Exploiting SUMOylation to target MYC-driven cancers

Abstract

The transcription factor MYC is one of the most frequently amplified oncogenes in cancer and has therefore attracted much attention as a potential anti-cancer target. However, targeting MYC directly through small molecule inhibitors has proved challenging and so an alternative approach involving indirectly targeting MYC by exploiting synthetic lethality interactions has also been explored. One such synthetic lethal gene is SAE2, the catalytic component of the SUMO E1 activating enzyme. Depletion of SAE2 was shown to drive polyploidy and apoptosis exclusively in MYC-overexpressing cells. This synthetic lethal relationship between MYC and SAE2 has since been validated in several different cancer contexts. Meanwhile, the recent development of small molecule inhibitors of SUMOylation (SAEi) has presented the exciting opportunity of exploiting this synthetic lethality to target MYC-driven cancers. However, the underlying mechanism driving the dependency of MYC-overexpressing cells on a functioning SUMOylation pathway is still unclear. SAEi disrupts the entire downstream SUMOylation pathway and may therefore have pleiotropic effects. A better understanding of the relationship between MYC and SAE2 could lead to more specific targets downstream of the SUMOylation pathway that can be exploited therapeutically. Therefore, the aim of this thesis was to further investigate potential mechanisms driving the synthetic lethality between MYC and SAE2. The synthetic lethal relationship between MYC and SAE2 was first validated utilising a MYC-tuneable cell line and a recently developed SAEi. Fluorescent microscopy and DNA content analysis revealed that SAEi drives mitotic defects such as the formation of micronuclei and chromatin bridges exclusively in MYC-overexpressing cells, culminating in polyploidy and a reduction in proliferation. This mitotic phenotype is reflective of the numerous roles of SUMOylation in the regulation of mitotic progression. Interestingly, transcriptomic profiling of MYC-high and MYC-low cells treated with SAEi also revealed a role for SUMOylation in the regulation of gene expression. SUMOylation was found to drive a gene expression programme that opposes the effects of MYC, restricting expression of MYC-target genes and of MYC itself, whilst supporting TGF-b signalling by promoting expression of key TGF-b pathway members. This poses a model in which SUMOylation is required to maintain regulation of cell cycle progression. Inhibition of SUMOylation in MYC-overexpressing cells results in bypassing of regulatory checkpoints and uncontrolled progression through the cell cycle, leaving cells vulnerable to mitotic perturbations. Loss of the cytokinesis protein CIT-K was found to be one such vulnerability, exacerbating the effects of SUMOylation inhibition on cells that overexpress MYC cells and sensitising them to concentrations SAEi that are otherwise tolerable. The finding that MYC-overexpressing cells treated with SAEi are particularly vulnerable to further disruptions of cytokinesis, such as the loss of CIT-K, could lead to the identification of novel targets to use in combination with SAEi as a strategy for targeting MYC-driven cancers.

Date of Award	11 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stephen Taylor (Main Supervisor)