Podocalyxin as a Basal-like Breast Cancer Stem Cell Marker

Institution: University of Southern California
Investigator(s): Graham Casey, Ph.D. -
Award Cycle: 2009 (Cycle 15) Grant #: 15IB-0076 Award: $243,676
Award Type: IDEA
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease



Initial Award Abstract (2009)

Breast cancers are heterogeneous in their clinical course and their response to therapy. This is largely due to differences in their underlying biology, with at least five different types of breast cancers being recognized through gene expression and pathology studies. Of these, basal-like is one of the most aggressive forms of breast cancer is associated with poor prognosis and a high risk of metastases. One of the current prevailing theories in breast cancer recurrence and metastasis is that metastases may develop from a small population of breast cancer stem cells that may be resistant to established therapies. However this theory is not without controversy. In support of this theory, populations of cancer cells can be identified in breast cancers and cancer cell lines that have some stem cell characteristics such as prolonged growth capacity in tissue culture and the ability (albeit limited) to reestablish the phenotypic heterogeneity of the primary tumor. Recent studies have shown such cells to be basal-like in phenotype and often to be associated with reduced expression of the gene BRCA1. Our new and unpublished studies implicate another gene, podocalyxin as a potential regulator of the basal-like phenotype. Podocalyxin is a cell surface glycoprotein structurally related to the vascular endothelium marker CD34 that is expressed on the surface of a wide range of cells including kidney podocytes, vascular endothelial cells, hematopoietic stem cells, and platelets. Podocalyxin is best characterized in the kidney where it functions as an anti-adhesion molecule that maintains open filtration pathways between neighboring podocyte foot processes.

We will use complementary cellular and molecular approaches using mouse models to establish a role for podocalyxin in basal-like breast cancer and breast cancer stem cell development. The expression of the podocalyxin gene in established breast cancer cell lines will be modulated. This will allow us to determine if increased expression leads to a basal-like phenotype with an increased population of cancer stem cells. Likewise, we will test whether decreased expression leads to a reversal of this phenotype. The subsequent aggressiveness of these “cancer stem cells” will be determined by assessing their growth as tumors in mice.

This project focuses on trying to establish the role of podocalyxin signaling in the development of basal-like breast cancer and breast cancer stem cells (CSCs) with the intermediate goal of identifying underlying molecular mechanisms and future goal of identifying novel targets for the treatment of this devastating disease. Basal-like breast cancer accounts for around 15% of all breast cancers. However, basal-like breast cancers arising in premenopausal African American women account for nearly 40% of breast cancers arising in this population. If our hypothesis is correct we will have identified a signaling pathway that represents a potential novel therapeutic target.




Final Report (2011

At least five different types of breast cancer have been recognized through gene expression and pathology studies. The basal-like breast cancer sub type is one of the most aggressive forms of breast cancer and is associated with poor prognosis and a high risk of metastases. One of the current theories of breast cancer recurrence is that metastases develop from a small population of breast cancer stem cells that may be resistant to established therapies. Indeed, cell populations can be identified from breast tumors that exhibit cancer stem cell characteristics such as prolonged growth in tissue culture and the limited ability to re-establish the phenotypic heterogeneity of the primary tumor. Recent studies suggest that breast cancer stem cells have a basal-like phenotype and are associated with reduced BRCA 1 expression.

Our studies have implicated the protein podocalyxin-like (PODXL) as a potential negative regulator of BRCA 1 and therefore possibly as a regulator of the basal-like phenotype. We have now been able to confirm our original findings that PODXL expression in breast cancer cells correlates with a reduction in BRCA 1 gene expression, as well as a reduction in expression of the estrogen receptor alpha gene. Most importantly we were able to demonstrate that PODXL expression led to an increase in the cancer stem cell population in these cells.

Studies are now in progress to confirm these findings and to determine the relationship between the appearance of these cancer stem cells and the basal-like phenotype. In parallel with these studies, we also investigated the effect of POD XL expression on tumor growth in a mouse model. Our preliminary results suggest that PODXL leads to an increase in the number of tumors that can grow and metastasize in this model system – however these results remain preliminary and need to be replicated in larger studies.

These findings support a role for podocalyxin in the down regulation of BRCAI and an increased cancer stem cell population. These findings are consistent with the basal-like breast cancer phenotype; although to date we have not confirmed that relationship. These data suggest that podocalyxin represents a marker of more aggressive breast cancer and potentially a target for treatment of aggressive breast cancer.