MARK T. GROUDINE, M.D., Ph.D.
What do Locus Control
Regions (LCRs) Actually Control?
Our laboratory focuses on the control of gene expression at the levels of DNA replication, chromatin structure, and initiation and elongation of RNA polymerase.
(1) The ß-globin locus control region (LCR). Analysis of naturally occurring deletion mutants and functional assays suggest that the ß-globin LCR is important in the establishment of an active chromatin structure, early timing of replication of the ß-globin locus in red cells, as well as selection of the origin of replication in the ß-globin locus. Our recent evidence also suggests that the LCR, as well as classical enhancers, may function to establish and/or maintain domains permissive for transcription, rather than to increase the rate of transcription per se. This hypothesis, as well as the function of individual components of the LCR, is being tested by altering the LCR via homologous recombination in both the mouse and human globin loci.
(2) A novel mode of control of cellular gene expression: a conditional block to RNA polymerase II elongation. This conditional elongation block controls the amount of initiated transcription that elongates past sites of polymerase arrest in the first exon of the c-myc gene and thereby regulates the amount of full-length c-myc transcripts. The simple combination of proximal upstream activators and core promoter elements is sufficient to generate paused RNA polymerase complexes in the promoter proximal region. Our working hypothesis is that two distinct RNA polymerase II complexes, which differ in their ability to read through arrest signals, are assembled in the promoter proximal region, and that classes of transcriptional activators including enhancers and acetyltransferaces, can alter gene expression, at least in part, by influencing elongation aspects of transcription. Current experiments focus on determining the role of the transcribed template, including its chromatin structure, in promoter proximal pausing of polymerase, and characterization of polymerase complexes that differ in elongation efficiency.
(3) A novel LCR in the immunoglobulin (Ig) heavy chain locus that deregulates c-myc expression. In human Burkitt's lymphoma (BL) and murine plasmacytoma cells, characterized by translocations that juxtapose c-myc and Ig sequences, the c-myc elongation block is abrogated, resulting in constitutive synthesis of c-myc RNA. We have discovered a novel LCR in the Ig heavy chain locus, which becomes linked in cis to c-myc as a consequence of these translocations. Our preliminary functional analysis of this element supports a model in which deregulation of c-myc expression in BL and plasmacytomas results from cis effects imposed on c-myc by this LCR through a combination of preventing the silencing of c-myc in mature B cells and promoting the assembly of elongation competent transcription complexes in the c-myc gene.