Clinical presentation and natural history of Barth Syndrome: An overview

Rescue of mitochondrial dysfunction through alteration of extracellular matrix composition in barth syndrome cardiac fibroblasts

Fibroblast-ECM (dys)regulation is associated with a plethora of diseases. The ECM acts as a reservoir of inflammatory factors and cytokines that mediate molecular mechanisms within cardiac cell populations. The role of ECM-mitochondria crosstalk in the development and progression of cardiac disorders remains uncertain. We evaluated the influence of ECM produced by stromal cells from patients with the mitochondrial cardiomyopathy (Barth syndrome, BTHS) and unaffected healthy controls on cardiac fibroblast (CF) metabolic function. To do this, cell-derived matrices CDMs were generated from BTHS and healthy human pluripotent stem cell-derived CFs (hPSC-CF) and used as cell culture substrates. BTHS CDMs negatively impacted the mitochondrial function of healthy hPSC-CFs while healthy CDMs improved mitochondrial function in BTHS hPSC-CFs. Mass spectrometry comparisons identified 5 matrisome proteins differentially expressed in BTHS compared to healthy CDM. Our results highlight a key role for the ECM in disease through its impact on mitochondrial function.

Health-related quality of life and family functioning in parents of children with Barth syndrome: an application of the Double ABCX model

BACKGROUND

Living with children with disabilities has a significant impact on parental health-related quality of life (HRQoL) and family functioning. Barth syndrome (BTHS) is a rare, X-linked disorder that primarily affects males, presenting symptoms such as cardiomyopathy, neutropenia, muscle weakness, and growth delays. In this study, we investigated how a child's functional performance, family cohesion, and satisfaction with healthcare affect parents of children with BTHS.

METHODS

Thirty-three parents of children with BTHS and 31 parents of age-matched unaffected children participated in this study. The parents completed a series of questionnaires. The Double ABCX model was applied to select measurement variables for this study. An independent samples t-test was used to compare HRQoL and family functioning between the two groups. Regression analysis was conducted to determine how a child's functional performance, family cohesion, and satisfaction with healthcare affect HRQoL and family functioning of parents of children with BTHS.

RESULTS

The HRQoL and family functioning of parents of children with BTHS were significantly lower than those of unaffected children (p <.05). In the regression analysis, the child's functional performance was a significant predictor of HRQoL and family functioning (F(3, 32) = 6.047, p =.003) for parents of children with BTHS.

CONCLUSIONS

This study lays the groundwork for examining the impact of raising children with BTHS on parents and families. It is crucial for health professionals to understand the clinical features of BTHS and to consider not only the child but also the family in order to address their unmet needs and provide holistic healthcare services.

Tafazzin regulates neutrophil maturation and inflammatory response

Barth syndrome (BTHS) is a rare genetic disease caused by mutations in the TAFAZZIN gene. It is characterized by neutropenia, cardiomyopathy and skeletal myopathy. Neutropenia in BTHS is associated with life-threatening infections, yet there is little understanding of the molecular and physiological causes of this phenomenon. We combined bone marrow analysis, CRISPR/Cas9 genome editing in hematopoietic stem cells and functional characterization of circulating BTHS patient neutrophils to investigate the role of TAFAZZIN in neutrophils and their progenitors. We demonstrate a partial cell intrinsic differentiation defect, along with a dysregulated neutrophil inflammatory response in BTHS, including elevated degranulation and formation of neutrophil extracellular traps (NETs) in response to calcium flux. Developmental and functional alterations in BTHS neutrophils are underpinned by perturbations in the unfolded protein response (UPR) signaling pathway, suggesting potential therapeutic avenues for targeting BTHS neutropenia.

Stem cell models of TAFAZZIN deficiency reveal novel tissue-specific pathologies in Barth syndrome

Barth syndrome (BTHS) is a rare mitochondrial disease caused by pathogenic variants in the gene TAFAZZIN, which leads to abnormal cardiolipin (CL) metabolism on the inner mitochondrial membrane. Although TAFAZZIN is ubiquitously expressed, BTHS involves a complex combination of tissue specific phenotypes including cardiomyopathy, neutropenia, skeletal myopathy, and growth delays, with a relatively minimal neurological burden. To understand both the developmental and functional effects of TAZ-deficiency in different tissues, we generated isogenic TAZ knockout (TAZ-KO) and WT cardiomyocytes (CMs) and neural progenitor cells (NPCs) from CRISPR-edited induced pluripotent stem cells (iPSCs). In TAZ-KO CMs we discovered evidence of dysregulated mitophagy including dysmorphic mitochondria and mitochondrial cristae, differential expression of key autophagy-associated genes, and an inability of TAZ-deficient CMs to properly initiate stress-induced mitophagy. In TAZ-deficient NPCs we identified novel phenotypes including a reduction in CIV abundance and CIV activity in the CIII2&CIV2 intermediate complex. Interestingly, while CL acyl chain manipulation was unable to alter mitophagy defects in TAZ-KO CMs, we found that linoleic acid or oleic acid supplementation was able to partially restore CIV abundance in TAZ-deficient NPCs. Taken together, our results have implications for understanding the tissue-specific pathology of BTHS and potential for tissue-specific therapeutic targeting. Moreover, our results highlight an emerging role for mitophagy in the cardiac pathophysiology of BTHS and reveal a potential neuron-specific bioenergetic phenotype.

What can ATP content tell us about Barth syndrome muscle phenotypes?

Adenosine triphosphate (ATP) is the energy currency within all living cells and is involved in many vital biochemical reactions, including cell viability, metabolic status, cell death, intracellular signaling, DNA and RNA synthesis, purinergic signaling, synaptic signaling, active transport, and muscle contraction. Consequently, altered ATP production is frequently viewed as a contributor to both disease pathogenesis and subsequent progression of organ failure. Barth syndrome (BTHS) is an X-linked mitochondrial disease characterized by fatigue, skeletal muscle weakness, cardiomyopathy, neutropenia, and growth delay due to inherited TAFAZZIN enzyme mutations. BTHS is widely hypothesized in the literature to be a model of defective mitochondrial ATP production leading to energy deficits. Prior patient data have linked both impaired ATP production and reduced phosphocreatine to ATP ratios (PCr/ATP) in BTHS children and adult hearts and muscles, suggesting a primary role for perturbed energetics. Moreover, although only limited direct measurements of ATP content and ADP/ATP ratio (an indicator of the energy available from ATP hydrolysis) have so far been carried out, analysis of divergent BTHS animal models, cultured cell types, and diverse organs has failed to uncover a unifying understanding of the molecular mechanisms linking TAFAZZIN deficiency to perturbed muscle energetics. This review mainly focuses on the energetics of striated muscle in BTHS mitochondriopathy.

ECG Findings Are Poor Predictors for Adverse Events and Cardiac Death in Barth Syndrome

Background

Patients with Barth syndrome (BTHS) can present with cardiomyopathy. BTHS subjects are at risk for cardiac adverse outcomes throughout life, including malignant arrhythmias and death. Electrocardiogram (ECG) parameters have never been assessed as a tool to predict adverse outcomes in individuals with BTHS.

Objectives

The purpose of this study was to identify any ECG parameters including QRS fragmentation, presence of arrhythmia, or abnormal intervals that could predict adverse outcomes and cardiac death among the BTHS population.

Methods

We performed a retrospective case referent study on subjects with BTHS (n=43), and compared them with our reference group, subjects with idiopathic dilated cardiomyopathy (DCM) from a single institution (n=53) from 2007-2021. BTHS data was obtained from subjects attending the biennial Barth Syndrome Foundation International Scientific, Medical, and Family Conferences (BSFISMFC) from 2002-2018. ECG data from first and last available ECG's prior to an adverse event or cardiac death was analyzed, and then multivariable regression was performed to determine odd ratios between ECG characteristics and adverse events/cardiac death.

Results

No ECG variables were statistically significant predictors of adverse events or cardiac death in the BTHS group. Last ECG QRS fragmentation trended to statistically significance (OR 13.3, p=0.12) in predicting adverse events in the DCM group.

Conclusion

No ECG parameters, including QRS fragmentation, presence of arrhythmia, or abnormal interval values predict adverse events or cardiac death among BTHS patients. QRS fragmentation may be a predictor of adverse events in the DCM population.

Barth syndrome: a rare cause of cardiomyopathy in neonates

Barth syndrome (BTHS) is one of the rare X linked recessive diseases that appear in infancy with a triad of myocardial and skeletal muscle diseases, neutropenia and growth retardation. The pathogenic variant of TAFAZZIN gene leads to BTHS, which encodes the TAFAZZIN protein of the inner membrane of the mitochondria, a phosphatidyltransferase involved in cardiolipin remodelling and functional maturation. We present a case of a neonate presenting with early-onset cardiomyopathy, neutropenia and failure to thrive with no family history of cardiac diseases. Echocardiography suggested a dilated left ventricle with non-compaction and a low ejection fraction. The baby was managed with diuretics and decongestive measures. Clinical exome sequencing detected a hemizygous novel splice site variant c.541+2 T>C in TAFAZZIN, confirming the diagnosis of BTHS.

The subtherapeutic dose of valproic acid induces the activity of cardiolipin-dependent proteins

A mood-stabilizing anticonvulsant valproic acid (VPA) is a drug with a pleiotropic effect on cells. Here, we describe the impact of VPA on the metabolic function of human HAP1 cells. We show that VPA altered the biosynthetic pathway of cardiolipin (CL) and affected the activities of mitochondrial enzymes such as pyruvate dehydrogenase, α-ketoglutarate dehydrogenase and NADH dehydrogenase. We demonstrate that a therapeutic dose of VPA (0.6 mM) has a harmful effect on cell growth and increases the production of reactive oxygen species and superoxides. On the contrary, less concentrated VPA (0.06 mM) increased the activities of CL-dependent enzymes leading to an increased level of oxidative phosphorylation and ATP production. The effect of VPA was also tested on the Barth syndrome model, which is characterized by a reduced amount of CL and an increased level of monolyso-CL. In this model, VPA treatment slightly attenuated the mitochondrial defects by altering the activities of CL-dependent enzymes. However, the presence of CL was essential for the increase in ATP production by VPA. Our findings highlight the potential therapeutic role of VPA in normalizing mitochondrial function in BTHS and shed light on the intricate interplay between lipid metabolism and mitochondrial physiology in health and disease. SUMMARY: This study investigates the dose-dependent effect of valproate, a mood-stabilizing drug, on mitochondrial function. The therapeutic concentration reduced overall cellular metabolic activity, while a subtherapeutic concentration notably improved the function of cardiolipin-dependent proteins within mitochondria. These findings shed light on novel aspects of valproate's effect and suggest potential practical applications for its use. By elucidating the differential effects of valproate doses on mitochondrial activity, this research underscores the drug's multifaceted role in cellular metabolism and highlights avenues for further exploration in therapeutic interventions.

Case Report: A Chinese child with Barth syndrome caused by a novel TAFAZZIN mutation

Barth syndrome (BTHS) is a rare X-linked recessive genetic disorder characterized by a broad spectrum of clinical features including cardiomyopathy, skeletal myopathy, neutropenia, growth delay, and 3-methylglutaconic aciduria. This disease is caused by loss-of-function mutations in the TAFAZZIN gene located on chromosome Xq28, resulting in cardiolipin deficiency. Most patients are diagnosed in childhood, and the mortality rate is highest in the early years. We report a case of acute, life-threatening metabolic decompensation occurring one day after birth. A novel TAFAZZIN splice site mutation was identified in the patient, marking the first reported case of such a mutation in BTHS identified in China. The report aims to expand our understanding of the spectrum of TAFAZZIN mutations in BTHS.

SS-31 treatment ameliorates cardiac mitochondrial morphology and defective mitophagy in a murine model of Barth syndrome

Barth syndrome (BTHS) is a lethal rare genetic disorder, which results in cardiac dysfunction, severe skeletal muscle weakness, immune issues and growth delay. Mutations in the TAFAZZIN gene, which is responsible for the remodeling of the phospholipid cardiolipin (CL), lead to abnormalities in mitochondrial membrane, including alteration of mature CL acyl composition and the presence of monolysocardiolipin (MLCL). The dramatic increase in the MLCL/CL ratio is the hallmark of patients with BTHS, which is associated with mitochondrial bioenergetics dysfunction and altered membrane ultrastructure. There are currently no specific therapies for BTHS. Here, we showed that cardiac mitochondria isolated from TAFAZZIN knockdown (TazKD) mice presented abnormal ultrastructural membrane morphology, accumulation of vacuoles, pro-fission conditions and defective mitophagy. Interestingly, we found that in vivo treatment of TazKD mice with a CL-targeted small peptide (named SS-31) was able to restore mitochondrial morphology in tafazzin-deficient heart by affecting specific proteins involved in dynamic process and mitophagy. This agrees with our previous data showing an improvement in mitochondrial respiratory efficiency associated with increased supercomplex organization in TazKD mice under the same pharmacological treatment. Taken together our findings confirm the beneficial effect of SS-31 in the amelioration of tafazzin-deficient dysfunctional mitochondria in a BTHS animal model.

Dietary linoleic acid supplementation fails to rescue established cardiomyopathy in Barth syndrome

Barth syndrome (BTHS) is a mitochondrial lipid disorder caused by mutations in TAFAZZIN (TAZ), required for cardiolipin (CL) remodeling. Cardiomyopathy is a major clinical feature, with no curative therapy. Linoleic acid (LA) supplementation is proposed to ameliorate BTHS cardiomyopathy by enhancing linoleoyl group incorporation into CL. While the beneficial effect of dietary LA supplementation in delaying the development of BTHS cardiomyopathy has been recently tested, its potential to reverse established BTHS cardiomyopathy remains unclear. Our study revealed that LA supplementation cannot rescue established BTHS cardiomyopathy in mice, highlighting the importance of early initiation of LA supplementation for maximum benefits.

Tafazzin deficiency causes substantial remodeling in the lipidome of a mouse model of Barth Syndrome cardiomyopathy

Barth Syndrome (BTHS) is a rare X-linked disease, characterized clinically by cardiomyopathy, skeletal myopathy, neutropenia, and growth retardation. BTHS is caused by mutations in the phospholipid acyltransferase tafazzin (Gene: TAFAZZIN, TAZ). Tafazzin catalyzes the final step in the remodeling of cardiolipin (CL), a glycerophospholipid located in the inner mitochondrial membrane. As the phospholipid composition strongly determines membrane properties, correct biosynthesis of CL and other membrane lipids is essential for mitochondrial function. Mitochondria provide 95% of the energy demand in the heart, particularly due to their role in fatty acid oxidation. Alterations in lipid homeostasis in BTHS have an impact on mitochondrial membrane proteins and thereby contribute to cardiomyopathy. We analyzed a transgenic TAFAZZIN-knockdown (TAZ-KD) BTHS mouse model and determined the distribution of 193 individual lipid species in TAZ-KD and WT hearts at 10 and 50 weeks of age, using electrospray ionization tandem mass spectrometry (ESI-MS/MS). Our results revealed significant lipid composition differences between the TAZ-KD and WT groups, indicating genotype-dependent alterations in most analyzed lipid species. Significant changes in the myocardial lipidome were identified in both young animals without cardiomyopathy and older animals with heart failure. Notable alterations were found in phosphatidylcholine (PC), phosphatidylethanolamine (PE), lysophosphatidylethanolamine (LPE), lysophosphatidylcholine (LPC) and plasmalogen species. PC species with 2-4 double bonds were significantly increased, while polyunsaturated PC species showed a significant decrease in TAZ-KD mice. Furthermore, Linoleic acid (LA, 18:2) containing PC and PE species, as well as arachidonic acid (AA, 20:4) containing PE 38:4 species are increased in TAZ-KD. We found higher levels of AA containing LPE and PE-based plasmalogens (PE P-). Furthermore, we are the first to show significant changes in sphingomyelin (SM) and ceramide (Cer) lipid species Very long-chained SM species are accumulating in TAZ-KD hearts, whereas long-chained Cer and several hexosyl ceramides (HexCer) species accumulate only in 50-week-old TAZ-KD hearts These findings offer potential avenues for the diagnosis and treatment of BTHS, presenting new possibilities for therapeutic approaches.

Stem cell models of TAFAZZIN deficiency reveal novel tissue-specific pathologies in Barth Syndrome

Barth syndrome (BTHS) is a rare mitochondrial disease caused by pathogenic variants in the gene TAFAZZIN, which leads to abnormal cardiolipin (CL) metabolism on the inner mitochondrial membrane. Although TAFAZZIN is ubiquitously expressed, BTHS involves a complex combination of tissue specific phenotypes including cardiomyopathy, neutropenia, skeletal myopathy, and growth delays, with a relatively minimal neurological burden. To understand both the developmental and functional effects of TAZ-deficiency in different tissues, we generated isogenic TAZ knockout (TAZ- KO) and WT cardiomyocytes (CMs) and neural progenitor cells (NPCs) from CRISPR-edited induced pluripotent stem cells (iPSCs). In TAZ-KO CMs we discovered evidence of dysregulated mitophagy including dysmorphic mitochondria and mitochondrial cristae, differential expression of key autophagy-associated genes, and an inability of TAZ-deficient CMs to properly initiate stress-induced mitophagy. In TAZ-deficient NPCs we identified novel phenotypes including a reduction in CIV abundance and CIV activity in the CIII2&CIV2 intermediate complex. Interestingly, while CL acyl chain manipulation was unable to alter mitophagy defects in TAZ-KO CMs, we found that linoleic acid or oleic acid supplementation was able to partially restore CIV abundance in TAZ-deficient NPCs. Taken together, our results have implications for understanding the tissue-specific pathology of BTHS and potential for tissue-specific therapeutic targeting. Moreover, our results highlight an emerging role for mitophagy in the cardiac pathophysiology of BTHS and reveal a potential neuron-specific bioenergetic phenotype.

Blood cytopenias as manifestations of inherited metabolic diseases: a narrative review

Inherited Metabolic Diseases (IMD) encompass a diverse group of rare genetic conditions that, despite their individual rarity, collectively affect a substantial proportion, estimated at as much as 1 in 784 live births. Among their wide-ranging clinical manifestations, cytopenia stands out as a prominent feature. Consequently, IMD should be considered a potential diagnosis when evaluating patients presenting with cytopenia. However, it is essential to note that the existing scientific literature pertaining to the link between IMD and cytopenia is limited, primarily comprising case reports and case series. This paucity of data may contribute to the inadequate recognition of the association between IMD and cytopenia, potentially leading to underdiagnosis. In this review, we synthesize our findings from a literature analysis along with our clinical expertise to offer a comprehensive insight into the clinical presentation of IMD cases associated with cytopenia. Furthermore, we introduce a structured diagnostic approach underpinned by decision-making algorithms, with the aim of enhancing the early identification and management of IMD-related cytopenia.

The Impact of Raising Children with Barth Syndrome on Parental Health-Related Quality of Life and Family Functioning: Preliminary Reliability and Validity of the PedsQL™ Family Impact Module

Objective

This study examined the preliminary reliability and validity of the PedsQL™ Family Impact Module (PedsQL FIM) in families of children with Barth syndrome (BTHS).

Method

A total of 72 parents with children or youth between the ages of 5 and 19 participated in this study. Thirty-three parents of children with BTHS and 39 parents of unaffected children completed the PedsQL FIM and a demographic information form. Internal consistency reliability and item-total correlations were calculated to test the reliability of the PedsQL FIM. Construct validity was examined using the known-groups method. We estimated the mean score differences of the PedsQL FIM between the two groups using three different models, including unadjusted, multivariate regression, and propensity score matching with inverse probability of treatment weighting (PS-IPTW) models.

Results

The Cronbach's alpha coefficients were greater than 0.70 for all scales of the PedsQL FIM, except for the communication scale. The item-total correlations were significant for all scales with moderate to high correlations (p < .05). In construct validity, the mean scores of the PedsQL FIM between the two groups were significantly different (p < .05) for all scales and total score in the unadjusted and PS-IPTW models. However, in the multivariate regression model, the family relationships scale was not significant between the two groups.

Conclusion

The PedsQL FIM demonstrated adequate measurement properties of preliminary reliability and validity in assessing the impact of children with BTHS on parental health-related quality of life (HRQoL) and family functioning. Further research needs to be conducted to examine the psychometric properties of the PedsQL FIM with a large sample of BTHS and with other pediatric rare diseases.

Cardiolipin deficiency leads to the destabilization of mitochondrial magnesium channel MRS2 in Barth syndrome

Barth syndrome (BTHS) is a debilitating X-linked cardio-skeletal myopathy caused by loss-of-function mutations in TAFAZZIN, a cardiolipin (CL)-remodeling enzyme required for the maintenance of normal levels of CL species in mitochondrial membranes. At present, how perturbations in CL abundance and composition lead to many debilitating clinical presentations in BTHS patients have not been fully elucidated. Inspired by our recent findings that CL is essential for optimal mitochondrial calcium uptake, we measured the levels of other biologically important metal ions in BTHS mitochondria and found that in addition to calcium, magnesium levels are significantly reduced. Consistent with this observation, we report a decreased abundance of the mitochondrial magnesium influx channel MRS2 in multiple models of BTHS including yeast, murine myoblast, and BTHS patient cells and cardiac tissue. Mechanistically, we attribute reduced steady-state levels of MRS2 to its increased turnover in CL-deficient BTHS models. By expressing Mrs2 in well-characterized yeast mutants of the phospholipid biosynthetic pathways, we demonstrate a specific requirement of CL for Mrs2 abundance and assembly. Finally, we provide in vitro evidence for the direct binding of CL with human MRS2. Together, our study has identified a critical requirement of CL for MRS2 stability and suggests perturbation of mitochondrial magnesium homeostasis as a novel contributing factor to BTHS pathology.

FGF21 and GDF15 are elevated in Barth Syndrome and are correlated to important clinical measures

Barth Syndrome (BTHS) is a rare X-linked disorder that is caused by defects TAFAZZIN, which leads to an abnormal cardiolipin (CL) profile of the inner mitochondrial membrane and clinical features including cardiomyopathy, neutropenia and skeletal myopathy. The ratio of monolysocardiolipin (MLCL, the remodeling intermediate of cardiolipin) to remodeled CL is always abnormal in Barth Syndrome and 3-methylglutaconic acid is often elevated affected patients, however neither of these biomarkers has been shown to temporally correlate to clinical status. In this study, we measured plasma FGF21 and GDF15 levels in 16 individuals with Barth Syndrome and evaluated whether these biomarkers were correlated to the MLCL/CL ratio in patient bloodspots and clinical laboratory parameters indicative of organ involvement in Barth Syndrome including: neutrophil and monocyte counts, liver function, and cardiac function (NT-proBNP). We found that FGF21 and GDF15 were elevated in all 16 patients and that FGF21 was significantly correlated to AST, ALT GGT, percentage of neutrophils comprising total white blood cells, percent monocytes comprising total white blood cells, and NT-proBNP levels. GDF-15 was significantly positively associated with NT-proBNP. We conclude that clinical measurements of FGF21 and GDF-15 may be relevant in the monitoring multi-organ system involvement in Barth Syndrome.

A novel panel of Drosophila TAFAZZIN mutants in distinct genetic backgrounds as a resource for therapeutic testing

Barth Syndrome is a rare, X-linked disorder caused by mutation of the gene TAFAZZIN (TAZ). The corresponding Tafazzin protein is involved in the remodeling of cardiolipin, a phospholipid with critical roles in mitochondrial function. While recent clinical trials have been promising, there is still no cure for Barth Syndrome. Because TAZ is highly conserved, multiple animal and cell culture models exist for pre-clinical testing of therapeutics. However, since the same mutation in different patients can lead to different symptoms and responses to treatment, isogenized experimental models can't fully account for human disease conditions. On the other hand, isogenized animal models allow for sufficient numbers to thoroughly establish efficacy for a given genetic background. Therefore, a combined method for testing treatments in a panel of isogenized cohorts that are genetically distinct from each other would be transformative for testing emerging pre-clinical therapies. To aid in this effort, we've created a novel panel of 10 Drosophila lines, each with the same TAZ mutation in highly diverse genetic backgrounds, to serve as a helpful resource to represent natural variation in background genetics in pre-clinical studies. As a proof of principle, we test our panel here using nicotinamide riboside (NR), a treatment with established therapeutic value, to evaluate how robust this therapy is across the 10 genetic backgrounds in this novel reference panel. We find substantial variation in the response to NR across backgrounds. We expect this resource will be valuable in pre-clinical testing of emerging therapies for Barth Syndrome.

Case report: Variability in clinical features as a potential pitfall for the diagnosis of Barth syndrome

Background

Barth syndrome is a rare genetic disease characterized by cardiomyopathy, skeletal muscle weakness, neutropenia, growth retardation and organic aciduria. This variable phenotype is caused by pathogenic hemizygous variants of the TAFAZZIN gene on the X chromosome, which impair metabolism of the mitochondrial phospholipid cardiolipin. Although most patients are usually diagnosed in the first years of life, the extremely variable clinical picture and the wide range of clinical presentations may both delay diagnosis. This is the case reported here of a man affected with severe neutropenia, who was not diagnosed with Barth syndrome until adulthood.

Case presentation

We describe herein a family case, specifically two Caucasian male cousins sharing the same mutation in the TAFAZZIN gene with a wide phenotypic variability: an infant who was early diagnosed with Barth syndrome due to heart failure, and his maternal cousin with milder and extremely different clinical features who has received the same diagnosis only at 33 years of age.

Conclusions

Our report supports the underestimation of the prevalence of Barth syndrome, which should be always considered in the differential diagnosis of male patients with recurrent neutropenia with or without signs and symptoms of cardiomyopathy.

Genetic modifiers modulate phenotypic expression of tafazzin deficiency in a mouse model of Barth syndrome

Barth syndrome is an X-linked disorder caused by loss-of-function mutations in Tafazzin (TAZ), an acyltransferase that catalyzes remodeling of cardiolipin, a signature phospholipid of the inner mitochondrial membrane. Patients develop cardiac and skeletal muscle weakness, growth delay and neutropenia, although phenotypic expression varies considerably between patients. Taz knockout mice recapitulate many of the hallmark features of the disease. We used mouse genetics to test the hypothesis that genetic modifiers alter the phenotypic manifestations of Taz inactivation. We crossed TazKO/X females in the C57BL6/J inbred strain to males from eight inbred strains and evaluated the phenotypes of first-generation (F1) TazKO/Y progeny, compared to TazWT/Y littermates. We observed that genetic background strongly impacted phenotypic expression. C57BL6/J and CAST/EiJ[F1] TazKO/Y mice developed severe cardiomyopathy, whereas A/J[F1] TazKO/Y mice had normal heart function. C57BL6/J and WSB/EiJ[F1] TazKO/Y mice had severely reduced treadmill endurance, whereas endurance was normal in A/J[F1] and CAST/EiJ[F1] TazKO/Y mice. In all genetic backgrounds, cardiolipin showed similar abnormalities in knockout mice, and transcriptomic and metabolomic investigations identified signatures of mitochondrial uncoupling and activation of the integrated stress response. TazKO/Y cardiac mitochondria were small, clustered and had reduced cristae density in knockouts in severely affected genetic backgrounds but were relatively preserved in the permissive A/J[F1] strain. Gene expression and mitophagy measurements were consistent with reduced mitophagy in knockout mice in genetic backgrounds intolerant of Taz mutation. Our data demonstrate that genetic modifiers powerfully modulate phenotypic expression of Taz loss-of-function and act downstream of cardiolipin, possibly by altering mitochondrial quality control.

The critical role of cardiolipin in metazoan differentiation, development, and maturation

Cardiolipins are phospholipids that are central to proper mitochondrial functioning. Because mitochondria play crucial roles in differentiation, development, and maturation, we would also expect cardiolipin to play major roles in these processes. Indeed, cardiolipin has been implicated in the mechanism of three human diseases that affect young infants, implying developmental abnormalities. In this review, we will: (1) Review the biology of cardiolipin; (2) Outline the evidence for essential roles of cardiolipin during organismal development, including embryogenesis and cell maturation in vertebrate organisms; (3) Place the role(s) of cardiolipin during embryogenesis within the larger context of the roles of mitochondria in development; and (4) Suggest avenues for future research.

Reduction in mRNA Expression of the Neutrophil Chemoattract Factor CXCL1 in Pseudomonas aeruginosa Treated Barth Syndrome B Lymphoblasts

Barth Syndrome (BTHS) is a rare X-linked genetic disease caused by a mutation in the TAFAZZIN gene, which codes for the protein tafazzin involved in cardiolipin remodeling. Approximately 70% of patients with BTHS exhibit severe infections due to neutropenia. However, neutrophils from BTHS patients have been shown to exhibit normal phagocytosis and killing activity. B lymphocytes play a crucial role in the regulation of the immune system and, when activated, secrete cytokines known to attract neutrophils to sites of infection. We examined the expression of chemokine (C-X-C motif) ligand 1 (CXCL1), a known chemotactic for neutrophils, in Epstein-Barr virus transformed control and BTHS B lymphoblasts. Age-matched control and BTHS B lymphoblasts were incubated with Pseudomonas aeruginosa for 24 h and then cell viability, CD27+, CD24+, CD38+, CD138+ and PD1+ surface marker expression and CXCL1 mRNA expression determined. Cell viability was maintained in lymphoblasts incubated in a ratio of 50:1 bacteria:B cells. Surface marker expression was unaltered between control and BTHS B lymphoblasts. In contrast, CXCL1 mRNA expression was reduced approximately 70% (p < 0.05) in untreated BTHS B lymphoblasts compared to control and approximately 90% (p < 0.05) in bacterial treated BTHS B lymphoblasts compared to the control. Thus, naïve and bacterial-activated BTHS B lymphoblasts exhibit reduced mRNA expression of the neutrophil chemoattractant factor CXCL1. We suggest that impaired bacterial activation of B cells in some BTHS patients could influence neutrophil function via impairing neutrophil recruitment to sites of infection and this could potentially contribute to these infections.

Temporal evolution of the heart failure phenotype in Barth syndrome and treatment with elamipretide

Barth syndrome (BTHS) is a rare genetic disorder caused by pathogenic variants in TAFAZZIN leading to reduced remodeled cardiolipin (CL), a phospholipid essential to mitochondrial function and structure. Cardiomyopathy presents in most patients with BTHS, typically appearing as dilated cardiomyopathy (DCM) in infancy and evolving to hypertrophic cardiomyopathy (HCM) resembling heart failure (HF) with preserved ejection fraction (HFpEF) in some patients ≥12 years. Elamipretide localizes to the inner mitochondrial membrane where it associates with CL, improving mitochondrial function, structure and bioenergetics, including ATP synthesis. Numerous preclinical and clinical studies in BTHS and other forms of HF have demonstrated that elamipretide improves left ventricular relaxation by ameliorating mitochondrial dysfunction, making it well suited for therapeutic use in adolescent and adult patients with BTHS.

Phenotypic Characterization of Male Tafazzin-Knockout Mice at 3, 6, and 12 Months of Age

Barth syndrome (BTHS) is an X-linked mitochondrial disease caused by mutations in the gene encoding for tafazzin (TAZ), a key enzyme in the remodeling of cardiolipin. Mice with a germline deficiency in Taz have been generated (Taz-KO) but not yet fully characterized. We performed physiological assessments of 3-, 6-, and 12-month-old male Taz-KO mice, including measures of perinatal survival, growth, lifespan, gross anatomy, whole-body energy and substrate metabolism, glucose homeostasis, and exercise capacity. Taz-KO mice displayed reduced viability, with lower-than-expected numbers of mice recorded at 4 weeks of age, and a shortened lifespan due to disease progression. At all ages, Taz-KO mice had lower body weights compared with wild-type (Wt) littermates despite similar absolute food intakes. This finding was attributed to reduced adiposity and diminutive organs and tissues, including heart and skeletal muscles. Although there were no differences in basal levels of locomotion between age-matched genotypes, indirect calorimetry studies showed higher energy expenditure measures and respiratory exchange ratios in Taz-KO mice. At the youngest age, Taz-KO mice had comparable glucose tolerance and insulin action to Wt mice, but while these measures indicated metabolic impairments in Wt mice with advancing age that were likely associated with increasing adiposity, Taz-KO mice were protected. Comparisons across the three age-cohorts revealed a significant and more severe deterioration of exercise capacity in Taz-KO mice than in their Wt littermate controls. The Taz-KO mouse model faithfully recapitulates important aspects of BTHS, and thus provides an important new tool to investigate pathophysiological mechanisms and potential therapies.

Beneficial effects of SS-31 peptide on cardiac mitochondrial dysfunction in tafazzin knockdown mice

Barth Syndrome (BTHS), a genetic disease associated with early-onset cardioskeletal myopathy, is caused by loss-of-function mutations of the TAFAZZIN gene, which is responsible for remodeling the mitochondrial phospholipid cardiolipin (CL). Deregulation of CL biosynthesis and maturation in BTHS mitochondria result in a dramatically increased monolysocardiolipin (MLCL)/CL ratio associated with bioenergetic dysfunction. One of the most promising therapeutic approaches for BTHS includes the mitochondria-targeted tetrapeptide SS-31, which interacts with CL. Here, we used TAFAZZIN knockdown (Taz^KD) mice to investigate for the first time whether in vivo administration of SS-31 could affect phospholipid profiles and mitochondrial dysfunction. The CL fingerprinting of Taz^KD cardiac mitochondria obtained by MALDI-TOF/MS revealed the typical lipid changes associated with BTHS. Taz^KD mitochondria showed lower respiratory rates in state 3 and 4 together with a decreased in maximal respiratory rates. Treatment of Taz^KD mice with SS-31 improved mitochondrial respiratory capacity and promoted supercomplex organization, without affecting the MLCL/CL ratio. We hypothesize that SS-31 exerts its effect by influencing the function of the respiratory chain rather than affecting CL directly. In conclusion, our results indicate that SS-31 have beneficial effects on improving cardiac mitochondrial dysfunction in a BTHS animal model, suggesting the peptide as future pharmacologic agent for therapy.

Stimulating myocardial pyruvate dehydrogenase activity fails to alleviate cardiac abnormalities in a mouse model of human Barth syndrome

Barth syndrome (BTHS) is a rare genetic disorder due to mutations in the TAFAZZIN gene, leading to impaired maturation of cardiolipin and thereby adversely affecting mitochondrial function and energy metabolism, often resulting in cardiomyopathy. In a murine model of BTHS involving short-hairpin RNA mediated knockdown of Tafazzin (TazKD mice), myocardial glucose oxidation rates were markedly reduced, likely secondary to an impairment in the activity of pyruvate dehydrogenase (PDH), the rate-limiting enzyme of glucose oxidation. Furthermore, TazKD mice exhibited cardiac hypertrophy with minimal cardiac dysfunction. Because the stimulation of myocardial glucose oxidation has been shown to alleviate diabetic cardiomyopathy and heart failure, we hypothesized that stimulating PDH activity would alleviate the cardiac hypertrophy present in TazKD mice. In order to address our hypothesis, 6-week-old male TazKD mice and their wild-type (WT) littermates were treated with dichloroacetate (DCA; 70 mM in the drinking water), which stimulates PDH activity via inhibiting PDH kinase to prevent inhibitory phosphorylation of PDH. We utilized ultrasound echocardiography to assess cardiac function and left ventricular wall structure in all mice prior to and following 6-weeks of treatment. Consistent with systemic activation of PDH and glucose oxidation, DCA treatment improved glycemia in both TazKD mice and their WT littermates, and decreased PDH phosphorylation equivalently at all 3 of its inhibitory sites (serine 293/300/232). However, DCA treatment had no impact on left ventricular structure, or systolic and diastolic function in TazKD mice. Therefore, it is unlikely that stimulating glucose oxidation is a viable target to improve BTHS-related cardiomyopathy.

Myocardial disturbances of intermediary metabolism in Barth syndrome

Barth Syndrome (BTHS) is a rare X-linked mitochondrial disorder due to mutations in the gene TAFAZZIN, which leads to immature cardiolipin (CL) remodeling and is characterized by the development of cardiomyopathy. The immature CL remodeling in BTHS results in electron transport chain respiratory defects and destabilization of supercomplexes, thereby impairing ATP production. Thus, BTHS-related cardiomyopathy appears to share metabolic characteristics of the failing heart being an "engine out of fuel." As CL associates with numerous mitochondrial enzymes involved in ATP production, BTHS is also characterized by several defects in intermediary energy metabolism. Herein we will describe the primary disturbances in intermediary energy metabolism relating to the heart's major fuel sources, fatty acids, carbohydrates, ketones, and amino acids. In addition, we will interrogate whether these disturbances represent potential metabolic targets for alleviating BTHS-related cardiomyopathy.

Mitochondrial Membranes and Mitochondrial Genome: Interactions and Clinical Syndromes

Mitochondria are surrounded by two membranes; the outer mitochondrial membrane and the inner mitochondrial membrane. They are unique organelles since they have their own DNA, the mitochondrial DNA (mtDNA), which is replicated continuously. Mitochondrial membranes have direct interaction with mtDNA and are therefore involved in organization of the mitochondrial genome. They also play essential roles in mitochondrial dynamics and the supply of nucleotides for mtDNA synthesis. In this review, we will discuss how the mitochondrial membranes interact with mtDNA and how this interaction is essential for mtDNA maintenance. We will review different mtDNA maintenance disorders that result from defects in this crucial interaction. Finally, we will review therapeutic approaches relevant to defects in mitochondrial membranes.

Barth Syndrome Cardiomyopathy: An Update

Barth syndrome (BTHS) is an X-linked mitochondrial lipid disorder caused by mutations in the TAFAZZIN (TAZ) gene, which encodes a mitochondrial acyltransferase/transacylase required for cardiolipin (CL) biosynthesis. Cardiomyopathy is a major clinical feature of BTHS. During the past four decades, we have witnessed many landmark discoveries that have led to a greater understanding of clinical features of BTHS cardiomyopathy and their molecular basis, as well as the therapeutic targets for this disease. Recently published Taz knockout mouse models provide useful experimental models for studying BTHS cardiomyopathy and testing potential therapeutic approaches. This review aims to summarize key findings of the clinical features, molecular mechanisms, and potential therapeutic approaches for BTHS cardiomyopathy, with particular emphasis on the most recent studies.

Studying Lipid-Related Pathophysiology Using the Yeast Model

Saccharomyces cerevisiae, commonly known as baker's yeast, is one of the most comprehensively studied model organisms in science. Yeast has been used to study a wide variety of human diseases, and the yeast model system has proved to be an especially amenable tool for the study of lipids and lipid-related pathophysiologies, a topic that has gained considerable attention in recent years. This review focuses on how yeast has contributed to our understanding of the mitochondrial phospholipid cardiolipin (CL) and its role in Barth syndrome (BTHS), a genetic disorder characterized by partial or complete loss of function of the CL remodeling enzyme tafazzin. Defective tafazzin causes perturbation of CL metabolism, resulting in many downstream cellular consequences and clinical pathologies that are discussed herein. The influence of yeast research in the lipid-related pathophysiologies of Alzheimer's and Parkinson's diseases is also summarized.