Stephen X. Skapek, M.D.

Studies at the Intersection of Cancer and Developmental Biology

Research Summary

At the intersection of cancer and developmental biology

The major theme is my laboratory is that cancer can be viewed, in part, as a developmental disease.  To be sure, many of the essential “hallmarks” of cancer – such as the capacity to migrate throughout the body, survive in stressful situations, and promote new blood vessel growth – represent developmental programs that have seemingly been co-opted by the malignant cell.  Further, many types of tumor cells seem to represent cells that have become “arrested” at a particular phase of cell lineage development.  In some situations, this apparent blockade in differentiation allows cells to continue to proliferate – another of the essential hallmarks of cancer. Interestingly, higher organisms are endowed with certain genes that act to block cancer formation.  These so-called tumor suppressor genes act in a variety of ways to impede aspects of cancer development.  In my laboratory, we are exploring how certain, key tumor suppressor genes promote critical aspects of cellular or organism development with ultimate goal of developing novel therapies based on the understanding of the developmental functions of the tumor suppressor genes.  To accomplish our goals, we take advantage of a wide variety of molecular and cell biology tools and histological techniques with cell culture-based systems, mouse models and human tissue samples.

The tumor suppressor gene Arf promotes vascular involution during eye development:   Its role as a regulator of pericyte-endothelial interactions and implications for physiological and pathological processes

The Arf gene is a bona fide tumor suppressor gene that is disrupted in many human cancers.  The current dogma holds that its gene product, p19Arf, is a nuclear protein that controls cell proliferation and can promote apoptosis by signaling to another nuclear protein, p53.  Interestingly, p19Arf has a several other effects that are independent of p53 – but their importance is less well established.

Several years ago, while studying the tumor suppressor effects of Arf, we discovered that mice lacking this gene are blind.  Without Arf, the developmentally regulated involution of certain blood vessels in the eye fails to occur in the first two weeks of life.  This is very interesting to us because it could indicate that Arf may also block the growth of blood vessels into malignant tumors, a process that is critical for their growth and metastasis.  Our findings show that this developmental function for p19Arf relates to its capacity to control a key growth factor (Platelet-derived growth factor), which also plays a role in promoting cancer cell growth.   We are taking several approaches to explore (1) how p19Arf controls certain vascular cells to promote the regression of these blood vessels; (2) how it controls the expression of Platelet-derived growth factor; (3) what controls the expression of Arf during development and in nascent cancer cells; (4) whether other developmental or physiological processes are controlled by Arf; (5) if tumor cell growth control by p19Arf is mediated by its control of blood vessels or this particular growth factor.
 

Mechanisms regulating the initiation of skeletal muscle differentiation: potential insight into rhabdomyosarcoma biology

The Retinoblastoma susceptibility gene (Rb) represents another crucial mammalian tumor suppressor gene.  Rb was first discovered in children who develop highly-malignant tumors of the retina (retinoblastoma).  As a tumor suppressor, it was originally thought that the Rb protein merely functioned to block the transition from the G1 phase into the S phase of the cell cycle – essentially acting as a “brake” to cell proliferation.  If Rb acts as a brake, one could imagine that an accelerator would also exist to foster cell proliferation when needed.  This process is fulfilled by proteins called Cyclins and their catalytic partners, Cyclin-dependent kinases (Cdks), which phosphorylate and inactivate Rb.  Essentially every human cancer has some disruption in Rb function, either by Rb gene mutation as noted above or by deregulation of Cyclin/Cdk complexes to impede Rb protein function. 

It is very clear that Rb also facilitates a variety of cellular developmental processes.  Indeed, it is absolutely required for a variety of aspects of mouse embryo development.  One classic example centers on the developing skeletal muscle.  As skeletal myoblasts mature, they must cease to proliferate in order to fully express proteins needed to make a normal skeletal myocyte.  Without Rb, cell cycle arrest does not occur and skeletal muscle maturation is severely impaired.

Interestingly, there is a highly-malignant cancer that is composed of skeletal muscle-like cells (rhabdomyosarcoma).  At a molecular level, rhabdomyosarcoma cells seem to represent skeletal myoblast-like cells that have somehow lost the capacity to cease proliferating and complete the terminal differentiation process.  In my laboratory, we have established that impairment of Rb protein function by deregulation of Cyclin/Cdk activity may underlie the arrested development.  We are currently using candidate-pathway approaches to explore why rhabdomyosarcoma cells do not differentiate; and we are developing screens that can be used to explore whether drug-like compounds can promote muscle differentiation and its attendant cell proliferation arrest as a new treatment approach.

Selected Papers

Skapek SX, Jansen D, Wei TF, McDermott T, Huang W, Olson EN, and Lee EYHP.  Cloning and characterization of a novel KRAB family transcriptional repressor that interacts with the retinoblastoma gene product, RB.  J Biol Chem 275: 7212-7223; 2000.

Lin SCL, Skapek SX, Papermaster DS, Hankin M, and Lee EYHP.  The proliferative and apoptotic activities of E2F1 in the mouse retina.  Oncogene 20: 7073-7084, 2001.

Skapek SX, Lin SCL, Jablonski M, McKeller R, Hu NP, and Lee EYHP.  Persistent expression of Cyclin D1 disrupts normal photoreceptor differentiation and retina development.  Oncogene 20: 6742-6751, 2001.

McKeller RN, Fowler JL, Cunningham JJ, Warner NW, Smeyne RJ, Zindy F, and Skapek SX.  The Arf tumor suppressor gene promotes hyaloid vascular regression during mouse eye development. Proceedings of the National Academy of Science 99: 3848-3853, 2002.

Martin AC, Thornton JD, Liu J, Wang XF, Zuo J, Jablonski MM, Chaum E, Zindy F, and Skapek SX.  Pathogenesis of persistent hyperplastic primary vitreous in mice lacking the Arf tumor suppressor gene.  Investigative Ophthalmology and Visual Sciences 45: 3387 – 3396, 2004.

Silva RLA, Martin AC, Thornton JD, Rehg JE, Berwistle D, Zindy F, and Skapek SX.  Arf-dependent regulation of Pdgf signaling in perivascular cells in the developing eye.  The EMBO Journal 24: 2803-2814; 2005.

Thornton JD, Silva RLA, Martin AC, and Skapek SX. The Arf tumor suppressor regulates Platelet-derived growth factor receptor b signaling: a new view through the eyes of Arf -/- mice. Cell Cycle 4: 1316-1319; 2005.

Saab RH, Bill JL, Miceli A, Anderson CM, Khoury J, Fry DW, Navid F, Houghton PJ, and Skapek SX.  Pharmacological inhibition of cyclin-dependent kinase 4 arrests proliferation in myoblasts and rhabdomyosarcoma-derived cells.  Molecular Cancer Therapeutics 5: 1299-1308; 2006.

Thornton JD, Swanson D, Mary MN, Pei D, Martin AC, Pounds S, Goldowitz D, and Skapek SX.  Persistent hyperplastic primary vitreous due to somatic mosaic deletion of the Arf tumor suppressor.  Investigative Ophthalmology and Visual Sciences 48: 491-499, 2007.