Reprogramming 2.0

Areas of investigation
Our laboratory focuses on the fundamental events involved in cell fate determination and differentiation and organogenesis. Specifically, we investigate the molecular events regulating developmental decisions that instruct cardiac progenitor cells to adopt a cardiac cell fate and subsequently fashion a functioning heart. We focus on transcriptional and post-transcriptional steps, including those involving microRNAs. We have leveraged this knowledge to reprogram fibroblasts directly into cardiomyocyte-like cells for regenerative purposes. We also investigate the causes of human cardiovascular disease by applying modern genetic technologies for the study of complex traits such as congenital heart disease. By using fly, mouse, and human genetics, we hope to develop a broad understanding of the biology underlying cardiogenesis and cardiovascular disorders.

Significance
Heart disease is the number one killer on both sides of the age spectrum, resulting in significant mortality and morbidity in children and adults. Congenital heart disease occurs in one of every one hundred live births and results from abnormalities of early cues that guide embryonic stem cells to form the four-chambered heart. Discovery of the genetic causes and molecular mechanisms of congenital heart disease will provide potential for novel preventive and therapeutic approaches in children. We have shown that the same knowledge can be used to devise clever ways of helping the injured adult heart repair itself by using some of the tools common to younger, embryonic hearts. Finally, the molecular instructions that enable the embryo to make a heart will be useful in directing stem cells to make new heart cellsóa potential treatment for heart failure or severe heart damage.

Approaches
We use many approaches to answer our research questions, including mouse and human genetics, molecular and developmental biology, and biochemistry. Transgenic mouse models are instrumental for elucidating the molecular pathways that regulate cardiac differentiation and the three-dimensional organization of the heart in vivo. To understand their contribution to disease, pathways discovered in mice and other model systems can be interrogated by mutational analysis of cells from humans with heart disease. In addition, we use traditional human genetic approaches to study families with autosomal dominant congenital heart defects to discover the genetic causes of human disease. Molecular and biochemical analyses of normal and mutated human genes, including the study of disease-specific induced pluripotent stem (iPS) cells, provide insights into the mechanisms underlying normal and abnormal cardiac developmental decisions. Finally, we use mouse models of myocardial infarctions (heart attacks) to evaluate developmental genes for their efficacy as therapeutic agents for acquired heart disease. The potential for cardiac developmental genes to reprogram stem cells into the cardiac fate will be an important approach for future studies.

Contributions
Our laboratory has elucidated a cascade of transcriptional and signaling events that control the early steps of cardiomyocyte differentiation and expansion into ventricular chambers. We found that muscle-specific histone methyltransferases and microRNAs regulate the activity of Hand2, a transcription factor essential for ventricle formation and more recently showed that microRNAs can efficiently guide stem cell fate decisions. We generated the first mouse ìknockoutî of a microRNA and showed that even decreasing dosage of a microRNA can have dramatic consequences on multiple aspects of cardiovascular function. We have subsequently found miRNAs that direct cardiac muscle, smooth muscle, and endothelial cells from pluripotent stem cells. In addition, we discovered a series of signaling events beginning with the morphogen Sonic hedgehog (Shh) that are essential for guiding a population of late cardiac progenitor cells in the outflow tract of the heart. These same cells form niches of cardiac progenitor cells postnatally. This pathway involves the transcription factor Tbx1, heterozygosity of which causes cardiac defects associated with DiGeorge syndrome. Finally, we used human genetics to discover the cause of some human cardiac septal defects (GATA4) and valve diseases (NOTCH1) and revealed the mechanisms through which mutations in these genes result in anomalies. As hoped, we found that a developmental gene, thymosin β4, has potent properties for cardioprotection in the setting of heart attacks in mice. We are now moving this discovery into Phase II clinical trials (FDA approved) in patients suffering ischemic damage to the heart.

Some questions addressed in ongoing studies:

  1. What are the direct targets of key transcription factors that regulate cardiogenesis and cardiomyocyte differentiation?
  2. How do mutations in human disease genes, such as TBX1, GATA4, and NOTCH1, actually cause disease and how could anomalies be prevented even in the setting of mutations?
  3. Do combinatorial human mutations/polymorphisms in cardiac developmental genes cause predisposition to disease?
  4. How do microRNAs regulate cardiogenesis and cardiac stem cells?
  5. Are microRNAs involved in human disease?
  6. How do microRNAs recognize their targets and how can one predict targets?
  7. How does thymosin β4, or related pathway members, protect tissues from ischemic damage?