The development of intratumoral heterogeneity in ovarian tumors: role of cancer stem cells in disease progression
Lunsford, Elaine Patricia
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Like with many cancers, a single ovarian tumor can display remarkable diversity in genetics, epigenetics, expression profiles, microenvironment and cell differentiation and plasticity. This so-called intratumoral heterogeneity (ITH) is thought to greatly increase mortality by enabling tumors to adapt quickly to therapy, metastasize, and recur, thus the study of ITH holds great clinical significance. Clonal evolution and cancer stem cell (CSC) theory are two models for the initiation and propagation of a tumor, which offer differing views on the way that ITH is developed and maintained. In the clonal evolution model, cancer arises from a single cell and, through genetic instability, proliferates into a diverse population of daughter cells, which develop additional mutations and undergo Darwinian selection under the influence of the tumor microenvironment. Each cell of the clonal evolution model may be capable of initiating a tumor independently. In CSC theory, cancer arises from the transformation of a stem cell that has the capacity to self-renew and differentiate into a diverse population of daughter cells. Each cell is NOT capable of tumorigenesis as most are terminally differentiated and do not harbor self-renewing capabilities. According to CSC theory, small, rare subpopulations of CSCs persist throughout chemotherapy and are responsible for repopulating the heterogeneous tumor post-treatment. The hypothesis that CSCs may play a role in ovarian cancer progression is the subject of this thesis. Many studies have detected the presence of stem cell markers and dysregulated stem cell signaling pathways in ovarian cancer, but doubts remain as to the existence of ovarian CSCs; critics have pointed out inherent flaws in experimental designs meant to identify and characterize CSCs. For example, the presence of cancer cells which express the stem cell marker CD133 has been correlated to both positive and negative impacts on prognosis. Further challenging the study of ovarian CSCs is the lack of consensus on the true cell of origin for ovarian cancer - whether it be from the fallopian tube epithelium or ovarian surface epithelium, or elsewhere in the peritoneal cavity - this will have important implications for the identification and characterization of tumorigenic ovarian CSCs. Advocates of clonal evolution theory have put forth incredible effort to reveal the extent of inter and intra-tumoral heterogeneity in ovarian cancer, and from these data there has arisen a general consensus that cancer cell populations do evolve in a step-wise fashion, accumulating additional mutations over time. The involvement of cancer stem cells in this progression and how exactly they fit in (as a cell of origin or arising from genetic mutations), as well as their significance for different cancer types, is a question worth answering. Despite the challenges facing the study of ovarian CSCs, the clinical impact of cells with stem-like properties has been repeatedly demonstrated, especially with regard to metastatic processes and chemoresistance. Moreover, new drugs which target stem cell pathways have proven effective in the treatment of ovarian cancer. The existence of a rare subset of cells that have enhanced tumor-initiating properties is apparent in ovarian cancer, and more work is needed to characterize the unique identifiers and behavior of these cells in vivo. Future experiments involving lineage tracing promise to deepen our understanding of the nature of ovarian CSCs and address whether normal stem cells might serve as the cell of origin.