Eponym: Barth syndrome

A case of infantile Barth syndrome with severe heart failure: Importance of splicing variants in the TAZ gene

Barth syndrome (BTHS) is an X-linked disorder characterized by cardiomyopathy, skeletal myopathy, and 3-methylglutaconic aciduria. The causative pathogenic variants for BTHS are in TAZ, which encodes a putative acyltransferase named tafazzin and is involved in the remodeling of cardiolipin in the inner mitochondrial membranes. Pathogenic variants in TAZ result in mitochondrial structural and functional abnormalities. We report a case of infantile BTHS with severe heart failure, left ventricular noncompaction, and lactic acidosis, having a missense c.640C>T (p.His214Tyr) variant in TAZ, which is considered a pathogenic variant based on the previously reported amino acid substitution at the same site (c.641A>G, p.His214Arg). However, in this previously reported case, heart function was compensated and not entirely similar to the present case. Silico prediction analysis suggested that c.640C>T could alter the TAZ messenger RNA (mRNA) splicing process. TAZ mRNAs in isolated peripheral mononuclear cells from the patient and in vitro splicing analysis using minigenes of TAZ found an 8 bp deletion at the 3' end of exon 8, which resulted in the formation of a termination codon in the coding region of exon 9 (H214Nfs*3). These findings suggest that splicing abnormalities should always be considered in BTHS.

Cardiomyopathies in Children and Systemic Disorders When Is It Useful to Look beyond the Heart?

Cardiomyopathy (CMP) is a rare disease in the pediatric population, with a high risk of morbidity and mortality. The genetic etiology of CMPs in children is extremely heterogenous. These two factors play a major role in the difficulties of establishing standard diagnostic and therapeutic protocols. Isolated CMP in children is a frequent finding, mainly caused by sarcomeric gene variants with a detection rate that can reach up to 50% of analyzed cohorts. Complex multisystemic forms of pediatric CMP are even more heterogenous. Few studies in literature take into consideration this topic as the main core since it represents a rarity (systemic CMP) within a rarity (pediatric population CMP). Identifying etiology in this cohort is essential for understanding prognosis, risk stratification, eligibility to heart transplantation and/or mechanical-assisted procedures, preventing multiorgan complications, and relatives' recurrence risk calculation. The previous points represent a cornerstone in patients' empowerment and personalized medical care approach. The aim of this work is to propose a new approach for an algorithm in the setting of the diagnostic framework of systemic pediatric CMP. On the other hand, during the literature review, we noticed a relatively common etiologic pattern in some forms of complex/multisystem CMP. In other words, certain syndromes such as Danon, Vici, Alström, Barth, and Myhre syndrome share a common pathway of directly or indirectly defective "autophagy" process, which appears to be a possible initiating/triggering factor for CMPs. This conjoint aspect could be important for possible prognostic/therapeutic implications in this category of patients. However, multicentric studies detailed functional and experimental models are needed prior to deriving conclusions.

Mitochondrial dysfunction, AMPK activation and peroxisomal metabolism: A coherent scenario for non-canonical 3-methylglutaconic acidurias

The occurrence of 3-methylglutaconic aciduria (3-MGA) is a well understood phenomenon in leucine oxidation and ketogenesis disorders (primary 3-MGAs). In contrast, its genesis in non-canonical (secondary) 3-MGAs, a growing-up group of disorders encompassing more than a dozen of inherited metabolic diseases, is a mystery still remaining unresolved for three decades. To puzzle out this anthologic problem of metabolism, three clues were considered: (i) the variety of disorders suggests a common cellular target at the cross-road of metabolic and signaling pathways, (ii) the response to leucine loading test only discriminative for primary but not secondary 3-MGAs suggests these latter are disorders of extramitochondrial HMG-CoA metabolism as also attested by their failure to increase 3-hydroxyisovalerate, a mitochondrial metabolite accumulating only in primary 3-MGAs, (iii) the peroxisome is an extramitochondrial site possessing its own pool and displaying metabolism of HMG-CoA, suggesting its possible involvement in producing extramitochondrial 3-methylglutaconate (3-MG). Following these clues provides a unifying common basis to non-canonical 3-MGAs: constitutive mitochondrial dysfunction induces AMPK activation which, by inhibiting early steps in cholesterol and fatty acid syntheses, pipelines cytoplasmic acetyl-CoA to peroxisomes where a rise in HMG-CoA followed by local dehydration and hydrolysis may lead to 3-MGA yield. Additional contributors are considered, notably for 3-MGAs associated with hyperammonemia, and to a lesser extent in CLPB deficiency. Metabolic and signaling itineraries followed by the proposed scenario are essentially sketched, being provided with compelling evidence from the literature coming in their support.

Barth Syndrome with Late-Onset Cardiomyopathy: A Missed Opportunity for Diagnosis

A male infant presented with neutropenia, growth delay, and death of a maternal uncle at age 2 years. Despite extensive evaluation over 10 years, Barth syndrome was not diagnosed until he presented in acute heart failure. Although late-onset cardiomyopathy is rare, persistence of common Barth features should have enabled earlier diagnosis.

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.

Mis-sesnse mutations in Tafazzin (TAZ) that escort to mild clinical symptoms of Barth syndrome is owed to the minimal inhibitory effect of the mutations on the enzyme function: In-silico evidence

Tafazzin (EC 2.3.1.23) is a Phospholipid Transacylase involved in Cardiolipin remodeling on mitochondrial membrane and coded by TAZ gene (Cytogenetic Location: Xq28) in human. Its mutations cause Barth syndrome (MIM ID: #302060)/3-Methyl Glutaconyl Aciduria Type II, an inborn error of metabolism often leading to foetal or infantile fatality. Nevertheless, some mis-sense mutations result in mild clinical symptoms. To evaluate the rationale of mild symptoms and for an insight of Tafazzin active site, sequence based and structure based ramifications of wild and mutant Tafazzins were compared in-silico. Sequence based domain predictions, surface accessibilities on substitution & conserved catalytic sites with statistical drifts, as well as thermal stability changes for the mutations and the interaction analysis of Tafazzin were performed. Crystal structure of Tafazzin is not yet resolved experimentally, therefore 3D coordinates of Tafazzin and its mutants were spawned through homology modeling. Energetically minimized and structurally validated models were used for comparative docking simulations. We analyzed active site geometry of the models in addition to calculating overall substrate binding efficiencies for each of the enzyme-ligand complex deduced from binding energies instead of comparing only the docking scores. Also, individual binding energies of catalytic residues on conserved HX4D motif of Acyltransferase superfamily present in Tafazzins were estimated. This work elucidates the basis of mild symptoms in patients with mis-sense mutations, identifies the most pathogenic mutant among others in the study and also divulges the critical role of HX4D domain towards successful transacylation by Taffazin. The in-silico observations are in complete agreement with clinical findings reported for the patients with mutations.

Inborn errors of metabolism underlying primary immunodeficiencies

A number of inborn errors of metabolism (IEM) have been shown to result in predominantly immunologic phenotypes, manifesting in part as inborn errors of immunity. These phenotypes are mostly caused by defects that affect the (i) quality or quantity of essential structural building blocks (e.g., nucleic acids, and amino acids), (ii) cellular energy economy (e.g., glucose metabolism), (iii) post-translational protein modification (e.g., glycosylation) or (iv) mitochondrial function. Presenting as multisystemic defects, they also affect innate or adaptive immunity, or both, and display various types of immune dysregulation. Specific and potentially curative therapies are available for some of these diseases, whereas targeted treatments capable of inducing clinical remission are available for others. We will herein review the pathogenesis, diagnosis, and treatment of primary immunodeficiencies (PIDs) due to underlying metabolic disorders.

Novel mutations in the TAZ gene in patients with Barth syndrome

Barth syndrome is an X-linked recessive disorder that is caused by mutations in Taffazin gene (TAZ), leading to severe cardiolipin deficiency which results in respiratory chain dysfunction. Barth syndrome is characterized by cardiomyopathy, neutropenia, skeletal myopathy, growth deficiency and 3-methylglutaconic aciduria. In this paper, we present clinical, biochemical and molecular data of the first four Czech patients from four unrelated families diagnosed with this rare disease. The mean age of onset was 5.5 ± 3.8 months. One child suffered from sudden cardiac death at the age of 2 years, the age of living patients is between 3 and 13 years. Muscle hypotonia was present in all four patients; cardiomyopathy and growth retardation in three and neutropenia in two of them. Two patients manifested a dilated and one patient a hypertrophic cardiomyopathy. A characteristic laboratory abnormality was the intermittently increased excretion of 3-methylglutaconic acid. Three novel hemizygous mutations in the TAZ gene were found (c.584G>T; c.109+6T>C; c.86G>A). We conclude that Barth syndrome should be included in differential diagnosis of cardiomyopathy in childhood, especially in the cooccurrence of dilated cardiomyopathy and 3-methylglutaconic aciduria.

Cardiolipin deficiency affects respiratory chain function and organization in an induced pluripotent stem cell model of Barth syndrome

Barth syndrome (BTHS) patients carrying mutations in tafazzin (TAZ1), which is involved in the final maturation of cardiolipin, present with dilated cardiomyopathy, skeletal myopathy, growth retardation and neutropenia. To study how mitochondrial function is impaired in BTHS patients, we generated induced pluripotent stem cells (iPSCs) to develop a novel and relevant human model system for BTHS. BTHS-iPSCs generated from dermal fibroblasts of three patients with different mutations in TAZ1 expressed pluripotency markers, and were able to differentiate into cells derived from all three germ layers both in vitro and in vivo. We used these cells to study the impact of tafazzin deficiency on mitochondrial oxidative phosphorylation. We found an impaired remodeling of cardiolipin, a dramatic decrease in basal oxygen consumption rate and in the maximal respiratory capacity in BTHS-iPSCs. Simultaneous measurement of extra-cellular acidification rate allowed us a thorough assessment of the metabolic deficiency in BTHS patients. Blue native gel analyses revealed that decreased respiration coincided with dramatic structural changes in respiratory chain supercomplexes leading to a massive increase in generation of reactive oxygen species. Our data demonstrate that BTHS-iPSCs are capable of modeling BTHS by recapitulating the disease phenotype and thus are important tools for studying the disease mechanism.

Natural history of Barth syndrome: a national cohort study of 22 patients

Background

This study describes the natural history of Barth syndrome (BTHS).

Methods

The medical records of all patients with BTHS living in France were identified in multiple sources and reviewed.

Results

We identified 16 BTHS pedigrees that included 22 patients. TAZ mutations were observed in 15 pedigrees. The estimated incidence of BTHS was 1.5 cases per million births (95%CI: 0.2–2.3). The median age at presentation was 3.1 weeks (range, 0–1.4 years), and the median age at last follow-up was 4.75 years (range, 3–15 years). Eleven patients died at a median age of 5.1 months; 9 deaths were related to cardiomyopathy and 2 to sepsis. The 5-year survival rate was 51%, and no deaths were observed in patients ≥3 years. Fourteen patients presented with cardiomyopathy, and cardiomyopathy was documented in 20 during follow-up. Left ventricular systolic function was very poor during the first year of life and tended to normalize over time. Nineteen patients had neutropenia. Metabolic investigations revealed inconstant moderate 3-methylglutaconic aciduria and plasma arginine levels that were reduced or in the low-normal range. Survival correlated with two prognostic factors: severe neutropenia at diagnosis (<0.5 × 10⁹/L) and birth year. Specifically, the survival rate was 70% for patients born after 2000 and 20% for those born before 2000.

Conclusions

This survey found that BTHS outcome was affected by cardiac events and by a risk of infection that was related to neutropenia. Modern management of heart failure and prevention of infection in infancy may improve the survival of patients with BTHS without the need for heart transplantation.

A novel mutation of the TAZ gene in Barth syndrome: acute exacerbation after contrast-dye injection

A 14-month-old boy was transferred because of dilated and hypertrophied left ventricle, neutropenia, and developmental delay. After checking computed tomographic angiography with contrast-dye, the patient showed acute exacerbation and finally died from multi-organ failure despite intensive cares. From genetic analysis, we revealed that the patient had Barth syndrome and found a novel hemizygous frame shift mutation in his TAZ gene, c.227delC (p.Pro76LeufsX7), which was inherited from his mother. Herein, we report a patient with Barth syndrome who had a novel mutation in TAZ gene and experienced unexpected acute exacerbation after contrast dye injection for computed tomographic angiography.

New clinical and molecular insights on Barth syndrome

Background

Barth syndrome (BS) is an X-linked infantile-onset cardioskeletal disease characterized by cardiomyopathy, hypotonia, growth delay, neutropenia and 3-methylglutaconic aciduria. It is caused by mutations in the TAZ gene encoding tafazzin, a protein involved in the metabolism of cardiolipin, a mitochondrial-specific phospholipid involved in mitochondrial energy production.

Methods

Clinical, biochemical and molecular characterization of a group of six male patients suspected of having BS. Three patients presented early with severe metabolic decompensation including respiratory distress, oxygen desaturation and cardiomyopathy and died within the first year of life. The remaining three patients had cardiomyopathy, hypotonia and growth delay and are still alive. Cardiomyopathy was detected during pregnancy through a routine check-up in one patient. All patients exhibited 3-methylglutaconic aciduria and neutropenia, when tested and five of them also had lactic acidosis.

Results

We confirmed the diagnosis of BS with sequence analysis of the TAZ gene, and found five new mutations, c.641A>G p.His214Arg, c.284dupG (p.Thr96Aspfs*37), c.678_691del14 (p.Tyr227Trpfs*79), g.8009_16445del8437 and g.[9777_9814del38; 9911-?_14402del] and the known nonsense mutation c.367C>T (p.Arg123Term). The two gross rearrangements ablated TAZ exons 6 to 11 and probably originated by non-allelic homologous recombination and by Serial Replication Slippage (SRS), respectively. The identification of the breakpoints boundaries of the gross deletions allowed the direct detection of heterozygosity in carrier females.

Conclusions

Lactic acidosis associated with 3-methylglutaconic aciduria is highly suggestive of BS, whilst the severity of the metabolic decompensation at disease onset should be considered for prognostic purposes. Mutation analysis of the TAZ gene is necessary for confirming the clinical and biochemical diagnosis in probands in order to identify heterozygous carriers and supporting prenatal diagnosis and genetic counseling.

Mitochondriale Erkrankungen

Mitochondriale Erkrankungen sind – wie auch die übrigen Artikel dieses Schwerpunkthefts verdeutlichen – ein weites Feld in der klinischen Genetik. Aufgrund des bunten klinischen Bildes sowohl bei Kindern als auch bei Erwachsenen ergeben sich relativ häufig Konstellationen, in denen auch eine mitochondriale Erkrankung in das differenzialdiagnostische Spektrum mit einbezogen wird. In der genetischen Beratung stellt dies eine besondere Herausforderung dar, da unter dieser Verdachtsdiagnose grundsätzlich alle Erbgänge, inklusive einer maternalen Vererbung, möglich sind und gleichzeitig eine zielgerichtete Diagnostik zum molekulargenetischen Beleg oder auch Ausschluss der Diagnose durch die außerordentliche genetische Heterogenität erschwert ist. Im Folgenden erläutern wir ein rationales Vorgehen in der molekulargenetischen Diagnostik und besondere Aspekte der genetischen Beratung.