Ovarian cancer is an extremely aggressive disease, that in most cases, is able to grow undetected deep within the abdominal cavity for years, until fully disseminated disease causes overt symptoms which cannot be ignored. Despite its insidious nature, ovarian cancer has an Achilles heel – extreme sensitivity to chemotherapy. The vast majority of patients do respond well to initial therapies and a small subset of women can survive more than 10 years without further treatment. Recent work has just begun to focus on identifying markers capable of predicting which patients will survive long-term. In preliminary work, we have identified a tumor marker that can forecast whether a patient with the most aggressive subtype of ovarian cancer will respond well to chemotherapy.
The identified marker is a member of a distinct class of proteins which are only expressed in cancer cells and in the human germ line called cancer/testis (CT) antigens. Due to its restricted expression, it could potentially be used to stimulate the immune system to attack and kill the tumor cells just as it would a virus or bacteria that had infected the body. This strategy is called immunotherapy. Studies have shown that proteins of the same class as our candidate protein can in fact be targets (or antigens) for the immune system, but it is not known if the identified marker has the same capability. Therefore, one part of this proposal seeks to elucidate the candidate protein’s role as a tumor antigen. I will do this by determining if patients produce antibodies targeting the candidate protein, as well as, test if T cells can target and destroy tumor cell lines expressing the candidate protein.
The second part of my proposal is rooted in the observation that cell lines expressing the candidate protein have increased levels of DNA damage and form smaller tumors in a mouse model following treatment with chemotherapy. Follow-up experiments showed increased phosphorylation of 53BP1 at a previously uncharacterized site. 53BP1 is a key protein involved in the DNA damage repair response. I propose to clarify if the candidate protein modulates the function of 53BP1 via phosphorylation by measuring changes in 53BP1 recruitment to sites of damage within the DNA. We will also examine the effect of phosphorylation of 53BP1 on the two DNA damage repair pathways, non-homologous end joining and homologous recombination, using reporter plasmids that when repaired through each unique pathway will become fluorescent.
While immunotherapy has been extremely effective in certain cancers, the promise of this breakthrough therapy has yet to materialize in ovarian cancer. Therefore, the identification of the candidate protein as a bona fide tumor antigen in ovarian cancer is novel and suggests that it could be used as an immunotherapy target. In addition, though many CT antigens have been identified, very few have described functions. Identifying the mechanism by which this CT antigen promotes chemosensitivity will contribute a new understanding to the biological role of CT antigens. Finally, characterization of a previously undescribed phosphosite on 53BP1 could reveal a novel mechanism in the DNA damage repair response.
This grant was made possible by a generous donation from Jack Dubnicek, in memory of Jamison Manning.