Barth syndrome is an X-linked, recessive multisystem disease in men, usually diagnosed in infancy and characterized by growth retardation (reduced weight, short stature), dilated cardiomyopathy, endocardial fibroelastosis, noncompaction, arrhythmias, exercise intolerance, mitochondrial myopathy, 3-methylglutaconic aciduria and cyclic neutropenia [1▪,2]. More rare phenotypic features include hypercholesterolaemia, monocytosis, low prealbumin, low plasma carnitine and lactacidosis [3,4]. Barth syndrome is caused by loss-of-function mutations in the TAZ gene (G4.5) on chromosome Xq28, which encodes for tafazzin, a phospholipid transacylase involved in the remodelling of cardiolipin, which is located on the inner mitochondrial membrane [1▪] and necessary for proper functioning of the electron transport along the respiratory chain . Accordingly, TAZ mutations cause loss of cardiolipin, particularly its tetra-linoleoyl form, at the inner mitochondrial membrane, resulting in respiratory chain dysfunction [1▪,4]. This review focuses on recent advances concerning the pathogenesis, clinical presentation, diagnosis and treatment of Barth syndrome with particular regard to the haematological abnormalities.
Among various phenotypic manifestations of Barth syndrome mentioned above, the facial appearance has been largely ignored. In a study of boys  with Barth syndrome, characteristic facial dysmorphology, such as tall and broad forehead, prominent chin, full cheeks, large ears and deep-set eyes, has been found. In a study  of the sensory processing and sensory responsiveness in 21 boys with Barth syndrome, it turned out that these patients had a strong gag reflex since the early stages, a strong preference for salty, cheesy and spicy foods, a restricted repertoire of foods they eat (picky eaters) and impaired auditory sensitivity and auditory filtering. Children reported having difficulties with filtering out sounds from the environment or being bothered by loud sounds . These findings indicated that patients with Barth syndrome also develop sensory modulation disorder (SMD) characterized by sensory nonresponsiveness, sensory over-responsiveness or sensation seeking or craving .
When studying the neurocognitive performance of 19 patients with Barth syndrome by means of the Test of Early Mathematics Ability and standardized measures of intelligence quotient (IQ), visual perception and vocabulary, it turned out that preschoolers and kindergartners with Barth syndrome had age-appropriate maths, vocabulary, spatial and IQ scores, except for lower maths scores in kindergartners . Maths difficulties in Barth syndrome appear to emerge by kindergarten but do not include early number sense. Executive functions may underlie or may mediate maths performance in Barth syndrome .
Although cardiomyopathy and myopathy are the most frequent presenting manifestations of Barth syndrome, single patients present with neutropenia at onset . In a 17-year-old boy, neutropenia was first diagnosed at an age of 6 weeks, when he presented with pneumonia . The white blood cell count at that time was 6.3 × 103/μl with 2% neutrophils and 7% bands (absolute neutrophil count of 441). Subsequently, he had been repeatedly hospitalized for fever, each time associated with neutropenia . Although diagnostic workup, including bone marrow biopsy, had been negative to date, he began receiving biweekly injections of granulocyte colony stimulating factor with a beneficial effect . Generally, Barth syndrome is no longer regarded as a lethal infantile disease because the age of patients living with Barth syndrome ranges between 0 and 49 years and peaks around puberty . However, mortality is still highest during the first 4 years of life .
As Barth syndrome is an X-linked disease, women are usually not affected due to the presence of one compensatory wild-type gene. Recently, however, the first manifesting female carrier has been reported [1▪]. The female infant developed severe dilated cardiomyopathy already at age 1 month. Cyclic neutropenia and noncompaction were also present. Activity of complexes I, III and IV of the respiratory chain was decreased in skin fibroblasts [1▪]. Barth syndrome in this girl was due to a deletion of exons 1 to 5 of 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 [1▪]. The patient developed recurrent episodes of heart failure, progressive muscle weakness and a fatal septic shock due to neutropenia at age 3 years [1▪].
The pathogenesis of neutropenia in Barth syndrome is largely unknown, but recently, new conflicting results have emerged. Phosphatidylserine is a phospholipid of the inner mitochondrial membrane, the exposure of which is generally regarded as a marker of apoptosis. When investigating the neutrophil function in patients with Barth syndrome, directed motility and killing activity of neutrophils was found normal in seven patients . In contrast, circulating neutrophils and eosinophils, but not monocytes or lymphocytes, showed annexin-V binding, suggesting phosphatidylserine exposure due to apoptosis . Caspase activity, however, was absent in neutrophils, shape and mass of mitochondria were normal, there was neither clustering of mitochondria nor Bax translocation upon apoptosis, and there was no phagocytosis of neutrophils with almost no cardiolipin . Early clearance of neutrophils was excluded as the cause of neutropenia in patients with Barth syndrome .
Although neutrophils of patients with Barth syndrome avidly expose phosphatidylserine , they do not show other markers of apoptosis and apparently function normally . In all tissues of patients with Barth syndrome, however, supercomplex organization of the respiratory chain is disturbed resulting in insufficient electron transport along the respiratory chain [11,12]. From their findings, the authors concluded that neutropenia does not result from apoptosis of myeloid precursors or end-stage neutrophils . More likely, reactive oxygen species trigger the exposure of phosphatidylserine, which in turn leads to increased clearance of neutrophils by tissue macrophages .
Previously, it has been shown that supercomplex organization of the respiratory chain is generally disturbed in all tissue cells of patients with Barth syndrome, even in clinically unaffected patients . Disruption of the tafazzin gene impairs assembly and stability of complex IV and its supercomplex form [11,12]. Disturbed supercomplex organization in mitochondria of patients with Barth syndrome was confirmed by another study  showing that the supercomplex I/III/IV was unstable and complex IV more readily dissociated from the supercomplex. In addition, the interaction between complex I and complex III was less stable than normal with decreased levels of supercomplex I/III . Furthermore, the amount of complex I was reduced in patients with Barth syndrome . Accordingly, the electron transport along the respiratory chain was insufficient [11,12].
In a cell model of Barth syndrome, human HL60 myeloid progenitor cells were transfected with TAZ-specific shRNAs [13▪]. Transfection of shRNAs resulted in downregulation of tafazzin expression, mimicking the effect of a truncation mutation in the TAZ gene [13▪]. Flow cytometry of cells carrying shRNAs showed a two-fold increase in the proportion of annexin V-positive cells. In addition, there was increased dissipation of the mitochondrial membrane potential, aberrant release of cytochrome-c from mitochondria and elevated levels of activated caspase-3 in response to the TAZ knockdown [13▪]. Transfection of TAZ-specific shRNS had a similar effect in U937 myeloid cells [13▪]. Application of the caspase-specific inhibitor zVAD-fink resulted in reduction of apoptosis to near normal values [13▪]. It was concluded that neutropenia in Barth syndrome is attributable to increased dissipation of the mitochondrial membrane potential, aberrant release of cytochrome-c, activation of caspase-3 and accelerated apoptosis of myeloid progenitor cells [13▪]. This effect could be restored in vitro by treatment with caspase-specific inhibitors [13▪].
In a mouse model of Barth syndrome, it has been recently shown that knockout of tafazzin results in a significant reduction of tetra-linoleoyl cardiolipin in skeletal and cardiac muscles with consecutive accumulation of monolysocardiolipins and cardiolipin molecular species with aberrant acyl-groups [14▪]. Electron microscopy showed aggregated mitochondria, variability in size and shape, and multiple concentric layers of densely packed cristae with onion-shaped morphology, suggesting degeneration and mitophagy. Mitochondria in cardiac muscle contained patches of swollen tubular cristae and large vacuoles, and mitochondrion-associated membranes, formed from endoplasmatic reticulum membrane vesicles, were observed [14▪].
Studying insulin-like growth factor (IGF)-1 and growth hormone and the inflammatory cytokines interleukin (IL)-6 and tumour necrosis factor (TNF)-alpha by high-sensitivity, enzyme-linked immunosorbent assays in 22 patients with Barth syndrome, IL-6 and the IL-6:IGF ratio were significantly increased in patients with Barth syndrome . Growth hormone was age-dependent in patients with Barth syndrome, with lower levels below 14 years of age and elevated levels above 14 years of age . The TNF-alpha:growth hormone ratio was decreased in patients with Barth syndrome . The authors concluded that inflammatory processes may contribute to the catabolic nature of Barth syndrome . Interactions between growth hormones and cytokines may explain the high phenotypic variability of Barth syndrome .
In a study of the substrate metabolism during hyperinsulinemic conditions in five patients with Barth syndrome, it turned out that patients with Barth syndrome had a lower fat-free mass (FFM), reduced systolic function, increased insulin-stimulated glucose disposal rate per kilogram of FFM, reduced basal and hyperinsulinemic lipolytic rate per kilogram of fat mass and a trend towards increased higher basal leucine rate of appearance per kilogram of FFM compared with controls . The study showed that fatty acids, glucose and amino acid metabolism are impaired in patients with Barth syndrome and apparently related to cardiomyopathy and myopathy .
Histopathological abnormalities on muscle biopsy, so far reported, include increased number of fat droplets in type-I muscle fibres, ultrastructural abnormalities of mitochondria and reduced activity of respiratory chain complexes III (cytochrome c1 + c, cytochrome b) and IV (cytochrome aa3) . Neutrophil bone marrow cells show membrane-bound vacuoles . By electron microscopic tomography of lymphoblasts from patients with Barth syndrome, it has been shown that mitochondrial size is more variable than in controls, and that the mitochondrial volume per cell is increased due to clustering of fragmented mitochondria inside nuclear invaginations . Mitochondria of lymphoblasts from patients with Barth syndrome showed reduced cristae density, reduced cristae alignment and heterogenous cristae distribution . Three-dimensional reconstrucion of mitochondria from patients with Barth syndrome showed areas of adhesion of opposing inner membranes, resulting in obliteration of the inter-cristae space . Extensions of adhesion zones were short or extended, resulting in sheets of collapsed cristae, which are packaged as multiple concentric layers . There may be also tubular structures, which most likely also derive from adhesion zones .
Diagnosing Barth syndrome relies on the history, clinical presentation, blood tests, cardiologic investigations, electromyography, muscle biopsy and the confirmation of a TAZ mutation in blood lymphocytes. The family history may be positive for a paediatric male death of unknown cause . Blood tests may show decreased levels of cardiolipin , particularly tetra-linoleoyl cardiolipin . The monolysocardiolipin:cardiolipin ratio can be reliably measured in blood spots as a screening method for Barth syndrome in neonates . Recently, a confirmatory tetra-linoleoyl cardiolipin high-pressure liquid chromatography–tandem mass spectrometry blood test has become available to confirm 3-methylglutaconic aciduria . The most reliable biochemical marker for diagnosing Barth syndrome is 3-methylglutaconic aciduria . Early diagnosis is essential because it improves survival of patients with Barth syndrome .
Treatment of Barth syndrome is symptomatic and mainly directed towards complications of cardiomyopathy, such as heart failure, systolic dysfunction or severe ventricular arrhythmias, complications of neutropenia, such as severe infections, myopathy (physiotherapy), lactic acidosis by application of lactate-lowering agents, or carnitine deficiency by means of substitution. In single patients, neutropenia may respond to biweekly injections of granulocyte colony stimulating factor . A single patient with Barth syndrome with heart failure required mechanical circulatory support by means of a ventricular assist device as a bridge to heart transplantation . Due to an acute lung injury, he additionally required the placement of an in-line oxygenator to maintain end-organ function .
A number of new insights have been gained during recent years concerning the pathogenesis, clinical manifestations, diagnosis and treatment of Barth syndrome. Barth syndrome may also present with facial dysmorphism, sensory abnormalities and cognitive dysfunction. Contrary to previous standards, female carriers also may be affected in cases of specific chromosomal constellations. Concerning the pathogenesis of neutropenia, there are some indications that reactive oxygen species trigger the exposure of phosphatidylserine in the absence of other markers of apoptosis. Exposure of phosphatidylserine may lead to increased clearance of neutrophils by tissue macrophages. Why eosinophils also show exposure of phosphatidylserine in the absence of increased clearance remains speculative. Further studies are required to confirm that application of granulocyte colony stimulating factors is indeed an effective treatment of neutropenia in Barth syndrome.
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
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Additional references related to this topic can also be found in the Current World Literature section in this issue (p. 70).
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