Our research focuses on the genetic and cellular mechanisms that control embryonic development. In particular we are interested in the processes required for closure of the neural tube and for development of the vertebrate limb and lung. Our approaches include a forward genetic screen in mice to identify mutations that affect specific processes of embryonic development and embryological and molecular manipulation of the chick embryo.
ENU-mutagenesis screen for genes required for mouse development. A forward genetic approach is ideal for identifying key genes that regulate specific developmental processes of interest. In an on-going screen in mice for ENU (ethylnitrosourea)- generated mutations, we are identifying recessive mutations that disrupt specific aspects of embryonic development. We have focused on mutations that affect neural tube closure, limb and lung development. To learn more about the screen, to view pictures of the various mutants, and to follow our progress in identifying the genes responsible and their mechanism of action, please go to our Website - http://mouse.ski.mskcc.org/
Neural tube defects (NTD) are the second most common human birth defect, affecting 1:1,000 births. However, the underlying genetic basis of NTD in humans is very poorly understood. Failure to close the neural tube in the brain leads to exencephaly (anencephaly in humans), failure to close the spinal neural tube causes spina bifida. Neural tube closure involves a variety of coordinated developmental processes including pattern formation, proliferation, cell differentiation, and changes in cell shape and cell movement. The mouse provides an excellent model to systematically identify the genes that are required for closure of the neural tube. The chick embryo provides an excellent experimental system to test the function of the identified genes.
Our studies seek to identify the genes required and their mechanism of action.
Genetics of neural tube closure.
To advance an understanding of the causes of birth defects of the brain and spinal cord in humans, we are using the mouse as a model to systematically identify the genes involved in closure of the neural tube, followed by a comprehensive cellular and molecular characterization of their mechanism of action. From the ENU mutagenesis screen we have identified a large number of mouse lines with NTDs. Our approach is to identify the gene by genetic mapping and to characterize the phenotype to elucidate the developmental process that is affected. Priority will be given to mutations that are linked to potential human disease loci. Moreover, we will test whether the NTD can be suppressed by dietary supplementation to understand the genetics underlying the effects of, for instance, folic acid which can suppress the incidence of NTDs by an unknown mechanism.
Experimental studies of neural development - BMPs and WNTs control patterning and growth of the neural tube
The chick provides an experimentally amenable and rapid system to determine the function of a gene. We are using the chick to functionally characterize the genes arising from the mutagenesis screen, as well as other developmentally important genes. In the latter category, we have explored the role of the Bone Morphogenetic Protein (BMP) and WNT signaling in coordinating patterning and growth in the embryonic spinal neural tube using gain and loss of function approaches including in vivo knockdown of gene expression by siRNAs. We found that BMPs are necessary to regulate pattern formation, including the establishment of discrete expression domains of Wnt signaling components along the dorsal-ventral axis of the neural tube and that WNTs function as mitogenic signals regulating neural tube growth. Thus, BMPs, acting through WNTs, couple patterning and growth to generate dorsal neuronal progenitor populations in the appropriate proportions within the neural tube.
Key signals that control limb growth and patterning have been identified in the past 10 years, yet surprisingly little is known about how these regulators function, what are their upstream activators and downstream targets, or how patterning information is translated into skeletal elements of appropriate size and shape.
Our interests in limb development continue to revolve around the formation of the skeletal primordia and the function of the apical ectodermal ridge (AER). The AER is the critical signaling center that controls limb outgrowth via the production of Fibroblast growth factors (FGFs). We are exploring the mechanisms that regulate AER formation and function through molecular manipulations of the chick embryonic limb, imaging techniques and mouse genetics. Our studies of the chick limb have determined that BMPs are upstream of two important developmental processes, that of AER formation and dorsal-ventral patterning. The BMP signal bifurcates at the level of two different transcription factors to mediate AER formation and dorsal-ventral patterning differentially.
A disruption in AER formation and function appears to be the primary defect in a mouse mutant we found in the mutagenesis screen that leads to missing digits, dorsal-ventral digit duplications, and aberrant ossification. We have identified other limb mutants that cause polydactyly (extra digits), soft tissue syndactyly (the webbing between the digits does not undergo regression), and other skeletal defects including shortening of some limb elements. Moreover, we are further exploring genetic interactions between various mutant lines to determine whether they regulate similar functions or act within a genetic pathway. These studies provide an unbiased means to identify key developmental regulators and will contribute significantly to a greater understanding of the genetic and cellular control of vertebrate limb development. (Grants from the National Institutes of Health provided support for the limb work.)
Lung Development and Disease
The lung and other highly branched organs such as the kidney and lacrimal gland develop from a simple epithelial bud into a complex three dimensionally patterned functional organ. This happens through a process called branching morphogenesis. We wish to elucidate the fundamental processes underlying the development of the lung, as well as the pathogenesis of lung disease. To do so we use a combination of forward and reverse genetics in mice and molecular manipulations in organ cultures to identify key regulators of branching morphogenesis. We have identified novel regulators of lung development through the mouse ENU mutagenesis screen. Moreover we have manipulated key signaling pathways such as BMP and WNT to elucidate the mechanisms by which the position and shape of the bud are determined. The future goal is to study if and how developmentally important genes are involved in tissue injury and repair, in particular in diseased lungs. (This work is supported by the National Institutes of Health and by the Sandler Program for Asthma Research)