HDAC1 and HDAC2 as regulators of epidermal homeostasis

Histone deacetylases (HDACs) are important epigenetic regulators that remove acetyl moieties from lysine residues of histones and other proteins. Inhibition of HDACs by small molecule inhibitors is a promising strategy in the treatment of various diseases and has been approved for anti-cancer therapy. The highly homologous deacetylases HDAC1 and HDAC2 play important roles in many signal transduction pathways and are crucial regulators of proliferation, differentiation and development in the mammalian organism. Epidermal development and homeostasis is regulated by a well-orchestrated sequence of proliferation, differentiation and cell death. By using a genetic mouse model with epidermal-specific HDAC1 and HDAC2 deletion we have revealed a dosage-dependent effect of HDAC1/HDAC2 activity on epidermal proliferation and differentiation. Loss of function studies in mice are complicated by the fact that loss of HDAC1 or HDAC2 is at least partially compensated by upregulation of its paralog. Importantly reduction of HDAC1/HDAC2 activity below a critical threshold level results in severe defects in skin development including hyperkeratosis and epidermal hyperproliferation indicating a novel and unexpected function of HDAC1/HDAC2 in proliferation control. In this project we will study the impact of HDAC1 and HDAC2 on development and homeostasis in the mouse epidermis. We will analyze in detail the defects of mice with reduced HDAC1/HDAC2 activity and the underlying deregulated mechanisms. In addition, we will examine in a systematic approach using mass spectrometry, RNA-seq and ChIP-seq changes in HDAC1/HDAC2 co-repressor complex formation and recruitment as well as target gene expression upon epidermal deletion of Hdac1 and Hdac2 alleles to identify the relevant deregulated signalling pathways. The results of this study will enable us to dissect individual and redundant functions of HDAC1 and HDAC2 in the regulation of epidermal cell proliferation and differentiation thereby providing important insights into the epigenetic control of cell fate decisions.

In this project we have revealed a surprising role for histone deacetylase 1 (HDAC1) - a member of a family of chromatin modifying proteins - in the development of skin tumors. The results from our group emphasize that care must be taken when using HDAC inhibitors as anti-cancer drugs. The skin is the largest organ of the human body, protecting us from dehydration and external impacts. It is a self-renewing tissue, meaning that in response to wounding, new skin cells will replace the old damaged ones and our wound will heal. On a molecular level skin development and regeneration is controlled by a wide range of factors, ensuring that the right number of undifferentiated progenitor cells differentiate into skin cells and make their way to replace the old or damaged ones. If something goes wrong during this process, pathologies, including skin tumors, can be the consequence. With non-melanoma skin cancers being the most frequent human tumors, there is clearly a tremendous need to understand the underlying molecular mechanisms, to allow the development of drugs to treat these types of cancer.One regulatory mechanism known to be important for skin development and renewal is the modification of chromatin. In self-renewing tissues, such as the skin, chromatin modifications are also involved in regulating the replacement of old or damaged cells in later stages in life. One type of chromatin modifiers is called histone deacetylase (HDAC), of which 20 variants are known in humans. HDACs are important epigenetic regulators, which remove acetyl moieties from histones and other proteins. HDAC inhibitors are already approved for cancer treatment but most of these drugs are not specific and act on several HDAC variants. Two of these enzymes, HDAC1 and HDAC2, were based on previous results promising targets for tumor therapy and are known to have redundant functions. When we ablated the function of either of these proteins in the skin of a mouse model system it had no obvious effect.However, when we ablated HDAC1 function completely and HDAC2 function partly we saw severe developmental defects and the spontaneous development of skin tumors. We further investigated HDAC1/HDAC2 function in a skin tumor model system, and found that deactivating HDAC1 actually accelerates tumor development, while HDAC2 deactivation has no effect. These results are surprising and highlight that it is crucial to understand the individual role the various HDAC variants in different cell types and hence different types of cancer, before HDAC inhibitors can be used safely as therapeutics.