Alice Fang '27

Poly-ADP ribose polymerases (PARPs) are proteins that participate in the process of repairing single strand DNA breaks (SSBs). PARPs carry out surveillance in the cell and when DNA damage occurs, PARPs bind to and recognize DNA damage, setting off DNA repair pathways (Morales et al., 2014). PARP inhibitors (PARPi) block the DNA damage repair pathway, leaving the SSBs unrepaired. Eventually, the SSBs will accumulate and form double stranded breaks (DSBs).

Normal ovarian cells use BRCA proteins to recruit repair complexes to the DSB region and help repair the DSBs using a method called homologous recombination (HR) (Dilmac and Ozpolat, 2023). Homologous recombination is a process in which identical and undamaged nucleotide sequences from homologous chromosomes are used to repair DSBs. In BRCA-mutant ovarian cancer, DSBs are unable to be repaired because BRCA proteins lose function and are unable to recruit repair complexes.

While PARPi is a viable treatment for BRCA-mutant ovarian cancer, 40-50% of BRCA-mutant patients do not respond to PARPi due to innate or acquired PARPi resistance (Dilman and Ozoplat, 2023). Acquired resistance happens because cancer cells reproduce and adapt quickly to overcome the PARPi lethality. In addition, more than 50% of HGSC patients are do not have BRCA mutations, meaning that PARPi treatment are not substantially beneficial to a large majority of patients (Zhang et al., 2023). This highlights the importance of finding a different mechanism by which we can potentiate the therapeutic effect of PARPi treatment. The solution may lie in polyploid giant cancer cells (PGCCs).

PGCCs are large cancer cells that are usually thought to be senescent, meaning that the cells age but do not die or divide. However, scientists are not entirely certain on the formation or function of PGCCs (Zhou et al., 2022).

In Targeting polyploid giant cancer cells potentiates a therapeutic response and overcomes resistance to PARP inhibitors in ovarian cancer, Zhang et al. investigates the association of PGCCs with the PARPi (specifically, Olaparib) resistance in ovarian cancer. Their research looks at high-grade serous carcinoma (HGSC), the most common subtype of ovarian cancer. First, they found that PGCCs grow in Olaparib-resistant HGSC patient-derived xenografts (PDX) and that they were able to induce PGCC formation using Olaparib. PDX is a model of cancer where cancer tissues are collected from patients and planted into an immunodeficient mouse. Second, while PGCCs are usually thought to be senescent, they found that PGCCs are also able to escape senescence and produce daughter cells that are able to divide. A portion of these mitotic competent daughter cells also have therapeutic resistance, increasing PARPi resistance within the tumor. So far, Zhang et al. found that not only can PGCCs become resistant to PARPi treatment, but they can also produce more cancer cells that are also resistant to PARPi treatment. However, this isn’t all bad news.

From previous studies, we know that PGCCs dedifferentiate and mimic early embryonic development (Niu et al., 2017). Leveraging this knowledge, Zhang et al. reasoned that certain contraceptives, such as Mifepristone, may be able to block the life cycle of PGCCs. Indeed, they found that Mifepristone was able to suppress tumor growth in both Olaparib-naive and Olaparib-resistant HGSC cells, treating the original issue of PGCC and daughter cells being resistant to Olaparib. The results showed that PGCCs are most sensitive to a combination of Olaparib and Mifepristone treatment. In conclusion, Zhang et al. found a way to grow PGCCs in HGSC cells and then use the PGCCs to suppress cancer growth while also overcoming PARPi resistance.

Zhang et al. mentioned that PGCCs can be used to potentiate the therapeutic response of PARPi treatment and combat more than one HGSC subtype. Previously, PARPi treatment worked best with BRCA mutant patients. However, Zhang et al. suggest that PGCCs, rather than the BRCA status of tumor, are associated with acquired Olaparib resistance. This means that PGCCs can be used as an alternative treatment regime, broadening applications of PARPi to HGSC patients who do not have BRCA mutations.

 

References:

Dilmac, S., & Ozpolat, B. (2023). Mechanisms of PARP-Inhibitor-Resistance in BRCA-Mutated Breast Cancer and New Therapeutic Approaches. Cancers, 15(14), 3642. https://di.org/10.3390/cancers15143642

Morales, J. C., Li, L., Fattah, F. J., Dong, Y., Bey, E. A., Patel, M., Gao, J., & Boothman, D. A. (2014). Review of Poly (ADP-ribose) Polymerase (PARP) Mechanisms of Action and Rationale for Targeting in Cancer and Other Diseases. Critical Reviews in Eukaryotic Gene Expression, 24(1), 15–28.

Niu, N., Mercado-Uribe, I., & Liu, J. (2017). Dedifferentiation into blastomere-like cancer stem cells via formation of polyploid giant cancer cells. Oncogene, 36(34), 4887–4900. https://doi.org/10.1038/onc.2017.72

Zhang, X., Yao, J., Li, X., Niu, N., Liu, Y., Hajek, R. A., Peng, G., Westin, S., Sood, A. K., & Liu, J. (2023). Targeting polyploid giant cancer cells potentiates a therapeutic response and overcomes resistance to PARP inhibitors in ovarian cancer. Science Advances, 9(29), eadf7195. https://doi.org/10.1126/sciadv.adf7195

Zhou, X., Zhou, M., Zheng, M., Tian, S., Yang, X., Ning, Y., Li, Y., & Zhang, S. (2022). Polyploid giant cancer cells and cancer progression. Frontiers in Cell and Developmental Biology, 10, 1017588. https://doi.org/10.3389/fcell.2022.1017588

There is an ongoing search for better and more effective treatments for cancer. The reason cancer is difficult to treat is because many cancer cells can evade the immune system. Chimeric antigen receptor (CAR) T cell therapy uses the patient’s own immune cells to treat their cancer. During CAR-T therapy, T cells, which are immune system cells that help kill off pathogens, are extracted from a patient’s blood. These T cells are then mixed with a virus that allows the T cells to develop a chimeric antigen receptor (CAR), an artificial receptor that will navigate toward the location of the cancer. Then, the CAR-T cells are released back into the patient’s bloodstream intravenously. The CAR-T cells track and home in on a tumor-associated antigen (TAA) that is specific to the patient’s cancer. The CAR-T cells then release molecules that trigger cell death pathways within the tumor cells. This process is illustrated in Figure 1. 

Figure 1 CAR-T cell therapy process from the National Cancer Institute. 

 CAR-T cells are shown to be effective at treating blood cancers such as leukemia and lymphoma. Despite the success CAR-T cells have with these cancer types, this therapy is much less effective for treating solid tumors. CAR-T cells can only induce lysis once they bind to the cell surface. However, solid tumors have a small surface area to volume ratio, making it difficult for CAR-T cells to infiltrate and reach all the tumor cells. Even if it does, the rate at which the tumor cells are killed by this process is much slower than the rate at which the tumor cells replicate, rendering the system inefficient. Another challenge lies in the identification of markers that are “specifically and uniformly expressed on heterogeneous solid tumors” (Vincent et al., 2023). Heterogeneous solid tumors are ones that have a sub-population of cells with different features to that of the tumor cells. TAAs identified are rarely tumor-specific, meaning most antigens are present in healthy cells too. Thus, the use of these TAAs carries a high risk of fatal on-target, off-tumor toxicity that kills the vital, healthy cells of the patient.   

To better treat solid tumors, Vincent et al. from Columbia University took a slightly different approach whereby they created probiotic-guided CAR-T cells (ProCARs) that responded to synthetic CAR targets released by tumor-colonizing colonizing probiotic bacteria. The researchers engineered a probiotic strain, Escherichia coli Nissle 1917 (EcN), which is used as a delivery vesicle for synthetic CAR targets called Tags. EcN is able to colonize tumor cells and grow in tumor microenvironments (TMEs) and contains quorum-regulated circuits, meaning bacteria will die after reaching a certain population density threshold and release Tags within the tumor. When Tags are released, they broadly bind to surface and matrix proteins of the tumor cell. CAR-T cells are engineered with GFP28z receptors, which will bind to Tags and activate the CAR-T cell. Thus, activation of CAR-T cells allows for Tag-mediated in situ lysis of the tumor cells.  

Overall, this system overcomes the two previous challenges faced when treating solid tumors. By using Tag, this system allows for antigen-agnostic immunotherapy, meaning eliciting an immune response against tumor cells without the identification of TAAs. Furthermore, since the probiotic bacteria migrates to the tumor location, this treatment mechanism minimizes off-tumor toxicity. Last but not least, the advantage of using bacteria in this system lies in its ability to colonize tumor cells and grow within TMEs, enabling treatment options for difficult-to-infiltrate tumors.  

This system has shown significant lysis across seven human cancer cell lines. Empirical data reflect no significant weight loss shown in mice, no bacterial growth outside of tumor cells or in any healthy organs, and Tag plasmids were well maintained in vivo. While this treatment has been shown to be effective, the potential toxicity from repeated dosing of bacterial therapy in humans is still an important concern for clinical translation, since humans are more sensitive to toxins inside the bacterial cell than mice. Overall, the researchers conclude that probiotic-guided CAR-T cells may provide a novel and safe treatment method for solid tumors.  

 

Works Cited: 

T-cell Transfer Therapy—Immunotherapy—NCI (nciglobal,ncienterprise). (2022, April 1). [cgvArticle]. https://www.cancer.gov/about-cancer/treatment/types/immunotherapy/t-cell-transfer-therapy 

Vincent, R. L., Gurbatri, C. R., Li, F., Vardoshvili, A., Coker, C., Im, J., Ballister, E. R., Rouanne, M., Savage, T., de los Santos-Alexis, K., Redenti, A., Brockmann, L., Komaranchath, M., Arpaia, N., & Danino, T. (2023). Probiotic-guided CAR-T cells for solid tumor targeting. Science, 382(6667), 211–218. https://doi.org/10.1126/science.add7034