Michael A. Burke

Phenomapping for Novel Classification of Heart Failure With Preserved Ejection Fraction

Background— Heart failure with preserved ejection fraction (HFpEF) is a heterogeneous clinical syndrome in need of improved phenotypic classification. We sought to evaluate whether unbiased clustering analysis using dense phenotypic data (phenomapping) could identify phenotypically distinct HFpEF categories. Methods and Results— We prospectively studied 397 patients with HFpEF and performed detailed clinical, laboratory, ECG, and echocardiographic phenotyping of the study participants. We used several statistical learning algorithms, including unbiased hierarchical cluster analysis of phenotypic data (67 continuous variables) and penalized model-based clustering, to define and characterize mutually exclusive groups making up a novel classification of HFpEF. All phenomapping analyses were performed by investigators blinded to clinical outcomes, and Cox regression was used to demonstrate the clinical validity of phenomapping. The mean age was 65±12 years; 62% were female; 39% were black; and comorbidities were common. Although all patients met published criteria for the diagnosis of HFpEF, phenomapping analysis classified study participants into 3 distinct groups that differed markedly in clinical characteristics, cardiac structure/function, invasive hemodynamics, and outcomes (eg, phenogroup 3 had an increased risk of HF hospitalization [hazard ratio, 4.2; 95% confidence interval, 2.0–9.1] even after adjustment for traditional risk factors [P<0.001]). The HFpEF phenogroup classification, including its ability to stratify risk, was successfully replicated in a prospective validation cohort (n=107). Conclusions— Phenomapping results in a novel classification of HFpEF. Statistical learning algorithms applied to dense phenotypic data may allow improved classification of heterogeneous clinical syndromes, with the ultimate goal of defining therapeutically homogeneous patient subclasses.

Blockade of the erbB2 receptor induces cardiomyocyte death through mitochondrial and reactive oxygen species-dependent pathways

Overexpression of the receptor tyrosine kinase erbB2 (Her2 in humans) is correlated with a poor prognosis in breast and ovarian cancers. Treatment with trastuzumab (a monoclonal antibody against erbB2) improves survival; however, it also causes cardiomyopathy. We hypothesized that blockade of the erbB2 receptor induces cardiomyocyte death through a mitochondrial pathway that is dependent on the production of reactive oxygen species (ROS). We first showed that levels of erbB2 receptor are significantly decreased in an animal model of ischemic heart disease and in human ischemic cardiomyopathy. We treated neonatal rat cardiomyocytes with an inhibitory erbB2 antibody to study the mechanism behind the deleterious effects of erbB2 blockade. These cells displayed a dose-dependent increase in ROS production and cell death compared with control IgG-treated cells; these processes were reversed by the antioxidant, N-acetylcysteine. The effects of erbB2 antibody on both cell death and ROS production were also reversed by cyclosporine A and diazoxide, chemicals that regulate the pro- and anti-apoptotic channels in the mitochondria, respectively. Furthermore, mouse embryonic fibroblasts lacking Bax and Bak (proteins that mediate cell death through a mitochondrial pathway) were resistant to the deleterious effects of erbB2 antibody. These effects of erbB2 blockade appear to occur through a pathway involving AKT and PKC-α. Our results suggest that erbB2 plays a role in cardiomyocyte survival, and that the deleterious effects of trastuzumab on the heart occur through a mitochondrial pathway and is mediated by ROS production. Manipulation of redox signaling may be beneficial in cancer patients receiving trastuzumab.

Prognostic Importance of Pathophysiologic Markers in Patients With Heart Failure and Preserved Ejection Fraction

Background— Heart failure with preserved ejection fraction (HFpEF) is a heterogeneous syndrome associated with multiple pathophysiologic abnormalities, including left ventricular (LV) diastolic dysfunction, longitudinal LV systolic dysfunction, abnormal ventricular-arterial coupling, pulmonary hypertension, and right ventricular (RV) remodeling/dysfunction. However, the relative prognostic significance of each of these pathophysiologic abnormalities in HFpEF is unknown. Methods and Results— We prospectively studied 419 patients with HFpEF using echocardiography and sphygmomanometry to assess HFpEF pathophysiologic markers. Cox proportional hazards analyses were used to determine the associations between pathophysiologic markers and outcomes. Mean age was 65±12 years; 62% were women; 39% were black; comorbidities were common; and study participants met published criteria for HFpEF. RV abnormalities were frequent: 28% had abnormal tricuspid annular plane systolic excursion, 15% had reduced RV fractional area change, and 34% had RV hypertrophy. During a median follow-up time of 18 months, 102 (24%) were hospitalized for HF and 175 (42%) experienced the composite end point of cardiovascular hospitalization or death. Decreased LV compliance, measured as reduced LV end-diastolic volume at an idealized LV end-diastolic pressure of 20 mm Hg (EDV20), and RV remodeling, as indicated by increased RV wall thickness, were the 2 pathophysiologic markers most predictive of worse outcomes: adjusted hazard ratio per 1 SD decrease in EDV20=1.39 (95% confidence interval [CI], 1.10–1.75; P=0.006), and hazard ratio per 1 SD increase in RV wall thickness=1.37 (95% CI, 1.16–1.61; P<0.001). These associations persisted after additional adjustment for markers of HF severity. By contrast, markers of LV relaxation, longitudinal LV systolic dysfunction, and ventricular-arterial coupling were not significantly associated with adverse outcomes. Conclusions— In patients with HFpEF, reduced LV compliance and RV remodeling are the strongest pathophysiologic predictors of adverse outcomes.

Prevalence, Clinical Phenotype, and Outcomes Associated with Normal B-Type Natriuretic Peptide Levels in Heart Failure with Preserved Ejection Fraction

B-type natriuretic peptide (BNP) is used widely to exclude heart failure (HF) in patients with dyspnea. However, most studies of BNP have focused on diagnosing HF with reduced ejection fraction (EF). The aim of this study was to test the hypothesis that a normal BNP level (≤100 pg/ml) is relatively common in HF with preserved EF (HFpEF), a heterogenous disorder commonly associated with obesity. A total of 159 consecutive patients enrolled in the Northwestern University HFpEF Program were prospectively studied. All subjects had symptomatic HF with EF >50% and elevated pulmonary capillary wedge pressure. BNP was tested at baseline in all subjects. Clinical characteristics, echocardiographic parameters, invasive hemodynamics, and outcomes were compared among patients with HFpEF with normal (≤100 pg/ml) versus elevated (>100 pg/ml) BNP. Of the 159 patients with HFpEF, 46 (29%) had BNP ≤100 pg/ml. Subjects with normal BNP were younger, were more often women, had higher rates of obesity and higher body mass index, and less commonly had chronic kidney disease and atrial fibrillation. EFs and pulmonary capillary wedge pressures were similar in the normal and elevated BNP groups (62 ± 7% vs 61 ± 7%, p = 0.67, and 25 ± 8 vs 27 ± 9 mm Hg, p = 0.42, respectively). Elevated BNP was associated with enlarged left atrial volume, worse diastolic function, abnormal right ventricular structure and function, and worse outcomes (e.g., adjusted hazard ratio for HF hospitalization 4.0, 95% confidence interval 1.6 to 9.7, p = 0.003). In conclusion, normal BNP levels were present in 29% of symptomatic outpatients with HFpEF who had elevated pulmonary capillary wedge pressures, and although BNP is useful as a prognostic marker in HFpEF, normal BNP does not exclude the outpatient diagnosis of HFpEF.

Targeting myocardial substrate metabolism in heart failure: potential for new therapies

The incidence and prevalence of heart failure have increased significantly over the past few decades. Available data suggest that patients with heart failure independent of the aetiology have viable but dysfunctional myocardium that is potentially salvageable. Although a great deal of research effort has focused on characterizing the molecular basis of heart failure, cardiac metabolism in this disorder remains an understudied discipline. It is known that many aspects of cardiomyocyte energetics are altered in heart failure. These include a shift from fatty acid to glucose as a preferred substrate and a decline in the levels of ATP. Despite these demonstrated changes, there are currently no approved drugs that target metabolic enzymes or proteins in heart failure. This is partly due to our limited knowledge of the mechanisms and pathways that regulate cardiac metabolism. Better characterization of these pathways may potentially lead to new therapies for heart failure. Targeting myocardial energetics in the viable and potentially salvageable tissue may be particularly effective in the treatment of heart failure. Here, we will review metabolic changes that occur in fatty acid and glucose metabolism and AMP‐activated kinase in heart failure. We propose that cardiac energetics should be considered as a potential target for therapy in heart failure and more research should be done in this area.

The Sulfonylurea Receptor, an Atypical ATP-Binding Cassette Protein, and Its Regulation of the KATP Channel

ATP-binding cassette (ABC) proteins are highly conserved and widely expressed throughout nature and found in all organisms, both prokaryotic and eukaryotic. They mediate myriad critical cellular processes, from nutrient import to toxin efflux using the energy derived from ATP hydrolysis. Most ABC proteins mediate transport of substances across lipid membranes. However, there are atypical ABC proteins that mediate other processes. These include, but are not limited to, DNA repair (bacterial MutS), ion transport (cystic fibrosis transmembrane receptor), and mRNA trafficking (yeast Elf1p). The sulfonylurea receptor (SUR) is another atypical ABC protein that regulates activity of the potassium ATP channel (KATP). KATP is widely expressed in nearly all tissues of higher organisms and couples cellular energy status to membrane potential. KATP is particularly important in the regulation of insulin secretion from pancreatic &bgr;-cells and in regulating action potential duration in muscle cells. SUR is indispensable for normal channel function, and mutations in genes encoding SURs increase the susceptibility to diabetes, myocardial infarction, and heart failure. Here, we review the structure and function of ABC proteins and discuss SUR, its regulation of the KATP channel, and its role in cardiovascular disease.

Interpretation of B-type natriuretic peptide in cardiac disease and other comorbid conditions

B-Type natriuretic peptide (BNP) is elevated in states of increased ventricular wall stress. BNP is most commonly used to rule out congestive heart failure (CHF) in dyspneic patients. BNP levels are influenced by age, gender and, to a surprisingly large extent, by body mass index (BMI). In addition, it can be elevated in a wide variety of clinical settings with or without CHF. BNP is elevated in other cardiac disease states such as the acute coronary syndromes, diastolic dysfunction, atrial fibrillation (AF), amyloidosis, restrictive cardiomyopathy (RCM), and valvular heart disease. BNP is elevated in non-cardiac diseases such as pulmonary hypertension, chronic obstructive pulmonary disease, pulmonary embolism, and renal failure. BNP is also elevated in the setting of critical illness such as in acute decompensated CHF (ADHF) and sepsis. This variation across clinical settings has significant implications given the increasing frequency with which BNP testing is being performed. It is important for clinicians to understand how to appropriately interpret BNP in light of the comorbidities of individual patients to maximize its clinical utility. We will review the molecular biology and physiology of natriuretic peptides as well as the relevant literature on the utilization of BNP in CHF as well as in other important clinical situations, conditions that are commonly associated with CHF and or dyspnea.

Clinical and Mechanistic Insights into the Genetics of Cardiomyopathy

Over the last quarter-century, there has been tremendous progress in genetics research that has defined molecular causes for cardiomyopathies. More than a thousand mutations have been identified in many genes with varying ontologies, therein indicating the diverse molecules and pathways that cause hypertrophic, dilated, restrictive, and arrhythmogenic cardiomyopathies. Translation of this research to the clinic via genetic testing can precisely group affected patients according to molecular etiology, and identify individuals without evidence of disease who are at high risk for developing cardiomyopathy. These advances provide insights into the earliest manifestations of cardiomyopathy and help to define the molecular pathophysiological basis for cardiac remodeling. Although these efforts remain incomplete, new genomic technologies and analytic strategies provide unparalleled opportunities to fully explore the genetic architecture of cardiomyopathies. Such data hold the promise that mutation-specific pathophysiology will uncover novel therapeutic targets, and herald the beginning of precision therapy for cardiomyopathy patients.

ATP-binding cassette B10 regulates early steps of heme synthesis

Rationale: Heme plays a critical role in gas exchange, mitochondrial energy production, and antioxidant defense in cardiovascular system. The mitochondrial transporter ATP-binding cassette (ABC) B10 has been suggested to export heme out of the mitochondria and is required for normal hemoglobinization of erythropoietic cells and protection against ischemia–reperfusion injury in the heart; however, its primary function has not been established. Objective: The aim of this study was to identify the function of ABCB10 in heme synthesis in cardiac cells. Methods and Results: Knockdown of ABCB10 in cardiac myoblasts significantly reduced heme levels and the activities of heme-containing proteins, whereas supplementation with &dgr;-aminolevulinic acid reversed these defects. Overexpression of mitochondrial &dgr;-aminolevulinic acid synthase 2, the rate-limiting enzyme upstream of &dgr;-aminolevulinic acid export, failed to restore heme levels in cells with ABCB10 downregulation. ABCB10 and heme levels were increased by hypoxia, and reversal of ABCB10 upregulation caused oxidative stress and cell death. Furthermore, ABCB10 knockdown in neonatal rat cardiomyocytes resulted in a significant delay of calcium removal from the cytoplasm, suggesting a relaxation defect. Finally, ABCB10 expression and heme levels were altered in failing human hearts and mice with ischemic cardiomyopathy. Conclusions: ABCB10 plays a critical role in heme synthesis pathway by facilitating &dgr;-aminolevulinic acid production or export from the mitochondria. In contrast to previous reports, we show that ABCB10 is not a heme exporter and instead is required for the early mitochondrial steps of heme biosynthesis.

ABCB10 Regulates Early Steps of Heme Synthesis

Rationale: Heme plays a critical role in gas exchange, mitochondrial energy production, and antioxidant defense in cardiovascular system. The mitochondrial transporter ATP-binding cassette (ABC) B10 has been suggested to export heme out of the mitochondria and is required for normal hemoglobinization of erythropoietic cells and protection against ischemia–reperfusion injury in the heart; however, its primary function has not been established. Objective: The aim of this study was to identify the function of ABCB10 in heme synthesis in cardiac cells. Methods and Results: Knockdown of ABCB10 in cardiac myoblasts significantly reduced heme levels and the activities of heme-containing proteins, whereas supplementation with &dgr;-aminolevulinic acid reversed these defects. Overexpression of mitochondrial &dgr;-aminolevulinic acid synthase 2, the rate-limiting enzyme upstream of &dgr;-aminolevulinic acid export, failed to restore heme levels in cells with ABCB10 downregulation. ABCB10 and heme levels were increased by hypoxia, and reversal of ABCB10 upregulation caused oxidative stress and cell death. Furthermore, ABCB10 knockdown in neonatal rat cardiomyocytes resulted in a significant delay of calcium removal from the cytoplasm, suggesting a relaxation defect. Finally, ABCB10 expression and heme levels were altered in failing human hearts and mice with ischemic cardiomyopathy. Conclusions: ABCB10 plays a critical role in heme synthesis pathway by facilitating &dgr;-aminolevulinic acid production or export from the mitochondria. In contrast to previous reports, we show that ABCB10 is not a heme exporter and instead is required for the early mitochondrial steps of heme biosynthesis.