Abstract 1025: Controlled and endogenous depletion of a synthetic lethal target in human cancer cell lines via SMASh degron engineering

The SWI/SNF multi-subunit complexes play an important role in regulating processes such as gene transcription through ATP-dependent remodeling of chromatin. The importance of this role is highlighted by the prevalence of mutations in various SWI/SNF subunits across approximately 20% of cancers encompassing various lineages and subtypes. Functional genomics screening via pooled shRNAs has led to the discovery of distinct vulnerabilities of SWI/SNF mutant cancers to loss of mutually exclusive subunits. In particular, cancer cells with BRG1 loss become exquisitely dependent on the remaining BRG1 counterpart, BRM. To further understand the synthetic lethal relationship between BRM and BRG1, we have adapted a recently described degron, the small molecule assisted shutoff (SMASh) tag, showing for the first time the ability to apply this system to control endogenous levels of target protein, in this case BRM. We used the SMASh tag to target the endogenous BRM locus in both a BRG1 wild-type and mutant cancer cell line. Using these engineered cell lines, we were able to show that we can achieve tunable protein levels by varying the concentration of the effector molecule. The ability to tune BRM depletion allowed us to investigate the nature of the BRG1/BRM synthetic lethal relationship. Our data interestingly suggests that while modest depletion of BRM is sufficient to observe changes in the expression of a target gene regulated by SWI/SNF in a dose-dependent manner, a higher level of BRM depletion is required to elicit a growth arrest phenotype in the BRG1-mutant non-small cell lung cancer line. Together these results uncover a novel relationship between the catalytic SWI/SNF subunit and gene expression, where we find that the latter responds in a dose dependent manner to BRM levels. Additionally, we elucidate the level of BRM depletion required to observe the synthetic lethal phenotype, providing insight and informing on strategies for therapeutic targeting whether by protein degradation approaches, such as PROTAC, or other small molecule approaches. Finally, having provided a proof-of-concept, such tools can be readily adapted for application to endogenous degradation of other target proteins in different systems.

Citation Format: Florencia Rago, Matthew T. Dimare, Geoffrey Bushold, Avnish Kapoor, Hyo-eun C. Bhang, Zainab Jagani. Controlled and endogenous depletion of a synthetic lethal target in human cancer cell lines via SMASh degron engineering [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1025. doi:10.1158/1538-7445.AM2017-1025

Dipeptide boronic acid inhibitors of dipeptidyl peptidase IV: determinants of potency and in vivo efficacy and safety

Dipeptidyl peptidase IV (DPP-IV; E.C. 3.4.14.5), a serine protease that degrades the incretin hormones GLP-1 and GIP, is now a validated target for the treatment of type 2 diabetes. Dipeptide boronic acids, among the first, and still among the most potent DPP-IV inhibitors known, suffer from a concern over their safety. Here we evaluate the potency, in vivo efficacy, and safety of a selected set of these inhibitors. The adverse effects induced by boronic acid-based DPP-IV inhibitors are essentially limited to what has been observed previously for non-boronic acid inhibitors and attributed to cross-reactivity with DPP8/9. While consistent with the DPP8/9 hypothesis, they are also consistent with cross-reactivity with some other intracellular target. The results further show that the potency of simple dipeptide boronic acid-based inhibitors can be combined with selectivity against DPP8/9 in vivo to produce agents with a relatively wide therapeutic index (>500) in rodents.