Chromatin Regulation by Mammalian SIRT7 in Aging and Disease

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Members of the Sirtuin family of enzymes are important regulators of genomic stability, stress responses, and metabolic programs that impact on human health and disease. SIRT6 and SIRT7 are closely related mammalian sirtuins that regulate fundamental nuclear processes. These enzymes catalyze highly selective histone deacetylation reactions at chromatin, whereby important acetyl marks are removed from specific lysine residues. Proper regulation of histone acetylation patterns at chromatin is essential for establishing specialized chromatin states that control processes such as gene expression, DNA repair, and DNA replication. Dysregulation of histone acetylation patterns, as occurs when SIRT6 or SIRT7 are inactivated, can therefore have pathological consequences at the cellular and whole organism levels. Indeed, SIRT6 has numerous demonstrated functions relevant for aging, metabolism, and cancer. By contrast, much less is understood about SIRT7. We recently showed that SIRT7 is a highly selective H3K18Ac (acetylated histone H3 lysine 18) deacetylase that plays a pivotal role in modulating oncogenic transformation programs and tumor formation. Now, our preliminary studies provide evidence for additional epigenetic functions of SIRT7 in cellular metabolic processes that impact on both cancer and fatty liver disease. This proposal aims to elucidate the role and mechanisms of SIRT7 in these processes. In addition, our preliminary data reveal that the specific substrate of SIRT7, H3K18Ac, can also be deacetylated by SIRT6. We will ask how deacetylation of H3K18Ac is coordinated between SIRT6 and SIRT7 in specific genomic and physiologic settings, and whether these sirtuins provide compensatory mechanisms that protect against pathological consequences of defective H3K18Ac deacetylation. In Aim 1, we will elucidate novel roles and mechanisms of SIRT7 in oncogenic transformation and cancer pathways. We will use genetic and biochemical strategies to study a new link between SIRT7 and the oncogenic Myc transcription factor in regulating cancer cell translational capacity, proliferation, and survival. In addition, we will ask whether SIRT7 influences the efficiency of oncogenic transformation of primary cells in culture and overall tumor susceptibility in mice. In Aim 2, we will characterize the role and mechanisms of SIRT7 in preventing fatty liver disease. In humans, this disease is highly prevalent and predisposes to liver failure and cancer. However, its underlying mechanisms are poorly understood. We will combine molecular, genomic, and cellular approaches with studies of SIRT7 mutant mice to investigate the molecular mechanisms through which SIRT7 influence fatty liver disease pathogenesis. Growing evidence implicates endoplasmic reticulum (ER) stress in the development and progression of fatty liver disease. We will test the hypothesis that SIRT7 prevents fatty liver disease by attenuating the pathogenic effects of ER stress, as well as by directly regulating the expression of genes involved in lipid metabolism. In Aim 3, we will examine the functional interplay and overlap between SIRT6 and SIRT7 in H3K18Ac homeostasis and cancer cell biology. We will employ newly generated systems to inactivate both enzymes simultaneously, using double RNAi strategies and double mutant mice, and assay genomic, cellular and whole organism phenotypes. Together, these studies should elucidate fundamental chromatin mechanisms in human physiology and disease and the potential of SIRT6 and SIRT7 as therapeutic targets.