Breast Cancer Stem Cells; Tumour Suppressor pRb

There is growing but incomplete evidence that cancer may be organized in a hierarchy in which only a fraction of cells, termed Tumor Initiating Cells (TICs; also referred to as Cancer Stem Cells, CSC), is capable of instigating cancer and metastatic disease. In contrast, the majority of tumor cells represents progenitor and partially differentiated cells that have lost their tumorigenic potential. Thus, targeted killing of TICs may be curative.

Our laboratory is using several mouse models of breast cancer to analyze TICs, their biology, the pathways by which they divide and their unique properties relative to mammary stem cells (MSCs). The use of mouse models provides ample supply of primary tumors with defined genetic background to study these rare tumorigenic cells. In addition, powerful genetic manipulations in the mouse allow us to test the basic paradigms of the CSC model in vivo in isogenic mice.

We recently reported on the first identification of TICs in a mouse model of Her2/Neu, one of the most aggressive forms of breast cancer in human. We showed that TICs are functionally indistinguishable from tumorsphere initiating cells, which give rise to spheres in non-adherent conditions. These tumorspheres provide a means by which to screen small molecule libraries for TIC specific therapeutic targets. We are also attempting to purify Her2/Neu TICs to near homogeneity so we can test whether a single TIC can induce Her2/Neu tumors following transplantation into the mammary gland of a recipient mouse, as well as establish a gene expression signature for the Her2/Neu TIC. Our long-term goals are to develop TIC specific inhibitors for the major breast cancer subtypes.

One novel model that we have recently developed involves the targeted inactivation of the tumor suppressor pRb in the mammary gland. Rb is mutated in nearly 30% of human breast tumors and the mice we created produce similar spectrum of breast tumors as in human. We will next attempt to identify TICs and therapeutic targets in these Rb mutant tumors as well as define the effect of Rb status on the response of tumor cells to chemo- and hormonal therapies in vivo.

In addition to mutations that disrupt the Rb gene, pRb is often inactivated in cancer by phosphorylation induced by cyclin dependent kinases. To study the effect of pRb phosphorylation in vivo, we created mutant mice in which key Ser/Thr phospho-acceptor sites are substituted to Ala residues by homologous re-combination. Analysis of these phospho-mutant Rb knock-in mice reveals unanticipated roles for this tumor suppressor in genomic stability and aging.

Graduate Students:

Selected References:

• Jeff C. Liu, Tao Deng, Rajwinder S. Lehal , Jinny Kim and Eldad Zacksenhaus. Identification of tumorsphere- and tumor-initiating cells in Her2/Neu mammary tumors. Cancer Research (2007) 67:8671-8681.

• Jiang Z. and E. Zacksenhaus. Activation of retinoblastoma protein in mammary gland leads to ductal growth suppression, precocious differentiation and adenocarcinoma. J. Cell Biology (2002) 156:185-198.

• Ho, A., H. Li,, R. Hakem, T. W. Mak and E. Zacksenhaus. Coupling of Caspase-9 to Apaf-1 in response to loss of pRb or cytotoxic drugs is cell type dependent. EMBO J (2004) 23:460- 472

• Ho, T. A., H. Li, Okada, H., T. W. Mak and E. Zacksenhaus. XIAP dictates Apaf-1 dependency for caspase-9 activation. Mol. Cell. Biol. (2007) 27:5673-5685