Chromatin structure, gene expression and cell cycle: the role of Histone Deacetylase 1

Histone acetylation regulates dynamic changes in the chromatin structure and thereby chromatin-associated events like replication and transcription. The modification is reversible and is controlled by histone acetyltransferases and deacetylases. Previously, we have identified histone deacetylase HDAC1 as growth factor-inducible gene in T cells (Bartl, Taplick, Lagger, Khier, Kuchler and Seiser (1997), Mol.Cell.Biol. 17:5033-5043). Overexpression of the histone deacetylating enzyme in mouse fibroblasts resulted in transcriptional repression of reporter constructs and a partial block in the G2/M phases of the cell cycle. The objective of this proposal is to understand the molecular and cellular function of mouse HDAC1. The specific aims of this project are: 1. To analyze growth- and differentiation-dependent regulation of HDAC1 expression in mouse fibroblasts, chicken erythrocytes and mouse adipocytes. 2. To identify the molecular mechanisms that mediate HDAC1 regulation. 3. To examine histone H4 acetylation patterns and kinetics during growth induction, cell cycle and differentiation. For this purpose we have created a set of four antibodies that specifically recognize histone H4 acetylated on Lys-5, Lys-8, Lys-12 and Lys-16, respectively. 4. To reveal if HDAC1 prefers specific histones as substrates. 5. To characterize HDAC1 protein domains that are necessary for enzymatic activity and nuclear localization. 6. To analyze the effect of HDAC1 overexpression. 7. To disrupt the HDAC1 gene in mice.

The genetic information of higher organisms is stored as DNA in the nucleus. To fit into the tiny nucleus the DNA is organized as chromatin. The basic unit of chromatin organization, the nucleosome, consists of a histone octamer with DNA wrapped around it. In this way nucleosomal DNA resembles pearls on a string. To read the information stored on the DNA and to duplicate it prior to cell division, specific parts of the chromatin have to be accessible. However, the high degree of compaction limits the accessibility for high molecular weight complexes. Acetylation and deacetylation of histones by histone acetylases and deacetylases modulates the degree of chromatin condensation thereby regulating the accessibility to particular regions of the genome. In this project we have investigated one of the histone deacetylases, HDAC1, in more detail. To analyze the function of the enzyme during mammalian development we have disrupted the HDAC1 gene in mice. Loss of HDAC1 leads to embryonic lethality around day 10.5 after gestation. Proliferation and developmental programs are severely impaired in HDAC1-deficient embryos suggesting a crucial role of this protein for these processes. HDAC1-deficient stem cells are also affected in their proliferation rate and display increased levels of the CDK inhibitor p21. In addition specific changes in chromatin modifications and gene expression were observed in HDAC1 null cells. The p21 gene seems to be a crucial target gene for HDAC1. Expression of p21 is significantly increased in HDAC1 null embryos and stem cells and accompanied by reduced cyclin A- and cyclin E-associated kinase activities. One of the function of the p21 protein is the inhibition of specific cell cycle-regulating kinases thereby inducing cell cycle arrest. Our data indicate that HDAC1 might be an important target for deacetylase inhibitors which are currently tested as tumor drugs in clinical trials.