Delineating the Mechanisms Through Which ZNF165 Supports Growth and Survival of Triple-Negative Breast Cancer

Cancer/testis (CT) antigens are proteins whose expression is normally restricted to germ cells yet aberrantly activated in tumors, where their functions remain relatively cryptic. The transcription factor ZNF165, a CT antigen frequently expressed in triple-negative breast cancer (TNBC), was previously identified as essential for the growth and survival of TNBC and sustained transforming growth factor beta (TGFβ) signaling. However, the mechanisms through which ZNF165 functions in this context to regulate both TGFβ signaling and survival were poorly understood. To investigate its mechanism of action, I first sought to determine how ZNF165 interfaces with the TGFβ pathway in TNBC using a combination of genomics and biochemical approaches. I found that ZNF165 associates with SMAD3, a key signal transducing factor in the TGFβ pathway, to modulate transcription of TGFβ-dependent genes and thereby promote growth and survival of human TNBC cells. Importantly, my data demonstrate that through functioning as a cofactor for SMAD3, ZNF165 is able to specify a TGFβ-dependent gene expression program that promotes oncogenic phenotypes while repressing tumor-suppressive functions. In addition, I identified the KRAB zinc finger protein, ZNF446, and its associated tripartite motif protein, TRIM27, as obligate components of the ZNF165-SMAD3 complex that also support tumor cell viability. I found that while ZNF446 is bound to chromatin with the ZNF165-SMAD3 complex, TRIM27 alone is necessary for ZNF165 transcriptional activity and is required for TNBC tumor growth in vivo using an orthotopic xenograft model in immunocompromised mice. Moreover, my data also suggest that ZNF165 is SUMOylated by TRIM27 to enhance its protein stability, providing further insight into the mechanisms that regulate ZNF165 activity in TNBC. Together, my findings indicate that aberrant expression of a testis-specific transcription factor is sufficient to co-opt somatic transcriptional machinery to drive a pro-tumorigenic gene expression program in TNBC.