Research Interests
Our research interests involve many aspects of retroviral replication. We study HIV-1, Moloney murine leukemia virus, and XMRV, a novel xenotropic murine-like retrovirus that has recently been implicated in human prostate cancer.

Our studies with XMRV consist of elucidating the role of this newly discovered virus in human prostate cancer. XMRV is the first gammaretrovirus known to infect humans. While gammaretroviruses have well-characterized oncogenic effects in animals, they have not been shown to cause human cancers. Our experiments show that XMRV is indeed a gammaretrovirus with protein composition and particle ultrastructure highly similar to Moloney murine leukemia virus, another gammaretrovirus that we have studied for the last several years. We analyzed 334 consecutive prostate resection specimens using a quantitative PCR assay and immunohistochemistry with an anti-XMRV specific antiserum. We found the virus in 27% of prostate cancers. XMRV proteins are expressed primarily in malignant epithelial cells, suggesting that retroviral infection may be directly linked to tumorigenesis. We have shown that XMRV-infection is associated with prostate cancer, especially higher-grade cancers.  We are currently investigating possible mechanisms of oncogenesis by XMRV in cultured cells, in human tumors and in a mouse model.

XMRV virions, as visualized by transmission electron microscopy.

We are also involved in a detailed analysis of different steps of the retroviral lifecycle.  Using genetic footprinting, a technique that we developed, we performed a saturating mutagenesis of retroviral sequences. This study led to the identification of several retroviral sequences that are essential for viral replication. We analyzed some of these regions in greater detail and determined their precise role in the viral life cycle, which included uncoating, nuclear transport of the viral replication intermediates, capsid assembly and viral release. We are currently using genetic footprinting to identify what domains of HIV-1 are important for transport of the viral DNA into the nucleus. Transport into the nucleus needs to occur for the viral DNA to integrate into the host chromosome. Yet, we understand very little about how it occurs. HIV DNA can enter the nucleus of non-dividing cells, and these cells can serve as a reservoir for virus that remains inaccessible to anti-viral drugs. Understanding nuclear transport of HIV and designing methods to block it, will be very important to address this otherwise inaccessible fraction of virus that is resistant to prolonged anti-viral therapy.

More recently we have identified a domain of retroviral CA protein that is important for formation of viral cores. Mutations in this domain result in the assembly of viral proteins in flat or dome shaped patches on the plasma membrane, and a complete failure of spherical core formation, or of particle release. Electron microscopy, structural comparisons and molecular modeling suggest that this domain is important for the formation of pentamers, which provide curvature in the core. Our data are most consistent with a novel model for formation of pentamers, leading in turn to a novel model for retroviral assembly that may have general relevance to other spherical protein assemblies. More specifically, blocking pentamer formation by focusing on a few residues of a small loop, may offer novel therapeutic approaches for retroviral diseases in general.