Elucidating the mechanism of action of selective histone deacetylase inhibitors in cancer

Abstract

Given the fundamental roles of histone deacetylases (HDACs) in the regulation of DNA repair, replication, transcription and chromatin structure, it is fitting that therapies targeting HDAC activities are now being explored as anti-cancer agents. In fact, two histone deacetylase inhibitors (HDIs), SAHA and Depsipeptide, are FDA approved for single-agent treatment of refractory cutaneous T cell lymphoma (CTCL). An important target of these HDIs, histone deacetylase 3 (HDAC3), regulates processes such as DNA repair, metabolism, and tumorigenesis through the regulation of chromatin structure and gene expression. The data in this dissertation show that HDAC3 inhibition using a selective inhibitor, RGFP966, resulted in decreased cell growth in CTCL cell lines due to increased apoptosis that was associated with DNA damage and impaired S phase progression. Interestingly, through isolation of proteins on nascent DNA (iPOND), HDAC3 was associated with chromatin and is present at and around DNA replication forks. DNA fiber labeling analysis showed that inhibition of HDAC3 resulted in a significant reduction in DNA replication fork velocity within the first hour of drug treatment. This work also explores whether inhibition of HDACs 1/2 or HDACs 1-3 using selective inhibitors (RGFP233 and RGFP963 respectively) affects DNA replication. DNA fiber labeling analysis showed that treatment with either of these inhibitors resulted in reduction in DNA replication fork velocity. These results suggest that selective inhibition of HDAC3, HDACs 1/2, or HDACs 1-3 could be useful in treatment of CTCL by disrupting DNA replication of the rapidly cycling tumor cells, ultimately leading to cell death.