| Summary: | Therapeutic responses to Histone deacetylase (HDAC) inhibitors (HDACi) in many cancers are well described but development of resistance to HDACi is a major stumbling block. Whether HDACis induce epigenetic reprogramming and how this contributes to relapse is not reported. A CTCL cell line HuT78, and a CLL cell line MEC1, were used to develop HDACi resistant clones (RHuT78 and RMEC1 respectively) that persistently grow in the presence of the clinically used HDAC inhibitor Romidepsin. RHuT78 cells show perturbed trimethylation of histone H3 lysine K4 on Romidepsin treatment which linked to higher protein expression levels of the implicated demethylase KDM5A. Following on from these experiments, a qRT-PCR epigenetic gene expression array was used to quantify levels of 84 epigenetic gene transcripts in RHuT78 cells and significantly altered genes were taken forward for further investigation. Studies of gene expression patterns in parental, resistant and ‘drug holiday’ cell lines of both HuT78 and MEC1 led to particular interest in HDAC8, DNMT3A and DNMT3B. Functional studies showed that HDAC8 overexpression increased proliferation and resistance of HuT78 cells to Romidepsin. Parallel observations suggested an increase in proliferation of resistant cell lines cultured in the presence of the HDACi. This increased proliferation was seen even with lower concentrations of Romidepsin and argues against prolonged monotherapy using HDACis. Significantly, inhibitors of DNA methyltransferases synergised with Romidepsin in a dose and schedule dependent manner, reversing the changes in epigenetic gene expression associated with resistance and causing increased apoptosis in RHuT78 cells. Taken together this thesis identifies and characterises an unacknowledged contribution of epigenetic reprogramming to drug resistance and provides insights into the effects of Romidepsin on the epigenome that could potentially contribute to HDACi resistance.
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