Mechanism of HIV latency and role of INI1 in LTR regulation

Tokameh Mahmoudi, J. David Gladstone Institutes
Biomedical and Clinical Sciences

Over the last decade treatment of the Human Immunodeficiency Virus type 1 (HIV-1) infection has made significant progress. However, one of the main problems remaining is that subsequent to integration, HIV can establish a dormant, latent state within its target resting CD4 T cells. These infected cells persist in patients despite treatment with highly active antiretroviral therapy. Reactivation of latently infected cells leads to reactivation of viral gene expression and propagation of the viral life cycle. Thus, the continuous presence of a pool of HIV-1 infected latent cells generates problems in drug targeting and treatment of the infection. Therefore it is pertinent to understand the underlying molecular mechanisms of latency establishment and re-activation. Expression of HIV genes and its replication is controlled by the virus Long Terminal Repeat (LTR) element, which directs transcription of the viral genome. Latently infected cells contain replication-competent integrated HIV-1 genomes that are blocked at the transcriptional level. A critical factor in viral re-activation is the LTR-directed transcription of the viral genome. This involves complex interactions among the virus DNA, viral proteins as well as cellular factors. The aim of this study is to elucidate the underlying molecular mechanisms, which contribute to the establishment of HIV latency and which regulate the expression of HIV genes. One candidate, which may be involved in controlling the activity of the HIV LTR and HIV gene expression, is the cellular factor Integrase Interactor-1 (INI-1). INI-1 was first identified as a binding partner for the HIV Integrase protein, important for virus integration into the genome, and is packaged and present in the incoming HIV virus particle. INI-1 is also a component of chromatin remodelling complexes, which by changing the structure of the genome regulate expression of genes. Therefore INI-1 is an attractive candidate to regulate the HIV LTR. Our preliminary results indicate that INI-1 is a co-factor for the viral protein Tat in activating the HIV LTR. We will further delineate the mechanism by which INI-1 acts to activate HIV expression. We have shown previously that the site of integration of the HIV genome in the host is an important determinant of HIV LTR activity and generation of HIV latency. In order to understand how the site of integration is involved in silencing of HIV it is important to understand the mechanism of silencing. Several mechanisms have been proposed to mediate silencing of genes. Recent studies have elucidated an important role for double stranded RNA (dsRNA) and the RNA interference machinery in the establishment of silencing at certain genomic regions. In the case of an integrated HIV genome, dsRNA can arise from integration of the virus in opposite orientation to that of a host gene, by transcription of the viral LTR repeat elements and by integration near silent genomic regions called heterochromatin. Such dsRNA products may trigger RNA interference based silencing mechanism at the site of virus integration. Thus, we will examine whether dsRNA and the RNA interference machinery play a role in silencing of HIV and generation of latency. Understanding the underlying molecular mechanisms involved in HIV latency and in regulation of HIV LTR activity will identify novel targets to eradicate latent HIV infection in addition to current treatments directed against the active infection.