Role of Chromatin Structure in Regulating MHC Class I Expression

Division Of Basic Sciences - Nci

Investigators

Abstract

To examine the role of chromatin in transcriptional regulation of the MHC class I gene, we determined nucleosome occupancy and positioning, histone modifications and H2A.Z occupancy across its regulatory region in murine tissues that have widely different expression levels. Surprisingly, nucleosome occupancy and positioning were indistinguishable between spleen, kidney and brain. In all three tissues, the 200 bp upstream of the transcription start site had low nucleosome occupancy. In contrast, nuclease hypersensitivity, histone modifications and H2A.Z occupancy, showed tissue-specific differences. Thus, tissue-specific differences in MHC class I transcription correlate with histone modifications, not nucleosomal organization. Further, activation of class I transcription by -Interferon or its inhibition by α-amanitin did not alter nucleosome occupancy, positioning, nuclease hypersensitivity, histone modifications or H2A.Z occupancy in any of the tissues examined. Thus, chromatin remodeling was not required to dynamically modulate transcriptional levels. These findings suggest that the MHC class I promoter remains poised and accessible to rapidly respond to infection and environmental cues. To further assess the role of chromatin modifications, we will examine the association of Poly(ADP-ribose)polymerase-1 (PARP) and histone H1 with the MHC class I as a function of the levels of transcription. The relative levels of these two chromatin binding factors has been shown to be inversely correlated, with PARP being preferentially associated with actively transcribed regions. We hypothesize that the levels of PARP bound to the class I gene will vary among the tissues and in response to interferon stimulation. PARP occupancy may also be determined by the presence or absence of the class I boundary element. Chromosomes are nonrandomly positioned within the nucleus and as a result individual genes could be subject to distinct local environments in terms of proximity to the nuclear envelope, proteinaceous sub-compartments and chromatin domains. Currently, the view is that both nuclear localization as well as the ability of genes/loci to relocate in response to/facilitate activation or repression may be functionally important. Given the relevance of nuclear localization and chromatin movement with respect to gene expression, we would like to investigate its contribution to MHC Class I gene expression.