Our Science – Singer Website

Dinah S. Singer, Ph.D.

Experimental Immunology Branch Head, Molecular Regulation Section Senior Investigator Building 10, Room 4B36 10 Center Drive Bethesda, MD 20892 Phone:   301-496-9097 Fax:   301-480-8499 E-Mail:   dinah_singer@nih.gov

Biography

Dr. Singer is chief of the Molecular Regulation Section of the Experimental Immunology Branch and director of the Division of Cancer Biology, NCI. After receiving her B.S. from the Massachusetts Institute of Technology and her Ph.D. from Columbia University, Dr. Singer was a postdoctoral fellow in the Laboratory of Biochemistry, NCI, and a senior investigator in the Immunology Branch, NCI. She serves on a number of scientific and advisory boards, is a member of the American Association of Immunologists, and has served as a senior science officer at the Howard Hughes Medical Institute. Dr. Singer has received a number of awards, including the NIH Director's Award. Her research interests are in the areas of regulation of gene expression and molecular immunology.

Research

Molecular Mechanisms Regulating MHC Gene Expression

Major histocompatibility complex (MHC) class I molecules are ubiquitously expressed cell surface molecules which function as receptors for intracellularly derived peptide antigens, displaying them for inspection by cells of the immune system. Whereas self-peptides complexed with class I elicit no response, peptides derived from viruses or other intracellular pathogens trigger specific immune responses. Given the critical role that MHC class I molecules play in immune surveillance, and the multiplicity of viral mechanisms that have evolved to reduce class I expression and avert immune recognition, it is important to understand how MHC class I genes are regulated.

Regulation of MHC class I gene expression is primarily transcriptional and governed by two distinct mechanisms: homeostatic and dynamic. Among tissues, class I expression varies markedly, ranging from high expression in cells and tissues of the immune system to very low levels in kidney, liver, and endocrine tissues. Homeostatic mechanisms establish tissue-specific setpoints. Within an individual cell type, class I expression fluctuates dynamically from the setpoint in response to external signals. For example, cytokines such as interferons increase class I transcription, whereas hormones such as TSH reduce it.

The central hypothesis underlying our studies is that this regulatory network ensures a proper balance between immune surveillance and the maintenance of self-tolerance. Thus, the need to enhance immune surveillance by maximizing class I expression is balanced by the need to reduce the risk of autoreactivity by minimizing class I expression. We hypothesize that these two opposing forces have determined the array of factors that regulate expression of class I genes. Further, we postulate that this optimal balance differs among different tissues and among cells of the same tissue under different physiological conditions. Failure to regulate class I expression appropriately would result in dysregulation of the immune system. For example, viruses capable of repressing class I expression could permit the accumulation of a resistant reservoir of infected cells. Conversely, overexpression of class I could lead to activation of autoreactive cells and the generation of autoimmune disease.

A major research focus of the laboratory continues to be the precise characterization of this regulatory network. Our studies of class I transgenic mice have shown that normal patterns of expression are established by a DNA segment containing 1 kB of upstream regulatory sequences and extending 1 kB downstream of the coding sequences. Within the upstream 1 kB segment, we have identified a series of regulatory elements that contribute to both homeostatic and dynamic regulation. The core promoter, which spans a region between approximately -30 bp and +12 bp is a complex region that integrates signals emanating from the upstream homeostatic and dynamic regulatory domains. The class I core promoter consistes of distinct elements that are differentially utilized in homeostatic and dynamic transcriptional pathways and assemble distinct transcriptional complexes. Our studies are designed to provide a basis to achieve a complete mechanistic understanding of how homeostatic and dynamically modulated transcription factors regulate class I gene expression.

Among our findings are that:

- class I core promoter consists of distinct elements that are differentially utilitzed by homeostatic and dynamic transcriptional pathways. Homeostatic transcription initiates predominantly from upstream sites, while dynamic transcription initiates at downstream sites.

- class I promoter activity depends upon distinct transcription initiation complexes: homeostatic transcription requires the basal transcription factor TAF1 and its acetyl transferase (AT) while g-interferon induced dynamic transcription depends on the coactivator, CIITA.

- the class I promoter is activated by CIITA and repressed by ICER, both of which function through an upstream CRE element, which is within the hormone-specific regulatory domain;

- CIITA has AT activity, which is modulated by the downstream GTP-binding domain; this AT activity is necessary for CIITA-mediated activation;

- CIITA can functionally replace TAF1 in supporting class I expression. In contrast, the b/HLH transcription factor USF, which activates class I expression via an upstream E-box, requires TAF1;

- HIV Tat represses class I transcription by binding to the AT domain of TAF1and inhibiting its AT activity.


Collaborators on this research include John Brady, Leonard Kohn, and Paul Roche, NIH; and Edna Mozes, Weizmann Institute, Israel.

This page was last updated on 6/12/2008.