Novel Tumor Suppressors in Breast Development and Cancer

Institution: Stanford University
Investigator(s): Margaret Fuller, Ph.D. -
Award Cycle: 2009 (Cycle 15) Grant #: 15IB-0068 Award: $230,373
Award Type: IDEA
Research Priorities
Biology of the Breast Cell>Biology of the Normal Breast: the starting point

Initial Award Abstract (2009)

Cancers commonly arise in adult stem cell lineages. The cells that line the milk producing ducts of the breast are produced through proliferation then differentiation of adult stem cells, in response to hormonal signals dependent on gender, puberty and pregnancy. The normal mechanisms that “switch” precursor cells from proliferation to differentiation states in stem cell lineages may protect against cancer by blocking continued cell proliferation. Loss of these tumor suppressor mechanisms may underlie initiation of cancer in breast tissue and adult stem cell lineages other organs prone to cancer.

We plan to test the hypothesis that a novel tumor suppressor mechanism recently shown to control the switch from proliferation to differentiation in stem cell lineages in a model organism may also be required to stop precursor cell proliferation and allow terminal differentiation of duct cells in the mammalian breast. We will “knock down” the function of two novel mammalian tumor suppressors, called Bgcn and MeiP26/Brat, in mouse mammary gland precursor cells. We will test whether loss of Bgcn and/or MeiP26/Brat leads to an inability of the breast precursor cells to cease proliferation and correctly differentiate when cells are transplanted into the mouse mammary fat pad. If so, we will then test if loss of function of these tumor suppressors leads to mammary gland tumors in mice. The stem cell lineage tumor suppressors we are studying work by a novel mechanism involving translational control, most likely directed by microRNAs, and these have not previously been tested in mammalian cells.

If our experiments demonstrate that these tumor suppressors block proliferation and turn on differentiation in the breast stem cell lineage, it will open a new field of research. Our proposed research is also innovative in that it takes advantage of recent enabling breakthroughs in isolation, culture and transplantation of breast stem cells in the laboratory of our colleague and collaborator Dr. Roel Nusse. We envision these studies, if successful, having the potential to provide new genetic tests to identify women at risk.

Progress Report 1 (2010)

The system of ducts and support cells that form the milk-producing structure of the breast develops in the mammary fat pad during puberty from mammary gland stem cells. As in many other adult stem cell lineages, breast precursor cells first proliferate, then must stop dividing and differentiate to form the ducts. Cancers commonly arise in adult stem cell lineages and may initiate when cells fail to correctly make the developmentally programmed switch from proliferation to terminal differentiation. Thus genes that activate the switch from proliferation to differentiation in adult stem cell lineages may be a new class of developmental tumor suppressors.

We discovered two genes required for precursor cells to stop proliferating and initiate differentiation in a model adult stem cell lineage in the laboratory fruit fly, Drosophila. These genes act at the level of translational control in a mechanism that involves microRNAs. In this IDEA award, we are testing the hypothesis that mammalian homologs of these potential developmental tumor suppressors might be required for precursor cells to stop proliferation and turn on terminal differentiation in the stem cell lineage of the breast. If so, our work may identify a new class of tumor suppressors and a novel mechanism (translational control) involved in the initiation of breast cancer.

In the current funding period, we identified 4 homologs of the tumor suppressors expressed in mouse mammary gland and made lentiviral particles containing shRNAs to knock down expression of two of these candidates in mouse mammary precursor cells. We have now successfully transplanted the lentivirus infected precursor cells into the cleared mammary fat pad of recipient mice to test whether loss of function of the tumor suppressor homologs causes mammary gland precursor cells to overproliferate rather than differentiate into normal mammary gland ducts.

In the next funding period, we will analyze the effects of loss of function of these first two tumor suppressor homologs on mammary gland development in vivo from the transplantation experiment above. In addition, we will create the shRNA constructs and lentivirus reagents to test the effect of lowering function of three other homologs of the tumor suppressors. For these three additional tumor suppressor homologs, we will transfect mouse mammary gland stem and precursor cells with the shRNA-lentiviruses then transplant the cells into cleared mammary fat pad to assay the effects of loss of function on proliferation and differentiation of mammary gland ductal cells in vivo.

If we find that any of the genes are required to stop proliferation and turn on differentiation of mammary gland cells, we will test if mice that carry one mutant copy of any of the genes (-/+) are predisposed to develop breast cancer. These studies are highly relevant to breast cancer and could lead to new genetic diagnostic tests for predisposition to breast cancer.