(383d) Tat-Regulated Promoter Occupancy Mediates Open Loop Stochastic Bimodality In HIV1 Gene Expression And Latency

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 semi-randomly 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.

We have conducted a rigorous experimental and computational analysis of the gene expression dynamics in a retroviral model of HIV-1. This model retroviral system is derived from HIV-1 and is missing all viral pathogenic functions but contains its ability to integrate chromosomally, its full enhancer and promoter elements, and its implementation of the Tat-mediated positive feedback loop governing transcriptional activation. We have previously reported that stochastic fluctuations in Tat lead to phenotypic diversity, or stochastic bimodality, in HIV-1 gene expression, a phenomenon that can be observed experimentally and predicted computationally with stochastic modeling. We have hypothesized that Tat-mediated feedback is activated sporadically such that after the virus infects a target activated T cell and integrates into the host genome, significant periods of time may elapse before viral expression reaches the point where reproduction and propagation can occur. This time could be enough to allow an activated T cell to transition to its memory state, thereby trapping the lentivirus in an inactive form until such time as that memory cell is reactivated.

We now build on our previous discovery with experimental and computational approaches which utilize a mechanistic model of the HIV-1 promoter, regulated by cis-regulatory factors such as NFκB and Sp1, in addition to viral transactivator Tat. We show that stochastic bimodality may result from a combination of occupancy states of this promoter. Ultimately, these findings may give insight into the establishment of HIV-1 latency and may lead to the mechanistic understanding and design of therapies to purge and eradicate latent HIV-1 reservoirs.