Barth syndrome without tetralinoleoyl cardiolipin deficiency: a possible ameliorated phenotype

Kaempferol Improves Cardiolipin and ATP in Hepatic Cells: A Cellular Model Perspective in the Context of Metabolic Dysfunction-Associated Steatotic Liver Disease

Targeting mitochondrial function is a promising approach to prevent metabolic dysfunction-associated steatotic liver disease (MASLD). Cardiolipin (CL) is a unique lipid comprising four fatty acyl chains localized in the mitochondrial inner membrane. CL is a crucial phospholipid in mitochondrial function, and MASLD exhibits CL-related anomalies. Kaempferol (KMP), a natural flavonoid, has hepatoprotective and mitochondrial function-improving effects; however, its influence on CL metabolism in fatty liver conditions is unknown. In this study, we investigated the effects of KMP on mitochondrial function, focusing on CL metabolism in a fatty liver cell model (linoleic-acid-loaded C3A cell). KMP promoted mitochondrial respiratory functions such as ATP production, basal respiration, and proton leak. KMP also increased the gene expression levels of CPT1A and PPARGC1A, which are involved in mitochondrial β-oxidation. Comprehensive quantification of CL species and related molecules via liquid chromatography/mass spectrometry showed that KMP increased not only total CL content but also CL72:8, which strongly favors ATP production. Furthermore, KMP improved the monolysocardiolipin (MLCL)/CL ratio, an indicator of mitochondrial function. Our results suggest that KMP promotes energy production in a fatty liver cell model, associated with improvement in mitochondrial CL profile, and can serve as a potential nutrition factor in preventing MASLD.

Advances in methods to analyse cardiolipin and their clinical applications

Cardiolipin (CL) is a mitochondria-exclusive phospholipid, primarily localised within the inner mitochondrial membrane, that plays an essential role in mitochondrial architecture and function. Aberrant CL content, structure, and localisation have all been linked to impaired mitochondrial activity and are observed in the pathophysiology of cancer and neurological, cardiovascular, and metabolic disorders. The detection, quantification, and localisation of CL species is a valuable tool to investigate mitochondrial dysfunction and the pathophysiological mechanisms underpinning several human disorders. CL is measured using liquid chromatography, usually combined with mass spectrometry, mass spectrometry imaging, shotgun lipidomics, ion mobility spectrometry, fluorometry, and radiolabelling. This review summarises available methods to analyse CL, with a particular focus on modern mass spectrometry, and evaluates their advantages and limitations. We provide guidance aimed at selecting the most appropriate technique, or combination of techniques, when analysing CL in different model systems, and highlight the clinical contexts in which measuring CL is relevant.

Cardiolipin Regulates Mitochondrial Ultrastructure and Function in Mammalian Cells

Cardiolipin (CL) is a unique, tetra-acylated diphosphatidylglycerol lipid that mainly localizes in the inner mitochondria membrane (IMM) in mammalian cells and plays a central role in regulating mitochondrial architecture and functioning. A deficiency of CL biosynthesis and remodeling perturbs mitochondrial functioning and ultrastructure. Clinical and experimental studies on human patients and animal models have also provided compelling evidence that an abnormal CL content, acyl chain composition, localization, and level of oxidation may be directly linked to multiple diseases, including cardiomyopathy, neuronal dysfunction, immune cell defects, and metabolic disorders. The central role of CL in regulating the pathogenesis and progression of these diseases has attracted increasing attention in recent years. In this review, we focus on the advances in our understanding of the physiological roles of CL biosynthesis and remodeling from human patients and mouse models, and we provide an overview of the potential mechanism by which CL regulates the mitochondrial architecture and functioning.

Matrix produced by diseased cardiac fibroblasts affects early myotube formation and function

The extracellular matrix (ECM) provides both physical and chemical cues that dictate cell function and contribute to muscle maintenance. Muscle cells require efficient mitochondria to satisfy their high energy demand, however, the role the ECM plays in moderating mitochondrial function is not clear. We hypothesized that the ECM produced by stromal cells with mitochondrial dysfunction (Barth syndrome, BTHS) provides cues that contribute to metabolic dysfunction independent of muscle cell health. To test this, we harnessed the ECM production capabilities of human pluripotent stem-cell-derived cardiac fibroblasts (hPSC-CFs) from healthy and BTHS patients to fabricate cell-derived matrices (CDMs) with controlled topography, though we found that matrix composition from healthy versus diseased cells influenced myotube formation independent of alignment cues. To further investigate the effects of matrix composition, we then examined the influence of healthy- and BTHS-derived CDMs on myotube formation and metabolic function. We found that BTHS CDMs induced lower fusion index, lower ATP production, lower mitochondrial membrane potential, and higher ROS generation than the healthy CDMs. These findings imply that BTHS-derived ECM alone contributes to myocyte dysfunction in otherwise healthy cells. Finally, to investigate potential mechanisms, we defined the composition of CDMs produced by hPSC-CFs from healthy and BTHS patients using mass spectrometry and identified 15 ECM and related proteins that were differentially expressed in the BTHS-CDM compared to healthy CDM. Our results highlight that ECM composition affects skeletal muscle formation and metabolic efficiency in otherwise healthy cells, and our methods to generate patient-specific CDMs are a useful tool to investigate the influence of the ECM on disease progression and to investigate variability among diseased patients. STATEMENT OF SIGNIFICANCE: Muscle function requires both efficient metabolism to generate force and structured extracellular matrix (ECM) to transmit force, and we sought to examine the interactions between metabolism and ECM when metabolic disease is present. We fabricated patient-specific cell derived matrices (CDMs) with controlled topographic features to replicate the composition of healthy and mitochondrial-diseased (Barth syndrome) ECM. We found that disease-derived ECM negatively affects metabolic function of otherwise healthy myoblasts, and we identified several proteins in disease-derived ECM that may be mediating this dysfunction. We anticipate that our patient-specific CDM system could be fabricated with other topographies and cell types to study cell functions and diseases of interest beyond mitochondrial dysfunction and, eventually, be applied toward personalized medicine.

Longitudinal Observational Study of Cardiac Outcome Risk Factor Prediction in Children, Adolescents, and Adults with Barth Syndrome

Barth Syndrome (BTHS) is an X-linked mitochondrial cardioskeletal myopathy caused by defects in TAFAZZIN, a gene responsible for cardiolipin remodeling. Altered mitochondrial levels of cardiolipin lead to cardiomyopathy (CM), muscle weakness, exercise intolerance, and mortality. Cardiac risk factors predicting outcome are unknown. Therefore, we conducted a longitudinal observational study to determine risk factors for outcome in BTHS. Subjects with minimum two evaluations (or one followed by death or transplant) were included. Cardiac size, function, and QTc data were measured by echocardiography and electrocardiography at 7 time points from 2002 to 2018. Analysis included baseline, continuous, and categorical variables. Categorical risk factors included prolonged QTc, abnormal right ventricle fractional area change (RV FAC), left ventricle (LV) or RV non-compaction, and restrictive CM phenotype. The association between variables and cardiac death or transplant (CD/TX) was assessed. Median enrollment age was 7 years (range 0.5-22; n = 44). Transplant-free survival (TFS) was 74.4% at 15 years from first evaluation. The cohort demonstrated longitudinal declines in LV size and stroke volume z-scores (end-diastolic volume, p = 0.0002; stroke volume p < 0.0001), worsening RV FAC (p = 0.0405), and global longitudinal strain (GLS) (p = 0.0001) with stable ejection (EF) and shortening (FS) fraction. CD/TX subjects (n = 9) displayed worsening LV dilation (p = 0.0066), EF (p ≤ 0.0001), FS (p = 0.0028), and RV FAC (p = .0032) versus stability in TFS. Having ≥ 2 categorical risk factors predicted CD/TX (p = 0.0073). Over 15 years, 25% of BTHS subjects progressed to CD/TX. Those with progressive LV enlargement, dysfunction, and multiple cardiac risk factors warrant increased surveillance and intense therapy.

Clinical presentation and natural history of Barth Syndrome: An overview

Barth Syndrome is a rare X-linked disorder caused by pathogenic variants in the gene TAFAZZIN, which encodes for an enzyme involved in the remodeling of cardiolipin, a phospholipid primarily localized to the inner mitochondrial membrane. Barth Syndrome is characterized by cardiomyopathy, skeletal myopathy, neutropenia, and growth abnormalities, among other features. In this review, we will discuss the clinical presentation and natural history of Barth Syndrome, review key features of this disease, and introduce less common clinical associations. Recognition and understanding of the natural history of Barth Syndrome are important for ongoing patient management and developing endpoints for the demonstration of efficacy of new and emerging therapies.

An improved functional assay in blood spot to diagnose Barth syndrome using the monolysocardiolipin/cardiolipin ratio

Barth syndrome is an X-linked disorder characterized by cardiomyopathy, skeletal myopathy, and neutropenia, caused by deleterious variants in TAFAZZIN. This gene encodes a phospholipid-lysophospholipid transacylase that is required for the remodeling of the mitochondrial phospholipid cardiolipin (CL). Biochemically, individuals with Barth syndrome have a deficiency of mature CL and accumulation of the remodeling intermediate monolysocardiolipin (MLCL). Diagnosis typically relies on mass spectrometric measurement of CL and MLCL in cells or tissues, and we previously described a method in blood spot that uses a specific MLCL/CL ratio as diagnostic biomarker. Here, we describe the evolution of our blood spot assay that is based on the implementation of reversed phase-UHPLC separation followed by full scan high resolution mass spectrometry. In addition to the MLCL/CL ratio, our improved method also generates a complete CL spectrum allowing the interrogation of the CL fatty acid composition, which considerably enhances the diagnostic reliability. This addition negates the need for a confirmatory test in lymphocytes thereby providing a shorter turn-around-time while achieving a more certain test result. As one of the few laboratories that offer this assay, we also evaluated the diagnostic yield and performance from 2006 to 2021 encompassing the use of both the original and improved assay. In this period, we performed 796 diagnostic analyses of which 117 (15%) were characteristic of Barth syndrome. In total, we diagnosed 93 unique individuals with Barth syndrome, including three females, which together amounts to about 40% of all reported individuals with Barth syndrome in the world.

Cardiolipin, Mitochondria, and Neurological Disease

Over the past decade, it has become clear that lipid homeostasis is central to cellular metabolism. Lipids are particularly abundant in the central nervous system (CNS) where they modulate membrane fluidity, electric signal transduction, and synaptic stabilization. Abnormal lipid profiles reported in Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and traumatic brain injury (TBI), are further support for the importance of lipid metablism in the nervous system. Cardiolipin (CL), a mitochondria-exclusive phospholipid, has recently emerged as a focus of neurodegenerative disease research. Aberrant CL content, structure, and localization are linked to impaired neurogenesis and neuronal dysfunction, contributing to aging and the pathogenesis of several neurodegenerative diseases, such as AD and PD. Furthermore, the highly tissue-specific acyl chain composition of CL confers it significant potential as a biomarker to diagnose and monitor the progression in several neurological diseases. CL also represents a potential target for pharmacological strategies aimed at treating neurodegeneration. Given the equipoise that currently exists between CL metabolism, mitochondrial function, and neurological disease, we review the role of CL in nervous system physiology and monogenic and neurodegenerative disease pathophysiology, in addition to its potential application as a biomarker and pharmacological target.

Monolysocardiolipin (MLCL) interactions with mitochondrial membrane proteins

Monolysocardiolipin (MLCL) is a three-tailed variant of cardiolipin (CL), the signature lipid of mitochondria. MLCL is not normally found in healthy tissue but accumulates in mitochondria of people with Barth syndrome (BTHS), with an overall increase in the MLCL:CL ratio. The reason for MLCL accumulation remains to be fully understood. The effect of MLCL build-up and decreased CL content in causing the characteristics of BTHS are also unclear. In both cases, an understanding of the nature of MLCL interaction with mitochondrial proteins will be key. Recent work has shown that MLCL associates less tightly than CL with proteins in the mitochondrial inner membrane, suggesting that MLCL accumulation is a result of CL degradation, and that the lack of MLCL–protein interactions compromises the stability of the protein-dense mitochondrial inner membrane, leading to a decrease in optimal respiration. There is some data on MLCL–protein interactions for proteins involved in the respiratory chain and in apoptosis, but there remains much to be understood regarding the nature of MLCL–protein interactions. Recent developments in structural, analytical and computational approaches mean that these investigations are now possible. Such an understanding will be key to further insights into how MLCL accumulation impacts mitochondrial membranes. In turn, these insights will help to support the development of therapies for people with BTHS and give a broader understanding of other diseases involving defective CL content.

Mitochondrial dysfunction in metabolic and cardiovascular diseases associated with cardiolipin remodeling

Cardiolipin (CL) is an acidic phospholipid almost exclusively found in the inner mitochondrial membrane, that not only stabilizes the structure and function of individual components of the mitochondrial electron transport chain, but regulates relevant mitochondrial processes, like mitochondrial dynamics and cristae structure maintenance among others. Alterations in CL due to peroxidation, correlates with loss of such mitochondrial activities and disease progression. In this review it is recapitulated the current state of knowledge of the role of cardiolipin remodeling associated with mitochondrial dysfunction in metabolic and cardiovascular diseases.

Neutropenia in Barth syndrome: characteristics, risks, and management

PURPOSE OF REVIEW

Barth syndrome (BTHS) is an X-linked disease characterized by defective remodeling of phospholipid side chains in mitochondrial membranes. Major features include neutropenia, dilated cardiomyopathy, motor delay and proximal myopathy, feeding problems, and constitutional growth delay. We conducted this review of neutropenia in BTHS to aid in the diagnosis of this disease, and to improve understanding of both the consequences of neutropenia and the benefits of treatment with granulocyte colony-stimulating factor (G-CSF).

RECENT FINDINGS

In 88 patients with BTHS, neutropenia, that is, at least one count below 1.5 × 10/l, was detected in 74 (84%) and 44% had severe chronic neutropenia, with multiple counts below 0.5 × 10/l. The pattern of neutropenia varied between intermittent and unpredictable, chronic and severe, or cyclical with mathematically regular oscillations. Monocytosis, that is, monocytes more than 1.0 × 10/l, was observed at least once in 64 of 85 (75%) patients. G-CSF was administered to 39 of 88 patients (44%). Weekly average G-CSF doses ranged from 0.12 to 10.92 μg/kg/day (mean 1.16 μg/kg/day, median 1.16 μg/kg/day). Antibiotic prophylaxis was additionally employed in 21 of 26 neutropenic patients. Pretreatment bone marrow evaluations predominantly showed reduced myeloid maturation which normalized on G-CSF therapy in seven of 13 examined. Consistent clinical improvement, with reduced signs and symptoms of infections, was observed in response to prophylactic G-CSF ± prophylactic antibiotics. However, despite G-CSF and antibiotics, one adult patient died with multiple infections related to indwelling medical devices and gastrostomy site infection after 15.5 years on G-CSF and a pediatric patient required gastrostomy removal for recurrent abdominal wall cellulitis.

SUMMARY

BTHS should be considered in any men with neutropenia accompanied by any of the characteristic features of this syndrome. Prophylaxis with G-CSF ± antibiotics prevents serious bacterial infections in the more severe neutropenic patients although infections remain a threat even in patients who are very compliant with therapy, especially in those with indwelling devices.

Understanding the life experience of Barth syndrome from the perspective of adults: a qualitative one-on-one interview study

BACKGROUND

Barth syndrome (BTHS, OMIM 302060) is a rare, life-threatening, x-linked genetic disorder that occurs almost exclusively in males and is characterized by cardiomyopathy, neutropenia, skeletal muscle myopathy primarily affecting larger muscles, and shorter stature in youth. A greater number of individuals with BTHS are now surviving into adulthood due to advancements in diagnosis and disease management. Given these improvements in life expectancy, understanding the disease experience over time has become increasingly important to individuals with the condition, treatment developers, and regulatory agencies. A study was conducted to explore the experience of BTHS from the perspective of adult males at least 35 years of age with the condition via in-depth qualitative interviews.

RESULTS

Findings showed that adults with BTHS experienced a variety of signs/symptoms with variable onset and severity throughout their lives, the most frequently reported being the symptoms of tiredness, muscle weakness, and a fast and/or irregular heart rate, and the sign of short stature in youth. These signs/symptoms negatively impacted individuals' emotional, physical, social, and role functioning. Tiredness and weakness impacted some individuals' physical functioning from an early age and into adulthood. These symptoms generally worsened over time, increasingly interfering with individuals' ability to fully participate in paid and unpaid labor and to partake in family and leisure activities.

CONCLUSIONS

This research complements recent studies characterizing the potentially degenerative and progressive nature of BTHS and can encourage future research into the natural history and progression of BTHS in untreated individuals. Participants' interview responses revealed a range of symptoms and the potential for multiple impacts on individuals' physical, social, emotional, and role functioning as a result of BTHS symptoms, yet also revealed variability in severity of experience as well as the possibility of resilience and adaptation to the condition.

TAZ encodes tafazzin, a transacylase essential for cardiolipin formation and central to the etiology of Barth syndrome

Tafazzin, which is encoded by the TAZ gene, catalyzes transacylation to form mature cardiolipin and shows preference for the transfer of a linoleic acid (LA) group from phosphatidylcholine (PC) to monolysocardiolipin (MLCL) with influence from mitochondrial membrane curvature. The protein contains domains and motifs involved in targeting, anchoring, and an active site for transacylase activity. Tafazzin activity affects many aspects of mitochondrial structure and function, including that of the electron transport chain, fission-fusion, as well as apoptotic signaling. TAZ mutations are implicated in Barth syndrome, an underdiagnosed and devastating disease that primarily affects male pediatric patients with a broad spectrum of disease pathologies that impact the cardiovascular, neuromuscular, metabolic, and hematologic systems.

A critical appraisal of the tafazzin knockdown mouse model of Barth syndrome: what have we learned about pathogenesis and potential treatments?

Pediatric heart failure remains poorly understood, distinct in many aspects from adult heart failure. Limited data point to roles of altered mitochondrial functioning and, in particular, changes in mitochondrial lipids, especially cardiolipin. Barth syndrome is a mitochondrial disorder caused by tafazzin mutations that lead to abnormal cardiolipin profiles. Patients are afflicted by cardiomyopathy, skeletal myopathy, neutropenia, and growth delay. A mouse model of Barth syndrome was developed a decade ago, which relies on a doxycycline-inducible short hairpin RNA to knock down expression of tafazzin mRNA (TAZKD). Our objective was to review published data from the TAZKD mouse to determine its contributions to our pathogenetic understanding of, and potential treatment strategies for, Barth syndrome. In regard to the clinical syndrome, the reported physiological, biochemical, and ultrastructural abnormalities of the mouse model mirror those in Barth patients. Using this model, the peroxisome proliferator-activated receptor pan-agonist bezafibrate has been suggested as potential therapy because it ameliorated the cardiomyopathy in TAZKD mice, while increasing mitochondrial biogenesis. A clinical trial is now underway to test bezafibrate in Barth syndrome patients. Thus the TAZKD mouse model of Barth syndrome has led to important insights into disease pathogenesis and therapeutic targets, which can potentially translate to pediatric heart failure.

Increased mtDNA Abundance and Improved Function in Human Barth Syndrome Patient Fibroblasts Following AAV-TAZ Gene Delivery

Barth syndrome (BTHS) is a rare, X-linked, mitochondrial disorder caused by mutations in the gene encoding tafazzin. BTHS results in cardiomyopathy, muscle fatigue, and neutropenia in patients. Tafazzin is responsible for remodeling cardiolipin, a key structural lipid of the inner mitochondrial membrane. As symptoms can vary in severity amongst BTHS patients, we sought to compare mtDNA copy numbers, mitochondrial fragmentation, and functional parameters between primary dermal BTHS fibroblasts isolated from patients with two different mutations in the TAZ locus. To confirm cause‒effect relationships and further support the development of gene therapy for BTHS, we also characterized the BTHS cells following adeno-associated virus (AAV)-TAZ transduction. Our data show that, in response to AAV-TAZ transduction, these remarkably dynamic organelles show recovery of mtDNA copy numbers, mitochondrial structure, and mitochondrial function, providing additional evidence to support the therapeutic potential of AAV-mediated gene delivery for BTHS. This study also demonstrates the direct relationship between healthy mitochondrial membrane structure and maintenance of proper levels of mtDNA copy numbers.

Barth syndrome: mechanisms and management

Objectives: Barth syndrome is an ultra-rare, infantile-onset, X-linked recessive mitochondrial disorder, primarily affecting males, due to variants in TAZ encoding for the cardiolipin transacylase tafazzin. This review aimed to summarize and discuss recent and earlier findings concerning the etiology, pathogenesis, clinical presentation, diagnosis, treatment, and outcome of Barth syndrome.

Method: A literature review was undertaken through a MEDLINE search.

Results: The phenotype of Barth syndrome is highly variable but most frequently patients present with hypertrophic/dilated/non-compaction cardiomyopathy, fibroelastosis, arrhythmias, neutropenia, mitochondrial myopathy, growth retardation, dysmorphism, cognitive impairment, and other, rarer features. Lactic acid and creatine kinase, and blood and urine organic acids, particularly 3-methylglutaconic acid and monolysocardiolipin, are often elevated. Cardiolipin is decreased. Biochemical investigations may show decreased activity of various respiratory chain complexes. The diagnosis is confirmed by documentation of a causative TAZ variant. Treatment is symptomatic and directed toward treating heart failure, arrhythmias, neutropenia, and mitochondrial myopathy.

Conclusions: Although Barth syndrome is still an orphan disease, with fewer than 200 cases described so far, there is extensive ongoing research with regard to its pathomechanism and new therapeutic approaches. Although most of these approaches are still experimental, it can be expected that causative strategies will be developed in the near future.

Mitochondrial dysfunctions in barth syndrome

Barth syndrome (BTHS) is a rare multisystemic genetic disorder caused by mutations in the TAZ gene. TAZ encodes a mitochondrial enzyme that remodels the acyl chain composition of newly synthesized cardiolipin, a phospholipid unique to mitochondrial membranes. The clinical abnormalities observed in BTHS patients are caused by perturbations in various mitochondrial functions that rely on remodeled cardiolipin. However, the contribution of different cardiolipin-dependent mitochondrial functions to the pathology of BTHS is not fully understood. In this review, we will discuss recent findings from different genetic models of BTHS, including the yeast model of cardiolipin deficiency that has uncovered the specific in vivo roles of cardiolipin in mitochondrial respiratory chain biogenesis, bioenergetics, intermediary metabolism, mitochondrial dynamics, and quality control. We will also describe findings from higher eukaryotic models of BTHS that highlight a link between cardiolipin-dependent mitochondrial function and its impact on tissue and organ function. © 2019 IUBMB Life, 9999(9999):1-11, 2019.

Chlorella diet alters mitochondrial cardiolipin contents differentially in organs of Danio rerio analyzed by a lipidomics approach

The zebrafish (Danio rerio) is an important and widely used vertebrate model organism for the study of human diseases which include disorders caused by dysfunctional mitochondria. Mitochondria play an essential role in both energy metabolism and apoptosis, which are mediated through a mitochondrial phospholipid cardiolipin (CL). In order to examine the cardiolipin profile in the zebrafish model, we developed a CL analysis platform by using liquid chromatography-mass spectrometry (LC-MS). Meanwhile, we tested whether chlorella diet would alter the CL profile in the larval fish, and in various organs of the adult fish. The results showed that chlorella diet increased the chain length of CL in larval fish. In the adult zebrafish, the distribution patterns of CL species were similar between the adult brain and eye tissues, and between the heart and muscles. Interestingly, monolyso-cardiolipin (MLCL) was not detected in brain and eyes but found in other examined tissues, indicating a different remodeling mechanism to maintain the CL integrity. While the adult zebrafish were fed with chlorella for four weeks, the CL distribution showed an increase of the species of saturated acyl chains in the brain and eyes, but a decrease in the other organs. Moreover, chlorella diet led to a decrease of MLCL percentage in organs except the non-MLCL-containing brain and eyes. The CL analysis in the zebrafish provides an important tool for studying the mechanism of mitochondria diseases, and may also be useful for testing medical regimens targeting against the Barth Syndrome.

A novel mutation in TAZ causes mitochondrial respiratory chain disorder without cardiomyopathy

Tafazzin, encoded by the TAZ gene, is a mitochondrial membrane-associated protein that remodels cardiolipin (CL), an important mitochondrial phospholipid. TAZ mutations are associated with Barth syndrome (BTHS). BTHS is an X-linked multisystemic disorder affecting usually male patients. Through sequence analysis of TAZ, we found one novel mutation c.39_60del p.(Pro14Alafs*19) by whole-exome sequencing and a reported missense mutation c.280C>T p.(Arg94Cys) by Sanger sequencing in two male patients (Pt1 and Pt2). Patient with c.280C>T mutation had dilated cardiomyopathy, while another patient with c.39_60del mutation had no feature of cardiomyopathy. A reported m.1555A>G homoplasmic variant was also identified in the patient having mutation c.39_60del by whole mitochondrial DNA sequencing method. This variant was not considered to be the main cause of mitochondrial dysfunction based on a cytoplasmic hybrid (cybrid) assay. Tafazzin expression was absent in both patient-derived fibroblast cells. Complementation of TAZ expression in fibroblasts from the patient with the novel mutation c.39_60del restored mitochondrial respiratory complex assembly. High-performance liquid chromatography-tandem mass spectrometry-based metabolic analysis revealed the decline of CL and the accumulation of monolysocardiolipin, indicating the loss of tafazzin activity. Owing to phenotypic variability, it is difficult to diagnose BTHS based on clinical features only. We conclude that genetic analysis should be performed to avoid underdiagnosis of this potentially life-threatening inborn error of metabolism.

When silence is noise: infantile-onset Barth syndrome caused by a synonymous substitution affecting TAZ gene transcription

Barth syndrome (BTHS) is an X-linked inborn error of metabolism which affects males. The main manifestations are cardiomyopathy, myopathy, hypotonia, growth delay, intermittent neutropenia and 3-methylglutaconic aciduria. Diagnosis is confirmed by mutational analysis of the TAZ gene and biochemical dosage of the monolysocardiolipin/tetralinoleoyl cardiolipin (MLCL:L4-CL) ratio. We report a 6-year-old boy who presented with severe hypoglycemia, lactic acidosis and severe dilated cardiomyopathy soon after birth. The MLCL:L4-CL ratio confirmed BTHS (3.90 on patient's fibroblast, normal: 0-0.3). Subsequent sequencing of the TAZ gene revealed only the new synonymous variant NM_000116.3 (TAZ):c.348C>T p.(Gly116Gly), which did not appear to affect the protein sequence. In silico prediction analysis suggested the new c.348C>T nucleotide change could alter the TAZ mRNA splicing processing. We analyzed TAZ mRNAs in the patient's fibroblasts and found an abnormal skipping of 24 bases (NM_000116.3:c.346_371), with the consequent ablation of 8 amino acid residues in the tafazzin protein (NP_000107.1:p.Lys117_Gly124del). Molecular analysis of at risk female family members identified the patient's sister and mother as heterozygous carriers. Apparently harmless synonymous variants in the TAZ gene can damage gene expression. Such findings widen our knowledge of molecular heterogeneity in BTHS.

New targets for monitoring and therapy in Barth syndrome

PURPOSE

Barth syndrome (BTHS), an X-linked disorder caused by defects in TAZ, is the only known single-gene disorder of cardiolipin remodeling. We hypothesized that through analysis of affected individuals, we would gain a better understanding of the range of clinical features and identify targets for monitoring and therapy.

METHODS

We conducted a multidisciplinary investigation involving 42 patients with BTHS, including echocardiograms, muscle strength testing, functional exercise capacity testing, physical activity assessments, cardiolipin analysis, 3-methylglutaconic acid analysis, and review of genotype data. We analyzed data points to provide a quantitative spectrum of disease characteristics and to identify relationships among phenotype, genotype, and relevant metabolites.

RESULTS

Echocardiography revealed considerable variability in cardiac features. By contrast, almost all patients had significantly reduced functional exercise capacity. Multivariate analysis revealed significant relationships between cardiolipin ratio and left ventricular mass and between cardiolipin ratio and functional exercise capacity. We additionally identified genotypes associated with a less severe metabolic and clinical profile.

CONCLUSION

We defined previously unrecognized metabolite/phenotype/genotype relationships, established targets for therapeutic monitoring, and validated avenues for clinical assessment. In addition to providing insight into BTHS, these studies also provide insight into the myriad of multifactorial disorders that converge on the cardiolipin pathway.Genet Med 18 10, 1001-1010.

Clinical Characteristics and Outcomes of Cardiomyopathy in Barth Syndrome: The UK Experience

Barth syndrome (BTHS) is an X-linked disorder characterised by cardiomyopathy, neutropenia, skeletal myopathy and growth delay. This study describes the UK national clinical experience and outcome of cardiomyopathy in BTHS. The clinical course and echocardiographic changes of all patients with BTHS in the UK were reviewed from 2004 to 2014. In addition, strain analysis using 2D speckle tracking echocardiography was performed to further assess left ventricular function in the most recent follow-up. At last follow-up, 22 of 27 patients were alive with a median age of 12.6 (2.0-23.8) years; seven underwent cardiac transplantation at a median age of 2 (0.33-3.6) years, and five died (18.5%) at a median age of 1.8 (0.02-4.22) years. All deaths were related to cardiomyopathy or its management. Left ventricular diastolic dimension and systolic function measured by fractional shortening tended to normalise and stabilise after the first 3 years of life in the majority of patients. However, patients with BTHS (n = 16) had statistically significant reduction in global longitudinal and circumferential strain compared to controls (n = 18), (p < 0.001), despite apparent normal conventional measures of function. There was also reduced or reversed apical rotation and reduced left ventricular twist. Sustained ventricular arrhythmia was not seen at follow-up. Cardiac phenotype in BTHS is variable; however, longer-term outcome in our cohort suggests good prognosis after the first 5 years of life. Most patients appeared to have recovered near normal cardiac function by conventional echocardiographic measures, but strain analysis showed abnormal myocardial deformation and rotational mechanics.

The Role of Cardiolipin in Cardiovascular Health

Cardiolipin (CL), the signature phospholipid of mitochondrial membranes, is crucial for both mitochondrial function and cellular processes outside of the mitochondria. The importance of CL in cardiovascular health is underscored by the life-threatening genetic disorder Barth syndrome (BTHS), which manifests clinically as cardiomyopathy, skeletal myopathy, neutropenia, and growth retardation. BTHS is caused by mutations in the gene encoding tafazzin, the transacylase that carries out the second CL remodeling step. In addition to BTHS, CL is linked to other cardiovascular diseases (CVDs), including cardiomyopathy, atherosclerosis, myocardial ischemia-reperfusion injury, heart failure, and Tangier disease. The link between CL and CVD may possibly be explained by the physiological roles of CL in pathways that are cardioprotective, including mitochondrial bioenergetics, autophagy/mitophagy, and mitogen activated protein kinase (MAPK) pathways. In this review, we focus on the role of CL in the pathogenesis of CVD as well as the molecular mechanisms that may link CL functions to cardiovascular health.

Successful management of Barth syndrome: a systematic review highlighting the importance of a flexible and multidisciplinary approach

This review describes and summarizes the available evidence related to the treatment and management of Barth syndrome. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) standards were used to identify articles published between December 2004 and January 2015. The Cochrane Population, Intervention, Control, Outcome, Study Design (PICOS) approach was used to guide the article selection and evaluation process. Of the 128 articles screened, 28 articles matched the systematic review inclusion criteria. The results of this review indicate the need for a flexible and multidisciplinary approach to manage the symptoms most commonly associated with Barth syndrome. It is recommended that a comprehensive care team should include individuals with Barth syndrome, their family members and caregivers, as well as medical, rehabilitative, nutritional, psychological, and educational professionals. The evidence for specific treatments, therapies, and techniques for individuals with Barth syndrome is currently lacking in both quality and quantity.

Cardiolipin fingerprinting of leukocytes by MALDI-TOF/MS as a screening tool for Barth syndrome

Barth syndrome (BTHS), an X-linked disease associated with cardioskeletal myopathy, neutropenia, and organic aciduria, is characterized by abnormalities of card-iolipin (CL) species in mitochondria. Diagnosis of the disease is often compromised by lack of rapid and widely available diagnostic laboratory tests. The present study describes a new method for BTHS screening based on MALDI-TOF/MS analysis of leukocyte lipids. This generates a "CL fingerprint" and allows quick and simple assay of the relative levels of CL and monolysocardiolipin species in leukocyte total lipid profiles. To validate the method, we used vector algebra to analyze the difference in lipid composition between controls (24 healthy donors) and patients (8 boys affected by BTHS) in the high-mass phospholipid range. The method of lipid analysis described represents an important additional tool for the diagnosis of BTHS and potentially enables therapeutic monitoring of drug targets, which have been shown to ameliorate abnormal CL profiles in cells.

Intra-individual plasticity of the TAZ gene leading to different heritable mutations in siblings with Barth syndrome

Infantile-onset skeletal myopathy Barth syndrome (OMIM #302060) is caused by mutations in the X-linked TAZ gene and hence usually manifests itself only in hemizygous males. Confirmatory testing is provided by mutational analysis of the TAZ gene and/or by biochemical dosage of the monolysocardiolipin/tetralinoleoyl cardiolipin ratio. Heterozygous females do not usually display a clinical phenotype but may undergo molecular genetic prenatal diagnosis during pregnancy. We characterized two novel and non-identical TAZ gene rearrangements in the offspring of a single female carrier of Barth syndrome. The hg19chrX:g.153634427_153644361delinsKP_123427.1 TAZ gene rearrangement was identified in her affected son, whereas the NM_000116.3(TAZ)c.-72_109+51del TAZ gene deletion was identified in a male foetus during a subsequent pregnancy. The unaffected mother was surprisingly found to harbour both variants in addition to a wild-type TAZ allele. A combination of breakpoint junction sequencing, linkage analysis and assessment of allelic dosage revealed that the two variants had originated independently from an apparently unstable/mutable TAZ maternal allele albeit via different mutational mechanisms. We conclude that molecular prenatal diagnosis in Barth syndrome families with probands carrying TAZ gene rearrangements should include investigation of the entire coding region of the TAZ gene. The identification of the breakpoint junctions of such gross gene rearrangements is important to ensure accurate ascertainment of carriership with a view to providing appropriate genetic counselling.