Susan R. Ross, Ph.D.

Professor of Microbiology

Office Address:
University of Pennsylvania School of Medicine
313 BRBII/III
421 Curie Blvd
Philadelphia, PA 19104-6141

TEL 215-898-9764
LAB 215-898-2986
FAX 215-573-2028
rosss@mail.med.upenn.edu

RESEARCH SUMMARY

All animals show differential susceptibility to infection with viruses. We use mouse mammary tumor virus (MMTV), an endemic mouse retrovirus, to understand virus/host interactions, since the genetics of susceptibility is easily studied with naturally-occurring pathogens in inbred and genetically-manipulated mice. Infectious MMTV is passed from mothers to offspring through milk and first spreads in lymphoid cells before infecting mammary epithelial cells. MMTV causes breast cancer when the viral genome inserts next to cellular oncogenes, thereby activating their expression. Our studies focus on understanding the mechanisms that determine susceptibility to MMTV infection and virus-induced mammary tumors.

One area of investigation in the lab is how retroviruses infect their initial targets in vivo, since most require activated cells as their targets for infection. We showed that MMTV activates dendritic and B cells via toll-like receptor 4 (TLR4), a component of the innate immune system; we also demonstrated that dendritic cells are the initial targets of infection in vivo. The dendritic and B cells of mice with tlr4 mutations show diminished activation by MMTV. MMTV interacts with TLR4 through binding of its envelope protein; more recently, we have discovered that MMTV signals through and binds to another toll-like receptor, TLR2. We are currently focused on determining how dendritic cell infection and TLR signaling affects host immune responses to MMTV.

Another gene which we recently discovered is involved in the control of MMTV infection is apobec3. The genomes of all mammals encode apobec3 genes which play a role in intrinsic cellular immunity to a number of viruses, including human immunodeficiency virus type 1. APOBEC3 proteins are packaged into virions and inhibit retroviral replication in newly infected cells, at least in part by deaminating cytosine on the negative strand DNA intermediates. We found that mouse APOBEC3 protein is packaged into MMTV particles in vitro and dramatically reduces viral titers. Most importantly, APOBEC3 knockout mice are more susceptible to MMTV infection compared to their wild type littermates. These findings indicate that the APOBEC3 provides protection to mice against MMTV infection and represent the first demonstration that it functions during retroviral infection in vivo. Whether the accelerated infection and spread of MMTV in mA3 -/- mice affects its ability to cause breast cancer is currently under investigation.

We also identified another genetic locus in C3H/HeN mice that confers dominant susceptibility to MMTV infection in backcrosses with B10.BR mice, which are relatively resistant to milk-borne infection and mammary tumorigenesis. We mapped the locus responsible for this phenotype on G2 backcross mice ([C3H x B10] x B10) to a region on mouse Chr. 4. This locus affects virus spread in lymphocytes. We are currently carrying out more detailed mapping and testing candidate genes for linkage with the phenotype. The results obtained from this combined functional/genetic approach will greatly increase our understanding of the mechanisms which viruses use to infect their hosts and how genetic resistance to such viruses in the hosts occurs.

Numerous reports in recent years have implicated a virus highly related to MMTV in human breast cancer and patients with primary biliary cirrhosis, an autoimmune disease, making it important to determine whether the MMTV can infect human cells. We showed that the mouse mammary tumor virus cell entry receptor is mouse transferrin receptor 1 (TfR1) and that mouse and rat but not human, hamster, dog or cat TfR1 function as MMTV entry receptors. Comparison of the 6 protein sequences revealed a small number of amino acids conserved between mouse and rat that differ from the human, hamster, dog and cat TfR1s. By constructing hybrid mouse/human TfR1s we mapped the segments of the mouse receptor important for virus infection, thus confirming that human TfR1 does not function as an MMTV entry receptor. We also tested whether the envelopes of the “human” MTVs had undergone changes that altered virus tropism for human TfR1. However, introduction of the hMTV changes into MMTV did not alter virus tropism, making it unlikely that the mouse virus jumped species. We are currently studying how MMTV traffics with TfR1 within the cell during virus entry.

A final area of research in the lab is on the role of the MMTV envelope protein in breast cancer induction. In collaboration with John Monroe’s lab, we found that ectopic expression of the MMTV envelope protein in normal mammary epithelial cells resulted in phenotypic transformation and that an immuno-tyrosine based activation motif (ITAM) in this protein was critical to this activity. Moreover, mutation of the ITAM motif in an infectious MMTV dramatically attenuated its ability to cause mammary tumors, without affecting its infectivity in vivo. ITAMs are commonly found in receptors expressed in hematopoietic cells and are negatively regulated by cell-type specific modulators. We speculate that uncontrolled signaling by the envelope protein in epithelial cells, which lack such negative modulators, is an early step in the MMTV transformation process. Because ITAMs are found both in viral and cellular proteins, inappropriate expression of such signaling molecules represents a novel mechanism of transformation and is of potential importance in developing new treatment paradigms for breast and other cancers, especially those associated with viruses that encode proteins that activate ITAM-mediated signaling.

RECENT PUBLICATIONS