Clinical presentation and natural history of Barth Syndrome: An overview

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.