Cardiology; Heart Diseases; Kartagener Syndrome; Situs Inversus; Heterotaxy Syndrome
My laboratory focuses on the cause(s) of a type of congenital heart disease called heterotaxy. Patients with heterotaxy don’t have normal development of their organs along the left-right body axis (some have the heart on the right or in the middle, instead of the normal left), and more than 90% of them have severe congenital heart disease. Currently, we are developing a large-scale international collaboration using state of the art genomic technology to identify the genes causing heterotaxy in humans. In addition, we study mice with CHD to better understand how the left-right axis is formed, and how the heart develops.
Although our medical and surgical management of patients with congenital heart disease has made tremendous progress in the past 25 years, the understanding of why CHD develops remains relatively limited. One challenge in the care of patients with CHD is that different patients with anatomically very similar disease can have greatly disparate long-term outcomes. The mechanism underlying any individual patient’s CHD may have as of yet unknown impact on how well they do, and it is hoped that eventually it will become possible to tailor medical management and surgery more specifically based on an individual’s combination of anatomical abnormality and underlying developmental defect. If ciliary defects are responsible for some human CHD, it may become possible to treat the later manifestations of the disease, such as the myocardial dysfunction observed in many adult patients who have had successful surgery for their CHD, with drugs aimed at restoring more normal ciliary signaling.
Specialized Terms: Development of left-right asymmetry; Heterotaxy syndrome; Kartagener syndrome; Situs inversus
Extensive Research Description
Development of vertebrate left-right asymmetry
Cilia in development
The development of non-random asymmetry along the left-right axis is a unique feature of vertebrate development. Defects in this process in mouse and man commonly affect the development of the heart and result in severe congenital cardiac anomalies. The goal of my laboratory is to understand the mechanism by which embryonic cilia create and signal left-right positional information, and to investigate whether cilia have essential roles in other developmental processes. We have previously shown that the vertebrate LR axis is initiated at the mammalian node late in gastrulation. Dynein-driven motility of monocilia found on node cells generates directional flow of extraembryonic fluid, termed “nodal flow”. The direction of nodal flow is determined by the inherent chirality of the cilium itself, and artificial reversal of nodal flow is able to reverse the LR axis. Nodal flow sets up an asymmetric calcium signal found in the cells at the left border of the node. The development of LR asymmetry is also abnormal in mice with defects in the polycystin gene Pkd2, which functions in kidney monocilia as a mechanotransducer by mediating an intracellular calcium signal in response to fluid flow in the renal tubule. We have shown that Polycystin-2 protein localizes to a subset of node monocilia that are non-motile. This data suggests that embryonic cilia may be required to both create and sense nodal flow. The major foci of interest in my lab are currently:
How is the asymmetric perinodal calcium signal transmitted to the developing organs to result in asymmetric morphogenesis? Calcium signaling is being investigated by live-imaging of mouse embryos with a range of mutations affecting cilia biogenesis and function. To evaluate the relationship between perinodal calcium signals and asymmetric morphogenesis, asymmetric gene expression patterns are being correlated with calcium signals. At the cellular level, we are focusing on the inversin protein as a potential transducer of the perinodal calcium signal. Inversin is a highly conserved protein required for LR development, and it has calmodulin binding domains that are essential for its function in LR development. Studies are addressing the mechanism of inversin function in mice and in cultured polarized epithelial cells. We are using time-lapse imaging of live mouse embryos with fluorescently tagged node cells to study the potential link between perinodal calcium signals and migration of node cells.
What is the role of ciliary genes in human heterotaxy? We are collaborating with the laboratory of Richard Lifton in the Dept. of Genetics at Yale to perform a large-scale genomic analysis of human patients with heterotaxy. Analysis of copy-number variation in a cohort of patients with Htx identified >40 novel candidate genes for human Htx. These genes cluster in several developmental pathways, some of which have not previously been linked to LR development. We are validating the role of these genes in animal systems including mouse, Xenopus and zebrafish, and beginning more detailed analysis of their function within the molecular framework established for LR development.
Do cilia play roles in cardiac development independent of their function in LR development? Since cilia function as mechanosensors in other fluid-filled organs they could also be fluid flow sensors in heart development. We have shown that cilia are found in the mouse embryo heart at e8.5 – e12.5, and that heart development in mice with absent cilia is much more severely affected than in mice with paralyzed cilia and isolated abnormalities of LR development. We are using mice with conditional mutations affecting cilia structure and function to generate mice with cardiac-specific ciliary abnormalities in order to investigate the mechanism by which cardiac cilia function directly in heart morphogenesis.
- Nodal flow and Calcium signaling in vertebrate left-right development
- Mechanism of Inversin function in cilia and asymmetry development
- Cardiac cilia and epithelial mesenchymal transformation in cardiac morphogenesis
- TGFbeta signaling in heart development
- Genetic control of human heterotaxy
- J. J Essner, K.J.Vogan, M.K.Wagner, C.J.Tabin, H.J.Yost, and M.Brueckner. Conserved function for embryonic nodal cilia. Nature 418:37-38 (2002)
- J.McGrath, S.Somlo, S.Makova, X.Tian and M.Brueckner. Two populations of node monocilia initiate left-right asymmetry in the mouse. Cell 114:61-73 (2003)
- M.Brueckner. Heterotaxia, congenital heart disease, and primary ciliary dyskinesia. Circulation 115:2793-5 (2007)
- D.B. Tashjian, B. Weeks, M.Brueckner, R.J.Touloukian. Outcomes after Ladd procedure for intestinal malrotation with heterotaxia. Journal of Pediatric Surgery 42:528-31 (2007)
- J.Slough, L.Cooney, M.Brueckner. Monocilia in the embryonic mouse heart suggest a direct role for cilia in cardiac morphogenesis. Developmental Dynamics 237:2304-2314 (2008)