Dissecting the roles of histone deacetylase 1 and 2 in the hematopoietic system

Reversible histone acetylation on lysine residues has been intensively studied as an epigenetic mark for gene activation in a variety of eukaryotes. Histone deacetylases (HDACs) act to repress gene expression by removing acetyl groups from histones and possibly other proteins. Based on their expression pattern, distribution and biochemical activity, HDAC1 andhave been considered to be the major HDACs to regulate gene expression in a wide range of tissues in mammals. Experiments using transformed cells and HDAC inhibitors suggest that HDACs are important for cell cycle progression, apoptosis regulation and induction of differentiation; however their in vivo roles in a higher organism as well as their molecular mechanism of actions remain largely unexplored. I have generated mice conditionally targeted at the HDAC1 or HDAC2 locus and crossed these to different deleter mice in order to dissect the function of these enzymes in vivo; I focused on B cell specific inactivation in the first part of the thesis and on hematopoieticlineage specific inactivation in the second part. Mice completely lacking HDAC1 die during embryogenesis while animals lacking HDAC2 show partial perinatal mortality. In contrast, ablation of HDAC1 orin the B cell lineage led to no obvious developmental defect. Simultaneous deletion of both HDAC1 andin the B cell lineage causes a very severe blockade in the early B cell development accompanied by severe defects in cell cycle progression, apoptosis regulation and recombination of immunoglobulin heavy chain (IgH) variable (VH) segments. Using three-dimensional DNA fluorescence in situ hybridization, we show that pre-BI cells lacking HDAC1 andare severely impaired in the contraction of the IgH locus, a mechanism whereby distantly located variable genes are brought in close proximity to the rest of the locus. Analysis of histone modifications revealed that these pre-BI cells lshow global histone hyperacetylation as well as increased histone lysinemethylation. Taken together, we conclude that HDAC1 andare crucial factors for B cell development, survival and IgH recombination by modulating histone modifications which may affect gene expression and the higher order of chromatin structure. In the second part I have analyzed the roles of HDAC1 in the hematopoietic lineage. Loss of HDAC1 in the hematopoietic lineage has only minor impacts on the myeloid lineage development, but impaired IgG secretion and reduced germinal center formation were observed upon induction of a T cell dependent immune response. Using transcriptome analysis, we found that these germinal center B cells lacking HDAC1 exhibit higher expression of genes involved in cell cycle regulation as well as apoptosis. These results suggest that HDAC1 has important roles to regulate the adaptive immune response by controlling the expansion of B cells upon antigen stimulation.