Dr. Michael Bender

Models for tissue specific gene activation propose a multi-step process in which a locus is re-localized from areas of silenced heterochromatin to ¡§active¡¨ centers where a nuclease sensitive ¡§open¡¨ conformation is assumed, and chromatin remodeling and epigenetic modifications such as histone hyper-acetylation occur. Once this state is achieved, long-range enhancers and promoter proximal sequences lead to full gene activation. Locus control regions (LCRs) are sequence elements originally thought to lead to high-level position independent expression of linked genes in transgenic mice.
To address the role of LCRs and other distal regulatory elements I have been studying the murine bƒ{globin locus during erythroid maturation, a process that is characterized by profound chromatin condensation and silencing of all but a few genes. The bƒ{globin LCR is comprised of six DNaseI hypersensitive sites (HSs) located far upstream of the bƒ{like globin genes. Transgenic experiments and the analysis of a human bƒ{globin locus carrying a naturally occurring deletion the LCR and 27kb upstream (Hispanic thalassemia) had led to the notion that the LCR was responsible for an activated state of the locus defined by an ¡§open¡¨ active chromatin structure, localization away from peri-centromeric heterochromatin (PCH), generalized histone hyper-acetylation, and high-level b-globin transcription. The focus of my research is understanding how this ¡§open¡¨ activated state of the bƒ{globin locus is established and maintained during erythroid maturation. My initial attempts to address this involved deletion of the endogenous murine LCR by homologous recombination. Surprisingly, while a dramatic reduction in expression of the b-like globin genes was observed, the deletion resulted in no chromatin phenotype (no measurable effect on nuclease sensitivity, promoter hypersensitive site formation or core histone hyper-acetylation) or alteration in nuclear location of the locus. These results raise several major questions which are the focus of my research including: 1) which non-LCR sequences are capable of establishing an open bƒ{globin chromatin domain in this generally repressive environment, 2) what epigenetic modifications are associated with the transition from ¡§open¡¨ to ¡§closed¡¨ chromatin, 3) how do these elements affect the state of the open domain and transcription during erythroid maturation and 4) what is the mechanism by which the LCR activates gene expression.