faculty photo

Stephen Aaron Liebhaber, M.D.

Professor of Genetics
Department: Genetics
Graduate Group Affiliations

Contact information
560A Clinical Research Building
415 Curie Boulevard
Philadelphia, PA 19104-6145
Office: 215-898-7834
Fax: 215-573-5157
B.A. (Chemistry)
Brandeis University, Waltham, Massachusetts (Magna Cum Laude), 1968.
Yale University, New Haven, Connecticut (with honors), 1972.
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Description of Research Expertise

Research Interests
- Roles of chromatin structure and epigenetic controls in eucaryotic gene activation
- Roles of mRNA-protein interactions in control of eucarytic mRNA stability and expression

Key words: Chromatin structure, histone modifications, Growth Hormone gene, pituitary, placenta, transcriptional controls, mRNA expression, αCP RNA binding proteins, mRNA stability, globin mRNA.

Description of Research
Roles of chromatin structure and epigenetic controls in eucaryotic gene activation

In eukaryotic organisms gene regulation is dependent upon developmentally controlled alterations in chromatin structure. Epigenetic modifications in histones and DNA result in selective activation of gene expression profiles. In many cases these modifications in chromatin structure result in long-range control of gene promoters, reaching over hundreds of kbs. Understanding how these epigenetic modifications are themselves controlled and the mechanisms by which they selectively activate and/or silence cohorts of genes is central to our understanding of development and cellular differentiation. We are approaching these questions using as a model the human Growth Hormone gene cluster. The genes in this cluster are robustly expressed, physiologically and developmentally controlled, and highly specific to either pituitary somatotropes or placental syncytiotrophoblasts. These controls are conserved between mouse and human. Thus our studies are heavily dependent on the use of transgenic mouse models and are complemented where appropriate with in vitro chromatin analyses and cell culture models.

Roles of mRNA-protein interactions in control of eucaryotic mRNA stability and expression

At any given level of gene transcription, the final level of expression for a particular gene can be altered over a span of several orders of magnitude by modulation in mRNA stability or translational control. For example the half-lives of specific mammalian mRNAs can be as short as 15 minutes and as long as several days. Such post-transcriptional controls are dependent on sequence specific interactions between RNA binding proteins and target mRNAs. We are using a series of model mRNAs and expression systems to explore the role of a specific and highly abundant family of mRNA binding proteins, the αCPs, in mRNA stabilization and a variety of additional post-translational controls. These studies are based on in vitro RNA-protein interaction assays and cell-based expression models. Comprehensive studies of αCP functions are being carried out using immunoaffinity RNP-isolation, Affymetrix chip expression platforms, artificial tethering of RNA binding proteins to mRNAs, and biochemical analysis of RNP sub-localization. We are testing novel models by which mRNAs are stabilized by alterations in the stability of their 3' polyA tails. These analyses of mRNA decay are also revealing novel pathways of mRNA surveillance pathways and mRNA structural modification that impact on the expression of both wild-type and mutant gene expression.

Activation of placental hormone gene expression.
The hGH multigene locus contains a subset of GH-related genes expressed robustly and exclusively in the placenta. We are able to model this expression in transgenic mice carrying the intact hGH gene cluster with remote regulatory elements. Our current studies are aimed at identifying the cis-acting chromatin-based determinants of this highly restricted and developmentally controlled pattern of expression. These studies are carried out in transgenic mice and in ES and trophoblast stem (TS) cells derived from our various mouse lines carrying the hGH locus and addtional site-directed mutations. These studies, in conjunction with our studies of the hGH locus in the pituitary comprise an integrated approach to a chromatin locus with multiple developmental expression profiles and potentials.

Rotation Projects
Lab rotations are available in the areas of chromatin structure/gene activation and in the area of mRNA-protein interactions/mRNA stability control. Studies making extensive use of mouse transgenic models, stem cell cultures, and a variety of in vitro approaches can be explored. Students are encouraged to contact Dr. Liebhaber directly to discuss potential projects for rotation studies.

Lab personnel:
Please see our lab web-site for this information. Our laboratory has a steady state of approximately 10 doctoral and postdoctoral scientists.

Selected Publications

Ji, Xinjun, Kong, J, and Liebhaber, SA: An RNA-protein Complex Links Enhanced Nuclear 3’ Processing with Cytoplasmic mRNA Stabilization. EMBO J. 2011 Notes: In Press.

Ho, Y., Liebhaber, S. A., Cooke, N. E.: The Role of the hGH Locus Control Region in Somatotrope Restriction of hGH-N Gene Expression. Molecular endocrinology 2011.

Vishnu, M. R., Sumaroka, M., Klein, P. S., Liebhaber, S. A.: The poly(rC)-binding protein {alpha}CP2 is a noncanonical factor in X. laevis cytoplasmic polyadenylation. RNA 17(5): 944-56, 2011.

Chiefari, E., Iiritano, S., Paonessa, F., Le Pera, I., Arcidiacono, B., Filocamo, M., Foti, D., Liebhaber, S. A., Brunetti, A.: Pseudogene-mediated posttranscriptional silencing of HMGA1 can result in insulin resistance and type 2 diabetes. Nature communications 1: 40, 2010.

Eiring, A. M., Harb, J. G., Neviani, P., Garton, C., Oaks, J. J., Spizzo, R., Liu, S., Schwind, S., Santhanam, R., Hickey, C. J., Becker, H., Chandler, J. C., Andino, R., Cortes, J., Hokland, P., Huettner, C. S., Bhatia, R., Roy, D. C., Liebhaber, S. A., Caligiuri, M. A., Marcucci, G., Garzon, R., Croce, C. M., Calin, G. A., Perrotti, D.: miR-328 functions as an RNA decoy to modulate hnRNP E2 regulation of mRNA translation in leukemic blasts. Cell 140(5): 652-65, 2010.

Kini, H. K., Vishnu, M. R., Liebhaber, S. A.: Too much PABP, too little translation. The Journal of clinical investigation 120(9): 3090-3, 2010.

Sizova, D., Ho, Y., Cooke, N. E., Liebhaber, S. A.: Research resource: T-antigen transformation of pituitary cells captures three novel cell lines in the Pit-1 lineage. Molecular endocrinology 24(11): 2232-40, 2010.

Hamanaka, R. B., Bobrovnikova-Marjon, E., Ji, X., Liebhaber, S. A., Diehl, J. A.: PERK-dependent regulation of IAP translation during ER stress. Oncogene 28(6): 910-20, 2009.

Waggoner, S. A., Johannes, G. J., Liebhaber, S. A.: Depletion of the poly(C)-binding proteins alphaCP1 and alphaCP2 from K562 cells leads to p53-independent induction of cyclin-dependent kinase inhibitor (CDKN1A) and G1 arrest. J Biol Chem 284(14): 9039-49, 2009.

Ho, Y., Tadevosyan, A., Liebhaber, S. A., Cooke, N. E.: The juxtaposition of a promoter with a locus control region transcriptional domain activates gene expression. EMBO Rep 9(9): 891-8, 2008.

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Last updated: 07/25/2012
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