Characterization of Ubiquitin Ligase Targeting by Anticancer Sulfonamides

Aryl sulfonamides are small molecules that are selectively toxic to a subset of human cancer cell lines. Clinical trials of the aryl sulfonamide indisulam have resulted in modest clinical activity against a subset of solid tumors. Recent work revealed that indisulam recruits the RNA binding protein RBM39 to DCAF15, a component of the CRL4-DCAF15 E3 ubiquitin ligase. This recruitment results in RBM39 ubiquitination and degradation, leading to splicing defects and cancer cell death (Han et al., 2017; Uehara et al., 2017). The mechanism of action of sulfonamides is similar to that of immunomodulatory drugs (IMiDs), which recruit substrates to the closely related CRL4-CRBN E3 ubiquitin ligase for ubiquitination. Known for their roles in inhibiting embryonic development and cancer cell growth, IMiDs exert their pleiotropic effects by targeting a variety of substrate proteins to the CRL4-CRBN E3. Despite major advances in our understanding of aryl sulfonamides, it is unclear whether sulfonamides also target multiple substrates or modulate the endogenous function of the CRL4-DCAF15 E3 ligase. This dissertation describes our efforts to define the requirements for RBM39 ubiquitination, identify other substrates that are recruited to the CRL4-DCAF15 E3 ligase, and further our understanding of the cellular consequences of indisulam treatment. In Chapters 2 and 3, we define the components required for RBM39 ubiquitination using a combination of in vitro and in vivo techniques. In Chapters 4 and 5, we identify putative endogenous substrates and a previously undescribed neo-substrate recruited to the CRL4-DCAF15 for ubiquitination. In Chapter 6, we characterize the cellular consequences of indisulam treatment and neo-substrate degradation. In aggregate, this work aims to contribute to our understanding of the sulfonamide mechanism of action and the field of targeted protein degradation.