doi: 10.1161/CIRCRESAHA.108.178061
© 2008 American Heart Association, Inc.
Editorials |
Teed Off
Cardiac Conduction System Development Requires T-box Transcription Factors
From the Division of Molecular Cardiovascular Biology, Cincinnati Children’s Medical Center, Ohio.
Correspondence to Katherine E. Yutzey, Division of Molecular Cardiovascular Biology, Cincinnati Children’s Medical Center ML 7020, 240 Albert Sabin Way, Cincinnati, OH 45229. E-mail katherine. yutzey{at}cchmc.org
See related article, pages 1340–1349
Key Words: T-box transcription factors • Tbx3 • conduction system • development
In this issue of Circulation Research, Bakker et al1 report that the T-box transcription factor Tbx3 is required for development of the mouse atrioventricular conduction system. This study adds to the increasingly complex roles for T-box genes in the regulation of cell lineage maturation and proliferation in the developing heart. Previous studies from this same group have implicated Tbx3 in control of mouse sinoatrial node development and pacemaker activity.2 A closely related factor Tbx2 also contributes to atrioventricular (AV) canal specification by repressing genes expressed in the chamber myocardium.3,4 Mutations in human TBX5 cause Holt–Oram syndrome, which includes conduction system disease, and mice with heterozygous mutations in Tbx5 develop progressive AV block and other conduction anomalies.5–7 Additional T-box factors including Tbx1, Tbx20, and Tbx18 are expressed in distinct compartments of the heart and have critical functions in cardiac cell lineage maturation, proliferation, and morphogenesis.8 It is becoming increasingly apparent that multiple T-box transcription factors are important regulators of conduction system differentiation and patterning while also contributing to many other aspects of heart cell lineage development and morphogenesis.
Bakker et al1 report that Tbx3 is required for maturation of the atrioventricular conduction system based on loss of function studies in mice. Mice lacking Tbx3 do not survive past embryonic day (E)14.5 and exhibit abnormal gene expression in conduction system progenitor cells, as well as severe structural malformations, including double outlet right ventricle and ventricular septal defects. Particular attention was paid to the crest of the interventricular septum, where the AV bundle components of the central conduction system will develop. In normal mice, the myocytes in the crest of the interventricular septum diversify from the rest of the septum myocardium with increased expression of Tbx3, Tbx5, and Nkx2.5, as well decreased expression of connexin 43 (Cx43), Tbx18, Tbx20, and atrial natriuretic factor (Nppa). In the mice lacking Tbx3, increased expression of Tbx18, Tbx20, Nppa, and Cx40 is apparent in the crest of the interventricular septum, but Tbx5 and the Tbx3 mutant allele are expressed normally. These observations indicate that the diversification of the crest myocardium from the interventricular myocardium is dependent on Tbx3 but that the crest progenitors are present as indicated by Tbx5 and Tbx3 gene expression. In addition, cell proliferation is increased in the crest of the septum in embryos lacking Tbx3. Together, these data support a role for Tbx3 in repressing expression of septal myocardial genes and inhibiting proliferation in AV bundle progenitor cells.
Heterozygous mutations in the human TBX3 gene cause ulnar mammary syndrome (UMS), characterized by upper limb malformations and mammary gland hypoplasia, as well as dental and genital abnormalities.9 Strikingly, cardiac structural anomalies and conduction system disease are not part of the syndrome, although sporadic incidence of cardiovascular disease has been reported in individuals with TBX3 mutations.10 The initial reported human TBX3 mutations were predicted to result in haploinsufficiency, but additional point mutations with potential dominant negative function were identified in a family with UMS and congenital heart malformations. Therefore, the cardiovascular and ulnar mammary phenotypes of individuals with TBX3 mutations are likely dependent on the specific function of the mutant allele. Heterozygous Tbx3 mutant mice are apparently normal and do not exhibit any of the characteristic anomalies of UMS.11 Given the abnormal differentiation of conduction system in Tbx3 null embryos, it is surprising that no conduction system functional anomalies were observed in the null embryos or heterozygous mice.1 The apparently normal function of the cardiac conduction system in mice lacking Tbx3 and in human UMS suggests that Tbx3 is not absolutely required for normal cardiac conduction but that compensatory or additional molecular pathways contribute to specialization and function of these myocyte populations.
The T-box gene family can be divided into subfamilies based on conservation of protein structure and function as transcriptional repressors and/or activators (Figure).12 Tbx3 and Tbx2 are strong transcriptional repressors and are coexpressed in the AV canal and primitive myocardium. They are differentially expressed in the peripheral and central conduction system, where they inhibit cell proliferation and control lineage-specific gene expression.1,2 Tbx5 is a transcriptional activator expressed in the central conduction system, as well as the atrial and left ventricular chamber myocardium.7 Heterozygous mutations in Tbx5 in humans or mice cause conduction system disease, and Tbx5 also promotes cell proliferation and gene expression characteristic of chamber maturation.3,7 Members of the Tbx1 subfamily, including Tbx1, Tbx20, and Tbx18, have both transcriptional activation and repressor functions and can promote proliferation while inhibiting maturation of diverse cardiac cell populations.8
|
Multiple T-box transcription factors of different subfamilies are often coexpressed and can modulate the expression of one another. This is apparent in the increased expression of Tbx18 and Tbx20 in the crest of the interventricular septum of embryos lacking Tbx3.1 Tbx20 also has been shown to repress expression of Tbx2 in the primitive myocardium, thereby affecting cell proliferation through dysregulation of N-myc gene expression.8 Because T-box factors in each subfamily bind the same consensus DNA sequence, the balance of activator and repressor T-box factors present likely determines the levels of target gene expression. Multiple T-box proteins can affect the transcription of the Nppa promoter, with Tbx2, -3, -20, and -18 acting as repressors and Tbx5 functioning as an activator in conjunction with other cardiac transcription factors such as Nkx2.5 and GATA4.3,7,12,13 Therefore, the complex network of T-box gene expression and protein function regulates diverse aspects of cardiac cell differentiation and maturation with profound effects on heart morphogenesis and function.
There is emerging evidence that multiple T-box proteins control specialization and patterning of the developing conduction system. Tbx2, Tbx3, and Tbx5 are expressed in distinct compartments of the central and peripheral conduction system.14 Loss of Tbx3 inhibits specialization of the AV bundle and sinoatrial node precursor cells and increased expression of Tbx3 in the atrial myocardium promotes pacemaker function. Tbx5 is expressed throughout the central conduction system, and mice with heterozygous mutation of Tbx5 or individuals with Holt–Oram syndrome commonly exhibit AV block and conduction system disease.7 T-box downstream target genes include Cx43, which can be repressed by Tbx3, and Cx40, which can be activated by Tbx5.15 These genes are differentially expressed in conduction system and chamber myocardial cell lineages. Likewise Tbx2 can antagonize Tbx5 downstream gene expression by acting as a transcriptional repressor.3 Because Tbx2 and Tbx3 are coexpressed in the atria, AV canal, and primitive myocardium, it is possible that they can compensate for each other in establishing the earliest cardiac pacemaker activity of the midgestation embryo. Potential overlap in Tbx2 and Tbx3 function may underlie the apparently normal conduction system function in embryos lacking Tbx3, and it would be interesting to know if mouse embryos lacking both Tbx2 and Tbx3 can establish initial pacemaker activity in the primitive heart. However, loss of either Tbx2 or Tbx3 leads to embryonic lethality at E12 to E14, demonstrating that each has independent nonredundant functions in embryogenesis.
The apparently normal cardiac conduction and pacemaker activity in Tbx3 mutant mouse embryos underscores the complexity of conduction system development and function. The initial specialization of conduction system myocytes involves secreted factors including neuregulin and endothelin, but their effects on T-box expression and function have not been reported.16 It is likely that multiple signaling pathways and transcriptional regulatory networks contribute to the patterning and diversification of components of the central and peripheral conduction system. T-box transcription factors are key components of complex regulatory networks in many parts of the embryo, and all evidence supports similar complex mechanisms in the specialization of myocyte populations in the heart. Future studies will likely identify additional T-box–interacting proteins and modifying factors critical for specialization and patterning of the conduction system. The embryonic lethality of mice lacking Tbx2 or Tbx3 at E12 to E14 has precluded the assessment of their roles in later stages of development or the adult cardiac conduction system. Therefore, more targeted approaches will be necessary to determine all of the requirements for these T-box proteins in conduction system development and function. The work from the laboratory of Christoffels1 and other investigators has provided new insights into the role of T-box proteins in the early development and maturation of the specialized conduction system. These studies form the foundation for future work in the assessment of the contributions of these factors to adult conduction system function and disease mechanisms.
 |
Acknowledgments |
|---|
Sources of Funding
Research in the laboratory of the author is supported by the NIH/National Heart, Lung, and Blood Institute and the American Heart Association.
Disclosures
None.
|
Footnotes |
|---|
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
|
References |
|---|
 Top References |
|---|
1. Bakker ML, Boukens BJ, Mommersteeg MTM, Brons JF, Wakker V, Moorman AFM, Christoffels VM. Transcription factor Tbx3 is required for the specification of the atrioventricular conduction system. Circ Res. 2008; 102: 1340–1349.
2. Hoogaars WMH, Engel A, Brons JF, Verkerk AO, de Lange FJ, Wong LYE, Bakker ML, Clout DE, Wakker V, Barnett P, Revesloot JH, Moorman AFM, Verheijck EE, Christoffels VM. Tbx3 controls the sinoatrial node gene program and imposes pacemaker function on the atria. Genes Dev. 2007; 21: 1098–1112.
3. Habets PE, Moorman AF, Clout DE, van Roon MA, Lingbeek M, van Lohuizen M, Campione M, Christoffels VM. Cooperative action of Tbx2 and Nkx2.5 inhibits ANF expression in the atrioventricular canal: implications for cardiac chamber formation. Genes Dev. 2002; 16: 1234–1246.
4. Harrelson Z, Kelly RG, Goldin SN, Gibson-Brown JJ, Bollag RJ, Silver LM, Papaioannou VE. Tbx2 is essential for patterning the atrioventricular canal and for morphogenesis of the outflow tract during heart development. Development. 2004; 131: 5041–5052.
5. Li QY, Newbury-Ecob RA, Terrett JA, Wilson DI, Curtis AR, Yi CH, Gebuhr T, Bullen PJ, Robson SC, Strachan T, Bonnet D, Lyonnet S, Young ID, Raeburn JA, Buckler AJ, Law DJ, Brook JD. Holt-Oram syndrome is caused by mutations in TBX5, a member of the Brachyury (T) gene family. Nat Genet. 1997; 15: 21–29.[CrossRef][Medline] [Order article via Infotrieve]
6. Basson CT, Bachinsky DR, Lin RC, Levi T, Elkins JA, Soults J, Grayzel D, Kroumpouzou E, Traill TA, Leblanc-Straceski J, Renault B, Kucherlapati R, Seidman JG, Seidman CE. Mutations in human TBX5 cause limb and cardiac malformation in Holt-Oram syndrome. Nat Genet. 1997; 15: 30–35.[CrossRef][Medline] [Order article via Infotrieve]
7. Bruneau BG, Nemer G, Schmitt JP, Charron F, Robitaille L, Caron S, Conner DA, Gessler M, Nemer M, Seidman CE, Seidman JG. A murine model of Holt-Oram syndrome defines roles of the T-box transcription factor Tbx5 in cardiogenesis and disease. Cell. 2001; 106: 709–721.[CrossRef][Medline] [Order article via Infotrieve]
8. Stennard FA, Harvey RP. T-box transcription factors and their roles in regulatory hierarchies in the developing heart. Development. 2005; 132: 4897–4910.
9. Bamshad M, Lin RC, Law DJ, Watkins WC, Krakowiak PA, Moore ME, Franceschini P, Lala R, Holmes LB, Gebuhr TC, Bruneau BG, Schinzel A, Seidman JG, Seidman CE, Jorde LB. Mutations in human TBX3 alter limb, apocrine and genital development in ulnar-mammary syndrome. Nat Genet. 1997; 16: 311–315.[CrossRef][Medline] [Order article via Infotrieve]
10. Meneghini V, Odent S, Platonova N, Egeo A, Merlo GR. Novel TBX3 mutation data in families with Ulnar-Mammary syndrome indicate a genotype-phenotype relationship: mutations that do not disrupt the T-domain are associated with less severe limb defects. Eur J Med Genet. 2006; 49: 151–158.[CrossRef][Medline] [Order article via Infotrieve]
11. Davenport TG, Jerome-Majewska LA, Papaioannou VE. Mammary gland, limb and yolk sac defects in mice lacking Tbx3, the gene mutated in human ulnar mammary syndrome. Development. 2003; 130: 2263–2273.
12. Plageman TF Jr, Yutzey KE. T-box genes and heart development: putting the "T" in hearT. Dev Dynam. 2005; 232: 11–20.[CrossRef][Medline] [Order article via Infotrieve]
13. Farin HF, Bussen M, Schmidt MK, Singh MK, Schuster-Gossler K, Kispert A. Transcriptional repression by the T-box proteins Tbx18 and Tbx15 depends on Groucho corepressors. J Biol Chem. 2007; 282: 25748–25759.
14. Hoogaars WMH, Barnett P, Moorman AFM, Christoffels VM. T-box factors determine cardiac design. Cell Mol Life Sci. 2007; 64: 646–660.[CrossRef][Medline] [Order article via Infotrieve]
15. Hoogaars WMH, Tessari A, Moorman AF, de Boer PAJ, Hagoort J, Soufan AT, Campione M, Christoffels VM. The transcriptional repressor Tbx3 delineates the developing central conduction system of the heart. Cardiovasc Res. 2004; 62: 489–499.
16. Mikawa T, Hurtado R. Development of the cardiac conduction system. Semin Cell Dev Biol. 2007; 18: 90–100.[CrossRef][Medline] [Order article via Infotrieve]
Related Article:
Circ. Res. 2008 102: 1340-1349.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||