Faculty Bibliography

OBJECTIVE:To determine the prevalence of Barth syndrome in the pediatric population. STUDY DESIGN/METHODS:Data were collected from the Barth Syndrome Foundation Registry and relevant literature. With the advent of genetic testing and whole-exome sequencing, a multipronged Bayesian analysis was used to estimate the prevalence of Barth syndrome based on published data on the incidence and prevalence of cardiomyopathy and neutropenia, and the respective subpopulations of patients with Barth syndrome indicated in these publications. RESULTS:Based on 7 published studies of cardiomyopathy and 2 published studies of neutropenia, the estimated prevalence of Barth syndrome is approximately 1 case per million male population. This contrasts with 99 cases in the Barth Syndrome Foundation Registry, 58 of which indicate a US location, and only 230-250 cases known worldwide. CONCLUSIONS:It appears that Barth syndrome is greatly underdiagnosed. There is a need for better education and awareness of this rare disease to move toward early diagnosis and treatment.

BACKGROUND:Sudden death in children is a tragic event that often remains unexplained after comprehensive investigation. We report two asymptomatic siblings who died unexpectedly at approximately 1 year of age found to have biallelic (compound heterozygous) variants in PPA2. METHODS:The index case, parents, and sister were enrolled in the Sudden Unexplained Death in Childhood Registry and Research Collaborative, which included next-generation genetic screening. Prior published cases of PPA2 variants, along with the known biology of PPA2, were also summarized. RESULTS:Whole exome sequencing in both siblings revealed biallelic rare missense variants in PPA2: c.182C > T (p.Ser61Phe) and c.380G > T (p.Arg127Leu). PPA2 encodes a mitochondrially located inorganic pyrophosphatase implicated in progressive and lethal cardiomyopathies. As a regulator and supplier of inorganic phosphate, PPA2 is central to phosphate metabolism. Biological roles include the following: mtDNA maintenance; oxidative phosphorylation and generation of ATP; reactive oxygen species homeostasis; mitochondrial membrane potential regulation; and possibly, retrograde signaling between mitochondria and nucleus. CONCLUSIONS:Two healthy and asymptomatic sisters died unexpectedly at ages 12 and 10 months, and were diagnosed by molecular autopsy to carry biallelic variants in PPA2. Our cases add additional details to those reported thus far, and broaden the spectrum of clinical and molecular features of PPA2 variants.

Pediatric heart failure remains poorly understood, distinct in many aspects from adult heart failure. Limited data point to roles of altered mitochondrial functioning and in particular, changes in mitochondrial lipids, especially cardiolipin. Barth syndrome is a mitochondrial disorder caused by tafazzin mutations that lead to abnormal cardiolipin profiles. Patients are afflicted by cardiomyopathy, skeletal myopathy, neutropenia, and growth delay. A mouse model of Barth syndrome was developed a decade ago, which relies on a doxycycline-inducible shRNA to knock down expression of tafazzin mRNA ("TAZKD"). Our objective was to review published data from the TAZKD mouse to determine its contributions to our pathogenetic understanding of, and potential treatment strategies for, Barth syndrome. In regard to the clinical syndrome, the reported physiological, biochemical, and ultrastructural abnormalities of the mouse model mirror those in Barth patients. Using this model, the PPAR pan-agonist bezafibrate has been suggested as potential therapy because it ameliorated the cardiomyopathy in TAZKD mice, while increasing mitochondrial biogenesis. A clinical trial is now underway to test bezafibrate in Barth syndrome patients. Thus, the TAZKD mouse model of Barth syndrome has led to important insights into disease pathogenesis and therapeutic targets, which can potentially translate to pediatric heart failure.

Mitochondria contain cardiolipin (CL), an organelle-specific phospholipid that carries four fatty acids with a strong preference for unsaturated chains. Unsaturation is essential for the stability and for the function of mitochondrial CL. Surprisingly, we found tetrapalmitoyl-CL (TPCL), a fully saturated species, in the testes of humans and mice. TPCL was absent from other mouse tissues but was the most abundant CL species in testicular germ cells. Most intriguingly, TPCL was not localized in mitochondria but was in other cellular membranes even though mitochondrial CL was the substrate from which TPCL was synthesized. During spermiogenesis, TPCL became associated with the acrosome, a sperm-specific organelle, along with a subset of authentic mitochondrial proteins, including Ant4, Suox, and Spata18. Our data suggest that mitochondria-derived membranes are assembled into the acrosome, challenging the concept that this organelle is strictly derived from the Golgi apparatus and revealing a novel function of mitochondria.

Real-time imaging of the embryonic murine cardiovascular system is challenging due to the small size of the mouse embryo and rapid heart rate. High-frequency, linear-array ultrasound systems designed for small-animal imaging provide high-frame-rate and Doppler modes but are limited in regards to the field of view that can be imaged at fine-temporal and -spatial resolution. Here, a plane-wave imaging method was used to obtain high-speed image data from in utero mouse embryos and multi-angle, vector-flow algorithms were applied to the data to provide information on blood flow patterns in major organs. An 18-MHz linear array was used to acquire plane-wave data at absolute frame rates >/=10 kHz using a set of fixed transmission angles. After beamforming, vector-flow processing and image compounding, effective frame rates were on the order of 2 kHz. Data were acquired from the embryonic liver, heart and umbilical cord. Vector-flow results clearly revealed the complex nature of blood-flow patterns in the embryo with fine-temporal and -spatial resolution.

AIM: To extend our previously-published experience in estimating pressure gradients (PG) via physical examination in a large patient cohort. METHODS: From January 1, 1997 through December 31, 2009, an attending pediatric cardiologist compared clinical examination (EXAM) with Doppler-echo (ECHO), in 1193 patients with pulmonic stenosis (PS, including tetralogy of Fallot), aortic stenosis (AS), and ventricular septal defect (VSD). EXAM PG estimates were based primarily on a murmur's pitch, grade, and length. ECHO peak instantaneous PG was derived from the modified Bernoulli equation. Patients were 0-38.4 years old (median 4.8). RESULTS: For all patients, EXAM correlated highly with ECHO: ECHO = 0.99 (EXAM) + 3.2 mmHg; r = +0.89; P < 0.0001. Agreement was excellent (mean difference = -2.9 +/- 16.1 mmHg). In 78% of all patients, agreement between EXAM and ECHO was within 15 mmHg and within 5 mmHg in 45%. Clinical estimates of PS PG were more accurate than of AS and VSD. A palpable precordial thrill and increasing loudness of the murmur predicted higher gradients (P < 0.0001). Weight did not influence accuracy. A learning curve was evident, such that the most recent quartile of patients showed ECHO = 1.01 (EXAM) + 1.9, r = +0.92, P < 0.0001; during this time, the attending pediatric cardiologist had been > 10 years in practice. CONCLUSION: Clinical examination can accurately estimate PG in PS, AS, or VSD. Continual correlation of clinical findings with echocardiography can lead to highly accurate diagnostic skills.