Human African Trypanosomiasis (HAT), also known as sleeping sickness, is a neglected tropical disease caused by the protozoan parasite Trypanosoma brucei. T. brucei evades the host immune system by switching between thousands of variant surface glycoproteins (VSGs) expressed on its cell surface in a process called antigenic variation. VSG genes are monoallelically expressed from one of ~15 telomeric expression sites, while the rest are kept transcriptionally silent. Switching occurs when a new VSG is copied into the active expression site (gene conversion), or when a new expression site becomes active (transcriptional switching). Immune evasion via antigenic variation requires tight control of two processes: monoallelic VSG expression and VSG switching, yet the molecular processes that control monoallelic expression and switching remain mysterious (McCulloch, 2004). The established role for transcription in enhancing recombination (Gottipati and Helleday, 2009) suggests that processes affecting monoallelic expression of the VSGs will also affect switching. While the mechanisms that maintain silencing at the inactive expression sites are largely unknown, it is clear that chromatin-interacting factors play a role (reviewed in Horn and McCulloch, 2010). My preliminary data indicate that treatment of trypanosomes with a small molecule that inhibits bromodomain-containing proteins results in transcriptional changes at the silent expression sites, expression of multiple VSGs simultaneously, and an increase in VSG switching frequency. I hypothesize that bromodomain-containing proteins, a subclass of proteins responsible for "reading" the histone code, have a role in regulating monoallelic expression and switching of the VSGs. To test this, I will functionally characterize bromodomain-containing proteins in trypanosomes through localization and knockdown studies, and further ask whether these proteins influence the transcriptional activity of the expression sites. Lastly, I will ask whether modulating the transcriptional state of the expression site affets the frequency of VSG switching. These studies will shed light on the mechanism of immune evasion for this deadly parasite. Understanding these mechanisms could provide new ideas for therapeutic intervention and thus have a positive impact on human health.
The goal of this project is to elucidate mechanisms of immune evasion by the protozoan parasite Trypnosoma brucei, the causative agent of Human African Trypanomiasis (HAT). Specifically, I will examine how the parasite regulates switching of the protein coat expressed on its cell surface to elude recognition by the host immune system.