ABSTRACT: Bernasocchi T, El Tekle G, Bolis M, Svinkina T, Zoma M, Rinaldi A, Ceserani V, Janouskova H, Schram P, Carbone G, Alimonti A, Moch H, Carr SA, Udeshi ND, Theurillat JP. 2018.
While the co-operation of cancer driver genes in tumorigenesis has been studied, little is known about the interplay of driver mutations that never co-occur within the same cancer cells. The latter scenario has been identified in prostate cancer where recurrent gene fusions involving the oncogenic ERG transcription factor and point mutations in the ubiquitin ligase adaptor SPOP are strictly mutually exclusive. Nevertheless, the underlying basis of this observation is poorly understood. Here, we show that ERG and mutant SPOP, even though oncogenic on their own, are together synthetic sick. At the molecular level, both driver genes inhibit each other in a reciprocal manner. In ERG-driven tumors, wild type SPOP is required to dampen androgen receptor (AR) signaling and sustain ERG activity in part through its ability to degrade the bromodomain histone reader ZMYND11. Consequently, loss-of-function mutations in SPOP unleash excessive AR signaling and reduce ERG function. Conversely, oncogenic androgen receptor signaling driven by mutant SPOP is repressed by ERG. The incompatibility of mutant SPOP and ERG may help to understand why SPOP mutant tumors frequently harbor gene deletions in the chromatin-modifying enzyme CHD1. We find that mutant SPOP promotes the generation of ERG rearrangements in a CHD1-dependent manner. Thus, CHD1 gene deletions may protect SPOP-mutant tumors from ERG-mediated growth inhibition. Taken together, our findings reveal the existence of divergent and incompatible paths towards prostate cancer that converge on SPOP function.
Steven A. Carr 
