(627i) Molecular Mechanisms of HIV-1 Latency: Stochastics in Gene Expression and Chromatin Regulation

All living organisms rely on precisely regulated gene expression to control the complex intra- and inter-cellular molecular interactions that orchestrate their overall development and health. Likewise, some viruses exploit host gene regulation machinery to control their own gene expression for maintenance and replication. All of these events involve complex interactions among collections of genes that regulate each other's function and thereby generate a network or ?circuit? with highly complex behavior such as feedback and amplification.

HIV, the retrovirus that causes AIDS, can establish rare, latent infections of cells, and the resulting latent pools of virus represent the most significant barrier to elimination of virus from a patient since they persist for decades and can reactivate at any time. After HIV enters a cell, it integrates its genetic material into the host genome and then utilizes the transcriptional machinery from the host cell to regulate its gene expression. Furthermore, efficient HIV transcriptional regulation also requires the viral protein Tat, a protein encoded within in the HIV genome. Since the viral protein Tat helps catalyze the transcription of HIV genome, this constitutes a strong positive feedback circuit. Genetic positive feedback circuits can often attain two distinct states (active and inactive), and the state of circuit may be susceptible to genetic noise which can lead to switching between states. Such sources of noise become increasingly important in systems with low concentrations of regulatory factors, such as an HIV infected cell with very low numbers of Tat molecules. We have previously reported that stochastic fluctuations in Tat lead to phenotypic diversity in HIV gene expression, a phenomenon that can be observed experimentally and predicted computationally with stochastic modeling [1]. We have hypothesized that stochastic delays in the onset of viral gene expression can contribute to the formation of latent viral infections.

The HIV viral promoter, a primary determinant of viral gene expression dynamics, is regulated by the cellular signaling state, the chromatin environment of the viral integration site, and the level of Tat. Computational studies predict that transcription factor binding sites in the viral promoter, as well as the surrounding chromatin state, can exert a strong influence on the dynamic behavior of the feedback loop. We are performing experiments to test these predictions, and the resulting combined experimental and computational system can provide quantitative predictions of the molecular mechanisms contributing to the establishment of post-integration viral latency. Furthermore, this computational approach of examining HIV gene expression will guide in the developments of new treatments for HIV latency, by either activating these cells so that they can be targeted by conventional drugs or by inducing all cells into a persistent latent state.

1. Weinberger, L.S., Burnett, J.C., Toettcher, J.E., Arkin, A.P., and Schaffer, D.V. (2005) Stochastic gene expression in a lentiviral positive-feedback loop: HIV-1 Tat fluctuations drive phenotypic diversity. Cell, 122, 169-82.