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Genetic Regulation of Development and Disease
Summary: Matthew Scott is investigating how embryonic and later development is governed by proteins that control gene activity and signaling processes. He is exploring how defects in the regulators of development, or in related proteins, lead to birth defects, cancer, and neurodegenerative disease.
We study Hedgehog (Hh) signaling in flies, mice, and humans. In general we have asked three questions: (1) Where does the signal go from and to? (2) What information does it carry? (3) How is the signal received, transduced, and interpreted?
One particular focus of our work has been Ptc, a Hh receptor protein encoded by a gene called patched (ptc). Hh binds to Ptc on the surfaces of receiving cells and causes them to choose a certain pathway of differentiation (e.g., motor neuron) or to divide. Since Hh and Ptc proteins act in opposition, the Ptc protein provides restraint designed to prevent excessive production of certain types of cells and to rein in growth. We showed that reduction or elimination of ptc function during mouse development leads to spina bifida, polydactyly, midbrain overgrowth, defects in the heart, and excessive body size. We found that mutations in human PATCHED (PTCH) cause a variety of birth defects, medulloblastoma of the cerebellum (the most common childhood malignant brain tumor), and basal cell carcinoma of the skin (the most common human cancer).
We have found that the primary cilium, an immotile appendage found on most cells, serves as an antenna for receiving the Sonic hedgehog (Shh) signal. The transduction machinery of the pathway is contained within such cilia. We have shown that Shh and Ptc control movements of proteins into or out of the cilia as they influence expression of genes. We are investigating the mechanisms through which cilium proteins affect Shh signaling.
We are using mutant mice to learn how tumors arise and to find new ways to stop them. We continue to study the mechanisms and impact of Shh signaling in the cerebellum. Much of our current research is directed at learning the molecular pathways of Hh signal transduction within cells, with a particular focus on the movements of transduction proteins within cells.
Among the genes that are highly expressed in tumors are some that encode components of the sterol synthesis pathway (SSP). These enzymes produce sterols, steroids, and bile acids. We found that medulloblastoma cells in culture have an exceptionally high requirement for activity of the SSP, both to grow and to activate target genes of the Hh pathway. We traced this requirement to a need for certain oxysterols, which are derivatives of cholesterol that have a hydroxyl group attached. Oxysterols are powerful activators of Hh target gene transcription, suggesting that oxysterols or a derivative of them play a role in transduction of the Hh signal. Manipulating such molecules may inhibit cancer cell growth.
Using genomics approaches, we identified many genes active during key early steps of cerebellar growth and in early tumorigenesis. Our general goal is to understand how cells are born, shaped, polarized, and connected. To explore the functions of genes in a way that is amenable to rapid genetic interference, we have collaborated with William Talbot (Stanford University) to study zebrafish cerebellum development. Planar cell polarity (PCP) genes organize cells in the developing Drosophila wing and eye, and in vertebrate gastrulation and inner ear development. We are studying PCP protein functions during cerebellum development.
A Neurodegeneration Syndrome Related to Intracellular Organelle Trafficking
The Niemann-Pick type C (NPC) syndrome has a major impact on the cerebellum. People with a mutated npc gene accumulate cholesterol deposits in their cells, and the Purkinje neurons of the cerebellum degenerate. Our hope is that by understanding the molecular and cellular basis of the disease we will find a way to lessen its severity. The Npc1 protein is closely related to Ptc and to the protein Npc1l1, the major cholesterol-uptake transporter in the mammalian gut. Npc1, Ptc, and Npc1l1 may all be transporter proteins related to the RND transporters, multidrug resistance pumps found in bacteria. We are using engineered mice and biochemical approaches to investigate npc gene functions in cerebellum neurons.
Npc1 and Npc2 are ancient proteins, present even in yeast, and we have developed genetic models of NPC disease in Drosophila to apply the power of genetics to its analysis. The mutant Drosophila, like mammals, accumulate sterols in aberrant intracellular compartments. This leads to deficient synthesis of the molting hormone ecdysone, because steroids like ecdysone are normally made in the mitochondria by enzymes that convert sterols to steroids.
Dramatic movements of proteins and organelles have become apparent in our studies of Hh signal transduction and of Niemann-Pick disease. Many trafficking events depend on a family of remarkable, small GTP-binding proteins called Rab proteins. To investigate the ~30 Drosophila Rab genes systematically, we have collaborated with Hugo Bellen (HHMI, Baylor College of Medicine) to construct a set of transgenic fly lines that allow all Drosophila Rab activities to be reduced or increased. We are using the transgenic flies to investigate how Rab proteins contribute to developmental signal transduction and to development.
Other Projects
We have begun to study the roles of Shh and other regulators in lung stem cells, as well as the roles of the stem cells themselves in development, cancer, and regeneration. Using light-regulated channel proteins that can stimulate or inhibit neurons, we are investigating the development of Drosophila neural circuitry. Proteins involved in gene regulation and signaling emerged from earlier genomics studies of fly and mouse embryos. Projects derived from the genomics data include analysis of chromatin-remodeling proteins in the context of mouse embryonic stem cell development, and studies of Drosophila nucleostemin proteins. These proteins, originally identified in mammals, have been implicated as nucleolar proteins with possible roles in stem cells and RNA processing. Using Drosophila mutations, we are investigating functions of nucleostemin genes during Drosophila development. Initial findings suggest a role in body size control.
Our studies are also supported by grants from the National Institutes of Health, the Ludwig Institute for Cancer Research, the Damon Runyon–Walter Winchell Cancer Fund, the American Cancer Society, the Jane Coffin Childs Foundation, and the Ara Parseghian Medical Research Foundation.
Last updated: March 17, 2008
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