Abstract 62: Functional analysis of endometrial cancer mutants of the E3 ubiquitin ligase adaptor protein, SPOP

Abstract

Abstract The purpose of this study is to functionally annotate endometrial cancer (EC) specific mutants of SPOP (Speckle-type POZ protein). SPOP is the substrate adaptor for the E3 ligase complex CUL3-SPOP-RBX1, which is responsible for mediating the ubiquitination, and subsequent proteasomal degradation of a number of protein substrates including the SRC3 (NCOA3/AIB1) oncoprotein. Our laboratory has previously shown that SPOP is frequently mutated in serous and clear cell ECs, two clinically aggressive subtypes of EC. Importantly, the vast majority of SPOP mutants localized within the MATH domain, which is responsible for substrate binding. We hypothesize that EC associated SPOP mutations encode loss-of-function mutants that abolish or attenuate substrate degradation. To test this hypothesis, we initially focused our attention on five SPOP mutations we uncovered in ECs (E47K, S80R, P94A, M117I and R121Q). HEK293 cells were transiently co-transfected with a HA-tagged SRC3-expressing construct and a MYC-tagged construct expressing either vector alone, wildtype SPOP, or mutant forms of SPOP. Protein lysates from transfected cells were subjected to Western blotting to measure exogenous levels of HA-SRC3 and MYC-SPOP. Cells exogenously expressing wildtype SPOP or the E47K, P94A, M117I or R121Q mutants exhibited lower levels of HA-SRC3 than cells expressing the vector control. In contrast, cells expressing the S80R mutant had elevated levels of HA-SRC3 compared to cells expressing either wildtype SPOP or the vector control. Co-immumoprecipitation experiments are being performed to assess the substrate binding capacity of SPOP mutants. Our findings indicate that the SPOP-S80R mutation, which is recurrent in EC, has an impaired ability to down-regulate SRC3 protein levels compared to wildtype SPOP. Ongoing and future studies aim to elucidate the mechanism that underlies the effect of SPOP-S80R, and to functionally annotate additional SPOP mutations in EC. Citation Format: Fred Lozy, Mary Ellen Urick, Meghan Rudd, Deena Maurer, Daphne W. Bell. Functional analysis of endometrial cancer mutants of the E3 ubiquitin ligase adaptor protein, SPOP. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 62.