Reptilian heart development and the molecular basis of cardiac chamber evolution.

The emergence of terrestrial life witnessed the need for more sophisticated circulatory systems. This has evolved in birds, mammals and crocodilians into complete septation of the heart into left and right sides, allowing separate pulmonary and systemic circulatory systems, a key requirement for the evolution of endothermy. However, the evolution of the amniote heart is poorly understood. Reptilian hearts have been the subject of debate in the context of the evolution of cardiac septation: do they possess a single ventricular chamber or two incompletely septated ventricles? Here we examine heart development in the red-eared slider turtle, Trachemys scripta elegans (a chelonian), and the green anole, Anolis carolinensis (a squamate), focusing on gene expression in the developing ventricles. Both reptiles initially form a ventricular chamber that homogenously expresses the T-box transcription factor gene Tbx5. In contrast, in birds and mammals, Tbx5 is restricted to left ventricle precursors. In later stages, Tbx5 expression in the turtle (but not anole) heart is gradually restricted to a distinct left ventricle, forming a left-right gradient. This suggests that Tbx5 expression was refined during evolution to pattern the ventricles. In support of this hypothesis, we show that loss of Tbx5 in the mouse ventricle results in a single chamber lacking distinct identity, indicating a requirement for Tbx5 in septation. Importantly, misexpression of Tbx5 throughout the developing myocardium to mimic the reptilian expression pattern also results in a single mispatterned ventricular chamber lacking septation. Thus ventricular septation is established by a steep and correctly positioned Tbx5 gradient. Our findings provide a molecular mechanism for the evolution of the amniote ventricle, and support the concept that altered expression of developmental regulators is a key mechanism of vertebrate evolution.

Shox2 mediates Tbx5 activity by regulating Bmp4 in the pacemaker region of the developing heart.

Heart formation requires a highly balanced network of transcriptional activation of genes. The homeodomain transcription factor, Shox2, is essential for the formation of the sinoatrial valves and for the development of the pacemaking system. The elucidation of molecular mechanisms underlying the development of pacemaker tissue has gained clinical interest as defects in its patterning can be related to atrial arrhythmias. We have analyzed putative targets of Shox2 and identified the Bmp4 gene as a direct target. Shox2 interacts directly with the Bmp4 promoter in chromatin immunoprecipitation assays and activates transcription in luciferase-reporter assays. In addition, ectopic expression of Shox2 in Xenopus embryos stimulates transcription of the Bmp4 gene, and silencing of Shox2 in cardiomyocytes leads to a reduction in the expression of Bmp4. In Tbx5(del/+) mice, a model for Holt-Oram syndrome, and Shox2(-/-) mice, we show that the T-box transcription factor Tbx5 is a regulator of Shox2 expression in the inflow tract and that Bmp4 is regulated by Shox2 in this compartment of the embryonic heart. In addition, we could show that Tbx5 acts cooperatively with Nkx2.5 to regulate the expression of Shox2 and Bmp4. This work establishes a link between Tbx5, Shox2 and Bmp4 in the pacemaker region of the developing heart and thus contributes to the unraveling of the intricate interplay between the heart-specific transcriptional machinery and developmental signaling pathways.

The significance of low DBP in US adults with isolated systolic hypertension.

OBJECTIVES: To determine the features of isolated systolic hypertension (ISH), the most common hypertensive subtype in persons at least age 50, associated with greater cardiovascular disease (CVD) risk when accompanied by low diastolic blood pressure (DBP). DESIGN: Data were obtained from adult hypertensives at least age 18 in the National Health Nutrition Examination Survey (NHANES) 1999-2006 (n = 19 808, projected to 199.3 million). METHOD: ISH were categorized by low DBP (< 70 mmHg) vs. higher DBP (≥ 70-89 mmHg), treatment status, age, sex, ethnicity, cardiometabolic risk factors, and comorbidities. RESULTS: A 58.6% of all untreated US hypertensives had ISH (mean blood pressure 154.3/73.8 mmHg). Untreated and treated persons with ISH and DBP less than 70 mmHg represented 30 and 35%, respectively, of the ISH population and had almost twice the prevalence of diabetes and CVD, but a lower prevalence of the metabolic syndrome (P < 0.05 to P < 0.01). There was a three-fold greater prevalence of CVD from the highest to the lowest DBP strata in untreated ISH (P < 0.01). Logistic regression showed that age, female sex, and diabetes, but not treatment status, were independently associated with lower DBP (all P < 0.01). Of those persons with ISH and DBP less than 70 mmHg, 45% remain untreated. CONCLUSION: Older persons with untreated ISH and DBP less than 70 mmHg, comprising almost one-third of the untreated ISH population, had greater prevalence of diabetes and CVD than persons with ISH in association with DBP 70-89 mmHg. Intensified efforts to identify and adequately treat these individuals are needed to reduce their associated CVD risk.

Chromatin remodelling complex dosage modulates transcription factor function in heart development.

Dominant mutations in cardiac transcription factor genes cause human inherited congenital heart defects (CHDs); however, their molecular basis is not understood. Interactions between transcription factors and the Brg1/Brm-associated factor (BAF) chromatin remodelling complex suggest potential mechanisms; however, the role of BAF complexes in cardiogenesis is not known. In this study, we show that dosage of Brg1 is critical for mouse and zebrafish cardiogenesis. Disrupting the balance between Brg1 and disease-causing cardiac transcription factors, including Tbx5, Tbx20 and Nkx2-5, causes severe cardiac anomalies, revealing an essential allelic balance between Brg1 and these cardiac transcription factor genes. This suggests that the relative levels of transcription factors and BAF complexes are important for heart development, which is supported by reduced occupancy of Brg1 at cardiac gene promoters in Tbx5 haploinsufficient hearts. Our results reveal complex dosage-sensitive interdependence between transcription factors and BAF complexes, providing a potential mechanism underlying transcription factor haploinsufficiency, with implications for multigenic inheritance of CHDs.

Tbx5-dependent rheostatic control of cardiac gene expression and morphogenesis.

Dominant mutations in the T-box transcription factor gene TBX5 cause Holt-Oram syndrome (HOS), an inherited human disease characterized by upper limb malformations and congenital heart defects (CHDs) of variable severity. We hypothesize that minor alterations in the dosage of Tbx5 directly influences severity of CHDs. Using a mouse allelic series, we show a sensitive inverse correlation between Tbx5 dosage and abnormal cardiac morphogenesis and gene expression. The CHDs found in mice harbouring a hypomorphic allele of Tbx5 (Tbx5(lox/+) mice) are less pronounced than those found in Tbx5 haploinsufficient mice (Tbx5(del/+)), and homozygous hypomorphic (Tbx5(lox/lox)) embryos have noticeably more advanced cardiac development than Tbx5 null (Tbx5(del/del)) embryos. Examination of target gene expression across the allelic series uncovers very fine sensitivity across the range of Tbx5 dosages, in which some genes respond dramatically differently to only 15% differences in Tbx5 mRNA levels. This analysis was expanded to a genome-wide level, which uncovered a Tbx5 dosage-sensitive genetic program involving a network of cardiac transcription factors, developmentally important cell-cell signaling molecules, and ion channel proteins. These results indicate an exquisite sensitivity of the developing heart to Tbx5 dosage and provide significant insight into the transcriptional and cellular mechanisms that are disrupted in CHDs.

Tbx20 dose-dependently regulates transcription factor networks required for mouse heart and motoneuron development.

To elucidate the function of the T-box transcription factor Tbx20 in mammalian development, we generated a graded loss-of-function series by transgenic RNA interference in entirely embryonic stem cell-derived mouse embryos. Complete Tbx20 knockdown resulted in defects in heart formation, including hypoplasia of the outflow tract and right ventricle, which derive from the anterior heart field (AHF), and decreased expression of Nkx2-5 and Mef2c, transcription factors required for AHF formation. A mild knockdown led to persistent truncus arteriosus (unseptated outflow tract) and hypoplastic right ventricle, entities similar to human congenital heart defects, and demonstrated a critical requirement for Tbx20 in valve formation. Finally, an intermediate knockdown revealed a role for Tbx20 in motoneuron development, specifically in the regulation of the transcription factors Isl2 and Hb9, which are important for terminal differentiation of motoneurons. Tbx20 could activate promoters/enhancers of several genes in cultured cells, including the Mef2c AHF enhancer and the Nkx2-5 cardiac enhancer. The Mef2c AHF enhancer relies on Isl1- and Gata-binding sites. We identified a similar Isl1 binding site in the Nkx2-5 AHF enhancer, which in transgenic mouse embryos was essential for activity in a large part of the heart, including the outflow tract. Tbx20 synergized with Isl1 and Gata4 to activate both the Mef2c and Nkx2-5 enhancers, thus providing a unifying mechanism for gene activation by Tbx20 in the AHF. We conclude that Tbx20 is positioned at a critical node in transcription factor networks required for heart and motoneuron development where it dose-dependently regulates gene expression.

TBX5 mutations and congenital heart disease: Holt-Oram syndrome revealed.

PURPOSE OF REVIEW: Mutations in the T-box transcription factor TBX5 cause Holt-Oram syndrome (HOS), an autosomal-dominant condition characterized by a familial history of congenital heart defects and preaxial radial ray upper limb defects. This review summarizes recent developments in the study of TBX5 as it relates to congenital heart disease and the pathology of HOS. RECENT FINDINGS: Currently, 37 mutations in TBX5 have been found in patients with HOS. Most of these mutations cause premature truncation of the primary TBX5 transcript, thereby presumably causing haploinsufficiency. Conversely, missense mutations diminish the interaction of TBX5 with other transcription factors and reduce nuclear localization of mutant protein. Although mutations are found throughout the TBX5 gene, no evidence exists to suggest that genotype affects the location of heart and limb defects or the severity of HOS manifestation. However, genetic background, and to a lesser extent, environmental and stochastic modifiers are believed to influence greatly the severity of HOS manifestation and may account for the large variation seen in the severity of defects, even among members of the same kindred. Careful clinical examination of patients who seek treatment with heart and limb malformations is necessary to avoid misdiagnosis of similar congenital conditions. With the proper examination, TBX5 mutations can be identified in more than 70% of patients with a clinical diagnosis of HOS. SUMMARY: Genetic analysis of patient populations and the biochemical characterization of the mutated proteins have provided considerable insight into the function of TBX5 in cardiac development and disease pathology. Novel discoveries await as these two paradigms merge.