Novel mutations of ABCA1 transporter in patients with Tangier disease and familial HDL deficiency.

The objective of the study was the characterization of ABCA1 gene mutations in 10 patients with extremely low HDL-cholesterol. Five patients (aged 6 months to 76 years) presented with splenomegaly and thrombocytopenia suggesting the diagnosis of Tangier disease (TD). Three of them were homozygous for novel mutations either in intron (c.4465-34A>G) or in exons (c.4376delT and c.5449C>T), predicted to encode truncated proteins. One patient was compound heterozygous for a nucleotide insertion (c.1758_1759insG), resulting in a truncated protein and for a nucleotide substitution c.4799A>G, resulting in a missense mutation (p.H1600R). The last TD patient, found to be heterozygous for a known mutation (p.D1009Y), had a complete defect in ABCA1-mediated cholesterol efflux in fibroblasts, suggesting the presence of a second undetected mutant allele. Among the other patients, four were asymptomatic, but one, with multiple risk factors, had severe peripheral artery disease. Three of these patients were heterozygous for known mutations (p.R130K+p.N1800H, p.R1068C, p.N1800H), while two were carriers of novel mutations (c.1195-27G>A and c.396_397insA), predicted to encode truncated proteins. The pathogenic effect of the two intronic mutations (c. 1195-27G>A and c.4465-34A>G) was demonstrated by the analysis of the transcripts of splicing reporter mutant minigenes expressed in COS-1 cells. Both mutations activated an intronic acceptor splice site which resulted in a partial intron retention in mature mRNA with the production of truncated proteins. This study confirms the allelic heterogeneity of TD and suggests that the diagnosis of TD must be considered in patients with an unexplained splenomegaly, associated with thrombocytopenia and hypocholesterolemia.

Ablation of triadin causes loss of cardiac Ca2+ release units, impaired excitation-contraction coupling, and cardiac arrhythmias.

Heart muscle excitation-contraction (E-C) coupling is governed by Ca(2+) release units (CRUs) whereby Ca(2+) influx via L-type Ca(2+) channels (Cav1.2) triggers Ca(2+) release from juxtaposed Ca(2+) release channels (RyR2) located in junctional sarcoplasmic reticulum (jSR). Although studies suggest that the jSR protein triadin anchors cardiac calsequestrin (Casq2) to RyR2, its contribution to E-C coupling remains unclear. Here, we identify the role of triadin using mice with ablation of the Trdn gene (Trdn(-/-)). The structure and protein composition of the cardiac CRU is significantly altered in Trdn(-/-) hearts. jSR proteins (RyR2, Casq2, junctin, and junctophilin 1 and 2) are significantly reduced in Trdn(-/-) hearts, whereas Cav1.2 and SERCA2a remain unchanged. Electron microscopy shows fragmentation and an overall 50% reduction in the contacts between jSR and T-tubules. Immunolabeling experiments show reduced colocalization of Cav1.2 with RyR2 and substantial Casq2 labeling outside of the jSR in Trdn(-/-) myocytes. CRU function is impaired in Trdn(-/-) myocytes, with reduced SR Ca(2+) release and impaired negative feedback of SR Ca(2+) release on Cav1.2 Ca(2+) currents (I(Ca)). Uninhibited Ca(2+) influx via I(Ca) likely contributes to Ca(2+) overload and results in spontaneous SR Ca(2+) releases upon beta-adrenergic receptor stimulation with isoproterenol in Trdn(-/-) myocytes, and ventricular arrhythmias in Trdn(-/-) mice. We conclude that triadin is critically important for maintaining the structural and functional integrity of the cardiac CRU; triadin loss and the resulting alterations in CRU structure and protein composition impairs E-C coupling and renders hearts susceptible to ventricular arrhythmias.

Systems approach to improving traumatic brain injury care in Myanmar: a mixed-methods study from lived experience to discrete event simulation.

OBJECTIVES: Traumatic brain injury (TBI) is a global health problem, whose management in low-resource settings is hampered by fragile health systems and lack of access to specialist services. Improvement is complex, given the interaction of multiple people, processes and institutions. We aimed to develop a mixed-method approach to understand the TBI pathway based on the lived experience of local people, supported by quantitative methodologies and to determine potential improvement targets. DESIGN: We describe a systems approach based on narrative exploration, participatory diagramming, data collection and discrete event simulation (DES), conducted by an international research collaborative. SETTING: The study is set in the tertiary neurotrauma centre in Yangon General Hospital, Myanmar, in 2019-2020 (prior to the SARS-CoV2 pandemic). PARTICIPANTS: The qualitative work involved 40 workshop participants and 64 interviewees to explore the views of a wide range of stakeholders including staff, patients and relatives. The 1-month retrospective admission snapshot covered 85 surgical neurotrauma admissions. RESULTS: The TBI pathway was outlined, with system boundaries defined around the management of TBI once admitted to the neurosurgical unit. Retrospective data showed 18% mortality, 71% discharge to home and an 11% referral rate. DES was used to investigate the system, showing its vulnerability to small surges in patient numbers, with critical points being CT scanning and observation ward beds. This explorative model indicated that a modest expansion of observation ward beds to 30 would remove the flow-limitations and indicated possible consequences of changes. CONCLUSIONS: A systems approach to improving TBI care in resource-poor settings may be supported by simulation and informed by qualitative work to ground it in the direct experience of those involved. Narrative interviews, participatory diagramming and DES represent one possible suite of methods deliverable within an international partnership. Findings can support targeted improvement investments despite coexisting resource limitations while indicating concomitant risks.

Casq2 deletion causes sarcoplasmic reticulum volume increase, premature Ca2+ release, and catecholaminergic polymorphic ventricular tachycardia.

Cardiac calsequestrin (Casq2) is thought to be the key sarcoplasmic reticulum (SR) Ca2+ storage protein essential for SR Ca2+ release in mammalian heart. Human CASQ2 mutations are associated with catecholaminergic ventricular tachycardia. However, homozygous mutation carriers presumably lacking functional Casq2 display surprisingly normal cardiac contractility. Here we show that Casq2-null mice are viable and display normal SR Ca2+ release and contractile function under basal conditions. The mice exhibited striking increases in SR volume and near absence of the Casq2-binding proteins triadin-1 and junctin; upregulation of other Ca2+ -binding proteins was not apparent. Exposure to catecholamines in Casq2-null myocytes caused increased diastolic SR Ca2+ leak, resulting in premature spontaneous SR Ca2+ releases and triggered beats. In vivo, Casq2-null mice phenocopied the human arrhythmias. Thus, while the unique molecular and anatomic adaptive response to Casq2 deletion maintains functional SR Ca2+ storage, lack of Casq2 also causes increased diastolic SR Ca2+ leak, rendering Casq2-null mice susceptible to catecholaminergic ventricular arrhythmias.

Modest reductions of cardiac calsequestrin increase sarcoplasmic reticulum Ca2+ leak independent of luminal Ca2+ and trigger ventricular arrhythmias in mice.

Cardiac calsequestrin-null mice (Casq2-/-) display catecholaminergic ventricular tachycardia akin to humans with CASQ2 mutations. However, the specific contribution of Casq2 deficiency to the arrhythmia phenotype is difficult to assess because Casq2-/- mice also show significant reductions in the sarcoplasmic reticulum (SR) proteins junctin and triadin-1 and increased SR volume. Furthermore, it remains unknown whether Casq2 regulates SR Ca2+ release directly or indirectly by buffering SR luminal Ca2+. To address both questions, we examined heterozygous (Casq2+/-) mice, which have a 25% reduction in Casq2 but no significant decrease in other SR proteins. Casq2+/- mice (n=35) challenged with isoproterenol displayed 3-fold higher rates of ventricular ectopy than Casq2+/+ mice (n=31; P<0.05). Programmed stimulation induced significantly more ventricular tachycardia in Casq2+/- mice than in Casq2+/+ mice. Field-stimulated Ca2+ transients, cell shortening, L-type Ca2+ current, and SR volume were not significantly different in Casq2+/- and Casq2+/+ myocytes. However, in the presence of isoproterenol, SR Ca2+ leak was significantly increased in Casq2+/- myocytes (Casq2+/- 0.18+/-0.02 F(ratio) versus Casq2+/+ 0.11+/-0.01 F(ratio), n=57, 60; P<0.01), resulting in a significantly higher rate of spontaneous SR Ca2+ releases and triggered beats. SR luminal Ca2+ measured using Mag-Fura-2 was not altered by Casq2 reduction. As a result, the relationship between SR Ca2+ leak and SR luminal Ca2+ was significantly different between Casq2+/- and Casq2+/+ myocytes (P<0.01). Thus, even modest reductions in Casq2 increase SR Ca2+ leak and cause ventricular tachycardia susceptibility under stress. The underlying mechanism is likely the direct regulation of SR Ca2+ release channels by Casq2 rather than altered luminal Ca2+.

A systems approach to trauma care in Myanmar: from health partnership to academic collaboration.

Experience from a variety of disciplines suggests that improving healthcare, particularly in resource-poor environments, can benefit from a systems approach. However, putting this into practice is challenging, especially in the context of an international institutional health partnership. In this article, we outline how a systems approach to the improvement of trauma care has informed both clinical improvement and academic collaboration as part of an ongoing partnership involving Cambridge University Hospitals NHS Foundation Trust, the University of Cambridge, and Cambridge Global Health Partnerships in the UK, and Yangon General Hospital, University of Medicine 1, and the Tropical Health and Education Trust (THET) in Myanmar. Improving and researching trauma care is an exemplar of a systems problem, requiring an understanding of the relevant people, equipment, processes, institutions, and power structures that result in the delivery of care at all points of the patient's journey from injury to rehabilitation. Exploring this in the explicit context of traumatic brain injury is one of the research themes of the NIHR Global Health Research Group on Neurotrauma, allowing systems research to directly inform efforts at practical improvement.

Phase 2 reentry as a trigger to initiate ventricular fibrillation during early acute myocardial ischemia.

BACKGROUND: Phase 2 reentry caused by heterogeneous loss of the transient outward potassium current (I(to))-mediated epicardial action potential (AP) dome can produce a closely coupled R-on-T extrasystole leading to ventricular fibrillation (VF) under conditions of ST-segment elevation unrelated to ischemia. The present study examined the role of phase 2 reentry in the initiation of VF during early myocardial ischemia. METHODS AND RESULTS: Regional myocardial ischemia was produced in an isolated, arterially perfused canine right ventricular wedge preparation. Transmembrane APs from 2 epicardial sites at each side of the ischemic border were simultaneously recorded together with measurements of extracellular potassium concentration ([K+]o) and a transmural ECG. Loss of the I(to)-mediated epicardial AP dome in the ischemic zone but not in the perfused tissue resulted in phase 2 reentry and associated R-on-T extrasystoles capable of initiating VF in 7 of 15 preparations during the first 3 to 9 minutes of myocardial ischemia, with marked ST-segment elevation and [K+]o accumulation. The I(to) and phase 1 magnitude of epicardium contributed importantly to the onset of VF. Phase 1 magnitude and I(to) density at +30 mV in the group with phase 2 reentry-related R-on-T extrasystoles were 32.2+/-1.3 mV and 30.3+/-0.5 pA/pF (n=7), respectively, significantly greater than those (24.0+/-1.8 mV and 23.2+/-1.0 pA/pF) in the group without the extrasystoles (n=8, P<0.01). CONCLUSIONS: Acute regional myocardial ischemia results in markedly heterogeneous loss of I(to)-mediated epicardial AP domes across the ischemic border, leading to phase 2 reentry. Phase 2 reentry can in turn produce an R-on-T extrasystole capable of initiating VF.

Biochemical markers of bone turnover - uses and limitations.

Bone turnover markers of resorption and formation are released during the process of bone remodelling. These markers have been extensively studied in a number of therapeutic trials of osteoporosis during the past decade. This has led to better understanding of their physiology, clinical applications and possible ways to optimize analytical techniques. Bone markers can complement the results of bone mineral density in the management of osteoporosis, but their use in clinical practice is challenged by pre-analytical and analytical variability. This review will discuss different types of bone markers, their limitations, use in different metabolic bone diseases and current recommendations from the International Osteoporosis Foundation and the International Federation of Clinical Chemistry and Laboratory Medicine bone marker standards working group.

Multicentre trial of weekly risedronate on bone density in adults with cystic fibrosis.

BACKGROUND: The aim of this study was to assess the efficacy, tolerability and safety of risedronate in adults with CF. METHODS: Patients with a lumbar spine (LS), total hip (TH) or femoral neck (FN) bone mineral density (BMD) Z-score of -1 or less were randomised to receive risedronate 35 mg weekly or placebo, and calcium (1g)+vitamin D(3) (800IU). RESULTS: At baseline, BMD Z-scores in the risedronate (n=17) and placebo (n=19) groups were similar. By 24 months, 7/17 risedronate patients vs 0/19 placebo patients stopped the study medication due to bone pain. After 24 months treatment, the mean difference (95% CI) in change in LS, TH and FN BMD between the risedronate vs placebo groups was 4.3% (0.4, 8.2) p=0.03; 4.0% (-0.5, 8.6) p=0.08; and 2.4% (-3.5, 8.2) p=0.41. CONCLUSIONS: After two years treatment there was a significant increase in LS BMD with weekly risedronate compared to placebo.

ECG repolarization waves: their genesis and clinical implications.

The electrocardiographic (ECG) manifestation of ventricular repolarization includes J (Osborn), T, and U waves. On the basis of biophysical principles of ECG recording, any wave on the body surface ECG represents a coincident voltage gradient generated by cellular electrical activity within the heart. The J wave is a deflection with a dome that appears on the ECG after the QRS complex. A transmural voltage gradient during initial ventricular repolarization, which results from the presence of a prominent action potential notch mediated by the transient outward potassium current (I(to)) in epicardium but not endocardium, is responsible for the registration of the J wave on the ECG. Clinical entities that are associated with J waves (the J-wave syndrome) include the early repolarization syndrome, the Brugada syndrome and idiopathic ventricular fibrillation related to a prominent J wave in the inferior leads. The T wave marks the final phase of ventricular repolarization and is a symbol of transmural dispersion of repolarization (TDR) in the ventricles. An excessively prolonged QT interval with enhanced TDR predisposes people to develop torsade de pointes. The malignant "R-on-T" phenomenon, i.e., an extrasystole that originates on the preceding T wave, is due to transmural propagation of phase 2 reentry or phase 2 early afterdepolarization. A pathological "U" wave as seen with hypokalemia is the consequence of electrical interaction among ventricular myocardial layers at action potential phase 3 of which repolarization slows. A physiological U wave is thought to be due to delayed repolarization of the Purkinje system.

The QT and Tp-e intervals in left and right chest leads: comparison between patients with systemic and pulmonary hypertension.

BACKGROUND: Action potential duration in the right ventricle is normally shorter than that in the left. We tested the hypothesis that there may be intrinsic differences in the QT and Tp-e (an interval from the peak to the end of the T wave) intervals between the left and right chest leads that can be exaggerated by systemic hypertension but attenuated by pulmonary hypertension in humans. METHODS: Electrocardiograms in the left (V4L-V6L) and right (V4R-V6R) chest leads were obtained in 40 healthy individuals, 29 patients with systemic hypertension and left ventricular hypertrophy, and 15 patients with pulmonary hypertension. RESULTS: In healthy individuals, the corrected QT (QTc) and corrected Tp-e [T(p-e)c] intervals were 421+/-5 and 86+/-3 milliseconds in V4L through V6L, respectively, significantly longer than those recorded from V4R through V6R (383+/-5 and 62+/-4 milliseconds, respectively; P<.01). Left ventricular hypertrophy prolonged the QTc interval in V4L through V6L (456+/-5 milliseconds), exaggerating the difference in the QTc interval between the left and right chest leads (61+/-4 vs 40+/-3 milliseconds in healthy control subjects; P<.01). Left ventricular hypertrophy also resulted in a small but significant increase in the T(p-e)c interval in V4L through V6L (97+/-3 vs 86+/-3 milliseconds in control subjects; P<.05) but exerted no significant effect on the T(p-e)c interval in the right. In contrast, pulmonary hypertension lengthened the QTc interval in the right chest leads, reducing the difference in the QTc interval between the left and right chest leads (3+/-8 vs 40+/-3 milliseconds in control subjects; P<.01). CONCLUSIONS: There are intrinsic differences in the QT and Tp-e intervals between V4L-V6L and V4R-V6R that are significantly amplified by systemic hypertension but markedly attenuated by pulmonary hypertension.