable to metabolize long-chain fatty-acids and suffer a variety of symptoms exacerbated during fasting. The three phenotypes associated with complete MTP deficiency are an early-onset cardiomyopathy and early death, an intermediate form with recurrent hypoketotic hypoglycemia and a sensorimotor neuropathy with episodic rhabdomyolysis with small amount of residual enzyme activities. This review aims to discuss the pathophysiological mechanisms and clinical manifestations of each phenotype, which appears different and linked to HADHB expression levels. Notably, the pathophysiology of the sensorimotor neuropathy is relatively unknown and we provide a hypothesis on the qualitative aspect of the role of acylcarnitine buildup in Schwann cells in MTP deficiency patients. We propose that acylcarnitine may exit the Schwann cell and alter membrane properties of nearby axons leading to axonal degeneration based on recent findings in different metabolic disorders.

Recent Advances in the Pathophysiology of Fatty Acid Oxidation Defects: Secondary Alterations of Bioenergetics and Mitochondrial Calcium Homeostasis Caused by the Accumulating Fatty Acids

Deficiencies of medium-chain acyl-CoA dehydrogenase, mitochondrial trifunctional protein, isolated long-chain 3-hydroxyacyl-CoA dehydrogenase, and very long-chain acyl-CoA dehydrogenase activities are considered the most frequent fatty acid oxidation defects (FAOD). They are biochemically characterized by the accumulation of medium-chain, long-chain hydroxyl, and long-chain fatty acids and derivatives, respectively, in tissues and biological fluids of the affected patients. Clinical manifestations commonly include hypoglycemia, cardiomyopathy, and recurrent rhabdomyolysis. Although the pathogenesis of these diseases is still poorly understood, energy deprivation secondary to blockage of fatty acid degradation seems to play an important role. However, recent evidence indicates that the predominant fatty acids accumulating in these disorders disrupt mitochondrial functions and are involved in their pathophysiology, possibly explaining the lactic acidosis, mitochondrial morphological alterations, and altered mitochondrial biochemical parameters found in tissues and cultured fibroblasts from some affected patients and also in animal models of these diseases. In this review, we will update the present knowledge on disturbances of mitochondrial bioenergetics, calcium homeostasis, uncoupling of oxidative phosphorylation, and mitochondrial permeability transition induction provoked by the major fatty acids accumulating in prevalent FAOD. It is emphasized that further in vivo studies carried out in tissues from affected patients and from animal genetic models of these disorders are necessary to confirm the present evidence mostly achieved from in vitro experiments.

A Rare Presentation of Cardiomyopathy in Pregnancy

Our patient presented in her third trimester of pregnancy with new onset of heart failure. A thorough workup in the initial postpartum period with detailed past medical history, advanced imaging modalities, and a multidisciplinary approach revealed a rare and treatable etiology of cardiomyopathy. (Level of Difficulty: Intermediate.)

Evaluation of earlier versus later dietary management in long-chain 3-hydroxyacyl-CoA dehydrogenase or mitochondrial trifunctional protein deficiency: a systematic review

Background

Mitochondrial trifunctional protein (MTP) and long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiencies are rare fatty acid β-oxidation disorders. Without dietary management the conditions are life-threatening. We conducted a systematic review to investigate whether pre-symptomatic dietary management following newborn screening provides better outcomes than treatment following symptomatic detection.

Methods

We searched Web of Science, Medline, Pre-Medline, Embase and the Cochrane Library up to 23rd April 2018. Two reviewers independently screened titles, abstracts and full texts for eligibility and quality appraised the studies. Data extraction was performed by one reviewer and checked by another.

Results

We included 13 articles out of 7483 unique records. The 13 articles reported on 11 patient groups, including 174 people with LCHAD deficiency, 18 people with MTP deficiency and 12 people with undifferentiated LCHAD/MTP deficiency. Study quality was moderate to weak in all studies. Included studies suggested fewer heart and liver problems in screen-detected patients, but inconsistent results for mortality. Follow up analyses compared long-term outcomes of (1) pre-symptomatically versus symptomatically treated patients, (2) screened versus unscreened patients, and (3) asymptomatic screen-detected, symptomatic screen-detected, and clinically diagnosed patients in each study. For follow up analyses 1 and 2, we found few statistically significant differences in the long-term outcomes. For follow up analysis 3 we found a significant difference for only one comparison, in the incidence of cardiomyopathy between the three groups.

Conclusions

There is some evidence that dietary management following screen-detection might be associated with a lower incidence of some LCHAD and MTP deficiency-related complications. However, the evidence base is limited by small study sizes, quality issues and risk of confounding. An internationally collaborative research effort is needed to fully examine the risks and the benefits to pre-emptive dietary management with particular attention paid to disease severity and treatment group.

Inborn Errors of Metabolism with Myopathy: Defects of Fatty Acid Oxidation and the Carnitine Shuttle System

Fatty acid oxidation disorders (FAODs) and carnitine shuttling defects are inborn errors of energy metabolism with associated mortality and morbidity due to cardiomyopathy, exercise intolerance, rhabdomyolysis, and liver disease with physiologic stress. Hypoglycemia is characteristically hypoketotic. Lactic acidemia and hyperammonemia may occur during decompensation. Recurrent rhabdomyolysis is debilitating. Expanded newborn screening can detect most of these disorders, allowing early, presymptomatic treatment. Treatment includes avoiding fasting and sustained extraneous exercise and providing high-calorie hydration during illness to prevent lipolysis, and medium-chain triglyceride oil supplementation in long-chain FAODs. Carnitine supplementation may be helpful. However, conventional treatment does not prevent all symptoms.

Mitochondrial β-oxidation of saturated fatty acids in humans

Mitochondrial β-oxidation of fatty acids generates acetyl-coA, NADH and FADH₂. Acyl-coA synthetases catalyze the binding of fatty acids to coenzyme A to form fatty acyl-coA thioesters, the first step in the intracellular metabolism of fatty acids. l-carnitine system facilitates the transport of fatty acyl-coA esters across the mitochondrial membrane. Carnitine palmitoyltransferase-1 transfers acyl groups from coenzyme A to l-carnitine, forming acyl-carnitine esters at the outer mitochondrial membrane. Carnitine acyl-carnitine translocase exchanges acyl-carnitine esters that enter the mitochondria, by free l-carnitine. Carnitine palmitoyltransferase-2 converts acyl-carnitine esters back to acyl-coA esters at the inner mitochondrial membrane. The β-oxidation pathway of fatty acyl-coA esters includes four reactions. Fatty acyl-coA dehydrogenases catalyze the introduction of a double bond at the C2 position, producing 2-enoyl-coA esters and reducing equivalents that are transferred to the respiratory chain via electron transferring flavoprotein. Enoyl-coA hydratase catalyzes the hydration of the double bond to generate a 3-l-hydroxyacyl-coA derivative. 3-l-hydroxyacyl-coA dehydrogenase catalyzes the formation of a 3-ketoacyl-coA intermediate. Finally, 3-ketoacyl-coA thiolase catalyzes the cleavage of the chain, generating acetyl-coA and a fatty acyl-coA ester two carbons shorter. Mitochondrial trifunctional protein catalyzes the three last steps in the β-oxidation of long-chain and medium-chain fatty acyl-coA esters while individual enzymes catalyze the β-oxidation of short-chain fatty acyl-coA esters. Clinical phenotype of fatty acid oxidation disorders usually includes hypoketotic hypoglycemia triggered by fasting or infections, skeletal muscle weakness, cardiomyopathy, hepatopathy, and neurological manifestations. Accumulation of non-oxidized fatty acids promotes their conjugation with glycine and l-carnitine and alternate ways of oxidation, such as ω-oxidation.

Clinical and genetic characteristics of patients with fatty acid oxidation disorders identified by newborn screening

BACKGROUND

Fatty acid oxidation disorders (FAODs) include more than 15 distinct disorders with variable clinical manifestations. After the introduction of newborn screening using tandem mass spectrometry, early identification of FAODs became feasible. This study describes the clinical, biochemical and molecular characteristics of FAODs patients detected by newborn screening (NBS) compared with those of 9 patients with symptomatic presentations.

METHODS

Clinical and genetic features of FAODs patients diagnosed by NBS and by symptomatic presentations were reviewed.

RESULTS

Fourteen patients were diagnosed with FAODs by NBS at the age of 54.8 ± 4.8 days: 5 with very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency, 5 with medium chain acyl-CoA dehydrogenase (MCAD) deficiency, 1 with primary carnitine deficiency, 1 with carnitine palmitoyltransferase 1A (CPT1A) deficiency, 1 with long-chain 3-hydroxyacyl-CoA dehydrogenase or mitochondrial trifunctional protein (LCAHD/MTP) deficiency, and 1 with short chain acyl-CoA dehydrogenase (SCAD) deficiency. Three patients with VLCAD or LCHAD/MTP deficiency developed recurrent rhabdomyolysis or cardiomyopathy, and one patient died of cardiomyopathy. The other 10 patients remained neurodevelopmentally normal and asymptomatic during the follow-up. In 8 patients with symptomatic presentation, FAODs manifested as LCHAD/MTP deficiencies by recurrent rhabdomyolysis or cadiomyopathy (6 patients), and VLCAD deficiency by cardiomyopathy (1 patient), and CPT1A deficiency by hepatic failure (1 patient). Two patients with LCHAD/MTP deficiencies died due to severe cardiomyopathy in the neonatal period, and developmental disability was noted in CPT1A deficiency (1 patient).

CONCLUSIONS

NBS helped to identify the broad spectrum of FAODs and introduce early intervention to improve the clinical outcome of each patient. However, severe clinical manifestations developed in some patients, indicating that careful, life-long observation is warranted in all FAODs patients.

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

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

Mitochondrial Trifunctional Protein Deficiency: Severe Cardiomyopathy and Cardiac Transplantation

We describe mitochondrial trifunctional protein deficiency (MTPD) in two male siblings who presented with severe cardiomyopathy in infancy. The first sibling presented in severe cardiac failure at 6 months of age and succumbed soon after. The second sibling came to attention after newborn screening identified a possible fatty acid oxidation defect. Dietary therapy and carnitine supplementation commenced in the neonatal period. Despite this the second child required cardiac transplantation at 3 years of age after a sudden and rapid decline in cardiac function. The outcome has been excellent, with no apparent extra-cardiac manifestations of a fatty acid oxidation disorder at the age of 7. Pathogenic HADHA mutations were subsequently identified via genome wide exome sequencing. This is the first reported case of MTPD to undergo cardiac transplantation. We suggest that cardiac transplantation could be considered in the treatment of cardiomyopathy in MTPD.

Short-term administration of Nicotinamide Mononucleotide preserves cardiac mitochondrial homeostasis and prevents heart failure

Heart failure is associated with mitochondrial dysfunction so that restoring or improving mitochondrial health is of therapeutic importance. Recently, reduction in NAD⁺ levels and NAD⁺-mediated deacetylase activity has been recognized as negative regulators of mitochondrial function. Using a cardiac specific KLF4 deficient mouse line that is sensitive to stress, we found mitochondrial protein hyperacetylation coupled with reduced Sirt3 and NAD⁺ levels in the heart before stress, suggesting that the KLF4-deficient heart is predisposed to NAD⁺-associated defects. Further, we demonstrated that short-term administration of Nicotinamide Mononucleotide (NMN) successfully protected the mutant mice from pressure overload-induced heart failure. Mechanically, we showed that NMN preserved mitochondrial ultrastructure, reduced ROS and prevented cell death in the heart. In cultured cardiomyocytes, NMN treatment significantly increased long-chain fatty acid oxidation despite no direct effect on pyruvate oxidation. Collectively, these results provide cogent evidence that hyperacetylation of mitochondrial proteins is critical in the pathogenesis of cardiac disease and that administration of NMN may serve as a promising therapy.

Pharmacological inhibition of carnitine palmitoyltransferase 1 restores mitochondrial oxidative phosphorylation in human trifunctional protein deficient fibroblasts

BACKGROUND

Mitochondrial Trifunctional Protein deficiency (TFPD) is a severe genetic disease characterized by altered energy metabolism and accumulation of long-chain (LC) acylcarnitines in blood and tissues. This accumulation could impair the mitochondrial oxidative phosphorylation (OxPhos), contributing to the non-optimal outcome despite conventional diet therapy with medium-chain triglycerides (MCT).

METHOD

Acylcarnitine and OxPhos parameters were measured in TFPD-fibroblasts obtained from 8 children and cultured in medium mimicking fasting (LCFA) or conventional treatment (MCT), with or without Etomoxir (ETX) an inhibitor of carnitine palmitoyltransferase 1 (CPT1) activity, and were compared to results obtained with fibroblasts from 5 healthy-control children. The effects of various acylcarnitines were also tested on control fibroblasts.

RESULTS

In the LCFA-condition, TFPD-fibroblasts demonstrated a large accumulation of LC-acylcarnitines associated with decreased O₂-consumption (63±3% of control, P<0.001) and ATP production (67±5%, P<0.001) without modification of coupling efficiency. A dose-dependent decrease in O₂-consumption was reproduced in control fibroblasts by addition of increasing dose of LC-acylcarnitines, while it was almost preserved with MC-acylcarnitines. The MCT-condition reduced LC-acylcarnitine accumulation and partially improved O₂-consumption (80±3%, P<0.01) in TFPD-fibroblasts. The addition of ETX in both LCFA- and MCT-conditions normalized acylcarnitine profiles and restored O₂-consumption and ATP production at the same levels than control.

CONCLUSION

Accumulation of LC-acylcarnitines plays a major role in the pathophysiology of TFPD, reducing OxPhos capacities. These deleterious effects could be partially prevented by MCT-therapy and totally corrected by ETX. Inhibition of CPT1 may be view as a new therapeutic target for patients with a severe form of TFPD.

[Mitochondrial trifunctional protein deficiency: an adult patient with similar progress to Charcot-Marie-Tooth disease]

A 45-year-old man presented to us due to slowly progressive muscle weakness and sensory disturbances in his lower limbs since his 40's. He reported multiple episodes of exercise-induced severe muscle fatigue and brown urine in his childhood, which disappeared by age 20. A nerve conduction study showed peripheral axonal neuropathy and then Charcot-Marie-Tooth disease (CMT) was considered as the most likely diagnosis; however, exome sequencing failed to identify a mutation in the known genes of CMTs. Since age 55, he recurrently developed severe rhabdomyolysis that required hospitalization. On suspicion of lipid metabolism disorders, we performed serum acylcarnitine analysis, and which revealed mildly elevated long-chain fatty acids. We re-examined variants obtained via exome sequencing and found a mutation in HADHB. Mitochondrial trifunctional protein (MTP) deficiency is a rare autosomal recessive disorder of mitochondrial fatty acid beta-oxidation caused by HADHA or HADHB mutation. It can be a life-threatening multiorgan disorder with early infantile onset, but it can also present in childhood or adolescence with peripheral neuropathy and recurrent rhabdomyolysis. This case of adult-diagnosed MTP deficiency was characterized by slowly progressive peripheral neuropathy masquerading CMT in addition to muscular symptoms. MTP deficiency should be considered in patients with the combination of peripheral neuropathy and recurrent rhabdomyolysis.

Disturbance of mitochondrial functions provoked by the major long-chain 3-hydroxylated fatty acids accumulating in MTP and LCHAD deficiencies in skeletal muscle

The pathogenesis of the muscular symptoms and recurrent rhabdomyolysis that are commonly manifested in patients with mitochondrial trifunctional protein (MTP) and long-chain 3-hydroxy-acyl-CoA dehydrogenase (LCHAD) deficiencies is still unknown. In this study we investigated the effects of the major long-chain monocarboxylic 3-hydroxylated fatty acids (LCHFA) accumulating in these disorders, namely 3-hydroxytetradecanoic (3HTA) and 3-hydroxypalmitic (3HPA) acids, on important mitochondrial functions in rat skeletal muscle mitochondria. 3HTA and 3HPA markedly increased resting (state 4) and decreased ADP-stimulated (state 3) and CCCP-stimulated (uncoupled) respiration. 3HPA provoked similar effects in permeabilized skeletal muscle fibers, validating the results obtained in purified mitochondria. Furthermore, 3HTA and 3HPA markedly diminished mitochondrial membrane potential, NAD(P)H content and Ca(2+) retention capacity in Ca(2+)-loaded mitochondria. Mitochondrial permeability transition (mPT) induction probably underlie these effects since they were totally prevented by cyclosporin A and ADP. In contrast, the dicarboxylic analogue of 3HTA did not alter the tested parameters. Our data strongly indicate that 3HTA and 3HPA behave as metabolic inhibitors, uncouplers of oxidative phosphorylation and mPT inducers in skeletal muscle. It is proposed that these pathomechanisms disrupting mitochondrial homeostasis may be involved in the muscle alterations characteristic of MTP and LCHAD deficiencies.

Metabolomics-assisted proteomics identifies succinylation and SIRT5 as important regulators of cardiac function

Cellular metabolites, such as acyl-CoA, can modify proteins, leading to protein posttranslational modifications (PTMs). One such PTM is lysine succinylation, which is regulated by sirtuin 5 (SIRT5). Although numerous proteins are modified by lysine succinylation, the physiological significance of lysine succinylation and SIRT5 remains elusive. Here, by profiling acyl-CoA molecules in various mouse tissues, we have discovered that different tissues have different acyl-CoA profiles and that succinyl-CoA is the most abundant acyl-CoA molecule in the heart. This interesting observation has prompted us to examine protein lysine succinylation in different mouse tissues in the presence and absence of SIRT5. Protein lysine succinylation predominantly accumulates in the heart whenSirt5is deleted. Using proteomic studies, we have identified many cardiac proteins regulated by SIRT5. Our data suggest that ECHA, a protein involved in fatty acid oxidation, is a major enzyme that is regulated by SIRT5 and affects heart function.Sirt5knockout (KO) mice have lower ECHA activity, increased long-chain acyl-CoAs, and decreased ATP in the heart under fasting conditions.Sirt5KO mice develop hypertrophic cardiomyopathy, as evident from the increased heart weight relative to body weight, as well as reduced shortening and ejection fractions. These findings establish that regulating heart metabolism and function is a major physiological function of lysine succinylation and SIRT5.

Altered Energetics of Exercise Explain Risk of Rhabdomyolysis in Very Long-Chain Acyl-CoA Dehydrogenase Deficiency

Rhabdomyolysis is common in very long-chain acyl-CoA dehydrogenase deficiency (VLCADD) and other metabolic myopathies, but its pathogenic basis is poorly understood. Here, we show that prolonged bicycling exercise against a standardized moderate workload in VLCADD patients is associated with threefold bigger changes in phosphocreatine (PCr) and inorganic phosphate (Pi) concentrations in quadriceps muscle and twofold lower changes in plasma acetyl-carnitine levels than in healthy subjects. This result is consistent with the hypothesis that muscle ATP homeostasis during exercise is compromised in VLCADD. However, the measured rates of PCr and Pi recovery post-exercise showed that the mitochondrial capacity for ATP synthesis in VLCADD muscle was normal. Mathematical modeling of oxidative ATP metabolism in muscle composed of three different fiber types indicated that the observed altered energy balance during submaximal exercise in VLCADD patients may be explained by a slow-to-fast shift in quadriceps fiber-type composition corresponding to 30% of the slow-twitch fiber-type pool in healthy quadriceps muscle. This study demonstrates for the first time that quadriceps energy balance during exercise in VLCADD patients is altered but not because of failing mitochondrial function. Our findings provide new clues to understanding the risk of rhabdomyolysis following exercise in human VLCADD.

Peripheral neuropathy in patients with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency - A follow-up EMG study of 12 patients

BACKGROUND

The neonatal screening and early start of the dietary therapy have improved the outcome of long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD). The acute symptoms of LCHADD are hypoketotic hypoglycemia, failure to thrive, hepatopathy and rhabdomyolysis. Long term complications are retinopathy and neuropathy. Speculated etiology of these long term complications are the accumulation and toxicity of hydroxylacylcarnitines and long-chain fatty acid metabolites or deficiency of essential fatty acids.

AIMS

To study the possible development of polyneuropathy in LCHADD patients with current dietary regimen.

METHODS

Development of polyneuropathy in 12 LCHADD patients with the homozygous common mutation c.G1528C was evaluated with electroneurography (ENG) studies. The ENG was done 1-12 times to each patient, between the ages of 3 and 40 years. Clinical data of the patients were collected from the patient records.

RESULTS

The first sign of polyneuropathy was detected between the ages of 6-12 years, the first abnormality being reduction of the sensory amplitudes of the sural nerves. With time, progression was detected by abnormalities in sensory responses extending to upper limbs, as well as abnormalities in motor responses in lower limbs. Altogether, eight of the patients had polyneuropathy, despite good compliancy of the diet.

CONCLUSIONS

This study is the first to report the evolution of polyneuropathy with clinical neurophysiological methods in a relative large LCHADD patient group. Despite early start, and good compliance of the therapy, 6/10 of the younger patients developed neuropathy. However, in most patients the polyneuropathy was less severe than previously described.

Proteomic analysis of physiological versus pathological cardiac remodeling in animal models expressing mutations in myosin essential light chains

In this study we aimed to provide an in-depth proteomic analysis of differentially expressed proteins in the hearts of transgenic mouse models of pathological and physiological cardiac hypertrophy using tandem mass tag labeling and liquid chromatography tandem mass spectrometry. The Δ43 mouse model, expressing the 43-amino-acid N-terminally truncated myosin essential light chain (ELC) served as a tool to study the mechanisms of physiological cardiac remodeling, while the pathological hypertrophy was investigated in A57G (Alanine 57 → Glycine) ELC mice. The results showed that 30 proteins were differentially expressed in Δ43 versus A57G hearts as determined by multiple pair comparisons of the mutant versus wild-type (WT) samples with P < 0.05. The A57G hearts showed differential expression of nine mitochondrial proteins involved in metabolic processes compared to four proteins for ∆43 hearts when both mutants were compared to WT hearts. Comparisons between ∆43 and A57G hearts showed an upregulation of three metabolically important mitochondrial proteins but downregulation of nine proteins in ∆43 hearts. The physiological model of cardiac hypertrophy (∆43) showed no changes in the levels of Ca(2+)-binding proteins relative to WT, while the pathologic model (A57G) showed the upregulation of three Ca(2+)-binding proteins, including sarcalumenin. Unique differences in chaperone and fatty acid metabolism proteins were also observed in Δ43 versus A57G hearts. The proteomics data support the results from functional studies performed previously on both animal models of cardiac hypertrophy and suggest that the A57G- and not ∆43- mediated alterations in fatty acid metabolism and Ca(2+) homeostasis may contribute to pathological cardiac remodeling in A57G hearts.

The Biochemistry and Physiology of Mitochondrial Fatty Acid β-Oxidation and Its Genetic Disorders

Mitochondrial fatty acid β-oxidation (FAO) is the major pathway for the degradation of fatty acids and is essential for maintaining energy homeostasis in the human body. Fatty acids are a crucial energy source in the postabsorptive and fasted states when glucose supply is limiting. But even when glucose is abundantly available, FAO is a main energy source for the heart, skeletal muscle, and kidney. A series of enzymes, transporters, and other facilitating proteins are involved in FAO. Recessively inherited defects are known for most of the genes encoding these proteins. The clinical presentation of these disorders may include hypoketotic hypoglycemia, (cardio)myopathy, arrhythmia, and rhabdomyolysis and illustrates the importance of FAO during fasting and in hepatic and (cardio)muscular function. In this review, we present the current state of knowledge on the biochemistry and physiological functions of FAO and discuss the pathophysiological processes associated with FAO disorders.

Deregulation of mitochondrial functions provoked by long-chain fatty acid accumulating in long-chain 3-hydroxyacyl-CoA dehydrogenase and mitochondrial permeability transition deficiencies in rat heart--mitochondrial permeability transition pore opening as a potential contributing pathomechanism of cardiac alterations in these disorders

Mitochondrial trifunctional protein and long-chain 3-hydroxyacyl-CoA dehydrogenase deficiencies are fatty acid oxidation disorders biochemically characterized by tissue accumulation of long-chain fatty acids and derivatives, including the monocarboxylic long-chain 3-hydroxy fatty acids (LCHFAs) 3-hydroxytetradecanoic acid (3HTA) and 3-hydroxypalmitic acid (3HPA). Patients commonly present severe cardiomyopathy for which the pathogenesis is still poorly established. We investigated the effects of 3HTA and 3HPA, the major metabolites accumulating in these disorders, on important parameters of mitochondrial homeostasis in Ca(2+) -loaded heart mitochondria. 3HTA and 3HPA significantly decreased mitochondrial membrane potential, the matrix NAD(P)H pool and Ca(2+) retention capacity, and also induced mitochondrial swelling. These fatty acids also provoked a marked decrease of ATP production reflecting severe energy dysfunction. Furthermore, 3HTA-induced mitochondrial alterations were completely prevented by the classical mitochondrial permeability transition (mPT) inhibitors cyclosporin A and ADP, as well as by ruthenium red, a Ca(2+) uptake blocker, indicating that LCHFAs induced Ca(2+)-dependent mPT pore opening. Milder effects only achieved at higher doses of LCHFAs were observed in brain mitochondria, implying a higher vulnerability of heart to these fatty acids. By contrast, 3HTA and docosanoic acids did not change mitochondrial homeostasis, indicating selective effects for monocarboxylic LCHFAs. The present data indicate that the major LCHFAs accumulating in mitochondrial trifunctional protein and long-chain 3-hydroxyacyl-CoA dehydrogenase deficiencies induce mPT pore opening, compromising Ca(2+) homeostasis and oxidative phosphorylation more intensely in the heart. It is proposed that these pathomechanisms may contribute at least in part to the severe cardiac alterations characteristic of patients affected by these diseases.

Deficiency of ECHS1 causes mitochondrial encephalopathy with cardiac involvement

OBJECTIVE

Short-chain enoyl-CoA hydratase (ECHS1) is a multifunctional mitochondrial matrix enzyme that is involved in the oxidation of fatty acids and essential amino acids such as valine. Here, we describe the broad phenotypic spectrum and pathobiochemistry of individuals with autosomal-recessive ECHS1 deficiency.

METHODS

Using exome sequencing, we identified ten unrelated individuals carrying compound heterozygous or homozygous mutations in ECHS1. Functional investigations in patient-derived fibroblast cell lines included immunoblotting, enzyme activity measurement, and a palmitate loading assay.

RESULTS

Patients showed a heterogeneous phenotype with disease onset in the first year of life and course ranging from neonatal death to survival into adulthood. The most prominent clinical features were encephalopathy (10/10), deafness (9/9), epilepsy (6/9), optic atrophy (6/10), and cardiomyopathy (4/10). Serum lactate was elevated and brain magnetic resonance imaging showed white matter changes or a Leigh-like pattern resembling disorders of mitochondrial energy metabolism. Analysis of patients' fibroblast cell lines (6/10) provided further evidence for the pathogenicity of the respective mutations by showing reduced ECHS1 protein levels and reduced 2-enoyl-CoA hydratase activity. While serum acylcarnitine profiles were largely normal, in vitro palmitate loading of patient fibroblasts revealed increased butyrylcarnitine, unmasking the functional defect in mitochondrial β-oxidation of short-chain fatty acids. Urinary excretion of 2-methyl-2,3-dihydroxybutyrate - a potential derivative of acryloyl-CoA in the valine catabolic pathway - was significantly increased, indicating impaired valine oxidation.

INTERPRETATION

In conclusion, we define the phenotypic spectrum of a new syndrome caused by ECHS1 deficiency. We speculate that both the β-oxidation defect and the block in l-valine metabolism, with accumulation of toxic methacrylyl-CoA and acryloyl-CoA, contribute to the disorder that may be amenable to metabolic treatment approaches.

Neuropsychological outcomes in fatty acid oxidation disorders: 85 cases detected by newborn screening

Mitochondrial fatty acid oxidation disorders include conditions in which the transport of activated acyl-Coenzyme A (CoA) into the mitochondria or utilization of these substrates is disrupted or blocked. This results in a deficit in the conversion of fat into energy. Most patients with fatty acid oxidation defects are now identified through newborn screening by tandem mass spectrometry. With earlier identification and preventative treatments, mortality and morbidity rates have improved. However, in the absence of severe health and neurological effects from these disorders, subtle developmental delays or neuropsychological deficits have been noted. Medical records were reviewed to identify outcomes in 85 children with FAOD's diagnosed through newborn screening and followed at one metabolic center. Overall, 54% of these children identified through newborn screening experienced developmental challenges. Speech delay or relative weakness in language was noted in 26 children (31%) and motor delays were noted in 24 children (29%). The majority of the 46 children receiving psychological evaluations performed well within the average range, with only 11% scoring <85 on developmental or intelligence tests. These results highlight the importance of screening children with fatty acid oxidation disorders to identify those with language, motor, or cognitive delay. Although expanded newborn screening dramatically changes the health and developmental outcomes in many children with fatty acid oxidation disorders, it also complicates the interpretation of biochemical and molecular findings and raises questions about the effectiveness or necessity of treatment in a large number of cases. Only by systematically evaluating developmental and neuropsychological outcomes using standardized methods will the true implications of newborn screening, laboratory results, and treatments for neurocognitive outcome in these disorders become clear.

A compound heterozygous mutation in HADHB gene causes an axonal Charcot-Marie-tooth disease

Background

Charcot-Marie-Tooth disease (CMT) is a heterogeneous disorder of the peripheral nervous system. So far, mutations in hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase/enoyl-CoA hydratase (trifunctional protein), beta subunit (HADHB) gene exhibit three distinctive phenotypes: severe neonatal presentation with cardiomyopathy, hepatic form with recurrent hypoketotic hypoglycemia, and later-onset axonal sensory neuropathy with episodic myoglobinuria.

Methods

To identify the causative and characterize clinical features of a Korean family with motor and sensory neuropathies, whole exome study (WES), histopathologic study of distal sural nerve, and lower limb MRIs were performed.

Results

WES revealed that a compound heterozygous mutation in HADHB is the causative of the present patients. The patients exhibited an early-onset axonal sensorimotor neuropathy without episodic myoglobinuria, and showed typical clinical and electrophysiological features of CMT including predominant distal muscle weakness and atrophy. Histopathologic findings of sural nerve were compatible with an axonal CMT neuropathy. Furthermore, they didn’t exhibit any other symptoms of the previously reported HADHB patients.

Conclusions

These data implicate that mutation in HADHB gene can also cause early-onset axonal CMT instead of typical manifestations in mitochondrial trifunctional protein (MTP) deficiency. Therefore, this study is the first report of a new subtype of autosomal recessive axonal CMT by a compound heterozygous mutation in HADHB, and will expand the clinical and genetic spectrum of HADHB.

When the usual symptoms become an unusual diagnosis: a case report of trifunctional protein complex

Disorders of mitochondrial fatty acid b-oxidation should be considered in any infant who presents with unexplained hypoglycemia and/or myopathy. Although disorders of trifunctional protein (TFP) complex including long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) and mitochondrial TFP deficiencies are extremely rare, the combined incidence of mitochondrial fatty acid disorders is quite frequent. With the expansion of newborn screening, what were once considered uncommon disorders are being identified with increasing frequency in asymptomatic infants. The following case scenario presents an infant who developed symptoms prior to the completion of newborn screening. This fairly routine course for a late-preterm infant reveals an extremely rare inborn error of metabolism, LCHAD deficiency. An overview of TFP complex, the differential diagnoses as the case unfolds, diagnostic test results, acute care management, and short-term patient follow-up is presented. With experience, health care providers often become accustomed to and expect to see common things regularly. This case presents a scenario which, as it unfolds, appears to be quite common. It turns out, however, to be very uncommon.

Peripheral neuropathy associated with mitochondrial disease in children

Mitochondrial diseases in children are often associated with a peripheral neuropathy but the presence of the neuropathy is under-recognized because of the overwhelming involvement of the central nervous system (CNS). These mitochondrial neuropathies are heterogeneous in their clinical, neurophysiological, and histopathological characteristics. In this article, we provide a comprehensive review of childhood mitochondrial neuropathy. Early recognition of neuropathy may help with the identification of the mitochondrial syndrome. While it is not definite that the characteristics of the neuropathy would help in directing genetic testing without the requirement for invasive skin, muscle or liver biopsies, there appears to be some evidence for this hypothesis in Leigh syndrome, in which nuclear SURF1 mutations cause a demyelinating neuropathy and mitochondrial DNA MTATP6 mutations cause an axonal neuropathy. POLG1 mutations, especially when associated with late-onset phenotypes, appear to cause a predominantly sensory neuropathy with prominent ataxia. The identification of the peripheral neuropathy also helps to target genetic testing in the mitochondrial optic neuropathies. Although often subclinical, the peripheral neuropathy may occasionally be symptomatic and cause significant disability. Where it is symptomatic, recognition of the neuropathy will help the early institution of rehabilitative therapy. We therefore suggest that nerve conduction studies should be a part of the early evaluation of children with suspected mitochondrial disease.

Necrotizing enterocolitis and respiratory distress syndrome as first clinical presentation of mitochondrial trifunctional protein deficiency

BACKGROUND

Newborn screening (NBS) for long-chain 3-hydroxy acyl-CoA dehydrogenase (LCHAD) deficiency does not discriminate between isolated LCHAD deficiency, isolated long-chain keto acyl-CoA (LCKAT) deficiency and general mitochondrial trifunctional protein (MTP) deficiency. Therefore, screening for LCHAD deficiency inevitably comprises screening for MTP deficiency, which is much less amenable to treatment. Furthermore, absence of a clear classification system for these disorders is still lacking.

MATERIALS AND METHODS

Two newborns screened positive for LCHAD deficiency died at the age of 10 and 31 days, respectively. One due to severe necrotizing enterocolitis (NEC), cardiomyopathy and multiorgan failure and the other due to severe infant respiratory distress syndrome (IRDS) and hypertrophic cardiomyopathy. (Keto)-acylcarnitine concentration and enzymatic analysis of LCHAD and LCKAT suggested MTP deficiency in both patients. Mutation analysis revealed a homozygous HADHB c.357+5delG mutation in one patient and a homozygous splice-site HADHB mutation c.212+1G>C in the other patient.Data on enzymatic and mutation analysis of 40 patients with presumed LCHAD, LCKAT or MTP deficiency were used to design a classification to distinguish between these disorders.

DISCUSSION

NEC as presenting symptom in MTP deficiency has not been reported previously. High expression of long-chain fatty acid oxidation enzymes reported in lungs and gut of human foetuses suggests that the severe NEC and IRDS observed in our patients are related to the enzymatic deficiency in these organs during crucial stages of development.Furthermore, as illustrated by the cases we propose a classification system to discriminate LCHAD, LCKAT and MTP deficiency based on enzymatic analysis.

Outcome in six patients with mitochondrial trifunctional protein disorders identified by newborn screening

Before the newborn screening era, disorders of the mitochondrial trifunctional protein (TFP) complex including long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) presented with high morbidity and mortality. Data on outcome and prognosis of TFP deficiency disorders since implementation of screening are scarce. We here characterize 6 screened patients with a disorder of the TFP complex (3 of those with LCHADD) with respect to clinical presentation and molecular features. Three of 6 patients were symptomatic prior availability of screening results on days 4-5 of life. Of the three asymptomatic patients recognised by screening, one acutely died at 3months at home during an infection. Two patients remained asymptomatic with preventive measures during follow-up until the age of 3years. One of them had an older sibling with identical genotype born before the screening era, who became symptomatic with 15months. We conclude that newborn screening for disorders of the TFP complex allows identification of asymptomatic cases; however, the acute presentation in 3/6 babies before screening is noteworthy and troublesome. TFP and LCHAD deficiencies remain life-threatening disorders. This is in clear contrast to other defects of long-chain fatty acid oxidation after identification by newborn screening.

Fatty acid oxidation disorders: outcome and long-term prognosis

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.

Mitochondrial fatty acid oxidation disorders: clinical presentation of long-chain fatty acid oxidation defects before and after newborn screening

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.

Cardiolipin remodeling by ALCAT1 links oxidative stress and mitochondrial dysfunction to obesity

Oxidative stress causes mitochondrial dysfunction and metabolic complications through unknown mechanisms. Cardiolipin (CL) is a key mitochondrial phospholipid required for oxidative phosphorylation. Oxidative damage to CL from pathological remodeling is implicated in the etiology of mitochondrial dysfunction commonly associated with diabetes, obesity, and other metabolic diseases. Here, we show that ALCAT1, a lyso-CL acyltransferase upregulated by oxidative stress and diet-induced obesity (DIO), catalyzes the synthesis of CL species that are highly sensitive to oxidative damage, leading to mitochondrial dysfunction, ROS production, and insulin resistance. These metabolic disorders were reminiscent of those observed in type 2 diabetes and were reversed by rosiglitazone treatment. Consequently, ALCAT1 deficiency prevented the onset of DIO and significantly improved mitochondrial complex I activity, lipid oxidation, and insulin signaling in ALCAT1(-/-) mice. Collectively, these findings identify a key role of ALCAT1 in regulating CL remodeling, mitochondrial dysfunction, and susceptibility to DIO.

Carnitine palmitoyltransferase 2: New insights on the substrate specificity and implications for acylcarnitine profiling

Over the last years acylcarnitines have emerged as important biomarkers for the diagnosis of mitochondrial fatty acid beta-oxidation (mFAO) and branched-chain amino acid oxidation disorders assuming they reflect the potentially toxic acyl-CoA species, accumulating intramitochondrially upstream of the enzyme block. However, the origin of these intermediates still remains poorly understood. A possibility exists that carnitine palmitoyltransferase 2 (CPT2), member of the carnitine shuttle, is involved in the intramitochondrial synthesis of acylcarnitines from accumulated acyl-CoA metabolites. To address this issue, the substrate specificity profile of CPT2 was herein investigated. Saccharomyces cerevisiae homogenates expressing human CPT2 were incubated with saturated and unsaturated C2-C26 acyl-CoAs and branched-chain amino acid oxidation intermediates. The produced acylcarnitines were quantified by ESI-MS/MS. We show that CPT2 is active with medium (C8-C12) and long-chain (C14-C18) acyl-CoA esters, whereas virtually no activity was found with short- and very long-chain acyl-CoAs or with branched-chain amino acid oxidation intermediates. Trans-2-enoyl-CoA intermediates were also found to be poor substrates for CPT2. Inhibition studies performed revealed that trans-2-C16:1-CoA may act as a competitive inhibitor of CPT2 (K(i) of 18.8 microM). The results obtained clearly demonstrate that CPT2 is able to reverse its physiological mechanism for medium and long-chain acyl-CoAs contributing to the abnormal acylcarnitines profiles characteristic of most mFAO disorders. The finding that trans-2-enoyl-CoAs are poorly handled by CPT2 may explain the absence of trans-2-enoyl-carnitines in the profiles of mitochondrial trifunctional protein deficient patients, the only defect where they accumulate, and the discrepancy between the clinical features of this and other long-chain mFAO disorders such as very long-chain acyl-CoA dehydrogenase deficiency.

Inborn errors of energy metabolism associated with myopathies

Inherited neuromuscular disorders affect approximately one in 3,500 children. Structural muscular defects are most common; however functional impairment of skeletal and cardiac muscle in both children and adults may be caused by inborn errors of energy metabolism as well. Patients suffering from metabolic myopathies due to compromised energy metabolism may present with exercise intolerance, muscle pain, reversible or progressive muscle weakness, and myoglobinuria. In this review, the physiology of energy metabolism in muscle is described, followed by the presentation of distinct disorders affecting skeletal and cardiac muscle: glycogen storage diseases types III, V, VII, fatty acid oxidation defects, and respiratory chain defects (i.e., mitochondriopathies). The diagnostic work-up and therapeutic options in these disorders are discussed.

Management and outcome in 75 individuals with long-chain fatty acid oxidation defects: results from a workshop

At present, long-chain fatty acid oxidation (FAO) defects are diagnosed in a number of countries by newborn screening using tandem mass spectrometry. In the majority of cases, affected newborns are asymptomatic at time of diagnosis and acute clinical presentations can be avoided by early preventive measures. Because evidence-based studies on management of long-chain FAO defects are lacking, we carried out a retrospective analysis of 75 patients from 18 metabolic centres in Germany, Switzerland, Austria and the Netherlands with special regard to treatment and disease outcome. Dietary treatment is effective in many patients and can prevent acute metabolic derangements and prevent or reverse severe long-term complications such as cardiomyopathy. However, 38% of patients with very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency had intermittent muscle weakness and pain despite adhering to therapy. Seventy-six per cent of patients with disorders of the mitochondrial trifunctional protein (TFP)-complex including long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency, had long-term myopathic symptoms. Of these, 21% had irreversible peripheral neuropathy and 43% had retinopathy. The main principle of treatment was a fat-reduced and fat-modified diet. Fat restriction differed among patients with different enzyme defects and was strictest in disorders of the TFP-complex. Patients with a medium-chain fat-based diet received supplementation of essential long-chain fatty acids. l-Carnitine was supplemented in about half of the patients, but in none of the patients with VLCAD deficiency identified by newborn screening. In summary, in this cohort the treatment regimen was adapted to the severity of the underlying enzyme defect and thus differed among the group of long-chain FAO defects.

Mitochondrial trifunctional protein deficiency with recurrent rhabdomyolysis

Rhabdomyolysis is an important clinical diagnosis. The differential diagnosis is extensive and includes various etiologies, such as infection, inflammation, trauma, endocrinopathies, and congenital muscular and metabolic disorders. Reported here is the case of an infant with recurrent rhabdomyolysis diagnosed as suffering from mitochondrial trifunctional protein deficiency -- a rare beta oxidation defect. The clinical course was unique, and a new mutation in the mitochondrial trifunctional protein gene was identified.