General mitochondrial trifunctional protein (TFP) deficiency as a result of either alpha- or beta-subunit mutations exhibits similar phenotypes because mutations in either subunit alter TFP complex expression and subunit turnover

A G1528C Hadha knock-in mouse model recapitulates aspects of human clinical phenotypes for long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency

Long chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) is a fatty acid oxidation disorder (FAOD) caused by a pathogenic variant, c.1528 G > C, in HADHA encoding the alpha subunit of trifunctional protein (TFPα). Individuals with LCHADD develop chorioretinopathy and peripheral neuropathy not observed in other FAODs in addition to the more ubiquitous symptoms of hypoketotic hypoglycemia, rhabdomyolysis and cardiomyopathy. We report a CRISPR/Cas9 generated knock-in murine model of G1528C in Hadha that recapitulates aspects of the human LCHADD phenotype. Homozygous pups are less numerous than expected from Mendelian probability, but survivors exhibit similar viability with wildtype (WT) littermates. Tissues of LCHADD homozygotes express TFPα protein, but LCHADD mice oxidize less fat and accumulate plasma 3-hydroxyacylcarnitines compared to WT mice. LCHADD mice exhibit lower ketones with fasting, exhaust earlier during treadmill exercise and develop a dilated cardiomyopathy compared to WT mice. In addition, LCHADD mice exhibit decreased visual performance, decreased cone function, and disruption of retinal pigment epithelium. Neurological function is affected, with impaired motor function during wire hang test and reduced open field activity. The G1528C knock-in mouse exhibits a phenotype similar to that observed in human patients; this model will be useful to explore pathophysiology and treatments for LCHADD in the future.

New Acylcarnitine Ratio as a Reliable Indicator of Long-Chain 3-Hydroxyacyl-CoA Dehydrogenase Deficiency

Long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) and mitochondrial trifunctional protein (MTP) deficiencies are rare fatal disorders of fatty acid β-oxidation with no apparent genotype-phenotype correlation. The measurement of acylcarnitines by MS/MS is a current diagnostic workup in these disorders. Nevertheless, false-positive and false-negative results have been reported, highlighting a necessity for more sensitive and specific biomarkers. This study included 54 patients with LCHAD/MTP deficiency that has been confirmed by biochemical and molecular methods. The analysis of acylcarnitines in dried blood spots was performed using ESI-MS/MS. The established "HADHA ratio" = (C16OH + C18OH + C18:1OH)/C0 was significantly elevated in all 54 affected individuals in comparison to the control group. Apart from 54 LCHAD deficiency patients, the "HADHA ratio" was calculated in 19 patients with very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency. As VLCAD-deficient patients did not show increased "HADHA ratio", the results emphasized the high specificity of this new ratio. Therefore, the "HADHA ratio" was shown to be instrumental in improving the overall performance of MS/MS-based analysis of acylcarnitine levels in the diagnostics of LCHAD/MTP deficiencies. The ratio was demonstrated to increase the sensitivity and specificity of this method and reduce the chances of false-negative results.

Oxaliplatin-induced cardiotoxicity in mice is connected to the changes in energy metabolism in the heart tissue

Oxaliplatin is a platinum-based alkylating chemotherapeutic agent used for cancer treatment. At high cumulative dosage, the negative effect of oxaliplatin on the heart becomes evident and is linked to a growing number of clinical reports. The aim of this study was to determine how chronic oxaliplatin treatment causes the changes in energy-related metabolic activity in the heart that leads to cardiotoxicity and heart damage in mice. C57BL/6 male mice were treated with a human equivalent dosage of intraperitoneal oxaliplatin (0 and 10 mg/kg) once a week for eight weeks. During the treatment, mice were followed for physiological parameters, ECG, histology and RNA sequencing of the heart. We identified that oxaliplatin induces strong changes in the heart and affects the heart's energy-related metabolic profile. Histological post-mortem evaluation identified focal myocardial necrosis infiltrated with a small number of associated neutrophils. Accumulated doses of oxaliplatin led to significant changes in gene expression related to energy related metabolic pathways including fatty acid (FA) oxidation, amino acid metabolism, glycolysis, electron transport chain, and NAD synthesis pathway. At high accumulative doses of oxaliplatin, the heart shifts its metabolism from FAs to glycolysis and increases lactate production. It also leads to strong overexpression of genes in NAD synthesis pathways such as Nmrk2. Changes in gene expression associated with energy metabolic pathways can be used to develop diagnostic methods to detect oxaliplatin-induced cardiotoxicity early on as well as therapy to compensate for the energy deficit in the heart to prevent heart damage.

Silencing of Mitochondrial Trifunctional Protein A Subunit (HADHA) Increases Lipid Stores, and Reduces Oviposition and Flight Capacity in the Vector Insect Rhodnius prolixus

Rhodnius prolixus is an obligatory hematophagous insect, vector of Chagas disease. After blood meal, lipids are absorbed, metabolized, synthesized, and accumulated in the fat body. When necessary, stored lipids are mobilized, transported to other organs, or are oxidized to provide energy. Mitochondrial β-oxidation is a cyclic conserved pathway, where degradation of long-chain fatty acids occurs to contribute to cellular energetic demands. Three of its reactions are catalyzed by the mitochondrial trifunctional protein (MTP), which is composed by hydroxyacyl-CoA dehydrogenase trifunctional multienzyme complex subunits alpha and beta (HADHA and HADHB, respectively). Here, we investigated the role of HADHA in lipid metabolism and reproduction of Rhodnius prolixus females. The expression of HADHA gene (RhoprHadha) was determined in the organs of starving adult insects. The flight muscle and ovary had higher expression levels when compared to the anterior and posterior midguts or the fat body. RhoprHadha gene expression was upregulated by blood meal in the flight muscle and fat body. We generated insects with RNAi-mediated knockdown of RhoprHadha to address the physiological role of this gene. RhoprHadha deficiency resulted in higher triacylglycerol content and larger lipid droplets in the fat body during starvation. After feeding, lifespan of the knockdown females was not affected, but they exhibited a decrease in oviposition, although hatching was the same in both groups. Silenced females showed lower forced flight capacity than the control ones, and their fat bodies had lower gene expression levels of Brummer lipase (RhoprBmm) and long-chain acyl-CoA synthetase 2 (RhoprAcsl2). Taken together, these findings indicate that HADHA is important to guarantee successful reproduction and efficient mobilization of lipid stores during starvation and flight.

Outcomes and genotype correlations in patients with mitochondrial trifunctional protein or isolated long chain 3-hydroxyacyl-CoA dehydrogenase deficiency enrolled in the IBEM-IS database

Purpose

Mitochondrial trifunctional protein deficiency (TFPD) and isolated long chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) are two related defects of fatty acid β -oxidation. While NBS has decreased mortality, morbidity remains significant. Additionally, the relationship of genotype to clinical outcome remains unclear. To better understand these issues, we collected natural history data for these conditions by reviewing seven years of retrospective data from 45 cases of TFPD or LCHADD in the Inborn Errors of Metabolism - Information System.

Methods

Available data included age at database entry, last datapoint, and development of various complications. Data were analyzed by clinical assigned diagnosis (LCHADD or TFPD), subdivided by method of ascertainment (newborn screening-NBS, or other than by newborn screening-NNBS), then re-analyzed based on four genotype groups: homozygous c.1528GC (p.E510Q) (common LCHAD variant); heterozygous c.1528GC (p.E510Q), other HADHA variants; and HADHB variants.

Results

Forty-five patients from birth to 34 years of age were analyzed by assigned diagnosis (30 LCHADD and 15 TFPD) and method of ascertainment. Thirty had further analysis by genotype (22 biallelic HADHA variants and 8 biallelic HADHB variants). With regards to maternal complications, retinopathy, cardiomyopathy and hypoglycemia, patients with biallelic HADHA variants (with or without the common LCHAD variant) manifest a traditional LCHADD phenotype, while those with HADHB gene variants more commonly reported neuromusculoskeletal type TFPD phenotype. While retinopathy, rhabdomyolysis and peripheral neuropathy tended to present later in childhood, many features including initial report of cardiomyopathy and hypoglycemia presented across a wide age spectrum.

Conclusion

This study demonstrates the utility of genotypic confirmation of patients identified with LCHADD/TFPD as variants in the HADHA and HADHB genes lead to different symptom profiles. In our data, biallelic HAHDA variants conferred a LCHADD phenotype, regardless of the presence of the common LCHAD variant.

Novel mutations in the HADHB gene causing a mild phenotype of mitochondrial trifunctional protein ( MTP ) deficiency

Mitochondrial trifunctional protein (MTP) deficiency is an ultrarare hereditary recessive disorder causing a broad spectrum of phenotypes with lethal infantile cardiomyopathy at the most severe end. Attenuated forms with polyneuropathy have been reported combined with myoglobinuria or rhabdomyolysis as key features. We here report three young adults (two siblings) in which three variants in the HADHB‐gene were identified. All three cases had a similar mild phenotype with axonal neuropathy and frequent intermittent weakness episodes but without myoglobinuria. Special dietary precautions were recommended to minimize complications especially during infections and other catabolic states. MTP deficiency is therefore an important differential diagnosis in patients with milder fluctuating neuromuscular symptoms.

Qualitative evaluation of the symptoms and quality of life impacts of long-chain fatty acid oxidation disorders

BACKGROUND

Long-chain fatty acid oxidation disorders (LC-FAOD) are a group of rare autosomal-recessive genetic disorders characterized by metabolic deficiencies in which the body is unable to convert long-chain fatty acids into energy. To date, however, there is limited understanding of the patient experience of LC-FAOD.

METHODS

The symptoms, observable signs, and quality of life (QoL) impacts associated with LC-FAOD were explored via a focus group (n = 8) and semi-structured interviews (n = 6) with patients and caregivers of patients with LC-FAOD, and interviews (n = 4) with expert clinicians. Data were analyzed via thematic analysis and summarized in a conceptual model.

RESULTS

Participants reported a wide range of signs and symptoms associated with LC-FAOD, broadly categorized as musculoskeletal, endocrine/nutritional/metabolic, neurological, gastrointestinal/digestive, sensory, cardiovascular, respiratory, urological, and constitutional. LC-FAOD were reported to have a significant impact on various aspects of patients' lives including physical functioning, participation in daily activities, emotional/psychological wellbeing, and social functioning. Lifestyle modifications (such as diet and exercise restrictions) were necessary because of the condition. Symptoms were typically episodic in presentation often arising or exacerbated during catabolic conditions such as prolonged exercise, fasting, physiological stress, and illness/infection. Symptoms were also commonly reported to lead to emergency room visits, hospitalization, and clinical complications.

CONCLUSION

LC-FAOD have a considerable impact on patients' lives. There is a high degree of concordance in the signs, symptoms, and impacts of LC-FAOD reported by patients, caregivers, and clinicians; however, there were many symptoms and impacts that were only reported by patients and caregivers, thus demonstrating that insights from patient/caregiver experience data are integral for informing medical product development and facilitating patient-centered care.

Hypoparathyroidism, neutropenia and nephrotic syndrome in a patient with mitochondrial trifunctional protein deficiency: A case report and review of the literature

INTRODUCTION

Mitochondrial trifunctional protein (TFP) deficiency is an autosomal recessive disorder that causes a clinical spectrum of diseases ranging from severe infantile cardiomyopathy to mild chronic progressive neuromyopathy, however, parathyroid glands, hematologic system and kidney damage are not the common presentations of this disease.

METHODS

We describe the clinical, biochemical and molecular features of the TFP deficiency patient at our institution. We also provide an extensive literature review of previous published cases with emphasis on the clinical/biochemical phenotype-genotype correlation of this disorder.

RESULTS

Our case is a complete TFP deficiency patient dominated presented with hypoparathyroidism, neutropenia and nephrotic syndrome, which caused by compound heterozygoues variants in HADHB gene. Based on the retrospective study of 157 cases, TFP patients presented with diverse clinical, biochemical and molecular features. The onset age is typically before early childhood. Neuromuscular system is more vulnerable involved. Severe form is generally characterized by multiorgan involvement. A notable feature of severe and intermediate form is respiratory failure. Neuropathy and rhabdomyolysis are the typical manifestations of mild form. Increased long-chain 3-OH-acylcarnitines (C16-OH, C18:1-OH) are the most common biochemical finding. The mortality of the present study is as high as 57.9%, which is linked with the onset age, phenotype, mutation type and muscular histology. Mutations in HADHB are more frequent in Asian descent with complete TFP deficiency and usually presented with atypical presentations. The type of mutation, rather than residual enzyme activity seem to be more related to the phenotype and prognosis. The most common HADHA variant is 1528G > C, no common HADHB variant were detected.

CONCLUSIONS

TFP deficiency is heterogeneous at both the molecular and phenotypic levels, generally a high mortality. Although there is no strict clinical/biochemical phenotype-genotype correlation, difference in ethnic and subunit mutations still have certain characteristics.

Optical Metabolic Imaging of Mitochondrial Dysfunction on HADH Mutant Newborn Rat Hearts

BACKGROUND

Mitochondrial [Formula: see text]-oxidation of fatty acids is the primary energy source for the heart and carried out by Hydroxy Acyl-CoA Dehydrogenase (HADH) encoded trifunctional protein. Mutations in the genes encoding mitochondrial proteins result in functionally defective protein complexes that contribute to energy deficiencies, excessive reactive oxygen species (ROS) production, and accumulation of damaged mitochondria. We hypothesize that a dramatic alternation in redox state and associated mitochondrial dysfunction is the underlying cause of Fatty Acid Oxidation (FAO) deficiency mutant, resulting in heart failure. Mitochondrial co-enzymes, NADH and FAD, are autofluorescent metabolic indices of cells when imaged, yield a quantitative assessment of the cells' redox status and, in turn, that of the tissue and organ.

METHOD

We utilized an optical cryo-imager to quantitively evaluate the three-dimensional distribution of mitochondrial redox state in newborn rats' hearts and kidneys. Redox ratio (RR) assessment shows that mitochondrial dysfunction is extreme and could contribute to severe heart problems and eventual heart failure in the mutants.

RESULTS

Three-dimensional redox ratio (NADH/FAD) rendering, and the volumetric mean value calculations confirmed significantly decreased cardiac RR in mutants by 31.90% and 12.32%, in renal mitochondrial RR compared to wild-type control. Further, histological assessment of newborn heart myocardial tissue indicated no significant difference in myocardial tissue architecture in both control and severe (HADHAe4-/-) conditions.

CONCLUSION

These results demonstrate that optical imaging can accurately estimate the redox state changes in newborn rat organs. It is also apparent that the FAO mutant's heart tissue with a low redox ratio is probably more vulnerable to cumulative damages than kidneys and fails prematurely, contributing to sudden death.

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.

Mitochondrial morphology, bioenergetics and proteomic responses in fatty acid oxidation disorders

Mutations in nuclear genes encoding for mitochondrial proteins very long-chain acyl-CoA dehydrogenase (VLCAD) and trifunctional protein (TFP) cause rare autosomal recessive disorders. Studies in fibroblasts derived from patients with mutations in VLCAD and TFP exhibit mitochondrial defects. To gain insights on pathological changes that account for the mitochondrial deficits we performed quantitative proteomic, biochemical, and morphometric analyses in fibroblasts derived from subjects with three different VLCAD and three different TFP mutations. Proteomic data that was corroborated by antibody-based detection, indicated reduced levels of VLCAD and TFP protein in cells with VLCAD and TFP mutations respectively, which in part accounted for the diminished fatty acid oxidation capacity. Decreased mitochondrial respiratory capacity in cells with VLCAD and TFP mutations was quantified after glucose removal and cells with TFP mutations had lower levels of glycogen. Despite these energetic deficiencies, the cells with VLCAD and TFP mutations did not exhibit changes in mitochondria morphology, distribution, fusion and fission, quantified by either confocal or transmission electron microscopy and corroborated by proteomic and antibody-based protein analysis. Fibroblasts with VLCAD and to a lesser extend cells with TFP mutations had increased levels of mitochondrial respiratory chain proteins and proteins that facilitate the assembly of respiratory complexes. With the exception of reduced levels of catalase and glutathione S-transferase theta-1 in cells with TFP mutations, the levels of 45 proteins across all major intracellular antioxidant networks were similar between cells with VLCAD and TFP mutations and non-disease controls. Collectively the data indicate that despite the metabolic deficits, cells with VLCAD and TFP mutations maintain their proteomic integrity to preserve cellular and mitochondria architecture, support energy production and protect against oxidative stress.

Newborn Screening for Long-Chain 3-Hydroxyacyl-CoA Dehydrogenase and Mitochondrial Trifunctional Protein Deficiencies Using Acylcarnitines Measurement in Dried Blood Spots-A Systematic Review of Test Accuracy

Background: Long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) and mitochondrial trifunctional protein (MTP) deficiencies are rare autosomal recessive fatty acid β-oxidation disorders. Their clinical presentations are variable, and premature death is common. They are included in newborn blood spot screening programs in many countries around the world. The current process of screening, through the measurement of acylcarnitines (a metabolic by-product) in dried blood spots with tandem mass spectrometry, is subject to uncertainty regarding test accuracy. Methods: We conducted a systematic review of literature published up to 19th June 2018. We included studies that investigated newborn screening for LCHAD or MTP deficiencies by tandem mass spectrometry of acylcarnitines in dried blood spots. The reference standards were urine organic acids, blood acylcarnitine profiles, enzyme analysis in cultured fibroblasts or lymphocytes, mutation analysis, or at least 10-year follow-up. The outcomes of interest were sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). Assessment of titles, abstracts, and full-text papers and quality appraisal were carried out independently by two reviewers. One reviewer extracted study data. This was checked by a second reviewer. Results: Ten studies provided data on test accuracy. LCHAD or MTP deficiencies were identified in 23 babies. No cases of LCHAD/MTP deficiencies were identified in four studies. PPV ranged from 0% (zero true positives and 28 false positives from 276,565 babies screened) to 100% (13 true positives and zero false positives from 2,037,824 babies screened). Sensitivity, specificity, and NPV could not be calculated as there was no systematic follow-up of babies who screened negative. Conclusions: Test accuracy estimates of screening for LCHAD and MTP deficiencies with tandem mass spectrometry measurement of acylcarnitines in dried blood were variable in terms of PPVs. Screening methods (including markers and thresholds) varied between studies, and sensitivity, specificity, and NPVs are unknown.

Effects of fasting, feeding and exercise on plasma acylcarnitines among subjects with CPT2D, VLCADD and LCHADD/TFPD

BACKGROUND

The plasma acylcarnitine profile is frequently used as a biochemical assessment for follow-up in diagnosed patients with fatty acid oxidation disorders (FAODs). Disease specific acylcarnitine species are elevated during metabolic decompensation but there is clinical and biochemical heterogeneity among patients and limited data on the utility of an acylcarnitine profile for routine clinical monitoring.

METHODS

We evaluated plasma acylcarnitine profiles from 30 diagnosed patients with long-chain FAODs (carnitine palmitoyltransferase-2 (CPT2), very long-chain acyl-CoA dehydrogenase (VLCAD), and long-chain 3-hydroxy acyl-CoA dehydrogenase or mitochondrial trifunctional protein (LCHAD/TFP) deficiencies) collected after an overnight fast, after feeding a controlled low-fat diet, and before and after moderate exercise. Our purpose was to describe the variability in this biomarker and how various physiologic states effect the acylcarnitine concentrations in circulation.

RESULTS

Disease specific acylcarnitine species were higher after an overnight fast and decreased by approximately 60% two hours after a controlled breakfast meal. Moderate-intensity exercise increased the acylcarnitine species but it varied by diagnosis. When analyzed for a genotype/phenotype correlation, the presence of the common LCHADD mutation (c.1528G > C) was associated with higher levels of 3-hydroxyacylcarnitines than in patients with other mutations.

CONCLUSIONS

We found that feeding consistently suppressed and that moderate intensity exercise increased disease specific acylcarnitine species, but the response to exercise was highly variable across subjects and diagnoses. The clinical utility of routine plasma acylcarnitine analysis for outpatient treatment monitoring remains questionable; however, if acylcarnitine profiles are measured in the clinical setting, standardized procedures are required for sample collection to be of value.

Efficacy of bezafibrate in two patients with mitochondrial trifunctional protein deficiency

Mitochondrial trifunctional protein (TFP) deficiency is a rare inherited metabolic disorder caused by defects in fatty acid β-oxidation (FAO) of long-chain fatty acids, leading to impaired energy production. Fasting avoidance, fatty acid-restricted diets, and supplementation with medium-chain triglycerides are recommended as a treatment, but there are no pharmaceutical treatments available with strong evidence of efficacy. Bezafibrate, which enhances the transcription of FAO enzymes, is a promising therapeutic option for FAO disorders (FAODs). The effectiveness of bezafibrate for FAODs has been reported in some clinical trials, but few clinical studies have investigated its in vivo efficacy toward TFP deficiency.

Herein, we describe two Japanese patients with TFP deficiency. Patient 1 presented with recurrent myalgia since the age of 5 years. Laboratory findings showed increased serum levels of long-chain fatty acids and reduced expression of TFPα and TFPβ in his skin fibroblasts. Based on these findings, he was diagnosed with the myopathic type of TFP deficiency. Patient 2 suddenly exhibited cardiopulmonary arrest one day after birth. Elevated levels of creatine kinase and long-chain acylcarnitines were observed. Genetic analysis identified compound heterozygous variants in HADHB (c.1175C>T/c.1364T>G). He was diagnosed with the lethal type of TFP deficiency. Although both patients were treated with dietary therapy and l-carnitine supplementation, they experienced frequent myopathic attacks associated with respiratory infections and exercise. After the initiation of bezafibrate, their myopathic manifestations were markedly reduced, leading to an improvement in quality of life without any side effects.

Our clinical findings indicate that bezafibrate combined with other treatments such as dietary therapy may be effective in improving myopathic manifestations in TFP deficiency.

Insight into the Regulatory Function of Human Hair Keratins in Wound Healing Using Proteomics

Keratins derived from human hair possess excellent wound healing qualities. However, their functional contribution to this process is poorly understood. In this study, the regulatory function of human hair keratins in wound healing is investigated using proteomic analysis by dividing keratins into different groups based on their molecular weight distributions: low molecular weight keratins (LMWK, 10-30 kDa), medium molecular weight keratins (MMWK, 30-50 kDa), and high molecular weight keratins (HMWK, >50 kDa). Keratin hydrogels with different molecular weights exhibit various morphologies, rheological properties, degradation rates, and wound healing activities. Using proteomic analysis, LMWK and HMWK hydrogels exhibit a stronger regulatory ability for wound healing at days 1 and 7, respectively. The major functions of LMWK during wound healing are regulation of cells communication and function. In contrast, proteins associated with energy metabolism are significantly expressed after HMWK hydrogel treatment at day 1, and these play an important role in cellular growth and reactive oxygen species scavenging at day 7. These results demonstrate that the wound healing qualities of human hair keratins are influenced by their molecular weight distribution, and the proteomic analysis sheds new light on the regulatory function of human hair keratins during wound healing.

Implications of the mitochondrial interactome of mammalian thioredoxin 2 for normal cellular function and disease

Multiple thioredoxin isoforms exist in all living cells. To explore the possible functions of mammalian mitochondrial thioredoxin 2 (Trx2), an interactome of mouse Trx2 was initially created using (i) a monothiol mouse Trx2 species for capturing protein partners from different organs and (ii) yeast two hybrid screens on human liver and rat brain cDNA libraries. The resulting interactome consisted of 195 proteins (Trx2 included) plus the mitochondrial 16S RNA. 48 of these proteins were classified as mitochondrial (MitoCarta2.0 human inventory). In a second step, the mouse interactome was combined with the current four-membered mitochondrial sub-network of human Trx2 (BioGRID) to give a 53-membered human Trx2 mitochondrial interactome (52 interactor proteins plus the mitochondrial 16S RNA). Although thioredoxins are thiol-employing disulfide oxidoreductases, approximately half of the detected interactions were not due to covalent disulfide bonds. This finding reinstates the extended role of thioredoxins as moderators of protein function by specific non-covalent, protein-protein interactions. Analysis of the mitochondrial interactome suggested that human Trx2 was involved potentially in mitochondrial integrity, formation of iron sulfur clusters, detoxification of aldehydes, mitoribosome assembly and protein synthesis, protein folding, ADP ribosylation, amino acid and lipid metabolism, glycolysis, the TCA cycle and the electron transport chain. The oxidoreductase functions of Trx2 were verified by its detected interactions with mitochondrial peroxiredoxins and methionine sulfoxide reductase. Parkinson's disease, triosephosphate isomerase deficiency, combined oxidative phosphorylation deficiency, and lactate dehydrogenase b deficiency are some of the diseases where the proposed mitochondrial network of Trx2 may be implicated.

Mitochondrial Genetic Disorders: Cell Signaling and Pharmacological Therapies

Mitochondrial fatty acid oxidation (FAO) and respiratory chain (RC) defects form a large group of inherited monogenic disorders sharing many common clinical and pathophysiological features, including disruption of mitochondrial bioenergetics, but also, for example, oxidative stress and accumulation of noxious metabolites. Interestingly, several transcription factors or co-activators exert transcriptional control on both FAO and RC genes, and can be activated by small molecules, opening to possibly common therapeutic approaches for FAO and RC deficiencies. Here, we review recent data on the potential of various drugs or small molecules targeting pivotal metabolic regulators: peroxisome proliferator activated receptors (PPARs), sirtuin 1 (SIRT1), AMP-activated protein kinase (AMPK), and protein kinase A (PKA)) or interacting with reactive oxygen species (ROS) signaling, to alleviate or to correct inborn FAO or RC deficiencies in cellular or animal models. The possible molecular mechanisms involved, in particular the contribution of mitochondrial biogenesis, are discussed. Applications of these pharmacological approaches as a function of genotype/phenotype are also addressed, which clearly orient toward personalized therapy. Finally, we propose that beyond the identification of individual candidate drugs/molecules, future pharmacological approaches should consider their combination, which could produce additive or synergistic effects that may further enhance their therapeutic potential.

Crystal structure of human mitochondrial trifunctional protein, a fatty acid β-oxidation metabolon

Membrane-bound mitochondrial trifunctional protein (TFP) catalyzes β-oxidation of long chain fatty acyl-CoAs, employing 2-enoyl-CoA hydratase (ECH), 3-hydroxyl-CoA dehydrogenase (HAD), and 3-ketothiolase (KT) activities consecutively. Inherited deficiency of TFP is a recessive genetic disease, manifesting in hypoketotic hypoglycemia, cardiomyopathy, and sudden death. We have determined the crystal structure of human TFP at 3.6-Å resolution. The biological unit of the protein is α₂β₂ The overall structure of the heterotetramer is the same as that observed by cryo-EM methods. The two β-subunits make a tightly bound homodimer at the center, and two α-subunits are bound to each side of the β₂ dimer, creating an arc, which binds on its concave side to the mitochondrial innermembrane. The catalytic residues in all three active sites are arranged similarly to those of the corresponding, soluble monofunctional enzymes. A structure-based, substrate channeling pathway from the ECH active site to the HAD and KT sites is proposed. The passage from the ECH site to the HAD site is similar to those found in the two bacterial TFPs. However, the passage from the HAD site to the KT site is unique in that the acyl-CoA intermediate can be transferred between the two sites by passing along the mitochondrial inner membrane using the hydrophobic nature of the acyl chain. The 3'-AMP-PPi moiety is guided by the positively charged residues located along the "ceiling" of the channel, suggesting that membrane integrity is an essential part of the channel and is required for the activity of the enzyme.

Fatal pitfalls in newborn screening for mitochondrial trifunctional protein (MTP)/long-chain 3-Hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency

Background

Mitochondrial trifunctional protein (MTP) and long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency are long-chain fatty acid oxidation disorders with particularly high morbidity and mortality. Outcome can be favorable if diagnosed in time, prompting the implementation in newborn screening programs. Sporadic cases missed by the initial screening sample have been reported. However, little is known on pitfalls during confirmatory testing resulting in fatal misconception of the diagnosis.

Results

We report a series of three patients with MTP and LCHAD deficiency, in whom diagnosis was missed by newborn screening, resulting in life-threatening metabolic decompensations within the first half year of life. Two of the patients showed elevated concentrations of primary markers C16-OH and C18:1-OH but were missed by confirmatory testing performed by the maternity clinic. A metabolic center was not consulted. Confirmatory testing consisted of analyses of acylcarnitines in blood and organic acids in urine, the finding of normal excretion of organic acids led to rejection and underestimation of the diagnosis, respectively. The third patient, a preterm infant, was not identified in the initial screening sample due to only moderate elevations of C16-OH and C18:1-OH and normal secondary markers and analyte ratios.

Conclusion

Our observations highlight limitations of newborn screening for MTP/LCHAD deficiency. They confirm that analyses of acylcarnitines in blood and organic acids in urine alone are not suitable for confirmatory testing and molecular or functional analysis is crucial in diagnosing MTP/LCHAD deficiency. Mild elevations of primary biomarkers in premature infants need to trigger confirmatory testing. Our report underscores the essential role of specialized centers in confirming or ruling out diagnoses in suspicious screening results.

MOXI Is a Mitochondrial Micropeptide That Enhances Fatty Acid β-Oxidation

Micropeptide regulator of β-oxidation (MOXI) is a conserved muscle-enriched protein encoded by an RNA transcript misannotated as non-coding. MOXI localizes to the inner mitochondrial membrane where it associates with the mitochondrial trifunctional protein, an enzyme complex that plays a critical role in fatty acid β-oxidation. Isolated heart and skeletal muscle mitochondria from MOXI knockout mice exhibit a diminished ability to metabolize fatty acids, while transgenic MOXI overexpression leads to enhanced β-oxidation. Additionally, hearts from MOXI knockout mice preferentially oxidize carbohydrates over fatty acids in an isolated perfused heart system compared to wild-type (WT) animals. MOXI knockout mice also exhibit a profound reduction in exercise capacity, highlighting the role of MOXI in metabolic control. The functional characterization of MOXI provides insight into the regulation of mitochondrial metabolism and energy homeostasis and underscores the regulatory potential of additional micropeptides that have yet to be identified.

Cryo-EM structure of human mitochondrial trifunctional protein

The mitochondrial trifunctional protein (TFP) catalyzes three reactions in the fatty acid β-oxidation process. Mutations in the two TFP subunits cause mitochondrial trifunctional protein deficiency and acute fatty liver of pregnancy that can lead to death. Here we report a 4.2-Å cryo-electron microscopy α2β2 tetrameric structure of the human TFP. The tetramer has a V-shaped architecture that displays a distinct assembly compared with the bacterial TFPs. A concave surface of the TFP tetramer interacts with the detergent molecules in the structure, suggesting that this region is involved in associating with the membrane. Deletion of a helical hairpin in TFPβ decreases its binding to the liposomes in vitro and reduces its membrane targeting in cells. Our results provide the structural basis for TFP function and have important implications for fatty acid oxidation related diseases.

A Leveraged Signal-to-Noise Ratio (LSTNR) Method to Extract Differentially Expressed Genes and Multivariate Patterns of Expression From Noisy and Low-Replication RNAseq Data

To life scientists, one important feature offered by RNAseq, a next-generation sequencing tool used to estimate changes in gene expression levels, lies in its unprecedented resolution. It can score countable differences in transcript numbers among thousands of genes and between experimental groups, all at once. However, its high cost limits experimental designs to very small sample sizes, usually N = 3, which often results in statistically underpowered analysis and poor reproducibility. All these issues are compounded by the presence of experimental noise, which is harder to distinguish from instrumental error when sample sizes are limiting (e.g., small-budget pilot tests), experimental populations exhibit biologically heterogeneous or diffuse expression phenotypes (e.g., patient samples), or when discriminating among transcriptional signatures of closely related experimental conditions (e.g., toxicological modes of action, or MOAs). Here, we present a leveraged signal-to-noise ratio (LSTNR) thresholding method, founded on generalized linear modeling (GLM) of aligned read detection limits to extract differentially expressed genes (DEGs) from noisy low-replication RNAseq data. The LSTNR method uses an agnostic independent filtering strategy to define the dynamic range of detected aggregate read counts per gene, and assigns statistical weights that prioritize genes with better sequencing resolution in differential expression analyses. To assess its performance, we implemented the LSTNR method to analyze three separate datasets: first, using a systematically noisy in silico dataset, we demonstrated that LSTNR can extract pre-designed patterns of expression and discriminate between "noise" and "true" differentially expressed pseudogenes at a 100% success rate; then, we illustrated how the LSTNR method can assign patient-derived breast cancer specimens correctly to one out of their four reported molecular subtypes (luminal A, luminal B, Her2-enriched and basal-like); and last, we showed the ability to retrieve five different modes of action (MOA) elicited in livers of rats exposed to three toxicants under three nutritional routes by using the LSTNR method. By combining differential measurements with resolving power to detect DEGs, the LSTNR method offers an alternative approach to interrogate noisy and low-replication RNAseq datasets, which handles multiple biological conditions at once, and defines benchmarks to validate RNAseq experiments with standard benchtop assays.

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.

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.

Thyroid Hormone Stimulation of Adult Brain Fatty Acid Oxidation

Thyroid hormone is a critical modulator of brain metabolism, and it is highly controlled in the central nervous system. Recent research has uncovered an important role of thyroid hormone in the regulation of fatty acid oxidation (FAO), an energetic process essential for neurodevelopment that continues to support brain metabolism during adulthood. Thyroid hormone stimulation of FAO has been shown to be protective in astrocytes and mouse models of brain injury, yet a clear mechanism of this relationship has not been elucidated. Thyroid hormone interacts with multiple receptors located in the nucleus and the mitochondria, initiating rapid and long-term effects via both genomic and nongenomic pathways. This has complicated efforts to isolate and study-specific interactions. This chapter presents the primary signaling pathways that have been identified to play a role in the thyroid hormone-mediated increase in FAO. Investigation of the impact of thyroid hormone on FAO in the adult brain has challenged classical models of brain metabolism and widened the window of potential neuroprotective strategies. A detailed understanding of these pathways is essential for any researchers aiming to expand the field of neuroenergetics.

Peripheral Neuropathy, Episodic Rhabdomyolysis, and Hypoparathyroidism in a Patient with Mitochondrial Trifunctional Protein Deficiency

A combination of unexplained peripheral neuropathy, hypoparathyroidism, and the inability to cope with metabolic stress could point to a rare inborn error of metabolism, such as mitochondrial trifunctional protein (MTP) deficiency.Here, we describe a 20-year-old woman who was known since childhood with axonal motor sensory polyneuropathy of unknown origin. She presented with progressive dyspnoea, and increased muscle weakness, preceded by 6 days of fever, vomiting, and diarrhoea. Laboratory testing showed rhabdomyolysis, and hypocalcaemia with low parathyroid levels. The patient was intubated because of respiratory insufficiency and a viral and bacterial pneumonia was diagnosed. She was discharged after 16 days of admission. Metabolic screening, performed at the time of rhabdomyolysis, showed increased concentrations of long-chain 3-hydroxyacyl carnitine species, together with elevated urinary excretion of 3-hydroxy dicarboxylic acids. Decreased activity of long-chain 3-hydroxyacyl-CoA dehydrogenase and long-chain 3-ketoacyl-CoA thiolase in peripheral lymphocytes and fibroblasts confirmed a MTP deficiency. Sequence analysis of the HADHB gene showed two heterozygous variants: c.209+1G>C (splicing defect) and c.980T>C (p.Leu327Leu). When the acylcarnitine profile was repeated after the episode of rhabdomyolysis had resolved it showed no abnormalities.Our case illustrates a cluster of peripheral neuropathy, episodic rhabdomyolysis, and hypoparathyroidism in a patient with MTP deficiency caused by mutations in the HADHB gene. It stresses the importance of performing metabolic screening when patients are most symptomatic, as normal results can be found at times when no metabolic stress is present. Screening is relatively easy and timely diagnosis has important implications for treatment.

UX007 for the treatment of long chain-fatty acid oxidation disorders: Safety and efficacy in children and adults following 24weeks of treatment

BACKGROUND

Long-chain fatty acid oxidation disorders (LC-FAOD) lead to accumulation of high concentrations of potentially toxic fatty acid intermediates. Newborn screening and early intervention have reduced mortality, but most patients continue to experience frequent hospitalizations and significant morbidity despite treatment. The deficient energy state can cause serious liver, muscle, and heart disease, and may be associated with an increased risk of sudden death. Triheptanoin is a medium odd-chain fatty acid. Anaplerotic metabolites of triheptanoin have the potential to replace deficient tricarboxylic acid (TCA) cycle intermediates, resulting in net glucose production as a novel energy source for the treatment of LC-FAOD.

STUDY DESIGN

A single-arm, open-label, multicenter Phase 2 safety and efficacy study evaluated patients with severe LC-FAOD evidenced by ongoing related musculoskeletal, cardiac, and/or hepatic events despite treatment. After a four-week run-in on current regimen, investigational triheptanoin (UX007) was titrated to a target dose of 25-35% of total daily caloric intake. Patients were evaluated on several age/condition-eligible endpoints, including submaximal exercise tests to assess muscle function/endurance (12-minute walk test; 12MWT) and exercise tolerance (cycle ergometry), and health related quality of life (HR-QoL). Results through 24weeks of treatment are presented; total study duration is 78weeks.

RESULTS

Twenty-nine patients (0.8 to 58years) were enrolled; most qualified based on severe musculoskeletal disease. Twenty-five patients (86%) completed the 24-week treatment period. At Week 18, eligible patients (n=8) demonstrated a 28% increase (LS mean=+181.9 meters; p=0.087) from baseline (673.4meters) in 12MWT distance. At Week 24, eligible patients (n=7) showed a 60% increase in watts generated (LS mean=+409.3W; p=0.149) over baseline (744.6W) for the exercise tolerance test. Improvements in exercise tests were supported by significant improvements from baseline in the adult (n=5) self-reported SF-12v2 physical component summary score (LS mean=+8.9; p<0.001). No difference from baseline was seen in pediatric parent-reported (n=5) scores (SF-10) at Week 24. Eighteen patients (62%) had treatment-related adverse events, predominantly gastrointestinal (55%), mild-to-moderate in severity, similar to that seen with prior treatment with medium chain triglyceride (MCT) oil. One patient experienced a treatment-related serious adverse event of gastroenteritis. One patient discontinued from study due to diarrhea of moderate severity; the majority of patients (25/29; 86%) elected to continue treatment in the extension period.

CONCLUSIONS

In patients with severe LC-FAOD, UX007 interim study results demonstrated improved exercise endurance and tolerance, and were associated with positive changes in self-reported HR-QoL.

Characterization of Chorioretinopathy Associated with Mitochondrial Trifunctional Protein Disorders: Long-Term Follow-up of 21 Cases

PURPOSE

To assess long-term effects of genotype on chorioretinopathy severity in patients with mitochondrial trifunctional protein (MTP) disorders.

DESIGN

Retrospective case series.

PARTICIPANTS

Consecutive patients with MTP disorders evaluated at a single center from 1994 through 2015, including 18 patients with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) and 3 patients with trifunctional protein deficiency (TFPD).

METHODS

Local records from all visits were reviewed. Every participant underwent a complete ophthalmic examination and was evaluated by a metabolic physician and dietitian. Nine patients underwent ancillary funduscopic imaging including optical coherence tomography (OCT) and OCT angiography.

MAIN OUTCOME MEASURES

The primary outcome measure was best-corrected visual acuity at the final visit. Secondary outcome measures included spherical equivalent refraction, visual fields, electroretinography B-wave amplitudes, and qualitative imaging findings.

RESULTS

Participants were followed up for a median of 5.6 years (range 0.3-20.2 years). The median age of LCHADD participants at initial and final visits was 2.3 and 11.9 years, whereas that for TFPD participants at initial and final visits was 4.7 and 15.5 years, respectively. Four long-term survivors older than 16 years were included (3 with LCHADD and 1 with TFPD). The LCHADD participants demonstrated a steady decline in visual acuity from an average of 0.23 logarithm of the minimum angle of resolution (logMAR; Snellen equivalent, 20/34) at baseline to 0.42 logMAR (Snellen equivalent, 20/53) at the final visit, whereas TFPD patients maintained excellent acuity throughout follow-up. Participants with LCHADD, but not TFPD, showed an increasing myopia with a mean decrease in spherical equivalent refraction of 0.24 diopters per year. Visual fields showed sensitivity losses centrally associated with defects on OCT. Multimodal imaging demonstrated progressive atrophy of the outer retina in LCHADD, often preceded by the formation of outer retinal tubulations and choriocapillaris dropout. Electroretinography findings support the more severe clinical profile of LCHADD patients compared with TFPD patients; the function of both rods and cones are attenuated diffusely in LCHADD patients, but are within normal limits for TFPD patients.

CONCLUSIONS

Despite improved survival with early diagnosis, medical management, and dietary treatment, participants with the LCHADD subtype of MTP disorder continue to demonstrate visually disabling chorioretinopathy. Multimodal imaging is most consistent with choriocapillaris loss exceeding photoreceptor loss.

Mitochondrial trifunctional protein deficiency in human cultured fibroblasts: effects of bezafibrate

Mitochondrial trifunctional protein (MTP) deficiency caused by HADHA or HADHB gene mutations exhibits substantial molecular, biochemical, and clinical heterogeneity and ranks among the more severe fatty acid oxidation (FAO) disorders, without pharmacological treatment. Since bezafibrate has been shown to potentially correct other FAO disorders in patient cells, we analyzed its effects in 26 MTP-deficient patient fibroblasts representing 16 genotypes. Overall, the patient cell lines exhibited variable, complex, biochemical profiles and pharmacological responses. HADHA-deficient fibroblasts showed markedly reduced alpha subunit protein levels together with decreased beta-subunit abundance, exhibited a -86 to -96% defect in LCHAD activity, and produced large amounts of C14 and C16 hydroxyacylcarnitines. In control fibroblasts, exposure to bezafibrate (400 μM for 48 h) increased the abundance of HADHA and HADHB mRNAs, immune-detectable alpha and beta subunit proteins, activities of LCHAD and LCKAT, and stimulated FAO capacities, clearly indicating that MTP is pharmacologically up-regulated by bezafibrate in human fibroblasts. In MTP-deficient patient fibroblasts, which were found markedly FAO-deficient, bezafibrate improved FAO capacities in six of 26 (23%) cases, including three cell lines heterozygous for the common c1528G > C mutation. Altogether, our results strongly suggest that, due to variable effects of HADHA and HADHB mutations on MTP abundance and residual activity, improvement of MTP deficiency in response to bezafibrate was achieved in a subset of responsive genotypes.

Fetal left ventricular noncompaction cardiomyopathy and fatal outcome due to complete deficiency of mitochondrial trifunctional protein

We report a fetal case with fatal outcome having a novel mutation in the HADHB gene, coding the beta-subunit of the mitochondrial trifunctional protein. Parents had a previous pregnancy loss due to fetal heart failure and hydrops. The next pregnancy led to left ventricular noncompaction and increasing pleural effusions after 29 gestational weeks. The fetus was small for gestational age, and long bones were abnormally short. The baby was born severely asphyxiated at 32 gestational weeks by cesarean section. Intensive care was withdrawn due to failure to thrive and suspicion of a severe mitochondrial disorder. Postmortem brain MRI suggested microcephaly with a simplified gyral pattern. The lateral cerebral ventricles were normal. Chromosome analysis was normal (46, XX). Fibroblasts cultured from a skin biopsy of the baby revealed the large homozygous deletion c.1109+243_1438-703del in the HADHB gene, and heterozygous mutations were detected in both parents. The deletion has not been reported earlier.

CONCLUSION

It is important to differentiate systemic metabolic diseases from disorders that affect only the cardiac muscle. Trifunctional protein deficiency is a relatively rare disorder of the fatty acid β-oxidation cycle. The mutation in the HADHB gene causes a systemic disease with early-onset cardiomyopathy. Understanding the molecular genetic defect of the patient allows appropriate genetic counseling of the family.

WHAT IS KNOWN

• Mitochondrial disorders as a group are an important etiology for fetal cardiomyopathies including human trifunctional protein (TFP) disorders and several other mitochondrial diseases.

WHAT IS NEW

• We report a fetal case with fatal outcome having a novel mitochondrial trifunctional protein mutation (c.1109+243_1438-703del in the HADHB gene).

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.

ECHS1 mutations cause combined respiratory chain deficiency resulting in Leigh syndrome

The human ECHS1 gene encodes the short-chain enoyl coenzyme A hydratase, the enzyme that catalyzes the second step of β-oxidation of fatty acids in the mitochondrial matrix. We report on a boy with ECHS1 deficiency who was diagnosed with Leigh syndrome at 21 months of age. The patient presented with hypotonia, metabolic acidosis, and developmental delay. A combined respiratory chain deficiency was also observed. Targeted exome sequencing of 776 mitochondria-associated genes encoded by nuclear DNA identified compound heterozygous mutations in ECHS1. ECHS1 protein expression was severely depleted in the patient's skeletal muscle and patient-derived myoblasts; a marked decrease in enzyme activity was also evident in patient-derived myoblasts. Immortalized patient-derived myoblasts that expressed exogenous wild-type ECHS1 exhibited the recovery of the ECHS1 activity, indicating that the gene defect was pathogenic. Mitochondrial respiratory complex activity was also mostly restored in these cells, suggesting that there was an unidentified link between deficiency of ECHS1 and respiratory chain. Here, we describe the patient with ECHS1 deficiency; these findings will advance our understanding not only the pathology of mitochondrial fatty acid β-oxidation disorders, but also the regulation of mitochondrial metabolism.

Uncoupling, metabolic inhibition and induction of mitochondrial permeability transition in rat liver mitochondria caused by the major long-chain hydroxyl monocarboxylic fatty acids accumulating in LCHAD deficiency

Patients with long-chain 3-hydroxy-acyl-CoA dehydrogenase (LCHAD) deficiency commonly present liver dysfunction whose pathogenesis is unknown. We studied the effects of long-chain 3-hydroxylated fatty acids (LCHFA) that accumulate in LCHAD deficiency on liver bioenergetics using mitochondrial preparations from young rats. We provide strong evidence that 3-hydroxytetradecanoic (3HTA) and 3-hydroxypalmitic (3HPA) acids, the monocarboxylic acids that are found at the highest tissue concentrations in this disorder, act as metabolic inhibitors and uncouplers of oxidative phosphorylation. These conclusions are based on the findings that these fatty acids decreased ADP-stimulated (state 3) and uncoupled respiration, mitochondrial membrane potential and NAD(P)H content, and, in contrast, increased resting (state 4) respiration. We also verified that 3HTA and 3HPA markedly reduced Ca2+ retention capacity and induced swelling in Ca2+-loaded mitochondria. These effects were mediated by mitochondrial permeability transition (MPT) induction since they were totally prevented by the classical MPT inhibitors cyclosporin A and ADP, as well as by ruthenium red, a Ca2+ uptake blocker. Taken together, our data demonstrate that the major monocarboxylic LCHFA accumulating in LCHAD deficiency disrupt energy mitochondrial homeostasis in the liver. It is proposed that this pathomechanism may explain at least in part the hepatic alterations characteristic of the affected patients.

The clinical spectrum of inherited diseases involved in the synthesis and remodeling of complex lipids. A tentative overview

Over one hundred diseases related to inherited defects of complex lipids synthesis and remodeling are now reported. Most of them were described within the last 5 years. New descriptions and phenotypes are expanding rapidly. While the associated clinical phenotype is currently difficult to outline, with only a few patients identified, it appears that all organs and systems may be affected. The main clinical presentations can be divided into (1) Diseases affecting the central and peripheral nervous system. Complex lipid synthesis disorders produce prominent motor manifestations due to upper and/or lower motoneuron degeneration. Motor signs are often complex, associated with other neurological and extra-neurological signs. Three neurological phenotypes, spastic paraparesis, neurodegeneration with brain iron accumulation and peripheral neuropathies, deserve special attention. Many apparently well clinically defined syndromes are not distinct entities, but rather clusters on a continuous spectrum, like for the PNPLA6-associated diseases, extending from Boucher-Neuhauser syndrome via Gordon Holmes syndrome to spastic ataxia and pure hereditary spastic paraplegia; (2) Muscular/cardiac presentations; (3) Skin symptoms mostly represented by syndromic (neurocutaneous) and non syndromic ichthyosis; (4) Retinal dystrophies with syndromic and non syndromic retinitis pigmentosa, Leber congenital amaurosis, cone rod dystrophy, Stargardt disease; (5) Congenital bone dysplasia and segmental overgrowth disorders with congenital lipomatosis; (6) Liver presentations characterized mainly by transient neonatal cholestatic jaundice and non alcoholic liver steatosis with hypertriglyceridemia; and (7) Renal and immune presentations. Lipidomics and molecular functional studies could help to elucidate the mechanism(s) of dominant versus recessive inheritance observed for the same gene in a growing number of these disorders.

Mitochondrial bioenergetics deregulation caused by long-chain 3-hydroxy fatty acids accumulating in LCHAD and MTP deficiencies in rat brain: a possible role of mPTP opening as a pathomechanism in these disorders?

Long-chain 3-hydroxylated fatty acids (LCHFA) accumulate in long-chain 3-hydroxy-acyl-CoA dehydrogenase (LCHAD) and mitochondrial trifunctional protein (MTP) deficiencies. Affected patients usually present severe neonatal symptoms involving cardiac and hepatic functions, although long-term neurological abnormalities are also commonly observed. Since the underlying mechanisms of brain damage are practically unknown and have not been properly investigated, we studied the effects of LCHFA on important parameters of mitochondrial homeostasis in isolated mitochondria from cerebral cortex of developing rats. 3-Hydroxytetradecanoic acid (3 HTA) reduced mitochondrial membrane potential, NAD(P)H levels, Ca(2+) retention capacity and ATP content, besides inducing swelling, cytochrome c release and H2O2 production in Ca(2+)-loaded mitochondrial preparations. We also found that cyclosporine A plus ADP, as well as ruthenium red, a Ca(2+) uptake blocker, prevented these effects, suggesting the involvement of the mitochondrial permeability transition pore (mPTP) and an important role for Ca(2+), respectively. 3-Hydroxydodecanoic and 3-hydroxypalmitic acids, that also accumulate in LCHAD and MTP deficiencies, similarly induced mitochondrial swelling and decreased ATP content, but to a variable degree pending on the size of their carbon chain. It is proposed that mPTP opening induced by LCHFA disrupts brain bioenergetics and may contribute at least partly to explain the neurologic dysfunction observed in patients affected by LCHAD and MTP deficiencies.

Pathophysiology of fatty acid oxidation disorders and resultant phenotypic variability

Fatty acids are a major fuel for the body and fatty acid oxidation is particularly important during fasting, sustained aerobic exercise and stress. The myocardium and resting skeletal muscle utilise long-chain fatty acids as a major source of energy. Inherited disorders affecting fatty acid oxidation seriously compromise the function of muscle and other highly energy-dependent tissues such as brain, nerve, heart, kidney and liver. Such defects encompass a wide spectrum of clinical disease, presenting in the neonatal period or infancy with recurrent hypoketotic hypoglycaemic encephalopathy, liver dysfunction, hyperammonaemia and often cardiac dysfunction. In older children, adolescence or adults there is often exercise intolerance with episodic myalgia or rhabdomyolysis in association with prolonged aerobic exercise or other exacerbating factors. Some disorders are particularly associated with toxic metabolites that may contribute to encephalopathy, polyneuropathy, axonopathy and pigmentary retinopathy. The phenotypic diversity encountered in defects of fat oxidation is partly explained by genotype/phenotype correlation and certain identifiable environmental factors but there remain many unresolved questions regarding the complex interaction of genetic, epigenetic and environmental influences that dictate phenotypic expression. It is becoming increasingly clear that the view that most inherited disorders are purely monogenic diseases is a naive concept. In the future our approach to understanding the phenotypic diversity and management of patients will be more realistically achieved from a polygenic perspective.

Observations regarding retinopathy in mitochondrial trifunctional protein deficiencies

Although the retina is thought to primarily rely on glucose for fuel, inherited deficiency of one or more activities of mitochondrial trifunctional protein results in a pigmentary retinopathy leading to vision loss. Many other enzymatic deficiencies in fatty acid oxidation pathways have been described, none of which results in retinal complications. The etiology of retinopathy among patients with defects in trifunctional protein is unknown. Trifunctional protein is a heteroctomer; two genes encode the alpha and beta subunits of TFP respectively, HADHA and HADHB. A common mutation in HADHA, c.1528G>C, leads to a single amino acid substitution, p. Glu474Gln, and impairs primarily long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) activity leading to LCHAD deficiency (LCHADD). Other mutations in HADHA or HADHB often lead to significant reduction in all three enzymatic activities and result in trifunctional protein deficiency (TFPD). Despite many similarities in clinical presentation and phenotype, there is growing evidence that they can result in different chronic complications. This review will outline the clinical similarities and differences between LCHADD and TFPD, describe the course of the associated retinopathy, propose a genotype/phenotype correlation with the severity of retinopathy, and discuss the current theories about the etiology of the retinopathy.

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.

A patient with mitochondrial trifunctional protein deficiency due to the mutations in the HADHB gene showed recurrent myalgia since early childhood and was diagnosed in adolescence

Mitochondrial trifunctional protein (MTP) is a multienzyme complex involved in the metabolism of long-chain hydroxyacyl-CoA, a product of the fatty acid β-oxidation cycle. MTP is an α4β4 hetero-octomer encoded by two different genes: HADHA (OMIM 600890) and HADHB (OMIM 143450). MTP deficiency induces three different types of presentation: (1) a lethal phenotype with neonatal onset (severe); (2) a hepatic phenotype with infant onset (intermediate); and (3) a neuromyopathic phenotype with late-adolescent onset (mild). While acylcarnitine analysis has revealed increased levels of long-chain hydroxyacylcarnitine in blood when an MTP deficiency exists, the neuromyopathic type is usually asymptomatic and does not always result in an abnormality in acylcarnitine analysis results. We report here the case of a 13-year-old girl with recurrences of intermittent myalgia since her early childhood, for whom the disorder had not been definitely diagnosed. Since she was referred to our hospital because of rhabdomyolysis, we have repeatedly performed blood acylcarnitine analysis and found slight increases in long-chain 3-OH-acylcarnitine levels, on the basis of which we made a chemical diagnosis of MTP deficiency. Immunoblot analysis of skin fibroblasts revealed loss of α- and β-subunits of MTP. In addition, analysis of the HADHB gene, which encodes long-chain 3-ketoacyl-CoA thiolase, one of the enzymes constituting MTP, identified compound heterozygous mutations of c.520C>T (p.R141C) and c.1331G>A (p.R411K). MTP deficiency is considered an extremely rare disorder, as only five cases (lethal phenotype, two patients; hepatic phenotype, two patients; and neuromyopathic phenotype, one patient) have thus far been reported in Japan. However, it is likely that the neuromyopathic phenotype of MTP deficiency has not yet been diagnosed among patients with recurrences of intermittent myalgia and rhabdomyolysis, as in our patient reported here.

Comprehensive cDNA study and quantitative analysis of mutant HADHA and HADHB transcripts in a French cohort of 52 patients with mitochondrial trifunctional protein deficiency

BACKGROUND

Deficiency of mitochondrial trifunctional protein (MTP) is caused by mutations in the HADHA and HADHB genes, which have been mostly delineated at the genomic DNA level and have not been always elucidated.

AIM

To identify mutations in a French cohort of 52 MTP deficient patients and the susceptibility of mutations generating premature termination codons (PTCs) to the nonsense mRNA mediated decay (NMD).

METHODS

Mutation screening in fibroblasts was performed at the cDNA level and real-time RT-PCR was used to compare the levels of the different PTC-bearing mRNAs before and after a treatment of fibroblasts by emetine, a translation inhibitor.

RESULTS

A mutation detection rate of 100% was achieved. A total of 22 novel mutations were identified, including a large-sized genomic deletion in HADHB gene. A high proportion of all identified mutations were non-sense, frameshift and splicing mutations, generating (PTCs), distributed essentially on HADHA coding regions. We could demonstrate that the majority of mutations resulting in PTCs conform to the established rules governing the susceptibility to NMD.

CONCLUSION

Our results emphasize the value of cDNA analysis in the characterization of HADHA and HADHB mutations and further strengthen the model of haploinsufficiency as a major pathomechanism in MTP defects.

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.

Mitochondrial fatty acid oxidation disorders: pathophysiological studies in mouse models

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

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.

Current issues regarding treatment of mitochondrial fatty acid oxidation disorders

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