Protein Kinase C Gene Fusions and Other Mechanisms for Loss of PKC Function in Cancer
Protein kinase C (PKC) plays a critical role in cell signaling and homeostasis, regulating biological processes such as proliferation, differentiation, and apoptosis. Its dysregulation is associated with a multitude of pathophysiological states, with recent analyses of disease-associated mutations indicating that loss of PKC function is generally associated with cancer and gain of function with degenerative diseases. This dissertation expands on the mechanisms of PKC dysregulation in cancer, focusing on how gene fusions or mutations that disrupt autoinhibition cause loss of PKC. In the first part of the dissertation, newly identified PKC gene fusions in cancer encoding proteins that retain either the PKC catalytic or regulatory domain were characterized. Overexpression of catalytic domain fusions in cells revealed that they are constitutively active, as assessed using biosensors and other cellular assays. However, their inability to adopt an autoinhibited conformation resulted in their marked stability compared to full-length PKC. To assess whether these fusions were too unstable to accumulate at endogenous levels, CRISPR/Cas9-mediated gene editing was used to engineer a fusion of TANC2 with PRKCA. While the fusion mRNA was detected in the engineered cells, no detectable protein was expressed. Thus, the catalytic domain fusions are paradoxically loss-of-function. Characterization of a regulatory domain fusion revealed a dominant-negative role for the protein, suppressing the activity of wild-type PKC. The second part of the dissertation focused on additional mechanisms by which PKC is dysregulated in cancer. This work showed that  PKC that cannot be autoinhibited is subject to dephosphorylation by the phosphatase PHLPP and unphosphorylated PKC is sensitive to degradation;  inactivating mutants of PKC can be dominant-negative by sequestering common titratable components; and  impairment of the regulatory domains causes mislocalization of PKC. Taken together, these studies illustrate the diverse ways in which PKC function is lost in cancer, allowing cancer cells to overcome this cellular brake to tumor growth.