Clinical manifestations

Presentation and course

Most children with Barth syndrome are hypotonic at birth and have clinical signs of cardiomyopathy in the newborn period or within the first few months of life. Barth syndrome has also been shown to play a significant role in fetal miscarriage and stillbirth, most likely as a consequence of cardiac complications (82). Severe neonatal lactic acidosis may occur (05). Occasionally, a patient will present with only neutropenia in the first year or even later. In a particular extended pedigree, an adult with typical biochemical features of the disease had a history of only moderate growth retardation and hypotonia during early childhood (44). Spencer and colleagues reported that 58% of patients under the age of 18 were below the fifth percentile for weight and at or below the fifth percentile for height (79).This is supported by Barth Syndrome Foundation Registry data in which 50% of Barth syndrome patients between the ages of 27 and 36 months have a weight that roughly overlaps the third percentile on a normative curve (66). Fifty percent of Barth syndrome patients between the ages of six and 36 months also roughly overlap the third percentile for height. This indicates a significant growth delay throughout childhood compared to the normal population. Constitutional growth delay in Barth syndrome boys is accompanied by delayed bone age, which can vary between eight months and two years six months (19). Height and weight remain below or parallel to the third percentile until the pubertal years, when growth accelerates rapidly (78). Patients may even exceed mid-parental height. This rapid growth may be coupled with worsening cardiac function and emaciation as food intake is not sufficiently increased to cope with amplified metabolic demands (19).

Almost all children with Barth syndrome have clinically significant muscular hypotonia and weakness, increased exertional fatigue, and exercise intolerance. Delayed gross motor milestones, myopathic facies, a waddling gait, and a positive Gowers sign are common. In addition to impaired balance, motor reaction time is deficient (37). Muscle mass in some children is substantially reduced and contributes to the appearance of failure to thrive. Although affected children usually have normal intelligence, some have learning disabilities, especially in the areas of visual-spatial and arithmetic reasoning. Other common school problems include rapid fatigue during sustained fine motor activities, such as handwriting (51).

Cardiac disease in Barth syndrome is typically a dilated cardiomyopathy with a partial component of myocardial thickening. In 70% of patients, cardiomyopathy was diagnosed before the age of one year, and in 12% of patients, was severe enough to require cardiac transplantation (66). The cardiac disease is most often identified at birth or in the first months of life. Dilated cardiomyopathy may suddenly become manifest during a febrile disease, masquerading as viral cardiomyopathy. In children with longstanding cardiac disease, endocardial fibroelastosis is not uncommon. Hypertrophic cardiomyopathy has also been reported in several instances, with hypertrophic cardiomyopathy and dilated cardiomyopathy even being reported within a single family (19). In rare cases heart failure can fluctuate unpredictably between hypertrophic cardiomyopathy and dilated cardiomyopathy within individual patients, termed an “undulating” phenotype. Patients may recover from severe cardiac failure during infancy and childhood. In others, the disease may be progressive, often showing a biphasic profile with progressive worsening following apparent recovery in mid-childhood. Cardiac transplantation is necessary in some cases. Another hazard is the sudden onset of cardiac arrhythmia, sometimes complicated by sudden death due to ventricular arrhythmia (80). A prolonged QTc interval may be found on routine EKG. This necessitates frequent cardiac monitoring. An intracardiac device may become necessary to manage this complication. Cardiac symptoms may appear late, and one patient was not diagnosed until the acute onset of cardiac failure at 10 years (95).

Cardiac transplantation has been done, and it appears that Barth syndrome offers no limitation to its success rate. Ages at transplantation vary from infancy to young adulthood (19). Despite the apparently increased risk of infection due to neutropenia, a so-called Berlin Heart EXCOR^® has been successfully applied to a 3-year-old child, bridging the period to heart transplantation (23; 03). The mild end of the spectrum shows that 5% of Barth syndrome patients present without signs of cardiomyopathy (66).

The combined impact of diminished cardiac output and muscle weakness has been studied by Spencer and colleagues who concluded that severe exercise intolerance in Barth syndrome is due to both cardiac and skeletal muscle impairments (79). This decrease in cardioskeletal muscle bioenergetics can be quantified by physiologic means (07). Oxygen uptake is also severely limited during times of peak exercise (‘VO2peak’), although this deficit does not appear to progress as patients grow into adulthood (15).

Some patients present with left ventricular noncompaction (10). This presents on cardiac ultrasound as a spongy structure of the left ventricular myocardium with deep trabeculae, possibly representing a persistent stage of fetal development. Left ventricular noncompaction may be seen in other genetic cardiomyopathies as well.

For many affected children, neutropenia can be as serious and life-threatening as the cardiac disease. Indeed, almost half of the patients examined by Barth and colleagues died of infections rather than heart disease (05). Severe chronic neutropenia (with multiple absolute neutrophil counts < 0.5 x 109/L) can be present, and some patients will have intermittent counts as low as zero. Neutropenia can be chronic, cyclic, or intermittent, though it is most commonly intermittent and extremely unpredictable (19). Neutropenic periods can leave patients vulnerable to recurrent bacterial infections, including respiratory infections, dermatological infections, and infections of the mouth and gums leading to soreness and ulcers; septicemia and lethal infection can also occur. There is also usually a "compensatory" monocytosis, which may explain why a number of children with Barth syndrome have few bacterial infections despite their severe neutropenia. However, the partial nature of the defective myelopoiesis is clear from the observations that once a bacterial infection occurs, children with Barth syndrome often develop an exuberant leukocytosis, and respond well to granulocyte colony-stimulating factor or granulocyte-macrophage colony-stimulating factor.

Typical facial features were defined in a sample of 12 patients with high forehead, round face, prominent cheek during infancy, and a gynoid stature and fat distribution in puberty and adolescence (32; 19).

Survival has long been thought to be short. Due to the work of the Barth Syndrome Foundation, an organization founded by parents, long-term studies have revealed many more affected males are surviving into adulthood (06; 19). The oldest living patient is currently 55 years of age.

Because Barth syndrome is an X-linked disease, the heterozygous state in women may theoretically lead to mild symptoms of the disease comparable to some other X-linked disorders (such as X-linked adrenoleukodystrophy). However, heterozygous clinical manifestations are not on record in this disease and survival in heterozygotes appears unaffected. X-inactivation studies by Orstavik and colleagues in tissues from female heterozygotes have shown selection against the allele carrying the mutation in the majority (58). This offers a good explanation for the lack of clinical symptoms in heterozygotes. An exceptional case study mentions Barth syndrome in a baby girl with clinical cardiomyopathy, cyclic neutropenia and multiple complex respiratory chain impairment (20). Genetic analyses confirmed the diagnosis of Barth syndrome by showing a large intragenic deletion of exons 1 through 5 in the TAZ gene. Cytogenetic analysis showed mosaicism for monosomy X and for a ring X chromosome with a large deletion of the long arm, including the Xq28 region. Cardiolipin analysis revealed a decreased monolysocardiolipin/cardiolipin. TAZ mutation analysis showed a large intragenic mutation involving the first 5 exons.

Important statistics on a larger patient group were provided in the study of Spencer and colleagues (78). Clinical findings in a group of 34 patients of 1.2 to 22.6 years were as follows: 90% of the patients had a cardiomyopathy with the mean age at diagnosis of 5.5 months. Echocardiographic examination revealed cardiac dilatation and reduced functioning. Left ventricular morphology demonstrated increased trabeculations or true noncompaction in 53%. Of 16 patients who were evaluated at or over 11 years of age, seven (43%) had documented ventricular arrhythmia. Moderate growth deficiency was present. 25% had low total blood leucocyte count. Hypocholesterolemia was present in 24%, decreased LDL cholesterol in 56%, and low prealbumin in 79%. Mildly increased creatine kinase was present in 15%.

In a cohort of 22 patients, all patients had increased excretion of 3-methylglutaconic acid (65). Vernon and colleagues determined 3-MGA in plasma in controls and patients and found significant increase in the latter without overlap: the average 3MGC level in affected patients was 1088 nmol/L ± 435 (range 393 to 2326) (91). Normal values for this substrate are 162 ± 68 (SD) nmol/L. A decrease of plasma arginine was observed in two cohorts (65; 91).

Clinical and growth data in Barth syndrome were recorded from a large group of 73 subjects enrolled in a follow-up study by Roberts and colleagues (66). Growth curves showed decreased weight and height, with a late catch-up. Reasons for hospitalization beside cardiac complications included various infections, including diarrhea and dehydration. Metabolism is altered in patients. Rarely, but significantly, 4% suffered from periods of hypoglycemia. The glucose turnover rate is increased in patients during rest, as well as during exercise and post-exercise periods; fat metabolism is decreased (14). Lymphocytes of affected patients demonstrate significantly increased glucose transport and utilization, resulting in hypoglycemia (53).

An intermediate form of the condition has been discovered. Following measurement of leukocyte mature cardiolipin (L4CL/CL4) levels in 156 controls and 34 confirmed, TAZ-mutation positive Barth syndrome patients, a subcategory of seven Barth syndrome patients from three unrelated families was identified. These patients had L4CL levels that were at the lower end of the control range, but had a skewed MLCL:L4CL ratio, which was significantly higher than controls, but lower than other Barth syndrome patients (13). These seven patients appeared to have a phenotype that was significantly less severe than other Barth syndrome patients. Although some symptoms were typical of Barth syndrome, eg, cardiomyopathy (five patients), male infant deaths (one family), growth delay (three patients), and 3-methylglutaconic aciduria (five patients), many aspects were unusual. None of the seven patients had persistent neutropenia, two of the patients were adults who were completely asymptomatic, and five patients had normal exercise tolerance.

Prognosis and complications

Today, the prognosis for Barth syndrome appears to be more favorable overall than suggested by the original family of Barth and colleagues (05). Experience suggests that with good cardiac management, at least 75% of patients who are attended closely will show gradual improvement and even normalization of their cardiac function. However, it is also possible that differences in intrinsic severity among pedigrees may make significant morbidity and mortality more common in some pedigrees (05; 04) and rare in others (44; 18). The American Heart Association has released a scientific statement for cardiac management in patients with associated neuromuscular disorders, highlighting their clinical heterogeneity and need for tailored approaches (27). One study emphasizes the risk of cardiac arrhythmia, which may lead to sudden death (80; 78). Another risk is deterioration of cardiac function during puberty or adulthood, even leading to the need for cardiac transplantation. Once neutropenia is recognized, its complications are largely preventable by close monitoring of the patients and early use of antibiotics. However, the partial nature of the defective myelopoiesis is clear from the observation that once a bacterial infection occurs, children with Barth syndrome often develop an exuberant leukocytosis. Granulocyte counts also respond well to granulocyte colony-stimulating factor. However, many other patients with severe neutropenia do not have recurrent infections, possibly because of a chronic, substantial monocytosis.

It is evident that patients and families face challenges at numerous levels - physical, social, and emotional. Levels of experience and individual resilience are variable (50).

Clinical vignette

The male patient belongs to the same family previously described (05). He had a normal, full-term birth, and normal growth and motor development in the first three months. Routine investigation occasioned by the family history at three months revealed neutropenia, right ventricular hypertrophy on EKG, and normal echocardiographic findings. Metabolic screening revealed increased excretion of 3-methylglutaconic acid and 3-methylglutaric acid. Plasma arterial lactate and free l-carnitine were normal. At the age of six months, cardiac function had declined with left ventricular dilatation and reduced shortening fraction between 20% and 30%, and arterial lactate had risen to 4.08 mmol/l (normal is less than 2.2 mmol/l). He was treated orally with l-carnitine 100 mg/kg, digoxin, and vitamin E. Cardiac dysfunction became stable, his growth declined with length at the 50th percentile during the first year and was at two standard deviations below normal at the age of two years. He started to walk unsupported at two years, showing mild proximal neuromuscular weakness with a positive Gowers phenomenon and inability to run. Extraocular and bulbar muscles were normal. Follow-up at the age of nine years revealed cardiomyopathy with gallop rhythm, decreased left ventricular functioning, a shortening fraction of 22% (normal 30% to 40%) dentogenic infections associated with persistent neutropenia, persistent excretion of 3-methylglutaconic acid, 3-methylglutaric acid, 2 ethylhydracrylic acid, and decreased total serum cholesterol: 1.77 mmol/l. When he was 10 years old, he died suddenly from ventricular tachycardia and cardiogenic shock. Cardiac resuscitation was attempted, but to no avail.

DNA from fibroblasts showed a Y51X nonsense mutation in exon 2 of the TAZ gene, and cardiolipin studies revealed decreased cardiolipin pool and a remodeling defect (92).

Biological basis

Etiology and pathogenesis

Barth syndrome is an X-linked disorder due to mutations in the TAZ gene, which encodes an enzyme activity responsible for the final step in the biosynthesis of mature cardiolipin. Cardiolipin is a phospholipid that is primarily localized in mitochondrial membranes and has important properties required for energy production, but also for a crucial step in mitochondrial apoptosis. Analysis of the TAZ genes of 91 unrelated patients with Barth syndrome revealed 73 different disease-causing mutations in all exons (31). The disease is associated with deficiency and increased turnover in cardiolipin (92), causing multiple abnormalities in the mitochondrial respiratory chain. Cardiolipin is composed of two phosphatidyl moieties linked through their phosphate groups to a central glycerol group (72). The C1 and C2 of each phosphatidyl moiety carry acyl side chains. Initially, during the biosynthesis of cardiolipin, these are predominantly oleate chains (C18:1), to be replaced by linoleate (C18:2) in the final product of most, but not all tissues.

All enzymes involved in de novo synthesis of cardiolipin are localized in the inner mitochondrial membrane. In the next step following cardiolipin synthesis, tafazzin regulates the remodeling of its 4 side chains. Tafazzin is localized on the outer side of the inner and the inner side of the outer mitochondrial membrane in S cerevisiae (101). Limitation of the number of different acyl species in cardiolipin to 1, the structure being dependent on the organ, is now regarded as the essential function of the remodeling step in cardiolipin biosynthesis that is impaired in Barth syndrome (70; 71). Also, the total amount of cardiolipin is decreased. This has been confirmed in myocardial muscle, skeletal muscle (73), thrombocytes (73; 87), cultured skin fibroblasts (88), and lymphoblasts (99). Present evidence confirms the enzymatic activity of tafazzin as a transacylase exchanging acyl groups between cardiolipin and monolysocardiolipin (71). Deficiency of cardiolipin (CL) and excess of monolysocardiolipin (MLC), resulting in an increased MLC/CL ratio, provides a sensitive base for testing (38).

The core biochemical defect in Barth syndrome is an abnormality in the metabolism of cardiolipin, a large complex phospholipid, affecting the composition of its side chains. Cardiolipin is the signature phospholipid of mitochondria, which is predominantly localized on the inner mitochondrial membrane where it constitutes 10% to 20% of the membrane lipids. This site contains the respiratory chain complexes. Proper functioning of the respiratory chain requires the presence of cardiolipin. Without cardiolipin, the electron transport system is destabilized, and the resultant oxidative stress is thought to lead to myopathy (42).

Deficiency of tetralinoleoylcardiolipin (L4CL) in several tissues was established as the typical biochemical effect of Barth syndrome in cultured fibroblasts and skeletal muscle (92; 73). The cardiolipin molecule has four acyl groups that are remodeled in the final step of the biosynthesis of cardiolipin. Remodeling involves the removal of one side chain from CL, rendering monolysocardiolipin and its replacement with another acyl-group from another phospholipid molecule, which acts as acyl-source in this pathway. The biosynthetic route of cardiolipin is known (33; 72). The deficiency of L4CL is caused by a defect in the normal remodeling of cardiolipin acyl groups, resulting in deficiency of cardiolipin with four linoleyl groups, the predominant cardiolipin species found in most tissues of normal persons, together with a severely diminished total concentration of cardiolipin and an excess of monolysocardiolipin. Later studies established that the tetralinoleoyl species of cardiolipin is the most prevalent cardiolipin in heart, skeletal muscle, thrombocytes, and cultured fibroblasts, but there are exceptions to this rule in some other tissues, such as lymphoblasts.

The source of linoleic acid used in remodeling of cardiolipin is not free linoleic acid but another phospholipid, such as phosphatidylcholine (PC) and phosphatidylethanolamine (PE) (98; 99). It was further shown that acyl species different from linoleic acid may be involved in different human tissues and in different species (70; 71). Their conclusion is that the normal remodeling process limits the number of acyl species, resulting in a symmetric overall structure rather than incorporating one specific type of acyl group for all tissues and species.

Deficiency of cardiolipin in the inner mitochondrial membrane causes dysfunction of the mitochondrial respiratory chain, resulting in multicomplex deficiency. A reduced mitochondrial membrane potential and proliferation of mitochondria was found in lymphoblasts from patients with Barth syndrome, although ATP production was found to be normal. The normal ATP production in these experiments might be explained by mitochondrial proliferation, compensating for reduced activity (99). The accumulation of monolysocardiolipin could be demonstrated in various tissues in patients with Barth syndrome. Interest has focused on the mechanism by which cardiolipin “protects” the respiratory chain. Stability of interaction between respiratory chain complexes was found to be impaired in lymphoblast mitochondria from patients with Barth syndrome (52). Increasing interest became focused on two functional aspects of cardiolipin:

Although cardiolipin is a component of all mitochondria, the effect of cardiolipin deficiency through TAZ mutations varies in different tissues. Experimental tafazzin deletion has shed light on this aspect. Tissues with a high packing density of mitochondrial cristae, such as differentiating mouse cardiomyocytes and flight muscle of Drosophila, showed increased susceptibility to morphological changes compared to mitochondria with less cristae packing density, as demonstrated by electron tomography in experimental tafazzin deficiency (01).

Although tafazzin acts as a transacylase, altering its acyl-chain composition by exchange with other phospholipids, it is not clear why the final product should be tetralinoleoyl (L4CL) cardiolipin rather than a random mixture of acyl-chains of different lengths and saturation. It was found, however, that tafazzin promotes the packing of lipid bilayers in curves, thereby favoring their normal structure (69; 68). The latter finding also offers a clue to the severe structural alterations in mitochondrial cristae affected by Barth syndrome, especially their abnormal packing density. Abnormal morphology and aggregation of mitochondrial inner membranes is a hallmark of Barth syndrome. The other findings, summarized by Schlame and colleagues, are reduced membrane potential, reduced stability of respiratory supercomplexes, release of cytochrome c and stimulation of apoptosis in myeloid progenitor cells, and resistance to Fas-induced extrinsic pathway of apoptosis (68). Cardiolipin is also required for the synthesis of iron-sulfur cofactors. Cardiolipin has also been found in the peroxisomal membrane (59).

Developments in genetic engineering allow the reprogramming of somatic cells to become pluripotent stem cells. Induced pluripotential stem cells (iPSCs) can be transformed into cardiomyocytes (iPSC-CMs). Induced pluripotential stem cells transformed into cardiomyocytes with TAZ deficiency can be obtained from fibroblasts of patients or from control fibroblasts with induced mutations. The effects can be studied in vitro. Induced pluripotential stem cells generated from patients with Barth syndrome transformed into cardiomyocytes provided the means to study the impact of the mutated TAZ gene in vitro on the most sensitive tissue involved (“heart-on-chip)”. This approach to the pathogenesis of Barth syndrome by Wang and colleagues provided several new unequivocal results (93). The procedure allowed the formation of highly enriched cultures of cardiomyocytes (iPSC-CMs) that were TAZ deficient and could be studied for the effects of the gene defect through direct observations in vitro. Sarcomere assembly and myocardial contraction abnormalities occurred despite normal ATP levels, proving that deficient sarcomere assembly was not the single result of deficient energy levels. Also, reactive oxygen species were found to be produced in excess. Compared to controls Barth syndrome iPSC-CMs had a proton leak across the inner mitochondrial membrane with decreased F1F0 ATP synthase activity, ultimately leading to ATP depletion (60; 93). The procedure also allowed for the preliminary testing of substitution therapies with small molecules such as the deficient amino-acids arginine and cysteine and linoleic acid. Some promising results were obtained with linoleic acid. Linoleic acid addition corrected the mitochondrial metabolic abnormalities, probably by substitution of the acyl chains of cardiolipin.

The pathogenesis of neutropenia is still poorly understood. Studies by Kuijpers and colleagues showed Annexin V binding to neutrophil and eosinophil cell membranes, indicative of serine on the outside of the membrane, a stage preceding apoptosis (45). Directed motility and killing activity of neutrophils was normal. No other signs of apoptosis were found. In a subsequent paper, van Raam and Kuijpers speculated that a mitochondrial mechanism, ie, reactive oxygen species, could be at the basis of the neutropenia (89). Makaryan and colleagues developed an in vitro model of Barth syndrome by transfecting human HL60 myeloid progenitor cells with TAZ-specific shRNA (short hairpin RNA), resulting in significant downregulation in TAZ expression (49). Annexin V binding was increased (analogous to the previous study by Kuijpers and colleagues), and mitochondrial membrane potential was decreased. Also, a decrease of cytochrome c from mitochondria and elevated levels of activated caspase-3 were found, indicating increased apoptosis of mitochondrial origin as the basic mechanism underlying neutropenia in Barth syndrome.

Another enigmatic finding in Barth syndrome, the moderate reduction in plasma cholesterol, may be due to decreased synthesis, as shown in lymphoblasts from Barth syndrome patients and controls (34). In the patients, biosynthesis from the labeled pyruvate and acetate precursors was diminished. The investigators found a decrease in the mRNA of the key cholesterol biosynthesis enzyme HMGCoA reductase.

The defective gene in Barth syndrome is the tafazzin (TAZ) gene, located on the long arm of the X chromosome. The TAZ gene includes 11 exons (08). Pathogenic mutations have been found in all exons (31). Two splice variants of the TAZ transcript translate to enzymatically active products in humans: full-length tafazzin and a copy lacking exon 5 (Delta5). It is not yet clear whether they are functionally different. Expression studies of human tafazzin in Drosophila have shown that the acyl composition of cardiolipin is symmetric, but length and saturation are dependent on the host and tissue environment (100). Taz deficient knockdown mouse models have been developed that closely mimic the human condition (02; 77).

Epidemiology

Barth syndrome is rare, with approximately 230 to 250 cases reported worldwide (54). The current estimated prevalence worldwide is one per one million males (54).

The condition has been reported in many different ethnic groups and does not appear to occur more frequently in any group.

In the United States, only 58 patients are registered, suggesting that the disorder is greatly underdiagnosed (54). With fewer than 10 new cases diagnosed each year in the United States, the incidence may be around one per 300,000 to 400,000 live births. On average, two new cases are recognized in the United Kingdom each year, from around 700,000 live births, suggesting a similar incidence. Because a metabolic evaluation of a pediatric cardiomyopathy (including urinary organic acid analysis) has become routine in many pediatric centers, ascertainment is improving. Awareness of Barth syndrome is also improving due to the work of the Barth Syndrome Foundation, a nonprofit organization of parents and scientists.

Prevention

Prevention of Barth syndrome is through genetic counseling as well as prenatal diagnosis of affected fetuses. Precise prenatal diagnosis is possible by mutation analysis of the TAZ gene. Ideally, the site of the mutation in the family at risk should be known at the time of referral for antenatal diagnosis. The first step should be identification of the gender of the fetus, followed by mutation analysis in the case of a male fetus.

Differential diagnosis

The metabolic differential for a dilated cardiomyopathy in combination with skeletal myopathy in children includes defects of fatty acid beta-oxidation and various other disorders of mitochondrial oxidative metabolism, such as mitochondrially inherited DNA mutations (24). However, many of these disorders characteristically have a hypertrophic rather than dilated cardiomyopathy, and are not associated with neutropenia. One novel TAZ mutation causes mitochondrial respiratory chain abnormalities without cardiomyopathy, indicating that clinical features alone are insufficient to diagnose Barth syndrome (11).

In the large majority of patients, organic acid analysis of the urine reveals an increased excretion of 3-methylglutaconic acid. Increased excretion of 3-methylglutaconic acid is due to either a defect in the catabolism of leucine, known as 3-methylglutaconyl-CoA hydratase defect, or a group of mitochondrial disorders. According to a proposal by Wortmann and colleagues, the mitochondrial group is broken down into two subgroups: (1) an association with defective phospholipid remodeling, which includes Barth syndrome and SERAC1 defect or MEGDEL syndrome, and (2) mitochondrial membrane–associated disorders, which include OPA3 defect or Costeff syndrome; DCMA syndrome, a dilated cardiomyopathy with cardiac conduction defects and ataxia (DCMA) syndrome found in a Canadian Hutterite population; and TMEM70 defect, which also features (hypertrophic) cardiomyopathy, dysmorphic features, cataracts, and lactic acidosis (22; 36; 96). DCMA syndrome (DNAJC19 mutation) was found to result in altered cardiolipin acylation similar to Barth syndrome, but no increase of monolysocardiolipin (64).

X-linked dilated cardiomyopathy has been reported in adults in several pedigrees. Mutations of the Duchenne-related dystrophin gene have been found in some of these pedigrees (55).

Diagnostic workup

The diagnosis of Barth syndrome should be considered for any male child who presents with dilated cardiomyopathy, neutropenia, or idiopathic myopathy with growth retardation. It is also possible that the diagnosis will be made for the first time in adults (75). In all cases, a complete family history should be obtained, looking for possible cases of cardiac disease, failure-to-thrive, unexplained infantile deaths, and unexplained sudden deaths with pedigree analysis supporting X-linked inheritance. It is customary to investigate patients with dilated cardiomyopathy by urinary organic acid analysis, but multiple cases of false negative diagnosis have occurred, and measurement of L4CL:MLCL ratio is advisable.

Cardiolipin content analysis can be performed on either platelets or lymphocytes. More specificity is gained by determination of monolysocardiolipins, the intermediates in the remodeling of cardiolipin in lymphocytes (86; 38) or fibroblasts (90). A method was developed for the determination of cardiolipin in blood spots (46). A method using leukocytes for the quick determination of the monolysocardiolipin/cardiolipin ratio (MLCL/CL4) (increased in Barth syndrome) is reported by Bowron and colleagues (12). The ranges of MLCL/CL4 between patients and controls do not overlap. Bowron and colleagues identified a group of mutation proven Barth syndrome patients having an intermediate ratio between controls and the previously established range and a tendency to milder disease expression (13). Further proof of diagnosis can then be obtained by finding a pathogenic mutation in the TAZ gene. A database of all reported mutations (more than 150) is available at www.barthsyndrome.org.

Predominant muscle involvement and lactic aciduria may lead to muscle biopsy and a workup for respiratory chain disorders. The biopsy will show moderate accumulation of neutral fat, and biochemical evaluation may reveal involvement of two or more complexes. This finding can also be caused by mitochondrial DNA mutations, including the MELAS type or by Barth syndrome (24).

Neutropenia may be permanent, erratic, or cyclic. A single determination may, therefore, miss it, and two or three determinations at weekly intervals may yield a more reliable result. Due to the extreme unpredictability of occurrence of neutropenia in many patients, it is sometimes best to perform blood counts on multiple occasions when patients have mouth ulcers, sore gums, skin spots, or exacerbation of lethargy and fatigue because these can be indicative of accompanying neutropenia. Neutropenia has not been detected in one sixth of patients, including some who have had more than 50 blood counts performed over many years (Steward et al unpublished data).

Screening of the family for TAZ mutations should include the mother, sibs, and matrilineal relatives. Most of the mutations found in affected males are also identified in their mother’s somatic DNA, with only 13% of known mutations being de novo (19). These new mutations may either be sporadic or the result of gonadal mosaicism, explaining the potential finding of multiple affected males within a family in which blood DNA testing suggests that a mother is not a carrier (17).

Management

Cardiac failure due to cardiomyopathy can be well-controlled in many patients by conventional medication (including cardiac glycosides, diuretics, angiotensin-converting enzyme inhibitors, and beta-blockers). However, some patients will fail this therapy, either by not coming under control or by relapsing, and in the most completely ascertained cohort in the United Kingdom, approximately 25% of patients required cardiac transplantation (19). This figure is higher than the 14% reported to the Barth Syndrome Foundation USA Registry, but ascertainment bias may contribute to this difference because a national service for Barth syndrome has been in place in the United Kingdom since 2010. Monitoring for arrhythmia should be undertaken and, if present, may necessitate placement of an implantable cardioverter defibrillator (80). It is arguable that Barth syndrome patients should be kept on lifelong cardiac medication, even if conventional echocardiographic parameters of left ventricular function normalize, because speckle tracking studies show persistence of ventricular strain abnormalities on longitudinal follow-up in all patients (43).

Decreased myeloid maturation in bone marrow may be corrected by the administration of G-CSF (81). In fact, Barth Syndrome Foundation registry data indicate that 49% of patients had been on G-CSF in order to manage their neutropenia, and 27.5% were still on continued treatment (66). G-CSF is typically started as a dose of 2 to 3 µg/kg/dose injected subcutaneously two to three times weekly, depending on the severity of neutropenia (19). This may be continued long-term (with the dose adjusted according to neutrophil counts following administration) or be given only to cover periods of infection or severe mouth ulcers. Patients generally respond exceptionally well to G-CSF, with neutrophil counts rising significantly and neutropenic symptoms and bacterial infections being alleviated. However, there can be issues in achieving an optimal dose of G-CSF, which is due to the marked variability of neutrophil counts that characterize the untreated disease. The aim is, therefore, to increase average neutrophil count rather than to permanently raise it into the normal (19). Preventative antibiotics are also given to neutropenic patients, either alone or alongside G-CSF, to further prevent bacterial infections. Patients with indwelling devices are at particular risk (81). When patients have been shown to be neutropenic at some stage, it is advisable to initially treat any fever as a suspected episode of febrile neutropenia with urgent hospital review. It is hoped that this strategy will avoid some of the devastating septic episodes that have historically affected patients.

Growth delay and skeletal myopathy seen in Barth syndrome patients are difficult to treat. The role of growth hormone treatment is debatable. Several patients have been shown to have growth hormone deficiency and have been reported to respond well to growth hormone therapy (19). However, it is conceivable that this was a consequence of their constitutional growth delay and that growth hormone levels would have subsequently normalized spontaneously. Given the delays in growth, dietary composition is important in growing children. However, this is complicated by altered taste sensitivity (25). It has been hypothesized that arginine supplements may help with muscle growth and strength, and some boys take supplements of arginine or citrulline, as the latter can be converted to arginine (62). This followed the discovery that reduced plasma arginine levels appear to be a feature observed in a considerable fraction of Barth syndrome patients, which may, in turn, limit protein synthesis in the boys (65). However, the use of arginine has not yet been subjected to clinical trial. Physiotherapy may help with muscle weakness, fatigue, and reaching motor milestones (40). Cornstarch may also be given in the evening to try and avoid muscle wasting while fasting overnight (19). Resistance exercise training has been successful in increasing muscle strength in a small cohort (N=9) of patients (09). Occupational therapy may be required, even for seemingly routine efforts like swallowing pills (63).

Although there is no specific therapy for Barth syndrome, there is interest in performing clinical trials of the lipid-lowering drug, bezafibrate. When tested on both TAZ-knockdown Barth syndrome mice and human Barth syndrome fibroblasts, bezafibrate was shown to improve (but not normalize) the L4-CL:MLCL ratio. In the intermediate form of Barth syndrome, a reduced abnormal MLCL:L4CL ratio appeared to correspond with a less severe disease phenotype (13). It is, therefore, hoped that bezafibrate therapy could lead to a reduction in severity of symptoms and so result in an improved quality of life. Continuing work in knockdown mice suggests that this may be the case (61).

A number of attempts at therapy have been made in knockdown mice. Experimental gene therapy to restore tafazzin function is in early stages (83). Attempts to limit oxidative stress and prevent cardioskeletal myopathy experimentally have not been successful to date, nor has exogenous administration of cardiolipin been successful in improving the cardiolipin profile (39; 42). Adeno-associated TAZ transduction in patient fibroblasts has led to the recovery of structure and function of mitochondria, providing further support for future gene therapy (84).

Special considerations

Pregnancy

Evidence suggests that Barth syndrome is an underrecognized cause of fetal miscarriage and stillbirth, primarily due to cardiac complications (82). In one study, for example, TAZ mutations were identified in four of 114 (3.5%) male patients with primary cardiomyopathy (age range one to 20 months) (94). Prenatal diagnosis based on mutation analysis of the TAZ gene is now available. Heterozygous women do not appear to have different or more frequent pregnancy complications than women not at risk for Barth syndrome.

Anesthesia

There are no known anesthetic complications associated with Barth syndrome.